Selfish MAC Layer Misbehavior in Wireless Networks

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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

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Index
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Review of Details about DCF
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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.

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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.

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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

Normal Host Backoff rand0,CW
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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.

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Conclusion on Definition of Misbehavior in This
Paper

• In this paper, selfish misbehavior only consider
  the senders behavior intending to obtain unfair
  share of channel access.
• Does not consider high layer solutions.
• Does not consider malicious attacks.

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Index
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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.

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Illustration Graph

• R provides backoff B to S in ACK
• S uses B for backoff
• NewBackoff f(backoff, ID, attempt)CW

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Penalty Scheme

• Penalty is introduced in two time scale per
  transmission and for the last w transmissions.

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Penalty Scheme

• For each transmission, a penalty proportional to
  the deviation D is added to the next backoff
  counter.
• D max (aBexp Bact, 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.

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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.

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Index
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Problems and Consequence

• Problem
• Receiver may sense the channel under a different
  status than the sender.
• Consequence
• Unjustified penalty to well behaved nodes.

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Scenario

• Classic Hidden Terminal Scenario

S reduces its backoff counter now, which will be
considered as misbehavior later.
M
Y
S
D
X
K
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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

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Index
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Throughput no misbehavior
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Persistent Misbehavior -Diagnosis Accuracy
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Persistent Misbehavior- throughput
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Thank You !

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