## 3 Mobility Management Architecture Using CoAP
### Message Format of CoMP  
In this subsection, we present the message format using CoAP. Figure 5 shows the IP address information during CoMP handover. We assume that the CoAP node moves from WSN BS1 to WSN BS2. In this situation, Cur_T_Addr and Cur_Lifetime of CoAP node are changed to New_T_Addr as a temporary IP address, _i.e._, T_Addr, and New_Lifetime as Lifetime, respectively. However, P_Addr as the permanent IP address, _i.e._, P_Addr, does not change. P_Addr, T_Addr, and Lifetime of CoAP nodes are cached on the WMMT of the WMMS. W_Addr indicates the IP address of the WMMS. Figure 6 shows the request message and response message format in the CoAP standard. The detailed information refers to the CoAP standard document [2]. The message format is based on the RESTful format. In Figure 6, Ver indicates version and Type indicates the message type, _i.e._, Confirmable (0), Non-confirmable (1), Acknowledgement (2), and Reset (3). The code indicates the message type of request or response, _i.e._, GET (1), POST (2), PUT (3), and DELETE (4). The message ID indicates the identifier of the message, which is created by the sender of the CoAP node. The token is intended for use as a client-local identifier for differentiating between concurrent requests. A CoAP node should generate tokens in a way that tokens currently in use for a given source/destination pair are unique. In a response message, the response code indicates that the status code that the client requested was successfully received, understood, and accepted. Figure 6 shows the CoAP message format [2]. We use the CoAP message format and extend the option delta to perform the CoMP signaling procedure. In the CoAP message format in Figure 6, we define the option delta and option length to specify the resource constraints. We extend the option delta value to support the CoMP. Table 2 shows the extended option delta and its description of the CoMP message. In the option delta, the 2048-2054 are newly defined. The range of 2048 to 64,999 of option  
Figure 5: IP address change during CoMP handover. delta gives the designated expert in CoAP standard [2]. The message type field includes CoAP methods, _i.e._, GET, PUT, POST, and DELETE. Figures 7-11 show the CoMP message format. Figure 7 shows the GET request message for discovery that retrieves the T_Addr and Lifetime of corresponding CoAP nodes. The GET request message for discovery, as shown in Figure 7, is an example of the request message of CoAP Client. CoAP Client constructs and sends a GET request message for discovery, including the P_Addr of CoAP node. As a response, CoAP Client receives the ACK response message for discovery, including the P_Addr, Cur_T_Addr, and Cur_Lifetime. Figure 8 shows a PUT BU request and an ACK binding response. CoAP Client constructs and sends a PUT binding update request message, including P_Addr, and New_T_Addr, in option value. As a response, CoAP Client receives an ACK binding response message. Figure 9 shows a PUT holding request and an ACK holding response message. CoAP node creates the PUT request message, including the P_Addr and H flag \"1\" and sends its message to CoAP Client. As a response, CoAP Client responds to the ACK response message, including the sequence number.