Communications Technology

Lecture 6: Data Link Layer

June 2009

References

Computer Network by A. S. Tanenbaum, Chapter 3

Data & Computer Communications by William Stallings

Understand principles behind data link layer services: error detection, correction sharing a broadcast channel:

multiple access MAC protocols

continued

Learning Objectives

Data Link Layer (L2)

Introduction and services Error detection and correction Multiple/Media access protocols

L2 IntroductionSome terminologies hosts and routers are nodes communication channels

that connect adjacent nodes along communication path are links

wired links wireless links LANs

layer-2 packet is a frame, encapsulates datagram

“link”

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

L2: Context Datagram transferred

by different link protocols over different links:

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

Each link protocol provides different services

E.g., reliable data transfer (rdt)

transportation analogy trip from HCMC to Hanoi

limo: HUT to TSN plane: TSN to Noi Bai bus: Noi Bai to Hanoi

tourist = datagram transport segment =

communication link transportation mode =

link layer protocol travel agent = routing

algorithm

L2 Services Framing, link access:

encapsulate datagram into frame, adding header, trailer

channel access if shared medium “MAC” addresses used in frame headers to

identify source, dest different from IP address!

Reliable delivery between adjacent nodes seldom used on low bit error link (fiber,

some twisted pair) wireless links: high error rates

L2 Services (more) Flow Control

pacing between adjacent sending and receiving nodes

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 Half-duplex and full-duplex

with half duplex, nodes at both ends of link can transmit, but not at same time

Adaptors Communicatingsendingnode

frame

receivingnode

datagram

frame

adapter adapter

link layer protocol

link layer implemented in “adaptor” (aka NIC)

Ethernet card, modem, 802.11 card

adapter is semi-autonomous, implementing link & physical layers

sending side encapsulates datagram in

a frame adds error checking bits,

rdt, flow control, etc. receiving side

looks for errors, rdt, flow control, etc

extracts datagram, passes to receiving node

Data Link Layer (L2)

Introduction and services Error detection and correction Multiple/Media access protocols

Error Detection

D = Data protected by error checking, may include header fieldsED = Error Detection bits (redundancy)

Error detection is not 100% reliable!• a good error detector may miss some errors, but rarely• larger ED field generally yields better detection

‘

Parity CheckingSingle Bit Parity:Detect single bit errors

Two Dimensional Bit Parity:Detect and correct single bit errors

0 0

Error Detection: Parity Checking

- add a parity bit for every d data bits so that the number of ones of each d+1 data and parity bits is even

Issue: errors generally are bursty, i.e. several consecutive bits are flipped;

10111 0001 1010 1011

1010 1010 0101 0101

Internet checksum

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/TCP checksum field

Receiver: compute checksum of

received segment check if computed

checksum equals checksum field value:

NO - error detected YES - no error detected. But

maybe errors nonetheless?

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

Checksumming: 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 (ATM, HDCL)

Cyclic Redundancy Check: Background

Widely used in practice, e.g., Ethernet, FDDI, PKZIP, WinZip, PNG

For a given data D, consider it as a polynomial D(x) consider the string of 0 and 1 as the

coefficients of a polynomial e.g. consider string 10011 as x4+x+1

addition and subtraction are modular 2, thus the same as xor

Choose generator polynomial G(x) with r+1 bits, where r is called the degree of G(x)

Cyclic Redundancy Check: Objective

Given data G(x) and D(x), choose R(x) with r bits, such that

D(x)xr+R(x) is exactly divisible by G(x)

The bits correspond to D(x)xr+R(x) are sent to the receiver

Since G(x) is global, when the receiver receives the transmission T’(x), it divides T’(x) by G(x)

if non-zero remainder: error detected! if zero remainder, assumes no error

+x

CRC: Steps and an Example

Suppose the degree of G(x) is r

Append r zero to D(x), i.e. consider D(x)xr

Divide D(x)xr by G(x). Let R(x) denote the reminder

Send <D, R> to the receiver

The Power of CRC Let T(x) denote D(x)xr+R(x), and E(x) the polynomial of the

error bits the received signal is T(x)+E(x)

Since T(x) is divisible by G(x), we only need to consider if E(x) is divisible by G(x)

Examples a single bit of error: E(x) = xi

if G(x) contains two or more terms, E(x) is not divisible by G(x)

an odd number of errors: E(x) has an odd number of terms: lemma: if E(x) has an odd number of terms, E(x) cannot be divisible

by (x+1) suppose E(x) = (x+1)F(x), let x=1, the left hand will be 1, while the

right hand will be 0 thus if G(x) contains x+1 as a factor, E(x) will not be divided by G(x)

Many more errors can be detected by designing the right G(x)

Example G(x) 16 bits CRC:

CRC-16: x16+x15+x2+1, CRC-CCITT: x16+x12+x5+1 both can catch

all single or double bit errors all odd number of bit errors all burst errors of length 16 or less >99.99% of the 17 or 18 bits burst errors

32 bits CRC: CRC32: x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7

+ x5 + x4 + x2 + x + 1 used by Ethernet, FDDI, PKZIP, WinZip, and PNG

For more details see the link below and further links it contains:

http://en.wikipedia.org/wiki/Cyclic_redundancy_check

Data Link Layer (L2)

Introduction and services Error detection and correction Multiple/Media access protocols

Multiple Access Links and Protocols

Two types of “links”: point-to-point

PPP for dial-up access point-to-point link between Ethernet switch and host

broadcast (shared wire or medium) traditional Ethernet upstream HFC 802.11 wireless LAN

Multiple Access Protocols Single shared broadcast channel

thus, if two or more simultaneous transmissions by nodes, due to interference, only one node can send successfully at a time (see CDMA later for an exception)

multiple access protocol Protocol that determines how nodes share

channel, i.e., determines when nodes can transmit Communication about channel sharing must use

channel itself ! Discussion: properties of an ideal multiple access

protocol.

Ideal Multiple Access Protocol

Broadcast channel of rate R bps Efficiency: when one node wants to

transmit, it can send at rate R Fairness: when N nodes want to

transmit, each can send at average rate R/N

Decentralized: no special node to coordinate

transmissions no synchronization of clocks

Simple

MAC Protocols: a taxonomyThree broad classes: Channel Partitioning

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

allocate piece to node for exclusive use Random Access

channel not divided, allow collisions “recover” from collisions

“Taking turns” Nodes take turns, but nodes with more to

send can take longer turns

Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by

nodes: interference collision if node receives two or more signals at

the same timemultiple access protocol distributed algorithm that determines how

nodes share channel, i.e., determine when node can transmit

communication about channel sharing must use channel itself! no out-of-band channel for coordination

Random Access Protocols When node has packet to send

transmit at full channel data rate R. no a priori coordination among nodes

two or more transmitting nodes == “collision”,

random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed

retransmissions) Examples of random access MAC protocols:

slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA

Evolution of Contention Protocols

Aloha

SlottedAloha

Improvement: Start transmission only at fixed times (slots)

CSMA

CSMA = Carrier Sense Multiple AccessImprovement: Start transmission only if no transmission is ongoing

CD = Collision DetectionImprovement: Stop ongoing transmission if a collision is detected (e.g. Ethernet)

CSMA/CD

Developed in the 1970s for a packet radio network

Slotted ALOHAAssumptions all frames same size time is divided into

equal size slots, time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized

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

Operation when node obtains fresh

frame, it transmits in next slot

no collision, node can send new frame in next slot

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

Pure (unslotted) ALOHA unslotted ALOHA: simpler, no

synchronization when frame first arrives

transmit immediately collision probability increases:

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

CSMA (Carrier Sense Multiple Access)

CSMA: listen before transmit:If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission

Human analogy: don’t interrupt others!

CSMA collisionscollisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wasted

spatial layout of nodes

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

CSMA/CD (Collision Detection)

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

wastage collision detection:

easy in wired LANs: measure signal strengths, compare transmitted, received signals

difficult in wireless LANs: receiver shut off while transmitting

human analogy: the polite conversationalist

CSMA/CD collision detection

“Taking Turns” MAC protocols

channel partitioning MAC protocols: share channel efficiently and fairly at high load inefficient at low load: delay in channel access,

1/N bandwidth allocated even if only 1 active node!

Random access MAC protocols efficient at low load: single node can fully

utilize channel high load: collision overhead

“taking turns” protocolslook for best of both worlds!

“Taking Turns” MAC protocols

Polling: master node

“invites” slave nodes to transmit in turn

concerns: polling overhead latency single point of

failure (master)

Token passing: control token passed

from one node to next sequentially.

token message concerns:

token overhead latency single point of failure

(token)

Summary of MAC protocols

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

code Time Division, Frequency Division

Random partitioning (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD carrier sensing: easy in some technologies (wire),

hard in others (wireless) CSMA/CD used in Ethernet CSMA/CA used in 802.11

Taking Turns polling from a central site, token passing

Chapter Summary

principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access MAC protocols

Backup slides

Overview Ethernet and Wi-Fi are both “multi-

access” technologies Broadcast medium, shared by many hosts Simultaneous transmissions will result in

collisions Media Access Control (MAC) protocol

required Rules on how to share medium

Media Access Control Protocols

Channel partitioning Divide channel into smaller “pieces” (e.g., time

slots, frequency) Allocate a piece to node for exclusive use E.g. Time-Division-Multi-Access (TDMA) cellular

network

Taking-turns Tightly coordinate shared access to avoid

collisions E.g. Token ring network

Contention Allow collisions “recover” from collisions E.g. Ethernet, Wi-Fi

Contention Media Access Control Goals

Share medium If two users send at the same time, collision results

in no packet being received (interference) If no users send, channel goes idle Thus, want to have only one user send at a time

Want high network utilization TDMA doesn’t give high utilization

Want simple distributed algorithm no fancy token-passing schemes that avoid

collisions

Evolution of Contention Protocols

Aloha

SlottedAloha

Improvement: Start transmission only at fixed times (slots)

CSMA

CSMA = Carrier Sense Multiple AccessImprovement: Start transmission only if no transmission is ongoing

CD = Collision DetectionImprovement: Stop ongoing transmission if a collision is detected (e.g. Ethernet)

CSMA/CD

Developed in the 1970s for a packet radio network

(Pure) ALOHA

Topology: Broadcast medium with multiple stations

Aloha Protocol: Whenever a station has data, it

transmits immediately

Receivers ACK all packets

No ACK = collision. Wait a random time and retransmit

Simple, but Radical Previous attempts all partitioned

channel TDMA, FDMA, etc.

Aloha optimized the common case (few senders) and dealt with collisions through retries

Trade-off Compared to TDMA

In TDMA, you always have to wait your turn

delay proportional to number of sites

In Aloha, can send immediately

Aloha gives much lower delays, at the price of lower utilization (as we will see)

Collisions in (Pure) ALOHA

1.1 1.2

TransmissionTime

(F)

Station 1

2.1Station 2

3.1 3.2Station 3

Broadcastchannel

2.2

1.3

CompleteCollision

PartialCollision

Performance of (Pure) ALOHA Performance questions:

What is the collision probability?

What is the maximum throughput?

Notation:C : link capacity in (bits/sec)

s : packet size (bits/packet) F : packet transmission time (sec)

C

sF

Frame which collideswith start of red frame

Frame

t0-F t0 t0+F

VulnerablePeriod of red frame

Time

Frame which collideswith end of red frame

Collisions and Vulnerable Period

A frame (red frame) will be in a collision if and only if another transmission begins in the vulnerable period of the frame

Vulnerable period has the length of 2 frame times

Traffic Model

FsC

rateservicetotal

ratearrivaltotalloadoffered

+

1

2

3

i

1

3

2

i

Poisson rate from all stations (Poisson Process’s are additive)

tetf )(

Inter-arrival time is exponentially distributed

System Model

p

1 -p

p r

(1 -p )rr

pGG

c o llis io n

s e rvic e d

p

1 -pG *+

Fact: Bernoulli sampling of a PP is also a PP

p Probability of collision

G Total carried load (incl. Retransmissions)

Probability of Collision

 

FGeFineventsomePp 21)2__(

   

Frame which collideswith start of red frame

Frame

t0-F t0 t0+F

VulnerablePeriod of red frame

Time

Frame which collideswith end of red frame

If Poisson events occur at rate ,

P (no event in T seconds) = e-T

2F

Throughput and Total Carried Load

 

pGG GFGe 2

RF 2Re

From

)( stable DelayE

FGsC

GR

/ load carried totalnormalized

If stable, all offered traffic is serviced, and r is also the throughput.

Expression characterizes throughput vs. total carried load including retransmissions. What is ALOHA’s maximum throughput?

Maximum Throughput

 

18.02

1 1max e

Maximum achievable throughput:

0dR

d0)2( 22 RR eeR

2

1R

Observe: if offered load > 0.18*C, unstable

Performance of ALOHA

• Maximum throughput approximately 18% of the capacity• Can do better with improved control • However, ALOHA is still used for its simplicity

•Ex. Cell phone call establishment

0 0.5 1 1.5 2 2.50

0.18

0.36

0.54

R

Thr

ough

put

(Pur

e A

LOH

A)

Ideal (no collisions):R

Pure ALOHA: Re-2R

Slotted ALOHA (S-ALOHA)

The Slotted Aloha Protocol Slotted Aloha - Aloha with an additional constraint Time is divided into discrete time intervals (=slot) A station can transmit only at the beginning of a

frame

As a consequence: Frames either collide completely or do not collide

at all Vulnerable period = ?

Collisions in S-ALOHA1.1 1.2

TransmissionDelay

Station 1

2.1Station 2

3.1 3.2

Station 3

Broadcastchannel

2.2

1.3

CompleteCollision

GFi eFYPp 1)(

Total Throughput in S-ALOHA:

Maximum achievable throughput:

Performance gain but requires nodes to have synchronized frame boundaries

Performance of S-ALOHA

37.01max e

RRe

Comparison of ALOHA and S-ALOHA

0 0.5 1 1.5 2 2.5 30

0.1

0.2

0.3

0.4

0.5

R

Thr

ough

put

(ALO

HA

)

Slotted ALOHA: Re-R

Pure ALOHA: Re-2R

Ideal (no collisions): R

802.3 Ethernet

Broadcast technology

Carrier-sense multiple access with collision detection (CSMA/CD). MA = multiple access CS = carrier sense CD = collision detection

Base Ethernet standard is 10 Mbps. Original design was ~2 Mbps 100Mbps, 1Gbps, 10Gbps

host host host host

host host host host

Hub

CSMA/CD Algorithm Sense for carrier If carrier present, wait until carrier ends

Sending would force a collision and waste time Send packet and sense for collision If no collision detected, consider packet

delivered Otherwise, abort immediately, perform

“exponential back off” and send packet again. Start to send at a random time picked from an

interval Length of the interval increases with every

retransmission

CSMA/CD: Some Details

When a sender detects a collision, it sends a “jam signal” Make sure that all nodes are aware

of the collision Length of the jam signal 48 bits

Exponential backoff operates in multiples of 512 bit time

CSMA collisions

Collisions can occur:propagation delay means two nodes may not hear each other’s transmissionCollision:entire packet transmission time wasted

spatial layout of nodes along ethernet

Note:role of distance and propagation delay in determining collision prob.

CSMA/CD (Collision Detection)

Collisions detected within short time

Colliding transmissions aborted, reducing channel wastage

Easy in wired LANs: measure signal strengths, compare transmitted, received signals

Difficult in wireless LANs

CSMA/CD collision detection

Minimum Packet Size

Why put a minimum packet size? Give a host enough time to detect collisions In Ethernet, minimum packet size = 64

bytes (two 6-byte addresses, 2-byte type, 4-byte CRC, and 46 bytes of data)

If host has less than 46 bytes to send, the adaptor pads (adds) bytes to make it 46 bytes

What is the relationship between minimum packet size and the length of the LAN?

Minimum Packet Size (more)

propagation delay (d)a) Time = t; Host 1 starts to send frame

Host 1 Host 2

propagation delay (d)

Host 1 Host 2b) Time = t + d; Host 2 starts to send a frame just before it hears fromhost 1’s frame

propagation delay (d)

Host 1 Host 2c) Time = t + 2*d; Host 1 hears Host 2’s frame detects collision

LAN length = (min_frame_size)*(light_speed)/(2*bandwidth) = = (8*64b)*(2.5*108mps)/(2*107 bps) = 6400m approx

Exponential Backoff Algorithm

Ethernet uses the exponential backoff algorithms to determine when a station can retransmit after a collision

Algorithm:• Set “slot time” equal to 512bit time• After first collision wait 0 or 1 slot times• After i-th collision, wait a random number between 0 and 2i-1 time slots• Do not increase random number range, if i=10• Give up after 16 collisions

Frame Frame Frame Frame

t0 Contention Slotst1

Contention interval Idle

CSMA/CD Contention Interval

Contention slots end in a collision Contention interval is a sequence of contention slots Length of a slot in contention interval is 512 bit time

time

Ethernet Frame Structure Sending adapter encapsulates IP

datagram

Preamble: 7 bytes with pattern 10101010 followed by

one byte with pattern 10101011 Used to synchronize receiver, sender clock

rates

Ethernet Frame Structure (more)

Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match

Type: 2 bytes, is actually a length field in 802.3

CRC: 4 bytes, checked at receiver, if error is detected, the frame is simply dropped

Data payload: maximum 1500 bytes, minimum 46 bytes

Ethernet Technologies: 10Base2

10: 10Mbps; 2: under 200 meters max cable length Thin coaxial cable in a bus topology

Repeaters used to connect up to multiple segments Repeater repeats bits it hears on one interface to its other

interfaces: physical layer device only!

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

Ethernet” T stands for Twisted Pair Hub to which nodes are connected by

twisted pair, thus “star topology”

10BaseT and 100BaseT (more) Max distance from node to Hub is 100 meters Hub can gather monitoring information,

statistics for display to LAN administrators

Hubs still preserve one collision domain Every packet is forwarded to all hosts

Use bridges to address this problem Bridges forward a packet only to the destination

leading to the destination Next lecture

Gbit Ethernet

Use standard Ethernet frame format Allows for point-to-point links and

shared broadcast channels In shared mode, CSMA/CD is used;

short distances between nodes to be efficient

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

Building Large LAN Using Bridges

Bridges connect multiple IEEE 802 LANs at layer 2 Datagram packet switching Only forward packets to the right port Reduce collision domain

In contrast, hubs rebroadcast packets

host host host host host

host host host host host

host

host

Bridge

Transparent Bridges Overall design goal: Complete

transparency “Plug-and-play” Self-configuring without hardware or software

changes Bridges should not impact operation of existing LANs

Three parts to transparent bridges:(1) Forwarding of Frames(2) Learning of Addresses(3) Spanning Tree Algorithm

Frame Forwarding Each bridge maintains a forwarding

database with entries< MAC address, port, age>

MAC address: host address or group address

port: port number of bridge

age: aging time of entry

interpretation: a machine with MAC address lies in direction of the

port number from the bridge. The entry is age time units old

Assume a frame arrives on port x

Frame Forwarding 2

Bridge 2Port A Port C

Port x

Port B

Search if MAC address of destination is listed for ports A, B, or C

Forward the frame on theappropriate port

Flood the frame, i.e., send the frame on all

ports except port x

Found?

Notfound ?

In principle, the forwarding database could be set statically (=static routing)

In the 802.1 bridge, the process is made automatic with a simple heuristic:

The source field of a frame that arrives on a port tells which hosts are reachable from this port.

Address Learning

Bridge 2Port A Port C

Port x

Port BLAN 3

host n

Algorithm: For each frame received, stores the source

address in the forwarding database together with the port where the frame was received.

An entry is deleted after some time out (default is 15 seconds).

Address Learning 2

Bridge 2Port A Port C

Port x

Port BLAN 3

host n

Example

Bridge 2

Port1

LAN 1

A

LAN 2

CB D

LAN 3

E F

Port2

Bridge 2

Port1 Port2

•Consider the following packets: <Src=A, Dest=F>, <Src=C, Dest=A>, <Src=E, Dest=C>

•What have the bridges learned?XX YY

Consider the two LANs that are connected by two bridges.

Assume host n is transmitting a frame F with unknown destination.

What is happening? Bridges A and B flood the frame

to LAN 2. Bridge B sees F on LAN 2 (with

unknown destination), and copies the frame back to LAN 1

Bridge A does the same. The copying continuesWhere’s the problem? What’s

the solution ?

Danger of Loops

LAN 2

LAN 1

Bridge BBridge A

host n F

The solution to the loop problem is to not have loops in the topology

IEEE 802.1 has an algorithm that builds and maintains a spanning tree in a dynamic environment.

Bridges exchange messages (Configuration Bridge Protocol Data Unit (BPDU)) to configure the bridge to build the tree.

Spanning Trees

What’s a Spanning Tree?

A subset of edges of a graph that spans all the nodes without creating any cycle (i.e. a tree)

4

5

7

1 2

6

3

802.1 Spanning Tree Approach (Sketch)

Elect a bridge to be the root of the tree Every bridge finds shortest path to the

root Union of these paths become the

spanning tree

4

5

7

Root 2

6

3

What do the BPDU messages do?

With the help of the BPDUs, bridges can: Elect a single bridge as the root bridge. Calculate the distance of the shortest path to

the root bridge Each LAN can determine a designated bridge,

which is the bridge closest to the root. The designated bridge will forward packets towards the root bridge.

Each bridge can determine a root port, the port that gives the best path to the root.

Select ports to be included in the spanning tree.

Concepts Each bridge as a unique identifier:

Bridge ID = <MAC address + priority level>Note that a bridge has several MAC addresses

(one for each port), but only one ID

Each port within a bridge has a unique identifier (port ID).

Root Bridge: The bridge with the lowest identifier is the root of the spanning tree.

Path Cost: Cost of the least cost path to the root from the port of a transmitting bridge; Assume it is measured in # of hops to the root.

Root Port: Each bridge has a root port which identifies the next hop from a bridge to the root.

Concepts Root Path Cost: For each bridge, the cost of the

min-cost path to the root Designated Bridge, Designated Port: Single

bridge on a LAN that provides the minimal cost path to the root for this LAN:

- if two bridges have the same cost, select the one with highest priority (smallest bridge ID)

- if the min-cost bridge has two or more ports on the LAN, select the port with the lowest identifier

Note: We assume that “cost” of a path is the number of “hops”.

A Bridged Network

B3B3

B5B5

B2B2B7B7

B1B1

B4B4B6B6

Steps of Spanning Tree Algorithm

1. Determine the root bridge2. Determine the root port on all other

bridges3. Determine the designated bridge on

each LAN

Each bridge is sending out BPDUs that contain the following information:root bridge (what the sender thinks it is)

root path cost for sending bridgeIdentifies sending bridge

root IDroot ID costcost bridge ID/port IDbridge ID/port ID

Ordering of Messages We can order BPDU messages with the following

ordering relation ““ (let’s call it “lower cost”):

If (R1 < R2)M1 M2

elseif ((R1 == R2) and (C1 < C2)) M1 M2

elseif ((R1 == R2) and (C1 == C2) and (B1 < B2))M1 M2

elseM2 M1

ID R1ID R1 C1C1 ID B1 ID B1 ID R2ID R2 C2 C2 ID B2ID B2M1 M2

Initially, all bridges assume they are the root bridge.

Each bridge B sends BPDUs of this form on its LANs:

Each bridge looks at the BPDUs received on all its ports and its own transmitted BPDUs.

Root bridge is the smallest received root ID that has been received so far (Whenever a smaller ID arrives, the root is updated)

Determine the Root Bridge

BB 00 BB

At this time: A bridge B has a belief of who the root is, say R.

Bridge B determines the Root Path Cost (Cost) as follows:

If B = R : Cost = 0. If B ≠ R: Cost = {Smallest Cost in any of BPDUs that

were received from R} + 1

B’s root port is the port from which B received the lowest cost path to R (in terms of relation ““).

Knowing R and Cost, B can generate its BPDU (but will not necessarily send it out):

Calculate the Root Path CostDetermine the Root Port

RR CostCost BB

At this time: B has generated its BPDU

B will send this BPDU on one of its ports, say port x, only if its BPDU is lower (via relation ““) than any BPDU that B received from port x.

In this case, B also assumes that it is the designated bridge for the LAN to which the port connects.

Calculate the Root Path CostDetermine the Root Port

RR CostCost BB

Bridge BPort A Port C

Port x

Port B

Selecting the Ports for the Spanning Tree At this time: Bridge B has calculated the root,

the root path cost, and the designated bridge for each LAN.

Now B can decide which ports are in the spanning tree:

B’s root port is part of the spanning tree All ports for which B is the designated bridge are part

of the spanning tree. B’s ports that are in the spanning tree will

forward packets (=forwarding state) B’s ports that are not in the spanning tree will

not forward packets (=blocking state)

A Bridged Network (End of Spanning Tree Computation)

B3B3

B5B5

B2B2B7B7

B1B1

B4B4B6B6

x

x

x

Ethernet Switches Bridges make it possible to increase LAN

capacity. Packets are no longer broadcasted - they are only

forwarded on selected links Adds a switching flavor to the broadcast LAN

Ethernet switch is a special case of a bridge: each bridge port is connected to a single host.

Can make the link full duplex (really simple protocol!)

Simplifies the protocol and hardware used (only two stations on the link) – no longer full CSMA/CD

Can have different port speeds on the same switch Unlike in a hub, packets can be stored

Can the Internet be One Big Switched Ethernet?

Inefficient Too much flooding

Explosion of forwarding table

Need to have one entry for every Ethernet address in the world!

Poor performance Tree topology does

not have good load balancing properties

Hot spots Etc…

LAN 2

Bridge 2

LAN 5

LAN 3

LAN 1

LAN 4

Bridge 5

Bridge 4Bridge 3

d

Bridge 1