MQTT Encoding and Decoding: Difference between revisions

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{| class="wikitable" style="border: 2px solid"
{| class="wikitable" style="border: 2px solid"
! rowspan="2" | Type
! rowspan="2" | Packet Type
! rowspan="2" | Number
! rowspan="2" | Direction
! rowspan="2" | Direction
! rowspan="2" | Flags
! rowspan="1" colspan="2" | Fixed Header
! rowspan="1" colspan="2" | Variable Header
! rowspan="1" colspan="2" | Variable Header
! rowspan="2" | Payload
! rowspan="2" | Payload
|-
|-
! Packet Type
! Flags
! Msg ID
! Msg ID
!  
!  
|-
|-
| CONNECT
| CONNECT
| Device to Broker
| 1
| 1
| Device to Broker
| 0
| 0
| -
| -
Line 55: Line 56:
|-
|-
| CONNACK
| CONNACK
| Broker to Device
| 2
| 2
| Broker to Device
| 0
| 0
| -
| -
Line 72: Line 73:
|-
|-
| PUBLISH
| PUBLISH
| Both
| 3
| 3
| Both
|  
|  
* b3: DUP
* b3: DUP
Line 85: Line 86:
|-
|-
| PUBACK
| PUBACK
| Both
| 4
| 4
| Both
| 0
| 0
| y
| y
Line 94: Line 95:
|-
|-
| PUBREC
| PUBREC
| Both
| 5
| 5
| Both
| 0
| 0
| y
| y
Line 103: Line 104:
|-
|-
| PUBREL
| PUBREL
| Both
| 6
| 6
| Both
| 1 (you may read this as "fixed QoS of 1")
| 1 (you may read this as "fixed QoS of 1")
| y
| y
Line 112: Line 113:
|-
|-
| PUBCOMP
| PUBCOMP
| Both
| 7
| 7
| Both
| 0
| 0
| y
| y
Line 121: Line 122:
|-
|-
| SUBSCRIBE
| SUBSCRIBE
| Device to Broker
| 8
| 8
| Device to Broker
| 1 (you may read this as "fixed QoS of 1")
| 1 (you may read this as "fixed QoS of 1")
| y
| y
Line 133: Line 134:
|-
|-
| SUBACK
| SUBACK
| Broker to Device
| 9
| 9
| Broker to Device
| 0
| 0
| y
| y
Line 146: Line 147:
|-
|-
| UNSUBSCRIBE
| UNSUBSCRIBE
| Device to Broker
| 10
| 10
| Device to Broker
| 1 (you may read this as "fixed QoS of 1")
| 1 (you may read this as "fixed QoS of 1")
| y
| y
Line 157: Line 158:
|-
|-
| UNSUBACK
| UNSUBACK
| Broker to Device
| 11
| 11
| Broker to Device
| 0
| 0
| y
| y
Line 166: Line 167:
|-
|-
| PINGREQ
| PINGREQ
| Device to Broker
| 12
| 12
| Device to Broker
| 0
| 0
| -
| -
Line 174: Line 175:
|-
|-
| PINGRESP
| PINGRESP
| Broker to Device
| 13
| 13
| Broker to Device
| 0
| 0
| -
| -
Line 182: Line 183:
|-
|-
| DISCONNECT
| DISCONNECT
| Device to Broker
| 14
| 14
| Device to Broker
| 0
| 0
| -
| -

Revision as of 08:48, 4 July 2017

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
      • 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").
  • 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.

Packet Type Direction Fixed Header Variable Header Payload
Packet Type Flags Msg ID
CONNECT Device to Broker 1 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 Broker to Device 2 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 Both 3
  • b3: DUP
  • b2-1: QoS
  • b0: RETAIN
 QoS > 0 ? y
  • topic name:String
  • message ID:word (is 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

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