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Constrained Application Protocol (CoAP) is a specialized Internet application Protocol for constrained devices, as defined in RFC7252 - The Constrained Application Protocol (CoAP). It enables those constrained devices called “ Nodes” to communicate with the wider Internet using similar protocols. CoAP is designed for use between devices on the same constrained network (e.g., low-power, lossy networks), 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 Short Message Service (SMS) on mobile communication networks.

CoAP is a service layer protocol that is intended for use in resource-constrained internet devices, such as wireless sensor network nodes. CoAP is designed to easily translate to Hypertext Transfer Protocol (HTTP) for simplified integration with the World Wide Web (WWW), while also meeting specialized requirements such as Multicast support, very low overhead, and simplicity. Multicast, low overhead, and simplicity are important for Internet of Things (IOT) and Machine to Machine (M2M) communication, which tend to be deeply embedded and have much less memory and power supply than traditional internet devices have. Therefore, efficiency is very important. CoAP can run on most devices that support User Datagram Protocol (UDP) or a UDP analogue.

Source: https://en.wikipedia.org/wiki/Constrained_Application_Protocol