Reduce RTT for preemptively built circuits

by ra for Tor

Round-Trip-Times (RTT) of circuits can be measured by violating the exit policy of exit nodes by sending a "STREAM CONNECT" request to 127.0.0.1. Since the exit policy denies this connection, the request results in an error that can be timed in a measuring client. It is assumed that the RTTs are Fréchet-distributed and its tail can be approximated to be Pareto-distributed. This information can be used to reject a preemptively built circuit if its RTT is below a certain threshold value. It will be checked if the above assumption is valid and if performance of circuits can be increased with this approach with only a minor decrease in anonymity.