Free Riding in BitTorrent and Countermeasures

We show that, contrary to common belief, free riding is indeed possible in BitTorrent. We present a BitTorrent client implementation in Java which successfully downloads shared files without ever contributing a single byte of user data to the network. Surprisingly, this client often achieves the same download speed as the original BitTorrent client. This client is of particular interest for people living in countries where sharing of copyrighted material is forbidden by law but acquiring it is legal, no matter from what source. We also present possible modifications of BitTorrent to effectively reduce free riding. The key concept is a strict tit for tat algorithm paired with ideas from network coding. The new scheme results in a robust, scalable and fair peer-to-peer file sharing system.