Link%layer+addressing,+Ethernet,+VLANs+LAN+addresses+and+ARP+...
Transcript of Link%layer+addressing,+Ethernet,+VLANs+LAN+addresses+and+ARP+...
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Link-‐layer addressing, Ethernet, VLANs
Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley
Some materials copyright 1996-2012 J.F Kurose and K.W. Ross, All Rights Reserved
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
137.196.7.23
137.196.7.78
137.196.7.14
137.196.7.88
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Link layer, LANs: outline 5.1 Introduc>on,
services 5.2 Error detec>on,
correc>on 5.3 Mul>ple access
protocols 5.4 LANs
§ Addressing, ARP § Ethernet § Switches § VLANS
5.5 Link virtualiza>on: MPLS
5.6 Data center networking
5.7 A day in the life of a web request
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Link-‐layer addressing • Media Access Control address (MAC)
– 48-‐bit globally unique address • 281,474,976,710,656 possible addresses • Should last >ll 2100 • e.g. 01:23:45:67:89:ab
– Address of all 1's is broadcast • FF:FF:FF:FF:FF:FF
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LAN addresses and ARP Each adapter on LAN has unique MAC address Including routers, NAT boxes, etc.
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 or wireless)
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Address transla>on • Problem:
– How does host send a message to someone on their own network? Their default router?
– IP address is not the link-‐level address (e.g. MAC)
• Solu>on: – Host maintains table: IP address -‐> link address – Using the Address Resolu>on Protocol (ARP)
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ARP procedure • If des>na>on IP in sender's ARP table:
– Fire off link-‐layer packet – Otherwise send ARP query using broadcast address
• ARP query: – IP address you're looking for – Your own IP and hardware address – Des>na>on responds with hardware address – Other hosts can ignore or refresh their ARP tables
• Plug-‐and-‐play, no interven>on from admin
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Walkthrough: Send datagram from A to B via R – Focus on addressing
• At IP (datagram) and MAC layer (frame)
– Assume: • A knows B's IP address • A knows IP address of first hop router, R (how?) • A knows R's MAC address (how?)
R
1A-‐23-‐F9-‐CD-‐06-‐9B 222.222.222.220
111.111.111.110 E6-‐E9-‐00-‐17-‐BB-‐4B CC-‐49-‐DE-‐D0-‐AB-‐7D
111.111.111.112
111.111.111.111 74-‐29-‐9C-‐E8-‐FF-‐55
A
222.222.222.222 49-‐BD-‐D2-‐C7-‐56-‐2A
222.222.222.221 88-‐B2-‐2F-‐54-‐1A-‐0F
B
Addressing: rou>ng to another LAN
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R
1A-‐23-‐F9-‐CD-‐06-‐9B 222.222.222.220
111.111.111.110 E6-‐E9-‐00-‐17-‐BB-‐4B CC-‐49-‐DE-‐D0-‐AB-‐7D
111.111.111.112
111.111.111.111 74-‐29-‐9C-‐E8-‐FF-‐55
A
222.222.222.222 49-‐BD-‐D2-‐C7-‐56-‐2A
222.222.222.221 88-‐B2-‐2F-‐54-‐1A-‐0F
B
IP Eth Phy
IP src: 111.111.111.111 IP dest: 222.222.222.222
v A creates IP datagram with IP source A, des>na>on B
v A creates link-‐layer frame with R's MAC address as dest, frame contains A-‐to-‐B IP datagram
MAC src: 74-‐29-‐9C-‐E8-‐FF-‐55 MAC dest: E6-‐E9-‐00-‐17-‐BB-‐4B
Addressing: rou>ng to another LAN
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R
1A-‐23-‐F9-‐CD-‐06-‐9B 222.222.222.220
111.111.111.110 E6-‐E9-‐00-‐17-‐BB-‐4B CC-‐49-‐DE-‐D0-‐AB-‐7D
111.111.111.112
111.111.111.111 74-‐29-‐9C-‐E8-‐FF-‐55
A
222.222.222.222 49-‐BD-‐D2-‐C7-‐56-‐2A
222.222.222.221 88-‐B2-‐2F-‐54-‐1A-‐0F
B
IP Eth Phy
v Frame sent from A to R
IP Eth Phy
v Frame received at R, datagram extracted from frame, passed up to IP
MAC src: 74-‐29-‐9C-‐E8-‐FF-‐55 MAC dest: E6-‐E9-‐00-‐17-‐BB-‐4B
IP src: 111.111.111.111 IP dest: 222.222.222.222
IP src: 111.111.111.111 IP dest: 222.222.222.222
Addressing: rou>ng to another LAN
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R
1A-‐23-‐F9-‐CD-‐06-‐9B 222.222.222.220
111.111.111.110 E6-‐E9-‐00-‐17-‐BB-‐4B CC-‐49-‐DE-‐D0-‐AB-‐7D
111.111.111.112
111.111.111.111 74-‐29-‐9C-‐E8-‐FF-‐55
A
222.222.222.222 49-‐BD-‐D2-‐C7-‐56-‐2A
222.222.222.221 88-‐B2-‐2F-‐54-‐1A-‐0F
B
IP src: 111.111.111.111 IP dest: 222.222.222.222
v R forwards datagram with IP source A, des>na>on B
v R creates link-‐layer frame with B's MAC address as dest, frame contains A-‐to-‐B IP datagram
MAC src: 1A-‐23-‐F9-‐CD-‐06-‐9B MAC dest: 49-‐BD-‐D2-‐C7-‐56-‐2A
IP Eth Phy
IP Eth Phy
Addressing: rou>ng to another LAN
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R
1A-‐23-‐F9-‐CD-‐06-‐9B 222.222.222.220
111.111.111.110 E6-‐E9-‐00-‐17-‐BB-‐4B CC-‐49-‐DE-‐D0-‐AB-‐7D
111.111.111.112
111.111.111.111 74-‐29-‐9C-‐E8-‐FF-‐55
A
222.222.222.222 49-‐BD-‐D2-‐C7-‐56-‐2A
222.222.222.221 88-‐B2-‐2F-‐54-‐1A-‐0F
B
IP src: 111.111.111.111 IP dest: 222.222.222.222
MAC src: 1A-‐23-‐F9-‐CD-‐06-‐9B MAC dest: 49-‐BD-‐D2-‐C7-‐56-‐2A
IP Eth Phy
IP Eth Phy
Addressing: rou>ng to another LAN
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v R forwards datagram with IP source A, des>na>on B
v R creates link-‐layer frame with B's MAC address as dest, frame contains A-‐to-‐B IP datagram
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R
1A-‐23-‐F9-‐CD-‐06-‐9B 222.222.222.220
111.111.111.110 E6-‐E9-‐00-‐17-‐BB-‐4B CC-‐49-‐DE-‐D0-‐AB-‐7D
111.111.111.112
111.111.111.111 74-‐29-‐9C-‐E8-‐FF-‐55
A
222.222.222.222 49-‐BD-‐D2-‐C7-‐56-‐2A
222.222.222.221 88-‐B2-‐2F-‐54-‐1A-‐0F
B
IP src: 111.111.111.111 IP dest: 222.222.222.222
MAC src: 1A-‐23-‐F9-‐CD-‐06-‐9B MAC dest: 49-‐BD-‐D2-‐C7-‐56-‐2A
IP Eth Phy
Addressing: rou>ng to another LAN
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v R forwards datagram with IP source A, des>na>on B
v R creates link-‐layer frame with B's MAC address as dest, frame contains A-‐to-‐B IP datagram
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Classic Ethernet • Ethernet
– Luminferous ether through which electromagne>c radia>on once thought to propagate
– Carrier Sense, Mul>ple Access with Collision Detec>on (CSMA/CD)
– IEEE 802.3
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Robert Metcalfe, co-‐inventor of Ethernet
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Classic Ethernet • Ethernet
– Xerox Ethernet standardized as IEEE 802.3 in 1983
– Xerox not interested in commercializing
– Metcalfe leaves and forms 3Com
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Classic Ethernet connec>vity
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• Shared medium – All hosts hear all traffic on cable – Hosts tapped the cable – 2500m maximum length – May include repeaters amplifying signal – 10 Mbps bandwidth
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Classic Ethernet cabling
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Thick Ethernet cable (yellow), 10BASE-‐5 transceivers, cable tapping tool (orange), 500m maximum length.
Thin Ethernet cable (10BASE2) with BNC T-‐connector, 185m maximum length.
Cable aKer being "vampire" tapped.
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Ethernet frame format • Frame format
– Preamble produces 10-‐Mhz square wave • Allows clock synch between sender & receiver
– Pad to at least 64-‐bytes • Allows collision detec>on
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Ethernet
802.3
AlternaOng 0's and 1's (except start of frame of 11)
48-‐bit MAC addresses
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Ethernet receivers • Hosts listens to medium
– Deliver to host: • Any frame with host's MAC address • All broadcast frames (all 1's) • Mul>cast frames (if subscribed to) • Or all frames if in promiscuous mode (e.g. Wireshark)
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MAC sublayer • Media Access Control (MAC) sublayer
– Who goes next on a shared medium – Ethernet hosts can sense if medium in use – Algorithm for sending data:
1. Is medium idle? If not, wait. 2. Start transmiing data, listen for collision. 3. If collision detected, transmit 32-‐bit jamming
sequence. Stop transmiing and go to backoff procedure.
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Backoff procedure • Binary exponen>al backoff
– First collision • Wait 0-‐1 >meslots (chosen at random)
– Second collision • Wait 0-‐3 >meslots
– In general, ith collision • Wait a random number of >meslots between 0 and 2i -‐ 1 (max of 1023 slots)
– Give up aler 16 or so retries – Timeslot = 51.2 µs
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Ethernet hubs • Long single cable
– Hard to find breaks or loose connec>ons • Different wiring paoern
– Each host wired straight to hub – Hub simply connected all wires together – Using exis>ng office twisted pair phone lines
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Switched Ethernet • Hubs
– Made network easier to manage – But did not address capacity problem
• Switches – High-‐speed backplane connec>ng all ports – Only output frame to des>na>on port – Isolates traffic, no collisions, beoer security
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Q: How does switch know A' is reachable via interface 4, B' is reachable via interface 5?
Switch with six interfaces (1,2,3,4,5,6)
A
A'
B
B' C
C'
1 2
3 4 5
6 v A: Each switch has a switch table, each entry: § (MAC address of host, interface to reach host, >me stamp)
§ Looks like a rou>ng table!
Q: How are entries created, maintained in switch table?
§ Perhaps something like a rou>ng protocol?
Switch forwarding table
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A
A'
B
B' C
C'
1 2
3 4 5
6
• Switch learns which hosts can be reached through which interfaces – When frame received, switch learns loca>on of sender: incoming LAN segment
– Records sender/loca>on pair in switch table
A A'
Source: A Dest: A'
MAC addr interface TTL Switch table
(iniOally empty) A 1 60
Switch: self-‐learning
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When frame received at switch: 1. Record incoming link, MAC address of sending host 2. Index switch table using MAC des>na>on address 3. if entry found for des>na>on then {
if des>na>on on segment from which frame arrived then drop frame
else forward frame on interface indicated by entry } else flood /* fwd on all ports except arriving interface */
Switch: frame filtering/forwarding
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A
A'
B
B' C
C'
1 2
3 4 5
6
A A'
Source: A Dest: A'
MAC addr interface TTL switch table
(iniOally empty) A 1 60
A A’ A A’ A A’ A A’ A A'
• Frame des>na>on, A', loca>on unknown: flood
A' A
v Des>na>on A loca>on known:
A' 4 60
selec>vely send on just one link
Self-‐learning, forwarding: example
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v Switches can be connected together
Q: Sending from A to G -‐ how does S1 know to forward frame des>ned to F via S4 and S3? v A: Self-‐learning! (works exactly the same as in single-‐switch case!)
A
B
S1
C D
E
F S2
S4
S3
H
I
G
Interconnec>ng switches
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To external network
router
IP subnet
mail server
web server
Ins>tu>onal network
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switch
switch switch
switch switch
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Both are store-‐and-‐forward: § Router: network-‐layer device
§ Layer-‐3 packet switch § Switch: link-‐layer device
§ Layer-‐2 packet switch
Both have forwarding tables: § Router:
§ Compute tables using rou>ng algorithms, IP addresses
§ Switch: § Learn forwarding table using
flooding, self-‐learning, MAC addresses
application transport network
link physical
network link
physical
link physical
switch
datagram
application transport network
link physical
frame
frame
frame datagram
Switches vs. routers
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router
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Fast Ethernet • Fast Ethernet
– IEEE 802.3u, 1995 – Keep all the classic Ethernet frame formats, etc. – Reduce bit >me from 100 nsec to 10nsec – 100 Mbps – No more mul>drop cables or vampire taps
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Gigabit Ethernet • Gigabit Ethernet
– IEEE 802.3ab, 1999 – 1000 Mbps – Unacknowledged datagram service – Addi>on of flow control – Unofficial support for jumbo frames
• Up to 9KB (instead of limit of 1500 bytes)
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Even faster • 10-‐Gigabit Ethernet
– 1000x faster than original Ethernet, 2003 – Inside data centers, long haul trunks
• 40 and 100-‐Gigabit Ethernet – Star>ng to be deployed, 2010
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Ethernet retrospec>ve • Why so popular?
– Easy to administer, no rou>ng or config tables – Cheap hardware and wiring – Plays nice with TCP/IP
• Ethernet and IP are connec>onless protocols • Alternates like ATM were not
– Periodic speed increases • Order of magnitude every few years without throwing away exis>ng infrastructure
– Borrowed good ideas from other (failed) networking technologies
• e.g. FDDI, Fiber Channel 33
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Consider: v CS user moves office to EE,
but wants connect to CS switch?
v Single broadcast domain: § All layer-‐2 broadcast traffic (ARP, DHCP, unknown loca>on of des>na>on MAC address) must cross en>re LAN
§ Security, privacy, and efficiency issues
Computer Science Electrical
Engineering
Computer Engineering
VLANs: mo>va>on
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VLANs Port-‐based VLAN: Switch ports grouped (by switch management solware) so that single physical switch ……
Switches suppor>ng VLAN capabili>es can be configured to define mul>ple virtual LANS over single physical LAN infrastructure.
Virtual Local Area Network
1
8
9
16 10 2
7
…
Electrical Engineering (VLAN ports 1-‐8)
Computer Science (VLAN ports 9-‐15)
15
…
Electrical Engineering (VLAN ports 1-‐8)
…
1
8 2
7 9
16 10
15
…
Computer Science (VLAN ports 9-‐16)
… operates as mul>ple virtual switches
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Port-‐based VLAN
1
8
9
16 10 2
7
…
Electrical Engineering (VLAN ports 1-‐8)
Computer Science (VLAN ports 9-‐15)
15
…
v Traffic isolaOon: Frames to/from ports 1-‐8 can only reach ports 1-‐8 § Can also define VLAN based on
MAC addresses of endpoints, rather than switch port
v Dynamic membership: Ports can be dynamically assigned among VLANs
router
v Forwarding between VLANS: done via rou>ng § Just as with separate switches § In prac>ce vendors sell combined
switches plus routers
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VLANS spanning mul>ple switches
• Trunk port: Carries frames between VLANs defined over mul>ple physical switches
– Frames forwarded within VLAN between switches can't be vanilla 802.1 frames, must carry VLAN ID info
– 802.1q protocol adds/removed addi>onal header fields for frames forwarded between trunk ports
1
8
9
10 2
7
…
Electrical Engineering (VLAN ports 1-‐8)
Computer Science (VLAN ports 9-‐15)
15
…
2
7 3
Ports 2,3,5 belong to EE VLAN Ports 4,6,7,8 belong to CS VLAN
5
4 6 8 16
1
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type
2-‐byte Tag Protocol Iden>fier (value: 81-‐00)
Tag Control Informa>on (12 bit VLAN ID field, 3 bit priority field like IP TOS)
Recomputed CRC
Ethernet frame
802.1Q frame
dest. address
source address data (payload) CRC preamble
dest. address
source address preamble data (payload) CRC
type
802.1Q VLAN frame format
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Summary
39
• Address Reserva>on Protocol (ARP) – Mapping between link-‐layer addresses (MAC) and network-‐layer addresses (IP)
– Cached table in opera>ng system – Broadcast queries for IP des>na>ons with unknown MAC
• Wired Ethernet – Long history and widely adopted – Hubs vs. switches vs. routers – Order of magnitude bit rate increase every few years – Careful aoen>on to backwards compa>bility
• VLANs – Allows virtual segrega>on of hosts into isolated groups