  BitTorrent is a widely-deployed, peer-to-peer file transfer protocol
engineered with a "tit for tat" mechanism that encourages cooperation.
Unfortunately, there is little incentive for nodes to altruistically provide
service to their peers after they finish downloading a file, and what altruism
there is can be exploited by aggressive clients like Bit- Tyrant. This
altruism, called seeding, is always beneficial and sometimes essential to
BitTorrent's real-world performance. We propose a new long-term incentives
mechanism in BitTorrent to encourage peers to seed and we evaluate its
effectiveness via simulation. We show that when nodes running our algorithm
reward one another for good behavior in previous swarms, they experience as
much as a 50% improvement in download times over unrewarded nodes. Even when
aggressive clients, such as BitTyrant, participate in the swarm, our rewarded
nodes still outperform them, although by smaller margins.
