## 4 Case Study of Interworking of QKD Networks and the Role of SDN Orchestrator
### Implementation Results of SDN Controller and SDN Orchestrator for KOREN
#### 4.2.2 Implementation of QSDNO  
The QKDNO of KOREN holds information on two interconnected networks, each with its own vendor, in advance. When a request for interconnection between the two networks is received, it decides the route for processing the received request using the information of the interconnected networks it holds in advance and delivers the route to the QKDNC of each network domain. Afterward, the QKDNC of each network domain carries out the internal processing for the specific route that requires processing in the segment of the network domain it governs.  
For the end-to-end QKD-derived key delivery, the QKDNO can request path calculations from the path calculation engine (PCE). The PCE repeatedly provides a list of candidate paths that satisfy the constraints received from the QKDNO until the QKDNO finally chooses a path and decides to deploy it for the QKD service link in the QKD network. When a QKD service provider such as KOREN deploys different KMSs for the different parts of a network, it needs to know where common trusted nodes are located with APIs to allow the KMSs to interoperate to pass keys between the two different QKD networks. If there are multiple common trusted nodes to interconnect different QKD networks, a path needs to be calculated in advance for an end-to-end key delivery from the initiating application to the target application; these each belong to different QKD networks, which means a common trusted node must be selected in advance. The QKDNO can perform this role with the information from each QKDNC of each QKD network. For the QKDNO to coordinate between network parts, an interface between the QKDNO and the QKDNC needs to be defined. This interface describes the flow of information between both entities, with the QKDNC performing as a server and the QKDNO operating as a client. The QKDNO can orchestrate QKD networks through this interface in terms of network configuration and topology, management policy, and performance management.  
Figure 6 illustrates the process of transmitting keys between different QKD networks. The QKDNO is connected to each QKDNC that manages a different QKD network, and this connection occurs through the SDN orchestrator interface. When an application request for key delivery between two applications, which are indicated as SAE_ID1 and SAE_ID2 in Figure 2, belonging to different QKD networks is received, a service link connecting the two applications in the two networks is calculated by QKDNO, and a path is designed to pass through a specific border node. Subsequently, each service link belonging to each network is established by the QKDNC of each network, which are represented as \"Service Link for QKD network A\" and \"Service Link for QKD network B\" in different colors in Figure 2, respectively, and at this border node, a separate API is used to deliver the key, which was discussed in clause 3. An example of this is standardized in ETSI GS QKD 020 [14].  
Figure 6: An example of the creation of the service_link with the multi-segment for the use case of delivering the key between two QKD networks via the border node is depicted.  
To create a service link between two applications, SAE_ID1 and SAE_ID2, which exist in two different networks, the QKDNO calculates the path that the service link must traverse through the path computation engine. If the service link needs to pass through different network domains to be realized, an ID corresponding to the multi-segment, \"ms_app_id\", is set to achieve the service link. The application must include relevant information, such as \"server_ms_info\" and \"client_ms_info\" in Table 2, to perform the procedure of transmitting the keys between the two networks, expressed in the figure as \"extkey API\", and this paper refers to ETSI GS QKD 020 to implement it. Subsequently, the QKDNC overseeing each network domain copies this \"ms_app_id\" to set up the service link for each network segment.  
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