## 3 Classifications of Routing Protocols in UAV Networks
### Routing Based on Network Structure
#### 3.1.4 Hybrid Routing Protocol  
Hybrid routing protocols (HRP) are based on the combination of proactive and reactive routing methods. HRPs combine the best features of both routing protocol systems. HRPs overcome the message overload problem in proactive routing and the delay caused by reactive routing. These hybrid protocols are particularly suitable for large networks based on the division of the network into subnets or regions. Therefore, a proactive routing protocol operates in each region, and a reactive routing protocol is used for communication between regions. Two of the prominent HRP-based routing protocols are Zone Routing Protocol (ZRP) [45] and the Temporally-Ordered Routing Algorithm (TORA) [46]. The prominent variants of these protocols will be described in detail below. In the ZRP regional routing protocol, each UAV is assigned to a separate operating region, and these regions may overlap with the regions of nearby UAVs. Data packets within a single region are routed using intra-region routing, while data packets between regions are routed using inter-region routing. Intra-region routing is implemented through proactive routing to maintain paths, and inter-region routing uses reactive routing mechanisms to maintain and find optimal paths. In [47], the authors propose a location-assisted delay-tolerant routing protocol (LADTR) for UAV networks for use in post-disaster operations. Exploiting location-assisted forwarding is combined with store-and-forward (SCF) techniques. Future UAV locations are estimated based on the location and velocity information of GPS-integrated UAVs. The forwarding UAV node predicts the location of the destination UAV node and then decides the forwarding route. Relay UAVs improve the availability of links between the search UAVs and the ground station, thereby reducing end-to-end delay and increasing the packet delivery ratio. In the simulation results of NS-3, LADTR shows its superiority over AODV, GPSR, Spray-and-Wait, and Epidemic routing protocols in terms of packet delivery ratio, average end-to-end delay, and normalized routing cost. The TORA time-ordered routing protocol allows each UAV to influence only the routing tables of its closest neighbors. The main advantage of this protocol is its ability to reduce the reaction to topological changes. Furthermore, it eliminates invalid paths and searches for new paths in a single run of the distributed algorithm. TORA mainly uses reactive routing, but in some cases, it also uses a proactive approach. These routing protocols construct and maintain a directed acyclic graph (DAG) from the source UAV to  
Figure 11: Structure of the proposed CLEA-AODV. the destination. Multiple paths are created between UAVs, based on the DAG to transmit packets. TORA is chosen to quickly compute updated paths in case of disconnected paths and to enhance adaptability. To reduce the signaling cost, TORA often chooses longer paths instead of the shortest path method. The authors in [48] proposed a fast reconstruction time-order routing protocol (RTORA) based on the original TORA protocol. RTORA applies a cost-reduction mechanism to overcome the adverse effects caused by link reversal errors in TORA. The simulation results using OPNET to compare the RTORA protocol with the original TORA protocol show that the route reestablishment time of RTORA is reduced by about 25%, while the control cost is reduced by about 56%. Table 3 summarizes the advantages and limitations of some prominent routing algorithms based on UAV network architecture, including their classification and a description of their roles in specific applications.