[ntp:questions] Strange jumps in PPM

A C agcarver+ntp at acarver.net
Wed Aug 21 20:24:01 UTC 2013

On 8/21/2013 10:01, unruh wrote:
> NTP of course cannnot tell the difference between your source going
> wild, or your computer ( say a sidden temp rise due to heavy work load)
> clock going wild. It HAS to trust the sources. Now if you have 5 sources
> all of similar accuracy, then it could throw away one or two as false
> tickers. but with only one PPS, it HAS to assume that the signal from
> the source is good. It has nothing else to go by.
> You with hindsight could say-- clearly it was the source going wild--
> but how could ntpd in the heat of the battle.

In general it can't tell but if I give it the gift of clairvoyance (i.e. 
my knowledge of my specific system) then it CAN figure out what 
happened.  I know from data collection that the system is stable, 
temperatures are relatively stable and there's no sudden loading (the 
sole purpose of the machine is ntpd).  If the PPS signal, within one 
polling interval, goes from a few microsecond offset to several 
millisecond offset then something drastic has happened.  Since the 
polling interval is fixed at 16 seconds (minpoll/maxpoll 4) that type of 
acceleration of the offset is inconsistent with any normal behavior thus 
PPS is wrong and should be ignored until it comes back into alignment.

So the idea isn't that ntpd should figure out on its own without user 
intervention.  Instead there would be some kind of 
configuration/fudge/tinker options available so that the user can supply 
baseline information that can help ntpd identify a rogue source versus 
normal system behaviors.

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