MQTT Encoding and Decoding

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