# Review of Unmanned Aerial Vehicle Swarm Communication Architectures and Routing Protocols
## 3 Routing Protocols
### Topology-Based Routing Protocols
#### 3.3.2 Proactive Routing Protocols  
A proactive routing protocol (PRP) can update the routing table periodically to keep it up to date. The advantages are that it can accurately reflect the topology of the network, and the delay in obtaining routes is minimal. Therefore, PRP is suitable for applications with real-time requirements. However, to update the routing tables periodically, a number of messages exchange continuously between nodes. That is why PRPs are not suitable for UAV swarm network whose nodes move fast. When network scale and mobility increase to a certain degree, most PRP schemes are not applicable. At present, the most popular proactive routing protocols are the Optimized Link State Routing (OLSR) protocol, Destination Sequenced Distance Vector (DSDV) protocol and their variations. (1) Optimized Link State Routing (OLSR) protocol is a link-state optimized routing protocol originally for flat topology [56]. By flooding the link-state of neighbors to other nodes, the topology information is exchanged between nodes. Nodes use the shortest path algorithm to destinations to choose the next hop. [PERSON] et al. [57] applied the OLSR routing in flying ad hoc networks and then analyzed the applicability of OLSR for a flying ad hoc network [58]. Some other researchers also compared the OLSR with other routing protocols [58; 59]. In a UAV Ad hoc network, the location of nodes and the state of links change rapidly. This would cause not only packed loss but would also put  
Figure 8: Multilevel Hierarchical Routing in a UAV swarm Ad hoc network. a strain on already constrained bandwidth [38]. Optimizations involve abandoning the traditional approach of non-selectively forwarding topology control (TC) packets and instead adopting the method of selecting neighbors as multiple point relays (MPR). Through this mechanism, OLSR reduces the protocol overhead. The MPR technology, TC strategy, and routing optimization technology have become the main means for designing efficient OLSR protocols. Therefore, in recent years, the OLSR protocols and the MPR optimization algorithm have been extensively studied, becoming of the most commonly used routing algorithms for ad hoc networks. Although the overhead caused by the periodic flooding strategy is reduced by MPR, problems such as recalculation of routes and selection of MPR still plague researchers. Therefore, many variants have been designed and proposed. [PERSON] et al. in [60] proposed a cross layer design for UAV communication networks and further proposed a Directional OLSR (DOLSR) protocol based on directional antennas for UAV ad hoc networks [61]. The ODSR minimizes latency by reducing the number of MPR members. [PERSON] et al. [62] proposed a Mobility and Load-aware OLSR (ML-OLSR) protocol, which added mobility and load-aware algorithms to the OLSR protocol. This protocol successfully obtained a good delivery rate and latency performance. (2) DSDV [17; 63; 64]: In the DSDV protocol, the network has nodes for every path from source to destination, before demand has emerged. Each node in the network has a routing table that lists all valid destination nodes and the number of nodes that can reach the destination node. Each entry in the routing table of the destination node is marked with a node serial number. For the information in the routing table to be consistent with regards to the dynamically changing topology, it must be updated frequently. DSDV routing table updates are classified into \"full-dump\" and \"incremental-dump\". \"Full-dump\" sends the entire routing table to its neighbors, which requires multiple packets; \"incremental-dump\" only sends the modified portions of routing information to its neighbors, and therefore only one packet is needed. When the network is relatively stable, \"incremental-dump\" can avoid extra traffic and reduces the burden on the network. However, in a network with rapidly changing topology such as a UAV swarm network, the update process increases network bandwidth sharply and also puts the DSDV protocol at a disadvantage.