MQTT

MQTT is a publish/subscribe model whereby clients subscribe to topics and maintain an always-on TCP connection to a message broker. As new messages are sent to the broker, they include the topic with the message, allowing the broker to determine which clients should receive the message. Messages are pushed to the clients through the always-on connection:

This model supports flexible communication use cases, allowing sensors to publish their data and brokers to pass that data onto subscribing systems that wish to consume or further process the sensor data. Although MQTT is primarily suited for use over TCP-based networks, the MQTT for Sensor Networks (MQTT-SN) specification provides an optimized version of MQTT for use within WSNs. 

For more information, see Stanford-Clark and Linh Truong, MQTT for Sensor Networks protocol specification, Version 1.2. International Business Machines (IBM). 2013. URL: http://mqtt.org/new/wp-content/uploads/2009/06/MQTT-SN_spec_v1.2.pdf.

MQTT-SN is optimized for use with battery-operated devices possessing limited processing and storage resources. It allows sensors and actuators to make use of the publish/subscribe model on top of ZigBee and similar RF protocol specifications.