Selfish MAC Layer Misbehavior in Wireless Networks

1
Selfish MAC Layer Misbehavior in Wireless Networks

• Pradeep Kyasanur and Nitin H. Vaidya
• 2005 IEEE
• Reviewed by Dean Chiang

2
How is network contention resolved?

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

Ok everyone better wait the same amount!
Im transmitting guys!
What if I dont want to?
You got it!
3
Problem Definition
Normal Host Backoff rand0,CW
t
Normal Channel Usage
t
Channel Usage with a Selfish Host
4
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.

5
Solution

• Modified MAC Protocol
• Goal 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

6
Protocol Modifications - Backoff

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

7
Protocol Modifications - Retransmission

• Sender adds attempt number to RTS
• Receiver estimates expected backoff from attempt
  number.

8
Penalty Scheme

• Penalize if
• B_actual lt 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.

9
What happens after identification?

• Possibilities
• MAC Layer
• Receiver exiles sender by not responding to RTS.
• Network Layer
• Network routes around host.

10
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.

11
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.

12
Results Throughput

• Throughput comparison to 802.11
• Adheres closely to throughput of 802.11

13
Results Fairness

• Fairness Index comparison to 802.11
• Fairness decreases with of nodes
• Fairness deviates more when background traffic is
  high (TWO-FLOW)

14
Results What about crafty clients?
Correct Diagnosis Protocol is worse at
detecting adaptive misbehavior
HOWEVER
Throughput gain is not large because of penalty
scheme
15
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

16
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.

17
Discussion/Consideration

• What does the police do?
• Why did the government create a police force?
• At what intrinsic layer does the governments
  control end?

http//www.thunderroadlaser.com/images/thunderimag
es/05_Police20Badge20-2072dpi.jpg
18
Discussion/Consideration

• Is this really the best place to handle selfish
  behavior?

Base Station
PC, Driver
Wireless Network Adapter
Not Regulated by IEEE
Regulated by IEEE
Regulated by IEEE
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