A PUBACK message is the response to a PUBLISH message with QoS level 1. A PUBACK message is sent by a broker in response to a PUBLISH message from a publishing client, and by a subscriber in response to a PUBLISH message from the broker.
The table below shows the format of the fixed header.
bit | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
---|---|---|---|---|---|---|---|---|
byte 1 | Message Type (4) | DUP flag | QoS level | RETAIN | ||||
0 | 1 | 0 | 0 | x | x | x | x | |
byte 2 | Remaining Length (2) | |||||||
0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 |
Contains the Message Identifier (Message ID) for the PUBLISH 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 client receives the PUBACK message, it discards the original message, because it is also received (and logged) by the broker.
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