2Fast: Collaborative downloads in P2P networks

P2P systems that rely on the voluntary contribution of bandwidth by the individual peers may suffer from freeriding. To address this problem, mechanisms enforcing fairness in bandwidth sharing have been designed, usually by limiting the download bandwidth to the available upload bandwidth. As in real environments the latter is much smaller than the former these mechanisms severely affect the download performance of most peers. In this paper we propose a system called Mast, which solves this problem while preserving the fairness of bandwidth sharing. In Mast, we form groups of peers that collaborate in downloading a file on behalf of a single group member which can thus use its full download bandwidth. A peer in our system can use its currently idle bandwidth to help other peers in their ongoing downloads, and get in return help during its own downloads. We assess the performance of Mast analytically and experimentally, the latter in both real and simulated environments. We find that in realistic bandwidth limit settings, Mast improves the download speed by up to a factor of 3.5 in comparison to state-of-the-art P2P download protocols.