Anarchy in Tor: Performance Cost of Decentralization

Like many routing protocols, the Tor anonymity network has decentralized path selection, in clients locally and independently choose paths. As a result, network resources may be left idle, leaving the system in a suboptimal state. This is referred to as the price of anarchy, where agents acting in their own self interest can make poor decisions when viewed in a global context. In this paper we explore the cost of anarchy in Tor by examining the potential performance increases that can be gained by centrally optimizing circuit and relay selection using global knowledge. In experiments with both offline and online algorithms, we show that centrally coordinated clients can achieve up to 75% higher bandwidth compared to traditional Tor. Drawing on these findings, we design and evaluate a decentralized version of our online algorithm, in which relays locally distribute information enabling clients to make smarter decisions locally and perform downloads 10-60% faster. Finally, we perform a privacy analysis of the decentralized algorithm against a passive and active adversary trying to reduce anonymity of clients and increase their view of the Tor network. We conclude that this decentralized algorithm does not enable new attacks, while providing significantly higher performance.