[ntp:questions] jumpy offset/drift

Brad Knowles brad at stop.mail-abuse.org
Thu Aug 11 14:07:36 UTC 2005

At 11:15 AM +0200 2005-08-11, Folkert van Heusden wrote:

>  I did some investigating and found that ntpd constantly switches between
>  servers and my local DCF receiver:

	Yup.  If you've got four falsetickers that are all in reasonably 
close agreement, they can result in the truechimer being kicked off 
the island.

	If you don't want this to happen, you've got to take extra steps 
to try to guarantee that you only ever sync to your DCF receiver.

>  What I now don't understand is why it sometimes marks the dcf receiver
>  as a bad clock. The delay is constantly 0 and the offset as well as the
>  jitter are always low.

	See above.  You've got too many other clocks that are all 
reasonably close to each other, so they're pulling the overall 
average away from your DCF receiver.  As a result, instead of the 
three or four falsetickers being kicked off the island, your one 
truechimer is being kicked off.

	Of course, that's assuming that your DCF receiver really is the 
one truechimer -- maybe it's the falseticker and the others are 
truechimers instead.  That's a hard question to answer without a lot 
more investigation.

Brad Knowles, <brad at stop.mail-abuse.org>

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."

     -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
     Assembly to the Governor, November 11, 1755

   SAGE member since 1995.  See <http://www.sage.org/> for more info.

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