[ntp:questions] Re: Box not synchronizing

mooron at mooron.com mooron at mooron.com
Thu Jun 8 16:04:46 UTC 2006


Richard B. Gilbert wrote:
> mooron at mooron.com wrote:
> > Richard B. Gilbert wrote:
> >
> >>JCA wrote:
> >>
> >>>  I have three Linux boxes, namely, A, B, and C, in my LAN. I use C
> >>>as the NTP server for my LAN. C, in turn, gets its synchronization
> >>>from some external NTP server.
> >>>
> >>> Both A and B use the same (very simple) ntp.conf configuration file:
> <snip>
> >
> > This is what I consider a minimal NTP.CONF:
> >
> > server x.x.x.x maxpoll 6
> > driftfile /etc/ntp.drift
> > enable auth
> >
> > The "maxpoll 6" forces ntpd to update every 64 seconds.
> > "enable auth" prevents unauthorized people/systems
> > from messing with your system over the network.
> >
> > That simple configuration is all you really need in almost
> > all cases.
> >
> > - Mooron
> >
>
> Omit the "maxpoll 6".  ntpd will adjust its poll interval within the
> limits of the default values of MINPOLL and MAXPOLL to the currently
> optimal value.  Tampering with the values of MINPOLL and MAXPOLL will
> generally result in sub optimal performance.  Typical behavior is for
> ntpd to start polling at 64 second intervals (MINPOLL) and gradually
> increase the polling interval.  The polling interval will sometimes
> increase to MAXPOLL (2^10 or 1024 seconds) if conditions are ideal.
>
> Translating the math I don't really follow into English, the shorter
> poll intervals let ntpd correct large errors quickly and the long
> intervals allow ntpd to correct small errors accurately.

I don't follow it either. NTPD appears to use something like a PID
control algorithm with some filtering and clustering of the sample
data.  How NTPD could become more accurate with less data
is something I don't understand.  I doubt anyone can give a real
explanation.  Probably because it just isn't so.

I have noticed that with bad clock hardware, such as a SUN Sparc,
it does a poor job of controlling the poll interval.

- Mooron




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