Developing MQTT Clients – Guide

Introduction

MQTT Client development guide will help you to develop Clients as per the Oasis Standard of MQTT Protocol. The developed clients can be connected to the MQTTRoute, a message broker which follows the OASIS standard MQTT version 3.1.1.

The MQTTRoute is a client Broker which is a publish/subscribe messaging transport protocol oriented that is supporting MQTT. It is a lightweight messaging protocol. This feature made MQTT suitable for Machine 2 Machine communication and the Internet of Things, where a small code footprint is required, and network bandwidth is precious. The protocol runs over TCP/IP. The publish/subscribe message pattern provides one too many message distribution and decoupling of MQTT clients. Other necessary features include quality of service levels, Last Will & Testament and Retained messages.

Getting Started

Machine-to-Machine, Machine-to-User, Machine-to-Mobile communication cannot happen without a powerful central server. MQTT edge devices and applications communicate via the MQTT Broker which acts as the central server. You can use any open source MQTT Broker or download the Enterprise MQTT Broker for FREE which can be run on Windows, Linux, Mac & Raspberry Pi now and get started.

 

MQTT Packet Format

The Broker will respond to the following structured packet controls. The packet control consists of three parts in the following order.

  • Fixed header – presents on all MQTT control packets
  • Variable header – presents in some MQTT control packets
  • Payload – presents in some MQTT control packets

1. Fixed Header

x is the bit value variation for each packet.

Example-connect packet

Bit 7 6 5 4 3 2 1 0
Byte 1 MQTT Control Packet type (1) Reserved
0 0 0 1 0 0 0 0
Byte 2 Remaining length
Here,0001 will represent 1st packet type CONNECT

The fixed header contains two byte

  • 1st byte is splited into two as 4 bit unsigned, one is for control packet another one is for flag
  • 2nd byte is for remaining length

2. Variable Header

The Variable header must be defined inside the fixed header and payload. The content of a variable header will vary depend on the packet type. Common format of a packet identifier in most packet is

 

Bit 7 6 5 4 3 2 1 0
Byte 1 Packet Identifier MSB
Byte 2 Packet Identifier LSB
The following packets contains a Packet identifier

  • PUBLISH
  • PUBACK
  • PUBREC
  • PUBREL
  • PUBCOMP
  • SUBSCRIBE
  • SUBACK
  • UNSUBSCRIBE
  • UNSUBACK

3. Payload

The payload is the final part of a packet. It is the application message. The Control packets which contain a payload are

  • CONNECT
  • PUBLISH
  • SUBSCRIBE
  • SUBACK
  • UNSUBSCRIBE

CONNECT

After MQTT connection establishment between an MQTT client and Broker, the first packet must be a CONNECT packet. The CONNECT packet must be sent only once over a network connection. The second CONNECT Packet sent from an MQTT Client will be ignored and disconnected.

Fixed Header

Bit 7 6 5 4 3 2 1 0
byte1 MQTT control packet type (1) Reserved
0 0 0 1 0 0 0 0
byte2… Remaining length

Variable Header

The variable header has to contain four parts

  • Protocol name bytes
  • Protocol level
  • Connect flags
  • Keepalive

Protocol name bytes

 

description 7 6 5 4 3 2 1 0
Protocol Name
Byte 1 Length MSB (0) 0 0 0 0 0 0 0 0
Byte2 Length LSB (4) 0 0 0 0 0 1 0 0
Byte3 M 0 1 0 0 1 1 0 1
Byte4 Q 0 1 0 1 0 0 0 1
Byte5 T 0 1 0 1 0 1 0 0
Byte6 T 0 1 0 1 0 1 0 0

 

A incorrect protocol name will be disconnected by MQTTRoute

Protocol level

 

7 6 5 4 3 2 1 0
Protocol Level
Byte 7 Leve(4) 0 0 0 0 0 1 0 0

 

Protocol level for the MQTTRoute is 4, other values will lead to disconnection

 

Connect flags

 

Bit 7 6 5 4 3 2 1 0
User name flag Password flag Will retain Will qos Will flag cleansession reserved
Byte 8 x x x x x x x X

 

This field has to mention the presence or absence of data in a payload.

 

Keepalive

 

Bit 7 6 5 4 3 2 1 0
Byte 9 Keep Alive MSB
Byte 10 Keep Alive LSB

 

The MQTT client must send the control packets not exceeding the value of defined keepalive. The keep-alive value will depend on the client’s control packet duration of one transmission.

 

Payload

The payload contains the MQTT Client identifiers for the client eg: username, password. The client id must be unique to each client the Broker will respond to the unique client id for each client. Sending client id with an empty field leads to rejection of the client. The following fields must be taken in the following order only Client Identifier, Will Topic, Will Message, User Name, Password. The above fields must be in string format. The packet which does not send CONNECT packet after a reasonable time will be rejected by the Broker. After the above validation success, the Broker will respond in 2 format

  • Check whether the client is already present if present the Broker will respond with a disconnection.
  • The Broker will send a CONNACK packet with zero value.

After a successfull CONNACK the Broker will do a keepalive monitoring.

CONNACK

The Broker will respond with a CONNACK for a successful connection. If a CONNACK packet is not received by an MQTT client within a reasonable time the client must close the connection the response time will depend on the infrastructure.

Fixed Header

 

Bit 7 6 5 4 3 2 1 0
Byte1 MQTT Control Packet Reserved
0 0 1 0 0 0 0 0
Byte2 Remaining Length
0 0 0 0 0 0 1 0

 

Remaining value is the length of variable header in CONNACK it has to be 2.

 

Variable header

 

bit 7 6 5 4 3 2 1 0
Connect Acknowledgement flag Reserved sp
Byte 1 0 0 0 0 0 0 0 x
Connect Return code
Byte 2 x x x x x x x x

 

Byte1 is the connection acknowledge flags and bits 7 to 1 are reserved. And bit 0 is for session present(sp) flag -it indicates the presence of a clientid in Broker.

PINGRESP

It is the response for PINGREQ from MQTT Route to show that the Broker is alive.

Fixed header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 MQTT Control Packet type (13) Reserved
1 1 0 1 0 0 0 0
Byte 2 Remaining length(0)
0 0 0 0 0 0 0 0

 

It does not have variable header.

PUBLISH

 

The client who wants to send a MQTT messages to a specified topic can use the PUBLISH control packet and the broker will forward this MQTT messages to the clients who subscribed to that same topic.

 

Fixed header

 

Bit 7 6 5 4 3 2 1 0
byte1 MQTT control packet type (3) DUP QoS Retain
  0 0 1 1 x x x x
byte2 Remaining length

 

 

Flag section

3.DUP flag-fresh publish has to be set as 0 and redelivery has to be set as 1.
2,1.QOS level-this flag represent the level of assurance for an application message delivery.
0-at most one delivery
1-at least one delivery
2-exactly one delivery
0.retain-retain flag set as 1 application message must be stored
retain flag set as 0 application message must not be stored.

 

Variable header

 

Bit 7 6 5 4 3 2 1 0
Byte 3 Topic Name Length(MSB)  
Byte 4 Topic Name Length (LSB)
Byte 5…n Topic name
Byte n+1 Message Id(MSB)
Byte n+2 Message Id (LSB)

It contains a topic name, a packet identifier

Topic name

The matching topic name is only sent to subscribers. It has to be the first field of the PUBLISH packet and must be a string.

Packet Identifier

The packet identifier must be present in QoS (Quality of Service) 1 and 2.

Payload

The payload contains the application message to be published after Byte n+2. The client can expect the following response after a PUBLISH

For QoS 0-none
For QoS 1-PUBACK packet
For QoS 2-PUBREC packet

SUBACK

The Broker will respond with a SUBACK for a Subscribe

Fixed Header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 MQTT Control Packet type (9) Reserved
1 0 0 1 0 0 0 0
Byte 2 Remaining length

Remaining length

It should have the length of variable header plus the length of the payload. SUBACK will be response to the SUBSCRIBE.

Variable header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 Packet Identifier MSB
Byte 2 Packet Identifier LSB

Payload

 

bit 7 6 5 4 3 2 1 0
Return code
Byte 1 x 0 0 0 0 0 x x

A matching return code for a subscribe will be sent by the Broker. The Client have to expect the following return codes.

 

Bit 7 Bit 1 Bit 0
QoS 0 0 0 0
QoS 1 0 0 1
QoS 2 0 0 2
For Failure Not 0 Not 0 Not 0,1,2

UNSUBSCRIBE

The client has to send this UNSUBSCRIBE packet when it wants to UNSUBSCRIBE from a particular topic.

 

Bit 7 6 5 4 3 2 1 0
Byte 1 MQTT Control Packet type (10) Reserved
1 0 1 0 0 0 1 0
Byte 2 Remaining length

 

Bits 3,2,1,0 of the fixed header must be 0,0,0,1. Otherwise the Broker will disconnect the client. The remaining length has to be the length of variable header and the length of the payload.

Variable Header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 Packet Identifier MSB
Byte 2 Packet Identifier LSB

Payload

 

Byte N… Topic Name

The payload has to contain at least one topic name otherwise it will be discarded. The topic name must be a String. The unsubscribe must be done after the values on the buffer or the bending QoS 1 and QoS 2 messages have been sent.

UNSUBACK

The UNSUBACK is sent from Broker to show the response for an UNSUBSCRIBE.

Fixed Header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 MQTT Control Packet type (11) Reserved
1 0 1 1 0 0 0 0
Byte 2 Remaining length(2)
0 0 0 0 0 0 1 0

Variable Header

The variable header will contain the Packet id of the UNSUBSCRIBE packet and returned from the Broker for the response of UNSUBSCRIBE.

 

Bit 7 6 5 4 3 2 1 0
Byte 1 Packet Identifier MSB
Byte 2 Packet Identifier LSB

DISCONNECT

Fixed Header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 MQTT Control Packet type (14) Reserved
1 1 1 0 0 0 0 0
Byte 2 Remaining length(0)
0 0 0 0 0 0 0 0

 

There is no variable header and payload for Disconnect. The DISCONNECT packet is the final control packet to be sent from the client. The Broker will validate the reserved bits for 0 if not the client will be disconnected. After the DISCONNECT packet, the client must close the network connection and must not send any control packets.

PUBREL

It is the acknowledgment packet to the PUBREC

Fixed header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 MQTT Control Packet type (6) Reserved
0 1 1 0 0 0 1 0
Byte 2 Remaining length(2)
0 0 0 0 0 0 1 0

 

The bits in fixed header 3,2,1,0 are to be set as 0,0,1,0 respectively otherwise the connection will be closed.

Variable header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 Packet Identifier MSB
Byte 2 Packet Identifier LSB

 

Contains the packet identifier for which the PUBLISH acknowledged.

MQTT Packet Types

The MQTT Control packet consist of 14 packets

  1. CONNECT – MQTT Client requests a connection to a Broker
  2. CONNACK – Acknowledge connection request
  3. PUBLISH – Publish message
  4. PUBACK – Publish acknowledgement
  5. PUBREC – Publish received(QoS 2 publish received)
  6. PUBREL – Publish release(QoS 2 publish received)
  7. PUBCOMP – Publish complete
  8. SUBSCRIBE – Subscribes to a topic
  9. SUBACK – Subscribe acknowledgment
  10. UNSUBSCRIBE – Unsubscribe from a topic
  11. UNSUBACK – Unsubscribe acknowledgement
  12. PINGREQ – PING request
  13. PINGRESP – PING response
  14. DISCONNECT – Disconnect notification

PUBACK

The PUBACK is a response to a publish.

Fixed header

 

Bit 7 6 5 4 3 2 1 0
Byte1 MQTT Control Packet(4) Reserved
0 1 0 0 0 0 0 0
Byte2 Remaining Length
0 0 0 0 0 0 1 0

 

The remaining length of fixed header will represent the length of the variable header.

Variable Header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 Packet Identifier MSB
Byte 2 Packet Identifier LSB

 

This contains the packet identifier from PUBLISH for which it is acknowledging.

PUBREC

It is the response of PUBLISH in QoS 2

Fixed header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 MQTT Control Packet type (5) Reserved
0 1 0 1 0 0 0 0
Byte 2 Remaining length(2)
0 0 0 0 0 0 1 0

Variable header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 Packet Identifier MSB
Byte 2 Packet Identifier LSB

PUBCOMP

It is the response to the PUBREL in QoS 2.

Fixed header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 MQTT Control Packet type (7) Reserved
0 1 1 1 0 0 0 0
Byte 2 Remaining length(2)
0 0 0 0 0 0 1 0

Variable header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 Packet Identifier MSB
Byte 2 Packet Identifier LSB

SUBSCRIBE

The MQTT Client has to send a SUBSCRIBE packet for subscriptions. The Broker will respond with the PUBLISH packet with its payload.

Fixed header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 MQTT Control Packet type (5) Reserved
1 0 0 0 0 0 1 0
Byte 2 Remaining length

 

Bits 3,2,1,0 has to be set as 0,0,1,0. It contains the topic to which the MQTT Client subscribing. The topic must be a string.

Variable header

 

Bit 7 6 5 4 3 2 1 0
Byte 1 Packet Identifier MSB
Byte 2 Packet Identifier LSB

Payload

 

Description 7 6 5 4 3 2 1 0
Topic filter
Byte1 Length MSB
Byte2 Length LSB
Byte3…N Topic filter
Requested QoS
Reserved QoS
Byte N+1 0 0 0 0 0 0 x x

 

The Payload has to mention at least one topic/QoS mention otherwise it will be discarded. The subscriber with no payload is a violation of the structure and will be discarded. The topic Name must be a string. If any field in the reserved field is non-zero or the QoS is not 0,1,2 the client will be discarded.

PINGREQ

The MQTT Client must send a PINGREQ to the Broker to say it is alive on the network when it does not send any control packets and have to make a routine check to indicate it’s alive.

Bit 7 6 5 4 3 2 1 0
Byte 1 MQTT Control Packet type (12) Reserved
1 1 0 0 0 0 0 0
Byte 2 Remaining length(0)
0 0 0 0 0 0 0 0

 

It should not have variable header and payload.

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