Investigating the CoAP Congestion Control Strategies for 6TiSCH-Based IoT Networks

The Constrained Application Protocol (CoAP) is a RESTful protocol standardized by the IETF and widely used for IoT applications. CoAP includes a default congestion control algorithm to ensure efficient operation under high traffic conditions. Other congestion control algorithms for CoAP have been proposed and evaluated in the literature, including the very popular CoCoA algorithm. All these algorithms assume that the underlying wireless communication is regulated through the 802.15.4 CSMA-CA protocol. Today, many IoT systems are based on the 6TiSCH architecture that, instead, leverages the TSCH (Time Slotted Channel Hopping) mode of IEEE 802.15.4, i.e., a synchronous and time-slotted access protocol. In this paper we investigate, by simulation, the suitability of existing CoAP congestion-control algorithms to the 6TiSCH architecture. Our results show that the performance of the considered algorithms are strongly influenced by the Scheduling Function used to allocate communication resources to nodes. In addition, our analysis emphasizes that CoCoA does not provide a significant advantage over the default algorithm in 6TiSCH networks. We investigate the motivations for such a behavior and propose an optimized version of CoCoA, namely 6CoCoA, specifically tailored for 6TiSCH networks. 6CoCoA is able to provide up to a 15% improvement of the Transaction Delivery Ratio and up to a 25% reduction of the end-to-end Transaction Delay, when the network is congested.