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 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
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
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
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
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
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
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
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
Monitors can miss packets Monitor AP Client 2 Client 1 PAM - April 2008 On The Fidelity of 802.11 Packet Traces 6
Monitors can miss packets Monitor AP Client 2 Client 1 PAM - April 2008 On The Fidelity of 802.11 Packet Traces 6
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
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
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
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
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
Estimating completeness Monitor Client AP PAM - April 2008 On The Fidelity of 802.11 Packet Traces 8
Estimating completeness 1 Monitor Client 1 AP PAM - April 2008 On The Fidelity of 802.11 Packet Traces 8
Estimating completeness Missed 1 2 Monitor Client 1 2 AP PAM - April 2008 On The Fidelity of 802.11 Packet Traces 8
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
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
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
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
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
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
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
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
Percentage for trace completeness SIGCOMM 2004 Dataset Rodrig et al PAM - April 2008 On The Fidelity of 802.11 Packet Traces 11
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
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
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
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
Trace completeness score Packets collected i Packets expected i PAM - April 2008 On The Fidelity of 802.11 Packet Traces 13
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
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
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
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
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
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
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
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
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
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
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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