BDP-CoAP: Leveraging Bandwidth-Delay Product for Congestion Control in CoAP

CoAP is one of the most popular protocols for data transfer in IoT networks. Since COAP uses an unreliable transport protocol (UDP) to deliver application data, loss-based congestion control algorithms are introduced in CoAP to mitigate network congestion. In particular, CoCoA+, which is currently under standardisation by the IETF, leverages RTT-measurements to regulate the frequency of packet retransmissions. Recent studies have shown that CoCoA+ still suffers from some critical performance issues, and a few modifications were proposed. In this paper, we follow a different approach, and we design a rate-based congestion control algorithm for COAP, called BDP-COAP, which is derived from the TCP BBR protocol. More precisely, BDP-COAP paces the transmissions of a CoAP sender in order to match the estimated bandwidth of the bottleneck link and constrains the total amount of unacknowledged data to be upper-bounded by the estimated bandwidth-delay product. We compare our solution against standard CoAP and CoCoA+. Results demonstrate the BDP-COAP significantly improves throughput fairness while obtaining similar total goodput as CoAP and CoCoA+. Furthermore, BDP-COAP ensures more stable performance also in dynamic traffic scenarios and when competing with congestion-unaware traffic.