MQTT Encoding and Decoding

MQTT

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
  • packet type:byte
    • b7-4: MQTT packet type
  • 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

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

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

Type	Number	Direction	Flags	Variable Header		Payload
				Msg ID
CONNECT	1 (you may read this as "fixed QoS of 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 (is QoS > 0)	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 (you may read this as "fixed QoS of 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 (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	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 (you may read this as "fixed QoS of 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	-		-

MQTT