[ntp:questions] manual change on clock time seems to inhibit ntp service.

Martin Burnicki martin.burnicki at meinberg.de
Fri Aug 23 13:27:02 UTC 2013


Sebastien WILLEMIJNS wrote:
> Hello,
>
> i installed meinberg release on windows 7 with default conf.
>
> at startup, time is "well clocked" but after 2 minutes i decided to
> change time (10 minutes later or 10 minutes less), poll and when values
> seems to be frozen after this change
>
> why time isn't now updated  ?

This is a kind of test of run by beginners with NTP, but this doesn't 
work as you expected.

NTP has been designed to apply big time offsets only once after it has 
started. Usually it then queries the time from the NTP server in 
periodic intervals (this is called polling), e.g. once every 64 seconds, 
feeds the replies into a filter and adjusts the Windows system time 
slowly and smoothly according to the required adjustment determined by 
the filter algorithm.

So you can *not* check how NTP works if you occasionally set the system 
time off while NTP is already running, and expect NTP to step the time 
back quickly. If you do this then it takes a number of polling 
intervals, usually up to about 15 minutes, until NTP accepts the time 
step. Then it throws away all the filter data it has collected earlier, 
steps the system time, and starts over from scratch.

If in a case as mentioned above the sudden time offset exceeds about 
1000 seconds (which NTP calls the panic threshold) then NTP even stops 
itself with a message saying something like, "The system time has 
stepped. This must have been done by the administrator who knows what 
he's doing, so I can't do anything else". This is by design of NTP.

To see if NTP keeps adjusting the system time continuously and correctly 
you could run "ntpq -p" repeatedly in a command line window on your 
Windows machine, and watch how the reported offset (which is in 
milliseconds) decreases until it reaches and then stays at some minimum.


Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany



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