# A Probabilistic VDTN Routing Scheme Based on Hybrid Swarm-Based Approach
## 5 Proposed VDTN Solution
### Geography-Based Recovery Forwarding  
Both FA and GSO approaches are exposed to the unavailability of the next-SCF vehicles which offer better relay quality for undefined long periods. This situation forces to find a recovery mechanism to avoid the uncontrolled delivery delays generated by long buffer store time.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**VDTN Components** & **GSO Elements** \\ \hline VDTN vehicle & Glowworm agent \\ \hline Next-SCF vehicle & Adjacent neighboring glowworm \\ \hline Forwarding quality & Luciferin \\ \hline Destination vehicle & Prey or Food source \\ \hline \hline \end{tabular}
\end{table}
Table 2: VDTN/GSO mapping.  
Thus, the Minimum Estimated Time of Delivery (METD) concept, as proposed in GeOpps [31] and GeoSpray [32], is switched by passing bundle copies to the vehicles which can reach closer positions to the destination than the bundle's holder node.  
As introduced in the METD, the estimated time of arrival between the candidate vehicle \(\mathbf{i}\) and selected \(NP_{i}\) (ETA(\(Host\),\(NP_{i}\))) in Equation (17), and between the latter and the destination ETA(\(NP_{i}\),\(D\)) in Equation (18) is based.  
\[ETA_{Host,NP(i)} =Avg_{Path\_Speed(i)}\times Dist_{Host,NP(i)} \tag{17}\] \[ETA_{NP(i),D} =Avg_{Path\_Speed(i)}\times Dist_{NP(i),D} \tag{18}\]  
Considering:  
* \(Avg_{Path\_Speed(i)}\): the extracted average speed from the GPS-calculated path of candidate vehicle \(\mathbf{i}\).
* \(Dist_{Host,NP(i)}\): the distance between the bundle's carrier node and the nearest point of candidate vehicle \(\mathbf{i}\).
* \(Dist_{NP(i),D}\): the distance between the nearest point of candidate vehicle \(\mathbf{i}\) and the destination.  
For each candidate SCF vehicle \(\mathbf{i}\)'s NP (\(NP(i)\)), the ETA is recuperated from GPS to calculate the METD in Equation (19).  
\[METD_{i}=ETA_{Host,NP(i)}+ETA_{NP(i),D} \tag{19}\]  
The condition on which the decision to forward bundles to the next-hop nodes is based on Equation (20):  
\[METD_{Host}\leq\frac{1}{2}\times METD_{Cand(i)} \tag{20}\]  
Considering:  
* \(METD_{Host}\): the measured METD of the bundle's host node.
* \(METD_{Cand(i)}\): the measured METD of candidate node \(\mathbf{i}\).  
The proposed restriction of the number of METD-based selected vehicles is justified by the minimization of overheads, as the bundle forwarding is limited to candidates offering a least twice better METD quality than the bundle's host vehicle.  
The mapped illustrated use case in Figure 2 explains the proposed restricted METD mechanism.  
Figure 2: Restricted Minimum Estimated Time of Delivery (METD) mechanism of the hybrid FA–GSO VDTN router.  
As shown in this example, the GPS-traced path of the current bundle's carrier (vehicle **B**) is compared to other available SCF candidate nodes (vehicles **A** and **C**). We notice that both vehicles **B** and **C** gather better METD values than bundle host **B** with \(METD_{C}<METD_{A}<METD_{B}\) but only node **C** meets the condition \(METD_{C}<\frac{1}{2}\times METD_{B}\); thus, node **C** is selected as the next SCF vehicle of the forwarded bundle.