[time-nuts] NIST time services
albertson.chris at gmail.com
Mon Mar 24 03:38:44 EDT 2014
On Sun, Mar 23, 2014 at 11:50 PM, Hal Murray <hmurray at megapathdsl.net>wrote:
> albertson.chris at gmail.com said:
> > The assumption NTP makes is that you can judge the quality of a server
> > the variance (of "jitter") in the time it reports.
> I think it's more complicated than that.
> I think it also includes the non-jitter part of the round trip time.
Yes. NTP calls it "root distance" which includes a jitter and delay
component. I think (?) they project jitter and delay on a plane and
compute the distance to a centroid and call that "root distance". But it
does have both components as you say. I just re-read the docs.
The point is that NTP's assumption that one way equals 1/2 the round trip
if wrong causes some inaccuracy. I think you are right about a fixed
But if there are other clocks that are better they will cluster and the one
with the asymmetry will be removed from the set that is used. The offset
will cause it to be an out layer
NTP is pretty good at detecting bad clocks if it has enough clocks to
compare. I always like to use at least five.
> assumes the path is symmetric. Any constant asymmetry will turn into an
> apparent fixed offset. I think ntpd is smart enough to include that in
> calculations of the clock quality, but I don't understand the details.
> These are my opinions. I hate spam.
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