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MQTT

MQTT Encoding and Decoding

The MQTT protocol is very lightweight. It does not use many bytes to get the message across, and the handshake (sequence of messages) is pretty simple (with user-choosable options ranging from "primitive" to "modestly sophisticated").

Each MQTT packet consists of:

  • Fixed header
    • packet type:byte, consisting of the actual MQTT packet type in the higher nibble and some flags in the lower nibble
      • b7-4: MQTT packet type
      • b3: DUP flag (PUBLISH only)
      • b2-1: QoS (PUBLISH only)
      • b0: RETAIN flag (PUBLISH only)
    • 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"). This mechanism ensures that short messages stay short by using the fewest number of bytes needed to encode the length.
  • Variable header (optional), depending on the package type
  • Payload (optional), depending on the package type

Type "Array" is binary data, after the number of bytes in the array encoded as two bytes, MSB first.

Type "String" is like "Array", with the text encoded in UTF-8 without a terminating "0" byte.

Packet Type Direction Fixed Header Variable Header Payload
Packet Type Flags Msg ID? Content
CONNECT Device to Broker 1 0 -
  • protocol name:String (always "MQTT")
  • protocol level:byte (always 4)
  • 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 Broker to Device 2 0 -
  • connect acknowledge flags:byte
    • b0: session present
  • connect return code:byte
    • 0: connection accepted (success)
    • 1: unacceptable protocol version (fail)
    • 2: identifier rejected (fail)
    • 3: server unavailable (fail)
    • 4: bad user name or password (fail)
    • 5: not authorized (fail)
-
PUBLISH Both 3
  • b3: DUP
  • b2-1: QoS
  • b0: RETAIN
QoS > 0 ? y
  • topic name:String
  • message ID:word (if QoS > 0)
payload:Array (optional)
PUBACK Both 4 0 y
  • message ID of PUBLISH packet:word
-
PUBREC Both 5 0 y
  • message ID of PUBLISH packet:word
-
PUBREL Both 6 1 (you may read this as "fixed QoS of 1") y
  • message ID of PUBREC packet:word
-
PUBCOMP Both 7 0 y
  • message ID of PUBREL packet:word
-
SUBSCRIBE Device to Broker 8 1 (you may read this as "fixed QoS of 1") y
  • message ID:word
  • topic filter:String
  • requested QoS:byte (b1-0: QoS)

repeat sequence one or more times

SUBACK Broker to Device 9 0 y
  • message ID of SUBSCRIBE packet:word
  • return code:byte
    • b7: failure
    • b1-0: maximum QoS

repeat sequence one or more times

UNSUBSCRIBE Device to Broker 10 1 (you may read this as "fixed QoS of 1") y
  • message ID:word
  • topic filter:String

repeat sequence one or more times

UNSUBACK Broker to Device 11 0 y
  • message ID of SUBSCRIBE packet:word
-
PINGREQ Device to Broker 12 0 - -
PINGRESP Broker to Device 13 0 - -
DISCONNECT Device to Broker 14 0 - -

MQTT