Overlay routing has emerged as a promising approach to mitigating many problems with Internet routing, such as improving the reliability of Internet paths and supporting multicast communication. As overlay routing is gaining wider acceptance, we argue that it is time t o investigate how overlay networks can benefit Internet transport. This paper presents Slot, a framework that leverages overlay networks to improve the throughput of feedback-based transport protocols. Slot exploits the observation that the throughput of feedback-based transport protocols (e.g., TCP, XCP, VCP, DCCP) is inversely proportional to the length of their end-to-end feedback control loop, and effectively shortens an end-to-end control loop by breaking it up into multiple pipelined shortened sub-loops via intermediaries carefully chosen from an overlay network. As a result, Slot increases the throughput of an end-to-end transport connection to that of the longest sub-loop. This paper studies the potential of Slot and addresses key challenges in the design and the deployment of Slot. The contributions of this paper are three-fold. First, we make the case for Slot by measuring and analyzing the control loop lengths of close to 3.7 million node pairs and their potential benefit from Slot using PlanetLab as an example overlay network. Second, we identify key challenges in the design of Slot and show that a simple, low overhead solution can be used to select an overlay path that can achieve close to the maximum throughput improvement possible. Third, we implement a prototype of Slot and deploy it on PlanetLab to fetch a large set of files crawled from popular web servers. Our results show that compared to directly fetching the same documents, Slot improves the throughput of 95% of the large file transfers, and 50% of these transfers achieve more than 30% increase in throughput.

Date of this Version

July 2005