1. Cyclex An Efficient Cheating-Resistant MAC Protocol Jim Pugh Mentors: Imad Aad, Mario Čagalj, Prof. Jean-Pierre Hubaux

2. Outline Problem with current protocol Cyclex protocol description How Cyclex solves current problem Pros and cons of Cyclex

3. IEEE 802.11 Cheating Problem Nodes can modify their protocol to allow themselves to gain throughput without incurring any penalty (cheat) Instead of choosing a randomized backoff time, nodes will choose some low number Cheating nodes send packets more often than non-cheating nodes Difficult to detect, due to random nature of 802.11

4. Our Solution: Cyclex Alternative to IEEE 802.11 for infrastructure mode Uses a turn-based approach to achieve time division multiple access (TDMA) cycle structure Cycle structure allows for easy cheating detection Operates on two different channels instead of one Uses Fast Collision Recovery to preserve order in cycle

5. System Model Infrastructure mode with 1 access point, many nodes Reliable communication channels Nodes may join or leave network at any time

6. Two Channel Approach Based on “A Jamming-Based MAC Protocol to Improve the Performance of Wireless Multihop Ad Hoc Networks” by Ye et al Upon hearing transmission from node on channel A, access point (AP) begins sending “busy tone” on channel B Eliminates hidden node problem

7. Two Channel Approach

8. Fast Collision Recovery Can detect collisions at nodes more quickly using two channel approach If error detected in packet header, AP will change from busy tone to “error tone” on channel B Transmitting nodes that hear error tone will know collision occurred immediately; can attempt to transmit again sooner than nodes in IEEE 802.11

9. Fast Collision Recovery

10. Maintaining Order with Fast Collision Recovery In IEEE 802.11, colliding nodes may have long delay before next transmission With Fast Collision Recovery, colliding nodes can transmit soon after collision; all other nodes wait longer If the maximum random backoff is short enough, we can guarantee that colliding nodes will transmit before any other nodes Preserves order, which is important for turn- based algorithm

11. Full Protocol Description Nodes take turn transmitting; specific order Transmissions form a “cycle” Acknowledgements from access point include node identity; possible for all nodes to track who transmits

12. Cyclex Operation

13. Joining the Cycle To join, node must either wait X consecutive slot counts of channel idle time or hear a complete “cycle” If no nodes present, after X slot counts, node will start a cycle If nodes present, after hearing cycle (same node transmit twice), node will insert itself into cycle by preempting next node transmission

14. Joining the Cycle

15. Leaving the Cycle If node misses transmission, may have no data or may have left network If node misses transmission, next node waits one slot count, then transmits; if two nodes in a row miss transmission, next node waits two slot counts; etc If node misses Y transmissions in a row, it is removed from cycle

16. Leaving the Cycle

17. Collisions Nodes in cycle don’t collide since ordered Joining nodes don’t collide with nodes in cycle since preempting is faster Only collisions occur from nodes joining the cycle simultaneously Worst case analysis: 0.15% loss of time In case of glitch, fast collision recovery allows order of network to be maintained

18. Cyclex Cheating Detection Cheating difficult to detect in IEEE 802.11 due to random nature Cyclex is ordered; easy cheating detection Active nodes listen to who transmits between their transmissions; if some node transmits twice, it is cheating Once cheating detected, measures can be taken to punish node

19. Cheating Detection

20. Pros of Cyclex Solves hidden node problem; less collisions occur, and time saved from not using RTS and CTS Cheating-resistant Very little lost transmission time in stable network operation

21. Cons of Cyclex Two channels needed instead of one Busy tone means more power used by access point Current design only works in infrastructure mode

22. Possible Modifications of Protocol Consecutive slot counts required before joining network Missed transmissions before node removal from cycle Adapt protocol for use in ad hoc networks

23. References M. Raya, J. P. Hubaux, and I. Aad, “DOMINO: A System to Detect Greedy Behavior in IEEE 802.11 Hotspots.” M. Cagalj, S. Ganeriwal, I. Aad, and J. P. Hubaux, “On Cheating in CSMA/CA Ad Hoc Networks.” C. E. Koksal, H. Kassab, and H. Balakrishnan, “An Analysis of Short-Term Fairness in Wireless Media Access Protocols,” in Proc. ACM SIGMETRICS’00, Santa Clara, CA, June 2000. J. L. Sobrinho and A. S. Krishnakumar, “Quality-of-Service in Ad Hoc Carrier Sense Multiple Access Wireless Networks,” IEEE Journal on Selected Areas in Communications, Vol. 17, No. 8, August 1999. S. R. Ye, Y. C. Wang, and Y. C. Tseng, “A Jamming-Based MAC Protocol to Improve the Performance of Wireless Multihop Ad Hoc Networks.”