[ntp:questions] The libntp resumee...

David Woolley david at ex.djwhome.demon.co.uk.invalid
Mon Sep 8 07:14:14 UTC 2008

Kay Hayen wrote:
> They are owned by the same people who then own installations of our system, so 
> that wouldn't be an issue.

You will still have to ensure that they do not enable kiss of death on 
those servers.  Also you should make sure that they don't try to use 
w32time, especially older versions, and if your timing requirements are 
tighter than a few tens of milliseconds, that there are no Windows 
machines involved.

> 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.
> 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.

> 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.

> blame its own clock for that extra difference and assign it dispersion that 
> it doesn't deserve.
> That's a different model and I think Mr. Unruh already clarified to me that 
> it's not the model that NTP uses. I think "my" model has little experience or 
> qualification behind it. Current NTP on the other hand is proven.
Firstly, I don't know any time synchronisation software that doesn't 
have a large step by step element.

More importantly, if you are going to micro-manage ntpd, you need a deep 
understanding of how it works to know what the statistics really mean 
and know what are realistic expectations.

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