Re: [Ntp] I-D Action: draft-ietf-ntp-update-registries-09.txt

[Danny Mayer <[e-mail]>]

On 11/26/23 9:52 PM, Hal Murray wrote:
> Could you please save/reserve extensiion Field Type 0x0000?
Yes. This is actually a crypto-NAK as defined in RFC 5905 Section 9.2 
though it's not called an extension field there.
>
> I don't have a good writeup.  The idea is tangled up with 7822.  The problem
> is how to distinguish an extension from a MAC.
That was the problem I was grappling with in that RFC. As the author of 
that section you can blame me it it's a bit mangled!
>
> Shared key MACs have a 4 byte key number and 16 or 20 bytes of MAC.  The key
> number space was shared by autokey and manual assignments.  The manual mode
> was restricted to the lower 65K.  So sites that are not using autokey don't
> need the at least 28 byte length restriction from RFC 7822 for the last
> extension.  If the type field is 0, the rest is a MAC rather than an extension.

I did come up with an idea about handling a variable length MAC since 
the RFC was published though there may be some corner cases that would 
not be handled correctly. The problem is not having a length field to 
state how long it is supposed to be. The existing MAC should be replaced 
by an extension field MAC in a NTP v5 implementation so we have a length 
field we can depend on.

Danny