Is the Round-trip Time Correlated with the Number of Packets in Flight?

1
Is the Round-trip Time Correlated with the Number
of Packets in Flight?

• Saad Biaz, Auburn University
• Nitin H. Vaidya, University of Illinois at
  Urbana-Champaign

published in ACM IMC03
2
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 lt beta)
• if the quantity lt alpha, cwnd increases
• if the quantity gt 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
Slow Fast
frequency distribution
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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

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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