Please Slow Down! The Impact on Tor Performance from Mobility

The number of mobile devices connecting to the Internet is predicted to surpass desktop connections by 2014. The likely growth in their mobile client base will offer an additional challenge for anonymity networks, such as Tor, in maintaining an efficient privacy service. We have conducted a simple experiment that illustrates this challenge. We have simulated the performance achieved by a mobile Tor node as it roams at varying speeds between wireless networks. The results show that the impact on performance for the mobile user, and potentially the wider Tor network, is significant when roaming, and as expected, increases with higher mobility speeds and longer recovery times. We review a range of solutions and suggest that, although the use of a lighter transport protocol and/or adaptive client throttling may reduce the performance impact of mobility, a better strategy is to provide a persistent connection to the Tor network for roaming mobile users.