## 6 Open Issues and Future Research Directions  
Exciting and promising research areas and issues are addressed and discussed in this section. Since UAV routing protocols are still in the early stages of development, the network dynamic nature and link disconnect, delay and QoS, simulation tool, energy consumption, coordination and collaboration, and flying in 3D space are the main challenges for developing a topology-based routing protocol for UAV networks. The enhancement of routing scalability, the elimination of complexity in topology-based routing, the reduction of routing latency, energy-efficient routing, improved routing security, and equitable  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Protocol** & **Protocol** & **Year** & **Route** & **Mobility** & **Simulation** & **Performance** & **Application** \\
**Type** & **Name** & **Type** & **Model** & **Tool** & **Metrics * & **Scenario** \\ \hline \multirow{6}{*}{**Proactive**} & OLSR & 2003 & Dynamic & RWP & NS-2 & RO & FANETs \\ \cline{2-8}  & P-OLSR & 2013 & Dynamic & PPRZM & Test bed & DL & \begin{tabular}{c} Relay, \\ Open area coverage \\ \end{tabular} \\ \cline{2-8}  & ML-OLSR & 2014 & Dynamic & RWP & QualNet & PD, ED & FANETs \\ \cline{2-8}  & GPNC-SP & 2018 & Dynamic & GM & MATLAB & RO, LS & FANETs \\ \cline{2-8}  & OLSR-ETX & 2018 & Dynamic & RWP & NS-3 & ED, PD, RO & Ocean FANETs \\ \cline{2-8}  & TOLSR & 2020 & Dynamic & PPRZM & MATLAB & ED, PD & Search and rescue \\ \cline{2-8}  & QTAR & 2021 & Dynamic & GM & MATLAB & \begin{tabular}{c} PD, ED, RO, \\ EC \\ \end{tabular} & \begin{tabular}{c} Monitoring \\ applications. \\ \end{tabular} \\ \hline \multirow{6}{*}{**Reactive**} & AODV & 2003 & On demand & RWP & NS-2 & PD, ED, NR, \\  & ADRP & 2017 & On demand & RWP & NS-2 & \begin{tabular}{c} PD, ED, NR, \\ TH \\ \end{tabular} & FANETs \\ \cline{2-8}  & RM-AODV & 2017 & On demand & MG & NS-3 & \
\begin{tabular}{c} ED, PD, RO, \\ PS \\ \end{tabular} & Video Surveillance \\ \cline{2-8}  & BR-AODV & 2017 & On demand & N/ A & NS-2 & GO, DR, ED & Surveillance \\ \cline{2-8}  & SARP & 2018 & On demand & RWP & NS-2 & PD, TH, NR, & FANETs \\ \cline{2-8}  & EE-Hello & 2019 & On demand & GM & NS-3 &
\begin{tabular}{c} PD, TH, RO, \\ EC \\ \end{tabular} & Green UAVs \\ \cline{2-8}  & MDRMA & 2019 & On demand & RWP & NS-3 & ED, RO, PD, & FANETs \\ \hline \multirow{6}{*}{**Hybrid**} & ZRP & 2002 & Hybrid & RWP & GloMoSim & ED & FANETs \\ \cline{2-8}  & SHARP & 2003 & Hybrid & RWP & GloMoSim & PO, LR, DJ & FANETs \\ \cline{2-8}  & RTORA & 2013 & Hybrid & RWP & OPNET & RO, ED & Swarm Network \\ \cline{2-8}  & LERP & 2017 & Hybrid & RWP & NS-3 & PD & FANETs \\ \cline{2-8}  & RFLSR & 2019 & Hybrid & PPRZM & Others & EC, NK, TB & Agriculture \\ \hline \multirow{3}{*}{**Static**} & MLHR & 2000 & Static & RWP & GloMoSim & RO & FANETs \\ \cline{2-8}  & DCR & 2005 & Static & RWP & Others & ED & FANETs \\ \cline{2-8}  & LCAD & 2007 & Static & FP & Test bed & PD, TH & FANETs \\ \hline \hline \end{tabular}  
* Performance metrics, RO: routing overhead, DLR: datagram loss rate, PD: packet delivery ratio, ED: end-to-end delay, LS: link stability, EC: energy consumption, NR: normalized routing load, TH: throughput, PS: peak-signal-to-noise-ratio, GO: goodput, DR: drop rate, LR: loss rate, DJ: delay jitter, NK: number of killed parasites, TB: transmitted bytes.  
\end{table}
Table 4: Comparison of topology-based routing protocols.  
load distribution across nodes are the difficulties that need to be addressed. This section summarizes six complex challenges that will be useful to academics and engineers while deciding on a routing protocol or designing a new one for FANETs.