# Fast Reroute Mechanism for Satellite Networks Based on Segment Routing and Dual Timers Switching
## 2 Related Work
### Node Behavior Design
#### 3.2.1 PLR Node Behavior
This formula describes the mechanism of the backup path activation timer, which is used to control the switching between primary and backup paths and to mitigate the overhead caused by frequent path switches:  
\[T_{bpat}(t)= \begin{cases}T_{reset}&\text{new transient fault is detected}\\ 0&\text{backup path is deactivated}\end{cases}  \tag{2}\]  
where \(T_{bpat}(t)\) represents the value of the backup path timer at time \(t\). \(T_{reset}\) is the reset value of the timer, which is used to reset the timer in the event of a new fault occurrence. Once the fault is resolved and the backup path is deactivated, the timer value is reset to 0, indicating that the backup path is no longer active. To provide a clearer description of the synchronization mechanisms and potential race conditions between the hardware monitoring and link failure detection modules, Figure 3 illustrates the state transition of the PLR node. The following content analyzes the relationships between the primary and backup routes, the triggering conditions for switching between backup and primary paths, the role of timers, and other relevant details, based on the states of the PLR node. 1. Pre-Update Primary Route State:  
Under normal circumstances, the PLR operates in the pre-update primary route state. This is the initial state, in which the PLR forwards data packets using the primary route and precomputes the backup route. In this state, the priority of the backup route is lower than that of the primary route. The PLR will only transition from the initial state to the backup path state if a failure is detected by the hardware monitoring module. 2. Backup Path State:  
The backup path state is an intermediate state, where the priority of the backup route is adjusted to be higher than that of the primary route. This state is more complex, as its maintenance and transition are influenced by the backup path activation timer and the backup path deactivation timer. Two timers, which are unique to the PLR, are introduced as follows. After a transient fault is detected by hardware monitoring and the PLR switches to the backup path, a short-term failure timer (also known as the backup path activation timer) is set to avoid the frequent switching between the backup and primary paths, which would incur overhead. As long as the backup path activation timer has not expired, the path will remain on the backup route. In the case of long-lasting faults detected by the link failure detection mechanism, the PLR provides the capability to switch to the backup path. After the failure is detected, to mitigate micro-loops caused by flooding delay in the fault information, the PLR sets the backup path deactivation timer. The duration of this timer is recommended to be in seconds, covering the maximum flooding delay, the time needed for recalculating the primary route, and the propagation delay. This timer duration depends on the constellation scale and requires configuration by the network management system. Figure 3: The PLR node state transition diagram. 3. Post-Update Primary Route State:  
The post-update primary route state represents a stable state after the failure, when the primary route has converged. In this state, the priority of the backup route is adjusted to be lower than that of the converged primary route. The PLR then forwards data packets using the converged primary route.