A PUBREL message is the response either from a publisher to a PUBREC message from the broker, or from the broker to a PUBREC message from a subscriber. It is the third message in the QoS 2 protocol flow.
The table below shows the fixed header format.
bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
---|---|---|---|---|---|---|---|---|
byte 1 | Message Type (6) | DUP flag | QoS level | RETAIN | ||||
0 | 1 | 1 | 0 | x | x | x | x | |
byte 2 | Remaining Length (2) | |||||||
0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 |
The variable header contains the same Message ID as the PUBREC message that is being acknowledged. The table below shows the format of the variable header.
bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
---|---|---|---|---|---|---|---|---|
byte 1 | Message ID MSB | |||||||
byte 2 | Message ID LSB |
When the broker receives a PUBREL message from a publisher, the broker makes the original message available to interested subscribers, and sends a PUBCOMP message with the same Message ID to the publisher. When a subscriber receives a PUBREL message from the broker, the subscriber makes the message available to the subscribing application and sends a PUBCOMP message to the broker.
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