[ntp:questions] SNTP with 1ms of precision?

Rob nomail at example.com
Thu Jun 17 14:18:59 UTC 2010

Marcelo Pimenta <marcelopimentacs at gmail.com> wrote:
> Yep but I have an equipment for specific use with RFC2030 implemented
> because of a standard that I have follow that say it is mandatory SNTP -
> RFC2030. My equipment ask for time every minute and frequence tolerance of
> the crystal is less then 1ms/min. They are in a intranet with 2 switches
> isolated so... my question was about the SNTP because some people said to me
> that SNTP cannot garantee 1ms accuracy. Is it possible, in that described
> cenario to no have 1ms of accuracy? I expect that a implementation
> of RFC2030 made by a person at equipment X and the implementation of the
> same RFC at the same equipment X BUT by another person could give me
> different results??

SNTP is much worse than NTP because with SNTP a single query that somehow
gets delayed (e.g. momentary queue on a switchport) will immediately step
the time and it will be wrong for the entire period until you make the
next query.

With NTP a single query does not affect the time so much, because
successive queries are fed through a filter to find the average, and
the time is not stepped but slowly changed to the new value.  So when
there is an occasional delay in your network it will not affect the
time accuracy very much.

But having said that, there is nothing in NTP or SNTP that will guarantee
you an accuracy of 1ms.  It is a best effort protocol.  It will usually
do better but there is no guarantee.

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