Comparison of CoAP and CoCoA+ Congestion Control Mechanisms for Different IoT Application Scenarios

CoAP is a web transfer protocol that provides basicRESTful services for IoT devices. Since CoAP operates on topof UDP, CoAP must support additional congestion control andreliability mechanisms. However, CoAP is originally designedto allow implementations that do not maintain any end-to-endconnection information. Thus, the default congestion controlmechanisms are not capable of adapting to network conditions. Afew enhancements of the basic congestion control mechanism ofCoAP have been recently proposed, and CoCoA+ is commonlyconsidered the most mature of these proposals. However, theshortcomings of CoCoA+ have not been sufficiently investigatedin the literature. In this paper, we present an in-depth analysis ofthe congestion control mechanisms of CoCoA+ and CoAP for avariety of traffic patterns under realistic IoT setups. The resultsof this evaluation indicate that CoCoA+ can perform significantlyworse than default CoAP, especially with bursty traffic and innetworks with few clients as a result of an improper selectionof the retransmission timeouts. Furthermore, large variability ofRTO values in CoCoA+ makes difficult to meet end-to-end delayrequirements of typical IoT monitoring applications.