[ntp:questions] Re: How to force ntpd to set time first time if clock is withhin +-128ms range

David L. Mills mills at udel.edu
Thu Sep 11 16:16:10 UTC 2003


No, this is misleading. The problem is much more basic than that,
notwithstanding the iburst consideration. Unless you know the intrinsic
frequency offset, simply jamming the time correction in the discipline
loop will not always result in a specific time offset bound, especially
if the poll interval is allowed to increase above 64 s, which is
normally the case.

I'm really concerned that the simplistic advice offered by quick and
dirty references like the twichy frequently disguises the engineering
principles involved. There are things you can and cannot do based on
solid engineering principles, some of which apply in this case. The
twichy and even the faq needs to know when casual advice does not apply
and reference to ugly and boring documentation really and truly is
required. In this case, the principles are based on physics and
engineering principles of feedback loop theory and analysis.

Let's say you are driving a car and join a train of cars heading
downtown to work. You are told to join the train and follow the car in
front of you within a tolerance of one inch and do that within five
seconds of joining the train. The feedback loop consists of your eyeball
as phase detector, the engine and drive train as variable frequency
oscillator and your brain as loop filter. Can you do that within one
inch and five seconds, two inches and ten seconds? Does this get my
point across?


Harlan Stenn wrote:
> I think you want ntpd -gN, and use the "iburst" option for your server
> and peers.
> This is all documented at twiki.ntp.org, in the StartingNTP area.
> H

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