[ntp:questions] NTP tunning for OWD measurements

pret3nder at gmail.com pret3nder at gmail.com
Sat Oct 27 15:59:48 UTC 2012


Hello all, and thank you again for your valuable insight 
on this matter, it's been a great help to me.
I'll now try to reply to all your comments, please excuse 
me if I missed a particular comment.

Indeed I was wrong about the tunnel reference, that won't
correct the fact that a delay asymmetry will always exist
in the path. I'm sorry for that remark.

I already have ntpd setup, collecting statistics and I'm using
munin to graph some of those. Currently I'm only using
S1 <-> X1 one-way delay measurements to simplify.
Both S1 and X1 use the same stratum 1 time server, but
right now S1 is down because of a problem on the
datacenter. 
Nevertheless, here goes a link for the stats on 
the X1 server: http://imgur.com/a/Oer6z
The spikes show there are related to a network problem that
as been identified and fixed.

I'll also follow unruh's advice to plot the delay vs offset.
We are aware that we are sitting on 2nd class seats, and 
perfectly conscious that GPS on both ends of the link 
would give us the most accurate results. Nevertheless, I want
to be able to know what's the maximum error that will
affect my owamp measurements, using only ntpd synching
to stratum 1 servers on our network.
I'm also aware of the need to take the results with a grain of
salt, because there are also errors related to the OS overhead.

Since it's a fiber-optic network, we would like to have
microsecond accuracy, accounting of course the errors discussed
previously. I will try to get GPS signals at least in one of the links 
(on both ends) so I can compare results, and draw conclusions 
from there.

About plotting (delay+offset)/2 and (delay-offset)/2, I suspect
the interesting data would show in the condition the offset would
be negative on one way, and positive on the reserve way?

David Woolley is right, the project is about making the measurements,
although I don't like the idea of doing them blindly, without
knowing how to interpret the results. I won't have time to
investigate the deeps of NTP, but I do want to know the basics,
as that's one of the objectives of my thesis.


Kind Regards,
Pedro


On Saturday, October 27, 2012 3:56:22 PM UTC+1, John Hasler wrote:
> Terje Mathisen writes:
> 
> > The crux is this: You _MUST_ have a path to an absolute time
> 
> > reference, at both ends, which is totally independent of the path you
> 
> > are trying to measure!
> 
> 
> 
> And a tunnel is _not_ an independent path.
> 
> -- 
> 
> John Hasler 
> 
> jhasler at newsguy.com
> 
> Dancing Horse Hill
> 
> Elmwood, WI USA



More information about the questions mailing list