Abstract
Peer-to-Peer (P2P) networks can reduce the distribution cost of large media files for the original provider of the data significantly. Thereby, the BitTorrent protocol is widely used in the Internet today. Most research work studies the protocol analytically, by simulations at the flow-level or real world experiments. For flow-level simulations the influence of neglecting packet-level characteristics (e.g. TCP) is not yet quantified. Therefore, this paper compares packet-level simulation results with flow-level values and analytically derived bounds. Our findings show that BitTorrent is near to optimal at flow-level for different scenarios. Naturally, packet-level results deviate more from the optimal values but differences are at most around 30% in our simulations. Furthermore, we show that the propagation delay can significantly influence the peer selection in BitTorrent and the download performance of the peers. Hence, the unchoking/choking algorithm in BitTorrent exploits implicitly network proximity.
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