LASTor: A Low-Latency AS-Aware Tor Client


The widely used Tor anonymity network is designed to enable low-latency anonymous communication. However, in practice, interactive communication on Tor—which accounts for over 90% of connections in the Tor network [1]—incurs latencies over 5x greater than on the direct Internet path. One source of delays is propagate delay due to round about path on Tor which is shown in figure 1. In addition, since path selection to establish a circuit in Tor is oblivious to Internet routing, anonymity guarantees can breakdown in cases where an autonomous system (AS) can correlate traffic across the entry and exit segments of a circuit. For example in figure 2, AS2 can de-anonymiyze the path by profiling the traffic.


LASTor Overview

Both of these shortcomings in Tor can be addressed with only client-side modifications, i.e., without requiring a revamp of the entire Tor architecture. To this end, we design and implement a new Tor client, LASTor. First, LASTor can deliver significant latency gains over the default Tor client by simply accounting for the inferred locations of Tor relays while choosing paths. Second, since the preference for low latency paths reduces the entropy of path selection, we design LASTor’s path selection algorithm to be tunable. A user can choose an appropriate tradeoff between latency and anonymity by specifying a value between 0 (lowest latency) and 1 (highest anonymity) for a single parameter. Lastly, LASTor uses an efficient and accurate algorithm to identify paths on which an AS can correlate traffic between the entry and exit segments. This algorithm enables LASTor to avoid such paths and improve a user’s anonymity, while the low runtime of the algorithm ensures that the impact on end-to-end latency of communication is low.


   Round-about circuit

Common AS (AS2) on entry and exit segments

When using LASTor to visit the top 200 websites from several geographically-distributed end-hosts, we see that, in comparison to the default Tor client, LASTor reduces median latencies by 25% while also reducing the false negative rate of not detecting a potential snooping AS from 57% to 11%.



Masoud Akhoondi
Curtis Yu
Harsha V. Madhyastha



LASTor: A Low-Latency AS-Aware Tor Client. [ pdf ]
Masoud Akhoondi, Curtis Yu, and Harsha V. Madhyastha.
Proceedings of IEEE Symposium on Security and Privacy (Oakland'12), San Francisco, CA, May 2012.



 LASTor will be available for download soon. If you have any questions, please send email to Masoud.


[1] D. Mccoy, K. Bauer, D. Grunwald, T. Kohno, and D. Sicker, “Shining light in dark places: Understanding the Tor network,” in PETS, 2008.