[ntp:questions] What exactly does "Maximum Distance Exceded" mean?

Joseph Gwinn joegwinn at comcast.net
Sat Mar 14 03:52:29 UTC 2009


In article <49bae109$0$505$5a6aecb4 at news.aaisp.net.uk>,
 David Woolley <david at ex.djwhome.demon.co.uk.invalid> wrote:

> Joseph Gwinn wrote:
> > In article <49ba0e33$0$505$5a6aecb4 at news.aaisp.net.uk>,
> >  David Woolley <david at ex.djwhome.demon.co.uk.invalid> wrote:
> > 
> >> Joseph Gwinn wrote:
> >>
> >>> What is this error likely telling me?  What are the possibilities?  What 
> >>> tests will tell the tale?
> >> Your timeservers are unsynchronised, but for some reason not setting 
> >> their stratum to 16.
> >>
> >> Distance exceeded means that the combination of worst case round trip 
> >> time induced error and an assumed drift of 15ppm since the last valid 
> >> time on the root server (plus a few minor components) has exceeded 1 
> >> second.
> > 
> > How would it know of drift, in this isolated little island?
> 
> What is described here is not what is conventionally called an island. 
> An island is a system which does not and never has had a source of 
> correct time.

OK.  I was using it in the sense that this little two-box network has no 
other source of time save the one GPS receiver cum NTP timeserver.


> ntpd doesn't care about what the drift is in determining root distance. 
>   It simply takes the position that the actual local clock will be 
> somewhere within +/- 15ppm of the value which would achieve perfect 
> phase lock with true time.

This part seems to conflict with the max +/- 500 ppm steering authority 
of NTP.  How are these two limits related?

 
> The assumed maximum reasonable error therefore grows at 15 microseconds 
> per second.

One suspicion I have is that the drifts file has data from some other 
test still in it.  We will try deleting the drifts file.

Another suspicion is that the computer's sense of time is too far away 
from that provided by the timeserver.  I would have thought this would 
cause the daemon to balk and complain, but perhaps there is a window 
where it will not balk but will struggle mightily.  We will use ntpdate 
as part of the startup process and see if it matters.

Joe Gwinn




More information about the questions mailing list