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

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

    [ https://issues.oasis-open.org/browse/MQTT-286?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=63847#comment-63847 ] 

Ed Briggs commented on MQTT-286:
--------------------------------

I propose that references to QoS 2 Delivery Method A and Method B be removed from the non-normative text in MQTT 3.1.1. section 4.3, and the interaction diagram that accompanies it.   We need to bring this to a vote and close asap, as it has an impact on other parts of the specification.  Only one method will be shown in the text and diagram, that which corresponds to the current Method B (which forward the PUBLISH message when it is received, not when PUBREL arrives.

> Make Qos 2 Delivery 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
>          Components: core
>    Affects Versions: 5
>            Reporter: Ed Briggs
>            Assignee: Ed Briggs
>              Labels: Proposed
>
> 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."
> A brief containing additional details can be found here:
> https://www.oasis-open.org/apps/org/workgroup/mqtt/download.php/58364/Mqtt-286-MqttQoS2DeliveryProposal.pdf



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