Team Incentives in BitTorrent Systems

Although the popular BitTorrent protocol strives to limit free-riding via its tit-for-tat incentives, recent research efforts have shown that it does not strictly enforce fairness. Freeriding opportunities indeed exist, mainly via optimistic unchokes, a BitTorrent mechanism that facilitates the continuous discovery of better peers to interact with. Our results in this work also show that increasing numbers of free-riders can considerably hurt the performance of compliant peers. In an effort to address this problem, this paper proposes a new BitTorrent-like protocol that dynamically organizes peers of similar upload bandwidth in teams— groups of peers collaborating for mutual benefit. Team members mostly satisfy their data download needs inside their team and only perform optimistic unchokes when absolutely necessary. As a result, this team-based protocol improves peer performance via explicit cooperation. At the same time, it limits bandwidth spent on optimistic unchokes, thereby rendering the system more robust against free-riders. We have modified an existing BitTorrent implementation to implement the team protocol and have evaluated its impact by running real experiments on a controlled PlanetLab testbed. Our results show that the protocol enables slightly improved performance for compliant peers, while free-riders are unable to sustain high download rates, as compared to regular BitTorrent. In addition, we observe a higher degree of robustness: increasing numbers of free-riders in the system have a significantly lower negative impact on the performance of contributing peers.