Question on Loosely Synchronized Router Clocks

Date: Mon, 17 Sep 2007 18:22:12 -0400
From: Deepak Jain <>

>> 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
REALLY accurate.

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.