Yoran Heling graduates on peer selection in Direct Connect

by Yoran Heling

In a distributed Peer-to-peer (P2P) system such as Direct Connect, files are often distributed over multiple source peers. It is up to the downloading peer to decide from how many and from which source peers to download the particular file of interest. Biased Random Period Switching (BRPS) is an algorithm, implemented at the downloading peer, that determines at what point to download from which source peer. The number of source peers that a downloading peer downloads from at a certain point is called the Degree of Parallelism (DoP). This research focussed on implementing BRPS in an existing Direct Connect client and comparing the downloading performance against an unmodified client. Two implementations of BRPS in Direct Connect have been made. A simple implementation that follows the original BRPS algorithm as closely as possible, with minor modifications that were required to ensure that the downloading process would not get stuck on an unavailable source peer. An improved implementation has also been made with slight modifications to the original BRPS algorithm. The improved implementation incorporates two improvements to ensure that the DoP does not drop below its desired value in the face of unavailable source peers.

The original client and the two BRPS implementations have been evaluated in a controlled Direct Connect network with 50 downloading peers and a variable number of source peers. The source peers have been configured to throttle their available bandwidth to an average of 500 KB/s, and following a realistic bandwidth distribution based on measurements from the Tor P2P network. The experiments consisted of all downloading peers downloading the same file at the same time, and taking measurements on the side of these downloading peers. Four experiments have been performed, with one varying parameter in each experiment. The size of the file being downloaded was varied between 100 MB and 1024 MB in the first experiment, the second experiment varied the DoP between 1 and 15. The number of source peers was varied between 10 and 100 in the third experiment, and in the last experiment between 0% and 80% unavailable source peers were added to the network.

In all experiments, both BRPS implementations performed close to the optimal average download time, and were consistently faster than the original client by a factor of 2 to 5. In the last experiment, the improved BRPS implementation did keep the measured DoP closer to its desired value than the simple implementation, but this has not resulted in a significant difference in the measured download times.

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