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MAC Layer (Mis)behaviors

This paper explores the behaviors and misbehaviors of the 802.11 MAC (Medium Access Control) layer, focusing on its performance in multihop networks under CSMA (Carrier Sense Multiple Access) protocols. It compares different access methods such as DCF (Distributed Coordination Function) and PCF (Point Coordination Function), examines how TCP interacts with MAC, and assesses throughput fairness among well-behaved and misbehaving nodes. Solutions for improving MAC layer fairness and mechanisms to detect and penalize misbehaving nodes are proposed, contributing to the reliability and efficiency of wireless communication.

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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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