The delay region for P2P file transfer

Motivated by P2P file transfer applications (e.g., BitTorrent) on the Internet, this paper considers the problem of delivering a file from a server to multiple receivers in a P2P network. Each receiver has an associated delay in receiving the file. We aim at understanding the optimal delay region, i.e., the set of all possible delay vectors that can be achieved. Previous work has addressed the problem of delivering the file to all receivers in minimum amount of time (equivalently, minimizing the maximum delay to the receivers), assuming peer uplinks are the only bottleneck in the network. This paper shows that it is in fact possible to significantly reduce the average delay at a slight increase in the maximum delay. Moreover, given an order at which the receivers finish downloading, the optimal delay region is characterized by a system of linear inequalities. Any point in the optimal delay region can be achieved by linear network coding. We also propose a simple routing scheme that has near-optimal empirical performance.