MQTT Encoding and Decoding: Difference between revisions

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=MQTT Encoding and Decoding=
=MQTT Encoding and Decoding=


The MQTT protocol is very lightweight (does not use many bytes to get the message across).
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:
Each MQTT packet consists of:


* Fixed header
* Fixed header
** packet type:byte
** 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
*** 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").
*** 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
* Variable header (optional), depending on the package type
* Payload (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 "Array" is binary data, after the number of bytes in the array encoded as two bytes, MSB first.


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


{| 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
|-
|-
! Msg ID
! Packet Type
!  
! Flags
! Msg ID?
! Content
|-
|-
| CONNECT
| CONNECT
| Device to Broker
| 1
| 1
| Device to Broker
| 0
| 0
| -
| -
|  
|  
* "MQTT":String
* protocol name:String (always "MQTT")
* protocol level (4):byte
* protocol level:byte (always 4)
* connect flags:byte
* connect flags:byte
** b7: user
** b7: user
Line 42: Line 46:
** b4-3: will QoS
** b4-3: will QoS
** b2: will
** b2: will
** b1: clean session)
** b1: clean session
* keep alive:word
* keep alive:word
|
|
Line 52: Line 56:
|-
|-
| CONNACK
| CONNACK
| Broker to Device
| 2
| 2
| Broker to Device
| 0
| 0
| -
| -
Line 60: Line 64:
** b0: session present
** b0: session present
* connect return code:byte
* connect return code:byte
** 0: connection accepted
** 0: connection accepted (success)
** 1: unacceptable protocol version
** 1: unacceptable protocol version (fail)
** 2: identifier rejected
** 2: identifier rejected (fail)
** 3: server unavailable
** 3: server unavailable (fail)
** 4: bad user name or password
** 4: bad user name or password (fail)
** 5: not authorized
** 5: not authorized (fail)
| -
| -
|-
|-
| PUBLISH
| PUBLISH
| Both
| 3
| 3
| Both
|  
|  
* b3: DUP
* b3: DUP
Line 78: Line 82:
|
|
* topic name:String
* topic name:String
* message ID:word
* message ID:word (if QoS > 0)
| payload:Array (optional)
| payload:Array (optional)
|-
|-
| PUBACK
| PUBACK
| Both
| 4
| 4
| Both
| 0
| 0
| y
| y
Line 91: Line 95:
|-
|-
| PUBREC
| PUBREC
| Both
| 5
| 5
| Both
| 0
| 0
| y
| y
Line 100: Line 104:
|-
|-
| PUBREL
| PUBREL
| Both
| 6
| 6
| Both
| 1 (you may read this as "fixed QoS of 1")
| 1
| y
| y
|  
|  
Line 109: Line 113:
|-
|-
| PUBCOMP
| PUBCOMP
| Both
| 7
| 7
| Both
| 0
| 0
| y
| y
Line 118: Line 122:
|-
|-
| SUBSCRIBE
| SUBSCRIBE
| Device to Broker
| 8
| 8
| Device to Broker
| 1 (you may read this as "fixed QoS of 1")
| 1
| y
| y
|  
|  
Line 130: Line 134:
|-
|-
| SUBACK
| SUBACK
| Broker to Device
| 9
| 9
| Broker to Device
| 0
| 0
| y
| y
Line 143: Line 147:
|-
|-
| UNSUBSCRIBE
| UNSUBSCRIBE
| Device to Broker
| 10
| 10
| Device to Broker
| 1 (you may read this as "fixed QoS of 1")
| 1
| y
| y
|  
|  
Line 154: Line 158:
|-
|-
| UNSUBACK
| UNSUBACK
| Broker to Device
| 11
| 11
| Broker to Device
| 0
| 0
| y
| y
Line 163: Line 167:
|-
|-
| PINGREQ
| PINGREQ
| Device to Broker
| 12
| 12
| Device to Broker
| 0
| 0
| -
| -
Line 171: Line 175:
|-
|-
| PINGRESP
| PINGRESP
| Broker to Device
| 13
| 13
| Broker to Device
| 0
| 0
| -
| -
Line 179: Line 183:
|-
|-
| DISCONNECT
| DISCONNECT
| Device to Broker
| 14
| 14
| Device to Broker
| 0
| 0
| -
| -

Latest revision as of 12:49, 13 June 2018

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

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