[tor-talk] Reasoning behind 10 minute circuit switch?

[Greg Norcie]

Thanks for the detailed reply!

On 10/14/14, 5:10 PM, Roger Dingledine wrote:
> On Tue, Oct 14, 2014 at 12:17:27PM -0400, Greg Norcie wrote:
>> I'm working on doing a study on user tolerance of delays (for
>> example, latency on Tor).
>>
>> During our discussion, a bit of a debate occured about the TBB's
>> circuit switching. I was wondering if there's any research that's
>> been done to arrive at the 10 minute window for circuit switching,
>> or if that was number picked arbitrarily?
>
> It was alas picked arbitrarily. As Nick notes, it used to be 30 seconds,
> and then when we started getting users, all the relays complained of
> running at 100% cpu handling circuit handshakes. We changed it to 10
> minutes, and the complaints went away -- at least until the botnet
> showed up.
>
> We've had an open research question listed for years now -- see bullet
> point 4 on
> https://research.torproject.org/ideas.html
>
> """
> Right now Tor clients are willing to reuse a given circuit for ten
> minutes after it's first used. The goal is to avoid loading down the
> network with too many circuit creations, yet to also avoid having
> clients use the same circuit for so long that the exit node can build a
> useful pseudonymous profile of them. Alas, ten minutes is probably way
> too long, especially if connections from multiple protocols (e.g. IM and
> web browsing) are put on the same circuit. If we keep fixed the overall
> number of circuit extends that the network needs to do, are there more
> efficient and/or safer ways for clients to allocate streams to circuits,
> or for clients to build preemptive circuits? Perhaps this research item
> needs to start with gathering some traces of what requests typical
> clients try to launch, so you have something realistic to try to
> optimize.
> """
>
> Also note that if a stream request times out (or for certain similar
> failures), you move to a new circuit earlier than the 10 minute period.
> So it might be that users actively browsing will switch much more often
> than every 10 minutes. Somebody should study what happens in practice.
>
> The future plan is to isolate streams by domain, not by time interval:
> https://trac.torproject.org/projects/tor/ticket/5752
> But of course there are some tricky engineering and security
> considerations there.
>
> And lastly, see
> https://trac.torproject.org/projects/tor/ticket/5830
> for a standalone related analysis/research project that I wish somebody
> would do. :)
>
> --Roger
>