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 regardless of the network congestion state as long as no loss occurs (unless constrained by buffer size). Alternative congestion avoidance techniques (CATs) have been proposed to avoid such "agressive" 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. Such ability may be used to distinghish congestion losses from transmission losses. A previous study shows that these CATs do not yield interesting results for diagnosing the real reason of a loss. 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, we analyze the sample coefficient of correlation between round-trip time and the number of packets in fligh..