[ntp:questions] Sine wave offset patterns

Hans Jørgen Jakobsen hjj at wheel.dk
Sun Sep 8 16:46:40 UTC 2013

On Sat, 7 Sep 2013 17:51:02 GMT, Jared Watkins wrote:
> 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? 

Asymmetric delay variating by the day. NTP protocol assumes that delay is symmetric.
Depending on what accuracy you need sync to local clocks with low roundtrip delay
(and symmetric, and no load variance)
otherwise use ntp to measure oneway delay on links between centers (roundtrip +-offset)/2.

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