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Transcript of Selfish MAC Layer Misbehavior in Wireless Networks Pradeep Kyasanur and Nitin H. Vaidya 2005 IEEE...
Selfish MAC Layer Misbehavior in Wireless Networks
Pradeep Kyasanur and Nitin H. Vaidya
2005 IEEE
Reviewed by Dean Chiang
How is network contention resolved?
Distributed Contention Function (DCF) Relies on protocol adherence from all of the
network.
I’m transmitting
guys!
Ok everyone better wait the same amount!
You got it!
…What if I don’t want
to…?
Problem Definition
Selfish Host
Backoff = rand[1,1]
Normal Host
Backoff = rand[0,CW]
Normal Channel Usage
Channel Usage with a Selfish Host
t
t
Problem Challenges
Difference between a small backoff and differing usage by each node?
Node channel conditions are different. A node can claim an idle channel.
Solution
Modified MAC ProtocolGoal: retain performance of 802.11 while
ensuring detection of misbehavior in a short interval
Assumptions: Misbehaving senders only. Receivers are
assumed to be trusted. No collusion between sender and receiver
Protocol Modifications - Backoff
Receiver assigns random backoff and sends it in the CTS and ACK. Receiver is in control.
Protocol Modifications - Retransmission
Sender adds attempt number to RTS
Receiver estimates expected backoff from attempt number.
Penalty Scheme
Penalize if: B_actual < Alpha * B_expected (Alpha = 0.8)
Penalty Add to next backoff by an amount proportional to deviation and
then some.
Still not adhering? Identify as a selfish host. (Last W packets that deviated over total threshold T slots)
Receiver is in control. Misbehavior with backoff cannot be spoofed by senders.
What happens after identification?
Possibilities
MAC LayerReceiver exiles sender by not responding
to RTS.Network LayerNetwork routes around host.
Misdiagnosis
Receiver senses a busy channel from 2 hops away when sender does not.
Sender sends to receiver, receiver thinks sender did not adhere to assigned backoff.
Solution: receiver does not count a busy channel for transmission, only during overheard RTS/CTS.
Results: Diagnostic Accuracy
Notes:
•Low misdiagnosis rate
•Does not fully resolve selfish behavior problem. Only alleviates high end of deviation.
•Notice there is a threshold of misbehavior that can go undetected.
Results: Fairness
Fairness Index comparison to 802.11
•Fairness decreases with # of nodes
•Fairness deviates more when background traffic is high (TWO-FLOW)
Results: What about crafty clients?
Correct Diagnosis %
Protocol is worse at detecting adaptive misbehavior
Throughput gain is not large because of penalty scheme
HOWEVER
Pros and Cons
Pros Fast diagnosis, no need to collect tons of data Low Misdiagnosis rate Retains fairness and throughput of 802.11 Reduces workaround potential
Cons Additional overhead Backwards compatibility Spoofed MAC addresses
Addendum: Optimal Threshold
Authors show a proof for an optimal threshold
Threshold T is a function of assigned back-off value and # of slots.
However, authors chose a static threshold in simulations. This suggests the data could be better.
Discussion/Consideration
What does the police do?
Why did the government create a police force?
At what intrinsic layer does the government’s control end?
http://www.thunderroadlaser.com/images/thunderimages/05_Police%20Badge%20-%2072dpi.jpg
Discussion/Consideration
Is this really the best place to handle selfish behavior?
PC, DriverWireless Network Adapter
Regulated by IEEE
Regulated by IEEE
Not Regulated by IEEE
http://compuplus.com.mx/tienda/images/WAP54G.jpg
Base Station
http://store.madtux.org/images/PC319a.jpg
http://images.belkin.com/F6D3000/PRN1_F6D3000.jpg