International Journal of Communication Networks and Information Security ISSN: 2073-607X, 2076-0930 Volume 14 Issue 02 Year 2022 Page 111:123 _________________________________________________________________________________________________________________________________________________________ Available online at: https://ijcnis.org 111 A Survey on Congestion Control Protocols for CoAP 1 Sneha R Deshmukh, 2 Vijay T Raisinghani Department of Information Technology, Mukesh Patel School of Technology Management and Engineering, NMIMS Deemed-to-be University, Mumbai, India 1 sneha.deshmukh@nmims.edu, 2 vijay.raisinghani@nmims.edu Article History Received: 13 Feb 2022 Revised: 26 May 2022 Accepted: 12 August 2022 CC License CC-BY-NC-SA 4.0 Abstract The Internet of things (IoT) comprises things interconnected through the internet with unique identities. Congestion management is one of the most challenging tasks in networks. The Constrained Application Protocol (CoAP) is a low-footprint protocol designed for IoT networks and has been defined by IETF. In IoT networks, CoAP nodes have limited network and battery resources. The CoAP standard has an exponential backoff congestion control mechanism. This backoff mechanism may not be adequate for all IoT applications. The characteristics of each IoT application would be different. Further, the events such as unnecessary retransmissions and packet collision caused due to links with high losses and packet transmission errors may lead to network congestion. Various congestion handling algorithms for CoAP have been defined to enrich the performance of IoT applications. Our paper presents a comprehensive survey on the evolution of the congestion control mechanism used in IoT networks. We have classified the protocols into RTO-based, queue-monitoring, and rate-based. We review congestion avoidance protocols for CoAP networks and discuss directions for future work. Keywords: CoCoA+, pCoCoA, AdCoCoA, CoAP-Eifel 1. Introduction The internet of things (IoT) is a network of devices (physical and virtual) that can auto- configure. The protocols used for this network are standard and enable interoperability [33]. Further, the network is dynamic. These IoT devices exchange data over a network, which may have lossy links, packet transmission errors, and poor throughput. These conditions could lead to packet losses and unnecessary packet retransmissions, which could cause congestion in the network. Hence, a proficient congestion handling technique is required to reduce uncalled-for resending and determine the correct retransmission timeout (RTO) value. In the IoT protocol stack, Constrained Application Protocol (CoAP) [1] has reliable transmission as an option utilizing RTO and aggressive backoff. UDP is the transport CoAP protocol for CoAP. CoAP is designed for light applications which require congestion handling. CoAP has a request and response model for end-to-end operation. The RTO is initially set at random. After that, binary exponential backoff (BEB) is used for calculating the RTO. However, this random value at the start can cause retransmissions which could cause avoidable congestion. We broadly classify the congestion control mechanisms for CoAP networks into three categories – RTO-based, queue- monitoring, and rate-based. RTO-based mechanisms: CoCoA [2] calculates RTO by using the value of round-trip time