[ntp:questions] The libntp resumee...
Richard B. Gilbert
rgilbert88 at comcast.net
Mon Sep 8 18:23:24 UTC 2008
> kayhayen at gmx.de (Kay Hayen) writes:
>> Hello David,
>>>> When I say "restrict" it is our own system that decides that ">x ms"
>>>> offset is too bad and prevents ntpd from talking to it any further with a
>>>> "restrict" command. If all 2 servers of an "other host" are "restricted",
>>>> it will crash the software.
>>> You are overriding NTP's selection algorithms. Effectively you are no
>>> longer running NTP.
>> How would it be difference from using the restrict command manually?
>> And why would it not be NTP?
>>>> All of that is own our making and control.
>>>> Regarding the poll values. I am not sure why we do it the external NTPs
>>>> as well. Could be that the dispersion can be brought down quicker this
>>> You are misusing "dispersion". Dispersion is an estimate of worst case
>>> drift and reading resolution errors.
>> Well, dispersion is going down only with more samples to base estimation on,
>> isn't it? And we need that quick, if we want the server to influence the
>> hosts behind it quickly, say after a "NTP LAN" failure ended (some people
>> have dedicated LANs for NTP).
>>>> on "entry hosts" and allow the "other hosts" to synchronize faster with
>>>> them, or could be that we never considered it worthwhile to optimize it
>>>> away. Well yes, but between 2 queries from the same client the ntpd will
>>>> have made a certain adjustment. If the client gets to know this value, it
>>>> will have to
>>> ntpd is making adjustments at least every 4 seconds (old versions) and
>>> as often as every clock tick. It does this by adjusting frequency not
>>> by directly adjusting time.
>> I was not concerned with how the kernel makes the adjustments, but rather that
>> the a fixed time change over the period is known. The slew rate is known,
>> isn't it?
>> Let me use a car analogy, these things work. :-)
>> Lets assume a three lane high way with 3 cars that try to drive at the same
>> speed. The car to the left is driving at (near) constant speed. The driver in
>> the middle accelerates and braces according to his motor behaviour as well as
>> the observed difference in speed between him and the other one. Now what
>> should the driver to the right do?
> The cars have the road as a reference. However without the road, how does
> car 3 know that car 2 is accelerating and decelerating and that it is not
> hiw own car that is misbehaving? He does not. All he
> can do is collect more cars and use the average behaviour to determine who
> is behaving badly.
Car 3 has a speedometer!
> With two other cars only as a reference there is no way of deciding which
> is weird.
> And if he has the road as a reference, then use the road, not either of the
> other cars ( ie buy yourself a GPS receiver with PPS and then you will not
> have to worry about what other cars are doing).
>> In my view, he could take the acceleration of his neighbour into account when
>> making estimates of his own error.
>> Best regards,
>> Kay Hayen
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