Could you add a sort of non normative comments about handling version
3.x clients for backward compatibility?

For example:

What reason code should the broker send with CONNACK for protocol
error before determining the client version ?
(Version 3.x clients expect 0x01, while version 5 do 0x81 or 0x82.)
The broker cannot determine clients version immediately because
version 5 CONNECT magic word "MQTT" is as same as version 3.1.1.

How should the broker convert the messages between version 3.x clients
and version 5 ?
(Add or delete properties obviously. Anything else to concern ?)

Regards,
Tatsuzo Osawa