Most peer-to-peer content distribution sys- tems require the peers to privilege the welfare of the overall system over greedily maximizing their own utility. When downloading a file broken up into multiple pieces, peers are often asked to pass on some possible download opportunities of common pieces in order to favor rare pieces. This is to avoid the missing piece syndrome, which throttles the download rate of the peer-to-peer system to that of downloading the file straight from the server. In other situations, peers are asked to stay in the system even though they have collected all the file's pieces and have an incentive to leave right away. We propose a mechanism which allows peers to act greedily and yet stabilizes the peer-to-peer content shar- ing system. Our mechanism combines a fountain code at the server to generate innovative new pieces, and a prioritization for the server to deliver pieces only to new peers. While by itself, neither the fountain code nor the prioritization of new peers alone stabilizes the system, we demonstrate that their combination does, through both analytical and numerical evaluation. I. INTRODUCTION
Peer-to-peer (P2P) systems in general, and BitTorrent (BT) specifically, have been of significant interest to researchers and Internet users alike. Existing models of BT abstract away certain characteristics of the protocol that are important, which we address in this work. We present a simple yet accurate and easily extensible model of BT. The model's accuracy is validated through a rigorous simulation-based study and its extensibility is illustrated by incorporating recently proposed approaches to protocol changes in BT. In this paper, we develop simple models to study the performance of BitTorrent, a second generation peer-to-peer (P2P) application. We first present a simple fluid model and study the scalability, performance and efficiency of such a file-sharing mechanism. We then consider the built-in incentive mechanism of BitTorrent and study its effect on network performance. We also provide numerical results based on both simulations and real traces obtained from the Internet.