[OASIS Issue Tracker] (MQTT-286) Make Qos 2 Delibery Method 'B' Normative

[OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org>]

Ed Briggs created MQTT-286:
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             Summary: Make Qos 2 Delibery Method 'B' Normative
                 Key: MQTT-286
                 URL: https://issues.oasis-open.org/browse/MQTT-286
             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
          Issue Type: New Feature
            Reporter: Ed Briggs


I propose QoS 2 Delivery Method B should become the only forwarding method, and Method A be eliminated in future MQTT versions.

This can reduce delivery delays for PUBLISH messages over QoS 2 flows when the network propagation delay is substantial. In addition, Method B eliminates temporary storage required by Method A.  Both methods are described in MQTT 3.1.1 Section 4.3.3 in non-normative text accompanying diagram 4.3.

This change may also be beneficial for the following MQTT isses:

MQTT 197  Support Request Reply
MQTT 236  Consolidate ACKs, Enable NAKs
MQTT-271  Describe Small Device Limitations aka The Arduino problem.

I propose the existing non-normative text in section 4.3.3 : 

 "...there are two methods by which QoS 2 can be handled by the receiver. They differ in the point within the flow at which the message is made available for onward delivery. The choice of Method A or Method B is implementation specific. As long as an implementation chooses exactly one of these approaches, this does not affect the guarantees of a QoS 2 flow."

Be replaced by normative text along the lines of the following:

"Upon receiving a QoS 2 PUBLISH message which is both valid and not a duplicate, the receiver SHOULD immediately initiate onward delivery and send a PUBREC. The receiver MAY send a PUBREC before the onward delivery is complete.  If the forwarding operation fails, the receiver MUST send an appropriate error response, or terminate the transport as specified in section {TBD part of MQTT-236 }. The receiver MUST NOT send more than one PUBREC for the a single message arrival."





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