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On The Fidelity of 802.11 Packet Traces Aaron Schulman, Dave Levin, Neil Spring University of Maryland, College Park PAM - April 2008 On The Fidelity of 802.11 Packet Traces 1 Uses of 802.11 packet traces MAC Layer (Mahajan et al, Jardosh


  1. On The Fidelity of 802.11 Packet Traces Aaron Schulman, Dave Levin, Neil Spring University of Maryland, College Park PAM - April 2008 On The Fidelity of 802.11 Packet Traces 1

  2. Uses of 802.11 packet traces • MAC Layer (Mahajan et al, Jardosh et al) • Performance (Rodrig et al) • Troubleshooting (Cheng et al) These studies benefit from complete packet traces PAM - April 2008 On The Fidelity of 802.11 Packet Traces 2

  3. What is an incomplete trace? Transmissions are within range of the monitor but packets are missing AP Monitor Client 1 Client 2 PAM - April 2008 On The Fidelity of 802.11 Packet Traces 3

  4. What is an incomplete trace? Transmissions are within range of the monitor but packets are missing AP Monitor Client 1 Client 2 PAM - April 2008 On The Fidelity of 802.11 Packet Traces 3

  5. What is an incomplete trace? Transmissions are within range of the monitor but packets are missing AP Monitor Client 1 Client 2 PAM - April 2008 On The Fidelity of 802.11 Packet Traces 3

  6. Capturing complete 802.11 packet traces is hard • Monitor Hardware/Software • RF Interference • Monitor Placement • Merging requires accurate timestamps (Yeo et al, Portoles-Comeras et al) PAM - April 2008 On The Fidelity of 802.11 Packet Traces 4

  7. Trace Fidelity Completeness Accuracy Did we capture all of Did we timestamp the the packets? packets correctly? PAM - April 2008 On The Fidelity of 802.11 Packet Traces 5

  8. Main finding: Both are dependent on load Trace Fidelity Completeness Accuracy Did we capture all of Did we timestamp the the packets? packets correctly? PAM - April 2008 On The Fidelity of 802.11 Packet Traces 5

  9. Main finding: Both are dependent on load Trace Fidelity Completeness Accuracy Did we capture all of the packets? Did we timestamp the packets correctly? PAM - April 2008 On The Fidelity of 802.11 Packet Traces 5

  10. Monitors can miss packets Monitor AP Client 2 Client 1 PAM - April 2008 On The Fidelity of 802.11 Packet Traces 6

  11. Monitors can miss packets Monitor AP Client 2 Client 1 PAM - April 2008 On The Fidelity of 802.11 Packet Traces 6

  12. Monitors can miss packets Both the Monitor and AP receive a packet from Client 1 Monitor Monitor AP AP Client 2 Client 1 PAM - April 2008 On The Fidelity of 802.11 Packet Traces 6

  13. Monitors can miss packets Both the Monitor and AP receive a packet from Client 1 Monitor AP Client 2 Client 1 PAM - April 2008 On The Fidelity of 802.11 Packet Traces 6

  14. Monitors can miss packets Both the Monitor and AP receive a packet from Client 1 Monitor AP Client 2 Client 1 PAM - April 2008 On The Fidelity of 802.11 Packet Traces 6

  15. Monitors can miss packets Both the Monitor and AP receive a packet from Client 1 The Monitor misses a packet from Client 2 Monitor AP AP Client 2 Client 1 PAM - April 2008 On The Fidelity of 802.11 Packet Traces 6

  16. 802.11 protocol can show completeness Retry 802.11 Header Sequence # Bit Incremented when a Set when a packet is packet is sent a retransmission (Yeo et al) PAM - April 2008 On The Fidelity of 802.11 Packet Traces 7

  17. Estimating completeness Monitor Client AP PAM - April 2008 On The Fidelity of 802.11 Packet Traces 8

  18. Estimating completeness 1 Monitor Client 1 AP PAM - April 2008 On The Fidelity of 802.11 Packet Traces 8

  19. Estimating completeness Missed 1 2 Monitor Client 1 2 AP PAM - April 2008 On The Fidelity of 802.11 Packet Traces 8

  20. Estimating completeness 2 is missing 1 2 3 Monitor Client 1 2 3 AP PAM - April 2008 On The Fidelity of 802.11 Packet Traces 8

  21. Estimating completeness Missed 1 2 3 4 Monitor Client 1 2 3 4 AP Missed PAM - April 2008 On The Fidelity of 802.11 Packet Traces 8

  22. Estimating completeness Retransmitted 1 2 3 4 4 Monitor Client 1 2 3 4 4 AP Retransmitted PAM - April 2008 On The Fidelity of 802.11 Packet Traces 8

  23. Estimating completeness 4 is missing 1 2 3 4 4 Monitor Client 1 2 3 4 4 AP PAM - April 2008 On The Fidelity of 802.11 Packet Traces 8

  24. Estimating completeness The sequence number and retransmission bit show packets 2 and 4 are missing. 1 2 3 4 4 Monitor Client 1 2 3 4 4 AP PAM - April 2008 On The Fidelity of 802.11 Packet Traces 8

  25. How accurate is the estimate? • Start with SIGCOMM ’04 trace CHI • Randomly removed packets from trace • Compute estimated # of packets missing • Relative Error of Method = Estimate - Known Total packets PAM - April 2008 On The Fidelity of 802.11 Packet Traces 9

  26. Accuracy of estimate Relative error Fraction of Non-beacon Packets Removed Error bars show 95% confidence interval PAM - April 2008 On The Fidelity of 802.11 Packet Traces 10

  27. Accuracy of estimate The relative error is < 0.02 when up to 55% of the trace is removed. Relative error Fraction of Non-beacon Packets Removed Error bars show 95% confidence interval PAM - April 2008 On The Fidelity of 802.11 Packet Traces 10

  28. Percentage for trace completeness SIGCOMM 2004 Dataset Rodrig et al PAM - April 2008 On The Fidelity of 802.11 Packet Traces 11

  29. Percentage for trace completeness 81% Using the estimate the trace has of the packets sent by the AP SIGCOMM 2004 Dataset Rodrig et al PAM - April 2008 On The Fidelity of 802.11 Packet Traces 11

  30. Percentage for trace completeness 81% Using the estimate the trace has of the packets sent by the AP 37% of the AP’s packets were beacon packets sent when the network was idle SIGCOMM 2004 Dataset Rodrig et al PAM - April 2008 On The Fidelity of 802.11 Packet Traces 11

  31. Percentage for trace completeness 81% Using the estimate the trace has of the packets sent by the AP 37% of the AP’s packets were beacon packets sent when the network was idle 70% Excluding idle beacon packets of packets sent by the AP are in the trace SIGCOMM 2004 Dataset Rodrig et al PAM - April 2008 On The Fidelity of 802.11 Packet Traces 11

  32. One number is not enough • Problem: Completeness is only interesting when the network is under load - Example: Capturing a trace from an AP overnight • Solution: Estimate completeness within small trace intervals - Beacons are sent by AP every 100ms PAM - April 2008 On The Fidelity of 802.11 Packet Traces 12

  33. Trace completeness score Packets collected i Packets expected i PAM - April 2008 On The Fidelity of 802.11 Packet Traces 13

  34. Trace completeness score For all devices in-range Packets collected i Packets expected i PAM - April 2008 On The Fidelity of 802.11 Packet Traces 13

  35. Trace completeness score For all devices in-range Packets i Packets expected i PAM - April 2008 On The Fidelity of 802.11 Packet Traces 13

  36. Trace completeness score For all devices in-range Packets i Sequence Change i + Retransmissions i PAM - April 2008 On The Fidelity of 802.11 Packet Traces 13

  37. Trace completeness score For all devices in-range Packets i Sequence Change i + Retransmissions i Quantifies the completeness of interval i PAM - April 2008 On The Fidelity of 802.11 Packet Traces 13

  38. Visualizing trace completeness Portland ug 100000 1 10000 0.8 1000 Score 0.6 100 0.4 10 0.2 0 1 0 10 20 30 40 50 Load (change in sequence number) PAM - April 2008 On The Fidelity of 802.11 Packet Traces 14

  39. Visualizing trace completeness Portland ug • Y-Axis: Score 100000 1 • Completeness of 10000 0.8 an Interval 1000 Score 0.6 100 0.4 10 0.2 0 1 0 10 20 30 40 50 Load (change in sequence number) PAM - April 2008 On The Fidelity of 802.11 Packet Traces 14

  40. Visualizing trace completeness Portland ug • Y-Axis: Score 100000 1 • Completeness of 10000 0.8 an Interval 1000 Score 0.6 • X-Axis: Load 100 0.4 • Sequence # change 10 0.2 0 1 0 10 20 30 40 50 Load (change in sequence number) PAM - April 2008 On The Fidelity of 802.11 Packet Traces 14

  41. Visualizing trace completeness Portland ug • Y-Axis: Score 100000 1 • Completeness of 10000 0.8 an Interval 1000 Score 0.6 • X-Axis: Load 100 0.4 • Sequence # change 10 0.2 0 1 • Color: Frequency 0 10 20 30 40 50 Load (change in sequence number) PAM - April 2008 On The Fidelity of 802.11 Packet Traces 14

  42. Completeness with T -Fi plot Portland ug 100000 1 10000 0.8 1000 Score 0.6 100 0.4 10 0.2 0 1 0 10 20 30 40 50 Load (change in sequence number) PAM - April 2008 On The Fidelity of 802.11 Packet Traces 15

  43. Completeness with T -Fi plot • Complete loaded Portland ug 100000 trace has dark area 1 10000 on top 0.8 1000 Score 0.6 100 0.4 10 0.2 0 1 0 10 20 30 40 50 Load (change in sequence number) PAM - April 2008 On The Fidelity of 802.11 Packet Traces 15

  44. Completeness with T -Fi plot • Complete loaded Portland ug 100000 trace has dark area 1 10000 on top 0.8 1000 • Incomplete trace has Score 0.6 100 lower dark areas 0.4 10 0.2 0 1 0 10 20 30 40 50 Load (change in sequence number) PAM - April 2008 On The Fidelity of 802.11 Packet Traces 15

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