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From: David Magda <dmagda@ee.ryerson.ca>
Date: Fri, 1 Apr 2005 08:04:40 -0500
To: Peter Jeremy <PeterJeremy@optushome.com.au>
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cc: stable@freebsd.org
Subject: Re: Kernel NTP flipping between FLL and PLL modes
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On Apr 1, 2005, at 05:45, Peter Jeremy wrote:

> Any suggestions as to why this is happening?  (And how I can stop
> it regularly flipping)

I don't think this is really an issue. It may be annoying to see it in 
the logs, but NTPv4 uses each algorithm when it's appropriate to get 
the most accurate time. Since network conditions change, the way NTP 
has to  deal with them changes since it queries other NTP servers over 
the network.

This was actually freebsd-questions last year and one response pointed 
to this paper:

http://www.eecis.udel.edu/~mills/database/papers/allan.pdf

You may want to check-out the the netgroup comp.protocols.time.ntp if 
you want to ask the experts (and authors) on NTP.