[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.
> (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
> and initially (once) said "Time service corrected the clock error by
> 63 seconds".
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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