[tor-relays] "Fast" flag definition

[Igor Mitrofanov]

Thanks Tim and Roger.

I filed https://trac.torproject.org/projects/tor/ticket/24046.

(Not looking for any immediate fixes, just making sure those "Fast" 1
KB/s relays that you can see on blutmagie.de and the 100 KB/s
threshold are not at odds with whatever minimal user experience Tor
wants to provide).

On Sun, Oct 29, 2017 at 5:47 PM, Roger Dingledine <arma at mit.edu> wrote:
> On Sun, Oct 29, 2017 at 04:21:10PM -0700, Igor Mitrofanov wrote:
>> It looks like 94.7% of all Running relays have the "Fast" flag now. If
>> that percentage becomes 100%, the flag will become meaningless.
>> What were the reasons behind the current definition of "Fast", and are
>> those still valid? If not, should "Fast" become self-adjusting
>> ("faster than 2 Mbps or 70% of all Guard relays, whichever is
>> greater")?
>
> The goal of the Fast flag is to have some minimum threshold for whether
> a relay is useful at all.
>
> It actually is self-adjusting, in that it gets assigned to the top 7/8ths
> of the relays. But *also* it gets assigned to any relay that meets some
> minimum bandwidth threshold:
> https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2408
>
> The "give it to them anyway if they're above the threshold" is a security
> defense, else some jerk could sign up a whole lot of crummy relays,
> driving other legitimate relays out of the network, thus making Sybil
> attacks more effective.
>
> And the threshold is currently quite low -- 100KBytes/s.
>
> (It's actually more complicated than that, because some directory
> authorities assign it based on their own measurements ("I must be voting
> a consensus weight of at least 100 for this relay"), and others assign
> it based on the relay's self-reported number ("The relay must be claiming
> at least 100KBytes/s of capacity").)
>
> So think of Fast more as "worth using at all", where if you don't have
> the flag, you don't have a chance of being chosen, and if you do, then
> the consensus weights kick in to shift traffic towards bigger relays.
>
> --Roger
>
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