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

[ekr <[e-mail]>]

> @DavidSchinazi I think I might oppose to the idea due to the following reasons.
> 
> > * in v1 we say that receiving a version negotiation packet is a non-recoverable connection failure
> 
> This means that clients that prefer a newer version of QUIC cannot connect to a v1 server, unless it uses an Initial packet that is compatible with that of v1.

I don't believe that this is correct. In David's proposal, this would work as follows. In v1, we would just specify the VN/fail mechanism. So, if the client sends a v2 Initial, you get VN. However, in v2 we would specify a response similar to that in QUIC currently, namely that the client chooses the new version and then comes back with an Initial. So, if the client sends v2 to a v1-only server, they get VN and fall back to v1, with no change to the v1 server.

Note that this does not have an anti-downgrade mechanism, but we can add one retroactively: v2 clients send supported_versions and v2 servers have to check it, so if you are downgraded from v2 -> v1 you will detect it.

It's also worth noting that even though in principle QUIC now supports this use case, it requires a round trip from the client to the server, so I'm skeptical that clients will do it until the point where they are very unhappy about QUICv1.




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