[time-nuts] ISS NTP operation problems.

[Magnus Danielson]

On 2021-01-08 16:58, Hal Murray wrote:
> phk at phk.freebsd.dk said:
>> If you path is not stable, or you flip between different servers with
>> different delays and/or assymetries, your time will not be stable. 
> Ahh.  Thanks for the reminder.
>
> There is a huff-puff filter, I think it's optional.  It assumes the physical 
> path is stable and that increases in the round trip time are due to queuing 
> delays which are typically asymmetric so it drops answers if the delay is 
> longer than previous samples.  I'd have to look at the code for the details.

Huff-'n-puff is there only to handle packet jitter. The same method is
called min delay algorithm or lucky packet filtering in other
techniques. For the one-way delay estimate, over some window of raw
measurements, you produce the minimum delay one. Then you do the same
for the other direction. Only using these values you then do your
two-way time-transfer calculation.

You should also be cautioned that it works only because the time between
the nodes is stable, and thus the frequency difference between them
being virtually zero, because the assumption made breaks down whenever
there is a phase-drift / frequency error.

The success of this method depends on how large window of measurements
you have. You can get significant gains as you make the window cover
more samples. In the end, the success depends on packet rate, as you end
up having some maximum time between regulations.

This does not really solve problems with asymmetric routes,
route-changes etc.

> But that doesn't match the crazy graph with the drift way off.

I don't recall seeing the graph, did I forget to check it in the
original posting? Yes, lazy me forgot to check that. OK. Will look at it.

Cheers,
Magnus