Comparison of end-to-end and network-supported fast startup congestion control schemes

Abstract

A fundamental challenge for congestion control is the flow startup phase after the connection setup or after long idle periods. The Transmission Control Protocol (TCP) uses the time-consuming Slow-Start mechanism in that case. The objective of fast startup congestion control is to fully utilize a path much more rapidly. Faster startups can either be realized by new end-to-end congestion control mechanisms that change the Slow-Start algorithm, or by additional on-path signaling providing information about the path. This paper comprehensively compares end-to-end and network-supported fast startup schemes. We evaluate both the potential performance benefit and risk, using analytical models, simulation results, and experiments with real applications. Concerning end-to-end schemes, we study both a simple increase of the initial congestion window and the recently proposed Jump-Start mechanism. The considered network-supported schemes are the Quick-Start TCP extension and the Rate Control Protocol (RCP). They can all significantly reduce the transport delay of mid-sized data transfers, in particular over broadband networks with a non-negligible latency. As a result, fast startup schemes can significantly speed up Internet application with such traffic patterns. Our results also reveal that end-to-end approaches are not necessarily overly aggressive. Network-supported mechanisms can reduce the risk of congestion, but there are several unsolved open issues. In summary, we argue that end-to-end fast startup schemes such as an increased initial congestion window would be a promising solution for the Internet.