[tor-relays] Exit probability

[Roger Dingledine]

On Sun, Oct 29, 2017 at 03:20:47PM +0100, Sebastian Urbach wrote:
> "Exit" -- A router is called an 'Exit' iff it allows exits to at least one
> /8 address space on each of ports 80 and 443. (Up until Tor version 0.3.2,
> the flag was assigned if relays exit to at least two of the ports 80, 443,
> and 6667.)

Right. And see also my future plans to make it just "80 and 443":
https://bugs.torproject.org/23637

> The Exit probability is 0 without the Exit flag. 200 Exit connections
> without the Exit flag ? Seems ylto be weird...

I don't think it's that weird. You can read more about the goals of the
Exit flag here:
https://trac.torproject.org/projects/tor/ticket/22820#comment:3

but the simple summary here is that the Exit flag, and thus the exit
probability, is about *load balancing*, so other clients can choose
their paths in a way that produces globally useful choices.

That is, think of an exit probability of 0% as saying "Other people
should assume that I am not using any of my bandwidth for exit streams,
so I am fully available for use in other circuit positions", not "there
is no chance that you can exit from my relay".

--Roger