[ntp:questions] Re: NTP stepping issue

Brian Utterback Brian.Utterback at Sun.removeme.COM
Tue Oct 19 17:37:46 UTC 2004

Robert Rati wrote:
> recommended to do so.  In my testing, I saw the client daemon slew to 
> the correct time but then it continued slewing past the correct time. Is 
> this a bug?  Once it slews to the correct time provided by the servers, 
> the client should remain synchronized, right?

That is the correct behavior. The NTP system is designed as a big feedback
system (a phase locked loop actually) such the clock adjustment is
overcompensated at the beginning of an adjustment, with smaller and smaller
corrections applied as the time gets closer to the correct value. By limiting
the rate of correction to a maximum of 500 PPM, the system can be guaranteed
to remain stable and to eventually reach equilibrium at zero offset.

You might think that it should stop at zero, but you have to remember that
the system is not only trying to set the offset to zero, it is also
simultaneously trying to determine the correct clock frequency adjustment
to keep the offset at zero.

This is a rather subtle system, and somewhat counter intuitive to most of
us with a software background, but is quite familiar to DSP and analog hardware


Power corrupts. PowerPoint corrupts absolutely.
Brian Utterback - OP/N1 Revenue Product Engineering, Sun Microsystems, Inc.
Ph/VM: 877-259-7345, Em:brian.utterback-at-ess-you-enn-dot-kom

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