Exit Balancing Patch

Roger Dingledine arma at mit.edu
Wed Jul 18 09:25:55 UTC 2007

On Wed, Jul 18, 2007 at 02:18:56AM -0700, Mike Perry wrote:
> >  We do still
> > hear on or-talk from people running fast Tor servers on slow CPUs that
> > are bottlenecked by AES, though. Hm.
> Why does it matter if routers are CPU or network bound? From the Tor
> network routing point of view, it shouldn't matter, capacity is
> capacity. If it is a problem for node operators, they limit their
> bandwidth in the config, problem solved. If they don't, then they just
> run at 100% CPU, and Tor should still properly report their observed
> bandwidth rate (unless they lie, but again, that is a another,
> orthogonal matter).

Well, the problem is that Tor in fact doesn't properly report capacity at
the extremes. Our measure of capacity is the most you've seen yourself
burst in the past day -- it pretty much assumes that the pipe and other
resources you have are static throughout the day.

So if you somehow managed to push a lot briefly but you're too busy to
handle that level of traffic sustained, then you've overadvertised.

Putting a cap on advertised bandwidth when load balancing is a crude way
to account for this. Making our bandwidth estimate more complex may also
work, but then we have to figure out what's better. :)


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