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

[Kazuho Oku <[e-mail]>]

kazuho commented on this pull request.



>  
 The client MUST use the long header format and include its selected version on
 all packets until it has 1-RTT keys and it has received a packet from the server
 which is not a Version Negotiation packet.
 
 A client MUST NOT change the version it uses unless it is in response to a
-Version Negotiation packet from the server.  Once a client receives a packet
-from the server which is not a Version Negotiation packet, it MUST discard other
-Version Negotiation packets on the same connection.  Similarly, a client MUST
-ignore a Version Negotiation packet if it has already received and acted on a
-Version Negotiation packet.
+Version Negotiation packet from the server, or if the server picks a different,
+but compatible version (see {{version-upgrade}}).  Once a client receives a
+packet from the server which is not a Version Negotiation packet, it MUST
+discard other Version Negotiation packets on the same connection.  Similarly, a
+client MUST ignore a Version Negotiation packet if it has already received and
+acted on a Version Negotiation packet.

Generally speaking, a server cluster will indicate a common subset of versions that it supports in VN.

When adding support for a new version, each of the server start accepting the new version when found in Initial. After that completes, the next would be to modify each server to start indicating the use of the new version in VN.

In case of removing the support for an old version, we will remove that version from the VN list, then stop accepting Initial with that version.

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