[ntp:questions] Win7: ntpd adjusting time backwards

David Taylor david-taylor at blueyonder.co.uk.invalid
Mon Dec 24 11:52:47 UTC 2012


On 23/12/2012 00:11, Hal Murray wrote:
> In article <kb4hoe$9cm$1 at dont-email.me>,
>   David Taylor <david-taylor at blueyonder.co.uk.invalid> writes:
>
>> I wonder how I could get the delay figures for my own modem?  The only
>> data I can extract is: Latency: ~20 milliseconds, RTP jitter - up: 4
>> milliseconds, down: ~0.12 milliseconds.
>
> You can measure the delays.  After the typical client-server
> exchange you have 4 time stamps.  If you assume the network
> latency is symmetric you can compute the clock offset.
> If you assume both clocks are accurate you can compute
> the network delays.
>
> If you collect a bunch of data, it's reasonable to assume
> that the lowest delays are when the queues are empty.  Any
> longer delays are due to queuing.
>
>
>> Fascinating to see you have less delay on the slower upstream!
>
> The queuing delays depend upon the traffic.  You control that.

Thanks for that suggestion Hal.  I've written a program to do just that, 
and I am seeing down delays of a moderately consistent 4 ms, and up 
delays of between 20 and 30 ms.

The program was interesting in that measuring times to better than 15 ms 
resolution on Windows requires interpolation, and I had already 
developed that from for code I found on the Internet.  I think this was 
it's first use "in anger".

   http://www.satsignal.eu/ntp/TSCtime.html
-- 
Cheers,
David
Web: http://www.satsignal.eu



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