[tor-relays] Question about authority clock skew

Sebastian Hahn sebastian at torproject.org
Mon Apr 13 22:30:41 UTC 2020


Hi Luiz,


> On 13. Apr 2020, at 15:41, torjoy <south_america_bridges at protonmail.com> wrote:
> I was browsing the "Consensus health" page and something let me curious... What is the importance of the clock skew in the authorities with the resolution of microseconds? 
> <Imagem colada-Mon Apr 13 2020 103833 GMT-0300 (Horário Padrão de Brasília).png>
>                                                Picture: 2020/04/13 - 13:37 UTC
> 
> Also, how the authorities compare their clocks? Using ntp daemon for example? I'm asking this here because I work in a time and frequency reference lab and have two NTP (stratum 1, connected directly to UTC(LRTE) timescale. And a stratum 2 that is discilplinned with the stratum 1 timeserver). So considering the microssencond accuracy did the keepers of these authorities care about the time source? Or any time source is ok? 

I operate the directory authority gabelmoo. We do not synchronize our clocks amongst ourselves and accuracy of +/- a few seconds is really not important. Every operator does their best to keep the time roughly correct individually, for example gabelmoo uses a site-local NTP server that gets physical time from the German government's time broadcasting service.

The page you're referring to just shows the skew with a lot more digits than can actually be accurately measured. The reason to keep clock skew in check is that if the time differs by more than a few seconds, the voting process can get impacted which in the past has led to consensuses not being created even though enough directory authorities would theoretically be ready for it. Also some directory authorities had some historical trouble with keeping an accurate time due to virtual machine trouble - this was worrisome for relay operators, because they would get (wrong) warnings about a wrong time in their logfile.

Hope that helps
Sebastian


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