Re: [ntpwg] Parsing NTP packets regarding MACs and EXTs.

[Daniel Franke <[e-mail]>]

On 6/21/16, I <[e-mail]> wrote:
> If the packet is 48 bytes, it has no extensions and no MAC.
> If the packet is between 49 and 67 bytes inclusive, it's malformed; discard
> it.
> If the packet is 68 bytes, it has a legacy MAC and no extensions.
> If the packet is more than 68 bytes, the last 20 bytes are the keyid
> and MAC, the first 48 bytes are the standard fields, and everything in
> between is extension fields. If the keyid field is 0, then the MAC
> field is ignored, and these last 20 bytes are basically just filler to
> make the packet parse correctly.

I was just going through WG documents and saw that we already opined
on this issue last March with RFC7822. It's consistent with what I've
suggested so far, except that it allows extension fields without a MAC
as long as the final extension field is at least 28 bytes long. I
think this is too clever by half and I wish I'd been paying attention
before it was ratified so that I could have objected. Oh well, too
late now and no big deal.
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