[ntp:questions] time delta between clients

David L. Mills mills at udel.edu
Sat Jan 5 04:40:52 UTC 2008


Apparently, OWAMP uses NTP timestamps, but the article assumes there is 
a difference between "NTP time" and "system time". Statistically, NTP 
time is an unbiased estimator of UTC and system time is a lowpass 
version of it. The offsets you might consider NTP time represent 
statistical uncertanty of that estimate and should not be used as system 

Now for one-way delay calculations, which I can't find in the article. 
In cases of severe one-way congestion, which might be the objective, see 
the NTP huff-n'-puff filter, which is designed to prize this out. As for 
swish and sway due to congestion, Chapter 6 of das Buch has a really 
interesting example of a path from here to East Asia with awesome 
one-way congestion. Huff-n'-puff sliced through it like butter. It could 
be that the statistical methods used by huff-n'-puff be useful if 
exported to other applications.

Theoretically, you can't measure one-way propagation times unless you 
have some other measure such as bit rate. The first experiments to 
measure this was done 30 years ago using the ARPAnet and had some 
success. The code measures the data rate by launching packets of varying 
size and measuring the differential delays.

I recently tried the same thing in my earlier experiments using the NTP 
daemon ntpd, but wasn't thrilled with the results. For the experiment to 
work right the network should satisfy what is called the Kleinrock 
Assumption, and my dinkly ISDN line to campus fails that assumption. 
Nevertheless, the code is in the ntp-dev version, but currently disabled.


Rick Jones wrote:
> Ulrich Windl <Ulrich.Windl at rz.uni-regensburg.de> wrote:
>>oberman at es.net (Kevin Oberman) writes:
>>>Netperf is not really the best way to go. The appropriate tool for
>>>one-way latency is OWAMP.  http://e2epi.internet2.edu/owamp/
>>I think you missed the point: AFAIK, Rick is the author of netperf ;-)
> Shhhh! It's a secret!-)
> rick jones
> about to hit-up "management" for aproval to buy a GPS-18 for some
> experiments, unless someone knows of a better device...

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