[ntp:hackers] Further to the timestamping issue

Poul-Henning Kamp phk at phk.freebsd.dk
Tue Jun 17 11:24:54 UTC 2008


In message <48572897.7030607 at udel.edu>, "David L. Mills" writes:
>P-H,
>
>On reflect, I don't know whether your discain for the NTP statistics 
>budget is due to a conclusion that the maximum error and expected error 
>statistics are not useful and/or confusing or that my calculation of 
>them is faulty. It would be useful to find out what your replacements 
>might be.

Stratum 1 admins don't pay any attention to calibrating them correctly.

And since they are uncalibrated, their values have no or little
relation to the actual metrics.

Thus: Useless in practice.

But I don't know if they are by definition useless in theory: The
latest definitions I have been able to find are:

   Root Delay (rootdelay): Total round trip delay to the reference
   clock, in NTP short format.

   Root Dispersion (rootdisp): Total dispersion to the reference clock,
   in NTP short format.

But it is not obvious what is meant by "reference clock".

Is that the piece of hardware connected to the S1 server, or is it
the far-away atoms which governs the output of that piece of hardware ?

If they only measure back only to the S1 server, then they are
indeed useless in theory:  You would need to query the S1 server
to see where it lives and what refclock it uses, so you can compensate
for the diurnal changes in propagation delay in case it is a VLF
receiver, and for the jitter in case they used a cheap serial
alarm-clock type receiver. 

If they refer back to the distant atoms, then 99% of all VLF based
S1 servers on the net are badly uncalibrated (up to tens of
milliseconds) and most of the rest merely trivially uncalibrated
(tens of microseconds).

In that case, they are possibly not useless in theory, but because
the majority of servers are uncalibrated, we cannot know for sure.

Since there is no way to get all the servers out there to shape
up, any remedy would entail new fields with usable data, for
servers which are calibrated, and a pass-note from uncalibrated
servers.

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.


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