Which for some reason is not in the abnf spec, and thus will be ignored up until somebody tries to implement this and fails a compatibility test.

Please put it in your abnf spec. People use them, you know.

Yes, it's hard to specify. The problem is that you have both an uncapped ID and an uncapped PAYLOAD in the same message. I recommend giving ID a max length of, say, 32, and PAYLOAD then has a max length of 951 if I'm counting right.

Or you could consider that an IPv6 path MTU is at least 1280, and use that (or 1232) as your per message bound instead of 1024. You're sending a packet, might as well get full value.