[ntp:questions] Isolated Network Drift Problem

Richard B. Gilbert rgilbert88 at comcast.net
Thu Nov 20 23:39:07 UTC 2008

Cal Webster wrote:
> Our NTP servers are slowly loosing time. All are in nearly perfect sync
> but collectively drift backwards over time. Is there a way to apply a
> bias to the drift calculations?
> We had to disconnect from the Internet several months ago. Since then we
> have had serious drift problems. Shortly after the disconnect I
> discovered that we were predictably loosing 10 minutes every 15 days. I
> tried several things but not until I zeroed out the
> "driftfile" (/var/lib/ntp/drift) 10 days ago [Mon Nov 10 18:10:00 2008]
> did this large drift abate.
> Although it is much improved, we are still steadily loosing time. Three
> days after I zeroed the drift file [Thu Nov 13 15:04:00 EST 2008] we
> were 32 seconds behind. Today, 10 days later [Thu Nov 20 09:05:00 2008]
> we are 1 min 54 secs behind. This works out to roughly 12 secs per day -
> not bad I guess but still requires regular monitoring.
> Any suggestions would be welcome. I've outlined our NTP/Network topology
> and configuration below, as well as the output of some time reference
> tools that might be relevant.

About the only two things you can do are:
A. Purchase and install a hardware reference clock, or
B. Calculate an approximately correct value for for drift and store it 
in the drift file!

The hardware reference clock (GPS timing receiver, WWV, WWVH, or WWVB 
receiver) will give you both the exact time and also provide ntpd with 
the information needed to discipline your clock.

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