[ntp:questions] Sine wave offset patterns

Jared Watkins jared.watkins at gmail.com
Sat Sep 7 17:51:02 UTC 2013

Hello list... 

I'm setting up several ntp servers in data centers located in many different places around the world peering off sometimes "local" sometimes distant stratum1 internet servers. (GPS backed atomic oscillators are in the works but not yet available). I've tried different configurations including setting up the more distant systems as stratum3 to our other stratum2 boxes to keep the traffic on the private wan. In pretty much all cases though when graphing the offsets I'm seeing sine wave patterns.  The period is usually about 10 hours and the amplitude can vary by +-25ms for the 'closer' ones and as much as +-100ms for the more distant ones. 

I also noticed that when doing a restart on one of the worst boxes.. it initially returned to a near zero offset and then fell right back into the wave pattern at the same point as if uninterrupted which I thought was very strange.  These are normal off the shelf (read cheap) servers and are reporting drift file times of between 1-8... which I believe means the local clocks are ok. 

I've searched the docs and the archives but I've not seen any discussion of this.  Any ideas on what would cause it? 

For some specifics.. the most distant boxes that are now configured as stratum3 have delay times of between 200 and 300ms and corresponding jitter of <2 and <3ms   The 'better' ones acting as stratum2 have delays of <10 to 35ms for the external servers and jitter all <1ms. 


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