mTor: A multipath Tor routing beyond bandwidth throttling

One of the main obstacles that impede further expansion of Tor, the most popular anonymous communication system, is its large performance variance. The problem becomes worse when bandwidth-intensive applications, such as video streaming, contend with latency-sensitive applications, such as web browsing, for the scarce resources. Most of the existing solutions involve circuit-scheduling techniques to prioritize interactive traffic over bulk traffic or to completely throttle traffic of bandwidth-intensive applications. However, these approaches not only rely on accurate detection of traffic types but also adopt detection strategies that are easy to game. In this paper, we propose a different approach by exploring new capabilities of Tor to support bulk data transfers without degrading the performance of interactive traffic. Based on our observations that a large portion of low-bandwidth relays are under-utilized, we develop a multi-path Tor (mTor) routing algorithm to cater to bandwidth-intensive applications by constructing multiple circuits across low-bandwidth Tor relays. We present a self-adaptive “pulling” scheduling technique to dynamically allocate cells across multiple circuits, and an active congestion detection scheme to prevent slow circuits from becoming a bottleneck of the entire tunnel. Based on the results from experiments on the live Tor network and simulations over the Shadow simulator [1], we conclude that mTor not only achieves a desirable performance for bandwidth-intensive applications by utilizing multiple low-bandwidth relays, but also benefits latency-sensitive applications by reducing the load on high-bandwidth relays.