CoAP(Constrained Application Protocol)

Constrained Application Protocol is based on a subset of HTTP methods like REST but adds limited quality of service and works with UDP only, not TCP.

It was designed for constrained device connectivity in the early days of IoT’s emergence – it stands for “Constrained Application Protocol.”

Since MQTT’s arrival as a standard, with its equal handling of constrained devices, and much broader feature set beyond that, few people are choosing CoAP for new efforts. You are likely to choose CoAP only if it is the embedded choice for a sensor device, and you must support an application that already uses it.