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

[MikkelFJ <[e-mail]>]

mikkelfj commented on this pull request.



> +message sent in the first packet can be used in both versions.  A compatible
+version is also able to identify and acknowledge the first packet sent by the
+client in some fashion.  Other QUIC versions might have different constraints in
+determining what is compatible.  In order to facilitate this process, new QUIC
+versions could define a process for transforming the first packet from other
+compatible versions into the equivalent packet in the new version.
+
+Upgrading in this manner allows a server to upgrade without incurring the round
+trip imposed by sending a Version Negotiation packet.  It also allows clients to
+send their first packet using a widely deployed version, without the risk of
+having to use that version with servers that supports a more-preferred version.
+
+A server MUST NOT send a Version Negotiation packet if it prefers a version that
+is not compatible with the version the client initially chose; a server has to
+allow the client to choose between versions that are not compatible.
+

Since you wrote this, new text appeared requiring a connection abort if a VN with no overlap is received. If the server has implicit knowledge about what the client supports it doesn't change anything - either it goes forward with a compatible version or it sends a VN packet.

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