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

[Mike Bishop <[e-mail]>]

MikeBishop commented on this pull request.



> +list of QUIC versions and select an alternative version from the list of
+supported versions.  A server MAY choose a compatible version from that list and
+continue the handshake with that version.  The server sends its first packet as
+though it was continuing with the version it selects.
+
+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

The TLS comparison is somewhat inapt, since in TLS you actually do offer your highest (most preferred) version and let the server select a compatible, less-preferred version if it doesn't support that one.  QUIC can't do that because the contents of the Initial aren't invariant.

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