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! I’m transmitting guys! …What if I don’t want to…? You got it!

3. Selfish Host Backoff = rand[1,1] Problem Definition Normal Host Backoff = rand[0,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 < 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 government’s control end? http://www.thunderroadlaser.com/images/thunderimages/05_Police%20Badge%20-%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 http://images.belkin.com/F6D3000/PRN1_F6D3000.jpg http://store.madtux.org/images/PC319a.jpg http://compuplus.com.mx/tienda/images/WAP54G.jpg