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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