This whitepaper provides an overview of the MQTT protocol. MQTT is a is an open OASIS and ISO standard (ISO/IEC PRF 20922) lightweight, simple machine to machine TCP/IP based protocol which can be used for communication between IoT devices.
You can download it from the following link: https://packetstormsecurity.com/files/download/156416/mqtt-protocol.pdf
Source: https://packetstormsecurity.com/files/156416/MQTT-Protocol-Overview.html