Performance degradation of TCP Vegas in asymmetric networks and its remedies

Abstract

-TCP Vegas employs congestion avoidance, early detection of packet loss and conservative slow-start algorithms to improve TCP performance. With its proactive congestion detection, it utilizes network bandwidth more efficiently and achieves a higher throughput than TCP Reno. This paper shows that in asymmetric networks in which the bottleneck is on the reverse path rather than on the forward path, its performance can be significantly lower than that of Reno, in contrast to the 37% throughput improvement claimed in [4][5]. In particular, Vegas may erroneously converge to an operating region in which the available bandwidth on the forward path is under-utilized by a large margin. Even worse, when connections running Vegas and Reno co-exist and compete on the same network, Vegas suffers a severe penalty. We propose an approach to improve Vegas' throughput in asymmetric networks.I.

TCP Veno revisited

“…We may noticed that Veno's improvement is not so high as compared to its achieved amount in wireless networks, but at least, we know it is not worse than Reno, unlike others such as TCP Vegas, while it could get some improvements in wired networks, it may also lead to severer suffering in asymmetric networks [13,14]. This algorithm, lacking some sorts of flexible nature, is not our expected.…”

“…For example, Vegas TCP [1] adopts a moderate congestion control algorithms -additive increase and additive decrease, when competing with a TCP Reno connection, its window evolution is suppressed by Reno connections. This bias becomes very severe, and even unacceptable, when in the asymmetric networks [13][14].…”

“…If acks do not arrive at the sender fast enough, the sending rate will be throttled even if the forward path is congestion free, resulting in underutilization of the available bandwidth on the forward path. There have been quite a number of efforts looking into the performance issue of Reno TCP over asymmetric networks [13,21].…”