MQTT agreement with Aliyun IoT platform

1. Describes the MQTT protocol

1.1 the MQTT protocol

MQTT (Message Queue Telemetry Transmission) is a messaging protocol based on TCP/IP protocol stack to support asynchronous communication between parties. MQTT separates the sender from the receiver spatially and spatially, so it can scale in an unreliable network environment. Although called message queue telemetry transport, it has nothing to do with message queues and uses a publish and subscribe (Pub/Sub) model.

MQTT is a lightweight, flexible networking protocol dedicated to achieving the right balance for IoT developers:

• This lightweight protocol can be implemented over severely limited device hardware and high latency/limited bandwidth networks.
• Its flexibility makes it possible to support diverse application scenarios for IoT devices and services.

1.2 the MQTT Client library

The MQTT Client library is implemented in many languages, including Embedded C, C, Java, JavaScript, Python, C++, C#, Go, iOS, Android, etc. Eclipse Paho MQTT library download address: www.eclipse.org/paho/downlo…

The following development practices based on Nodejs version MQTT, access the address www.npmjs.com/package/mqt…

1.3 the MQTT messages

1.3.1 Fixed header

Control message Type

The name	value	Direction of packet Flow	describe
Reserved	0	ban	keep
CONNECT	1	Client -> Broker	Device Connects to the IoT platform
CONNACK	2	Broker -> Client	The IoT platform confirms the connection result
PUBLISH	3	two-way	news
PUBACK	4	two-way	QoS=1 Indicates that the message is received
PUBREC	5	two-way	IoT is not supported
PUBREL	6	two-way	IoT is not supported
PUBCOMP	7	two-way	IoT is not supported
SUBSCRIBE	8	Client -> Broker	Device subscribes to IoT Platform Topic
SUBACK	9	Broker -> Client	IoT platform confirms subscription results
UNSUBSCRIBE	10	Client -> Broker	Device unsubscribes to IoT Platform Topic
UNSUBACK	11	Broker -> Client	IoT platform confirms the result of unsubscription
PINGREQ	12	Client -> Broker	The device sends a heartbeat request to the IoT platform
PINGRESP	13	Broker -> Client	The IoT platform responds to the device heartbeat
DISCONNECT	14	Client -> Broker	Device Disconnects from the IoT platform
Reserved	15	ban	keep

Flag bit that controls the packet type

Control message	Fixed masthead logo	Bit 3	Bit 2	Bit 1	Bit 0
PUBLISH	MQTT 3.1.1 use	DUP	QoS	QoS	RETAIN

The length of the remaining

The number of bytes	The minimum value	The maximum
1	0 (0x00)	127 (0x7F)
2	128 (0x80, 0x01)	16 383 (0xFF, 0x7F)
3	16 384 (0x80, 0x80, 0x01)	2 097 151 (0xFF, 0xFF, 0x7F)
4	2 097 152 (0x80, 0x80, 0x80, 0x01)	268 435 455 (0xFF, 0xFF, 0xFF, 0x7F)

Note: The maximum payload of aliyun IoT is 256K

1.3.2 Variable Header

Some MQTT control messages contain a variable header section. It is between the fixed header and the load. The content of the mutable header varies according to the message type. The variable header Packet Identifier field exists in multiple types of packets.

Packet Identifier Bytes

Bit	7 – 0
byte 1	Message identifier MSB
byte 2	Packet identifier LSB

Control message	Message identifier
PUBLISH	Required (if QoS = 1,2)
PUBACK	Need to be
PUBREC	Need to be
PUBREL	Need to be
PUBCOMP	Need to be
SUBSCRIBE	Need to be
SUBACK	Need to be
UNSUBSCRIBE	Need to be
UNSUBACK	Need to be

1.3.3 Payload

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

Control message	The payload
CONNECT	Need to be
PUBLISH	optional
SUBSCRIBE	Need to be
SUBACK	Need to be
UNSUBSCRIBE	Need to be

2. Establish a connection with Aliyun IoT platform

2.1 the CONNECT

MQTT protocol of Aliyun IoT platform does not support will message, and the content parameters of CONNECT message are as follows:

ClientId, username, password by equipment triples (productKey, deviceName, deviceSecret) generated in accordance with the rules, the specific rules are as follows:

Official documentation: help.aliyun.com/document_de…

Device-side code example (Nodejs version) client.js

/**
"dependencies": { "mqtt": "2.18.8" }
*/
const crypto = require('crypto');
const mqtt = require('mqtt');
// Device identity triplet + region
const deviceConfig = {
    productKey: "Replacement".deviceName: "Replacement".deviceSecret: "Replacement".regionId: "cn-shanghai"
};
// Generate MQTT connection parameters based on triples
const options = initMqttOptions(deviceConfig);
const url = `tcp://${deviceConfig.productKey}.iot-as-mqtt.${deviceConfig.regionId}.aliyuncs.com:1883`;

//2. Establish a 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)
})


//IoT platform MQTT connection parameters initialized
function initMqttOptions(deviceConfig) {

    const params = {
        productKey: deviceConfig.productKey,
        deviceName: deviceConfig.deviceName,
        timestamp: Date.now(),
        clientId: Math.random().toString(36).substr(2),}/ / CONNECT parameters
    const options = {
        keepalive: 60.//60s
        clean: false.//cleanSession keeps the conversation going
        protocolVersion: 4 / / the 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;
}

/ * generated password based on HmacSha1 reference documentation: 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');
}
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2.2 CONNACK

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

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

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

3. Publish data

3.1 the PUBLISH

//3. Attribute data reporting
const topic = `/sys/${deviceConfig.productKey}/${deviceConfig.deviceName}/thing/event/property/post`;
setInterval(function() {
    // Publish data to 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);
}

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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 
}
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3.2 PUBACK

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

4.1 the SUBSCRIBE

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

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

SUBACK message body

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

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

4.4 UNSUBACK

receive unsuback packet => Packet {
  cmd: 'unsuback'.retain: false.qos: 0.dup: false.length: 2.topic: null.payload: null.messageId: 34323 
}
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5. Quality of service QoS

6. The device is disconnected and reconnected

The subscription relationship between the device and Ali Cloud IoT is maintained in the cloud, unless the device actively unsubscribe, the subscription relationship will not be cleared. After the device is reconnected, the subscription relationship remains, and the subscription does not need to be repeated.

7. Transport layer security TLS1.2

The link between the device and the IoT platform can be encrypted using TLS V1.2. If TLS encryption is used, you need to download the root certificate. In the CONNECT parameter, the Securemode of clientId is 2

Help.aliyun.com/document_de…