[ntp:questions] Server offset included in served time?

Unruh unruh-spam at physics.ubc.ca
Tue Sep 16 16:58:46 UTC 2008

David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:

>Unruh wrote:
>> Mike K Smith <mks-usenet at dsl.pipex.com> writes:

>>> So I can't plot the offsets in the loopstats file to show how accurate
>>> the time was on an NTP client over a period of time?
>> That is the best estimate that you have. Alternatively buy a GPS hook it up
>> and measure the offset against that. 

>But, if you use individual measurements, you will get a figure that, 
>most of the time, is several times the true error and not necessarily in 
>the right direction.

No idea what that sentence means. Are you refering to the gps readings
which will be at worst 2 orders of magnitude better than that offsets from
a generic network? 

>What you can do, is to use some hindsight, and make a slightly better 
>estimate of the true time by combining offsets from both before and 
>after the time the local clock was read.  That gives you an advantage 

Unfortunately those offsets are not very useful, because the clock that
read them has had its offsets and rates changed (by ntp) since then. Ie the
measured offsets are not a good estimate of the offsets from "true time".

>during startup and after transients, but, in the steady state, the local 
>clock time is going to be the same as such an enhanced statistic.

>In the steady state, all you can really deduce from the offsets is the 
>amount of noise in the measurements.  You can then expect the amount of 
>noise in the local clock time to be several times less.

No, the noise in the local clock may well dominate those offsets. This is
what happens for example to a system which is controlled by a hardware
refclock. The noise is mainly the temperature dependence of the local clock,
not the measurement noise. See www.theory.physics.ubc.ca/chrony/chrony.html
and the measurements for string, a GPS/ntp controlled machine. Much of the
noise is rate noise from the daily variations in temperature. 

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