[ntp:questions] Remaining synced on an unsynchronised peer?

Michael Butow michael.buetow at comsoft.de
Fri Dec 4 07:28:01 UTC 2009

> Why bother with the prefer anyway? What do you think it is doing for
> you?

Yeah, I'll explain that first.
This system is operating in closed environments where there are not a 
multitude of external NTP servers (e.g. the Internet) available.
Most times, the external NTP servers are GPS clocks of some type.
If lucky, there may be a proper architechture with 7 or more NTP servers 
available, but that is really seldom the case.
More often, there are only two GPS clocks. Sometimes, they are from 
different manufacturers.
The prefer is used to protect against divergence of these clocks during a 
leap second (it has been known to happen), otherwise the system is 
vulnerable to timekeeping inconsistencies as you can imagine:
- 1. Without "prefer", the host1/host2 nodes may, in the course of normal 
timekeeping, become synchronized on different external reference clocks, 
and in the case of two diverging external clocks (which may happen during 
leap seconds), they may suddenly make a step apart (1 second or more).
- 2. If the other 8 inner machines in the system may choose either host1/
host2 without preference, the inconsistency can propagate to the rest of 
the (redundant) system. The host1/host2 to which an inner machine is 
effectively synchronized is also determined by timing issues and other 
matters of circumstance.
- 3. A time divergence in the system can cause failures such as crashes 
and incorrect processing of input, the latter of which could conceivably 
occur undetected, constituting a safety hazard.

> The whole point of the prefer keyword is to say "use this server as long
> as it is sane even if there are other better servers". Stale is not
> insane.

I agree.

> When do you think that the prefer host should stop being used?

This is something I need to think deeper about to give you a reasoned 
answer, but off the cuff I would say I don't want to use a preferred host 
any more once its reachability is 0. In that case I would not mind 
switching to a non-preferred host with a higher poll which _is_ still 

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