What is MQTT? A Complete Guide on MQTT Protocol

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By: Cavli Wireless

The Internet of Things (IoT) is growing rapidly, with billions of connected devices deployed around the world. MQTT is a lightweight messaging protocol optimized for low-bandwidth networks. Prior to the MQTT protocol in IoT communication, devices struggled with complex protocols and limited bandwidth.

In this blog let’s explore how MQTT communication became the most reliable mode of communication in the connected world. This blog covers:

  • What is MQTT
  • MQTT Architecture
  • MQTT publish/ subscribe model
  • MQTT Servers/brokers
  • MQTT client and server communication
  • MQTT Message Types and Format
  • MQTT Security
  • Popular MQTT brokers and clients and lastly MQTT use cases in various applications.

What is the MQTT Protocol?

MQTT or otherwise MQ Telemetry Transport is a de-facto lightweight messaging protocol for IoT and M2M applications. The full form of MQTT is Message Queuing Telemetry Transport. It enables network users to easily transmit telemetry data and messages in limited-bandwidth scenarios. It defines a set of rules on how IoT devices can publish and subscribe to data over the Internet.

Publish/Subscribe Model of MQTT Protocol

MQTT is a publish-subscribe messaging protocol designed for constrained devices and works on top of the TCP/IP protocol. It carries out message exchange through an intermediary MQTT broker. It is event-driven and connects devices with the publish /subscribe (Pub/Sub) pattern. The MQTT publish/subscribe model is a very efficient way to communicate between MQTT clients.Publish/Subscribe Model of MQTT Protocol

Publish/Subscribe Model of MQTT Protocol

Architectural Components of an MQTT Protocol

The MQTT protocol in IoT has a set of components. The 4 major MQTT components are

  • Message
  • Client
  • MQTT Server or Broker
  • Topic

MQTT Message

MQTT messages are made up of three parts:

  • Fixed header
  • Variable header
  • Payload

The fixed header contains information about the type of message and the quality of service level.

The variable header contains information that is specific to each message type.

The MQTT message consists of a payload containing the actual message data published by a client. MQTT protocol limits the maximum payload size to 256 MB per message.Cost and space optimization imparted by iSIMs

MQTT Message Structure

Quality of Service (QoS)

MQTT offers a dependable information transmission system with three different levels of QoS agreement between the sender and receiver:

  • QoS (0) (At most once):No acknowledgement or retransmissions.
  • QoS (1) (At least once):Message is retransmitted until acknowledgement is received. Duplicate messages possible.
  • QoS (2) (Exactly once):Message is retransmitted until acknowledgement is received. Duplicates are not sent to the receiver.

Types of MQTT Messages

The three most common MQTT message types are:

  • Connect:Establishes the relationship among the networks while waiting for a link to be made with the server.
  • Disconnect:Pauses for the MQTT client to do whatever tasks it has to do before disconnecting the TCP/IP connection.
  • Publish:Following transmitting the message to the MQTT client, resumes to the application thread instantly.
  • Subscribe:To establish one or maybe more subject subscriptions, a subscribe package will always be delivered from client to server.

What are MQTT Clients?

MQTT Clients

MQTT Clients
MQTT Clients

MQTT clients can be categorized into two main roles: publishers and subscribers.

  • Publishers:These are MQTT clients that send or “publish” messages to the MQTT broker. Messages published by publishers are typically associated with specific topics. Topics are like message channels, and messages are sent to a particular topic.
  • Subscribers:These are MQTT clients that receive or “subscribe” to messages from the MQTT broker. Subscribers express their interest in specific topics, and the broker sends messages published to those topics to the subscribing clients.

With the publish/subscribe model, clients do not need to know about each other. They only need to know the topic that they are interested in. When a client publishes a message to a topic, the message is delivered to all of the clients that have subscribed to that topic.

What is MQTT Broker?

MQTT Broker

MQTT Broker

An MQTT broker is a server that receives all messages from the clients and then routes these messages to the appropriate destination clients. As a central hub in the MQTT protocol, the broker is responsible for managing message queues and ensuring message delivery based on the topic subscriptions. It also handles the authentication and authorization of clients, ensuring secure and efficient communication in an MQTT network.

MQTT Topic

MQTT Topic is a UTF-8 string that brokers use for message filtering. Topics are represented as hierarchical strings separated by forward slashes. Clients publish messages to specific topics. Subscribers register interest in topics and receive published messages matching those topics. The broker handles routing messages from publishers to the appropriate subscribed clients based on topic filtering.

Example of Topic

Topics are used to define the path of the message. Here the message (temperature) will be sent to the dining room which is on the first floor of the home.

Working of MQTT Protocol- Explained

Working of MQT

Working of MQTT

When a client connects to a broker, it must first identify itself. This is done using the MQTT connect packet. The connect packet contains the client’s name, the broker’s name, and the security credentials (if any).

Once the client has connected to the broker, it can start publishing and subscribing to messages.

To publish a message, the client sends an MQTT publish packet. The publish packet contains the message’s topic, the message’s payload, and the message’s QoS (quality of service).

To subscribe to a topic, the client sends an MQTT subscribe packet. The subscribe packet contains the topic that the client wants to subscribe to.

When a client publishes a message, the broker delivers the message to all of the clients that have subscribed to the topic.

When a client subscribes to a topic, the broker starts delivering messages that are published to that topic.

Understanding MQTT Security


MQTT supports client authentication, which is crucial for verifying the identity of devices connecting to the MQTT broker. Authentication is typically done using username and password credentials. Advanced implementations might use client certificates or tokens for more robust authentication.


After authentication, MQTT allows for the authorization of clients, specifying what resources a client can access and what actions it can perform. This is implemented by configuring topic permissions on the broker, which can restrict clients to publish or subscribe only to authorized topics.

Transport Layer Security (TLS/SSL):

For secure data transmission, MQTT can use TLS/SSL encryption at the transport layer. This ensures that the data exchanged between clients and the broker is encrypted, safeguarding it against eavesdropping and tampering. TLS/SSL also includes mechanisms for client and server authentication.

Payload Encryption:

In addition to transport layer encryption, MQTT allows for payload encryption. This means that the actual data (payload) of the MQTT messages can be encrypted, providing an additional layer of security. This is particularly useful when full transport layer encryption is not feasible or when additional security is required for sensitive data.

Popular MQTT Clients and Brokers in IoT

There are many MQTT clients and brokers available, both open source and commercial. Some of the popular MQTT clients and brokers include:

Eclipse Paho

The Paho Java Client, a Java-based MQTT client library, is designed for applications on JVM and Java-compatible platforms like Android, while paho-mqtt stands on top as a Python MQTT open-source client libraries.It also supports languages like C++, Java, JavaScript, and Golang.

Mosquitto Eclipse

Mosquitto is an open-source MQTT broker, lightweight and suitable for a range of devices from single-board computers to enterprise servers, available for Windows, Mac, Linux, and Raspberry Pi. It includes features like the “mosquitto-go-auth” plugin for enhanced security and a PHP wrapper “Mosquitto-PHP” for MQTT client development.

HiveMQ MQTT Client

HiveMQ is a commercial MQTT broker that enables client-to-client communication. It can be self-hosted on Windows or Linux. HiveMQ is an enterprise-grade MQTT broker optimized for large-scale deployments. It supports up to 10 million devices. It can be deployed on Docker, Kubernetes, and major cloud services like Azure, AWS, and Google Cloud.

RabbitMQ MQTT Broker

RabbitMQ is another open source message broker that supports MQTT, AMQP, STOMP and other protocols. Developed in Erlang, RabbitMQ is cross-platform compatible with Linux, Mac, Windows and FreeBSD. It enables MQTT connectivity through a plugin and also allows communicating over HTTP and WebSocket.

Applications of MQTT Protocol

MQTT Applications

MQTT Applications

MQTT in IoT powers mission-critical applications across industries.

In smart energy grids, MQTT enables reliable real-time transmission of meter readings to ensure accurate customer billing. No data packets are lost between smart meters and utility servers.

In precision agriculture, MQTT allows remote monitoring of soil moisture sensors and crop health analytics even from sparse farmlands with unstable connectivity. Farmers can receive alerts on droughts or pest infestations faster to protect yields and resources.

In device health monitoring, it enables continuous remote monitoring by instantly transmitting vibration, temperature and other sensor data to operators. Teams can identify deteriorating components and prevent unplanned downtime by mitigating issues before escalating into failures.

For billing and invoicing systems, MQTT powers accuracy and auditability. Its reliable message delivery with duplicate detection transforms invoice processing for telcos, utilities and SaaS companies. Duplicate record creation is eliminated, preventing costly inaccuracies and revenue leakage.

Advantages of MQTT Protocol

Its ease of use

MQTT’s ease of use is one of its most compelling features for IoT. It offers ready-made clients and brokers, allowing for quick and straightforward implementation.

It is reliable

MQTT queues messages and retries delivery, effectively handling intermittent connections that are common in IoT environments. Its Quality of Service (QoS) levels add another layer of reliability, ensuring that messages are delivered accordingly.

It is bidirectional

MQTT supports omnidirectional communication. This means any device in the network can act as a publisher or a subscriber to any topic.

It is scalable and future-proof

MQTT handles broadcasting a message to a hundred devices or a million effortlessly, the process remains the same – publish to a common topic, and all subscribed devices receive the message.

Closing Notes

In the fast-paced IoT domain, MQTT communications is ideal for IoT applications. With its publish/subscribe model, it unlocks the full potential of IoT in a wide variety of industries, ranging from health monitoring to manufacturing and many more. Leverage this simple architecture and easy-to-use API of MQTT to ensure reliable connectivity and optimum performance in our connected ecosystem.

To learn more, visit us at


  • https://mqtt.org/

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