Abstract

In this paper, we examine the factors that contribute to the variability in download time of a self-organizing P2P file distribution application such as BitTorrent. We conducted a series of side-by-side live experiments, involving two clients running on the same machine downloading the same file at the same time. We found that the download latency varied significantly, sometimes by a factor of 2. Surprisingly, the main contributing factor isn't the network bandwidths of the set of neighbors that a client is given. Rather, it has to do with the frequency of turn-overs in "close" neighbors, i.e. those that are in a stable data-exchange relationship with the client. Analysis of the log data shows that a client obtains over 90% of the file from a small set of close neighbors, and if a close neighbor leaves the network, it takes the client a long time, over half an hour, to find another one. This suggests that self-organizing P2P file distributions indeed need external help in order to provide QoS guarantees, but such guarantees are achievable with proper enhancements to the P2P network.