[ntp:questions] Re: MAXDISPERSE is too high! was Re: is there a way to "lock" the drift frequency

Tim Shoppa shoppa at trailing-edge.com
Sun Dec 7 13:30:05 UTC 2003


fredb at immanent.net (Frederick Bruckman) wrote in message news:<3fd1ee15_3 at corp.newsgroups.com>...
> It seems to me, that having MAXDISPERSE of "16." does absolutely nothing
> whatsoever. Can anyone recall why the limit is set so high?

Note that MAXDISPERSE isn't *just* a check on each packet; it's also used as
an input to the loop filter for missing data.  I feel a little guilty for
recommending that you change it, when in retrospect maybe adding a new test
with a new variable (probably tunable in ntp.conf and with ntpdc) would be more
reasonable.  But maybe it is appropriate, if you set MAXDISPERSE to a value
close to a second, to use that as the error bound on missing data?  Dave's
made enough comments about parameters tuned to Allan variances of PC clocks
that I don't want to venture that far!

The 16 sec value is probably useful as a sanity check against misbehaving
servers and/or networks which occasionally send packets to deep space
and back... you will note that 16 sec is plenty to get data back and forth
to the moon!

Tim.



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