Original text:
semantics and encodings for any version-specific field.  In particular, different packet protection keys might be used for different versions.  Servers that do not support a particular version <del> are unlikely to be able to decrypt the payload of the packet. Servers SHOULD NOT attempt to decode or decrypt a packet from an unknown version, but instead send a Version Negotiation packet, provided that the packet is sufficiently long. </del>  Packets with a supported version, or no version field, are matched to a connection using the connection ID or - for packets with zero-

Reviewer comments:
instead of saying servers SHOULD NOT attempt to parse unknown versions, stick with the factual statement that they will likely be unable to do so. Flips the SHOULD to the second half of the sentence, the expected response. I believe this is editorial, since we're not modifying expected behavior, but it's touching normative language, so please speak up if you're not convinced.
Actually, it seems like there should be a normative MUST NOT attempt.
I would have said "cannot". The contents are unknown, after all. Attempting to prohibit someone from doing something impossible seems like it might be unnecessary (or at least futile). Edit: of course, this change avoids having to deal with that question, which is better.
Why do you say it's impossible? You could simply hope that version n+1 was the same as version n in terms of structure and try to parse it as that, ignoring the version number.
Which is why the PR says "unlikely," not "impossible." But the impossible thing is for the server to know whether it's doing things correctly.
Right, but my point is that we should prohibit trying, not just give people advice that it's dumb.
(Broken out of .) This seems like tuning? Or is the intention to protect the endpoint from an attack, please explain the SHOULD.
encodings for any version-specific field. In particular, different packet protection keys might be used for different versions. Servers that do not support a particular version are unlikely to be able to decrypt the payload of the packet. Servers SHOULD NOT attempt to decode or decrypt a packet from an unknown version, but instead send a Version Negotiation packet, provided that the packet is sufficiently long. I'm having trouble seeing what's needed here. The endpoint has received a packet professing to be in a version it does not understand. It can't know the semantics or encoding of any field there -- and the paragraph already explains that. Yes, I'm sure there are potential attacks in interpreting a packet of unknown provenance and unknown semantics against a different set of semantics and expectations, but I'm not convinced we need to explain them. The text recommends sending a Version Negotiation packet, which seems like the only sensible response.
This refers to .
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I'm looking for two things: (i) I think this ought to clarify the intention for the SHOULD, I suspect it is clear to those working on this part, but not clear to me why this is a recommendation - i.e., specifically can you give me a use-case of why the Server would otherwise USE information from a packet from an unknown version, unless the intention is to say the server shouldn't waste resources to do try to do this? (ii) I think this implies understanding of Section 14.1 later in the spec, and a ref would help. So... Is it equally true that.. "Servers MUST NOT use the contents of a packet with an unknown version, and SHOULD NOT decrypt this packet, but instead send a Version Negotiation packet, provided that the packet is sufficiently long (see Section 14.1)."? Or perhaps simply explain something like: "Servers SHOULD NOT commit resources to attempt to decode or decrypt a packet from an unknown version, but instead send a Version Negotiation packet, provided that the packet is sufficiently long (see Section 14.1)."?
Fundamentally, it's a caution against assumptions which could prove incorrect, such as "this looks like an unencrypted TLS ClientHello, so I can go find the field that contains...." The server will either process information in a way that could be used maliciously, or the server will generate a response that won't be understood by the client. Ideally, this would be a MUST, except that it's entirely unenforceable by the client what the server does internally. Perhaps the right solution here is to remove the normative language with regard to that piece and stick to the statement of fact we already have:
That approach seems fine.

Revised text:
semantics and encodings for any version-specific field.  In particular, different packet protection keys might be used for different versions.  Servers that do not support a particular version  <ins> are unlikely to be able to decrypt the payload of the packet or properly interpret the result.  Servers SHOULD respond with a Version Negotiation packet, provided that the datagram is sufficiently long. </ins> Packets with a supported version, or no version field, are matched to a connection using the connection ID or - for packets with zero-