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

Spoon devnull at localhost.com
Mon Apr 2 11:02:40 UTC 2007


Richard B. gilbert wrote:

> Spoon wrote:
>
>> I've read this page:
>> http://ntp.isc.org/bin/view/Support/HowToCalibrateSystemClockUsingNTP
>> which explains how to let NTP determine the frequency offset (skew).
>>
>> I have a strange request:
>>
>> Is it possible to run NTP in a mode where it does not try to correct
>> the time offset, but only correct the frequency offset (skew)?
>>
>> In other words, assume my clock says it is some time last year, and
>> gains 1 second every day (11.6 ppm). I don't want NTP to either slew
>> or step my clock to the correct time, but I still would want it to fix
>> this 1 s per day (11.6 ppm) frequency offset.
>>
>> Has this ever been considered?
> 
> I doubt it very much!
> 
>> Is there a variable I could tinker with? :-)
> 
> I don't know of any.
> 
> What problem are you trying to solve?
> 
> Most people want the correct time rather than simply a clock keeping the
> wrong time but one that ticks at one second per second.

I'll try to explain my situation in detail.

Consider two systems, A and B.

A sends ~1000 UDP packets per second to B.

A timestamps each packet.

These packets travel over an IP network, and suffer delay and jitter.

B is supposed to re-send the packets it receives at the rate they
were originally sent by A.

B buffers N packets. Then it sends the first packet in the queue, 
computes the departure time of the next packet using the timestamps 
provided by A, and sleeps until that departure time.

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. Was that clear or did I forget something important?

Regards.




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