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

[Daniel Franke <[e-mail]>]

I don't think I follow. In what sense does it limit the length of the
chain, other than that stratum is capped at 16? And what
characteristic of RFC 5905's recommended input response address the
problem? (I need to read the book you've referenced before I fully
understand the problem in the first place; I've ordered it but it'll
take some time to arrive).

On Mon, Apr 13, 2020 at 3:15 PM Watson Ladd <[e-mail]> wrote:
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>
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> On Mon, Apr 13, 2020, 7:21 AM Daniel Franke <[e-mail]> wrote:
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>> 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.
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> 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.