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

Richard B. gilbert rgilbert88 at comcast.net
Mon Apr 2 13:10:50 UTC 2007


Spoon wrote:
> 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.

It's clear enough.

Ntpd should be able to both keep your clocks synchronized to the same 
time but also to the same rate.  There WILL be startup transients but 
once the two machines are synchronized they should both have the same 
time, within a millisecond or two and be ticking at 1.000000 seconds per 
second.

Is there some good reason why A's packets must be relayed through B? 
Does B do any processing of the packets or does it simply forward them?




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