1. 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 IMC ’ 03

2. 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?

3. 3 Outline Terminology Correlation coefficients used Experimental data Analysis results Conclusion

4. 4 Terminology P i, PW i, PF i, PT i, W(P i ) / W i, RTT(P i ) / RTT i, Sign(X), C l, B(C l )

5. 5 Correlation Coefficients

6. 6 Similar correlation coefficients for direction change (increase or decrease)

7. 7 Throughput and RTT against load

8. 8 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

9. 9 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

10. 10 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

11. 11 Partitioning connection sets to see if and how bottleneck bandwidth affects correlation coefficients, the TCP connections are divided into two sets: fast and slow

12. 12 Correlation bwt RTT and W frequency distribution Slow Fast

13. 13 Correlation in Direction of Change

14. 14 Can corrcoef characterize a path?

15. 15 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%

16. 16 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