MQTT Encoding and Decoding
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MQTT Encoding and Decoding
The MQTT protocol is very lightweight (does not use many bytes to get the message across).
Each MQTT packet consists of:
- Fixed header
- b7-4: MQTT Packet Typ, b3-0: Flags
- Remaining length - this one is a bit tricky and consumes between 1 and 4 bytes. Encoding is least significant bits first in b6-0 while b7 is a "continuation bit" ('1' meaning "another byte follows").
- Variable header (optional), depending on the package type
- Payload (optional), depending on the package type
Text is encoded in UTF-8 without a terminating "0" byte, after the number of bytes in the text encoded as two bytes, MSB first.
| Type | Number | Direction | Flags | Variable Header | Payload |
|---|---|---|---|---|---|
| CONNECT | 1 | Device to Broker | 0 | ||
| CONNACK | 2 | Broker to Device | 0 | ||
| PUBLISH | 3 | Both | b3: DUP, b2-1: QoS, b0: RETAIN | ||
| PUBACK | 4 | Both | 0 | ||
| PUBREC | 5 | Both | 0 | ||
| PUBREL | 6 | Both | 1 | ||
| PUBCOMP | 7 | Both | 0 | ||
| SUBSCRIBE | 8 | Device to Broker | 1 | ||
| SUBACK | 9 | Broker to Device | 0 | ||
| UNSUBSCRIBE | 10 | Device to Broker | 1 | ||
| UNSUBACK | 11 | Broker to Device | 0 | ||
| PINGREQ | 12 | Device to Broker | 0 | ||
| PINGRESP | 13 | Broker to Device | 0 | ||
| DISCONNECT | 14 | Device to Broker | 0 | ||