[ntp:questions] Fwd: Re: Best ways to get the reference times from ntp

Miroslav Lichvar mlichvar at redhat.com
Mon May 12 09:02:15 UTC 2014


On Wed, May 07, 2014 at 07:40:28PM +0000, William Unruh wrote:
> On 2014-05-07, mike cook <michael.cook at sfr.fr> wrote:
> > Le 7 mai 2014 ? 18:32, William Unruh a ?crit :
> >> The short answer is no, ntpd cannot play this game. You are trying to
> >> use A to discipline not only B but C as well but on machine B.
> >
> > My reading is that C is not being disciplined at all, but is to be used a reference (though non UTC) for B.
> 
> That is my reading as well. But something must be done to determine
> those values of x and y (Ctime= xT+y where T is UTC). Either that can be
> done on C using something like chrony (better) or ntpd, or B could run
> something to determine x and y for C and use those to help discipline B. 

The OP said the frequency offset of C is known, so only y is unknown
if I'm reading it right. But he also said that A and C are in the same
network, so I'm not sure if the frequency of C can be tranferred to B
with better accuracy than the frequency of A and if this idea of using
A to estimate offset and using C to estimate frequency can give better
results than just simply increasing the rate of polling of A.

I think support for frequency only sources wouldn't be very difficult
to add to chrony. Add a new selection option to bypass the selection
algorithm and just combine its frequency with other sources by
estimated skew. This could work with both NTP sources and reference
clocks.

-- 
Miroslav Lichvar


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