Duke Selfish MAC Layer Misbehavior in Wireless Networks Author: Pradeep Kyasanur and Nitin H. Vaidya Some slides are borrowed from the author and others Reviewed by Xuan Bao

Duke Company Logo Index Problem Definition 1 Proposed Methods 2 Modification 3 Simulation Result 4

Duke Company Logo Review of Details about DCF

Duke Company Logo Calculation of Contention Window (CW)  A host with data to transmit selects a random backoff counter from range [0, CW]  The backoff counter will decrease by one when the channel is idle for one time slot.  A host may access the channel when the counter reaches 0.  If a transmission is successful, CW is reset to the minimum value. Otherwise, CW is doubled, subject to the maximum value.

Duke Company Logo Opportunities of Misbehavior Using analogy, the contention process is like throwing a dice and the competitor with the least points gets channel access. Fairness is achieved because the loser in earlier contention will decrease their backoff counter and therefore has better chance to win the contention later.

Duke Company Logo Opportunities of Misbehavior A selfish host can choose a small backoff counter by: 1 Choosing the backoff counter from a smaller region than [0, CW] 2 Do not increase CW after collision Selfish Host Backoff = rand[1,1] Normal Host Backoff = rand[0,CW]

Duke Company Logo The Reason Behind Misbehavior Opportunities  Senders, while participating in contention, maintain the CW value by themselves. Therefore, they are in fact both the regulator and participator in this game.

Duke Company Logo Conclusion on Definition of Misbehavior in This Paper  In this paper, selfish misbehavior only consider the sender’s behavior intending to obtain unfair share of channel access.  Does not consider high layer solutions.  Does not consider malicious attacks.

Duke Company Logo Index Problem Definition 1 Proposed Methods 2 Methods 3 Simulation Results 4

Duke Company Logo Basic Frame  The receiver calculates and assign backoff counter to senders.  The receiver monitors the time interval between ACK and the next RTS. It identifies misbehavior based on the deviation between this interval and the assigned backoff counter.

Duke Company Logo Illustration Graph DATA Sender S Receiver R CTS ACK(B) RTS R provides backoff B to S in ACK S uses B for backoff NewBackoff = f(backoff, ID, attempt)*CW RTS B

Duke Company Logo Penalty Scheme  Penalty is introduced in two time scale: per transmission and for the last w transmissions.

Duke Company Logo Penalty Scheme  For each transmission, a penalty proportional to the deviation D is added to the next backoff counter.  D = max (a*B exp – B act, 0) (misdiagnose)  For nodes keeps deviating more than a threshold T for the last W transmissions.  Further punishment such as refusal of accepting further packets.

Duke Company Logo Tradeoff Here  Choice of parameters as factor a, threshold T.  Misdiagnose may lead to unwanted performance degrading especially when refuse accepting further transmission or drop packets.

Duke Company Logo Index Problem Definition 1 Proposed Methods 2 Modifications 3 Simulation Result 4

Duke Company Logo Problems and Consequence  Problem:  Receiver may sense the channel under a different status than the sender.  Consequence:  Unjustified penalty to well behaved nodes.

Duke Company Logo Scenario  Classic Hidden Terminal Scenario D Y S M K X S reduces its backoff counter now, which will be considere d as misbehavi or later.

Duke Company Logo Solution  The receiver only classifies a slot to be busy when an overheard RTS/CTS has reserved the slot or a packet is being received.  Assumptions behind this: Deep carrier sensing. Carrier sensing range = 2 * Communication range

Duke Company Logo Index Problem Definition 1 Proposed Methods 2 Modifications 3 Simulation Results 4

Duke Throughput – no misbehavior Proposed Scheme Number of sender nodes Throughput (Kbps \ node)

Duke Persistent Misbehavior -Diagnosis Accuracy Correct Diagnosis Misdiagnosis Percentage of Misbehavior (of misbehaving node) Percentage

Duke Persistent Misbehavior- throughput Proposed Scheme Percentage of Misbehavior Throughput (Kbps) Avg. with proposed scheme Avg. with

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