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Detection and Handling of MAC Layer Misbehavior

Detection and Handling of MAC Layer Misbehavior. Pradeep Kyasanur Nitin Vaidya University of Illinois at Urbana-Champaign. 802.11. Co-operative contention resolution using backoff intervals Greedy nodes can violate the protocol. Backoff Interval .

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Detection and Handling of MAC Layer Misbehavior

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  1. Detection and Handling of MAC Layer Misbehavior Pradeep Kyasanur Nitin Vaidya University of Illinois at Urbana-Champaign

  2. 802.11 • Co-operative contention resolution using backoff intervals • Greedy nodes can violate the protocol

  3. Backoff Interval • When transmitting a packet, choose a backoff interval in the range [0,cw] • cw is contention window • Count down the backoff interval when medium is idle • Count-down is suspended if medium becomes busy • When backoff interval reaches 0, transmit

  4. B1 = 25 B1 = 5 wait data data wait B2 = 10 B2 = 20 B2 = 15 DCF Example B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31

  5. Possible Misbehavior • Misbehavior in selecting back-off • Pick a smaller backoff as compared uniformly choosing from [0,cw] • Misbehavior after collision • Contention window not doubled on collision

  6. Detecting Misbehavior • Detection as an add-on to existing protocol • Verifiable protocols (U.Washington)

  7. Add-on to 802.11 • Host getting more than fair share may be misbehaving • How to determine fair share • Short-term behavior of 802.11 unfair • Observations must be over long intervals of time for greater reliability • Long latency

  8. Verifiable Approach • Short-term behavior in 802.11 not verifiable with high probability • Decrease randomness to increase verifiability

  9. Verifiable Approach • Per-receiver scheme • Focus on one receiver R and the senders S transmitting to this receiver • Receiver R dictates the backoff interval used by each sender • Backoff interval for next packet piggybacked on CTS or ACK for last packet

  10. RTS header Protocol Example • Receiver R RTS A R B CTS RTS packet: Attempt Number CTS packet: CTS header Next back-off

  11. Basic Idea • Having dictated the number of backoff slots, receiver R can determine misbehavior by observing deviation from the dictated value • Works best on wired networs where all hosts observe identical channel conditions

  12. Wireless Channel • Different hosts observe different channel conditions • Host R may see channel as busy, and B may see it as idle • R may incorrectly conclude that B is misbehaving A R B

  13. Proposed Scheme • Receiver R expects all senders to have R's view of the channel • Deviations from R's view (modulo some tolerance) considered to be misbehavior

  14. Devil is in Details • Deterministic function f used for retransmissions • Back-off value for retransmissions computed as • R computes expected back-off value as

  15. Details ... • Actual idle interval between transmissions is measured • actualTime = I1+I2+I3 • Deviation from protocol is signaled if • More than K deviations in last THRESH packets from the node indicates misbehavior

  16. Correcting Misbehavior • Backoff intervals assigned to misbehaving hosts larger • Compensation • To benefit despite compensation the misbehaving host must misbehave more radically • Making detection much easier

  17. Preliminary Simulations • Misbehavior is modeled as Percentage of Conformance (PC) • Misbehaving nodes always back-off for PC times the assigned back-off value • Single receiver with all other hosts within transmission range

  18. Metrics • Correct prediction: percentage of packets sent by misbehaving nodes that are correctly identified • Misprediction: percentage of packets sent by conforming nodes falsely identified as from misbehaving nodes • Average throughput over all flows • Misbehaving host's throughput

  19. Prediction Accuracy

  20. Throughput

  21. Misprediction Percentage

  22. Misprediction Percentage

  23. Prediction Accuracy

  24. Observations • Prediction accuracy is high • Can accurately detect misbehavior for nodes with small amounts of misbehavior also • Prediction accuracy can be traded with misprediction percentage by modifying protocol parameters • The correction scheme effectively controls the bandwidth obtained by greedy nodes as long as percentage of conformance is non-zero

  25. On-going Work • Allowing dynamic selection of protocol parameters for adaptively handling variations in channel conditions • Extending the mechanisms to multiple receivers • Applying proposed approach to improve fairness

  26. Caveats • This is work-in-progress • Protocols subject to change • Other mechanisms to detect receiver misbehavior, or sender-receiver pair misbehavior also need to be developed

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