FAQ: How do I throttle the bandwidth my Tor server uses?

[Roger Dingledine <arma@mit.edu>]

I just posted this on http://wiki.noreply.org/wiki/TheOnionRouter/TorFAQ
(It applies to clients too, though fewer people will need to throttle
their clients, I'd guess.)

== How do I throttle the bandwidth my Tor server uses? ==

As of the 0.0.8 release, clients choose servers proportional to their
advertised bandwidth. They consider both the bandwidth rate you've
configured, and the bandwidth capacity that your Tor server passively
estimates as it operates.

The BandwidthRate configuration parameter defaults to 800kB/s, which
is probably more than most people will use in practice. So the measured
capacity is going to be the limiting factor for those situations. Fast
servers tend to measure between 200kB/s and 600kB/s, and DSL lines tend
to measure about 90kB/s.

One way to do bandwidth throttling for Tor is to take advantage of
external systems, e.g. your operating systems's traffic shaping (Linux
has 'tc', I'm sure others have others). Tor will automatically detect
a lower capacity and scale back the attention you get from Tor clients.

Another way is to set your BandwidthRate lower in the Tor config. If you
want to handle about x gigabytes per day max, set it to x*10000. In this
case it would be polite to set your BandwidthBurst to something large
like fifty million bytes (50000000), so the throttling only actually
kicks in when the demand is consistently high.

We're working on algorithms to be able to handle statements like "I want
to use 30 gigabytes per month but no more", but you shouldn't expect
those until a later release.