[ntp:questions] Loopstats updated less frequently than expected

David J Taylor david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid
Sat May 23 19:38:56 UTC 2009


Unruh wrote:
[]
> If you mean you have a refclock attached, they have round trip time of
> 0. Thus there is never a minimum and all get used. Ntp's algorithm in
> short is: Save the last 8 polls. Check if the current poll has a
> larger
> roundtrip time than any of those 8. If it does, do not use it.
> If the round trip times are all 0, then the current one is never
> larger
> so it is always used.
>
> Now, the round trip time on the net  is a random variable, but from
> experience, on average only about 1 in 8 are used. ( actually it is a
> bit better than
> that so that about 1/7  are actually used.)
>
> This is to avert bias on the theory that longer round trip times are
> probably biased (  the delay is caused by one of the legs being
> longer than the other), but as a result, the actual average poll
> interval is
> roughly 7 times longer than the requested interval. Also since the
> time
> constants must be longer than the longest poll interval, this helps
> the system
> to have a very long time constant (ie respond extremely slowly to
> changes).

That makes some sense, Bill, and probably more if I study it more 
carefully.

Thanks,
David 




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