The Constrained Application Protocol (CoAP) is a simple and lightweight application layer protocol for constrained IoT devices, which provides a small memory and processing capacity. By default, the CoAP integrates a simple congestion control mechanism. A simple congestion control mechanism relies on a fixed interval Re-Transmission time-Out (RTO) and Binary Exponential Backoff (BEB). However, some existing works state that the default CoAP congestion control mechanism fails in effectively performing group communication and utilizing resources. Moreover, it cannot handle rapid and frequent requests. Thus, the basic congestion control mechanism results in a buffer overflow and packet loss. Recently, several optimized congestion control mechanisms have been designed according to RTT measurements, gradients of RTT measurements, the bandwidth of devices, traffic rates, and packet loss ratio.