RFC 7252 Constrained Application Protocol - The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained networks in the Internet of Things. The protocol is designed for machine-to-machine (M2M) applications such as smart energy and building automation.”
It enables those constrained devices to communicate with the wider Internet using similar protocols. CoAP is designed for use between Devices on the same constrained network, between Devices and general nodes on the Internet, and between Devices on different constrained networks both joined by an Internet. CoAP is also being used via other mechanisms, such as SMS on mobile communication networks. CoAP targets the type of operating environments defined in the ROLL and 6Lo working groups which have additional constraints compared to normal IP networks, but the CoAP protocol also operates over traditional IP networks.