Question on Loosely Synchronized Router Clocks
oberman at es.net
Mon Sep 17 22:37:04 UTC 2007
> Date: Mon, 17 Sep 2007 18:22:12 -0400
> From: Deepak Jain <deepak at ai.net>
> Valdis.Kletnieks at vt.edu wrote:
> > On Mon, 17 Sep 2007 14:28:45 PDT, Kevin Oberman said:
> >> I had a router that lost it's NTP servers and was off by about 20
> >> minutes. The only obvious problem was the timestamps in syslog. (That's
> >> what alarmed to cause us to notice and fix it.)
> > Trying to correlate logfiles with more than a several-second offset is
> > good and sufficient reason in itself to make sure everything is NTP-synched.
> So to bring the conversation to something more sequitur and relevant.
> 1) Its not hard <tm> to keep all of your devices in your network sync'd
> to the same clock. Especially if you use standardized configuration
> 2) And a reasonable number is on the order of seconds (or ~1 second)
> rather than minutes which is almost the same as being unsynch'd.
> 3) It is not guaranteed, but not hard to be sync'd to a level of
> precision on the order of a second or two using globally-available NTP
> sources to every other network you might directly connect with.
> I'm slightly suspicious of all the CDMA/atomic clock other NTP sources
> (for "higher precision") people point their IP gear at -- simply because
> IP doesn't need the same level of precision as SONET, at least, not yet.
> [exclusions for my suspicion include any NTP sources I run, but that's
> merely hubris ;)].
True atomic clocks are only of value for disciplining time, but atomic
time references tend to be a bit more accurate than GPS or anything else
of which I am aware. CDMA actually gets its time reference from GPS, gut
it is pretty accurate. I believe the spec calls for <1 usec error,
although the receiver still needs to allow for propagation delay to be
I have a mesh of NTP servers spread across the US that keep time within
5 usec based on CDMA clocks, but the operators of the CDMA clocks (cell
phone providers) are often rather slow in handling leap seconds. Took
weeks before the 1 second offset disappeared from all of them.
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4 EADA 927D EBB3 987B 3751
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