Transport and Application Layer Protocols for IoT: Comprehensive Review

The Internet of Things (IoT) connects billions of heterogeneous devices, necessitating lightweight, efficient, and secure communication protocols to support a diverse range of use cases. While physical and network-layer technologies enable connectivity, transport and application-layer protocols determine how IoT devices exchange, manage, and secure information. The diverse and constrained nature of IoT devices presents a challenge in selecting appropriate communication protocols, with no one-size-fits-all solution existing. This article provides a comprehensive review of key transport and application protocols in IoT, including MQTT, MQTT-SN, CoAP, LwM2M, AMQP, XMPP, WebSockets, HTTP/HTTPS, and OPC UA. Each protocol is examined in terms of its design principles, communication patterns, reliability mechanisms, and security features. The discussion highlights their suitability for different deployment scenarios, ranging from resource-constrained sensor networks to industrial automation and cloud-integrated consumer devices. By mapping protocol characteristics to IoT requirements, such as scalability, interoperability, power efficiency, and manageability, the article provides guidelines for selecting the optimal protocol stack to optimize IoT system performance and long-term sustainability. Our analysis reveals that while MQTT dominates cloud telemetry, CoAP and LwM2M are superior in IP-based constrained networks, and emerging solutions like OSCORE are critical for end-to-end security.