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

[Kazuho Oku <[e-mail]>]

@chris-wood 
> Moreover, absent a specific case where we will need to deal with complete incompatibility, which I don't foresee happening, the current generality seems to be a case of YAGNI. (That said, I recognize its relation to the invariants may be concerning. And also I am wrong a lot.)

FWIW, I see two inflexibilities in the Initial packet of QUIC v1 that might make us want to introduce an incompatible version in the future:
* The packet is authenticated using GCM, and it is impossible to introduce another authentication method.
* The only extension point is in the ClientHello extension, which means that the future versions of QUIC cannot signal different information when they retransmit the Initial.

The first point is what prohibits [I-D: Authenticated Handshake for QUIC](https://datatracker.ietf.org/doc/draft-kazuho-quic-authenticated-handshake/) from using an Initial packet format that is compatible with version 1. In case we decide to allow "downgrade" from using  authenticated Initials to pure-v1, we need incompatible version negotiation.

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