Tor is about to get faster: New version of Tor introduces congestion control feature, promises improved performance

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

In order for users to benefit from the new feature, exit relay operators will need to upgrade their installed versions of Tor to the new version.

According to an announcement posted on the Tor Project's blog, congestion control "is an adaptive feature of distributed networks, where the network and its endpoints operate in a way that maximizes utilization while minimizing constraint characteristics. It promises to "significantly improve Tor's performance, as well as improve the utilization of our network capacity.

The Onion Router, or Tor, is free, open-source software that enables anonymous communication, and Tor's primary mission is to provide Internet users with tools to combat censorship, tracking and surveillance. Users from all over the world can download the Tor browser to connect to the Web. One of its core mechanisms is that traffic is routed through a series of hops, with each hop knowing the previous and next hop, but no hop knowing the source and destination of a user's request.

Traffic congestion has long been a problem on the Tor network. While the situation has improved since Tor's early days, with extreme wait times of up to a minute for page load requests, it is clear that the lack of congestion control slows down users' network access.

Maintainers noted that Tor's encrypted design prevented it from using mechanisms such as packet loss or reordering to deal with traffic congestion. According to the project, "nearly 20 years" of research has resulted in "very few candidate algorithms to consider.

The volunteers maintaining the anonymous browser have been searching for a solution for nearly 20 years (Tor was first introduced in 2002), and now they believe they have found one, in the form of three new algorithms, Tor-Westwood, Tor-Vegas and Tor-NOLA, which together help reduce memory consumption and stabilize and minimize queueing latency.

Tor Westwood - minimizes packet loss in large pipes
Tor-Vegas - estimates queue length and introduces balancing elements
Tor-NOLA - works as a bandwidth-delay estimator.

Of the three algorithms, the maintainers chose Tor-Vegas because the other candidates "exhibit ack compression" which "causes them to grossly overestimate the bandwidth-delay product, which leads to uncontrolled congestion", while Vegas estimates the queue length and balances it.

Tests confirmed that Tor-Vegas removes speed limits from earlier versions of Tor without affecting end-to-end latency, and the Tor Project shared simulation results showing the difference in browsing speed and latency between version 0.4.7 and 0.4.6.

Once the Tor server is upgraded to the new version, the entire Tor network will run at full capacity. In order for everyone to benefit from the new system, all exit relay operators will need to upgrade to version 0.4.7. It also said that internal Tor node operators do not need to upgrade yet, but will need to set bandwidth limits.

"Because our network is about 25 percent utilized, we expect that throughput at the first few egress nodes that update to version 0.4.7 first could be very high until most egress nodes are upgraded and a new balance can be reached in terms of throughput and network utilization." The developers said, "For this reason, we are holding off on releasing a stable version of Tor Browser with congestion control until enough exit nodes have been upgraded to make the experience more uniform. We hope this will happen by May 31."

Once most exit nodes are upgraded to the new version 0.4.7, the Tor Project will release a new stable version of the Tor Browser with Congestion Control accordingly, and users should experience improved performance. In addition, users of the Brave browser, which supports Tor, should also benefit from this improvement.

For the next major stable release, version 0.4.8, the Tor Project plans to implement a traffic splitting mechanism to further improve network speeds.