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Subject: [OASIS Issue Tracker] (MQTT-209) QOS 2 delivery options must not be mixed


     [ https://tools.oasis-open.org/issues/browse/MQTT-209?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Richard Coppen updated MQTT-209:
--------------------------------

    Proposal: 
Following call (03.04.2014). Paul Freemantle proposes:

"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."


Previously:
The diagram is now non normative but change it to show Method A and Method B, clarify the either one method or the other must e followed.

  was:The diagram is now non normative but change it to show Method A and Method B, clarify the either one method or the other must e followed.


> QOS 2 delivery options must not be mixed
> ----------------------------------------
>
>                 Key: MQTT-209
>                 URL: https://tools.oasis-open.org/issues/browse/MQTT-209
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Bug
>          Components: core
>    Affects Versions: 3.1.1
>            Reporter: Paul Fremantle
>             Fix For: 3.1.1
>
>
> The QOS 2 delivery description says the following:
> "Figure 4.3 shows that there are two ways in 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 approach is implementation specific and does not affect the guarantees of a QoS 2 flow."
> Unfortunately while it is true that if you choose one approach it doesn't affect the guarantee, it seems like if you mix the two then messages can be lost. An academic I know did a formal analysis and proved this (I'll find a reference for you). 
> Therefore I would recommend that we clarify this.



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