Comments on transf-mqtt-v1.0-csd04.md

2.2 Default Topic Structure

Might be worth clarifying the difference between a subscription to "oc2/rsp" and "oc2/rsp/#".

2.6 MQTT Client Identifier

Some brokers allow a connection with a zero byte ClientId. This will cause the MQTT broker to generate a ClientID that is guaranteed to be unique.

2.8 Will Message

Might be better so say "must not use a will message", that way you can opt to use a will message in the future.

3.3 SUBSCRIBE Control Packet

If Qos=1 is used, the receiver will have to be capable of dealing with a repeated (duplicate) message.