Kumar : Performance Analysis of Versions of Tcp

|We use a stochastic model to study the through-put performance of various versions of TCP (Tahoe (including its older version that we call OldTahoe), Reno, and NewReno) in the presence of random losses on a wireless link in a local network. We model the cyclic evolution of TCP, each cycle starting at the epoch at which recovery starts from the losses in the previous cycle. TCP through-put is computed as the reward rate in a certain Markov renewal-reward process. Our model allows us to study the performance implications of various protocol features, such as, fast-retransmit and fast recovery. We show the impact of coarse timeouts. In the local network environment, the key issue is to avoid a coarse timeout after a loss occurs. We show the eeect of reducing the number of duplicate acknowlegements for triggering a fast-retransmit. A large coarse timeout granularity seriously affects the performance of TCP, and the various protocol versions diier in their ability to avoid a coarse timeout when random loss occurs; we quantify these diierences. As observed in simulations by other researchers, we show that, for large packet loss probabilities, TCP-Reno performs no better, or worse, than TCP-Tahoe. TCP-NewReno is a considerable improvement over TCP-Tahoe, and reducing the fast retransmit threshold from 3 to 1 yields a large gain in throughput; this is similar to one of the modiications in the recent TCP-Vegas proposal. We explain some of these observations in terms of the variation of fast-recovery probabilities with packet loss probability. Finally, we show that the results of our analysis compare well with a simulation that uses actual TCP code.