[ntp:questions] The libntp resumee...

Kay Hayen kayhayen at gmx.de
Mon Sep 8 15:43:55 UTC 2008

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
> > way
> 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?

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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