该消息是接收方收到 QoS 1 的PUBLISH 消息后，返回给发送方的应答消息。
该消息由于没有Payload，固定包头的剩余长度值为 2
1. 固定包头 Fixed Header
|
Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
|
byte 1 |
MQTT Control Packet type (4) |
Reserved |
||||||
|
|
0 |
1 |
0 |
0 |
0 |
0 |
0 |
0 |
|
byte 2 |
Remaining Length (2) |
|||||||
|
|
0 |
0 |
0 |
0 |
0 |
0 |
1 |
0 |
2. 可变包头
|
Bit |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
|
byte 1 |
Packet Identifier MSB |
|||||||
|
byte 2 |
Packet Identifier LSB |
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