# Review of Unmanned Aerial Vehicle Swarm Communication Architectures and Routing Protocols
## 3 Routing Protocols
### Topology-Based Routing Protocols
#### 3.3.3 Reactive Routing Protocols  
The reactive routing protocol (RRP) is also called on-demand routing protocol because it starts the route discovery procedure only when a message needs to be sent. Therefore, the required routing information cache is small [70]. When the network is not overburdened, the control overhead of RRPs is much smaller than the PRPs. However, because the source node needs to perform route discovery before data transmission, RRPs suffer from high transmission delays. Typical RRPs include Dynamic Source Routing (DSR), Ad hoc On-Demand Distance Vector (AODV), and more.  
(1) Dynamic Source Routing (DSR) protocol is a widely used and standardized RRP method for ad hoc networks. The most obvious advantage of the DSR protocol is that it does not have specific network infrastructure requirements. This protocol allows the network to self-organize and self-configure. Each data packet in a DSR protocol contains a complete list of routing nodes, so all nodes that forward or listen to this packet will store the routing information as a backup. Therefore, no matter how the nodes move and the network topology changes, they can maintain performance. There are many follow-up studies on DSR's adaptability in UAV Ad hoc networks. In [71], authors developed a UAVad hoc network test platform based on the DSR protocol. [PERSON] et al. [72] found that due to the high mobility and topology instability of UAV nodes, DSR is more suitable for UAV ad hoc networks than proactive protocols. [PERSON] et al. [73] proposed the Restrict DSR (RE-DSR) for limiting the maximum hop count of Route Requests (RREQ), which can save routing storage space and reduce routing overhead for UAV nodes. Then they proposed UAV Energy DSR (UE-DSR) for ad hoc networks consisting of small UAVs for reconnaissance missions [74].  
(2) Ad hoc On-Demand Distance Vector (AODV) [75] protocol is an RRP method based on DSDV and DSR. There is no maintenance of global routing information for the entire network, but only the destination addresses in data packets. Therefore, the overhead is low. Before the node sends data, it first looks up the routing table. If there is one path to the destination node, the data packet is sent to the next hop according to the routing table. However, if there is no available path, the source node broadcasts Route Request (RREQ) to neighboring nodes. After receiving the non-repeating RREQ, the neighboring nodes establish or update revers routes. Then they continue to broadcast the RREQ until the destination node receives. If the current node contains a valid route to the destination node, the route discovery process can also terminate itself. Then, a Route Reply (RREP) is returned to the source node along the reverse path. When the RREP returns, a forward route entry is established, and when the source node receives the RREP, it establishes a route from the source to the destination node. In recent years, there have been many studies on applying AODV to UAV communication networks, and some have also proposed variations of AODV. [PERSON] et al. [76] aimed at minimizing the number of hops in the route discovery process, and further proposed a route reliability criterion in which path update degree, path length, and path reliability are used as path selection criteria.