[ntp:questions] offset > .5 second. What does this mean?

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk
Mon Oct 30 09:30:31 UTC 2006

nicough at gmail.com wrote:
> Can someone clarify for me whether "offset" in the NTP world, is
> talking about:
> (a) The difference in time between the incorrect time on a local
> server, and the correct time on a correct NTP server.
> or
> (b) The time delay (caused by latency etc) to receive the correct time
> from an NTP server. Ie By the time the local server receives the
> "correct time", this "correct time" value is now slightly old.
> Remembering that eventlog regularly says "Time set (offset > .5
> second)"
> and initially (once) said "Time service corrected the clock error by
> 63 seconds".
> Thanks
> Nick


Please be aware that w32time in Windows 2000 DOES NOT use NTP!  It uses 
SNTP.  SNTP is a single query/response mechanism for getting a time 
estimate, but NTP involves software which queries multiple servers (to 
eliminate bad servers), and queries them continuously (with an adaptive 
interval) as part of a controlled time and frequency locking loop.

In NTP, offset refers to the time by which the OS time differs from the 
best current estimate of UTC (at least, that's what I would say, but the 
gurus will correct me).  In NTP, the delay to and from the server is 
measured and removed as an error source (although this is not perfect if 
the path to and from the server does not have the same time delay).

Also, once started, NTP will change the clock rate slightly to keep it in 
sync rather than stepping the clock.  I don't believe that the Windows 
2000 version of w32time does that - it uses the much cruder time steps.


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