[tor-relays] Authorithy clock skew in consensus health page

[Roger Dingledine]

On Tue, Dec 15, 2020 at 04:43:53AM +0000, torjoy wrote:
> Its just a curiosity question... What is the reference source used in comparsions of authorities clock skew in consensus health?

There are two "consensus health" tools, DepicTor and DocTor:
https://gitweb.torproject.org/depictor.git/tree/
https://gitweb.torproject.org/doctor.git/tree/

And I think if this is the page you're talking about:
https://consensus-health.torproject.org/#authorityclocks
it is generated by DepicTor.

> Are they synced throught NTP daemons (like chrony or opentpd for example)?

The directory authorities each use whatever tools they want to use
for keeping their time accurate. I imagine that yes, many of them use
some sort of ntpd.

> TOR can take advange of very accurate timing in the authorities and relays? I know that just few seconds (or miliseconds?) are fine of clock offset. And if the relay's clock has some fluctuation of hundreds of miliseconds? For example going from -100mS of offset to +300mS offset and them to -200 mS offset...

Correct, clock problems on the order of a second or two are fine.

In fact, I don't think that DepicTor's measurement will be more accurate
than that anyway, because I think the number comes from looking at the
Date stamp in the http header of the response, and comparing it to what
time the local DepicTor script thinks it is. So there will be issues
like network latency that affect it in tiny ways.

> I mean, too much clock noise can be a problem for TOR?

Correct. For directory authorities, they need to synchronize within
a minute or so, because of the timing required for voting about the
consensus documents:
https://spec.torproject.org/dir-spec

Relays can handle some more skew than that, but if they get too skewed
then they will start fetching and serving the wrong directory information.

Hope that helps,
--Roger