A PUBREC message is the response to a PUBLISH message with QoS level 2. It is the second message of the QoS level 2 protocol flow. A PUBREC message is sent by the broker in response to a PUBLISH message from a publishing client, or by a subscriber in response to a PUBLISH message from the broker.
The table below shows the fixed header format.
bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
---|---|---|---|---|---|---|---|---|
byte 1 | Message Type (5) | DUP flag | QoS level | RETAIN | ||||
0 | 1 | 0 | 1 | x | x | x | x | |
byte 2 | Remaining Length (2) | |||||||
0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 |
The variable header contains the Message ID for the acknowledged PUBLISH. 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 it receives a PUBREC message, the recipient sends a PUBREL message to the sender with the same Message ID as the PUBREC message.
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