In this letter, in order to solve problems mentioned above, we propose a reliable mobility management mechanism for IoT environments, called CoAP-based Mobility management Protocol (CoMP), which is based on IETF CoAP [3]. CoMP extends the CoAP message formats to provide the IP mobility management. More specifically, it extends the Confirmable (CON) message of the CoAP protocol [3] to reliably transmit the signaling message under unreliable IoT network environment. In addition, CoMP utilizes a separate location management server for keeping track of the changing IP addresses of the mobile sensor nodes, without using tunneling scheme. The usage of application layer signaling protocol, i.e., CoAP, separate location management server, and no tunneling makes the mobility management architecture to be very simplified. Furthermore, the reliable message handling mechanism of CoAP can maintain the IP connectivity under unreliable IoT environments. Finally, performance has been evaluated by both mathematical analysis and simulation.