Introduction to the MQTT Protocol and Alibaba Cloud's IoT Platform

MQTT Protocol

MQTT (Message Queuing Telemetry Transport) is a message protocol based on the TCP/IP protocol stack and supports asynchronous communication between the parties. MQTT separates the sender from the receiver in space and time, so it can be expanded in an unreliable network environment. Although it is called Message Queuing Telemetry Transport, it does not involve message queuing. It uses the publication and subscription model.

MQTT is a lightweight and flexible network protocol designed to achieve the right balance for IoT developers:

• The lightweight protocol can be implemented on heavily restricted device hardware and networks with high latency or limited bandwidth.
• Its flexibility makes it possible to support various application scenarios for IoT devices and services.

MQTT Client Library

The MQTT Client library is implemented in many languages, including Embedded C, C, Java, JavaScript, Python, C++, C#, Go, iOS, Android, and so on. Here is the download address of the MQTT library for Eclipse Paho.

The following development practice is based on the MQTT for Node.js, and the address can be found here.

MQTT Packet

Fixed Header

Control packet type

The flag bit of control packet type

Remaining length

Note: A single payload of Alibaba Cloud IoT is up to 256 K

Variable Header

Some MQTT control packets contain a variable header. It is between the fixed header and the payload. The content of the variable header varies depending on the packet type. The Packet Identifier field of the variable header exists in multiple types of packets.

Packet identifier bytes

Payload

The following MQTT control packets contain a payload at the end of the packets. For PUBLISH, a payload is a business message.

Connecting to the Alibaba Cloud IoT Platform

CONNECT

The MQTT protocol of the Alibaba Cloud IoT Platform does not support "will" messages, and the CONNECT message content parameters are as follows:

 clientId, username, and password are generated by the device trituples (productKey, deviceName, and deviceSecret) in accordance with the following rules:

Official documents: https://help.aliyun.com/document_detail/73742.html

Example of Device Code (Node.js) Client.js

/**
"dependencies": { "mqtt": "2.18.8" }
*/
const crypto = require('crypto');
const mqtt = require('mqtt');
// The trituples of the device identity + region
const deviceConfig = {
    productKey: "replace",
    deviceName: "replace",
    deviceSecret: "replace",
    regionId: "cn-shanghai"
};
//Generate mqtt connection parameters from the trituples
const options = initMqttOptions(deviceConfig);
const url = `tcp://${deviceConfig.productKey}.iot-as-mqtt.${deviceConfig.regionId}.aliyuncs.com:1883`;

//2. Establish the connection
const client = mqtt.connect(url, options);

client.on('packetsend', function (packet){
  console.log('send '+packet.cmd+' packet =>',packet)
})

client.on('packetreceive', function (packet){
  console.log('receive '+packet.cmd+' packet =>',packet)
})


//Initialization of mqtt connection parameters for IoT Platform
function initMqttOptions(deviceConfig) {

    const params = {
        productKey: deviceConfig.productKey,
        deviceName: deviceConfig.deviceName,
        timestamp: Date.now(),
        clientId: Math.random().toString(36).substr(2),
    }
    // CONNECT parameter
    const options = {
        keepalive: 60, //60s
        clean: false, //cleanSession maintains a persistent session
        protocolVersion: 4 //MQTT v3.1.1
    }
    //1. Generate clientid, username, password
    options.password = signHmacSha1(params, deviceConfig.deviceSecret);
    options.clientId = `${params.clientId}|securemode=3,signmethod=hmacsha1,timestamp=${params.timestamp}|`;
    options.username = `${params.deviceName}&${params.productKey}`;

    return options;
}

/*
  Generate a password based on HmacSha1
  Reference documents: https://help.aliyun.com/document_detail/73742.html?#h2-url-1
*/
function signHmacSha1(params, deviceSecret) {

    let keys = Object.keys(params).sort();
    // Sort by lexicographical order
    keys = keys.sort();
    const list = [];
    keys.map((key) => {
        list.push(`${key}${params[key]}`);
    });
    const contentStr = list.join('');
    return crypto.createHmac('sha1', deviceSecret)
            .update(contentStr)
            .digest('hex');
}

CONNACK

receive connack packet => Packet {
  cmd: 'connack',
  retain: false,
  qos: 0,
  dup: false,
  length: 2,
  topic: null,
  payload: null,
  sessionPresent: false,
  returnCode: 0 
}

PINGRESP

send pingreq packet => { cmd: 'pingreq' }

PINGRESP

receive pingresp packet => Packet {
  cmd: 'pingresp',
  retain: false,
  qos: 0,
  dup: false,
  length: 0,
  topic: null,
  payload: null 
}

DISCONNECT

Publishing Data

PUBLISH

//3. Report attribute data
const topic = `/sys/${deviceConfig.productKey}/${deviceConfig.deviceName}/thing/event/property/post`;
setInterval(function() {
    //Publish data to the topic
    client.publish(topic, getPostData(),{qos:1});
}, 5 * 1000);

function getPostData() {
    const payloadJson = {
        id: Date.now(),
        params: {
            temperature: Math.floor((Math.random() * 20) + 10),
            humidity: Math.floor((Math.random() * 20) + 60)
        },
        method: "thing.event.property.post"
    }

    console.log("===postData\n topic=" + topic)
    console.log(payloadJson)

    return JSON.stringify(payloadJson);
}

send publish packet => { cmd: 'publish',
  topic: '/sys/a1hQSwFledE/eud1jXfEgCsAiP2eId9Q/thing/event/property/post',
  payload: '{"id":1543896481106,"params":{"temperature":23,"humidity":73},"method":"thing.event.property.post"}',
  qos: 1,
  retain: false,
  messageId: 38850,
  dup: false 
}

PUBACK

receive puback packet => Packet {
  cmd: 'puback',
  retain: false,
  qos: 0,
  dup: false,
  length: 2,
  topic: null,
  payload: null,
  messageId: 38850 
}

Receiving Data

SUBSCRIBE

//4. Subscribe to topics and receive commands
const subTopic = `/${deviceConfig.productKey}/${deviceConfig.deviceName}/control`;
client.subscribe(subTopic)
client.on('message', function(topic, message) {
    console.log("topic " + topic)
    console.log("message " + message)
})

SUBSCRIBE to messages

send subscribe packet => { cmd: 'subscribe',
  subscriptions: 
   [ { topic: '/a1hQSwFledE/eud1jXfEgCsAiP2eId9Q/control', qos: 0 } ],
  qos: 1,
  retain: false,
  dup: false,
  messageId: 38851 
}

SUBACK

SUBACK messages

receive suback packet => Packet {
  cmd: 'suback',
  retain: false,
  qos: 0,
  dup: false,
  length: 3,
  topic: null,
  payload: null,
  granted: [ 128 ],
  messageId: 38851 
}

UNSUBSCRIBE

send unsubscribe packet => { cmd: 'unsubscribe',
  qos: 1,
  messageId: 34323,
  unsubscriptions: [ '/a1hQSwFledE/eud1jXfEgCsAiP2eId9Q/control' ] 
}

UNSUBACK

receive unsuback packet => Packet {
  cmd: 'unsuback',
  retain: false,
  qos: 0,
  dup: false,
  length: 2,
  topic: null,
  payload: null,
  messageId: 34323 
}

Quality of Service (QoS)

Reconnecting the Device

The subscription relationship between the device and Alibaba Cloud IoT is maintained in the cloud. Unless the device actively unsubscribes, the subscription relationship is not cleaned up. Once the device is reconnected, it still maintains the previous subscription relationship and does not need to repeat the subscription.

Transport Layer Security TLS1.2

The link between the device and the IoT Platform can be encrypted with TLS v1.2. If you are using TLS for encryption, you must download a root certificate. In the CONNECT parameter, the secure mode of clientId is 2.

To learn more about establishing MQTT connections on Alibaba Cloud IoT Platform, check out this page.