Re: [Ntp] The bump, or why NTP v5 must specify impulse response

[Watson Ladd <[e-mail]>]

On Mon, Apr 13, 2020, 7:21 AM Daniel Franke <[e-mail]> wrote:

> On Sun, Apr 12, 2020 at 6:11 PM Watson Ladd <[e-mail]> wrote:
> > Section 14.5 of Phase Lock Techniques by Floyd Gardner describes
> > difficult problems caused by the accumulation of errors in a chain of
> > PLLs under benign conditions, and section 2.2.4 describes the root
> > cause, namely an inevitable peak in the transfer function of a second
> > order filter. I don't think these are insolvable in NTP v5, but they
> > should give pause to the idea that we can avoid needing to specify the
> > the synchronization algorithm. The peaking of the PLL needs to be
> > controlled, or some thing more complex needs to be specified.
>
> Are the algorithms in RFC 5905 vulnerable to this? If so, I think that
> shoots down any implication that it's a practical necessity to solve
> this in the NTPv5 spec, since we've obviously been getting by okay so
> far.
>

v4 limits the length of the chain and specifies the input response to have,
which solves the problem.


> Just to be clear, though, I'm not proposing that the WG should be
> *silent* on synchronization algorithms in v5. Just that whatever we
> have to say about it should be in a separate, Informational document,
> and can discuss the design space and suggest multiple possibilities
> rather than require one particular thing.
>