Re: [chrony-dev] handling of unstable sources

[Frank Muzzulini <frank.muzzulini@xxxxxxxxxxxx>]

Miroslav Lichvar wrote:
> How do you define reasonable? :)
>
> The root dispersion value in NTP packet has only about 15us
> resolution. Ntpd assumes a fixed frequency error and adds to the
> dispersion 15 us every second since last reference update, but chrony
> uses the estimated skew instead, so with a good reference clock it
> will always transmit zero dispersion. 

My mind must have been twisted by years of contact with ntpd only ;-) 
Thinking about that once more, assuming a fixed error of 15ppm is not 
conservative but arbitrary. It will typically too high, but can also be 
way too low sometimes due to ntpd's reluctance to adapt the frequency 
when not allowed to jump.

I also noticed that even directly after an update the server reports a 
root dispersion of ~0.5ms. I don't know however whether this is a 
general feature of ntpd or it is a configured value trying to reflect 
the potential absolute error of the received signal (DCF77 in that case, 
which unlike GPS has no automatic correction for signal run times -- but 
than 0.5ms would be too less for any place more than 150km from the 
transmitter).

> Anyway, I'd like to rework the source selection function to use also
> some of the calculated statistics instead of just stratum and root
> distance.

Ok, I leave that up to you :)

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