1. Selfish MAC Layer Misbehavior in Wireless Networks ECE 299, Karthik Balasubramanian February 8, 2007 Nitin Vaidya and Pradeep Kyasanur

2. 2 The problem: a game theory approach CheatDon’t Cheat Cheat 1, 1 3, 0 Don’t cheat 0, 3 2, 2 Node B Node A Need incentive structure to prevent cheating

3. 3 Review: contention schemes Point Contention Function (PCF) Centralized controller required No ad-hoc networks 802.11 optional feature Distributed Contention Function (DCF) Suitable for ad-hoc networks Suitable for fluctuating number of nodes and hosts Uses Carrier Sense Multiple Access/Collision Avoidance What paper focuses on

4. 4 Review: collision avoidance and backoff Randomly choose backoff value B in range [0,CW] -CW is the Contention Window Count down backoff by 1 every idle slot wait Transmit wait B2=10 B1=20 B2=10 B1=0 S1 S2 CW=31 B1=15 B2=25 Source: Pradeep Kyasanur and Nitin Vaidya, UIUC

5. 5 Cheating Transmit wait B1 = 1 B2 = 20 Transmit wait B2 = 19 B1 = 1 Misbehaving node Well-behaved node Source: Pradeep Kyasanur and Nitin Vaidya, UIUC What if I always choose B=1? Good for me, bad for you

6. 6 Goal 1: detect cheating Method: Sender backoff data collection -Measure actual backoff durations of individual nodes -If backoff from skewed distribution, cheating suspected Method weaknesses -Cheating cannot be immediately diagnosed -High chance for misdiagnosis if small sample size -Requires significant computation Current 802.11 not sufficient for good detection

7. 7 802.11 modification: B selected by receiver 1. R provides backoff B to S in ACK B selected from [0,CW min ] DATA Sender S Receiver R CTS ACK(B) RTS 2. S uses B for backoff RTS B Source: Pradeep Kyasanur and Nitin Vaidya, UIUC

8. 8 Detection Receiver counts number of idle slots Bobsr Condition for detecting deviations: B obsr <  B 0 <  <= 1 Sender S Receiver R ACK(B) RTS Backoff B obsr Source: Pradeep Kyasanur and Nitin Vaidya, UIUC

9. 9 Goal 2: punish cheating Punishment -Must be effective deterrent -Must not hinder overall throughput If Σ( deviations) > threshold, cheater identified -Pass on to higher layers: “telling Daddy on you” -Appropriate action can be taken

10. 10 Punishment  When B obsr <  B, penalty P added  P proportional to  B– B obsr ACK(B+P) CTS DATA  Total backoff assigned = B + P B obsr Sender S Receiver R ACK(B) RTS Actual backoff < B Source: Pradeep Kyasanur and Nitin Vaidya, UIUC

11. 11 Simulation Setup Misbehaving Node Source: Pradeep Kyasanur and Nitin Vaidya, UIUC

12. 12 Diagnosis Correct Diagnosis Misdiagnosis Percentage of Misbehavior (of misbehaving node) Percentage Source: Pradeep Kyasanur and Nitin Vaidya, UIUC

13. 13 Throughput 802.11 Proposed Scheme Percentage of Misbehavior Throughput (Kbps per node) Avg. with penalty Avg. with 802.11 Source: Pradeep Kyasanur and Nitin Vaidya, UIUC

14. 14 Throughput with no misbehavior Proposed Scheme 802.11 Number of sender nodes Throughput (Kbps per node) Source: Pradeep Kyasanur and Nitin Vaidya, UIUC

15. 15 Discussion/Limitations Requires 802.11 modification for efficient detection Breakdown at high cheating levels No enforcement mechanism presented Questions?