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BitTorrent has emerged as a very popular peer-to-peer file sharing system, which uses an embedded set of incentive mechanisms to encourage contribution and prevent free-riding. However, the capability BitTorrent has of preventing free-riding needs further study. In this paper, we present a fluid model with two different classes of peers to capture the effect of free-riding on BitTorrent-like systems. With the model, we find that BitTorrent's incentive mechanism is successful in preventing free-riding in a system without seeds, but may not succeed in producing a disincentive for free-riding in a system with a high number of seeds. The reason for this is that BitTorrent does not employ any effective mechanisms for seeds to effectively guard against freeriding. Therefore, we propose a seed bandwidth allocation strategy for the BitTorrent system to reduce the effect of seeds on free-riding. Finally, simulation results are given, which validate what we have found in our analysis and demonstrate the effectiveness of the proposed strategy.
Bandwidth Allocation Strategy, BitTorrent, Free-riding, Incentive Mechanism, modeling

J. Yu, J. Wu and M. Li, "Free-Riding on BitTorrent-Like Peer-to-Peer File Sharing Systems: Modeling Analysis and Improvement," in IEEE Transactions on Parallel & Distributed Systems, vol. 19, no. , pp. 954-966, 2007.
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