03. Tencent cloud IOT device side learning -- overview of mqtt control package

The main purpose of this series is to record the learning notes of Tencent cloud Internet of things device , And on the device side SDK Make a supplementary explanation .

Overview of control package format

MQTT Control package format

MQTT All control packages are controlled by 3 Part of it is made up of ： Fixed head + Variable head + Message body . The fixed head is divided into Package type 、 Sign and remaining length .

		CONNECT	CONNACK	PUBLISH	PUBACK	SUBSCRIBE	SUBACK	UNSUBSCRIBE	UNSUBACK	PINGREQ	PINGRESP	DISCONNECT
Fixed head	Package type 4bits	0001	0010	0011	0100	0100	1001	1010	1011	1100	1101	1110
	sign 4bits	Retain 0000	Retain 0000	DUP 1bit QoS 2bits RETAIN 1bit	Retain 0000	Retain 0010	Retain 0000	Retain 0010	Retain 0000	Retain 0000	Retain 0000	Retain 0000
	Residual length 1-4 bytes	Indefinite	0x02	Indefinite	0x02	Indefinite	Indefinite	Indefinite	0x02	0x00	0x00	0x00
Variable head		The name of the agreement 6bytes Agreement level 1 byte Connection sign 1 byte Time to live 2 bytes	Connection confirmation mark 1 byte Connection return value 1 byte	The theme ( length + topic name ) ( Support 1 individual ) package ID 2bytes ( only QoS1)	package ID Same as PUBLISH	package ID 2bytes	package ID Same as SUBSCRIBE	package ID 2bytes	package ID Same as UNSUBSCRIBE	nothing	nothing	nothing
Message body		ClientIdusr name （ Optional ）password （ Optional ）	nothing	Application messages	nothing	The theme ( length + topic name ) ( Support 1 individual ) Expect the most QoS 1 byte	Return value 1 byte	The theme ( length + topic name )( Support 1 individual )	nothing	nothing	nothing	nothing

Control package functions

• CONNECT： The client initiates a connection to the platform , The first packet sent per connection
• CONNACK：CONNECT Reply to
• PUBLISH： The client publishes messages to the server , The server publishes messages to the client
• PUBACK：QoS1 Used as PUBLISH reply ,QoS0 No such reply
• SUBSCRIBE： Subscribe to topics , In a conversation , The client receives the corresponding topic The published message must be subscribed first
• SUBACK：SUBSCRIBE Reply to
• UNSUBSCRIBE： Unsubscribe from a topic
• UNSUBACK：UNSUBSCRIBE Reply to
• PINGREQ： To keep alive
• PINGRESP：PINGREQ Reply to
• DISCONNECT： Client disconnected , However, the reason for disconnecting through the client is that the network is disconnected , At this time, in fact DISCONNECT The packet cannot be sent out

MQTT flow chart

Control packet capture analysis

have access to SDK adopt TCP Not TLS Way to connect to the platform 1883 port , Run the example , Carry out grab bag analysis .

MQTT CONNECT

A brief analysis ,CONNECT The package structure is the same as above

• The head is basically fixed , The only one that can be set is Connect Flags Medium Clean Session, And the use of TLS Authentication may not be required User Name and Password
• Not TLS Under authentication , Connecting platform 1883 port , Authentication passed User Name and Password.

MQTT CONNACK

Mainly aimed at CONNECT Reply of package , No other special , Mainly the return value Retrun Code There will be differences on

---

MQTT SUBSCRIBE QoS0

MQTT SUBACK QoS0

MQTT SUBSCRIBE QoS1

MQTT SUBACK QoS1

SUBSCRIBE and SUBACK Is the corresponding relation , among Message Identifier To match ,QoS Settings should also correspond to .

---

MQTT PUBLISH QoS0

QoS0 Of PUBLISH The package didn't reply

MQTT PUBLISH QoS1

MQTT PUBACK QoS1

PUBLISH and PUBACK Is the corresponding relation , among Message Identifier To match ,QoS Settings should also correspond to .

---

MQTT DISCONNECT

When you disconnect , Sent by client , Usually not , Because in general MQTT It's all about maintaining a long connection , And the connection is often disconnected because of the network , This is the time DISCONNECT Packets generally cannot be sent out