[ntp:questions] Getting NTP to correct only the clock skew

Spoon root at localhost.invalid
Thu Apr 5 18:12:59 UTC 2007


Maarten Wiltink wrote:

> Spoon wrote:
> 
>> B is supposed to re-send the packets it receives at the rate they
>> were originally sent by A.
>> 
>> If the clocks on A and B do not tick at the same rate, the buffer
>> used by B will either overflow or underflow.
>> 
>> This is why I need A's clock and B's clock to tick at the same rate.
>> 
>> But it is not important to me that A and B's clock give the same
>> absolute time. ...
> 
> But nothing will break if they do, either. Right?

Right. Of course :-)

> The simplest solution
> would be to have both synchronized to UTC, or either one to the other,
> and accept that they will keep better time or at least closer time.
> From the sound of it, you do not care about what time A keeps _at all_,
> so if you simply slave it to B, your problem goes away.
> 
> The fact that you are concerned with reproducing timing but not with
> good time is, frankly, suspect and leads me to propose a different
> 'solution': could you monitor the buffer length and adjust frequency
> on system B from that? If it's slowly draining, slow down B a little;
> if it's growing, speed it up ever so slightly. Just like NTP does,
> really.

The very hard part (for me) is seeing that B's buffer is in fact slowly 
draining when there is a lot of jitter on the link between A and B.

I've tried using an exponentially-weighted moving average to filter the 
jitter out, but it didn't work as well as I had hoped. That is when I 
turned to NTP. I'm trying not to reinvent the wheel.

Are you saying I should use the theory in NTP but not the daemon?

Regards.




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