link layer fundamentals nwen302 computer network design

Link Layer Fundamentals

NWEN302 Computer Network Design

Topics to be covered

• Link layer services

• Link layer addressing and address resolution

• Ethernet

• Switches

© Winston Seah NWEN302 Computer Network Design

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Network data at different layersThe generic term for a piece of information at any layer is referred to as a packet.•Message – application layer data; stuff that comes from your application•Segment – transport layer data, typically TCP/UDP segments•Datagram – network layer data blocks moved by routers from source to destination•Frame – block of data transferred between neighboring devices at the link layer•bits – units of binary information “on the wire”


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application

transport

network

link

physical

Link Layer: Introduction


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Basic terminology:• hosts and routers are nodes• communication channels that

connect adjacent nodes along communication path are links– wired links– wireless links

• layer-2 packet 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• datagram transferred by different link

protocols over different links:– e.g., Ethernet on first link, frame relay on

intermediate links, IEEE 802.11 on last link

• each link protocol provides different services– e.g., may or may not provide reliable data

transfer over the link


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Link Layer Services

• Framing, link access: – Encapsulate datagram into frame, adding

header, trailer– Arbitrates channel access if shared medium– “MAC” addresses used in frame headers to

identify source and destination nodes

different from IP address!


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Link Layer Services• Reliable delivery between adjacent nodes

– Seldom used on low bit-error link (fiber, some twisted pair)

– Very often used on wireless links: high error rates• Q (for the future): why both link-level and

end-end reliability (e.g. TCP)?


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Link Layer Services

• Flow control: – pacing between adjacent sending and

receiving nodes– Another Q for the future How does this

compare against TCP flow control?

• Half-duplex and full-duplex– with half duplex, nodes at both ends of link

can transmit, but not at same time


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Link Layer Services

• Error detection: – Errors caused by signal attenuation, noise, …– receiver detects presence of errors

signals sender for retransmission or drops frame

• Error correction: – receiver identifies and corrects bit error(s)

without resorting to retransmission


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Where is the link layer implemented?

• In each and every host• Link layer implemented in

“adaptor” (aka network interface card – NIC)– Ethernet card, PCMCI card,

802.11 card– implements link, physical

layer• Connects to host’s system

buses• Combination of hardware,

software, and firmware


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controller

physicaltransmission

cpu memory

host bus (e.g., PCI)

network interface card / adaptor

host schematic

applicationtransportnetwork

link

linkphysical

Adaptors Communicating


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Receiving side:• Error detection, reliable data

transfer, flow control, etc• extracts datagram, passes to

upper layer at receiving side

Sending side:• encapsulates datagram in

frame• adds error checking bits,

reliable data transfer, flow control, etc.

controller controller

sending host receiving host

datagram datagram

datagram

frame

Error Detection


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EDC = 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

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00 correctable single bit error

Sender:• treat segment contents as

sequence of 16-bit integers• checksum: addition (1’s

complement sum) of segment contents

• sender puts checksum value into UDP checksum field

Internet checksum (more later)

Receiver:• compute checksum of

received segment• check if computed checksum

equals checksum field value:– NO - error detected– YES - no error detected

But maybe errors still exist?


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

https://www.youtube.com/watch?v=JqEvNxAJtDk



Checksum: Cyclic Redundancy Check• view data bits, D, as a binary number• choose r + 1 bit pattern (generator), G • goal: choose r CRC bits, R, such that

– <D,R> exactly divisible by G (modulo 2)

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

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

• widely used in practice (Ethernet, 802.11 WiFi, ATM)


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CRC ExampleWant:

D.2r XOR R = nGequivalently:

D.2r = nG XOR R equivalently:

if we divide D.2r by G, we get remainder R


R = remainder[ ]

D.2r

G

© Winston Seah 16

Try at home examples• G=1001, D=110010, r= ??

• Given that with polynomial

G(x)= x5+ x4+ x3+ x2+ x0

and D = AF1867C (in hexadecimal) the

r=0000 (in binary). Find the r, for

D= AF1867D.


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• Ethernet

• Switches


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MAC Addresses and ARP• 32-bit IP address:

– network-layer address– used to get datagram to destination IP subnet

• MAC / LAN / physical / Ethernet address: – function: send frame from one interface to

another physically-connected interface (same network)

– 48 bit MAC address (for most LANs)• “burned” in NIC ROM, and sometimes software

configurable


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LAN Addresses and ARP


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Each adapter on the LAN has a unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= 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)

LAN Address (more)• MAC address allocation administered by IEEE• Manufacturer buys portion of MAC address

space (to assure uniqueness) analogy:a) MAC address: like National Student Number (NZ)

and Social Security Number (US)b) IP address: like postal address

• MAC flat address portability ➜– can move LAN card from one LAN to another

• IP hierarchical address NOT portable– address depends on IP subnet to which node is

attached


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ARP: Address Resolution Protocol


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• Each IP node (host, router) on LAN has ARP table

• ARP table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; TTL>

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

Q: how to determine MAC addr of B knowing B’s IP addr?

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

ARP protocol: Same LAN• Node A wants to send datagram to node B,

and B’s MAC addr not in A’s ARP table.

• A broadcasts ARP query packet, containing B's IP address – dest MAC address = FF-FF-FF-FF-FF-FF– all nodes on LAN receive the ARP query

• B replies to A with its (B's) MAC address– frame sent to A’s MAC address (unicast)


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ARP protocol: Same LAN• Node A caches (saves) B’s IP-to-MAC

address pair in its ARP table until information becomes old (times out) – soft state: information that times out (goes away)

unless refreshed• ARP is “plug-and-play”:

– nodes create their ARP tables without intervention from net administrator

– “arp” command for displaying and modifying ARP table


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ARP usage in Routing to another LAN


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• Objective: send datagram from A to B via R• Assumption: A knows B’s IP address

• Router R maintains two ARP tables in, one for each IP (sub)network (LAN)

R

1A-23-F9-CD-06-9B

222.222.222.220111.111.111.110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111

A 74-29-9C-E8-FF-55

222.222.222.221

88-B2-2F-54-1A-0F

B222.222.222.222

49-BD-D2-C7-56-2A

ARP usage in Routing to another LAN1. A creates IP datagram with source A, destination B

and A sends IP datagram to next hop (router) R1. A uses ARP to get R’s MAC address 111.111.111.1102. A creates link-layer frame with R's MAC address as

destination, frame contains A-to-B IP datagram3. A’s NIC sends frame 4. R’s NIC receives frame 5. R removes IP datagram from Ethernet frame, and sees that

it is destined to B (in the same local subnet of its other NIC)6. R uses ARP to get B’s MAC address 222.222.222.2227. R creates frame containing A-to-B IP datagram sends to B


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• Ethernet

• Switches


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Ethernet“dominant” wired LAN technology: • cheap $20 for NIC• first widely used LAN technology• simpler, cheaper than token LANs and ATM• kept up with speed race: 10 Mbps – 10 Gbps

Metcalfe’s Ethernet sketch


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Ethernet: physical topology• bus: popular through mid 90s

– all nodes in same collision domain (can collide with each other)

• star: prevails today– active switch in center– each “spoke” runs a (separate) Ethernet protocol

(nodes do not collide with each other)

bus: coaxial cable

switch

star© Winston Seah NWEN302 Computer Network

Design29

Ethernet frame structure

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

preamble: • 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011• used to synchronize receiver, sender clock

rates

dest.address

sourceaddress

data (payload) CRCpreamble

type


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Ethernet frame structure (more)• addresses: 6 byte source, destination MAC

addresses– if adapter receives frame with matching destination

address, or with broadcast address (e.g. ARP packet), it passes data in frame to network layer protocol

– otherwise, adapter discards frame

• type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk)

• CRC: cyclic redundancy check at receiver– error detected: frame is dropped

dest.address

sourceaddress

data (payload) CRCpreamble

type


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Ethernet: unreliable, connectionless• connectionless: no handshaking

between sending and receiving NICs • unreliable: receiving NIC doesnt send

acks or nacks to sending NIC– data in dropped frames recovered only if

initial sender uses higher layer rdt (e.g., TCP), otherwise dropped data lost

• Ethernet’s MAC protocol: unslotted CSMA/CD wth binary backoff


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802.3 Ethernet standards: link & physical layers

• many different Ethernet standards– common MAC protocol and frame format– different speeds: 2 Mbps, 10 Mbps, 100

Mbps, 1Gbps, 10G bps– different physical layer media: fiber, cable


linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer


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• Ethernet

• Switches


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Ethernet switch• link-layer device: takes an active role

– store, forward Ethernet frames– examine incoming frame’s MAC address,

selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment

• transparent– hosts are unaware of presence of switches

• plug-and-play, self-learning– switches do not need to be configured


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Switch: multiple simultaneous transmissions• hosts have dedicated,

direct connection to switch

• switches buffer packets• Ethernet protocol used on

each incoming link, but no collisions; full duplex

– each link is its own collision domain

• switching: A-to-A’ and B-to-B’ can transmit simultaneously, without collisions

switch with six interfaces(1,2,3,4,5,6)

A

A’

B

B’ C

C’

1 2

345

6


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Switch forwarding table

Q: how does switch know A’ reachable via interface 4, B’ reachable via interface 5?

switch with six interfaces(1,2,3,4,5,6)

A

A’

B

B’ C

C’

1 2

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6A: each switch has a switch table, each entry:(MAC address of host, interface to reach host, time stamp)looks like a routing table!Q: how are entries created, maintained in switch table?

something like a routing protocol?


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A

A’

B

B’ C

C’

1 2

345

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Switch: self-learning• switch learns which

hosts can be reached through which interfaces– when frame received,

switch “learns” location of sender: incoming LAN segment

– records sender/location pair in switch table

A A’

Source: ADest: A’

Switch table (initially empty)

MAC addr interface TTL

A 1 60


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Switch: frame filtering/forwardingwhen 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 frame

else forward frame on interface indicated by entry

} else flood /* forward on all interfaces except

arriving interface */

© Winston Seah NWEN302 Computer Network

Design39

A

A’

B

B’ C

C’

1 2

345

6

Self-learning, forwarding: example

A A’

Source: ADest: A’

MAC addr interface TTL

switch table (initially empty)

A 1 60

A A’A A’A A’A A’A A’

• frame destination, A’, location unknown:flood

A’ A

• destination A location known:

A’ 4 60

selectively send

on just one link


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Interconnecting switches

• switches can be connected together

Q: sending from A to G - how does S1 know to forward frame destined to F via S4 and S3?A: self learning! (works exactly the same as in single-switch case!)

A

B

S1

C D

E

FS2

S4

S3

H

I

G


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Self-learning multi-switch exampleSuppose C sends frame to I, I responds to C

Q: show switch tables and packet forwarding in S1, S2, S3, S4

A

B

S1

C D

E

FS2

S4

S3

H

I

G


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to externalnetwork

router

IP subnet

mail server

web server

Institutional network


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Switches vs. routersboth are store-and-forward: routers: network-layer devices (examine network-layer headers)switches: link-layer devices (examine link-layer headers)

both have forwarding tables:routers: compute tables using routing algorithms, IP addressesswitches: learn forwarding table using flooding, learning, MAC addresses


linkphysical

networklink

physical

linkphysical

switch

datagram


linkphysical

frame

frame

frame

datagram


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Link Layer Fundamentals

• Link layer services (Sect 5.1 & 5.2)

• Link layer addressing and address resolution (Sect 5.4.1)

• Ethernet (Sect 5.4.2)

• Switches (Sect 5.4.3)


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link layer fundamentals nwen302 computer network design

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