Tor’s congestion control feature promises improved performance

The Tor Project released a new version of Tor, version 0.4.7.7, to the public on May 4, 2022. Tor 0.4.7.7 is the first version of the service to support congestion control, a new feature designed to reduce latency and eliminate speed limits on Tor.

For users to benefit from the new features, outbound relay operators must update the installed Tor version to the new version.

Congestion control “is an adaptive property of distributed networks, whereby a network and its endpoints operate in such a way that utilization is maximized, while a constraint property is minimized” according to the release information.

Tor’s primary mission is to provide Internet users with tools to fight censorship, tracking, and surveillance. Users from all over the world can download Tor Browser to connect to the network. One of its core mechanics is that traffic is routed through a series of hops. Each hop knows about the previous hop and the next hop, but no individual hop knows the source and destination of the user’s request.

Traffic congestion has long been a problem on the network. While things have improved since Tor’s inception, with extreme wait times of up to a minute for page load requests, it has become clear that a lack of congestion control was slowing down the network for its users.

Maintainers point out that Tor’s cryptographic design prevented it from using mechanisms like packet dropping or reordering to deal with traffic congestion. According to the project, “nearly two decades” of research resulted in “a very small class of candidate algorithms to consider.” Of these three, the maintainers chose Tor-Vegas, as the other candidates “exhibited ack compression”, which “caused them to grossly overestimate the Bandwidth-Delay product, leading to runaway congestion conditions”.

Testing confirmed that Tor-Vegas removes the speed limit of previous versions of Tor without affecting end-to-end latency. The entire Tor network will work at full capacity, once the Tor servers have been updated to the new version. Only output notes need to be updated for this effect, but dead-end relay operators may notice increased bandwidth usage.

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Tor users should notice performance improvements once most exit nodes have been upgraded to the new release version. Users of Brave Browser, which supports Tor, should also benefit from this improvement.

Now you: have you used tor? Did you experience slowdowns when using the network?

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