MAC Layer (Mis)behaviors
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Transcript of MAC Layer (Mis)behaviors
MAC Layer (Mis)behaviors
Christophe Augier - CSE802.11 Summer 2003
802.11 - MAC Based on CSMA like Ethernet Two different access methods
DCF – Distributed Coordination Function
PCF – Point Coordination Function
MAC – CSMA CSMA – Carrier Sense Multiple
Access
+ very effective under low loads
- possible collisions
CSMA/CACA: Collision Avoidance
Random back-off value
RTS / CTS
ACK scheme
Questions How MAC behaves with widely
used protocols?
Is the throughput fairly shared?
How MAC handles misbehaving nodes?
How… - Sender
Sense the medium
busy free
Transmit
CW=CWmin
repeat >= 7
CW= min(2 CW, Cwmax)
backoff counter = 0
Backoff value Selected from range [0, CW]
If medium is free for a time equal to DIFS – DCF InterFrame Space, backoff is counted down
If medium is busy, suspend backoff countdown
How… - Receiver
Check CRC
corruptedok
Send ACK
CRC: Cyclic Redundancy Check
How… - bothSender Receiver
RTS
CTS
data
data
ACK
Sense the medium
Transmit
MAC + TCP Why TCP?
Widely used – prevalent protocol used in Internet
Adaptability – network conditions, congestion control
Topology: ad hoc net, string of 7 nodes Assumptions:
No traffic – to get stable network and TCP throughput
Infinite flow – always data to send out
First test 1 sender, 1 receiver
2 3 4 51 6 7
TCP
First test results
Instability in the TCP
Connection lost
First test explication One node fails to reach its adjacent
node drops packets reports route failure
First test solution causes:
packet size – too big
number of packets sent back-to-back – too many
One node was capturing the medium
solution: decrease TCP window i.e. number of
packets sent back-to-back
Second test 2 TCP sessions started with a delay
of 20s
2 3 4 51 6 7
TCP TCP
interferences
Second test results The first session is forced down
Second test results causes:
Collisions between node 2 TCP packets and node 5 RTS packets
Route failure TCP session timeout
solutions: nullReducing the TCP window does not work
First conclusion
MAC is unchanged, the layer above MAC are changed to recover losses Link layer Transport layer
Change MAC - radical
MACs comparison Experiments to compare:
CSMA FAMA – RTS/CTS
802.11 – CSMA/CA, RTS/CTS + ACK
Under different multihop environments
Variable number of hops Var. TCP window sizes Var. number of hops
Variable number of hops Throughput is inversely proportional to
the hop distance
Max throughput with TCP Win= 1
Throughput: CSMA > FAMA > 802.11
802.11 stable for different values of W
Hidden terminal CSMA: unfair,
FAMA and 802.11: fair Thanks to CA mechanisms
Ring topology 802.11 not so fair compared to FAMA Increasing the DIFS period achieve
fairness
9x9 grid Good Throughput But capture
9x9 grid with mobility Evidence of captures effects But far better than CSMA and
FAMA
Conclusions 802.11 is promising
Good combination of throughput and fairness
Good behavior with mobility Need work:
To make TCP and MAC work well together
MAC layer timers - fairness
MAC layer misbehavior
Problem: No detection of misbehaviors A selfish node can get a better
throughput than well-behaved nodes
Solutions
Identify and punish misbehaving nodes: Avoid such nodes in routing Protocol penalizing misbehavior Game-theoretic techniques
Proposed solution Receiver selects the sender next
backoff value The sender have to use this assigned
backoff value
The receiver then can detect misbehavior
Detecting misbehavior Bact < * Bexp , 0 < a < 1
When a node is misbehaving? Deviation may come from:
Sender senses the medium as free, the receiver does not.
How to choose ?
Penalizing misbehaving nodes Select reasonably high Use a diagnosis scheme based on
a window W a threshold THRESH A node is misbehaving when
THRESHWpacket
actexp BB
Results
Issues misbehaving receiver Tradeoff between
Penalizing misbehaving nodes Ensuring the fairness of well-behaved
nodes
Conclusions 802.11 is promising (compared to previous MAC)
Good combination of throughput and fairness Good behavior with mobility
Need work: To make TCP and MAC work well together
Recover losses New protocol
MAC layer timers – fairness, avoid capture effects Mobility Safe against misbehaving nodes - fairness