Evaluation of Circuit Lifetimes in Tor

Tor is a popular anonymity network which achieves its anonymity by constructing paths over three Tor relays, so-called circuits. Multiple streams that correspond to TCP connections can be multiplexed over a single circuit. By default, circuits are used for about ten minutes before switching to new circuits. Once that time limit is reached the circuit cannot be used for any new streams. This time-window is called the maximum circuit dirtiness (MCD). This paper analyzes the consequences of changing the MCD for all clients in the network and provides data on how changing the MCD affects various metrics of the Tor network. Our analysis shows that reducing the MCD to a sane value has almost no impact on the clients. Neither performance nor anonymity of the clients are significantly affected by the MCD. On the relays however halving the default MCD reduces the memory usage by about 20% while maintaining the original throughput and no measurable increase in CPU usage. Raising the MCD shows the opposite effect and increases memory usage. By drastically reducing the MCD, a significant number of extra circuits are created. From a performance point of view, the MCD should be reduced. Building on this work, side effects on specific attacks on Tor should be investigated in future work.