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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

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MAC Layer (Mis)behaviors

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  1. MAC Layer (Mis)behaviors Christophe Augier - CSE802.11 Summer 2003

  2. 802.11 - MAC • Based on CSMA like Ethernet • Two different access methods • DCF – Distributed Coordination Function • PCF – Point Coordination Function

  3. MAC – CSMA • CSMA – Carrier Sense Multiple Access + very effective under low loads - possible collisions

  4. CSMA/CA CA: Collision Avoidance • Random back-off value • RTS / CTS • ACK scheme

  5. Questions • How MAC behaves with widely used protocols? • Is the throughput fairly shared? • How MAC handles misbehaving nodes?

  6. How… - Sender backoff counter = 0 Sense the medium busy free CW= min(2 CW, Cwmax) Transmit repeat >= 7 CW=CWmin

  7. 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

  8. How… - Receiver Check CRC corrupted ok Send ACK CRC: Cyclic Redundancy Check

  9. How… - both Sender Receiver RTS Sense the medium CTS data Transmit data ACK

  10. 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

  11. First test • 1 sender, 1 receiver TCP 1 2 3 4 5 6 7

  12. First test results • Instability in the TCP • Connection lost

  13. First test explication • One node fails to reach its adjacent node • drops packets • reports route failure

  14. 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

  15. Second test • 2 TCP sessions started with a delay of 20s TCP TCP 1 2 3 4 5 6 7 interferences

  16. Second test results • The first session is forced down

  17. Second test results • causes: • Collisions between node 2 TCP packets and node 5 RTS packets • Route failure • TCP session timeout • solutions: null Reducing the TCP window does not work

  18. First conclusion • MAC is unchanged, the layer above MAC are changed to recover losses • Link layer • Transport layer • Change MAC - radical

  19. MACs comparison • Experiments to compare: • CSMA • FAMA – RTS/CTS • 802.11 – CSMA/CA, RTS/CTS + ACK • Under different multihop environments

  20. Variable number of hops • Var. TCP window sizes • Var. number of hops

  21. 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

  22. Hidden terminal • CSMA: unfair, • FAMA and 802.11: fair • Thanks to CA mechanisms

  23. Ring topology • 802.11 not so fair compared to FAMA • Increasing the DIFS period achieve fairness

  24. 9x9 grid • Good Throughput • But capture

  25. 9x9 grid with mobility • Evidence of captures effects • But far better than CSMA and FAMA

  26. 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

  27. MAC layer misbehavior • Problem: • No detection of misbehaviors • A selfish node can get a better throughput than well-behaved nodes

  28. Solutions • Identify and punish misbehaving nodes: • Avoid such nodes in routing • Protocol penalizing misbehavior • Game-theoretic techniques

  29. Proposed solution • Receiver selects the sender next backoff value • The sender have to use this assigned backoff value • The receiver then can detect misbehavior

  30. Detecting misbehavior • Bact < a * 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 a ?

  31. Penalizing misbehaving nodes • Select a reasonably high • Use a diagnosis scheme based on • a window W • a threshold THRESH • A node is misbehaving when

  32. Results

  33. Issues • misbehaving receiver • Tradeoff between • Penalizing misbehaving nodes • Ensuring the fairness of well-behaved nodes

  34. 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

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