## 2 Related Works  
The directionality of FSOSNs demands efficient routing protocols for network communication. There have been several works focused on routing in FSOSNs. In [8], [PERSON] _et al._ proposed two protocols, namely simple-bro and simple-link. Simple-bro is a classical flooding based protocol that enables communication from the BS to sensor nodes. Correspondently, the simple-link protocol aims to establish a route from each sensor node to the BS by assigning each node an uplink node to orient. Consequently, the computed set of routes from sensor nodes to the BS forms an oriented forest with roots in the CHs. Figure 4(a) is the same example as Figure 2(a), and Figure 4(b) shows its corresponding network virtual topology after running the simple-link protocol. Obviously, this virtual topology is fragile when facing the death of nodes. That is because the death of a parent node in one tree would probably result in a tremendous amount of children nodes being separated from the network, and thus degrading the network communication. In Figure 4, when \(S_{7}\) died, \(S_{t}\), \(S_{5}\), and \(S_{6}\) would totally lose the communication with the BS, even if they still kept sufficient energy.  
In [9], two circuit-based algorithms, known as neighborhood discovery algorithm (NDA) and base station discovery algorithm (BDA), were introduced by [PERSON] _et al._ NDA attempts to acquire the local neighborhood by finding one most efficient circuit from a node through each of its successors. The destination of each circuit is the originating node itself. And the found circuit also represents the best routes for each of the nodes along that circuit. The NDA provides a significant reference for local neighborhood discovery and maintenance in directional networks. Our previous work in [13] has evaluated the performance of NDA affected by directionality. Another circuit-based algorithm, the BDA, enables BS to get the global network information by flooding the cluster route packets (CRPs). The global information reflecting the network topology is stored in the base stations' routing table  
Figure 4: **(a)** An example of FSOSN; **(b)** Virtual topology after running the simple-link protocol.  
(BSRT). This table is useful in constructing routing paths in the initial route discovery, and conducting affected nodes of alternative paths for route maintenance in the case of link failure. However, the details of the routing and maintenance strategies are not discussed in this paper, which is still an open research topic.  
Exploiting the security benefits of link directionality, [PERSON] [14] introduced a novel light-weight circuit-based, secure and integrated routing and localization paradigm within the FSOSNs. This scheme leverages the hierarchical cluster-based organization of the network, and the directionality of the links to deliver enhanced security performance including per hop and broadcast authentication, confidentiality, integrity and freshness of routing signals. Although the performance of this scheme was evaluated through comprehensive probability and simulation analysis on security, the author did not discuss or evaluate the performance in terms of energy conservation, which is of great significance in sensor networks.  
Actually, to guarantee efficient data communication in FSOSNs, a routing protocol with dynamic topology reconfiguration is crucial for two reasons. First, with reconfiguration, the network connectivity can be better maintained despite the constraint of directionality, which facilitates the delivery of data packets. Besides, energy expenditure can be managed more effectively, because the intensive strain of invalid transmission resulted from the lack of connectivity will be released through network reconfiguration. Therefore, RRP is proposed in this paper. To the best of our knowledge, this is the first work of reconfiguring sensor nodes in FSOSNs, the details of which are shown in the following section.