[ntp:questions] The libntp resumee...

Unruh unruh-spam at physics.ubc.ca
Mon Sep 8 17:13:40 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
>> > 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. 

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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