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

Paul Fremantle created MQTT-209:

             Summary: 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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