Out-of-the-box Multipath TCP as a Tor Transport Protocol: Performance and Privacy Implications

The transport design of Tor - the most popular anonymization network - has been identified as a key factor responsible for its performance unfairness. In Tor, traffic from multiple users is multiplexed in a single TCP connection between two relays. While this has positive effects on privacy, it negatively influences performance and is characterized by unfairness as TCP congestion control gives all the multiplexed Tor traffic as little of the available bandwidth as it gives to every single TCP connection that competes for the same resource. To counter this, we propose to use multipath TCP (MPTCP). It allows for better resource utilization and increases throughput of the Tor traffic to a fairer extent. Our evaluation in realworld settings shows that using out-of-the-box MPTCP leads to 15% performance gain. We analyze the privacy implications of MPTCP in Tor settings and discuss potential threats and mitigation strategies.