[time-nuts] ISS NTP operation problems.

[Magnus Danielson]

Poul-Henning,

On 2021-01-08 17:10, Poul-Henning Kamp wrote:
> --------
> Hal Murray writes:
>
>> There is a huff-puff filter, I think it's optional. 
> I think it is more likely that the median-filter is causing trouble.
>
> If you let the poll-rate ramp all the way up to 1024 seconds, the
> median filter can get "stuck" for almost an hour before it notices
> that something is horribly wrong[1].
>
> These days there are almost no circumstances under which you should not
> clamp your poll-rate to one minute "server bla maxpoll 6"
>
> On a strange path like the ISS, I would clamp it to the minimum
> 16 seconds ('maxpoll 4').
>
> When the backbone of the ARPAnet was 56kbit/s, being able to ramp
> up to 1024 seconds poll rate made sense.  It never makes sense now.
>
> Poul-Henning
>
>
> [1] The median filter should be automatically disabled and the most
> recent sample used, if the samples in the shift register are
> mononotonic or spread too much.  That's another change I never
> managed to "sell" to Dave Mills.

The Allan intercept concept comes out of the analysis of the ACTS system
where model signals was used over the public phone network and the noise
analysis in that context. For it's system view, it's a good analysis and
it allowed to reduce the number of calls, associated with significant
cost, into the ACTS system. Then, this was re-applied over to NTP under
the assumption that the noise models work. *NEWSFLASH* They don't. The
non-zero mean character of network delay variations just play havoc with
the underlying assumption. As one applies knowledge on how to handle
that noise, use min-delay algorithm, play with packet rates etc. you end
up with quite a different solution. Increasing packet-rate for NTP today
is for most scenarios very cheap, so the very fine property of low load
of NTP ends up being not so greatly needed at all times. If we do the
right processing, we can increase the packet rate for the benefit of
better performance.

Now, if one worked enough with these things, the things I say is really
not dramatic and new, but there is still a bit of history involved where
times have changes significantly.

Cheers,
Magnus