[ntp:questions] The libntp resumee...

Richard B. Gilbert rgilbert88 at comcast.net
Mon Sep 8 18:23:24 UTC 2008


Unruh wrote:
> 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
>>>> 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?
> 
> 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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