Do incentives build robustness in BitTorrent?

A fundamental problem with many peer-to-peer systems is the tendency for users to “free ride”—to consume resources without contributing to the system. The popular file distribution tool BitTorrent was explicitly designed to address this problem, using a tit-for-tat reciprocity strategy to provide positive incentives for nodes to contribute resources to the swarm. We show that although BitTorrent has been fantastically successful, its incentive mechanism is not robust to selfish clients. Through performance modeling parameterized by real-world traces, we demonstrate that high capacity peers provide low capacity peers with an unfair share of aggregate swarm resources. We use these results to drive the design and implementation of BitTyrant, a selfish BitTorrent client that provides a median 70% performance gain for a 1 Mbit client on real Internet swarms. We further show that this selfishness, when applied universally, can hurt average per-swarm performance compared to today’s BitTorrent client implementations.