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

David Woolley david at ex.djwhome.demon.co.uk.invalid
Fri Mar 13 22:41:03 UTC 2009

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.

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.

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

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