Internet ProtocolsFall 2006

Lectures 5 and 6MAC Layer

Andreas Terzis

Outline

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• MAC Protocols – MAC Protocol Examples

• Channel Partitioning– TDMA/FDMA– Token Ring

• Random Access Protocols– Aloha and Slotted Aloha– CSMA(/CD)– CSMA/CA

• Connecting Layer 2 networks• Interface to layer 3

MAC Protocols

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• Problem: How to share medium among multiple senders

• Different Solutions– Channel Partitioning (FDMA, TDMA,CDMA)– Taking Turns (Token Ring)– Random Access (Aloha, CSMA, CSMA/CD,

CSMA/CA)• Parameters

– Physical medium characteristics– End-point capabilities

Goals of MAC Protocols

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• MAC Protocols arbitrate access to a common shared channel among a population of users

• Goals– Fairness– High efficiency– Low delay– Fault tolerance– Low overhead– Low complexity– …

ALOHA

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• Packet radio network created by the University of Hawaii in the 70’s– Star topology– Two channels: Broadcast channel, Random Access

channel• Basic Idea:

– Let a node transmit when it has data to send– If frame was destroyed then retransmit after a

random period of time• Feedback about packet reception status is sent over the

broadcast channel

ALOHA Efficiency

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• Assumptions– Infinite number of users– Frame arrivals are modeled by a Poisson process

with rate λ– Retransmissions can also be modeled by a Poisson

process with rate G > λ– Throughput S=GP0

• P0 = probability frame does not suffer a collision

ALOHA: Collision Probability

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t0 t0+t t0+2t t0+3t

Vulnerable Interval

• Frame tx time:t• Vulnerable interval:2t• Prob[k arrivals]:

• P0=e-2G

• S=Ge-2G

• Max throughput is 0.184 when G=0.5

!2]Pr[

2

keGk

Gk −

=

Slotted ALOHA

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• Time is divided into slots equal to the frame transmission time– Central station sends clock tick signal– Satellite stations are allowed to send only

immediately after hearing clock tick• Vulnerable time is equal to t• Throughput S=Ge-G (max =0.37, G=1)• Expected number of transmissions:

GG

k

kGG

kk eeekekPE ==−== −

∞

=

−−−∞

=∑∑ 1)1(

1

1

1

Comparison

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CSMA (Carrier Sense Multiple Access)

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

• Human analogy: don’t interrupt others!

CSMA collisions

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

collision:entire packet transmission time wasted

spatial layout of nodes

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

CSMA Variants

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• Persistent CSMA: Backlogged nodes will immediately transmit after channel becomes idle– Higher probability of collisions at high loads

• P-Persistent CSMA: After channel is idle transmit with probability p– Higher delay at low loads

• Non-persistent CSMA: Transmit with probability p even when the channel is idle

Performance

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CSMA/CD (Collision Detection)

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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

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Ethernet uses CSMA/CD

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• Adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense

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

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

Ethernet CSMA/CD algorithm

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1. Adapter gets datagram from upper layer and creates frame

2. If adapter senses channel idle, it starts to transmit frame. If it senses 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 !

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

5. After aborting, adapter enters exponential backoff: after the m-thcollision, 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)

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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:• Goal: adapt retransmission

attempts to estimated current load– heavy load: random wait

will be longer• first collision: choose K from

{0,1}; delay is K x 512 bit transmission times

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

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

CSMA/CD efficiency

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Notation:• L : link length• B : link speed• c : speed of

light• F : frame size• τ : slot length

in bits

• Assume: k stations, prob transmit = p

cFBLeS

ceBLFF

eFF

AFFS

A

Aw

AA

keAkp

pkpA

j

k

211

222

2 interval contentionmean

1)1(=slots] j has interval contentionPr[

as 1then 1)1(=channel] acquires nodePr[

1

1

+=

+=

+≈

+=

=

=

−

∞→→=

−=

−

−

ττ

τ

Efficiency as a function of τ

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Ethernet “Classic”

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• 10Base2, 10Base5• 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!• Has become a legacy technology

Newer Ethernet Variants: 10BaseT and 100BaseT

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• 10/100 Mbps rate; latter called “fast ethernet”• T stands for Twisted Pair• Nodes connect to a hub: “star topology”; 100 m max

distance between nodes and hub• Hubs are essentially physical-layer repeaters:

– bits coming in one link go out all other links– no frame buffering– no CSMA/CD at hub: adapters detect collisions– provides net management functionality

hub

nodes

Gbit Ethernet

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• 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• Uses hubs, called here “Buffered Distributors”• Full-Duplex at 1 Gbps for point-to-point links• 10 Gbps now• Q: Who needs all this bandwidth?

Token Ring Overview

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• Examples– 16Mbps IEEE 802.5 (based on earlier IBM ring)– 100Mbps Fiber Distributed Data Interface (FDDI)– Resilient Packet Ring MAN (802.17)

Token Ring (cont)

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• Idea– Frames flow in one direction: upstream to downstream – special bit pattern (token) rotates around ring– must capture token before transmitting– release token after done transmitting

• immediate release• delayed release

– remove your frame when it comes back around– stations get round-robin service

Host

Host

Host

Host

From previousMSAU

To nextMSAU

MSAU

Host

From previoushost

To nexthost

Relay(a)

Host

Host Host

From previoushost

To nexthost

Relay(b)

Timed Token Algorithm

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• Token Holding Time (THT)– Upper limit on how long a station can hold

the token

• Token Rotation Time (TRT)– Upper limit on how long it takes the token

to traverse the ring– TRT <= ActiveNodes x THT + RingLatency

Additional Features

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• Successful delivery notification– Frame returning to sending host contains ACK

• Different levels of service– Token contains priority field (3-bit)– Only frames with at least as high priority can be

transmitted– Priority field is adjusted through reservation

mechanism

Body ChecksumSrcaddr

Variable48Destaddr

48 32Enddelimiter

8Framestatus

8Framecontrol

8Accesscontrol

8Startdelimiter

8

Token Maintenance

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• Lost Token– No token when initializing ring– Bit error corrupts token pattern– Node holding token crashes

• Generating a Token (and agreeing on monitor)– Execute when join ring or suspect a failure– Send a claim frame that includes the node’s MAC address– When receive claim frame forward if local MAC address

smaller– If your claim frame makes it all the way around the ring:

• Your are the ring monitor• You insert new token

Maintenance (cont)

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• Monitor duties– Regenerate token if current one is destroyed– Remove corrupted or orphaned frames

When to send token?

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Token

Frame

Token Frame

(a) (b)

Early Release Late Release

Relative advantages and drawbacks?

FDDI

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• Physical Properties– 100 Mbps, commonly uses fiber (although CDDI exists)– Two independent rings that transmit data in opposite directions– Second ring is used only when primary ring fails

• Tolerate failure of a stations or single cable break– 500 hosts max, 2 km between any pair of hosts, 100 km total

network size

(a) (b)

Wireless LANs

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• IEEE 802.11• Bandwidth: 1 - 54 Mbps• Physical Media

– spread spectrum radio (2.4GHz)– diffused infrared (10m)

Spread Spectrum

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• Idea– spread signal over wider frequency band than

required– originally designed to thwart jamming

• Frequency Hopping– transmit over random sequence of frequencies– sender and receiver share…

• pseudorandom number generator• seed

– 802.11 uses 79 x 1MHz-wide frequency bands

Spread Spectrum (cont)

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• Direct Sequence– for each bit, send XOR of that bit and n random bits– random sequence known to both sender and receiver – called n-bit chipping code – 802.11 defines an 11-bit chipping code

Random sequence: 0100101101011001

Data stream: 1010

XOR of the two: 1011101110101001

0

0

0

1

1

1

Collisions Avoidance

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• Similar to Ethernet• Problem: hidden and exposed nodes

A B C D

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MACAW• Sender transmits RequestToSend (RTS) frame• Receiver replies with ClearToSend (CTS) frame• Neighbors…

– see CTS: keep quiet– see RTS but not CTS: ok to transmit

• Receive sends ACK when has frame– neighbors silent until see ACK

• Collisions– no collisions detection– known when don’t receive CTS– exponential backoff

Interconnecting LANs

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• Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router.

• By partitioning a large LAN into multiple smaller networks, there are fewer collisions, and more parallel communications.

• It is now common for the port of an Ethernet switch to connect to just one (or a small number of) hosts.

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An Ethernet Network

RouterRouter OutsideworldProblem:

Shared network limits throughput.Lots of collisions reduces efficiency.

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Ethernet Switching

RouterRouter Outsideworld

Benefits: Number of collisions is reduced. If only one computer per port,no collisions can take place (each cable is now a self-contained point-to-point Ethernet link).Capacity is increased: the switch can forward multiple frames to different computers at the same time.

EthernetSwitch

One Ethernet Switch

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A typical LAN (IP network)

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Shared

Ethernet Switching

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1. Examines the header of each arriving frame.2. If the Ethernet DA is in its table, it forwards the

frame to the correct output port(s).3. If the Ethernet DA is not in its table, it broadcasts

the frame to all ports (except the one through which it arrived).

4. The table is learned by examining the Ethernet SA of arriving packets.

Ethernet SwitchingLearning addresses

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RouterRouter

OutsideworldEthernet

SwitchEthernetSwitch

EthernetSwitch

EthernetSwitch

A B C D E F G H

I J K L M N O P

Q1

2 345

6

J F

Switch learns that ‘F’ is reachable through port 5

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Ethernet SwitchingLearning addresses

RouterRouter Outsideworld

EthernetSwitch

EthernetSwitch

EthernetSwitch

EthernetSwitch

A B C D E F G H

I J K L M N O P

Q1

2 345

6

Ethernet address Port

A 1

B 2

C 3

D 4

E, F, G, H, Q 5

I, J, K, L, M, N, O, P 6

Q: How do we prevent loops?

LAN Addresses and ARP

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32-bit IP address:• network-layer address• used to get datagram to destination IP network (recall IP

network definition)LAN (or MAC or physical or Ethernet) address: • used to get datagram from one interface to another

physically-connected interface (same network)• 48 bit MAC address (for most LANs)

“burned” in the adapter EPROM

LAN Address (more)

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• MAC address allocation administered by IEEE• manufacturer buys portion of MAC address space (to assure

uniqueness)• Analogy:

(a) MAC address: like Social Security Number(b) IP address: like postal address

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

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

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)

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

ARP protocol

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

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

• A broadcasts ARP query packet, containing B's IP address – all machines on LAN receive

ARP query• B receives ARP packet, replies

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

(unicast)

• A caches (saves) 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 Protocol (II)

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• Proxy ARP– Reply on behalf of another node

• Gratuitous ARP– Node sends ARP asking for its own IP

address• Find out if address has been claimed• Change the mapping between MAC<->IP addr

• Do you see any problem with this?