Clarify text to insure unified packet identifier space

The text in the MQTT 3.1.1 specification is not clear that packet identifiers used for PUBLISH (and related ACKs) are taken from the same set of packet identifiers as those used for SUBSCRIBE related operations. I brought this up in the face-to-face meeting in Redmond, and both Andy Stanford-Clark and Ian Cragg responded that a single, unified packet id space was the intent of the specification.

Ken Borgendale commented that it was possible for implementations to operate successfully without using a unified packet identifier space, if they were careful to avoid ambiguity arising from a PUBLISH related command carrying the same packet identifier as a a SUBSCRIBE related command.

Ian Cragg observed that having separate spaces made it more likely for implementations to get into difficulties. I myself have seen several implementations in which some of these difficulties occurred.

I feel the text should be clarified to say the following two things:.

1. All packet identifiers are taken from a single set of packet identifiers, regardless of whether the message is a PUBLISH, PUBACK, PUBREC, PUBREL, PUBCOMP, SUBSCRIBE, UNSUBSCRIBE, SUBACK or UNSUBACK. (All packet identifiers refers to the set of packet identifiers used by either a client or a server when initiating transmission of commands carrying packet identifiers)

2. The receipt of a SUBACK or UNSUBACK MUST free the packet identifier for re-use, in the same sense that PUBACK and PUBCOMP free the packet identifier assigned to a PUBLISH.

The precise workding is a topic for further study.