Bjorn Scheuermann , Telematics Group University of Würzburg, Germany
Florian Tschorsch , Telematics Group University of Würzburg, Germany
Tor is one of the most popular network anonymization services. With increasing popularity, however, Tor is also faced with increasing load. Mechanisms for handling congestion and fairness in anonymization networks, where user privacy is of greatest significance, are not yet well understood. Thus current designs leave a lot to be desired: gross unfairness and largely suboptimal performance can be observed. In this paper, we focus on fairness aspects in the Tor network. We first show that interactions of multiple scheduling mechanisms in the current Tor design cause heavily unfair resource allocations to users. Subsequently, we develop a fairness model based on max-min fairness that takes the specifics of Tor into account. This leads us to a re-design of Tor's scheduling. We implement the new design in conjunction with a congestion feedback mechanism named N23, which has recently been proposed to be used in Tor. Our scheduling approach overcomes the unfairness problems which are exhibited by today's Tor implementation, and by Tor with N23 as well. It achieves global max-min fairness and thus a fair resource allocation despite selfish end-users.
Bjorn Scheuermann, Florian Tschorsch, "Tor is unfair -- And what to do about it", , vol. 00, no. , pp. 432-440, 2011, doi:10.1109/LCN.2011.6115503