Throttling Tor Bandwidth Parasites

Abstract

Tor's network congestion and performance problems stem from a small percentage of users that consume a large fraction of available relay bandwidth. These users continuously drain relays of excess bandwidth, creating new network bottlenecks and exacerbating the effects of existing ones. Attacking the problem at its source, we present the design of three new algorithms that throttle clients to reduce network congestion and increase interactive client performance. Unlike existing techniques, our algorithms adaptively adjust throttling parameters given only information local to a relay. We implement our algorithms in Tor and compare significant client performance benefits using network-wide deployments of our algorithms under a variety of network loads. We also analyze the effects of throttling on anonymity and compare the security of our algorithms under adversarial attack. Software patches for our algorithms will be submitted to Tor.