Peer-Assisted File Distribution: The Minimum Distribution Time

With the emergence of BitTorrent, Swarm-cast, and CDNs, peer-assisted file distribution has become a prominent Internet application, both in terms of user popularity and traffic volumes. We consider the following fundamental problem for peer-assisted file distribution. There are seed nodes, each of which has a copy of the file, and leecher nodes, each of which wants a copy the file. The goal is to distribute the file to all the leechers - with the assistance of the upload capacity of the leechers - in order to minimize the time to get the file to all the leechers (the distribution time). We obtain explicit expressions for the minimum distribution time of a general heterogeneous peer-assisted file distribution system. Derived with fluid-flow arguments, the expressions are in terms of the file size, the seeds´ upload rates and the leechers´ upload and download rates. We demonstrate the utility of the result by comparing the optimal distribution time with the measured distribution time when BitTorrent is used to distribute a file from a seed to ten leechers