[ntp:questions] Re: How to get combined clock offset

David Woolley david at djwhome.demon.co.uk
Sat May 7 17:58:02 UTC 2005

In article <mailman.78.1115371918.573.questions at lists.ntp.isc.org>,
David Lyons <David_Lyons at Raytheon.com> wrote:

> We need to get a measurement of "Time Quality" for our node.  We are using
> NTP Version 3.

Why are you interested in quality but using an obsolete version?

> It seems that the NTP system state variable "theta", the combined clock
> offset, would be the variable we need.

Overall theta is offset in loopinfo (ntpdc) and the individual thetas
are the offsets in the peers displays.  It's not a quality indicator.
If ntpd where confident of the value of theta, it would have corrected
the local clock by that amount, so theta would be reduced until it was
no longer confident in the value.  In fact, I think that there isn't
really an overall theta, but rather theta for the most recently selected
reference clock, at the time it was polled.

What would be a good measure depends on what you mean by quality.
ntpd's main quality measure is root dispersion, which is the worst case
error, assuming that the ultimate reference is a real reference clock and
not someone's local clock (basically it assumes that all the propagation
delay could be in one direction and that the clock has drifted at the
maximum reasonable rate since the last measurement).

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