## 2 Related Works  
For the constrained network, solutions of discovery can be considered as distributed and centralized for the IoT environment [19]. The discovery servers can be deployed for the large-scale distributed system that has multiple systems to handle the network where the server is deployed [20]. This fully distributed architecture can support an efficient discovery approach in low power and lossy networks such as the CoAP network [21; 22]. The CoAP network is a constrained network because the CoAP is designed for the M2M services which are provided by the constrained devices [23]. The constrained RESTful environments (CoRE) RD is a server that is deployed in the IoT network for providing services to discover the constrained nodes [24]. For the registration to the CoAP RD, the RD provides an interface to accept a POST from an CoAP device containing the list of resources in the CoAP message payload. The resources are described in the CoRE link format to be added to the RD. Therefore, the limitation of the CoAP RD is the registration interface of a CoAP RD and cannot involve the extra information for describing the details of registering devices. For constrained devices, the registration information shall be smaller. However, according to the edge computing, the devices quip with high-performance parts in the IoT network. Therefore, the CoAP RD shall provide the registration interface for the detail registration profile that can describe more detailed information of the registering device. Figure 1: The proposed system architecture. The server shall be based on the connected power supply to provide the lookup services for retrieving the registered device information that can involve URIs, accessing requirement, and status [25]. Therefore, the RD also can be used for the centralized solution with powerful equipment for large-scale IoT networks [26]. With the sufficient hardware specification, the RD also can support high-performance computing to provide intelligent services based on a large amount of data. The implementation of RD in the high-performance computer can be supported by the personal computer, cloud services, and virtualization solutions [27]. However, many small embedded boards are published for the high-performance such as Raspberry Pi, Intel Edison, and other devices which have a small size with the full ability to support sufficient computation and network communication [28]. Therefore, the embedded boards also enable deployment of the RD server for providing registration and discovery services in the constrained environment. The web-based service discovery solutions are built with a high-performance computer to provide searching services, such as web searching engines on the Internet. However, traditional web-based searching solutions cannot provide the service in the same way for the IoT network because of the heterogeneity and constrained network requirements [29]. In the IoT network, the discovery service provider shall support resource discovery regardless of the communication protocols and technologies used by IoT clients [30]. However, it is a challenge to fulfill all the communication interfaces for the heterogeneous devices. There are many IoT standard solutions to support the discovery mechanism in the framework level [31; 32; 33]. However, most of them present the discovery architecture for their specific communication protocol and service accessing mechanism [34]. The proposed solution is based on the OCF specification that refers to the CoAP-based service providing architecture. The proposal of RD in the OCF specification is focused to provide registration and discovery services for the OCF devices in the OCF network through CoAP communications. Also, the proposed RD enables the IoT devices and WSPs together to be discovered by the IoT clients in the OCF network. To provide the discovery service using the consistent scheme in order to discover the resource from the HTTP and OCF network, we present a new data model using RAML.