Let you understand the MQTT protocol

Author: Wang Yingyue, Unit: China Mobile Smart Home Operation Center

Labs Guide

At present, the MQTT protocol is widely used in the fields of autonomous driving, industry, communications, etc. In the smart home market, many IoT platforms such as China Mobile, Huawei, Alibaba, and Tencent support MQTT protocol access. This article will introduce the characteristics of the MQTT protocol, how it works, and other related issues in detail.

MQTT is an IoT messaging protocol under the Organization for the Advancement of Structured Information Standards (OASIS) standard. Its lightweight publish/subscribe transmission mechanism is very suitable for providing reliable message communication services for remote connected devices. Currently, the MQTT protocol is widely used in fields such as autonomous driving, industry, and communications. In the smart home market, many IoT platforms such as China Mobile, Huawei, Alibaba, and Tencent support MQTT protocol access.

Part 01 Characteristics of MQTT protocol

- Lightweight and efficient

Only very few resources are needed to implement the MQTT client, which makes it easy for the MQTT protocol to run on a microchip. The extremely simple message setting achieves minimal network overhead.

- Range Broadcast

Easily broadcast messages to millions of devices.

- Stable transmission

Three message transmission assurance levels provide reliability assurance for message delivery in different scenarios.

- Weak network support

The session persistence mechanism reduces the device reconnection time in a weak network environment.

- Security

Support multiple authentication protocols to ensure message security

Part 02How does the MQTT protocol work?

The MQTT protocol defines two types of network entities: message broker nodes (Broker) and MQTT clients (Client). Broker receives messages sent by Client and is responsible for forwarding these messages to the specified Client. Client is any entity that can interact with Broker and is responsible for sending and receiving messages. Common Clients include IoT sensors and cloud servers. It can be seen that in the MQTT protocol, messages are forwarded through Broker, rather than directly connecting and communicating between one Client and another. To distinguish, we can define Client as a message sender (Publisher-Client) and a message receiver (Subscriber-Client).

• The Client establishes a connection with the Broker, which can be a TCP/IP connection or an encrypted TLS connection.
• Subscriber-Client subscribes to a topic.
• Publisher-Client sends a Topic message to Broker.
• The Broker forwards the message to all Subscriber-Clients that subscribe to the Topic.

Part 03MQTT protocol quality assurance

As we all know, IoT devices are often in complex network environments, such as sensor devices in vegetable greenhouses in the agricultural IoT, temperature and humidity sensors in closed workshops in the industrial IoT, etc. In these complex network scenarios, the device status is uncertain. In order to ensure the connection in these scenarios, the MQTT protocol provides message service quality assurance levels (QoS).

• Qos-0 at most once

Qos 0 guarantees that the message will arrive at the destination no more than once. The message may arrive once or not at all. It is suitable for power-constrained devices, which can minimize the propagation time and reduce the message delivery cost; it is suitable for non-critical message delivery, such as messages that are frequently sent repeatedly.

• Qos-1 at least once

Qos 1 ensures that the message reaches the destination at least once. After confirming the receipt of the message, the Broker will return a PUBACK message to the Publisher-Client. If the message fails to be sent, the Publisher-Client will resend the message after a period of time. This type of message is suitable for the delivery of important messages that cannot be lost. This transmission mechanism can be used to ensure that the message can still be received after the offline device is online. The Subscriber-Client needs to handle duplicate messages.

• Qos-2 only once

Qos 2 ensures that the message reaches the destination only once. This type of message ensures that the receiving end receives a message only once. It is the safest and slowest form of message transmission in the MQTT protocol. It is suitable for scenarios where it is very important and duplicate messages are not allowed. Because the message is very important, the additional network overhead can be ignored.

Part 04 Minimum MQTT application construction

After understanding the basic theoretical concepts of MQTT, let's build a minimum MQTT service on the Windows system to implement the sending and subscription of MQTT messages.

(1) Broker Construction

We chose the open source broker platform Mosquitto as the single-point broker for this minimal MQTT application.

After downloading the Mosquitto Windows version, open the mosquitto.exe file through the Windows system terminal. Enter ./mosquitto.exe -p 1883 to start an MQTT service on the Windows system, with the listening address at 127.0.0.1 and the port at 1883.

(2) Client Construction

After completing the Broker setup, we also need the Client to send and subscribe messages. We choose the mqtt.fx software as the Client for this minimal MQTT application. Open the mqtt.fx software, set the MQTT Broker Address to 127.0.01, the Broker Port to 1883, and customize the Client ID. After completing the settings, click Connect on the main page. The connection indicator in the upper right corner of the page turns green, indicating that the connection is complete. At this time, we have achieved the connection from the Client to the Broker.

(3) Message subscription

Assume that the Client created in step 2 is a Subscriber-Client. At this time, click Subscribe on the main page and fill in the subscription Topic information. In this example, the Topic I subscribe to is test, and the Client can complete the subscription to the Topic.

(4) Message sending

Reopen an mqtt.fx client, create a Publisher-Client, set the MQTT Broker information to be consistent with the Subscriber-Client information, and after successfully connecting to the Broker, on the Publish page, send information to the specified Topic, i.e., test. At this time, we can see on the Subscriber-Client interface that the message is successfully forwarded from the Broker to the Subscriber-Client.

At this point, a minimal MQTT application has been built.

In summary, the MQTT protocol focuses on messages and has the characteristics of being lightweight, efficient, and low-overhead. Compared with protocols such as HTTP that are widely used in Internet scenarios, the MQTT protocol is more suitable for the field of IoT communications. ​