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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

 

 

 

 

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