[5] MQTT, mosquitto, Eclipse Paho --- MQTT Message format analysis overview, mqttpaho --- mqtt

Source: Internet
Author: User

[5] MQTT, mosquitto, Eclipse Paho --- MQTT Message format analysis overview, mqttpaho --- mqtt

We know that MQTT is a specification of the IOT protocol. The latest two versions of MQTT are: 3.1.1 and 3.1.0.

(1) The specification of 3.1.0 is as follows:
Http://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/mqtt-v3r1.html

(2) 3.1.1 specifications are as follows:
Http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/ OS /mqtt-v3.1.1-os.html#_Toc398718029

There are still many differences between the specification 3.1.0 and 3.1.1. For example, in the variable header, In the 3.1.0 specification, the keyword is "MQISdP"

Table 1. The first eight bytes of the MQTT V3.1 standard Variable Message Header

  Description 7 6 5 4 3 2 1 0
Protocol Name
Byte 1 Length MSB (0) 0 0 0 0 0 0 0 0
Byte 2 Length LSB (6) 0 0 0 0 0 1 1 0
Byte 3 'M' 0 1 0 0 1 1 0 1
Byte 4 'Q' 0 1 0 1 0 0 0 1
Byte 5 'I' 0 1 0 0 1 0 0 1
Byte 6 'S' 0 1 1 1 0 0 1 1
Byte 7 'D' 0 1 1 0 0 1 0 0
Byte 8 'P' 0 1 1 1 0 0 0 0

In MQTT V3.1.1, the keyword of the protocol name in the header of the variable message is "MQTT", as shown in the figure below.

Table 2. The first six bytes of the MQTT V3.1.1 Variable Message Header

 

Description

7

6

5

4

3

2

1

0

Protocol name

Byte 1

Length MSB (0)

0

0

0

0

0

0

0

0

Byte 2

Length LSB (4)

0

0

0

0

0

1

0

0

Byte 3

'M'

0

1

0

0

1

1

0

1

Byte 4

'Q'

0

1

0

1

0

0

0

1

Byte 5

'T'

0

1

0

1

0

1

0

0

Byte 6

'T'

0

1

0

1

0

1

0

0

Protocol Level

 

Description

7

6

5

4

3

2

1

0

Byte 7

Level (4)

0

0

0

0

0

1

0

0


The specifications mainly describe the specifications and rules of common message formats, message commands, message flows, and so on. In the previous four chapters, we have a rough understanding of MQTT. In the next series of articles, I will show you the following 14 command messages of MQTT through WireShark) in addition, we will repeat it with specific specifications. Note that the java library of the Eclipse Paho API that I use supports MQTT 3.1.1 and mosquitto also supports version 3.1.1, therefore, all Packet Capturing rules in this series are based on MQTT 3.1.1, rather than MQTT 3.1. The following table describes different message types in MQTT 14.

Table 3. 14 message types of MQTT (Command message)

Name

Value

Information Flow Direction

Description

Reserved

0

Forbidden

Reserved

CONNECT

1

Client to Server

Client request to connect to Server

CONNACK

2

Server to Client

Connect acknowledgment

PUBLISH

3

Client to Server

Or

Server to Client

Publish message

PUBACK

4

Client to Server

Or

Server to Client

Publish acknowledgment

PUBREC

5

Client to Server

Or

Server to Client

Publish received (assured delivery part 1)

PUBREL

6

Client to Server

Or

Server to Client

Publish release (assured delivery part 2)

PUBCOMP

7

Client to Server

Or

Server to Client

Publish complete (assured delivery part 3)

SUBSCRIBE

8

Client to Server

Client subscribe request

SUBACK

9

Server to Client

Subscribe acknowledgment

UNSUBSCRIBE

10

Client to Server

Unsubscribe request

UNSUBACK

11

Server to Client

Unsubscribe acknowledgment

PINGREQ

12

Client to Server

PING request

PINGRESP

13

Server to Client

PING response

DISCONNECT

14

Client to Server

Client is disconnecting

Reserved

15

Forbidden

Reserved


In the following sections, we will capture packets for WireShark and analyze the Protocols one by one. Thank you for your attention.

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