Congestion Control in Constrained Application Protocol for the Internet of Things: State-of-the-Art, Challenges, and Future Directions

The rapid development of smart sensors has enabled the integration of wireless sensor networks (WSNs) with the Internet. The emergence of the Internet of Things (IoT) represents a significant paradigm shift, with WSNs remaining essential to its framework. Most smart sensor nodes will serve as foundational components, which can cause network congestion. Congestion is a critical factor affecting the performance of IoT networks. The Constrained Application Protocol (CoAP) has been developed by the Internet Engineering Task Force (IETF) for IoT communications. CoAP is a lightweight, request/response-based RESTful protocol designed for resource-constrained nodes. Resource-constrained networks often face limitations in hardware and link capabilities, leading to challenges such as packet loss and increased delay. This review examines CoAP, highlighting its distinct fundamental features that facilitate deployment in constrained IoT environments. This paper provides an overview of WSNs as foundational elements for IoT applications, followed by a discussion of low-power wide area networks (LPWANs) and their standardisation as promising technologies for these applications. This paper discusses prominent state-of-the-art mechanisms for congestion control (CC) in CoAP, highlighting several research challenges and future directions. For controlling congestion in IoT resource-constrained networks, it suggests a three-state Markov model and machine learning-based strategies.