[tor-relays] write-history for exit relays only?

[teor]

> On 7 Sep 2016, at 06:36, Philipp Winter <phw at nymity.ch> wrote:
> 
> On Tue, Sep 06, 2016 at 12:10:06PM -0400, Aaron Johnson wrote:
>>> I suspect that one could approximate this number by accounting for the
>>> probability of all exits being selected as guard, middle, and exit, but
>>> I would prefer a simpler and more reliable approach.
>> 
>> This doesn’t seem like a bad approximation to me, given that for as
>> long as I have been aware, exits have had zero probability of being
>> chosen in any position other than the exit position.
> 
> Thanks, Aaron.  You are right.  Section 3.8.3 in dir-spec has the answer:
> <https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2611>
> 
> I just proved this to myself with the small attached Python script.
> Currently, exit bandwidth is the network's scarce resource, which is not
> surprising since running an exit is riskier than running a guard or a
> middle.  Since exits are scarce, the bandwidth weights in case 3,
> subcase A are currently in place:
> <https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2726>
> 
> In that case, the specification hard-codes the probability of an exit
> taking on a non-exit role (Wgd, Wmd, and Wme) to 0.
> <bandwidth-weights.py>

It's also worth noting that Exits will serve directory documents and hidden service descriptors, and act as introduction and rendezvous points, so your estimates could be a few percentage points off.

Tim

Tim Wilson-Brown (teor)

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