In Internet of Things (IoT), the simple IPv6 capable electronic devices with limited hardware resources like memory and power resources are called constrained devices. Congestion is a major issue in network communications of these devices. To solve congestion in networks of constrained devices, Internet Engineering Task Force (IETF) had designed Constrained Application Protocol (CoAP). CoAP deals congestion with a basic Congestion Control (CC) mechanism called Default CoAP. Afterward, CoCoA and CoCoA+, an Internet-draft-recommendations, has been introduced as the elective CC mechanisms for CoAP. However, limited evaluations had done on these CC mechanisms of CoAP. In this paper, the performance evaluation of Default CoAP, CoCoA, and CoCoA+ Congestion control mechanisms are evaluated through Simulations in different network topologies and varied in different Link Delivery Ratios (LDR) of sensor nodes in constant traffic scenario by using Cooja Simulator. The simulation results are generated and CoCoA+ shows a better performance in constant traffic scenario comparing to Default CoAP and CoCoA.