Exploring the Impact of Heterogeneous Reordering Robustness of TCP

The congestion control mechanisms in the standardized Transmission Control Protocol (TCP) may misinterpret packet reordering as congestive loss, leading to spurious congestion response and under-utilization of network capacity. Therefore, many TCP enhancements have been proposed to better differentiate between packet reordering and congestive loss, in order to enhance the reordering robustness (RR) of TCP. Since such enhancements are incrementally deployed, it is important to study the interactions of TCP flows with heterogeneous RR. This paper presents the first systematic study of such interactions by exploring how changing RR of TCP flows influences the bandwidth sharing among these flows. We define the quantified RR (QRR) of a TCP flow as the probability that packet reordering causes congestion response. We analyze the variation of bandwidth sharing as QRR changes. This leads to the discovery of several interesting properties. Most notably, we discover the counter-intuitive result that changing one flow’s QRR does not affect its competing flows in certain network topologies. We further characterize the deviation, from the ideal case of bandwidth sharing, as RR changes. We find that enhancing RR of a flow may increase, rather than decrease, the deviation in some typical network scenarios.