[ntp:questions] Single best NTP status indicator of clock accuracy?

unruh unruh at wormhole.physics.ubc.ca
Fri Dec 11 19:36:56 UTC 2009

On 2009-12-11, David Woolley <david at ex.djwhome.demon.invalid> wrote:
> unruh wrote:
>> Offset. That is ntp's best guess as to the difference between the time
>> on your clock and the true UTC time. ntp keeps no record of the rate
> No.  It is the difference between the time on your clock and the last 
> measurement from other clocks.  It includes the instantaneous 
> measurement error, but not systematic measurement error.  When ntpd is 
> operating in a stable environment and locked onto its sources, the 
> instantaneous offset readings can vary widely and the RMS value will be 
> quite a bit larger than the RMS variation in error in the actual machine 
> clock.

He asked for the best estimator of difference. And the measured offset
is it. Yes, there are sources of error, and you cannot know that those
have not affected your time, but if asked what is the difference in time
between your system and the remote system, that measured offset, using
the ntp protocol, is the best estimator of that difference. You have
nothing else from ntp. Now, one could argue that the approach used by
chrony-- doing a linear fit to the the past number of offset
measurements is a better estimator, and I would agree. But that is NOT
how ntp works. The only estimator it has is that offset, and it is thus
the best estimator.

> The big argument is about how often ntpd is actually operating in a 
> stable environment. When it is out of lock, offset approximates the 
> short term part of the error, but still does not include the systematic 
> error.

Agreed. But it is the only estimator you have. You might want more, but
ntp does not give it to you. 

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