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MQTT Client

MQTT Client is an optional extension which adds the Message Queue Telemetry Transport protocol to the stack. The MQTT add-on can be used with embOS/IP or with a different TCP/IP stack.

Message Queue Telemetry Transport (MQTT) is a publish/subscribe messaging transport protocol for use on top of the TCP/IP protocol. It is simple, light weight, and open. These characteristics make it ideal for use in many situations, including constrained environments such as for communication in Machine to Machine (M2M) and Internet of Things (IoT) contexts.

Feature list

  • Full MQTT version 3.1 support.
  • Publish/subscribe client included.
  • Support for Quality of Service data delivery.
  • Low memory footprint.
  • Independent of the TCP/IP stack: any stack with sockets can be used.
  • Publisher and subscriber demo included.
  • Project for executable on PC for Microsoft Visual Studio included.


TCP/IP stack

The MQTT Client requires a TCP/IP stack. It is optimized for embOS/IP, but any RFC-compliant TCP/IP stack can be used. The shipment includes a Win32 simulation, which uses the standard Winsock API and an implementation which uses the socket API of embOS/IP.

MQTT backgrounds

MQTT is a very light weight communication protocol originally designed for communication in M2M contexts. It is easy to implement on client side and has only a minimal packet overhead. This makes it ideal for the use with devices with limited resources.



It uses the publish/subscribe pattern, which is an alternative to the well-known client/server model. In opposite to the client/server model, where a client directly communicates with an endpoint, the publish/subscribe pattern decouples the sender and receiver of a particular message. In the MQTT context the sending client is called publisher, the receiving client subscriber.



Publisher and subscriber do not know about the existence of one another. To enable the transport of a message a third party is required. These third party is called broker in the MQTT context. The broker filters all incoming messages from the publishers and distributes them to the subscribers.

MQTT uses subject-based filtering of messages. Publishers sends topic related messages, subscribers receive messages if they have subscribed the topic.

To get messages a from an MQTT broker a subscriber establishes a connection to the broker. The broker checks if a publisher has sent a message for the subscribed topic and if so, sends it to the subscriber. The advantage of this approach is that publisher and subscriber do not need to know each other and that they do not need to run at the same time. All they need to know is the IP address of the broker.


Standard Description
MQTT Version 3.1.1 Direct external download: MQTT Version 3.1.1 Plus Errata 01

Shipping contents

The following table shows the contents of the MQTT Client root directory:

Directory Content
.\Application\ Contains the example applications to run the MQTT Client with embOS/IP.
.\Config\ Contains the MQTT Client configuration file.
.\Doc\ User & Reference Guide for embOS/IP.
.\IP\ Contains the MQTT sources: IP_MQTT_CLIENT.c and IP_MQTT_CLIENT.h
.\Windows\ Contains the source, the project files and executables to run the embOS/IP MQTT Client on a Microsoft Windows host.


Resource usage

The ROM usage depends on the compiler options, the compiler version and the used CPU. The memory requirements of the MQTT Client add-on presented in the tables below have been measured on a Cortex-M4 system. Details about the further configuration can be found in the sections of the specific example.

The following resource usage has been measured on a Cortex-M4 system using SEGGER Embedded Studio V3.10e, size optimization.

ROM usage

Add-on ROM
embOS/IP MQTT Client Approximately 2.5 kBytes

RAM usage

Add-on RAM
embOS/IP MQTT Client context Approximately 60 bytes