Evaluating stranger policies in P2P file-sharing systems with reciprocity mechanisms
Computer Networks: The International Journal of Computer and Telecommunications Networking
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In this paper, we present a simple mathematical model for studying the performance of the BitTorrent () file sharing system. We are especially interested in the distribution of peers in different states of the download job progress. With the model we find that the distribution of the download peers follows an asymmetric U-shaped curve under the stable state, due to BitTorrent's unchoking strategies. In addition, we find that the seeds' departure rate and the download peers' abort rate will influence the peer distribution in different ways notably. We also analyze the content availability under the dying process of the BitTorrent file sharing system. We find that the system's stability deteriorates with decreasing and unevenly distributed online peers, and BitTorrent's built-in ‘tit-for-tat’ unchoking strategy could not help to preserve the integrity of the file among the download peers. We propose an innovative ‘tit-for-tat’ unchoking strategy which enables more peers to finish the download job and prolongs the system's lifetime. By playing our innovative strategy, download peers could cooperate to improve the stability of the system by making a trade-off between the current downloading rate and the future service availability. Finally, experimental results are presented to validate our analytical results and support our proposals. Copyright © 2007 John Wiley & Sons, Ltd.