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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Question on Loosely Synchronized Router Clocks
> Date: Mon, 17 Sep 2007 18:22:12 -0400 > From: Deepak Jain <[email protected]> > > > > [email protected] 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: [email protected] Phone: +1 510 486-8634 Key fingerprint:059B 2DDF 031C 9BA3 14A4 EADA 927D EBB3 987B 3751 Attachment:
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