[ntp:questions] Re: Architecture / best practice for small/medium company setups

David Woolley david at djwhome.demon.co.uk
Sat Jul 1 08:49:09 UTC 2006

In article <ZLmdnecJjZxPgDjZnZ2dnUVZ_tednZ2d at comcast.com>,
Richard B. Gilbert <rgilbert88 at comcast.net> wrote:

> server.  If that single server fails, the client's clocks will start to 
> drift, perhaps by as much as four seconds per day but probably less than 
> that.  In twenty-four hours, two different systems might differ by as 
> much as eight seconds but again, probably less than that.  If he can 

Things would have to have been very bad before the server failed for this
to happen, and the NTP dispersion calculations would be invalid.

When the server drops out, individual machines will continue with the 
last known frequency correction, which, unless distorted by whatever 
caused the server loss, will typically be within 1ppm of true, so 
100ms a day is certainly not an unrealistic expectation.  If there 
was something that caused a high frequency correction error at the point
of failure, there is a good chance that it will affect both machines 
more or less equally.

(In my view, though, ntpd could handle this better.  At the moment, it
doesn't properly differentiate the following reasons for applying
long term (integral) frequency corrections:

1) environmental - fast changing but very limited in magnitude;
2) ageing - very slow except at end of life, but could be relatively
3) initial calibration - should only be performed on change of hardware, but
            can be very large and should be applied very quickly.

The result is that, when upset, it can end up applying a much larger
correction than could possibly be justified.)

More information about the questions mailing list