Reducing Source Load in BitTorrent

One of the main goals of BitTorrent is to reduce load on Web servers by encouraging clients to share content between themselves. However, BitTorrent's current design relies heavily on the original source to serve a disproportionate amount of the file. We modify standard BitTorrent software so that a source determines the current popularity of each of the blocks of a file and tries to serve only those blocks that are rare. Using extensive PlanetLab experiments, we show that this modification can save a significant amount of the source's upload bandwidth, with the tradeoff of some increased peer download time. In addition, there are individual experiments that both save bandwidth and have a faster download time than standard BitTorrent. We examine some of the more exceptional experiments, explore alternative algorithms, and provide insight for further improvements.