Analyzing and Improving BitTorrent Performance

In recent years, BitTorrent has emerged as a very popular and scalable peer-to-peer file distribution mechanism. It has been successful at distributing large files quickly and efficiently without overwhelming the capacity of the origin server. Early measurement studies verified that BitTorrent achieves excellent upload utilization, but raised several questions concerning utilization in settings other than those measured, fairness, and the choice of BitTorrent’s mechanisms. In this paper, we present a simulation-based study of BitTorrent. Our goal is to deconstruct the system and evaluate the impact of its core mechanisms, both individually and in combination, on overall system performance under a variety of workloads. Our evaluation focuses on several important metrics, including peer link utilization, file download time, and fairness amongst peers in terms of volume of content served. Our results confirm that BitTorrent performs near-optimally in terms of uplink bandwidth utilization, and download time except under certain extreme conditions. On fairness, however, our work shows that low bandwidth peers systematically download more than they upload to the network when high bandwidth peers are present. We find that the ratebased tit-for-tat policy is not effective in preventing unfairness. We show how simple changes to the tracker and a stricter, block-based tit-for-tat policy, greatly improves fairness.