Wednesday, December 22, 2004

Analysis of BitTorrent

The BitTorrent P2P file-sharing system | The Register: "Even though many P2P file-sharing systems have been proposed and implemented, only very few have stood the test of intensive daily use by a very large user community. The BitTorrent file-sharing system is one of these systems. Measurements on Internet backbones indicate that BitTorrent has evolved into one of the most popular networks [8]. In fact, BitTorrent traffic made up 53 per cent of all P2P traffic in June 2004 [12]. As BitTorrent is only a file-download protocol, it relies on other (global) components, such as websites, for finding files. The most popular website for this purpose is

There are different aspects that are important for the acceptance of a P2P system by a large user community. First, such a system should have a high availability. Secondly, users should (almost) always receive a good version of the content they request (no fake files) [10]. Thirdly, the system should be able to deal with flashcrowds. Finally, users should obtain a relatively high download speed.

In this paper we present a detailed measurement study of the combination of BitTorrent and Suprnova. This measurements study addresses all four aforementioned aspects. Our measurement data consist of detailed traces gathered over a period of 8 months (Jun'03 to Mar'04) of more than two thousand global components. In addition, for one of the most popular files we followed all 90,155 downloading peers from the injection of the file until its disappearance (several months). In a period of two weeks we measured the bandwidth of 54,845 peers downloading over a hundred newly injected files. This makes our measurement effort one of the largest ever conducted.

The contributions of this paper are the following: first, we add to the understanding of the operation of a P2P file-sharing system that apparently by its user-friendliness, the quality of the content it delivers, and its performance, has the right mechanisms to attract millions of users. Second, the results of this paper can aid in the (mathematical) modeling of P2P systems. For instance, in the fluid model in [13], it is assumed that the arrival process and the abort and departure processes of downloaders are Poisson, something that is in obvious contradiction with our measurements. One of our main conclusions is that within P2P systems a tension exists between availability, which is improved when there are no global components, and data integrity, which benefits from centralization."

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