[ntp:questions] Monitoring NTP - nptq -p

Richard B. gilbert rgilbert88 at comcast.net
Mon Mar 26 13:53:39 UTC 2007

Spoon wrote:
> Richard B. gilbert wrote:
>> Spoon wrote:
>>> Keith E. Brandt, M.D. wrote:
>>>> 'Poll' is the polling interval in seconds, which seems to vary quite 
>>>> a bit on my system, so here's the first real question - how does ntp 
>> Playing with MINPOLL and MAXPOLL is NOT recommended!!
>> Ntpd varies the polling interval between MINPOLL and MAXPOLL in order 
>> to best respond to existing conditions.  The math is over my head but 
>> it has something to do with the "Allan Intercept" q.v.
>> My oversimplified English explanation is that short polling intervals 
>> allow large errors to be corrected quickly while the longest polling 
>> interval allows small errors to be corrected very accurately.
> I would have thought that short polling intervals are always better,
> ignoring traffic overhead issues:
> If the current "correct" interval should have been e.g. 64 seconds
> instead of 16 seconds, just ignore 3 out of 4 replies.
> Where is the flaw in my logic?
> Regards.

The flaw is network jitter!  The round trip delays are not constant and 
the delays are not guaranteed to be symmetrical; consider, for example, 
a request packet that travels direct from New York to Los Angeles while 
the reply travels Los Angeles to Dallas to New York.  The routers use 
the best available path at the moment and the path may be different each 
time.  (This is one reason why servers close to you are usually "better".)

By using a longer poll interval you measure an error that is "large" 
with respect to the network jitter.

A great deal of research and thought went into the NTP algorithms.  You 
should not try to improve on them until you understand them.

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