[7] mqtt,mosquitto,eclipse Paho---MQTT message format connack message analysis

In the previous section ([6] mqtt,mosquitto,eclipse Paho---The MQTT message format for connect message format analysis) We analyzed the Connect message format, and we knew that the connect message was sent out by the client, As a connection request to the client, the server side will also have a return of the message, which is the connack message. After we send the Connect message, if the Wireshark capture tool is still turned on, it will catch a TCP message similar to the following, its 16 binary is: 20 02 00 00, then what does it mean?

First, let's look at the message format of Connack. The connack message does not have a message payload (Payload), only the message header: 2 bytes of fixed message header and 22 bytes of variable message headers. A total of 4 bytes

1. Fix header (fixed header): 16 binary data: 20 02

For the specific protocol format, see table 1 below

Table 1 – Format of fixed header

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

20 indicates that its current message type is: CONNACK
02 indicates that the following 2 bytes are followed.

2. Variable message header (Variable header): 16 binary data is: XX

For the specific protocol format, see table 2 below

Table 2 – Format of variable message header (Variable header)

	Describe	7	6	5	4	3	2	1	0
Connection answer flag bit		Reserved (not used)							SP1
BYTE 1		0	0	0	0	0	0	0	X
Connection results returned
BYTE 2		X	X	X	X	X	X	X	X

Let's analyze the 2 bytes of its variable header: 00 00

The first 00, which currently has no special meaning, is a reserved field for the MQTT protocol and may be used in future protocol versions.

A 00 of the second byte indicates a successful connection. The MQTT protocol defines a total of 6 types for the returned results (see table 3 below): Connection accepted, connection denied (unacceptable version), connection denied (Client ID server not allowed), connection denied ( server unreachable), connection denied (bad user name and password), connection denied (client not authenticated by authorization).

Table 3-Meaning of the return code value of the connection answer

Value	The connection code returned	Describe
0	0x00 Connection Accepted	Connection accepted
1	0x01 Connection denied (unacceptable version)	The service side does not support the protocol of the connected Mqtt
2	0x02 connection denied (Client ID server not allowed)	The client ID complies with the UTF-8 standard, but the server does not allow this client ID
3	0x03 Connection denied (server unreachable)	MQTT Server Unreachable
4	0x04 connection denied (bad user name and password)	Bad user name and password
5	0x05 Connection denied (client not authenticated by authorization)	Client does not have authorization authentication
6-255		Future reserved Fields

This message format is simple, hey, congratulations on your journey to learn the MQTT message format and move forward a step further, the next chapter, I will continue to explainSubscribe message Format, please pay attention and look forward to, thank you.

[7] mqtt,mosquitto,eclipse Paho---MQTT message format connack message analysis