MQTT

MQTT is a lightweight publish and subscribe messaging protocol in which devices publish readings to topics on a broker and any authorized subscriber receives them.

It suits sites with unreliable connectivity because it is built for small messages, keeps a session across reconnects, and offers three delivery guarantees. A site controller publishes its measurements and events to a cloud broker and subscribes to configuration and command topics from it. MQTT carries data between a site and the cloud; Modbus and DNP3 carry it between the controller and the equipment on site.