Chapter 5 Data Link Layer - ERNETisg/NETWORKS/SLIDES/Ethernet-supp.pdf · Chapter 5 Data Link Layer...

Chapter 5Data Link Layer

5: DataLink Layer 5a-1

Computer Networking: A Top Down Approach Featuring the Internet, 2nd edition. Jim Kurose, Keith RossAddison-Wesley, July 2002.

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All material copyright 1996-2002

J.F Kurose and K.W. Ross, All Rights Reserved

Chapter 5: The Data Link Layer

Our goals:r understand principles behind data link layer services:

m error detection, correction

m sharing a broadcast channel: multiple access

link layer addressing


m link layer addressing

m reliable data transfer, flow control: done!

r instantiation and implementation of various link layer technologies

Chapter 5 outline

r 5.1 Introduction and services

r 5.2 Error detection and correction

r 5.3Multiple access

r 5.6 Hubs, bridges, and switches

r 5.7 Wireless links and LANs

r 5.8 PPP


r 5.3Multiple access protocols

r 5.4 LAN addresses and ARP

r 5.5 Ethernet

r 5.8 PPP

r 5.9 ATM

r 5.10 Frame Relay

Link Layer: IntroductionSome terminology:r hosts and routers are nodes

(bridges and switches too)

r communication channels that connect adjacent nodes along communication path are links

wired links

“link”


m wired links

m wireless links

m LANs

r 2-PDU is a frame,encapsulates datagram

data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

Link layer: context

r Datagram transferred by different link protocols over different links:

m e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link

transportation analogyr trip from Princeton to

Lausanne

m limo: Princeton to JFK

m plane: JFK to Geneva

m train: Geneva to Lausanne

tourist = datagram


on last link

r Each link protocol provides different services

m e.g., may or may not provide rdt over link

r tourist = datagram

r transport segment = communication link

r transportation mode = link layer protocol

r travel agent = routing algorithm

Link Layer Services

r Framing, link access:m encapsulate datagram into frame, adding header, trailer

m channel access if shared medium

m ‘physical addresses’ used in frame headers to identify source, dest

• different from IP address!


• different from IP address!

r Reliable delivery between adjacent nodesm we learned how to do this already (chapter 3)!

m seldom used on low bit error link (fiber, some twisted pair)

m wireless links: high error rates

• Q: why both link-level and end-end reliability?

Link Layer Services (more)

r Flow Control:m pacing between adjacent sending and receiving nodes

r Error Detection:m errors caused by signal attenuation, noise.

receiver detects presence of errors:


m receiver detects presence of errors:

• signals sender for retransmission or drops frame

r Error Correction:m receiver identifies and corrects bit error(s) without resorting to retransmission

r Half-duplex and full-duplexm with half duplex, nodes at both ends of link can transmit, but not at same time

Adaptors Communicating

r link layer implemented in r receiving side

sendingnode

frame

rcvingnode

datagram

frame

adapter adapter

link layer protocol


r link layer implemented in “adaptor” (aka NIC)

m Ethernet card, PCMCI card, 802.11 card

r sending side:m encapsulates datagram in a frame

m adds error checking bits, rdt, flow control, etc.

r receiving sidem looks for errors, rdt, flow control, etc

m extracts datagram, passes to rcving node

r adapter is semi-autonomous

r link & physical layers

Chapter 5 outline






r 5.8 PPP




r 5.5 Ethernet

r 5.8 PPP

r 5.9 ATM

r 5.10 Frame Relay

Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields

• Error detection not 100% reliable!• protocol may miss some errors, but rarely• larger EDC field yields better detection and correction


Parity Checking

Single Bit Parity:Detect single bit errors

Two Dimensional Bit Parity:Detect and correct single bit errors


0 0

Internet checksum

Sender:r treat segment contents

as sequence of 16-bit

Receiver:r compute checksum of received

segment

Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)


as sequence of 16-bit integers

r checksum: addition (1’s complement sum) of segment contents

r sender puts checksum value into UDP checksum field

segment

r check if computed checksum equals checksum field value:

m NO - error detected

m YES - no error detected. But maybe errors nonetheless?More later ….

Checksumming: Cyclic Redundancy Check

r view data bits, D, as a binary number

r choose r+1 bit pattern (generator), G

r goal: choose r CRC bits, R, such thatm <D,R> exactly divisible by G (modulo 2)

m receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!

can detect all burst errors less than r+1 bits


m can detect all burst errors less than r+1 bits

r widely used in practice (ATM, HDCL)

CRC Example

Want:

D.2r XOR R = nG

equivalently:D.2r = nG XOR R

equivalently:


equivalently:if we divide D.2r by G, want remainder R

R = remainder[ ]D.2r

G

Chapter 5 outline






r 5.8 PPP




r 5.5 Ethernet

r 5.8 PPP

r 5.9 ATM

r 5.10 Frame Relay

Multiple Access Links and Protocols

Two types of “links”:r point-to-point

m PPP for dial-up access

m point-to-point link between Ethernet switch and host

r broadcast (shared wire or medium)m traditional Ethernet


m traditional Ethernet

m upstream HFC

m 802.11 wireless LAN

Multiple Access protocols

r single shared broadcast channel

r two or more simultaneous transmissions by nodes: interference

m only one node can send successfully at a time

multiple access protocol


r distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit

r communication about channel sharing must use channel itself!

r what to look for in multiple access protocols:

Ideal Mulitple Access Protocol

Broadcast channel of rate R bps

1. When one node wants to transmit, it can send at rate R.

2. When M nodes want to transmit, each can send at average rate R/M


average rate R/M

3. Fully decentralized:m no special node to coordinate transmissions

m no synchronization of clocks, slots

4. Simple

MAC Protocols: a taxonomy

Three broad classes:

r Channel Partitioningm divide channel into smaller “pieces” (time slots, frequency, code)

m allocate piece to node for exclusive use

r Random Access


r Random Accessm channel not divided, allow collisions

m “recover” from collisions

r “Taking turns”m tightly coordinate shared access to avoid collisions

Channel Partitioning MAC protocols: TDMA

TDMA: time division multiple accessr access to channel in "rounds"

r each station gets fixed length slot (length = pkt trans time) in each round

r unused slots go idle


r unused slots go idle

r example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle

Channel Partitioning MAC protocols: FDMA

FDMA: frequency division multiple accessr channel spectrum divided into frequency bands

r each station assigned fixed frequency band

r unused transmission time in frequency bands go idle

r example: 6-station LAN, 1,3,4 have pkt, frequency


r example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle

frequency bands

Channel Partitioning (CDMA)

CDMA (Code Division Multiple Access)r unique “code” assigned to each user; i.e., code set partitioning

r used mostly in wireless broadcast channels (cellular, satellite, etc)

r all users share same frequency, but each user has own


r all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data

r encoded signal = (original data) X (chipping sequence)r decoding: inner-product of encoded signal and chipping

sequence

r allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

CDMA Encode/Decode


CDMA: two-sender interference


Random Access Protocols

r When node has packet to sendm transmit at full channel data rate R.

m no a priori coordination among nodes

r two or more transmitting nodes -> “collision”,

r random access MAC protocol specifies:


r random access MAC protocol specifies: m how to detect collisions

m how to recover from collisions (e.g., via delayed retransmissions)

r Examples of random access MAC protocols:m slotted ALOHA

m ALOHA

m CSMA, CSMA/CD, CSMA/CA

Slotted ALOHA

Assumptions

r all frames same size

r time is divided into equal size slots, time to transmit 1 frame

Operation

r when node obtains fresh frame, it transmits in next slot

r no collision, node can send


transmit 1 frame

r nodes start to transmit frames only at beginning of slots

r nodes are synchronized

r if 2 or more nodes transmit in slot, all nodes detect collision

r no collision, node can send new frame in next slot

r if collision, node retransmits frame in each subsequent slot with prob. p until success

Slotted ALOHA

Pros Cons


Pros

r single active node can continuously transmit at full rate of channel

r highly decentralized: only slots in nodes need to be in sync

r simple

Cons

r collisions, wasting slots

r idle slots

r nodes may be able to detect collision in less than time to transmit packet

Slotted Aloha efficiency

r Suppose N nodes with many frames to send,

r For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1

r For many nodes, take limit of Np*(1-p*)N-1

as N goes to infinity,

Efficiency is the long-run fraction of successful slots when there’s many nodes, each with many frames to send


many frames to send, each transmits in slot with probability p

r prob that 1st node has success in a slot= p(1-p)N-1

r prob that any node has a success = Np(1-p)N-1

limit of Np*(1-p*)as N goes to infinity, gives 1/e = .37

At best: channelused for useful transmissions 37%of time!

Pure (unslotted) ALOHA

r unslotted Aloha: simpler, no synchronization

r when frame first arrivesm transmit immediately

r collision probability increases:m frame sent at t0 collides with other frames sent in [t0-1,t0+1]


frame sent at t0 collides with other frames sent in [t0-1,t0+1]

Pure Aloha efficiency

P(success by given node) = P(node transmits) .

P(no other node transmits in [p0-1,p0] .

P(no other node transmits in [p0,p0+1]

= p . (1-p)N-1 . (1-p)N-1

= p . (1-p)2(N-1)


= p . (1-p)2(N-1)

… choosing optimum p and then letting n -> infty ...

= 1/(2e) = .18 Even worse !

CSMA (Carrier Sense Multiple Access)

CSMA: listen before transmit:

r If channel sensed idle: transmit entire frame

r If channel sensed busy, defer transmission


r Human analogy: don’t interrupt others!

CSMA collisions

collisions can still occur:propagation delay means two nodes may not heareach other’s transmission

collision:

spatial layout of nodes


collision:entire packet transmission time wasted

note:role of distance & propagation delay in determining collision probability

CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMAm collisions detected within short timem colliding transmissions aborted, reducing channel wastage

r collision detection:


r collision detection:m easy in wired LANs: measure signal strengths, compare transmitted, received signals

m difficult in wireless LANs: receiver shut off while transmitting

r human analogy: the polite conversationalist

CSMA/CD collision detection


CSMA (Carrier-sense multiple access)

r If propagation time is much less than transmission time - all stations know that a transmission has started almost immediately

r First listen for clear medium (carrier sense)m If medium idle, transmit

r Collision occurs if another user starts transmitting within the time it takes for the first bit to reach this user


r Collision occurs if another user starts transmitting within the time it takes for the first bit to reach this user (propagation delay)

r Collision detected by waiting round trip plus ACK contentionm No ACK then retransmit

rMax utilization depends on propagation time (medium length) and frame length

m Longer frame and shorter propagation gives better utilization

CSMA/CDrWith CSMA, collision occupies medium for duration of transmission

m Even if the station next to transmitting station collided, collision will be detected after >= RTT

r Instead “CD”= collision detect:Stations listen whilst transmitting


m Stations listen whilst transmittingm If medium idle, transmitm If busy, listen for idle, then transmit (and listen)m If collision detected, jam (send noise) then cease transmission

rAfter jam, wait random time then start againm Binary exponential back off

Collision Detection

rCollision produces much higher signal voltage than signal

rCollision detected if cable signal greater than single station signal

rSignal attenuated over distance


rSignal attenuated over distancer Limit distance to 500m (10Base5) or 200m (10Base2)

rFor twisted pair (star-topology) activity on more than one port is collision

m Frames repeated, for CD to work

Why “Jam”?r Tanenbaum: “to make sure the sender does not miss the

collision” (48 bits)rHalsall: “Ensure that the collision is detected by all stations

involved”r Stallings: “Assure all staitons know that there has been a

collision”r


collision”r Keshav: “Sequence of 512 bits to ensure that every active

station on the network knows that a collision happened and increments its backoff counter”; “to ensure that all colliding stations agree that a collision has happened”

CSMA/CDOperation


Collision detection


Collision detection can still take as long as 2τ

Collision detection

rTransmitting stations may detect collisions almost immediately, and stop transmission

m Saves time and bandwidth

rWill improve upon just CSMA only if collision is detected during frame transmission


detected during frame transmissionrThis is possible if frames are long enough (and prop. Delay is short enough) so that collision is detected while transmission

m Guideline used in IEEE 802.3m Frame transmission time >= 2*prop + delta

CSMA/CD efficiency

r Tprop = max prop between 2 nodes in LAN

r ttrans = time to transmit max-size frame

tt /51

1efficiency

+=


r Efficiency goes to 1 as tprop goes to 0

r Goes to 1 as ttrans goes to infinity

r Much better than ALOHA, but still decentralized, simple, and cheap

transproptt /51

efficiency+

=

“Taking Turns” MAC protocols

channel partitioning MAC protocols:

m share channel efficiently and fairly at high load

m inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node!


node!

Random access MAC protocols

m efficient at low load: single node can fully utilize channel

m high load: collision overhead

“taking turns” protocols

look for best of both worlds!

“Taking Turns” MAC protocols

Polling:

r master node “invites” slave nodes to transmit in turn

r concerns:

Token passing:

r control token passed from one node to next sequentially.

r token message

concerns:


r concerns:m polling overhead

m latency

m single point of failure (master)

r concerns:m token overhead

m latency

m single point of failure (token)

Summary of MAC protocols

r What do you do with a shared media?m Channel Partitioning, by time, frequency or code

• Time Division,Code Division, Frequency Division

m Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD


• ALOHA, S-ALOHA, CSMA, CSMA/CD

• carrier sensing: easy in some technologies (wire), hard in others (wireless)

• CSMA/CD used in Ethernet

m Taking Turns• polling from a central site, token passing

LAN technologies

Data link layer so far:m services, error detection/correction, multiple access

Next: LAN technologiesm addressing


m addressing

m Ethernet

m hubs, bridges, switches

m 802.11

m PPP

m ATM

Ethernet

“dominant” LAN technology:

r cheap $20 for 100Mbs!

r first widely used LAN technology

r Simpler, cheaper than token LANs and ATM

r Kept up with speed race: 10, 100, 1000 Mbps


Kept up with speed race: 10, 100, 1000 Mbps

Metcalfe’s Ethernetsketch

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame


Preamble:

r 7 bytes with pattern 10101010 followed by one byte with pattern 10101011

r used to synchronize receiver, sender clock rates

Ethernet Frame Structure (more)r Addresses: 6 bytes

m if adapter receives frame with matching destination address, or with broadcast address, it passes data in frame to net-layer protocol

m otherwise, adapter discards frame

Type: indicates the higher layer protocol, mostly


r Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk)

r CRC: checked at receiver, if error is detected, the frame is simply dropped

Ethernet min frame length

rMin length needed for CD: for 2500m distance specification, RT prop delay is determined to be 50 µsec

m Frame transmission time >= 50 µsec

m At 10Mbps, bits transmitted in 50 µsec is 500 <= 512 =


m At 10Mbps, bits transmitted in 50 µsec is 500 <= 512 = 64*8 bits = 64 bytes

rWhen transmission interrupted, “bits & pieces” of frames appear on the cable

m Min frame length is one “filter” for valid frames

forward

Unreliable, connectionless service

r Connectionless: No handshaking between sending and receiving adapter.

r Unreliable: receiving adapter doesn’t send acks or nacks to sending adapter

m stream of datagrams passed to network layer can have


m stream of datagrams passed to network layer can have gaps

m gaps will be filled if app is using TCP

m otherwise, app will see the gaps

Ethernet uses CSMA/CD

r No slots

r adapter doesn’t transmit if it senses that some other adapter is transmitting, that is,

r Before attempting a retransmission, adapter waits a random time, that is, random access


transmitting, that is, carrier sense

r transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection

random access

Ethernet CSMA/CD algorithm

1. Adaptor gets datagram from and creates frame

2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until

4. If adapter detects another transmission while transmitting, aborts and sends jam signal

5. After aborting, adapter enters exponential


channel busy, waits until channel idle and then transmits

3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame !

enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K*512 bit times and returns to Step 2

Ethernet’s CSMA/CD (more)

Jam Signal: make sure all other transmitters are aware of collision; 48 bits;

Bit time: .1 microsec for 10 Mbps Ethernet ;for K=1023, wait time is about 50 msec

Exponential Backoff:

r Goal: adapt retransmission attempts to estimated current load

m heavy load: random wait will be longer

r first collision: choose K


about 50 msec r first collision: choose K from {0,1}; delay is K x 512 bit transmission times

r after second collision: choose K from {0,1,2,3}…

r after ten collisions, choose K from {0,1,2,3,4,…,1023}

See/interact with Javaapplet on AWL Web site:highly recommended !

Ethernet Technologies: 10Base2

r 10: 10Mbps; 2: under 200 meters max cable length

r thin coaxial cable in a bus topology


r repeaters used to connect up to multiple segments

r repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

r has become a legacy technology

10BaseT and 100BaseTr 10/100 Mbps rate; latter called “fast ethernet”

r T stands for Twisted Pair

r Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub

nodes


r Hubs are essentially physical-layer repeaters:

m bits coming in one link go out all other links

m no frame buffering

m no CSMA/CD at hub: adapters detect collisions

m provides net management functionality

hub

Fast EthernetrHigher bit rate media (100 Mbps) is available.

m Can it be used for Ethernet?

r Recall minimum frame length?m Set=512 bits by calculating time needed to detect collisions in Ethernets of upto 2.5km length, of 10Mbps bit rate

r Can higher bit rates be used without changing protocol specs, and still make it work?


specs, and still make it work?m Frame transmission time for 512 bit frame @100Mbps ~ 5µsec m 5 µsec >= twice prop. delay m Should be <= (1/10th) of 2.5 km => ~200m

This is what was Fast Ethernet: transmission media was available, Ethernet wires were anyway not stretching very far away -> perfect solution say, for e.g. “server room” LAN

Gbit Ethernet

r use standard Ethernet frame format

r allows for point-to-point links and shared broadcast channels

r in shared mode, CSMA/CD is used; short distances between nodes to be efficient


between nodes to be efficient

r uses hubs, called here “Buffered Distributors”

r Full-Duplex at 1 Gbps for point-to-point links

r 10 Gbps now !

Gigabit Ethernet

r 1000 Mbps transmission media available. m Cannot continue reducing max length

r Two enhancements to basic CSMA/CDm Carrier extension: Pad MAC frames to be at least 4096 bits


Carrier extension: Pad MAC frames to be at least 4096 bits• This means ~4 µsec frame transmission time

• 2*Prop delay < 4 µsec : Length restrictions

LAN Addresses and ARP

32-bit IP address:r network-layer addressr used to get datagram to destination IP network (recall IP network definition)

LAN (or MAC or physical or Ethernet) address:


LAN (or MAC or physical or Ethernet) address: r used to get datagram from one interface to another physically-connected interface (same network)

r 48 bit MAC address (for most LANs) burned in the adapter ROM

LAN Addresses and ARPEach adapter on LAN has unique LAN address


LAN Address (more)

r MAC address allocation administered by IEEE

r manufacturer buys portion of MAC address space (to assure uniqueness)

r Analogy:

(a) MAC address: like Social Security Number


(a) MAC address: like Social Security Number

(b) IP address: like postal address

r MAC flat address => portability m can move LAN card from one LAN to another

r IP hierarchical address NOT portablem depends on IP network to which node is attached

Recall earlier routing discussion

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.27

A

BE

Starting at A, given IP datagram addressed to B:

r look up net. address of B, find B on same net. as A

r link layer send datagram to B inside link-layer frame


223.1.3.2223.1.3.1

inside link-layer frame

B’s MACaddr

A’s MACaddr

A’s IPaddr

B’s IPaddr

IP payload

datagram

frame

frame source,dest address

datagram source,dest address

ARP: Address Resolution Protocol

r Each IP node (Host, Router) on LAN has ARP table

r ARP Table: IP/MAC address mappings for some LAN nodes

Question: how to determineMAC address of Bknowing B’s IP address?


address mappings for some LAN nodes< IP address; MAC address; TTL>

m TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

ARP protocol

r A wants to send datagram to B, and A knows B’s IP address.

r Suppose B’s MAC address is not in A’s ARP table.

r A broadcasts ARP query

r A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

m soft state: information that times out (goes


r A broadcasts ARP query packet, containing B's IP address

m all machines on LAN receive ARP query

r B receives ARP packet, replies to A with its (B's) MAC address

m frame sent to A’s MAC address (unicast)

that times out (goes away) unless refreshed

r ARP is “plug-and-play”:m nodes create their ARP tables without intervention from net administrator

Routing to another LANwalkthrough: send datagram from A to B via R

assume A knows B IP address

A


r Two ARP tables in router R, one for each IP network (LAN)

A

RB

r A creates datagram with source A, destination B

r A uses ARP to get R’s MAC address for 111.111.111.110

r A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram

r A’s data link layer sends frame

r R’s data link layer receives frame

r R removes IP datagram from Ethernet frame, sees its destined to B

r R uses ARP to get B’s physical layer address


R uses ARP to get B’s physical layer address

r R creates frame containing A-to-B IP datagram sends to B

A

RB

Chapter 5 outline






r 5.8 PPP




r 5.5 Ethernet

r 5.8 PPP

r 5.9 ATM

r 5.10 Frame Relay

Interconnecting LAN segments

r Hubs

r Bridges

r Switchesm Remark: switches are essentially multi-port


m Remark: switches are essentially multi-port bridges.

m What we say about bridges also holds for switches!

Interconnecting with hubs

r Backbone hub interconnects LAN segmentsr Extends max distance between nodesr But individual segment collision domains become one large collision domain

m if a node in CS and a node EE transmit at same time: collision

r Can’t interconnect 10BaseT & 100BaseT


r Can’t interconnect 10BaseT & 100BaseT

Bridges

r Link layer device

m stores and forwards Ethernet frames

m examines frame header and selectivelyforwards frame based on MAC dest address

m when frame is to be forwarded on segment, uses CSMA/CD to access segment


uses CSMA/CD to access segment

r transparent

m hosts are unaware of presence of bridges

r plug-and-play, self-learning

m bridges do not need to be configured

Bridges: traffic isolation

r Bridge installation breaks LAN into LAN segments

r bridges filter packets:

m same-LAN-segment frames not usually forwarded onto other LAN segments

m segments become separate collision domains


bridgecollision domain

collision domain

= hub

= host

LAN (IP network)

LAN segment LAN segment

Forwarding


How do determine to which LAN segment to forward frame?• Looks like a routing problem...

Self learning

r A bridge has a bridge table

r entry in bridge table:

m (Node LAN Address, Bridge Interface, Time Stamp)

m stale entries in table dropped (TTL can be 60 min)

r bridges learn which hosts can be reached through


r bridges learn which hosts can be reached through which interfaces

m when frame received, bridge “learns” location of sender: incoming LAN segment

m records sender/location pair in bridge table

Filtering/Forwarding

When bridge receives a frame:

index bridge table using MAC dest address

if entry found for destinationthen{

if dest on segment from which frame arrived


if dest on segment from which frame arrivedthen drop the frame

else forward the frame on interface indicated

}

else floodforward on all but the interface on which the frame arrived

Bridge example

Suppose C sends frame to D and D replies back with frame to C.


r Bridge receives frame from from Cm notes in bridge table that C is on interface 1

m because D is not in table, bridge sends frame into interfaces 2 and 3

r frame received by D

Bridge Learning: example


r D generates frame for C, sends

r bridge receives frame

m notes in bridge table that D is on interface 2

m bridge knows C is on interface 1, so selectively forwards frame to interface 1

Interconnection without backbone


r Not recommended for two reasons:- single point of failure at Computer Science hub

- all traffic between EE and SE must path over CS segment

Backbone configuration


Recommended !

Bridges Spanning Treer for increased reliability, desirable to have redundant, alternative paths from source to dest

r with multiple paths, cycles result - bridges may multiply and forward frame forever

r solution: organize bridges in a spanning tree by disabling subset of interfaces


disabling subset of interfaces

Disabled

Multiple LANs


Needed: Routing

rComplex large LANs need alternative routesm Load balancingm Fault tolerance

rBridge must decide whether to forward frameBridge must decide which LAN to forward frame on


m Bridge must decide which LAN to forward frame on

rRouting selected for each source-destination pair of LANs

m Done in configurationm Usually least hop routem Only changed when topology changes

Spanning Tree

r Bridge automatically develops routing table

r Automatically update in response to changesm Frame forwarding


Frame forwarding

m Address learning

m Loop resolution

Frame forwardingrMaintain forwarding database for each port

m List station addresses reached through each port

rFor a frame arriving on port X:m Search forwarding database to see if MAC address is listed for any port except XIf address not found, forward to all ports except X


m If address not found, forward to all ports except Xm If address listed for port Y, check port Y for blocking or forwarding state• Blocking prevents port from receiving or transmitting

m If not blocked, transmit frame through port Y

Address Learning

rWhen frame arrives at port X, it has come form the LAN attached to port X

rUse the source address to update forwarding database for port X to include that address


database for port X to include that address

rTimer on each entry in database (reset whenever frame received)

rEach time frame arrives, source address checked against forwarding database

Loop of Bridges


Spanning Tree Algorithm

r Creates a logical, or “active” topology that behaves like a spanning treem Makes alternate bridges redundant

m Is run periodically, so will discover failures and use alternate bridges if necessary


use alternate bridges if necessary

Reference: Fred Halsall: “Data Communications, Computer Networks and Open Systems”, 4th Edition.

Spanning Tree Algorithmr Variables:

1. Each bridge has a Priority Value and a unique Identifier

2. Each LAN segment has a Designated Cost (DC) inversely proportional to the bit rate

3. Each port of a bridge has a Path Cost (PC) = DC of the LAN segment to which it is attached


segment to which it is attached

Spanning Tree Algorithmr Working: Bridges regularly exchange frames known as Bridge Protocol

Data Units (BPDUs). This exchange does the following:1. Bridge with highest priority and smallest ID is selected as root bridge.2. Each bridge determines for each port, the least cost path from root bridge

to this port. This is the Root Path Cost (RPC) for this port.a) Select the port which has the least RPC and designate it as the Root Port (RP).

This is the port which will be used for communicating with the root.


This is the port which will be used for communicating with the root.

3. Once root port is determined, one bridge port is selected for each LAN segment as the designated bridge port (DP) to which frames will be sent for that LAN segment. a) This is a port (which is NOT a root port) which has the least path cost to the rootb) The ports of the root bridge are always DPs for the LAN segments connected to

the root bridge

4. The state of the bridge ports can be set either to forwarding or blocking.a) All ports that are either RPs or DPs are forwarding, the rest are blocking.

Topology Initializationr BPDUs are sent to a broadcast MAC address of all bridges on the LANr All bridges initially assume they are the root bridger Each BPDU contains (self ID, root ID, transmitting port ID, RPC of this

port)r If necessary,

m Update root ID based on received BPDUsAdd path cost of the port on which frame was received to the RPC in the


m Add path cost of the port on which frame was received to the RPC in the frame

m Sends out this new info on all other ports with all updated Idsm Procedure repeated by all bridges

• Will determine RPCs of each port• Will select Root Ports based on this

m Two or more bridges on the same segment will exchange BPDUs so that designated bridge-port can be seleted

Topology Changer Root bridge regularly transmits BPDUs, forwarded by all

bridges on all ports

r Bridges will keep timers associated with each of its forwarding ports

rWhen timers expire, procedure similar to topology initialization is done


initialization is donem Details…

Some bridge featuresr Isolates collision domains resulting in higher total max throughput

r limitless number of nodes and geographical coverage

r Can connect different Ethernet types

Transparent (“plug-and-play”): no configuration


r Transparent (“plug-and-play”): no configuration necessary

Bridges vs. Routers

r both store-and-forward devicesm routers: network layer devices (examine network layer headers)

m bridges are link layer devices

r routers maintain routing tables, implement routing algorithms


algorithms

r bridges maintain bridge tables, implement filtering, learning and spanning tree algorithms

Routers vs. Bridges

Bridges + and -

+ Bridge operation is simpler requiring less packet processing

+ Bridge tables are self learning

- All traffic confined to spanning tree, even when


- All traffic confined to spanning tree, even when alternative bandwidth is available

- Bridges do not offer protection from broadcast storms

Routers vs. Bridges

Routers + and -

+ arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols)

+ provide protection against broadcast storms

- require IP address configuration (not plug and play)


- require IP address configuration (not plug and play)

- require higher packet processing

r bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

Ethernet Switches

r Essentially a multi-interface bridge

r layer 2 (frame) forwarding, filtering using LAN addresses

r Switching: A-to-A’ and B-


r Switching: A-to-A’ and B-to-B’ simultaneously, no collisions

r large number of interfaces

r often: individual hosts, star-connected into switch

m Ethernet, but no collisions!

Ethernet Switches

r cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame

m slight reduction in latency


slight reduction in latency

r combinations of shared/dedicated, 10/100/1000 Mbps interfaces

Not an atypical LAN (IP network)

Dedicated

Shared


Summary comparison

hubs bridges routers switches

trafficisolation

no yes yes yes


isolation

plug & play yes yes no yes

optimalrouting

no no yes no

cutthrough

yes no no yes

Chapter 5 outline






r 5.8 PPP

5: DataLink Layer 5a-

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r 5.5 Ethernet

r 5.8 PPP

r 5.9 ATM

r 5.10 Frame Relay

IEEE 802.11 Wireless LAN

r 802.11bm 2.4-5 GHz unlicensed radio spectrum

m up to 11 Mbps

m direct sequence spread spectrum (DSSS) in

r 802.11am 5-6 GHz range

m up to 54 Mbps

r 802.11gm 2.4-5 GHz range


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spectrum (DSSS) in physical layer

• all hosts use same chipping code

m widely deployed, using base stations

m 2.4-5 GHz range

m up to 54 Mbps

r All use CSMA/CA for multiple access

r All have base-station and ad-hoc network versions

Base station approach

r Wireless host communicates with a base stationm base station = access point (AP)

r Basic Service Set (BSS) (a.k.a. “cell”) contains:

m wireless hosts

m access point (AP): base station

r BSSs combined to form distribution system (DS)


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r BSSs combined to form distribution system (DS)

Ad Hoc Network approach

r No AP (i.e., base station)

r wireless hosts communicate with each other

m to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z

r Applications:

“laptop” meeting in conference room, car


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m “laptop” meeting in conference room, car

m interconnection of “personal” devices

m battlefield

r IETF MANET (Mobile Ad hoc Networks) working group

IEEE 802.11: multiple access

r Collision if 2 or more nodes transmit at same time

r CSMA makes sense:m get all the bandwidth if you’re the only one transmitting

m shouldn’t cause a collision if you sense another transmission

r Collision detection doesn’t work: hidden terminal problem


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problem

IEEE 802.11 MAC Protocol: CSMA/CA

802.11 CSMA: sender

- if sense channel idle for DISF sec.

then transmit entire frame (no collision detection)

-if sense channel busy


105

-if sense channel busy then binary backoff

802.11 CSMA receiver

- if received OK

return ACK after SIFS

(ACK is needed due to hidden terminal problem)

Collision avoidance mechanisms

r Problem:m two nodes, hidden from each other, transmit complete frames to base station

m wasted bandwidth for long duration !

r Solution:

m small reservation packets


106

m small reservation packets

m nodes track reservation interval with internal “network allocation vector” (NAV)

Collision Avoidance: RTS-CTS exchanger sender transmits short RTS (request to send) packet: indicates duration of transmission

r receiver replies with


107

r receiver replies with short CTS (clear to send) packet

m notifying (possibly hidden) nodes

r hidden nodes will not transmit for specified duration: NAV

Collision Avoidance: RTS-CTS exchange

r RTS and CTS short:

m collisions less likely, of shorter duration

m end result similar to collision detection


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collision detection

r IEEE 802.11 allows:

m CSMA

m CSMA/CA: reservations

m polling from AP

A word about Bluetooth

r Low-power, small radius, wireless networking technology

m 10-100 meters

r omnidirectional

r Interference from wireless LANs, digital cordless phones, microwave ovens:

m frequency hopping helps


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r omnidirectionalm not line-of-sight infrared

r Interconnects gadgets

r 2.4-2.5 GHz unlicensed radio band

r up to 721 kbps

m frequency hopping helps

r MAC protocol supports:m error correction

m ARQ

r Each node has a 12-bit address

Chapter 5 outline






r 5.8 PPP




r 5.5 Ethernet

r 5.8 PPP

r 5.9 ATM

r 5.10 Frame Relay

Point to Point Data Link Control

r one sender, one receiver, one link: easier than broadcast link:

m no Media Access Control

m no need for explicit MAC addressing

m e.g., dialup link, ISDN line


m e.g., dialup link, ISDN line

r popular point-to-point DLC protocols:

m PPP (point-to-point protocol)

m HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!

PPP Design Requirements [RFC 1557]

r packet framing: encapsulation of network-layer datagram in data link frame

m carry network layer data of any network layer protocol (not just IP) at same time

m ability to demultiplex upwards

bit transparency: must carry any bit pattern in the


r bit transparency: must carry any bit pattern in the data field

r error detection (no correction)

r connection liveness: detect, signal link failure to network layer

r network layer address negotiation: endpoint can learn/configure each other’s network address

PPP non-requirements

r no error correction/recovery

r no flow control

r out of order delivery OK

r no need to support multipoint links (e.g., polling)


Error recovery, flow control, data re-ordering all relegated to higher layers!

PPP Data Frame

r Flag: delimiter (framing)

r Address: does nothing (only one option)

r Control: does nothing; in the future possible multiple control fields

r Protocol: upper layer protocol to which frame


r Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc)

PPP Data Frame

r info: upper layer data being carried

r check: cyclic redundancy check for error detection


Byte Stuffingr “data transparency” requirement: data field must be allowed to include flag pattern <01111110>

m Q: is received <01111110> data or flag?

Sender: adds (“stuffs”) extra < 01111110> byte


r Sender: adds (“stuffs”) extra < 01111110> byte after each < 01111110> data byte

r Receiver:

m two 01111110 bytes in a row: discard first byte, continue data reception

m single 01111110: flag byte

Byte Stuffing

flag bytepatternin datato send


flag byte pattern plusstuffed byte in transmitted data

PPP Data Control ProtocolBefore exchanging network-layer data, data link peers must

r configure PPP link (max. frame length, authentication)


authentication)

r learn/configure network

layer information

m for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

Chapter 5 outline






r 5.8 PPP




r 5.5 Ethernet

r 5.8 PPP

r 5.9 ATM

r 5.10 Frame Relay

Asynchronous Transfer Mode: ATM

r 1990’s/00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture

r Goal: integrated, end-end transport of carry voice, video, datameeting timing/QoS requirements of voice, video


120

m meeting timing/QoS requirements of voice, video (versus Internet best-effort model)

m “next generation” telephony: technical roots in telephone world

m packet-switching (fixed length packets, called “cells”) using virtual circuits

ATM architecture


121

r adaptation layer: only at edge of ATM network

m data segmentation/reassembly

m roughly analogous to Internet transport layer

r ATM layer: “network” layer

m cell switching, routing

r physical layer

ATM: network or link layer?Vision: end-to-end transport: “ATM from desktop to desktop”

m ATM is a network technology

Reality: used to connect


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Reality: used to connect IP backbone routers

m “IP over ATM”

m ATM as switched link layer, connecting IP routers

ATM Adaptation Layer (AAL)

r ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below

r AAL present only in end systems, not in switches

r AAL layer segment (header/trailer fields, data)


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r AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells

m analogy: TCP segment in many IP packets

ATM Adaptation Layer (AAL) [more]

Different versions of AAL layers, depending on ATM service class:

r AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation

r AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video

r AAL5: for data (e.g., IP datagrams)


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AAL PDU

ATM cell

User data

AAL5 - Simple And Efficient AL (SEAL)r AAL5: low overhead AAL used to carry IP datagramsm 4 byte cyclic redundancy check

m PAD ensures payload multiple of 48bytes

large AAL5 data unit to be fragmented into 48-


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m large AAL5 data unit to be fragmented into 48-byte ATM cells

ATM LayerService: transport cells across ATM network

r analogous to IP network layer

r very different services than IP network layer

NetworkArchitecture

ServiceModel Bandwidth Loss Order Timing

Congestionfeedback

Guarantees ?


126

Internet

ATM

ATM

ATM

ATM

best effort

CBR

VBR

ABR

UBR

none

constantrateguaranteedrateguaranteed minimumnone

no

yes

yes

no

no

no

yes

yes

yes

yes

no

yes

yes

no

no

no (inferredvia loss)nocongestionnocongestionyes

no

ATM Layer: Virtual Circuitsr VC transport: cells carried on VC from source to dest

m call setup, teardown for each call before data can flowm each packet carries VC identifier (not destination ID)

m every switch on source-dest path maintain “state” for each passing connection

m link,switch resources (bandwidth, buffers) may be allocated to


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m link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf.

r Permanent VCs (PVCs)

m long lasting connections

m typically: “permanent” route between to IP routers

r Switched VCs (SVC):

m dynamically set up on per-call basis

ATM VCs

r Advantages of ATM VC approach:

m QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter)

r Drawbacks of ATM VC approach:

m Inefficient support of datagram traffic


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m Inefficient support of datagram traffic

m one PVC between each source/dest pair) does not scale (N*2 connections needed)

m SVC introduces call setup latency, processing overhead for short lived connections

ATM Layer: ATM cellr 5-byte ATM cell header

r 48-byte payload

m Why?: small payload -> short cell-creation delay for digitized voice

m halfway between 32 and 64 (compromise!)


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halfway between 32 and 64 (compromise!)

Cell header

Cell format

ATM cell header

r VCI: virtual channel ID

m will change from link to link thru netr PT: Payload type (e.g. RM cell versus data cell)

r CLP: Cell Loss Priority bit

m CLP = 1 implies low priority cell, can be


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m CLP = 1 implies low priority cell, can be discarded if congestion

r HEC: Header Error Checksum

m cyclic redundancy check

ATM Physical Layer (more)

Two pieces (sublayers) of physical layer:r Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below

r Physical Medium Dependent: depends on physical medium being used


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medium being used

TCS Functions:

m Header checksum generation: 8 bits CRC

m Cell delineation

m With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send

ATM Physical Layer

Physical Medium Dependent (PMD) sublayerr SONET/SDH: transmission frame structure (like a container carrying bits);

m bit synchronization;

bandwidth partitions (TDM);


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m bandwidth partitions (TDM);

m several speeds: OC3 = 155.52 Mbps; OC12 = 622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps

r TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps

r unstructured: just cells (busy/idle)

IP-Over-ATMClassic IP only

r 3 “networks” (e.g., LAN segments)

r MAC (802.3) and IP addresses

IP over ATM

r replace “network” (e.g., LAN segment) with ATM network

r ATM addresses, IP addresses

ATM


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ATMnetwork

EthernetLANs

EthernetLANs

IP-Over-ATM

Issues:

r IP datagrams into ATM AAL5 PDUs

r from IP addresses to ATM addresses

ATMnetwork


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to ATM addresses

m just like IP addresses to 802.3 MAC addresses!

EthernetLANs

Datagram Journey in IP-over-ATM Network

r at Source Host:m IP layer maps between IP, ATM dest address (using ARP)

m passes datagram to AAL5

m AAL5 encapsulates data, segments cells, passes to ATM layer

r ATM network: moves cell along VC to destination

at Destination Host:


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r at Destination Host:

m AAL5 reassembles cells into original datagram

m if CRC OK, datagram is passed to IP

Chapter 5 outline






r 5.8 PPP


136



r 5.5 Ethernet

r 5.8 PPP

r 5.9 ATM

r 5.10 Frame Relay

Frame Relay

Like ATM:r wide area network technologies

r Virtual-circuit oriented

r origins in telephony world


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r origins in telephony world

r can be used to carry IP datagrams

mcan thus be viewed as link layers by IP protocol

Frame Relay

r Designed in late ‘80s, widely deployed in the ‘90s

r Frame relay service:

m no error control

m end-to-end congestion control


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Frame Relay (more)r Designed to interconnect corporate customer LANs

m typically permanent VC’s: “pipe” carrying aggregate traffic between two routers

m switched VC’s: as in ATM

r corporate customer leases FR service from public Frame Relay network (e.g., Sprint, ATT)


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corporate customer leases FR service from public Frame Relay network (e.g., Sprint, ATT)

Frame Relay (more)

r Flag bits, 01111110, delimit frame

r address:

m 10 bit VC ID field

addressflags data CRC flags


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m 10 bit VC ID field

m 3 congestion control bits

• FECN: forward explicit congestion notification (frame experienced congestion on path)

• BECN: congestion on reverse path

• DE: discard eligibility

Frame Relay -VC Rate Control

r Committed Information Rate (CIR)

m defined, “guaranteed” for each VC

m negotiated at VC set up time

m customer pays based on CIR


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r DE bit: Discard Eligibility bit

m Edge FR switch measures traffic rate for each VC; marks DE bit

m DE = 0: high priority, rate compliant frame; deliver at “all costs”

m DE = 1: low priority, eligible for congestion discard

Frame Relay - CIR & Frame Marking

r Access Rate: rate R of the access link between source router (customer) and edge FR switch(provider); 64Kbps < R < 1,544Kbps

r Typically, many VCs (one per destination router) multiplexed on the same access trunk; each VC has


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multiplexed on the same access trunk; each VC has own CIR

r Edge FR switch measures traffic rate for each VC; it marks (i.e. DE = 1) frames which exceed CIR (these may be later dropped)

r Internet’s more recent differentiated serviceuses similar ideas

Chapter 5: Summary

r principles behind data link layer services:m error detection, correction

m sharing a broadcast channel: multiple access

m link layer addressing, ARP

r link layer technologies: Ethernet, hubs, bridges, switches,IEEE 802.11 LANs, PPP,


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link layer technologies: Ethernet, hubs, bridges, switches,IEEE 802.11 LANs, PPP, ATM, Frame Relay

r journey down the protocol stack now OVER!m next stops: multimedia, security, network management

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