[ntp:questions] Need to verify the offset

Steve Kostecke kostecke at ntp.org
Wed Dec 10 04:17:20 UTC 2008


On 2008-12-09, David Woolley <david at djwhome.demon.co.uk> wrote:

> Hushpuppy wrote:
>
>> At my work (financial services) the market rules mandate
>> synchronizing our clocks to within a maximum 3 second difference
>> of the NIST atomic clock. We also need to log/document that we are
>> within 3 seconds.
>
> If ntpd is allowed to step and it has been more or less correct at
> some time, you will not get such a large error unless something is
> broken. If something is broken, it will invalidate the statistical
> measurements, anyway.

NTP can easily keep your clock within 3000 milliseconds of UTC. Unless
it is badly configured or the clock is broken.

> However, I think the worst case is something like root delay / 2
> + root dispersion, plus last hop delay / 2, plus precision, + 15
> microseconds for every second since the time was updated.

You're talking about calculating the synchronization distance (see
RFC-1305).  The ntptrace utility will do this for you.

Root Dispersion is a 32-bit signed fixed-point number indicating the
maximum error relative to the primary reference source, in seconds with
fraction point between bits 15 and 16. Only positive values greater than
zero are possible.

RFC-1305 may be found at http://www.faqs.org/rfcs/rfc1305.html

-- 
Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/




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