The Tor Project improves network speed performance with a new system

The Tor Project has released details of a newly introduced system called Congestion Control that promises to eliminate speed limits on the network.

This new system is operational in version 0.4.7.7 of the Tor protocol, the latest stable version available since last week.

Congestion control “will result in significant performance improvements to Tor, as well as increased utilization of our network capacity,” the project maintainers say.

Tor and Congestion

Tor (The Onion Router) is a volunteer-run overlay network made up of thousands of relays that serve as bounce points for encrypted user network traffic and exit nodes that are essentially the gateways to the public internet.

The mission of the project is to conceal users’ real location and browsing interests, aiming for the ultimate privacy and anonymity of Internet users.

One downside of such a system is slow browsing speeds, which are crippled by traffic congestion on Tor network nodes and queues on exit relays.

Regulating traffic congestion on the Tor network is a challenge without compromising on privacy safeguards. However, after nearly two decades of searching for solutions, the project finally introduced congestion control.

The new system implements three algorithms, namely Tor-Westwood, Tor-Vegas, and Tor-NOLA, which collectively help reduce memory consumption and stabilize and minimize queue delay and 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.

Results and implementation

The Tor Project ran simulations to compare 0.4.6 and 0.4.7, and the results are impressive across the board with smoother and improved browsing, no speed caps or bottlenecks, and no added latency end to end.

Comparison of throughput graphs
Comparison of throughput graphs (Tor)

However, for the wider community to benefit from the improvements, exit relay operators will need to upgrade to version 0.4.7 of the Tor protocol.

Internal Tor node operators do not need to upgrade but will need to set bandwidth limits. This is because traffic patterns will change as congestion control is expected to use relays to their full capacity.

The more customers that switch to version 0.4.7 (or later), the more the increase in performance on the network will become apparent to everyone, but the first results are already noticeable.

Advertised Relay Bandwidth Increase
Advertised Relay Bandwidth Increase (Tor)

“Because our network is about 25% utilized, we expect throughput to be very high for early adopters using 0.4.7 on fast circuits with fast 0.4.7 outputs up to the point where most of customers have upgraded. At this point, a new equilibrium will be reached in terms of throughput and network utilization.”

“For this reason, we are withholding the release of a stable Tor browser with congestion control until enough releases have been upgraded to make the experience more consistent. We hope this will happen by the time May 31” – the Tor Project

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

Kevin M. Risinger