M2M (Machine 2 Machine) – or M2M Protocol – refers to direct electronic communication among machines, devices and systems. For example a program in a PC communicating with smart sockets that measure the immediate current, without human intervention. Generally we use also the OPEN API term for this.
Modbus/TCP is an open M2M protocol designed for communication among various devices. The communication takes place by exchanging data messages between a client and a server in a master-slave mode.
MQTT(Message Queuing Telemetry Transport) is a simple protocol for exchanging messages among devices. It is frequently used in IoT applications.
The MQTT-flex version of the MQTT protocol can be configured in detail thanks to the “flex” extension.