Characterizing the effects of TCP's initial window

Abstract

Given the mice-elephant phenomenon (where 80% of flows in number contribute to only 20% of traffic in volume) of TCP traffic, TCP's slow-start algorithm plays an important role in today's Internet. Designed to slowly probe the network for available bandwidth, the current standard mandates TCP flows to start with an initial window (IW) size of at most ten segments. Given the speed of the today's networks, an IW of ten segments could be a good option; however, over years the IW value may need to be updated. This paper aids such future updates with a stochastic model that reveals the effects of IW on TCP traffic characteristics, queue length distribution, average packet loss probability, and response times of TCP flows. Our studies show that the optimal choice of IW for a flow depends on the bottleneck router's buffer size, number of active TCP connections, bottleneck router's link capacity, RTT, and flow size. We also observe that the IW value non-monotonously increases the number of RTTs required for flows to complete.