Analysis of TCP Vegas and TCP Reno

The best known congestion control mechanism for the TCP/IP protocols, is that proposed by Van Jacobson (1988) dubbed Tahoe. Revised two years after (Van Jacobson, 1990), by including fast recovery and fast retransmit (Stevens, 1994), the new version called Reno achieves more bandwidth utilization and retransmits less data than its predecessor. However its method of predicting the available bandwidth by provoking losses, is the main source of large averages and variances of rtt (round trip time) and also of aggressiveness (network saturation). Brakmo and Peterson (1995) proposed a new congestion avoidance algorithm dubbed Vegas based on some modifications to the source behavior of Reno. In this paper we use an analytic fluid approach in order to analyze the different features of both Vegas and Reno. We then use simulations to confirm our analytic results. When the available bandwidth is high, our results confirm the claims of Brakmo and Peterson (1995) and An et al. (1995); indeed Vegas can retransmit less than one-fifth as much data as Reno does, so that the higher the available bandwidth is, the more efficient Vegas is. However, under heavy congestion Vegas behaves like Reno and does not manage to make efficient use of its new mechanism for detection of congestion. The analytic results that we obtain are the evolution of the window size, round trip times and their averages, and the average throughput