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

[MikkelFJ <[e-mail]>]

mikkelfj commented on this pull request.



> +A QUIC version is compatible with this version if the cryptographic handshake
+message sent in the first packet can be used in both versions.  A compatible
+version is also able to identifying 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.
 

What if the client wants to use a specific other version if possible, but it does not want to do it in-flight within the current connection, for example to keep the client simple? It cannot omit that version from TP because then the current version will continue. It cannot include that version in TP because then it might get in-flight version change.

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