[ntp:questions] The libntp resumee...

Unruh unruh-spam at physics.ubc.ca
Mon Sep 8 21:36:16 UTC 2008

"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

>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!

Yes, that is with reference to the road. Car three should thus completely
ignore the other two cars and use his speedometer. 

Ie, put up a GPS receiver with a PPS and use that as your time source, and
ignore all the other ntp time sources, except perhaps as sanity checks (eg
if you r speedometer breaks you should get to know about it by occasionally
looking at the other cars)

>> 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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