Overview of the CoAP protocol for IoT

Abstract: Today (2021), several billions of devices are connected daily to the internet, so a very huge amount of a data is sent and managed every second. In the next years, it is expected that even more devices like sensors, actuators for domestic or industrial usage will connect to the internet. In this context, we review an HTTP like protocol called Constrained Application Protocol (CoAP) that was developed by the Internet of Engineering Task (IETF) to work on constrained IoT devices operating in lossy environments. Although this protocol is lightweight and efficient compared to other IoT protocols such as HTTP or MQTT, it has some limitation in some scenarios, for example when strong reliability is needed or in streaming applications. For this reason we also overview, from the most recent literature some modifications, one is CoAP over TCP that guarantee reliability, another is CoAP-SC that has a good mechanism to handle with error and flow control that are crucial in streaming applications.