DukeSelfish MAC Layer Misbehavior in Wireless
Networks
Author: Pradeep Kyasanur and Nitin H. VaidyaSome slides are borrowed from the author and
othersReviewed by Xuan Bao
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Index
Problem Definition1
Proposed Methods2
Modification3
Simulation Result4
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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
Selfish Host
Backoff = rand[1,1]
Normal Host
Backoff = rand[0,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 sender’s 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
Problem Definition1
Proposed Methods2
Methods3
Simulation Results4
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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
DA
T
A
Sender S
Receiver R
CTS
AC
K(B
)
RTS
• R provides backoff B to S in ACK
• S uses B for backoff
• NewBackoff = f(backoff, ID, attempt)*CW
RTS
B
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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 (a*Bexp – 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
Problem Definition1
Proposed Methods2
Modifications3
Simulation Result4
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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
D
Y
S
M
KX
S reduces its backoff counter now, which will be considered as misbehavior later.
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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
Problem Definition1
Proposed Methods2
Modifications3
Simulation Results4
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Throughput – no misbehavior
0
100
200
300
400
500
600
700
800
900
1000
1 2 4 8 16 32 64
Proposed Scheme
802.11
Number of sender nodes
Thr
ough
put
(Kbp
s \
node
)
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Persistent Misbehavior -Diagnosis Accuracy
0
10
20
30
40
50
60
70
80
90
100
100959080706050403020100
Correct Diagnosis
Misdiagnosis
Percentage of Misbehavior (of misbehaving node)
Per
cent
age
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Persistent Misbehavior- throughput
0
100
200
300
400
500
600
700
800
900
100959080706050403020100
802.11
Proposed Scheme
Percentage of Misbehavior
Thr
ough
put
(Kbp
s)
Avg. with proposed scheme
Avg. with 802.11