The Data Link Layer

Reading: Ch. 2.6, 3.1

2

Context

Application

Transport

Network

Data Link Ethernet Interface

Application

Presentation

Session

Transport

Network

(Data) Link

Physical

OSI

TCP/IP

Logical Link

Media Access Control (MAC)

Link Sublayers

Message, Segment, Packet, and Frame

3

HTTP

TCP

IP

Ethernet interface

HTTP

TCP

IP

Ethernet interface

IP IP

Ethernet interface

Ethernet interface

SONET interface

SONET interface

host host

router router

HTTP message

TCP segment

IP packet IP packet IP packet

Ethernet frame Ethernet frame SONET frame

4

Physical node-to-node

bit-by-bit

Network host-to-host

Link node-to-node

frame-by-frame

101101001000100101010

The Link Layer

Packet

Frame

5

Link Layer Services

• Encoding 

– Represent the 0s and 1s 

• Framing 

– Encapsulate packet into frame, adding header/trailer 

• Error detection 

– Receiver detecting errors with checksums 

• Error correction 

– Receiver optionally correcting errors 

• Flow control 

– Pacing between sending and receiving nodes 

– Often omitted; provided at higher level (transport) 

 

What is a Link?

6

Communication Medium Network Adapter

Link = Medium + Adapters

7

Adaptors Communicating

• Sending side – Encapsulates packet

in a frame

– Adds error checking bits, flow control, etc.

• Receiving side – Looks for errors, flow

control, etc.

– Extracts datagram and passes to receiving node

8

sending node

frame

receiving node

packet

frame

adapter adapter

link layer protocol packet

9

Simple Networks

• Multi-access link: Ethernet – Single physical link, shared by multiple nodes

• Point-to-point links: fiber-optic cable – Separate link per pair of nodes – Limitations on the number of adapters per node

multi-access link (broadcast network)

point-to-point links

Workstation Workstation

Workstation

Workstation Workstation

Workstation

10

Broadcast Networks (1)

• Bus topology

• Ethernet cable:

• All nodes share use of link, compete for access

. . .

10 Base 5

10Mbps Baseband (digital signalling)

Cable no longer than 500 m

11

Broadcast Networks (2)

• Star topology

• Fast Ethernet:

• The hub replicates the signal along all other links. (Switches are more common, and they forward frames only to the destination node.)

100 Base TX/FX

100Mbps Baseband (digital signalling)

Twisted Pairs / Fiber Optics

Hub

12

Broadcast Networks (3)

• Broadcast medium – All stations receive a copy of the message sent

– But most communication is intended to be only between two computers on a network

• To allow sender to specify destination, each station is assigned a hardware address (MAC address)

Sender Receiver

Signal propagates along the entire cable

13

Hardware Address (1)

• Example: Ethernet Addressing – Unique 48-bit MAC address

– First 24 bits is manufacturer code - assigned by IEEE

– Second 24 bits are sequentially assigned and UNIQUE

• Broadcast address: FF-FF-FF-FF-FF-FF

– Send the frame to all adapters 

14

Hardware Address (2)

• Where is the MAC address stored ? – On the network adapter

– When the adapter is manufactured

• The network adapter – Handles packet transmission and reception

– It operates independently of the CPU

– Compares the destination MAC address on each incoming packet to the MAC address of its own station and discards frames not destined for the station

• Interface hardware, not software, checks address

As an Aside: Promiscuous Mode

• Normal adapter: receives frames sent to – The local MAC address

– Broadcast address FF-FF-FF-FF-FF-FF

• Promiscuous mode – Receive everything, independent of destination MAC

• Useful for packet sniffing – Network monitoring

– E.g., wireshark, tcpdump

15

Why Not Just Use IP Addresses

?

16

Need to talk to the adapter to assign it an IP address

Who Am I: Acquiring an IP Address

17

71-65-F7-2B-08-53 1A-2F-BB-76-09-AD

0C-C4-11-6F-E3-98

???? 1.2.3.5

1.2.3.6

DHCP server

• Dynamic Host Configuration Protocol (DHCP) – Broadcast “I need an IP address, please!”

– Response “You can have IP address 1.2.3.4.”

Who Are You: Discovering the Receiver

• Address Resolution Protocol (ARP) – Broadcast “who has IP address 1.2.3.6?”

– Response “0C-C4-11-6F-E3-98 has 1.2.3.6!” 18

71-65-F7-2B-08-53 1A-2F-BB-76-09-AD

0C-C4-11-6F-E3-98

1.2.3.4 1.2.3.5

1.2.3.6

Sharing the Medium

Collisions

• Single shared broadcast channel – Avoid having multiple nodes speaking at once

– Otherwise, collisions lead to garbled data

20

71-65-F7-2B-08-53 1A-2F-BB-76-09-AD

Multi-Access Protocols

1. Divide the channel into pieces – In time

– In frequency

2. Take turns – Pass a token for the

right to transmit

3. Let collisions happen – … and detect and recover from them

– CSMA/CD (Carrier Sense Multiple Access /Collision Detection)

21

• Carrier sense (CS) – Listen before speaking

– Nodes can distinguish between an idle and a busy link

• Multiple Access (MA) – Multiple transmissions

– When a station is done transmitting, it can immediately start a new transmission

• Collision detection (CD) – Listen while transmitting

– Detect simultaneous talking … and stop!

– Wait for a period of time before trying to talk again!

CSMA/CD is Like Human Conversation…

22

23

Effect of Propagation Delay on CSMA

carrier sense = idle

Transmit a packet

Collision

A B

packet

C

Propagation Delay: Time to propagate a packet from one end to other

24

Collision detection time How long does it take to realize there has been a collision?

Worst case: 2 x

To detect the collision, A must transmit for at least 2x time.

A B

Time=0

A B

Time=-

A B

Time=2

= end-to-end propagation

delay

Comparing the Three Approaches

• Channel partitioning – Efficient and fair at high load

– Inefficient at low load

• “Taking turns” – Eliminates empty slots without collisions

– Vulnerable to failures (e.g. lost token)

• CSMA/CD with Random access – Efficient at low load

– Collision overhead at high load

25

Ethernet (802.3)

27

Ethernet

• Dominant wired LAN technology

• First widely used LAN technology

• Kept up with speed race: 10 Mbps – 40 Gbps

Metcalfe’s Ethernet sketch

Ethernet Uses CSMA/CD

• Carrier Sense: wait for link to be idle – Channel idle: start transmitting

– Channel busy: wait until idle

• Collision Detection: listen while transmitting – No collision: transmission is complete

– Collision: abort transmission, and send jam signal

• Random Access: exponential back-off – Double the wait interval between each retransmission

attempt

28

29

Ethernet Backoff Algorithm

• Binary Exponential Backoff: – If collision, choose one slot randomly from 2k slots, where k

is the number of collisions the frame has suffered.

– This algorithm can adapt to changes in network load.

– Retries limited to 15 times (although k is capped at 10)

slot length = 2 x end-to-end delay

A B

30

Binary Exponential Backoff

slot length = 2 x end-to-end delay = 51.2 s

A B

t=0s: Assume A and B collide (kA = kB = 1) A, B choose randomly from 21 slots: [0,1] Assume A chooses 1, B chooses 1 t=100s: A and B collide (kA = kB = 2) A, B choose randomly from 22 slots: [0,3] Assume A chooses 2, B chooses 0 t=150s: B transmits successfully t=250s: A transmits successfully

Limitations on Ethernet Link Length

• Latency depends on physical length of link

• Suppose – A sends a packet at time t

– B sends packet just before t+

• Then A doesn’t see collision till t+2

• Imposes restrictions on Ethernet – Maximum distance between two nodes: 2500 meters

– Minimum length of the packet: 512 bits (64 bytes) 31

latency A B

32

Limitations on Ethernet Frame Length

• IEEE 802.3 specifies max value of 2 to be 51.2s – This relates to maximum distance of 2500m between hosts

– At 10Mbps it takes 0.1s to transmit one bit so 512 bits (64B) take 51.2s to send

• Condition for CSMA/CD to work:

Transmission Time > 2 – So Ethernet frames must be at least 64 bytes long

– Padding is used if data is less than 64 bytes

• Maximum frame length is 1500 bytes – So that the adaptor doesn’t occupy the line for too long

• Sending adapter encapsulates packet in frame

• Preamble: synchronization – Seven bytes with pattern 10101010, followed by one byte

with pattern 10101011

– Used to synchronize receiver, sender clock rates

Ethernet Frame Structure

33

Preamble Source

MAC address

Destination MAC

address Type CRC Data

Ethernet Frame Structure

• Addresses: source and destination MAC addresses – Adaptor passes frame to network-level protocol only if

destination is local MAC address or multicast/broadcast address or adapter is in promiscuous mode

– Otherwise, adapter discards frame

• Type: indicates the higher layer protocol – Usually IP, but also Novell IPX, AppleTalk, …

• CRC: cyclic redundancy check – Checked at receiver; if error, the frame is dropped

34

Preamble Source

MAC address

Destination MAC

address Type CRC Data

Unreliable, Connectionless Service

• Connectionless – No handshaking between send and receive adapter

• Unreliable – Receiving adapter doesn’t send ACKs or NACKs

– Packets passed to network layer can have gaps (which get filled in by the TCP transport protocol)

35

36

Ethernet – Questions:

Q1: An Ethernet MAC sublayer receives 42 bytes of data from the LLC sublayer. How many bytes of padding must be added to the data?

Q2: An Ethernet MAC sublayer receives 1510 bytes of data from the LLC sublayer. Can the data be encapsulated in one frame? If not, how many frames need to be sent ? What is the size of the data in each frame ?

37

CSMA/CD – Question 1

Consider a 10Mb/s CSMA/CD network as shown below:

• Calculate the length of the shortest packet that the network above can support so that the CSMA/CD protocol will function correctly. Assume that bits travel on the wire at the speed c = 2 * 108 m/s.

H1

H2

H3

H10

Hub

100

100

100

100

38

CSMA/CD – Question 2

• The hub is now removed, but the computers remain in the same locations. A single cable is strung between the computers as shown below.

What is the length of a shortest packet in this case?

H1 H2

H10

H9

100 100

100

100

100 H3 100

100

39

CSMA/CD – Question 3

• Why do Ethernet adaptors select a random back-off time before trying to transmit a frame following a collision? Why do they pick the random back-off time from a larger range after each collision?

Extending Networks with

Interconnecting Devices

41

Interconnecting Devices

• There are many different interconnecting devices.

Ethernet

Router

Ethernet

Ethernet

Token- ring

Gateway

Bridge

Repeater

X.25 Network

42

Physical Layer: Repeaters

• Copy / Amplify signals between the two segments

• Analog devices, propagate valid signals as well as collisions

• Do not have hardware (MAC) addresses

• Ethernet allows at most 4 repeaters between 2 machines

Repeater

IP

LLC

802.3 MAC

IP

LLC

802.3 MACRepeater

43

Physical Layer: Hubs

• Joins multiple input lines electrically – Designed to hold multiple line cards

• Very similar to repeaters – Also operates at the physical layer

– Passive hubs may simply forward signals

– Active hubs may also amplify or refresh signals

4, 5, 8, 9, 16, 32, 64 Ports

44

Limitations of Repeaters and Hubs

• One large shared link – Each bit is sent everywhere

• Cannot support multiple LAN technologies – Does not buffer or interpret frames

– So, can’t interconnect between different rates or formats

– E.g., 10 Mbps Ethernet and 100 Mbps Ethernet

• Limitations on maximum nodes and distances – Shared medium imposes length limits

– E.g., cannot go beyond 2500 meters on Ethernet

45

Link Layer: Bridges/LAN Switches

• Interconnect multiple LANs, possibly of different types.

• Operate on frames, not signals.

• Have one or more NICs

BridgeToken-ring

BridgeIP

LLC

802.3 MAC 802.3 MAC 802.5 MAC

LLC

IP

LLC

802.5 MACLAN LAN

46

Bridges vs. LAN Switches

• A network switch is a computer networking device that connects network segments. The term commonly refers to a network bridge that processes and routes data at the Data link layer (layer 2) of the OSI model. One way to think of a layer 2 switch is as a multiport bridge.

• Switches that additionally process data at the Network layer (layer 3 and above) are often referred to as Layer 3 switches.

[Wikipedia] http://en.wikipedia.org/wiki/Network_switch

47

Bridge/LAN Switch Filtering

• Bridges learn from experience and build and maintain address tables of the nodes on the network.

– Extract destination address from the frame

– Look up the destination in a table

– Forward the frame to the appropriate LAN segment

• More about this later …

48

Advantages Over Hubs/Repeaters

• Only forward frames as needed

• Segments can support separate transmissions

• Can join segments using different technologies

hub hub hub

switch/bridge

segment segment segment

49

Disadvantages Over Hubs/Repeaters

• Delay in forwarding frames – Bridge/switch must receive and parse the frame

– … and perform a look-up to decide where to forward

– Storing and forwarding the packet introduces delay

– Solution: cut-through switching

• Need to learn where to forward frames – Bridge/switch needs to construct a forwarding table

– Ideally, without intervention from network administrators

– Solution: self-learning

• Higher cost – More complicated devices that cost more money

50

Motivation For Cut-Through Switching

• Buffering a frame takes time – Suppose L is the length of the frame

– And R is the transmission rate of the links

– Then, receiving the frame takes L/R time units

• Buffering delay can be a high fraction of total delay – Propagation delay is small over short distances

– Buffering delay may become a large fraction of total

A B

switches

51

Cut-Through Switching

• Start transmitting as soon as possible – Inspect the frame header and do the look-up

– If outgoing link is idle, start forwarding the frame

• Overlapping transmissions – Transmit the head of the packet via the outgoing link

– … while still receiving the tail via the incoming link

A B

switches

52

Learning Bridges

• What do bridges do if some LANs are reachable only in multiple hops ?

• What do bridges do if the path between two LANs is not unique ?

LAN 2

Bridge 2

LAN 5

LAN 3

LAN 1

LAN 4

Bridge 5

Bridge 4Bridge 3

d

Bridge 1

53

Transparent Bridges

• Not visible to end hosts

• Execute a spanning tree algorithm

• Two parts to transparent bridges: 1. Learning & Forwarding

2. Spanning Tree Algorithm

Bridge

LAN 5

LAN 3

LAN 1

LAN 4

Bridge

Bridge Bridge

Bridge

LAN 2

54

Transparent Bridges: Learning

• When a frame arrives – Inspect the source MAC address

– Associate the address with the incoming interface

– Store the mapping in the switch table

– Use a time-to-live field to refresh the mapping (default 15s)

A

B

C

D

Switch learns how to reach A.

55

Transparent Bridges: Forwarding (Miss)

• When frame arrives with unfamiliar destination – Forward the frame out all of the interfaces

– … except for the one where the frame arrived

– Hopefully, this case won’t happen very often

A

B

C

D

When in doubt, shout!

56

Transparent Bridges: Learning & Forwarding

When switch receives a frame:

Index switch table using MAC dest address

if entry found for destination then

{

if dest on segment from which frame arrived then

drop the frame

else

forward the frame on interface indicated

}

else flood

forward on all but the interface on which the frame arrived

57

Example

• Consider the following packets: (Src=A, Dest=F), (Src=C, Dest=A), (Src=E, Dest=C)

• What do the bridges learn?

Bridge 1

Port1

LAN 1

A

LAN 2

C B D

LAN 3

E F

Port2

Bridge 2

Port1 Port2

58

Danger of Loops

• Bridges sometimes need to broadcast frames – Upon receiving a frame with an unfamiliar destination

– Upon receiving a frame sent to the broadcast address

• Broadcasting can lead to loops – e.g., if the network contains a cycle of switches (reliability)

59

Loop Example (1) Bridge 1 Bridge 2

Host Port Host Port

A Top A Top

Bottom

60

Loop Example (2) Bridge 1 Bridge 2

Host Port Host Port

A Top A Bottom

Bottom

61

Solution: Spanning Trees

• Ensure the topology has no loops • Distributed algorithm constructing a spanning tree

– Sub-graph that covers all vertices but contains no cycles

– Switches cooperate to build the spanning tree

– … and adapt automatically when failures occur

62

Spanning Tree Algorithm - Key Ingredients

• Switches elect a “root” (smallest ID)

• Each switch identifies if its interface is on the shortest path from the root – And it exclude from the tree if not

• Switches collectively determine which interface is on a shortest path from a network segment to the root

• Configuration Messages (Y, d, X) – From bridge X

– Claiming Y is the root

– And the distance to root is d

root

one hop

three hops

BPDU (Bridge Protocol Data Unit)

Actual Information contained in BDPUs

63

64

Steps in Spanning Tree Algorithm

• Initially, each switch thinks it is the root – Switch sends a message out every interface

– … identifying itself as the root with distance 0

– Example: switch X announces (X, 0, X)

• Switches update their view of the root – Upon receiving a message, check the root id

– If the new id is smaller, start viewing that switch as root

• Switches compute their distance from the root – Add 1 to the distance received from a neighbor

– Identify interfaces not on a shortest path to the root

– … and exclude them from the spanning tree

65

Example From Switch #4’s Viewpoint

• Switch #4 thinks it is the root – Sends (4, 0, 4) message to 2 and 7

• Then, switch #4 hears from #2 – Receives (2, 0, 2) message from 2

– … and thinks that #2 is the root

• Then, switch #4 hears from #7 – Receives (2, 1, 7) from 7

– And realizes this is a longer path

– So, prefers its own one-hop path

– And removes 4-7 Iink from the tree (temporary view)

1

2

3 5

6 7

4

66

Example From Switch #4’s Viewpoint

• Switch #2 hears about switch #1 – Switch 2 hears (1, 1, 3) from 3

– Switch 2 starts treating 1 as root

– And sends (1, 2, 2) to neighbors

• Switch #4 hears from switch #2 – Switch 4 starts treating 1 as root

– And sends (1, 3, 4) to neighbors

• Switch #4 hears from switch #7 – Switch 4 receives (1, 3, 7) from 7

– And realizes this is a longer path

– So, prefers its own three-hop path

1

2

3 5

6 7

4

67

Root Ports and Designated Ports

• Each port attached to the spanning tree is a root port (RT) – Forwards frames towards the root as necessary

• Which bridge forwards frames to/from a LAN segment? – The one with a least cost path to the root

– Break ties by the lower bridge ID

– The port attaching that bridge to

the LAN segment is the

designated port for the LAN

• Blocked ports: – Each port that is not root

or designated is blocked

(does not receive or forward Ethernet frames)

1

2

3 5

6

7

4 RP

RP

RP RP

RP

RP

A

B

C

D

E

F

DP

DP

DP

DP

DP

DP

G

H

DP

DP

I DP

68

Spanning Tree Operation

• When the network has stabilized, it has converged and there is one spanning tree per network

• For every switched network the following elements exist: – One root bridge per network – One root port per non root bridge – One designated port per segment – Unused, non-designated (blocked) ports

• Root ports and designated ports forward data traffic • Non-designated (blocked) ports discard data traffic

– No addresses can be learned – Continue to receive and process BDPU packets

• so they can detect when an active path or device fails, and recalculate a new spanning tree

69

You Try It …

• Mark the Root, the Root Ports and the Designated Ports

70

Robust Spanning Tree Algorithm

• Algorithm must react to failures – Failure of the root node

• Need to elect a new root, with the next lowest identifier – Failure of other switches and links

• Need to recompute the spanning tree

• Root switch continues sending messages – Periodically reannouncing itself as the root (1, 0, 1) – Other switches continue forwarding messages

• every 2s by default

• Detecting failures through timeout (soft state!) – Switch waits to hear from others – Eventually times out and claims to be the root

See Section 3.1 in the textbook for details and another example

71

You Try It …

• If the link (4, 24) fails, what happens next?

72

Network Layer: Routers

• Router – A device that forwards data packets from one local area network (LAN) or wide area network (WAN) to another. Based on routing tables and routing protocols, routers read the network address in each transmitted frame and make a decision on how to send it based on the most expedient route (traffic load, line costs, speed, bad lines, etc.).

[TechWeb Encyclopedia]

73

Network Layer: Routers

Subnet-work

Router

Subnet-work

Router

Subnet-work

Application

TCP

IP

NetworkAccess

Application

TCP

IP

NetworkAccess

IP protocol IP protocol

DataLink

NetworkAccess

IP

NetworkAccess

NetworkAccess

IP

NetworkAccess

DataLink

DataLink

IP protocol

RouterRouter HostHost

74

Bridges vs. Routers (1)

• Bridges work at the data link layer, whereas routers work at the network layer.

• Bridges are faster than routers because they do not have to read the network protocol to get routing information.

75

Bridges vs. Routers (2)

• An enterprise network (e.g., university network) with a large number of local area networks (LANs) can use routers or bridges

• Until early 1990s: most LANs connected by routers

• Since mid1990s: LAN switches replace most routers

76

Internet

A Routed Enterprise Network

Router

Hubs

FDDI

FDDI

77

Internet

A Switched Enterprise Network

Router

Switch

78

Bridges vs. Routers (3)

Routers

• Each host’s IP address must be configured

• If network is reconfigured, IP addresses may need to be reassigned

• Routing done via protocols (RIP or OSPF)

• Each router manipulates packet IP header (e.g., reduces TTL field)

Bridges

• MAC addresses are hardwired

• No network configuration needed

• No routing protocol needed (sort of)

– learning bridge algorithm

– spanning tree algorithm

79

Comparing Hubs, Switches, Routers

Hub/

Repeater

Bridge/

Switch

Router

Traffic isolation

Plug and Play

Efficient routing

Cut through

no yes yes

yes yes no

no no yes

yes yes no

80

Transport Layer: Gateways

• Gateway – a computer that performs protocol conversion between different types of networks or applications. For example, a gateway can convert a TCP/IP packet to a NetWare IPX packet and vice versa, or from AppleTalk to DECnet, from SNA to AppleTalk and so on.

[TechWeb Encyclopedia]

81

This lecture

• Data Link Layer

– Transmission medium and adapters

• Multiple-Access (Broadcast) Networks

– Ethernet

• Interconnecting Devices

– Repeaters and hubs

– Bridges / LAN switches

– Routers

– Gateways