Abstract
BitTorrent as a peer-to-peer based file distribution system has received great attention recently. A file is divided into pieces for facilitating parallel downloading. However, peer dynamics usually causes the last block problem, where a file may never be completed because some pieces are missing in the network. This paper suggests a piece selection strategy to explicitly acquire rare pieces in order to avoid the last block problem. The proposed mechanism allows peers to be notified of the existence of rare pieces and thus these pieces can be easily retrieved. A hierarchical BitTorrent structure is also proposed to mitigate the potential overhead of this algorithm. Simulation results show that our scheme approximates the performance of the optimal globally rarest first piece-selecting algorithm and effectively enhances service availability. In addition, the hierarchical structure successfully bounds the message overhead and performs well at the forepart of simulations.