Why Tor Is Slow and What We Are Doing About It

Architecture and Background

Why Tor is Slow, and What We're Doing About It Roger Dingledine Many of you have probably tried Tor, and then stopped because you found it too slow. Now that Tor has several hundred thousand users, our original design decisions are showing their age. We need to figure out and deploy some major changes if we want the Tor network to scale up to the million-user mark.

Vulnerability Analysis

Problem #1 is that Tor's congestion control does not work well. We need to come up with ways to let "quiet" streams like web browsing co-exist better with "loud" streams like bulk transfer. Problem #2 is that some Tor users simply put too much traffic onto the network relative to the amount they contribute, so we need to work on ways to limit the effects of those users and/or provide priority to the other users.

Exploitation and Demonstrations

Problem #3 is that the Tor network simply doesn't have enough capacity to handle all the users that want privacy on the Internet. We need to develop strategies for increasing the overall community of relays, and consider introducing incentives to make the network more self-sustaining. In this talk I'll walk through these problems and more: why we think these are the right problems to solve, and how we're solving them.

Remediation and Defensive Strategies

Roger Dingledine is project leader for The Tor Project. The Tor network has grown to over 1500 relays handling traffic for hundreds of thousands of users daily. In the past few years The Tor Project has also gotten an increasingly diverse set of funders, become an official 501c3 nonprofit, and expanded its community of both volunteer and funded developers. In addition to all the hats he wears for Tor, Roger organizes academic conferences on anonymity and security, speaks at industry and hacker cons, and does tutorials on anonymity for national and foreign law enforcement. For copies of the slides and additional materials please see the DEF CON 17 Archive here: