MQTT, Message, Protocol, Topic, Subscribe, Publish → AWS IoT Core, Azure, Google, Smartthings IoT

 

MQTT Message Protocol

It is one of the message protocols required to send and receive messages between IoT devices.

MQTT protocol can send and receive messages many-to-many through a logical channel called “topic”,

MQTT is based on TCP and SSL/TLS can be applied for security.

→ a message protocol, CoAP is based on UDP, and DTLS can be used for security.

 

 

 

 

1. Operators in MQTT

There are roles of Broker, Publisher, and Subscriber in MQTT.

It relays, publishes messages, and subscribes to messages for a certain topic for each role.

MQTT Broker plays a key role in relaying messages to IoT Core, and IoT devices will mainly be the subjects of publishing and subscribing to messages.

 

Broker	Relaying the message, Broker Example) Mosquitto, HiveMQ, RabbitMQ, 등
Publisher	A device that publishes a message to a topic
Subscriber	A device that subscribes to messages on a topic

 

 

 

 

 

2. Format of MQTT topic

A topic is a target that publishes and subscribes to messages and is expressed in a hierarchical structure.

The way of expression is as follows.

 

To give an example by referring to the table below,

if you want to open the window in the living room of our home, you can issue an open message with the topic “smart-home/livingroom/window”.

Look, is the window open?

 

Hierarchical Structure			Expression as Topic
smart-home	livingroom	window	smart-home/livingrootm/window
		light	smart-home/livingrootm/light
	room1	humidity	smart-home/room1/humidity
		temperature	smart-home/room1/Temperature

 

 

 

 

3. MQTT message/packet structure

MQTT message format is divided into a fixed header(2 bytes), an optional header(variable length), and a payload((variable length)) as shown below.

 

 

 

 

4. MQTT Message Type

 

Message Type	Direction of flow	Description
CONNECT	Client to Server	Client request to connect to Server
CONNACK	Server to Client	Connect acknowledgment
PUBLISH	Client to Server or Server to Client	Publish message
PUBACK	Client to Server or Server to Client	Publish acknowledgment
PUBREC	Client to Server or Server to Client	Publish received (assured delivery part 1)
PUBREL	Client to Server or Server to Client	Publish release (assured delivery part 2)
PUBCOMP	Client to Server or Server to Client	Publish complete (assured delivery part 3)
SUBSCRIBE	Client to Server	Client subscribe request
SUBACK	Server to Client	Subscribe acknowledgment
UNSUBSCRIBE	Client to Server	Unsubscribe request
UNSUBACK	Server to Client	Unsubscribe acknowledgment
PINGREQ	Client to Server	PING request
PINGRESP	Server to Client	PING response
DISCONNECT	Client to Server	Client is disconnecting