'MQTT 5: When does the combination cleanSession=true + QoS1 make sense
The MQTT 5 spec says:
When a Client reconnects with Clean Start set to 0 and a session is present, both the Client and Server MUST resend any unacknowledged PUBLISH packets (where QoS > 0) and PUBREL packets using their original Packet Identifiers. This is the only circumstance where a Client or Server is REQUIRED to resend messages. Clients and Servers MUST NOT resend messages at any other time.
As re-transmission is only required if a clean session is disabled, I wonder if the combination cleanSession=true and QoS>0 makes sense at any time. Why are client libraries such as paho allowing those combinations? What is the difference between the combinations cleanSession=true, QoS=0 and cleanSession=true, QoS=1.
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