Re: [quicwg/base-drafts] introduce a version alias mechanism (#2573)

[Marten Seemann <[e-mail]>]

> I do fear that this feature will make a generic, independent DDoS solution practically impossible. The more I talk to folks on the DDoS side, the more push back I get from any type of coordination between backend servers and the DDoS device. If there is no coordination, then the device will not understand these aliased version numbers; and therefore will not be able to reply with a version specific response (Retry).

We had this discussion on the NY interim, and we concluded that it's a design feature of QUIC that middleboxes can't send retries **unless** they coordinate with the server. That's why we introduced the `original_connection_id` transport parameter, in which the server has to prove knowledge of the connection ID that the client initially used for its connection attempt. If a middlebox wants to send a Retry, it therefore must either communicate this connection ID directly to the server, or (which is the more practical solution) share a key with the server and encode it into the token.

To decode the version number of greased versions, it's not hard to imagine a similar arrangement between the middlebox and the server. Defining an algorithm to do so safely would probably belong in the LB draft.

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