Selfish MAC Layer Misbehavior in Wireless Networks

Pradeep Kyasanur and Nitin H. Vaidya

2005 IEEE

Reviewed by Dean Chiang

How is network contention resolved?

Distributed Contention Function (DCF) Relies on protocol adherence from all of the

network.

I’m transmitting

guys!

Ok everyone better wait the same amount!

You got it!

…What if I don’t want

to…?

Problem Definition

Selfish Host

Backoff = rand[1,1]

Normal Host

Backoff = rand[0,CW]

Normal Channel Usage

Channel Usage with a Selfish Host

t

t

Problem Challenges

Difference between a small backoff and differing usage by each node?

Node channel conditions are different. A node can claim an idle channel.

Solution

Modified MAC ProtocolGoal: retain performance of 802.11 while

ensuring detection of misbehavior in a short interval

Assumptions: Misbehaving senders only. Receivers are

assumed to be trusted. No collusion between sender and receiver

Protocol Modifications - Backoff

Receiver assigns random backoff and sends it in the CTS and ACK. Receiver is in control.

Protocol Modifications - Retransmission

Sender adds attempt number to RTS

Receiver estimates expected backoff from attempt number.

Penalty Scheme

Penalize if: B_actual < Alpha * B_expected (Alpha = 0.8)

Penalty Add to next backoff by an amount proportional to deviation and

then some.

Still not adhering? Identify as a selfish host. (Last W packets that deviated over total threshold T slots)

Receiver is in control. Misbehavior with backoff cannot be spoofed by senders.

What happens after identification?

Possibilities

MAC LayerReceiver exiles sender by not responding

to RTS.Network LayerNetwork routes around host.

Misdiagnosis

Receiver senses a busy channel from 2 hops away when sender does not.

Sender sends to receiver, receiver thinks sender did not adhere to assigned backoff.

Solution: receiver does not count a busy channel for transmission, only during overheard RTS/CTS.

Results: Diagnostic Accuracy

Notes:

•Low misdiagnosis rate

•Does not fully resolve selfish behavior problem. Only alleviates high end of deviation.

•Notice there is a threshold of misbehavior that can go undetected.

Results: Throughput

Throughput comparison to 802.11

•Adheres closely to throughput of 802.11

Results: Fairness

Fairness Index comparison to 802.11

•Fairness decreases with # of nodes

•Fairness deviates more when background traffic is high (TWO-FLOW)

Results: What about crafty clients?

Correct Diagnosis %

Protocol is worse at detecting adaptive misbehavior

Throughput gain is not large because of penalty scheme

HOWEVER

Pros and Cons

Pros Fast diagnosis, no need to collect tons of data Low Misdiagnosis rate Retains fairness and throughput of 802.11 Reduces workaround potential

Cons Additional overhead Backwards compatibility Spoofed MAC addresses

Addendum: Optimal Threshold

Authors show a proof for an optimal threshold

Threshold T is a function of assigned back-off value and # of slots.

However, authors chose a static threshold in simulations. This suggests the data could be better.

Discussion/Consideration

What does the police do?

Why did the government create a police force?

At what intrinsic layer does the government’s control end?

http://www.thunderroadlaser.com/images/thunderimages/05_Police%20Badge%20-%2072dpi.jpg

Discussion/Consideration

Is this really the best place to handle selfish behavior?

PC, DriverWireless Network Adapter

Regulated by IEEE

Regulated by IEEE

Not Regulated by IEEE

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Base Station

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