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

Unruh unruh-spam at physics.ubc.ca
Tue Sep 16 17:06:59 UTC 2008

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

>Uwe Klein wrote:

>> Does ntp serve "system" time or "correct" time to clients?

>NTP serves system time, which its algorithms believe to be the best 
>practical estimate of correct time within statistical error.  What we 
>are talking about here is making that assumption more valid during 
>startup transients.

>Basically, as far as NTP is concerned, there is no difference between 
>system time and correct time.  (Earlier versions used to serve a 
>corrected time during slew recoveries, but not during normal operation. 
>  That would not make a difference for the 120ms case discussed here, as 
>that uses the normal control loop.)

Note that this is a difference between ntp and chrony. Chrony uses past
offset measurements ( corrected for local clock rate changes) to make an
estimate from the measured offsets of the "correct time" and then slews the
clock quickly to that "correct time". Obviously on startup it, and ntp,
have no better estimates of correct time than the very few measurments of
offset they have. Chrony rapidly gets rid of those offset estimates (if
necessary much faster than 500PPM) by slewing the clock. If you ask it to
it will step on startup, but in general it rapidly slews to the correct time. 
I cannot remember if it corrects for the "known offset" in the time it
delivers to the outside.

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