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

Richard B. gilbert rgilbert88 at comcast.net
Thu Apr 5 19:24:40 UTC 2007


Spoon wrote:
> Richard B. gilbert wrote:
> 
>> 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?
>>>>
>>>>
>>>> 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?
>>
>>
>> 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.
> 
> 
> If I start ntpd when the system boots, and let it run 60-90 minutes 
> before I start my application, then this solution works very well!
> 
> However, my boss thinks it is unacceptable to have to wait so long to 
> start the application :-(
> 
>> 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?
> 
> 
> B sends the packets it receives to a local device. Why do you ask?
> 
> Regards.

I asked because I didn't understand why you were using server B to relay 
messages that it seemed that server A could send directly.




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