Of the many peer-to-peer (P2P) systems in existence, BitTorrent is one of the most appealing that has managed to attract millions of users in the past few years. In this paper we present an extensive study of BitTorrent availability through measurement and analysis. Previous studies are limited to tracker without distributed hash tables (DHT), which have been used widely already. We first discuss the difference of measurement methods, and then develop a new parallel method based on threshold with bounding measurement errors. Afterwards, we focus on three issues: the availability of tracker and DHT, the availability of pieces and the variability of availability over time. Our analysis provides several new findings: (1) Overall dynamics of peers are similar across different index. (2) Compared to tracker, DHT does not have better performance as expected. (3) Seeds contribute most of piece replicas. When they are of?ine, the availability of pieces will decrease dramatically. (4) The replicas of all pieces are almost the same without very rare pieces and the distribution of pieces is equal and effective. (5) The variability of availability shows a typical life cycle pattern over time, which means it is difficult for users to obtain files in the latter half of stage. In summary, this paper advances our understanding of availability by comparing different index, exploring the distribution of pieces and analyzing the ?uctuation of availability.
['Lin Ye', 'Hongli Zhang', 'Weizhe Zhang', 'J. Y. Tan']
Measurement and Analysis of BitTorrent Availability