# A Probabilistic VDTN Routing Scheme Based on Hybrid Swarm-Based Approach
## 2 Literature Review  
The probabilistic DTN routing has been developed widely in VANETs to face consistently-sparse networks. PRoPHET [12], a greedy-based predictive DTN routing scheme founded on the concept of delivery predictability (DP). The DP measures the ability of encountering nodes to forward bundles towards their destinations into a variable named the encounter predictability value. Nodes continuously exchange any updates indicating higher predictability values of neighbors than the one of bundle carrier node. Further, to the direct probability between two nodes ('A' and 'B' for instance), The DP supports transitive predictability values liaising between nodes having common direct contacts. This property extends the DP impact for larger neighborhood areas. Thus, a meeting table regrouping direct and indirect historic contacts is saved on each node. A third DP characteristic called the data aging is mandatory to temporarily update the predictability values. Probabilistic Bundle Relaying Scheme (PBRS) [17] hybrid DTN router combines knowledge-based and prediction-based forwarding modes. PBRS finds the prediction of the required duration to deliver every bundle to its destination for each candidate SCF node. Thus, the notion of release probability (Pr) is introduced for this purpose by referring to node speed as the evaluation parameter to calculate the probability of passing a bundle to another node. The Pr value is expected to cut down the bundle's store time in buffer cache so that the total delivery delay is cut down consequently. PRoPHET-based Spray-and-Wait (SnW) [18] is a combined ProPHET-SnW protocol which integrates the probabilistic concept of PRoPHET on SnW router [19]. The ProPHET's DP is performed during the spray phase, where \(L\) copies of each bundle are forwarded to the nodes providing higher delivery probability values. When the number of copies is reduced to 1, the SnW's wait phase is triggered. ProPHET-based SnW adopts also a buffer management mechanism to control buffer cache overflow efficiently by checking the bundles' size to meet the remaining buffer space of available candidate SCF nodes. This policy contributes to increase the delivery probability. Improved PRoPHET [20] is a hybrid Epidemic-ProPHET router. Epidemic (ER) [21] is enabled in early steps of forwarding operation which helps to propagate quickly bundle copies. While ProPHET follows in advanced stages of routing process to improve orientation of spread copies by ER. ProPHET is enabled when either a hop count threshold or a predefined number of forwarded bundles are reached. PRoPHET+ [22] is a modified version of PRoPHET conceived to improve the selection of bundles' carrier by introducing the delivery value (\(V_{D}\)) on the next-SCF node selection on the basis of a predefined threshold comparison. The \(V_{D}\) is calculated for every candidate next-SCF node following the \(V_{D}\)'s evaluation function in Equation (1) which is composed from four weighted parameters:  
* Buffer parameter (\(V_{B}\)), which is the difference between remaining buffer space ratio minus the bundle size. * Power parameter (\(V_{P}\)), which is the remaining power from the difference between device power and the minimum power for sending and receiving bundles. * Popularity parameter (\(V_{O}\)), which is the ratio of number of performed transmissions on the maximum number of transmissions. * Bandwidth parameter (\(V_{A}\)), which is the ratio between received node's bandwidth and host's bandwidth. \[V_{D}=(W_{B}\times V_{B})+(W_{P}\times V_{P})+(W_{A}\times V_{A})+(W_{O}\times V _{O})+(W_{R}\times V_{R}) \tag{1}\]  
\(W_{B}\), \(W_{P}\), \(W_{A}\) and \(W_{O}\) are respectively the affected weights for the predefined evaluation parameters. A candidate node handles the bundle if it has higher \(V_{D}\) than the predefined \(V_{D}\) threshold. The best \(V_{D}\) is used as a tiebreaker when numerous better candidate SCF nodes are available.