## 3 Simulation Analysis for the Designed ISTN Protocol Stack
### Test of the ISTN Protocol Stack in the Software Platform
Add UNI_DIRECTION_SEND and UNI_DIRECTION_RECEIVE to OSPF neighbor's states in addition to DOWN and FULL;
2. The uni-directional receiving node sends hello messages to the uni-directional sending node through other routing paths instead of the down link. Figure 11 is used as an example to illustrate the working principle of the OSPF uni-directional link as revised in Algorithm 1. If node #2 does not find its own node ID in the \begin{table}
\begin{tabular}{c c c c c} \hline  & **vx_simulator\_01** & **vx_simulator\_04** & **vx_simulator\_06** & **vx_simulator\_03** \\ \hline Forward LSP & Out-port: 3\({}^{1}\) & In-port: 1, Out-port: 5 & In-port: 1, Out-port: 3 & In-port: 1 \\ Backward LSP & In-port: 3\({}^{1}\) & Out-port: 1, In-port: 5 & Out-port: 1, In-port: 3 & Out-port: 1 \\ \hline \end{tabular}
\end{table} 
Table 2: A bi-directional label switching path under the circumstances of bi-directional link between node vx_simulator\_01 (SCID 1) and node vx_simulator\_03 (SCID 3) in the software platform. Figure 10: Simulation in software platform with 16 simulated nodes under the circumstances of bi-directional link. (**a**) The status of a neighbor’s ports, routing path for managing and controlling messages, and explicit constraint routing path for data under QoS requirements in node vx_simulator\_01 (SCID 1). (**b**) The status of a neighbor’s ports in node vx_simulator\_04 (SCID 4). OSPF HELLO packet sent from the peer node #1, then node #1 is considered to be in uni-directional sending status (i.e., UNI_DIRECTION_SEND). Because the former direct link from node #2 to node #1 is down, node #1 cannot receive the uni-directional receiving hello message from node #2 (i.e., UNI_DIRECTION_RECEIVE). In this case, node #2 sends this hello message to node #1 through other routing paths, i.e., from node #2 to node #3 and finally to node #1. When node #1 receives this message from node #3, the neighbor status of node #2 in node #1 is checked as uni-directional receiving, and thus the uni-directional link is established between node #1 and node #2. Note that if there is no other path between node #2 and node #1, the OSPF uni-directional link cannot be established. Figure 12a,b demonstrate the uni-directional link between nodevx_simulator_01 (SCID 1) and nodevx_simulator_04 (SCID 4) when the uni-directional link fromvx_simulator_04's port 1 tovx_simulator_01's port 3 and is down accidentally, leading to the status ofvx_simulator_04's port 1 changing from **FULL to UNI_DIRECTION_RECEIVE**, and the status ofvx_simulator_01's port 3 changing from **FULL to UNI_DIRECTION_SEND**. In addition, we improve the OSPF link-state database with the complement of the satellite link plan of connections (SLPC). SLPC is intended to hold scheduled/planned link information to update satellite network link state periodically, and thus control OSPF flood messages and reduce satellite link bandwidth usage. In [19], we propose a solution called the optimized OSPF with link plan (OOWLP) to overcome the shortcomings in the original rule of OSPF, e.g., the frequent route convergence induced by frequent satellite link validation, and long periods of satellite network instability. SLPC is meant to retain the link connection plan information of the satellite network for a period of time. The terrestrial network management system will upload the new SLPC regularly to the GEO satellites which are directly accessed by the network management system. Then, the GEO satellites will flood the SLPC information to all other satellites for synchronization, e.g., GEO, MEO, LEO satellites. Moreover, the judgment of the flooding procedure of the original OSPF is revised in Algorithm 1 lines 18-24. In the original OSPF, if the on-off action occurs in one link, the two nodes of the link would flood the updated message to all other nodes in the network. The process of route convergence is costly in terms of time and energy, and consumes the limited link resources. In general, the satellite link variation is induced by two scenarios:  
1.