ntpviz (rtt/2)

Gary E. Miller gem at rellim.com
Thu Sep 8 16:52:49 UTC 2016

Yo Dan!

Not the first time you have had to explain it.  I'll try to summarize
this in the index.html.  Maybe more can be said on the ntp.conf man page?

On Thu, 08 Sep 2016 10:39:07 -0500
Dan Drown <dan-ntp at drown.org> wrote:

> Quoting Hal Murray <hmurray at megapathdsl.net>:
> > I prefer RTT rather than RTT/2.
> >
> > RTT/2 suggests that the routing is symmetric which is wrong quite
> > often.  
> Adding +/- RTT/2 on the graph reverses the assumption ntp uses that  
> the routing is symmetric.  ntp already subtracts rtt/2 from the  
> calculated offset to estimate the one way latency.  Removing that
> from the equation will show you the relevant offsets of: when the
> request started and when the response was received.
> Here's an example of an upstream source that had the request latency  
> (in green) change due to routing paths:
> https://dan.drown.org/vps3/remote-ntpgps.mci.png
> You can see the response latency (in blue) did not change  
> significantly.  So the sudden jump in the NTP offset (in red) was
> not due to the clocks changing frequencies relative to each other,
> but due to routing effects.
> You can also see that after the 23rd, the new request path had a lot  
> more jitter that is not in the response path.  This also impacts the  
> offset.
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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