The Delicate Tradeoffs in BitTorrent-like File Sharing Protocol Design

The BitTorrent (BT) file sharing protocol is popular due to its scalability property and the incentive mechanism to reduce free-riding. However, in designing such P2P file sharing protocols, there is a fundamental "tussle" between keeping peers, specially the more resourceful ones, in the system for as long as possible to help the system achieve better performance and allowing peers finish their download as quickly as possible. The current BT protocol represents only "one" possible implementation in this whole design spectrum. In this paper, we characterize the "complete" design space of BT-like protocols. We use fairness index to measure the fairness that incorporates the contribution peers make. We show that there is a wide range of design choices, ranging from optimizing the performance of file download, to optimizing the fairness measure. More importantly, we show that there is a simple and easily implementable design knob which can be used to choose a particular operating point in the design space. We then discuss different algorithms (centralized versus distributed) in realizing the design knob. We also carry out performance evaluation to quantify the merits and properties of the BT-like file sharing protocols.