# Research on Multi-Strategy Routing Protocol in Flying Ad Hoc Networks
## 4 Forwarding Strategy Design  
### Reactive Mechanism Design Based on Joint Measurement Optimization  
Due to the highly dynamic nature of the FANET, the network topology can change at any time. Therefore, to ensure the stability of the constructed reactive path, the RGF protocol needs to add a minimum link expiration time word segment in the RREQ message and RREP message of the reactive mechanism used. If node \(i\) and node \(j\) are in communication range of each other, the three-dimensional position and speed vectors of the nodes are \(\left(x_{i},y_{i},z_{i}\right)\) and \(\left(v_{ix},v_{iy},v_{iz}\right)\), respectively. The three-dimensional position and velocity vectors of node \(j\) are \(\left(x_{j},y_{j};z_{j}\right)\) and \(\left(v_{jx},v_{y},v_{iz}\right)\), respectively. Then, the link expiration time (LET) between node \(i\) and node \(j\) is calculated according to Equations (2) and (3). \[LET_{ij}=\frac{\sqrt{\left(d^{2}+e^{2}+f^{2}\right)R^{2}-\left(ae-bd\right)^{ 2}-\left(af-cd\right)^{2}-\left(bf-ce\right)^{2}}-\left(ad+be+cf\right)}{d^{2 }+e^{2}+f^{2}} \tag{2}\]  
\[\left\{ \begin{array}{l}a=x_{i}-x_{j}\\ b=y_{i}-y_{j}\\ c=z_{i}-z_{j}\\ d=v_{ix}-v_{jx}\\ e=v_{iy}-v_{iy}\\ f=v_{iz}-v_{jz}\end{array} \right., \tag{3}\]  
When forwarding RREQ or RREP, each intermediate node needs to calculate the link expiration time with the previous hop node and then compare it with the value in the minimum link expiration time field. If it is less than the value recorded in this field, it will be replaced with a new value. Therefore, the \"minimum link expiration time\" field in RREQ or RREP will record the minimum value of all link expiration times on the path that the message passes, that is, the expiration time of the path, where RREQ records the expiration time on the reverse path to the source node, and RREP records the expiration time on the forward path to the destination node. The destination node must start a short timer after receiving the first RREQ. According to the received RREQ during the timing period, a joint metric is calculated for each path  
Figure 9: Switching conditions under the projection face. according to Equation (4). After the timer expires, the path with the largest joint metric value is activated by RREP. \[pathT=\frac{mLET}{n}, \tag{4}\]  
In Equation (4), \(pathT\) represents the joint metric, \(mLET\) represents the minimum link expiration time, and \(n\) represents the number of path hops. The path selected according to Equation (4) will have a certain stability, which can avoid the handoff of the forwarding strategy as much as possible, and can consider the number of hops, which can ensure a lower delay. After receiving the RREQ or RREP, other intermediate nodes also need to calculate the joint metric value for the newly discovered forward or reverse path according to Equation (4). Suppose there is active forward or reverse routing information in the routing table. In that case, intermediate nodes also need to calculate the current joint metric value on the old path as recorded in Equation (5). \[pathT_{current}=pathT_{record}-\frac{T_{current}-T_{record}}{n}, \tag{5}\]  
In the formula, \(pathT_{current}\) is the latest joint measure of the old path, \(pathT_{record}\) is the joint measure recorded in the routing table when the old path is constructed, \(T_{current}\) is the current time, \(T_{record}\) is the time stamp recorded in the routing table, and K is the number of hops recorded in the routing table. When one of the following conditions is met, the intermediate node needs to update the routing table:  
a. The new serial number is greater than the destination node's serial number stored in the routing table;  
b. The two serial numbers are equal, but the joint measure of the new path is greater than the current joint measure of the old path;  
c. The new serial number is an unknown serial number. The design algorithm of the reaction mechanism is as follows:  
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1.