Re: [chrony-users] Chrony source selection algorithm documentation

[Henrik Bieler <henrik.bieler@xxxxxx>]

Hey Miroslav,

thank you for your input. This works indeed. :-)
When chrony has selected the GPS and the stratum 1 ntp servers come
online, chrony selects one of them.
So far I could not yet observe chrony switching back to gps when the
servers go offline. The servers are still shown in "chronyc sources" as
selected. But maybe I just need to give it more time.

Thank you!
Henrik


Am 11.03.2016 um 08:59 schrieb Miroslav Lichvar:
> On Thu, Mar 10, 2016 at 07:58:09PM +0100, Henrik Bieler wrote:
>> I still would like to know a way of preferring the ntpd over the gps
>> when they are online. And switching back to gps when they go offline.
>>
>> Maybe someone can explain how chrony selects sources or point to
>> documentation.
> 
> What stratum do the NTP servers have? My guess is that the NTP sources
> are considered as unsynchronized for having stratum larger than the
> local clock. This is a requirement of the NTPv3 protocol which
> prevents synchronization loops (see "test 7" in RFC 1305). It was
> later removed in NTPv4 (implemented in chrony >= 2.0), which made it
> more vulnerable to synchronization loops, but allows synchronization
> to sources with larger stratum.
> 
> In your case, if chronyd (version 1.30) selected the GPS, it is
> stratum 1 and will ignore sources with stratum 2 or larger for a
> possibility they are directly or indirectly synchronized to it. You
> can check the measurements log to see if TEST7 is failing.
> 
> In order to outvote the GPS source as a falseticker you would need at
> least two stratum 1 servers. Considering that less than 10% of the
> pool.ntp.org servers are stratum 1, I think it's quite unlikely to
> happen. My suggestion would be to use two or three stratum 1 servers
> with open access from the list of servers maintained at the NTP
> website instead of the pool.
> 
> http://support.ntp.org/bin/view/Servers/StratumOneTimeServers
> 


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