Re: [quicwg/base-drafts] Compatible version upgrade (#1901)

[martinduke <[e-mail]>]

As I see it, there are two separable changes here:

1) Moving validation from client to server. The necessary change here is to move the transport parameter from EE to ClientHello. IMO this is no biggie, and addresses the server upgrade problem (i.e. clients can't distinguish between a server pool rolling upgrade and a downgrade attack).

2) Allowing seamless transition between "compatible" versions. This certainly reduces the latency penalty. However this creates a more complicated hierarchy of versions, which is difficult to understand. I also do not feel great that we have though through the implications of taking a v1 packet and acking it in v2. For instance, what if v2 requires a different set of client transport parameters?

I would suggest a separate PR to move the TPs over, which should be merged relatively quickly, and a more extended discussion on seamless upgrade.

If we separate out item (2), however, I think we should retain supported_versions in Encrypted Extensions  purely for next-connection informational purposes (and not for validation).

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