The fast-growing traffic of Peer-to-Peer (P2P) applications, most notably BitTorrent (BT), is putting unprecedented pressure to Internet Service Providers (ISPs). P2P locality has, therefore, been widely suggested to mitigate the costly inter-ISP traffic. In this paper, we for the first time examine the existence and distribution of the locality through a large-scale hybrid PlanetLab-Internet measurement. We find that even in the most popular Autonomous Systems (ASes), very few individual torrents are able to form large enough local clusters of peers, making state-of-the-art locality mechanisms for individual torrents quite inefficient. Inspired by peers' multiple torrent behavior, we develop a novel framework that traces and recovers the available contents at peers across multiple torrents, and thus effectively amplifies the possibilities of local sharing. We address the key design issues in this framework, in particular, the detection of peer migration across the torrents. We develop a smart detection mechanism with shared trackers, which achieves 45 percent success rate without any tracker-level communication overhead. We further demonstrate strong evidence that the migrations are not random, but follow certain patterns with correlations. This leads to torrent clustering, a practical enhancement that can increase the detection rate to 75 percent, thus greatly facilitating locality across multiple torrents. The simulation results indicate that our framework can successfully reduce the cross-ISP traffic and minimize the possible degradation of peers' downloading experiences.
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