Re: [quicwg/base-drafts] Simplify version negotiation (#2133)

[Mike Bishop <[e-mail]>]

> Yes. My argument is that this is basically the situation we have had with every TCP-using protocol forever. Nobody thinks you can send an HTTP request to a SSH server and have that work, but we do expect SSH clients and servers to be able to negotiate with each other despite some diversity of versions.

Except the equivalent of having QUIC version failure is an inability to do a TCP handshake, which does currently work between unrelated applications.  However, I think you're right that the larger point is:

> The invariant you need to retain isn't that all versions are compatible but that the client never has to guess whether to send an Initial of type A or B because the server might not understand A but might understand B (or vice versa). I've been trying to think of a non-hypothetical situation in which this might arise that can't be easily dealt with by retaining compat of the Initial. Do you have one?

To some extent, Google's use of QUIC Crypto is an existence proof.  There are servers which speak only QUIC Crypto, and there are servers which speak only TLS-based iQUIC. Even if QUIC Crypto is extinct by the time we ship this draft, the fact that the situation has arisen once suggests that it could again.

"Easily dealt with" is pretty dismissive of not only being able to parse old versions' framing, but being able to parlay the handshake contents into a different handshake protocol.

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