Estimating TCP flow completion time distributions

The flow completion time is an important performance metric for network users, and it is used in designing scheduling and congestion control algorithms. The existing TCP flow completion time distributions are calculated from complicated models, and they are only suitable for short TCP flows. This paper is motivated to build a simple model to derive TCP flow completion time distribution that is suitable for both short and long TCP flows. To build such a model, the distribution of the packet transmission latency under TCP congestion control is firstly calculated. The packet transmission latency is the time needed to successfully deliver a packet, including the time for retransmissions due to packet losses. Semi-Markov process is used to model the TCP congestion window evolution. Using the packet transmission latency distribution together with the TCP congestion window size information, a new parallel queueing model which describes how a file is transmitted by a TCP flow is formed. In the parallel queueing model, the flow completion time distribution is derived, and simulation results from NS2 demonstrate the accuracy of the proposed analysis.