Constrained Application Protocol (CoAP)

CoAP is point-to-point communication over User Datagram Protocol (UDP). It has low overhead requirements and was specifically developed for low-power devices operating over the internet. It was designed to work on microcontrollers with as low as 10 KiB of RAM and needs only 100 KiB for the operating codes.

CoAP was created by the Internet Engineering Task Force (IETF) to address the needs of power constrained IoT devices. It is also supported by the Eclipse foundation as an open standard.

CoAP follows a client/server architecture and is a one-to-one communication convention. It does allow for some multicast capabilities, which are early in development at the time of this writing.

CoAP is similar to HTTP in that it is a document transfer protocol and interoperates with the RESTful web. It is different from HTTP, in that it was designed for applications where networks are low-powered and often lossy.

CoAP operates on UDP protocol where data packets are much smaller than HTTP, which uses larger TCP packets. Clients and servers communicate without established connections. Information is transferred using self-contained datagrams. Datagrams are the core of the UDP protocol and each datagram contains all necessary information for routing without any reliance on previous exchanges between client and server.

It may help to think of a datagram like communicating with someone over mail (old school snail mail). The envelope has a delivery address and a return address and the payload would be the letter inside, but this is hidden from the mail man. You send regular letters to your cousin in Poland. She sends regular letters to you.

You have no guarantee that all your letters will be delivered or delivered in the same order that you sent them. It is your responsibility, once letters are received, to open them up, put them in the right order, and figure out if there are any missing. If you find one to be missing, you would handle mailing your cousin to request a resend of it.

CoAP works in the same manner where the sending and receiving applications play the role of you and your cousin. Applications need to be programmed appropriately to handle the required level of delivery reliability.