Abstract

A TCP flow is congestion responsive because it reduces its send window upon the appearance of congestion. An aggregate of nonpersistent TCP flows, however, may not be congestion responsive, depending on whether the flow (or session) arrival process reacts to congestion or not. In this paper, we describe a methodology for the passive estimation of traffic congestion responsiveness. The methodology aims to classify every TCP session as either "open-loop" or "closed-loop". In the closed-loop model, the arrival of a session depends on the completion of the previous session from the same user. When the network is congested, the arrival of a new session from that user is delayed. On the other hand, in the openloop model, TCP sessions arrive independently of previous sessions from the same user. The aggregate traffic that the open-loop model generates is not congestion responsive, despite the fact that each individual flow in the aggregate is congestion responsive. Our measurements at a dozen of access and core links show that more than 60-80% of the traffic that we could analyze (mostly HTTP traffic) follows the closed-loop model. Document type: Part of book or chapter of book

Abstract

Flow size based congestion control has the potential to improve user perceived Web performance due to the heavy tailed characteristic of file size distributions in the Web. After discussing the benefits and drawbacks of transport protocol and router based solutions for size based congestion control, guidelines for algorithm design are developed. Using these guidelines we find that size based congestion control needs to incorporate TCP models to avoid undesirable user incentives. Based on this insight we specify enhancements to TCP featuring size based congestion control and provide arguments for parameter settings. It is shown by simulation that our modified version of TCP significantly outperforms NewReno from a user perspective in scenarios using realistic models for Web traffic and topologies with multiple congested links. Document type: Part of book or chapter of book