A Network Formation Game Approach to Study BitTorrent Tit-for-Tat

The Tit-for-Tat strategy implemented in BitTorrent (BT) clients is generally considered robust to selfish behaviours. The authors of [1] support this belief studying how Tit-for-Tat can affect selfish peers who are able to set their upload bandwidth. They show that there is a “good” Nash Equilibrium at which each peer uploads at the maximum rate. In this paper we consider a different game where BT clients can change the number of connections to open in order to improve their performance. We study this game using the analytical framework of network formation games [2]. In particular we characterize the set of pairwise stable networks the peers can form and how the peers can dynamically reach such configurations. We also evaluate the loss of efficiency peers experience because of their lack of coordination: we find that the loss of efficiency is in general unbounded despite the utilization of the Tit-for-Tat strategy.