MAC Layer (Mis)behaviors

MAC Layer (Mis)behaviors

Christophe Augier - CSE802.11 Summer 2003

802.11 - MAC Based on CSMA like Ethernet Two different access methods

DCF – Distributed Coordination Function

PCF – Point Coordination Function

MAC – CSMA CSMA – Carrier Sense Multiple

Access

+ very effective under low loads

- possible collisions

CSMA/CACA: Collision Avoidance

Random back-off value

RTS / CTS

ACK scheme

Questions How MAC behaves with widely

used protocols?

Is the throughput fairly shared?

How MAC handles misbehaving nodes?

How… - Sender

Sense the medium

busy free

Transmit

CW=CWmin

repeat >= 7

CW= min(2 CW, Cwmax)

backoff counter = 0

Backoff value Selected from range [0, CW]

If medium is free for a time equal to DIFS – DCF InterFrame Space, backoff is counted down

If medium is busy, suspend backoff countdown

How… - Receiver

Check CRC

corruptedok

Send ACK

CRC: Cyclic Redundancy Check

How… - bothSender Receiver

RTS

CTS

data

data

ACK

Sense the medium

Transmit

MAC + TCP Why TCP?

Widely used – prevalent protocol used in Internet

Adaptability – network conditions, congestion control

Topology: ad hoc net, string of 7 nodes Assumptions:

No traffic – to get stable network and TCP throughput

Infinite flow – always data to send out

First test 1 sender, 1 receiver

2 3 4 51 6 7

TCP

First test results

Instability in the TCP

Connection lost

First test explication One node fails to reach its adjacent

node drops packets reports route failure

First test solution causes:

packet size – too big

number of packets sent back-to-back – too many

One node was capturing the medium

solution: decrease TCP window i.e. number of

packets sent back-to-back

Second test 2 TCP sessions started with a delay

of 20s

2 3 4 51 6 7

TCP TCP

interferences

Second test results The first session is forced down

Second test results causes:

Collisions between node 2 TCP packets and node 5 RTS packets

Route failure TCP session timeout

solutions: nullReducing the TCP window does not work

First conclusion

MAC is unchanged, the layer above MAC are changed to recover losses Link layer Transport layer

Change MAC - radical

MACs comparison Experiments to compare:

CSMA FAMA – RTS/CTS

802.11 – CSMA/CA, RTS/CTS + ACK

Under different multihop environments

Variable number of hops Var. TCP window sizes Var. number of hops

Variable number of hops Throughput is inversely proportional to

the hop distance

Max throughput with TCP Win= 1

Throughput: CSMA > FAMA > 802.11

802.11 stable for different values of W

Hidden terminal CSMA: unfair,

FAMA and 802.11: fair Thanks to CA mechanisms

Ring topology 802.11 not so fair compared to FAMA Increasing the DIFS period achieve

fairness

9x9 grid Good Throughput But capture

9x9 grid with mobility Evidence of captures effects But far better than CSMA and

FAMA

Conclusions 802.11 is promising

Good combination of throughput and fairness

Good behavior with mobility Need work:

To make TCP and MAC work well together

MAC layer timers - fairness

MAC layer misbehavior

Problem: No detection of misbehaviors A selfish node can get a better

throughput than well-behaved nodes

Solutions

Identify and punish misbehaving nodes: Avoid such nodes in routing Protocol penalizing misbehavior Game-theoretic techniques

Proposed solution Receiver selects the sender next

backoff value The sender have to use this assigned

backoff value

The receiver then can detect misbehavior

Detecting misbehavior Bact < * Bexp , 0 < a < 1

When a node is misbehaving? Deviation may come from:

Sender senses the medium as free, the receiver does not.

How to choose ?

Penalizing misbehaving nodes Select reasonably high Use a diagnosis scheme based on

a window W a threshold THRESH A node is misbehaving when

THRESHWpacket

actexp BB

Results

Issues misbehaving receiver Tradeoff between

Penalizing misbehaving nodes Ensuring the fairness of well-behaved

nodes

Conclusions 802.11 is promising (compared to previous MAC)

Good combination of throughput and fairness Good behavior with mobility

Need work: To make TCP and MAC work well together

Recover losses New protocol

MAC layer timers – fairness, avoid capture effects Mobility Safe against misbehaving nodes - fairness