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Is the Round-trip Time Correlated with the Number of Packets in Flight?
Saad Biaz, Auburn UniversityNitin H. Vaidya, University of Illinois at Urbana-Champaign
published in ACM IMC’03
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Overview
• TCP Reno keeps increasing sending rate till packet loss detected
• some Congestion Avoidance Techniques (CATs) are proposed to avoid such aggressive behavior
• some CATs use RTT / throughput to detect queue build-up and adjust congestion window size
• Can RTT reflect queue build-up?
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Outline
• Terminology• Correlation coefficients used• Experimental data• Analysis results• Conclusion
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Terminology
Pi , PWi , PFi , PTi , W(Pi) / Wi , RTT(Pi) / RTTi , Sign(X) , Cl , B(Cl)
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Correlation Coefficients
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Correlation Coefficients
• Similar correlation coefficients for direction change (increase or decrease)
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Throughput and RTT against load
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TCP-Vegas
• compare the quantity with two thresholds, alpha and beta (alpha < beta)
• if the quantity < alpha, cwnd increases
• if the quantity > beta, cwnd decreases
• otherwise, cwnd holds constant
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Experiments
• Data is collected by Paxson in 1994-95
• span many paths with different RTT• two data sets N1 and N2 are
collected, which are tcpdump traces over 37 sites across USA, Europe and Australia
• this study uses only N2 of 20000 tcpdump traces for bulk transfers of 100 KB between pairs of sites among the 37 concerned sites
• N2 contains both the tcpdump traces at sender and receiver, but only sender is consider in this study
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Experiments
• 14218 out of 20000 tcpdump traces are analyzed for technical reasons
• These 14218 connections were between 31 different sites
• Not all sites send to all sites; span only 737 paths
• RTT and data in flight (W) are found from those tcpdump traces
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Partitioning connection sets
• to see if and how bottleneck bandwidth affects correlation coefficients, the TCP connections are divided into two sets: fast and slow
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Correlation bwt RTT and W
frequency distribution
SlowFast
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Correlation in Direction of Change
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Can corrcoef characterize a path?
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Conclusion
•From corrcoef in direction of change in RTT and W for PW, about 88% connections have positive correlation •In general, RTT is sensitive to load•However, corrcoef is not strong enough to be relied (on RTT) to build congestion avoidance scheme
88%
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Conclusion
• RTT observed by a TCP connection is dependent on other traffic; effect is more significant if that TCP connection only share a small fraction of bandwidth
• vagaries of transport protocol like delayed ACK