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 | |
|---|---|---|---|---|---|---|
| Msg ID | ||||||
| CONNECT | 1 | Device to Broker | 0 | - | "MQTT":String protocol level (4):byte connect flags:byte (b7: user, b6: password, b5: will retain, b4-3: will QoS, b2: will, b1: clean session) keep alive:word |
client ID:String will topic:String (if present) will payload:Array (if present) user name:String (if present) password:Array (if present) |
| CONNACK | 2 | Broker to Device | 0 | - | connect acknowledge flags:byte (b0: session present) connect return code:byte (0: connection accepted, 1: unacceptable protocol version, 2: identifier rejected, 3: server unavailable, 4: bad user name or password, 5: not authorized) |
- |
| PUBLISH | 3 | Both | b3: DUP, b2-1: QoS, b0: RETAIN | QoS > 0 ? y | topic name:String message ID:word |
payload:Array (optional) |
| PUBACK | 4 | Both | 0 | y | message ID of PUBLISH packet:word | - |
| PUBREC | 5 | Both | 0 | y | message ID of PUBLISH packet:word | - |
| PUBREL | 6 | Both | 1 | y | message ID of PUBREC packet:word | - |
| PUBCOMP | 7 | Both | 0 | y | message ID of PUBREL packet:word | - |
| SUBSCRIBE | 8 | Device to Broker | 1 | y | message ID:word | topic filter:String requested QoS:byte (b1-0: QoS) (repeat sequence one or more times) |
| SUBACK | 9 | Broker to Device | 0 | y | message ID of SUBSCRIBE packet:word | return code:byte (b7: failure, b1-0: maximum QoS) (repeat sequence one or more times) |
| UNSUBSCRIBE | 10 | Device to Broker | 1 | y | message ID:word | topic filter:String (repeat sequence one or more times) |
| UNSUBACK | 11 | Broker to Device | 0 | y | message ID of SUBSCRIBE packet:word | - |
| PINGREQ | 12 | Device to Broker | 0 | - | - | |
| PINGRESP | 13 | Broker to Device | 0 | - | - | |
| DISCONNECT | 14 | Device to Broker | 0 | - | - | |