BitTorrent is an extremely effective and popular peer-to-peer file distribution application. It differs from traditional peer-to-peer file-sharing applications in that large files are decomposed into blocks, and in order to download a file, a peer concurrently retrieves blocks from multiple peers. Measurement and simulation studies have suggested that although BitTorrent achieves excellent utilization of upload capacity, its fairness properties are less impressive. In this paper, we seek to understand, primarily through simulation analysis, the fairness properties of the exchange mechanism that lies at the core of the BitTorrent protocol. We focus on a specific fairness metric, defined as the ratio of bytes uploaded to that downloaded by each individual peer. We propose three modifications to the protocol, and examine their impact on the fairness peers experience.
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