MQTT.pro Blog MQTT Protocol Overview


MQTT (Message Queuing Telemetry Transport) is a lightweight messaging protocol designed for efficient communication between devices, especially in low-bandwidth, high-latency, or unreliable networks. It is widely used in IoT (Internet of Things) applications due to its simplicity and efficiency. This blog will cover the fundamental concepts of MQTT to help you get started with this powerful protocol.

1. What is MQTT?

MQTT is a publish-subscribe-based messaging protocol that facilitates communication between devices. It was designed by IBM in the late 1990s and has since become an open standard under the OASIS consortium. Its primary goal is to provide a reliable communication mechanism with minimal overhead, making it ideal for constrained environments.

2. Key Components of MQTT


The broker is the central hub in an MQTT system. It receives messages from publishers and forwards them to the appropriate subscribers. The broker handles message routing, ensuring that messages are delivered to the correct clients based on their subscriptions.


Clients in an MQTT system can be publishers, subscribers, or both. Publishers send messages to the broker, while subscribers receive messages from the broker. Clients communicate with the broker over a TCP/IP connection.


Topics are hierarchical strings used to route messages between clients. They are organized in a tree structure, with levels separated by slashes (/). For example, home/livingroom/temperature is a topic that might represent temperature data from a sensor in the living room. Clients subscribe to topics to receive messages and publish messages to specific topics.

3. Quality of Service (QoS) Levels

MQTT provides three levels of Quality of Service to ensure reliable message delivery:

  • QoS 0 (At most once): The message is delivered at most once, and no acknowledgment is sent back. Suitable for applications where occasional message loss is acceptable.
  • QoS 1 (At least once): Ensures the message is delivered at least once, but duplicates are possible. Appropriate for scenarios where message duplication can be handled.
  • QoS 2 (Exactly once): Guarantees the message is delivered exactly once. Ideal for critical applications where message loss or duplication is unacceptable.

4. Last Will and Testament (LWT)

The LWT feature allows a client to specify a message that will be sent by the broker if the client disconnects unexpectedly. This helps other clients be aware of unexpected disconnections and take appropriate actions.

5. Retained Messages

A retained message is a special type of message that the broker stores and sends to any new subscribers to a topic. This ensures that subscribers receive the most recent message immediately upon subscription, even if the message was published before they subscribed.

6. Clean Session and Persistent Session

  • Clean Session: When a client connects with a clean session flag set to true, it indicates that the broker should not retain any subscription information or undelivered messages when the client disconnects.
  • Persistent Session: If the clean session flag is set to false, the broker retains the client's subscription information and undelivered messages even after the client disconnects. This is useful for clients that need to receive messages sent while they were offline.

7. Security

MQTT supports various security mechanisms to protect data integrity and confidentiality:

  • SSL/TLS Encryption: Encrypts the communication channel to prevent unauthorized access and ensure data privacy.
  • Authentication: Ensures that only authorized clients can connect to the broker.
  • Access Control Lists (ACLs): Manage permissions for clients to control their access to specific topics.


MQTT is a versatile and efficient protocol well-suited for IoT and other messaging applications. By understanding its core concepts and features, you can leverage MQTT to build robust, scalable, and reliable communication systems. Whether you are connecting sensors, managing smart devices, or developing real-time applications, MQTT provides a solid foundation for your messaging needs.

For more detailed guides and advanced topics on MQTT, stay tuned to our blog. Happy messaging!