## 3 Simulation Analysis for the Designed ISTN Protocol Stack
### Test of the ISTN Protocol Stack in the Software Platform  
For the verification and validation of network protocols and algorithms of the control plane, we firstly conduct the test in a software platform under the circumstances of large-scale network topology with multiple nodes. All managing and controlling messages are transmitted through user datagram protocol (UDP) frames in the software platform. For managing and controlling messages and service data, the multiple routing algorithms are initially evaluated in VxWorks Simulators, which are generated based on the shortest path and constraint path, respectively. Consider the calculation of a routing path from node vx_simulator_01 to node vx_simulator_03. In Figure 8, each node (i.e., vx_simulator_01 to vx_simulator_16, with spacecraft identification (SCID) from SCID 1 to SCID 16) has nine ports (i.e., 1 to 9) bi-directionally connected to neighbor node ports, while the priorities of the ports are increased from 1 to 9 and the costs of the ports are the same value equaling to 1, i.e., the priority and cost for the port 1 are 1 and 1, respectively, the priority and cost for the port 9 are 9 and 1, respectively, etc. According to the routing strategy (i.e., the Dijkstra algorithm) for managing and controlling messages, the computation of the routing path is based on the priority and cost of the port, namely: the path with high priority will be selected under the circumstances of equivalent costs. Link-state databases of all nodes are shown in Figure 9. So, the routing path for managing and controlling messages from node vx_simulator_01 (SCID 1) to node vx_simulator_03 (SCID 3) is shown in Table 1, Figure 10a, and Figure 10b, respectively, under the bi-directional link between vx_simulator_01 (SCID 1) and node vx_simulator_04 (SCID 4). In Figure 10a, the function _manage_control_message_routing_path_show_node_to_node\((1,3)\) shows the routing path from the source node (i.e., the first parameter 1) to the destination node (i.e., the second parameter 2). On the other hand, given the constraint of QoS requirements of a link duration time, forward and/or backward bandwidth, etc. the explicit bi-direction routing path of data from node vx_simulator_01 to node vx_simulator_03 is computed based on the CSPF-TE algorithm, which is shown in Table 2. In Figure 10a, the function \(cspf\_bi\_direction\_calculation(1,2,500,200,300,1000,1000)\) is used for the computation of the bi-direction routing path from the source node (i.e., the first parameter 1) to the destination node (i.e., the second parameter 2) under multiple QoS requirements, e.g., link duration time (i.e., the third parameter 500 s), forward bandwidth (i.e., the fourth parameter 200 megabits per second (Mbps)), backward bandwidth (i.e., the fifth parameter 300 Mbps), forward maximum latency (i.e., the sixth parameter 1000 milliseconds (ms)), and backward maximum latency (i.e., the seventh parameter 1000 ms). The function \(print\_cspf\_bi\_direction\_route\) is for the display of bi-direction explicit routing path. \begin{table}
\begin{tabular}{c c c c c} \hline  & **vx\_simulator\_01** & **vx\_simulator\_04** & **vx\_simulator\_06** & **vx\_simulator\_03** \\ \hline Forward path & Out-port: 4 & In-port: 2, Out-port: 6 & In-port 2, Out-port: 4 & In-port: 2, \\ Backward path & In-port: 4 & Out-port: 2, In-port: 6 & Out-port: 2, In-port: 4 & Out-port: 2 \\ \hline \end{tabular}
\end{table} 
Table 1: The routing path of managing and controlling messages between node vx_simulator_01 (SCID 1) and node vx_simulator_03 (SCID 3). Figure 8: The topology of VxWorks simulated network. We make full use of the uni-directional connection to transmit data while the opposite uni-directional link is down to improve the use of resource-limited satellite networks. The original OSPF protocol, on the other hand, does not allow for the calculation of a uni-directional routing path. We enhance the OSPF protocol in the following way to compute uni-directional routing paths:  
1.