An Analysis and Improvement of Congestion Control in the CoAP Internet-of-Things Protocol

An Internet of Things (IoT) environment typically consists of lossy and low powered networks (LLN) of interconnected sensors. An IoT messaging protocol that is gaining in popularity and importance is the Constrained Application Protocol (CoAP). CoAP runs at the application layer, handling reliability and congestion control over the unreliable UDP layer. Due to low bandwidth and high scale of communication, congestion can occur among sensors in an LLN, or when communicating to a border router, or when clients from the Internet access the resources in an LLN. While CoAP was originally specified with a primitive congestion control mechanism, to address its various limitations a new congestion control protocol named CoAP Congestion Control Advanced (CoCoA) is being developed. We present an analysis and evaluation of CoCoA, and based on our findings we propose and implement CoCoA 4-state-Strong, an adaptation of CoCoA that uses a 4-state estimator for variable backoffs to significantly improve throughput, even in highly lossy networks, while maintaining a higher goodput.