Date: Mon, 17 Sep 2007 18:22:12 -0400 From: Deepak Jain <deepak@ai.net>
Valdis.Kletnieks@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 control.
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. -- R. Kevin Oberman, Network Engineer Energy Sciences Network (ESnet) Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab) E-mail: oberman@es.net Phone: +1 510 486-8634 Key fingerprint:059B 2DDF 031C 9BA3 14A4 EADA 927D EBB3 987B 3751