Is the round-trip time correlated with the number of packets in flight?

TCP uses packet loss as a feedback from the network to adapt its sending rate. TCP keeps increasing its sending rate as long as no packet loss occurs (unless constrained by buffer size). Alternative congestion avoidance techniques (CATs) have been proposed to avoid such "aggressive" behavior. These CATs use simple statistics on observed round-trip times and/or throughput of a TCP connection in response to variations in congestion window size. These CATs have a supposed ability to detect queue build-up.The objective of this paper is to question the ability of these CATs to reliably detect queue build-up under real network conditions. For this purpose, the sample coefficient of correlation between round-trip time and the number of packets in flight is analyzed for 14,218 connections over 737 Internet paths. These coefficients of correlation were extracted from a set of tcpdump traces collected by Vern Paxson.The coefficients of correlation measured confirm that the correlation between RTT and window size is often weak.

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