The Deficit-based Distribution Algorithm in Bit Torrent Swarms for Peer-to-Peer File Sharing Allocation

Peer-to-peer file-sharing claims suffer from a fundamental problem of unfairness. Free-riders cause slower download times for others by contributing little or no upload bandwidth while consuming much download bandwidth. Previous attempts to address this fair bandwidth allocation problem suffer from slow peer discovery, inaccurate predictions of neighboring peers’ bandwidth allocations, underutilization of bandwidth, and complex parameter tuning. We present Fair Torrent, a new deficit-based distributed algorithm that accurately rewards peers in accordance with their contribution. A Fair Torrent peer simply uploads the next data block to a peer to whom it owes the most data as measured by a deficit counter. Fair Torrent is resilient to exploitation by free-riders and strategic peers, is simple to implement, requires no bandwidth over allocation, no prediction of peers’ rates, no centralized control, and no parameter tuning. We implemented Fair Torrent in a Bit Torrent client without modifications to the Bit Torrent protocol and evaluated its performance against other widely used Bit Torrent clients. Our results show that Fair Torrent provides up to two orders of magnitude better fairness, up to five times better download times for contributing peers, better performance on average in live Bit Torrent swarms.