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This article mainly explains "what is the principle of MQTT protocol". Interested friends may wish to have a look at it. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "what is the principle of MQTT protocol"?

I. brief introduction

MQTT (Message Queuing Telemetry Transport, message queuing Telemetry Transport Protocol) is a "lightweight" communication protocol based on publish / subscribe (publish/subscribe) mode, which is based on TCP/IP protocol and was released by IBM in 1999. The biggest advantage of MQTT is that it can provide real-time and reliable message service for connecting remote devices with very little code and limited bandwidth. As an instant messaging protocol with low overhead and low bandwidth consumption, it is widely used in Internet of things, small devices, mobile applications and so on.

MQTT is a client-server based message publish / subscribe transport protocol. MQTT protocol is lightweight, simple, open and easy to implement, which makes it suitable for a wide range of applications. In many cases, including constrained environments, such as machine-to-machine (M2M) communication and the Internet of things (IoT). It has been widely used in satellite link communication sensors, occasionally dialed medical devices, smart homes, and some miniaturized devices.

II. Design specifications

Because the environment of the Internet of things is very special, MQTT follows the following design principles:

(1) simplify and do not add optional features

(2) publish / subscribe (Pub/Sub) mode to facilitate the transmission of messages between sensors

(3) allow users to create topics dynamically with zero operation and maintenance cost

(4) reduce the transmission volume to a minimum to improve the transmission efficiency.

(5) low bandwidth, high latency, unstable network and other factors are taken into account.

(6) support continuous session control

(7) understanding that client computing power may be very low

(8) providing service quality management

(9) assume that the data is unknowable, do not force the type and format of the transmitted data, and maintain flexibility.

III. Main characteristics

MQTT protocol is a protocol designed to communicate between remote sensors and control devices working in low-bandwidth, unreliable networks. It has the following main characteristics:

(1) using the publish / subscribe message pattern to provide one-to-many message publishing and uncoupling the application.

This is similar to XMPP, but MQTT has much less information redundancy than XMPP, because XMPP uses XML-formatted text to pass data.

(2) message transmission that masks payload content.

(3) use TCP/IP to provide network connection.

The mainstream MQTT pushes data based on TCP connections, but there is also a UDP-based version called MQTT-SN. Because the two versions are based on different connection methods, their advantages and disadvantages are naturally different.

(4) there are three kinds of quality of service for message publishing:

"at most once," the release of messages depends entirely on the underlying TCP/IP network. Messages can be lost or duplicated. This level can be used in the case of environmental sensor data, it doesn't matter if you lose a read record, because there will be a second transmission soon. This method is mainly ordinary APP push, if your smart device is not online when the message is pushed, and the push is not received in the past, it will not be received when you connect to the Internet again.

"at least once" to ensure that the message arrives, but message repetition may occur.

"only once", make sure the message arrives once. This level can be used in some more stringent billing systems. In the billing system, duplicate or lost messages can lead to incorrect results. This highest quality messaging service can also be used for instant messaging APP push, ensuring that users receive it and only receive it once.

(5) small transmission, the overhead is very small (the fixed-length header is 2 bytes), and protocol switching is minimized to reduce network traffic.

This is why it is said in the introduction that it is very suitable for "in the field of the Internet of things, the communication between sensors and servers, the collection of information", knowing that the computing power and bandwidth of embedded devices are relatively weak, it is perfect to use this protocol to transmit messages.

(6) the mechanism for notifying the parties concerned of abnormal client interruptions using Last Will and Testament features.

Last Will: the last note mechanism, which is used to notify other devices under the same topic that the device that sent the last message has been disconnected.

Testament: a testamentary mechanism that functions like Last Will.

4. The principle of MQTT Protocol 4.1 the implementation of MQTT Protocol

The realization of MQTT protocol requires communication between client and server. In the process of communication, there are three identities in MQTT protocol: publisher (Publish), agent (Broker) (server) and subscriber (Subscribe). Among them, the publisher and subscriber of the message are both clients, the message broker is the server, and the message publisher can be the subscriber at the same time.

The message transmitted by MQTT is divided into two parts: topic (Topic) and load (payload):

(1) Topic, which can be understood as the type of message. After subscribing to (Subscribe), subscribers will receive the message content (payload) of the topic.

(2) payload, which can be understood as the content of the message, refers to the specific content that the subscriber wants to use.

4.2 Network transmission and application messages

MQTT builds the underlying network transport: it establishes a client-server connection, providing an orderly, lossless, byte-stream-based two-way transport between the two.

When the application data is sent over the MQTT network, the MQTT will associate the quality of service (QoS) associated with it with the subject name (Topic).

4.3 MQTT client

An application or device that uses the MQTT protocol that always establishes a network connection to the server. The client can:

(1) publish information that other clients may subscribe to

(2) subscribe to messages published by other clients

(3) unsubscribe or delete messages from the application

(4) disconnect from server.

4.4 MQTT Server

The MQTT server, known as a "message broker" (Broker), can be an application or a device. It is located between message publishers and subscribers, and it can:

(1) accept network connections from customers

(2) accept the application information released by the customer

(3) handle subscription and unsubscription requests from the client

(4) forward application messages to subscribed customers.

4.5 subscriptions, topics, sessions in the MQTT protocol

1. Subscription (Subscription)

Subscriptions include topic filters (Topic Filter) and maximum quality of service (QoS). Subscription is associated with a session (Session). A session can contain multiple subscriptions. Each subscription in each session has a different topic filter.

II. Conversation (Session)

After each client establishes a connection with the server, there is a session and there is a stateful interaction between the client and the server. A session exists between a network and may span multiple consecutive network connections between the client and the server.

III. Topic name (Topic Name)

A label that connects to an application message that matches the subscription of the server. The server sends a message to each client that subscribes to the matching label.

Topic filter (Topic Filter)

A wildcard filter for topic names, used in subscription expressions, represents multiple topics to which the subscription matches.

5. Load (Payload)

The specific content received by the message subscriber.

4.6 methods in the MQTT protocol

Methods (also known as actions) are defined in the MQTT protocol to represent operations on identified resources. This resource can represent pre-existing data or dynamically generated data, depending on the implementation of the server. Generally speaking, a resource refers to a file or output on a server. The main methods are:

(1) Connect. Waiting for a connection to be established with the server.

(2) Disconnect. Wait for the MQTT client to finish its work and disconnect the TCP/IP session from the server.

(3) Subscribe. Wait for the subscription to complete.

(4) UnSubscribe. Wait for the server to cancel one or more topics subscriptions from the client.

(5) Publish. The MQTT client sends a message request and returns to the application thread after the message is sent.

5. MQTT protocol packet structure

In MQTT protocol, a MQTT packet consists of three parts: fixed header (Fixed header), variable header (Variable header) and message body (payload). The structure of MQTT packets is as follows:

(1) fixed head (Fixed header). Exists in all MQTT packets and indicates the packet type and packet class identification of the packet.

(2) variable head (Variable header). It exists in some MQTT packets, and the packet type determines whether the variable header exists and its specific content.

(3) message Payload. It exists in some MQTT packets and indicates the specific content received by the client.

5.1 MQTT fixed head

Fixed headers exist in all MQTT packets and are structured as follows:

5.1.1 MQTT packet Typ

Location: bits 7-4 in Byte 1.

Compared to a 4-bit unsigned value, the type, value, and description are as follows:

5.1.2 Identification bit

Location: bits 3-0 in Byte 1.

In message types that do not use identity bits, identity bits are used as reserved bits. If an invalid flag is received, the receiver must close the network connection:

(1) DUP: publish a copy of the message. To ensure the reliable transmission of the message, if set to 1, MessageId is added in the following variable length, and a reply confirmation is required to ensure that the message transmission is complete, but cannot be used to detect message repeat transmission.

(2) QoS: the quality of service for publishing messages, that is, the number of times the messages are guaranteed to be delivered

At most one time, that is,: = 1: 10: once, that is: = 1: 11: reservation

(3) RETAIN: publish a reservation flag, indicating that the server wants to retain the information of this push. If a new subscriber appears, it will push the message to it. If it is available, it will be pushed to the current subscriber and released. 5.1.3 remaining length (Remaining Length)

Address: Byte 2.

The second byte of the fixed header is used to hold the total size of the variable length header and the message body, but not directly. This byte is expandable, and its preservation mechanism is that the first 7 bits are used to save the length, and the latter part is used to identify. When the last bit is 1, it is not long enough and needs to be saved with two bytes. For example, the latter size is calculated to be 0

5.2 MQTT variable head

The MQTT packet contains a variable header that resides between the fixed header and the payload. The content of the variable header varies depending on the type of packet, and it is more often used as the identification of the packet:

Many types of packets include a 2-byte packet identification field. These types of packets are: PUBLISH (QoS > 0), PUBACK, PUBREC, PUBREL, PUBCOMP, SUBSCRIBE, SUBACK, UNSUBSCRIBE, UNSUBACK.

5.3 Payload message body

The third part of the Payload message body MQTT packet, which contains four types of messages: CONNECT, SUBSCRIBE, SUBACK, and UNSUBSCRIBE:

(1) CONNECT, the main contents of the message body are: the ClientID of the client, the Topic and Message of the subscription, and the user name and password.

(2) SUBSCRIBE, the message body content is a series of subscribed topics and QoS.

(3) SUBACK, the content of the message body is that the server confirms and replies to the subject and QoS applied for by SUBSCRIBE.

(4) UNSUBSCRIBE, the body of the message is the topic to be subscribed to.

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