MQTT (formerly MQ Telemetry Transport) is a publish-subscribe based "light weight" messaging protocol for use on top of the TCP/IP protocol. It is designed for connections with remote locations where a "small code footprint" is required and/or network bandwidth is limited. The Publish-Subscribe messaging pattern requires a message broker. The broker is responsible for distributing messages to interested clients based on the topic of a message. Andy Stanford-Clark and Arlen Nipper of Cirrus Link Solutions authored the first version of the protocol in 1999.
The specification does not specify the meaning of "small code foot print" or the meaning of "limited network bandwidth". Thus, the protocol's availability for use depends on the context. In 2013 IBM submitted MQTT v3.1 to the OASIS specification body with a charter that ensured only minor changes to the specification could be accepted. MQTT-SN  is a variation of the main protocol aimed at embedded devices on non-TCP/IP networks, such as ZigBee.
There are several MQTT brokers available. They vary in their feature set and some of them implement additional features on top of the standard MQTT functionality.
Real world applications
In the real world, there are a number of projects that implement MQTT.
- Facebook Messenger. Facebook has used aspects of MQTT in Facebook Messenger. However, it is unclear how much of MQTT is used or for what; Moreover it is to be noted that this is a phone application not a sensor application.
- IECC Scalable DeltaRail's latest version of their IECC Signaling Control System uses MQTT for communications within the various parts of the system and other components of the signaling system. It provides the underlying communications framework for a system that is compliant with the CENELEC standards for safety-critical communications.
- MQTT 3.1.1 specification
- "OASIS Message Queuing Telemetry Transport (MQTT) Technical Committee". OASIS. Retrieved 9 May 2014.
- Andy, Stephen Clark. "MQTT For Sensor Networks (MQTT-SN) Protocol Specification Version 1.2" (PDF). Retrieved 9 May 2014.
- IBM WebSphere MQ Date accessed 2013-11-18
- Choosing Your Messaging Protocol: AMQP, MQTT, or STOMP | VMware vFabric Blog - VMware Blogs. Blogs.vmware.com (2013-02-19). Retrieved 2013-10-23.
- "Constrained Application Protocol (CoAP) draft-ietf-core-coap-18". The Internet Engineering Task Force (IETF®). Retrieved 9 May 2014.
- "InternetOfThings". XMPP WIKI. Retrieved 9 May 2014.
- "Internet Protocols for the Smart Grid RFC 6272". IETF. Retrieved 9 May 2014.
- MQTT Broker Feature Comparison Feature comparison of the most popular MQTT brokers.