Post on 15-Oct-2021
Chapter 6 Link Layer and LANs
UG3 Computer Communications amp Networks(COMN)
Mahesh Marinamaheshedacuk
Slides copyright of Kurose and Ross
Link layer introduction
terminologybull hosts and routers nodesbull communication channels that
connect adjacent nodes along communication path linksndash wired linksndash wireless linksndash LANs
bull layer-2 packet frameencapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
6-2Link Layer and LANs
Link layer context
bull datagram transferred by different link protocols over different linksndash eg Ethernet on first link
frame relay on intermediate links 80211 on last link
bull each link protocol provides different servicesndash eg may or may not
provide rdt over link
transportation analogybull trip from Princeton to Lausanne
ndash limo Princeton to JFKndash plane JFK to Genevandash train Geneva to Lausanne
bull tourist = datagrambull transport segment =
communication linkbull transportation mode = link
layer protocolbull travel agent = routing
algorithm
6-3Link Layer and LANs
Link layer services
bull framing link accessndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source destination bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this already (chapter 3)ndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
6-4Link Layer and LANs
bull flow controlndash pacing between adjacent sending and receiving nodes
bull error detection ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame
bull error correctionndash receiver identifies and corrects bit error(s) without resorting to
retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-5Link Layer and LANs
Where is the link layer implementedbull in each and every hostbull link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipndash Ethernet card 80211 card
Ethernet chipsetndash implements link physical
layerbull attaches into hostrsquos system
busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetworklink
linkphysical
6-6Link Layer and LANs
Adaptors communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-7Link Layer and LANs
Error detection
EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-8Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-9Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
senderbull treat segment contents
as sequence of 16-bit integers
bull checksum addition (1rsquos complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
receiverbull compute checksum of
received segmentbull check if computed
checksum equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transportnetwork layers only)
6-10Link Layer and LANs
Cyclic redundancy checkbull more powerful error-detection codingbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) Gbull goal choose r CRC bits R such that
ndash ltDRgt exactly divisible by G (modulo 2) ndash receiver knows G divides ltDRgt by G If non-zero remainder
error detectedndash can detect all burst errors less than r+1 bits
bull widely used in practice (Ethernet 80211 WiFi ATM)
6-11Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Link layer introduction
terminologybull hosts and routers nodesbull communication channels that
connect adjacent nodes along communication path linksndash wired linksndash wireless linksndash LANs
bull layer-2 packet frameencapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
6-2Link Layer and LANs
Link layer context
bull datagram transferred by different link protocols over different linksndash eg Ethernet on first link
frame relay on intermediate links 80211 on last link
bull each link protocol provides different servicesndash eg may or may not
provide rdt over link
transportation analogybull trip from Princeton to Lausanne
ndash limo Princeton to JFKndash plane JFK to Genevandash train Geneva to Lausanne
bull tourist = datagrambull transport segment =
communication linkbull transportation mode = link
layer protocolbull travel agent = routing
algorithm
6-3Link Layer and LANs
Link layer services
bull framing link accessndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source destination bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this already (chapter 3)ndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
6-4Link Layer and LANs
bull flow controlndash pacing between adjacent sending and receiving nodes
bull error detection ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame
bull error correctionndash receiver identifies and corrects bit error(s) without resorting to
retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-5Link Layer and LANs
Where is the link layer implementedbull in each and every hostbull link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipndash Ethernet card 80211 card
Ethernet chipsetndash implements link physical
layerbull attaches into hostrsquos system
busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetworklink
linkphysical
6-6Link Layer and LANs
Adaptors communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-7Link Layer and LANs
Error detection
EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-8Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-9Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
senderbull treat segment contents
as sequence of 16-bit integers
bull checksum addition (1rsquos complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
receiverbull compute checksum of
received segmentbull check if computed
checksum equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transportnetwork layers only)
6-10Link Layer and LANs
Cyclic redundancy checkbull more powerful error-detection codingbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) Gbull goal choose r CRC bits R such that
ndash ltDRgt exactly divisible by G (modulo 2) ndash receiver knows G divides ltDRgt by G If non-zero remainder
error detectedndash can detect all burst errors less than r+1 bits
bull widely used in practice (Ethernet 80211 WiFi ATM)
6-11Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Link layer context
bull datagram transferred by different link protocols over different linksndash eg Ethernet on first link
frame relay on intermediate links 80211 on last link
bull each link protocol provides different servicesndash eg may or may not
provide rdt over link
transportation analogybull trip from Princeton to Lausanne
ndash limo Princeton to JFKndash plane JFK to Genevandash train Geneva to Lausanne
bull tourist = datagrambull transport segment =
communication linkbull transportation mode = link
layer protocolbull travel agent = routing
algorithm
6-3Link Layer and LANs
Link layer services
bull framing link accessndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source destination bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this already (chapter 3)ndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
6-4Link Layer and LANs
bull flow controlndash pacing between adjacent sending and receiving nodes
bull error detection ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame
bull error correctionndash receiver identifies and corrects bit error(s) without resorting to
retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-5Link Layer and LANs
Where is the link layer implementedbull in each and every hostbull link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipndash Ethernet card 80211 card
Ethernet chipsetndash implements link physical
layerbull attaches into hostrsquos system
busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetworklink
linkphysical
6-6Link Layer and LANs
Adaptors communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-7Link Layer and LANs
Error detection
EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-8Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-9Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
senderbull treat segment contents
as sequence of 16-bit integers
bull checksum addition (1rsquos complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
receiverbull compute checksum of
received segmentbull check if computed
checksum equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transportnetwork layers only)
6-10Link Layer and LANs
Cyclic redundancy checkbull more powerful error-detection codingbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) Gbull goal choose r CRC bits R such that
ndash ltDRgt exactly divisible by G (modulo 2) ndash receiver knows G divides ltDRgt by G If non-zero remainder
error detectedndash can detect all burst errors less than r+1 bits
bull widely used in practice (Ethernet 80211 WiFi ATM)
6-11Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Link layer services
bull framing link accessndash encapsulate datagram into frame adding header trailerndash channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify
source destination bull different from IP address
bull reliable delivery between adjacent nodesndash we learned how to do this already (chapter 3)ndash seldom used on low bit-error link (fiber some twisted
pair)ndash wireless links high error rates
bull Q why both link-level and end-end reliability
6-4Link Layer and LANs
bull flow controlndash pacing between adjacent sending and receiving nodes
bull error detection ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame
bull error correctionndash receiver identifies and corrects bit error(s) without resorting to
retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-5Link Layer and LANs
Where is the link layer implementedbull in each and every hostbull link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipndash Ethernet card 80211 card
Ethernet chipsetndash implements link physical
layerbull attaches into hostrsquos system
busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetworklink
linkphysical
6-6Link Layer and LANs
Adaptors communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-7Link Layer and LANs
Error detection
EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-8Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-9Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
senderbull treat segment contents
as sequence of 16-bit integers
bull checksum addition (1rsquos complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
receiverbull compute checksum of
received segmentbull check if computed
checksum equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transportnetwork layers only)
6-10Link Layer and LANs
Cyclic redundancy checkbull more powerful error-detection codingbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) Gbull goal choose r CRC bits R such that
ndash ltDRgt exactly divisible by G (modulo 2) ndash receiver knows G divides ltDRgt by G If non-zero remainder
error detectedndash can detect all burst errors less than r+1 bits
bull widely used in practice (Ethernet 80211 WiFi ATM)
6-11Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
bull flow controlndash pacing between adjacent sending and receiving nodes
bull error detection ndash errors caused by signal attenuation noise ndash receiver detects presence of errors
bull signals sender for retransmission or drops frame
bull error correctionndash receiver identifies and corrects bit error(s) without resorting to
retransmission
bull half-duplex and full-duplexndash with half duplex nodes at both ends of link can transmit but not
at same time
Link layer services (more)
6-5Link Layer and LANs
Where is the link layer implementedbull in each and every hostbull link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipndash Ethernet card 80211 card
Ethernet chipsetndash implements link physical
layerbull attaches into hostrsquos system
busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetworklink
linkphysical
6-6Link Layer and LANs
Adaptors communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-7Link Layer and LANs
Error detection
EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-8Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-9Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
senderbull treat segment contents
as sequence of 16-bit integers
bull checksum addition (1rsquos complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
receiverbull compute checksum of
received segmentbull check if computed
checksum equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transportnetwork layers only)
6-10Link Layer and LANs
Cyclic redundancy checkbull more powerful error-detection codingbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) Gbull goal choose r CRC bits R such that
ndash ltDRgt exactly divisible by G (modulo 2) ndash receiver knows G divides ltDRgt by G If non-zero remainder
error detectedndash can detect all burst errors less than r+1 bits
bull widely used in practice (Ethernet 80211 WiFi ATM)
6-11Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Where is the link layer implementedbull in each and every hostbull link layer implemented in ldquoadaptorrdquo (aka network interface card NIC) or on a chipndash Ethernet card 80211 card
Ethernet chipsetndash implements link physical
layerbull attaches into hostrsquos system
busesbull combination of hardware
software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
applicationtransportnetworklink
linkphysical
6-6Link Layer and LANs
Adaptors communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-7Link Layer and LANs
Error detection
EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-8Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-9Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
senderbull treat segment contents
as sequence of 16-bit integers
bull checksum addition (1rsquos complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
receiverbull compute checksum of
received segmentbull check if computed
checksum equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transportnetwork layers only)
6-10Link Layer and LANs
Cyclic redundancy checkbull more powerful error-detection codingbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) Gbull goal choose r CRC bits R such that
ndash ltDRgt exactly divisible by G (modulo 2) ndash receiver knows G divides ltDRgt by G If non-zero remainder
error detectedndash can detect all burst errors less than r+1 bits
bull widely used in practice (Ethernet 80211 WiFi ATM)
6-11Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Adaptors communicating
bull sending sidendash encapsulates datagram in
framendash adds error checking bits
rdt flow control etc
bull receiving sidendash looks for errors rdt flow
control etcndash extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
6-7Link Layer and LANs
Error detection
EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-8Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-9Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
senderbull treat segment contents
as sequence of 16-bit integers
bull checksum addition (1rsquos complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
receiverbull compute checksum of
received segmentbull check if computed
checksum equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transportnetwork layers only)
6-10Link Layer and LANs
Cyclic redundancy checkbull more powerful error-detection codingbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) Gbull goal choose r CRC bits R such that
ndash ltDRgt exactly divisible by G (modulo 2) ndash receiver knows G divides ltDRgt by G If non-zero remainder
error detectedndash can detect all burst errors less than r+1 bits
bull widely used in practice (Ethernet 80211 WiFi ATM)
6-11Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Error detection
EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
6-8Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-9Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
senderbull treat segment contents
as sequence of 16-bit integers
bull checksum addition (1rsquos complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
receiverbull compute checksum of
received segmentbull check if computed
checksum equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transportnetwork layers only)
6-10Link Layer and LANs
Cyclic redundancy checkbull more powerful error-detection codingbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) Gbull goal choose r CRC bits R such that
ndash ltDRgt exactly divisible by G (modulo 2) ndash receiver knows G divides ltDRgt by G If non-zero remainder
error detectedndash can detect all burst errors less than r+1 bits
bull widely used in practice (Ethernet 80211 WiFi ATM)
6-11Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Parity checking
single bit paritysect detect single bit
errors
two-dimensional bit paritysect detect and correct single bit errors
0 0
6-9Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Internet checksum (review)
senderbull treat segment contents
as sequence of 16-bit integers
bull checksum addition (1rsquos complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
receiverbull compute checksum of
received segmentbull check if computed
checksum equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transportnetwork layers only)
6-10Link Layer and LANs
Cyclic redundancy checkbull more powerful error-detection codingbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) Gbull goal choose r CRC bits R such that
ndash ltDRgt exactly divisible by G (modulo 2) ndash receiver knows G divides ltDRgt by G If non-zero remainder
error detectedndash can detect all burst errors less than r+1 bits
bull widely used in practice (Ethernet 80211 WiFi ATM)
6-11Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Internet checksum (review)
senderbull treat segment contents
as sequence of 16-bit integers
bull checksum addition (1rsquos complement sum) of segment contents
bull sender puts checksum value into UDP checksum field
receiverbull compute checksum of
received segmentbull check if computed
checksum equals checksum field valuendash NO - error detectedndash YES - no error detected
But maybe errors nonetheless
goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transportnetwork layers only)
6-10Link Layer and LANs
Cyclic redundancy checkbull more powerful error-detection codingbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) Gbull goal choose r CRC bits R such that
ndash ltDRgt exactly divisible by G (modulo 2) ndash receiver knows G divides ltDRgt by G If non-zero remainder
error detectedndash can detect all burst errors less than r+1 bits
bull widely used in practice (Ethernet 80211 WiFi ATM)
6-11Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Cyclic redundancy checkbull more powerful error-detection codingbull view data bits D as a binary numberbull choose r+1 bit pattern (generator) Gbull goal choose r CRC bits R such that
ndash ltDRgt exactly divisible by G (modulo 2) ndash receiver knows G divides ltDRgt by G If non-zero remainder
error detectedndash can detect all burst errors less than r+1 bits
bull widely used in practice (Ethernet 80211 WiFi ATM)
6-11Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
CRC example
wantD2r XOR R = nG
equivalentlyD2r = nG XOR R
equivalentlyif we divide D2r by G want remainder R to satisfy
R = remainder[ ]D2r
G
6-12Link Layer and LANs
Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Multiple access links protocolstwo types of ldquolinksrdquobull point-to-point
ndash PPP for dial-up accessndash point-to-point link between Ethernet switch host
bull broadcast (shared wire or medium)ndash old-fashioned Ethernetndash upstream HFCndash 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF(eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
6-13Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Multiple access protocols
bull single shared broadcast channel bull two or more simultaneous transmissions by nodes interference
ndash collision if node receives two or more signals at the same time
multiple access protocolbull distributed algorithm that determines how nodes share
channel ie determine when node can transmitbull communication about channel sharing must use channel itself
ndash no out-of-band channel for coordination
6-14Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
An ideal multiple access protocol
given broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R2 when M nodes want to transmit each can send at average
rate RM3 fully decentralized
bull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
6-15Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
MAC protocols taxonomy
three broad classesbull channel partitioning
ndash divide channel into smaller ldquopiecesrdquo (time slots frequency code)ndash allocate piece to node for exclusive use
bull random accessndash channel not divided allow collisionsndash ldquorecoverrdquo from collisions
bull ldquotaking turnsrdquondash nodes take turns but nodes with more to send can take longer
turns
6-16Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Channel partitioning MAC protocols TDMA
TDMA time division multiple accessbull access to channel in rounds bull each station gets fixed length slot (length = packet
transmission time) in each round bull unused slots go idle bull example 6-station LAN 134 have packets to
send slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
6-17Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
FDMA frequency division multiple access bull channel spectrum divided into frequency bandsbull each station assigned fixed frequency bandbull unused transmission time in frequency bands go idle bull example 6-station LAN 134 have packet to send frequency
bands 256 idle
frequ
ency
ban
ds
time
FDM cable
Channel partitioning MAC protocols FDMA
6-18Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Random access protocols
bull when node has packet to sendndash transmit at full channel data rate Rndash no a priori coordination among nodes
bull two or more transmitting nodes ldquocollisionrdquobull random access MAC protocol specifies
ndash how to detect collisionsndash how to recover from collisions (eg via delayed
retransmissions)bull examples of random access MAC protocols
ndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA
6-19Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Slotted ALOHA
assumptionsbull all frames same sizebull time divided into equal size
slots (time to transmit 1 frame)
bull nodes start to transmit only slot beginning
bull nodes are synchronizedbull if 2 or more nodes transmit
in slot all nodes detect collision
operationbull when node obtains fresh
frame transmits in next slotndash if no collision node can send
new frame in next slotndash if collision node retransmits
frame in each subsequent slot with prob p until success
6-20Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Prosbull single active node can
continuously transmit at full rate of channel
bull highly decentralized only slots in nodes need to be in sync
bull simple
Consbull collisions wasting slotsbull idle slotsbull nodes may be able to
detect collision in less than time to transmit packet
bull clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
6-21Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
bull suppose N nodes with many frames to send each transmits in slot with probability p
bull prob that given node has success in a slot = p(1-p)N-1
bull prob that any node has a success = Np(1-p)N-1
bull max efficiency find p that maximizes Np(1-p)N-1
bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity givesmax efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
6-22Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Pure (unslotted) ALOHA
bull unslotted Aloha simpler no synchronizationbull when frame first arrives
ndash transmit immediately bull collision probability increases
ndash frame sent at t0 collides with other frames sent in [t0-1t0+1]
6-23Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Pure ALOHA efficiency
P(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0t0+1]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
even worse than slotted Aloha
6-24Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
framebull if channel sensed busy defer
transmission
bull human analogy donrsquot interrupt others
6-25Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
CSMA collisions
bull collisions can still occur propagation delay means two nodes may not hear each otherrsquos transmission
bull collision entire packet transmission time wastedndash distance amp
propagation delay play role in in determining collision probability
spatial layout of nodes
6-26Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
CSMACD (collision detection)
CSMACD carrier sensing deferral as in CSMAndash collisions detected within short timendash colliding transmissions aborted reducing channel wastage
bull collision detectionndash easy in wired LANs measure signal strengths compare
transmitted received signalsndash difficult in wireless LANs received signal strength
overwhelmed by local transmission strength
bull human analogy the polite conversationalist
6-27Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
CSMACD (collision detection)
spatial layout of nodes
6-28Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff ndash after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
ndash longer backoff interval with more collisions
6-29Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
CSMACD efficiency
bull Tprop = max prop delay between 2 nodes in LANbull ttrans = time to transmit max-size frame
bull efficiency goes to 1 ndash as tprop goes to 0ndash as ttrans goes to infinity
bull better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
511
+=
6-30Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
ldquoTaking turnsrdquo MAC protocols
channel partitioning MAC protocolssect share channel efficiently and fairly at high loadsect inefficient at low load delay in channel access 1N bandwidth
allocated even if only 1 active node
random access MAC protocolssect efficient at low load single node can fully utilize channelsect high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
6-31Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
pollingbull master node ldquoinvitesrdquo
slave nodes to transmit in turn
bull typically used with ldquodumbrdquo slave devices
bull concernsndash polling overhead ndash latencyndash single point of
failure (master)
master
slaves
poll
data
data
ldquoTaking turnsrdquo MAC protocols
6-32Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
token passingsect control token passed from
one node to next sequentially
sect token messagesect concerns
sect token overhead sect latencysect single point of failure
(token)
T
data
(nothingto send)
T
ldquoTaking turnsrdquo MAC protocols
6-33Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
cable headend
CMTS
ISP
cable modemtermination system
sect multiple 40Mbps downstream (broadcast) channelssect single CMTS transmits into channels
sect multiple 30 Mbps upstream channelssect multiple access all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cablemodemsplitter
hellip
hellip
Internet frames TV channels control transmitted downstream at different frequencies
upstream Internet frames TV control transmitted upstream at different frequencies in time slots
6-34Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
DOCSIS data over cable service interface spec sect FDM over upstream downstream frequency channelssect TDM upstream some slots assigned some have contention
bull downstream MAP frame assigns upstream slotsbull request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
MAP frame forInterval [t1 t2]
Residences with cable modems
Downstream channel i
Upstream channel j
t1 t2
Assigned minislots containing cable modemupstream data frames
Minislots containing minislots request frames
cable headend
CMTS
Cable access network
6-35Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Summary of MAC protocols
bull channel partitioning by time frequency or codendash Time Division Frequency Division
bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire) hard
in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211
bull taking turnsndash polling from central site token passingndash Bluetooth FDDI token ring
6-36Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
MAC addresses and ARP
bull 32-bit IP address ndash network-layer address for interfacendash used for layer 3 (network layer) forwarding
bull MAC (or LAN or physical or Ethernet) addressndash function used lsquolocallyrdquo to get frame from one interface to
another physically-connected interface (same network in IP-addressing sense)
ndash 48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
ndash eg 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation(each ldquonumeralrdquo represents 4 bits)
6-37Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
6-38Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
LAN addresses (more)
bull MAC address allocation administered by IEEEbull manufacturer buys portion of MAC address space (to
assure uniqueness)bull analogy
ndash MAC address like Social Security Numberndash IP address like postal address
bull MAC flat address portability ndash can move LAN card from one LAN to another
bull IP hierarchical address not portablendash address depends on IP subnet to which node is attached
6-39Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
ndash IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
ndash TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
6-40Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
ARP protocol same LAN
bull A wants to send datagram to Bndash Brsquos MAC address not in Arsquos
ARP tablebull A broadcasts ARP query
packet containing Bs IP address ndash destination MAC address =
FF-FF-FF-FF-FF-FFndash all nodes on LAN receive
ARP query bull B receives ARP packet replies
to A with its (Bs) MAC addressndash frame sent to Arsquos MAC
address (unicast)
bull A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
ndash soft state information that times out (goes away) unless refreshed
bull ARP is ldquoplug-and-playrdquondash nodes create their ARP
tables without intervention from net administrator
6-41Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
walkthrough send datagram from A to B via Rsect focus on addressing ndash at IP (datagram) and MAC layer (frame)sect assume A knows Brsquos IP addresssect assume A knows IP address of first hop router R (how)sect assume A knows Rrsquos MAC address (how)
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
6-42Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
IP src 111111111111IP dest 222222222222
sect A creates IP datagram with IP source A destination B sect A creates link-layer frame with Rs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
6-43Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IPEthPhy
sect frame sent from A to R
IPEthPhy
sect frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55MAC dest E6-E9-00-17-BB-4B
IP src 111111111111IP dest 222222222222
IP src 111111111111IP dest 222222222222
6-44Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LAN
IP src 111111111111IP dest 222222222222
sect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-45Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as destination address
frame contains A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
6-46Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Addressing routing to another LANsect R forwards datagram with IP source A destination B sect R creates link-layer frame with Bs MAC address as dest frame contains
A-to-B IP datagram
IP src 111111111111IP dest 222222222222
MAC src 1A-23-F9-CD-06-9BMAC dest 49-BD-D2-C7-56-2A
IPEthPhy
6-47Link Layer and LANs Check out the online interactive exercises for more examples httpgaiacsumassedukurose_rossinteractive
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Ethernet
ldquodominantrdquo wired LAN technology bull single chip multiple speeds (eg Broadcom BCM5761)bull first widely used LAN technologybull simpler cheapbull kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernet sketch6-48Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Ethernet physical topologybull bus popular through mid 90s
ndash all nodes in same collision domain (can collide with each other)
bull star prevails todayndash active switch in centerndash each ldquospokerdquo runs a (separate) Ethernet protocol (nodes
do not collide with each other)
switch
bus coaxial cablestar
6-49Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble bull 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011bull used to synchronize receiver sender clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-50Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Ethernet frame structure (more)bull addresses 6 byte source destination MAC addresses
ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
ndash otherwise adapter discards framebull type indicates higher layer protocol (mostly IP but
others possible eg Novell IPX AppleTalk)bull CRC cyclic redundancy check at receiver
ndash error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
6-51Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Ethernet unreliable connectionless
bull connectionless no handshaking between sending and receiving NICs
bull unreliable receiving NIC doesnt send acks or nacks to sending NICndash data in dropped frames recovered only if initial
sender uses higher layer rdt (eg TCP) otherwise dropped data lost
bull Ethernetrsquos MAC protocol unslotted CSMACD with binary backoff
6-52Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
8023 Ethernet standards link amp physical layers
bull many different Ethernet standardsndash common MAC protocol and frame formatndash different speeds 2 Mbps 10 Mbps 100 Mbps 1Gbps 10
Gbps 40 Gbpsndash different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
6-53Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Ethernet switch
bull link-layer device takes an active rolendash store forward Ethernet framesndash examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
bull transparentndash hosts are unaware of presence of switches
bull plug-and-play self-learningndash switches do not need to be configured
6-54Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Switch multiple simultaneous transmissions
bull hosts have dedicated direct connection to switch
bull switches buffer packetsbull Ethernet protocol used on each
incoming link but no collisions full duplexndash each link is its own collision
domainbull switching A-to-Arsquo and B-to-Brsquo
can transmit simultaneously without collisions switch with six interfaces
(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
6-55Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Switch forwarding table
Q how does switch know Arsquoreachable via interface 4 Brsquoreachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6sect A each switch has a switch table each entrysect (MAC address of host interface
to reach host time stamp)sect looks like a routing table
Q how are entries created maintained in switch table
sect something like a routing protocol
6-56Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Switch self-learning
bull switch learns which hosts can be reached through which interfaces
ndash when frame received switch ldquolearnsrdquo location of sender incoming LAN segment
ndash records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTLSwitch table
(initially empty)A 1 60
6-57Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Switch frame filteringforwarding
when frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frameelse forward frame on interface indicated by entry
else flood forward on all interfaces except arriving
interface 6-58Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Self-learning forwarding exampleA Arsquo
Source ADest Arsquo
MAC addr interface TTLswitch table
(initially empty)A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
bull frame destination Arsquo location unknown flood
Arsquo A
sect destination A location known
Arsquo 4 60
selectively send on just one link
6-59Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Interconnecting switches
self-learning switches can be connected together
Q sending from A to G ndash how does S1 know to forward frame destined to G via S4 and S3sect A self learning (works exactly the same as in
single-switch case)
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-60Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
sect Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
HI
G
6-61Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
6-62Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs
Switches vs routers
both are store-and-forward sect routers network-layer
devices (examine network-layer headers)
sect switches link-layer devices (examine link-layer headers)
both have forwarding tablessect routers compute tables using
routing algorithms IP addresses
sect switches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frameframe
framedatagram
6-63Link Layer and LANs