[ntp:questions] questions Digest, Vol 113, Issue 20

Jochen Bern Jochen.Bern at LINworks.de
Fri Mar 14 13:47:44 UTC 2014

On 14.03.2014 05:30, questions-request at lists.ntp.org digested:
> From: Mauricio Tavares <raubvogel at gmail.com>
>>> >> If you look at the website for time.gov it does display the time
>>> >> being off by 4 min.
>> >
>> > It's correct within wristwatch-and-eyeball accuracy for me right now.
>       Call me odd, but being off by 4 minutes does not seem to be bad.
> I mean, even a default kerberos install would be cool with that.

Well, that depends entirely on what you want to *do* with your
synchronized time. I don't know why Oliviers students would find a 4m
offset on their cell phones so outrageous (missing the bus? Or wasting
an extra 4m in the classroom waiting for a teacher whose arrival times
likely have more jitter than that?), but the computing center guys back
at my university installed their GPS receiver when they had to debug a
problem that repeatedly killed a pair of Cisco routers. The tens-of-ms
precision provided by a central ntpd with DCF77 reception wasn't enough
to correctly correlate the logs collected from each unit's NVRAM after

Do you remember the headlines a couple months ago when physicists
thought they had measured particles traveling a percent or so faster
than the speed of light? That was a clock out of sync by some fraction
of a second, too. They had miscalculated the correction needed to
counter the GPS signal's propagation delay from the surface to the
cavern where the particle detector's master clock sits.

>> > And prepare to use *several* pool servers instead of one, recommended
>> > precisely to avoid such a scenario.
>       I always thought the best thing to do is to have your own ntp
> servers which then are used by your servers. And then those ntp
> servers can query whatever pool you want to use.

It is, but as long as *one* randomly selected pool server may turn out
to have all sorts of issues, you want your "gateway" server(s) to use
*several* pool servers nonetheless. (Or several pools, or several types
(pool and non-pool), or a local clock on top, or whatever avoids the SPoF.)

								J. Bern
*NEU* - NEC IT-Infrastruktur-Produkte im <http://www.linworks-shop.de/>:
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Jochen Bern, Systemingenieur --- LINworks GmbH <http://www.LINworks.de/>
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