# Congestion-Aware Geocast Routing in Vehicular Delay-Tolerant Networks
## 5 Performance Evaluation and Results
### Delivery Latency  
Figure 8 shows the average message delivery latency for different protocols and geocast groups shown in Table 3.  
Latency is only averaged for delivered messages. This is why the GeoEpidemic Constrained protocol has the lowest (i.e., best) latencies. As messages are dropped as soon as nodes leave the ROI, longer paths through nodes that leave and return to the ROI do not result in a successful delivery and, for sparse networks, the delivery rate is so low that all messages are delivered rapidly near the center of the ROI or not delivered at all.  
GeoEpidemic disseminates messages through all nodes, even outside the ROI. Hence, it has the highest (i.e., worse) latencies, as some paths are very long. This is more significant for geocast groups two and three that have a smaller ROI, and for denser networks. The waste of resources for spreading messages outside the ROI causes congestion, degrading the protocol efficiency.  
The V-GRADIENT provides a moderate and adequate solution, putting a limit on latency, by means of a dynamic dropping policy, while still achieving good performance regarding the delivery  
Figure 7: Delivery ratio as a function of node density and protocol.  
ratio. For sparse networks, the V-GRADIENT has better delays, but they are similar to GeoS&W, since nodes are allowed to retain messages beyond their respective ROIs to ease further deliveries in the future. On the other hand, for denser networks, the latency slowly converges towards the GeoEpidemic Constrained results, since the buffer zone size is adapted dynamically according to network density and congestion level.  
Considering the average latency results obtained for messages pertaining to the geocast group with ID = 3, which encompasses 25% of the total number of nodes, while the V-GRADIENT performance shows improvements as the number of vehicles deployed in the network grows, the results associated with the MC-GeoDirectDelivery and GeoS&W protocols show minor variations. This effect is related to the low degree of cooperation shown among nodes for the latter protocols, which limits the exploitation of an increasing number of contact opportunities. Conversely, the V-GRADIENT protocol carefully selects regions where the replication is not restricted, allowing nodes to seize the surrounding contact opportunities with the aim of enabling a quick propagation of messages locally, even though the selected carrier nodes may not show interest in such relayed messages.  
The V-GRADIENT algorithm not only efficiently uses the density level and the buffer congestion metrics to infer the benefits of preserving messages being carried near their respective regions of interest, but also establishes location-based criteria to assess if vehicles that are not interested parties of a given message should cooperate in its dissemination process. Consequently, by making use of dynamic network information intelligently, the V-GRADIENT algorithm estimates which contact opportunities can improve the local spread of messages without compromising the delivery probability. Broadly  
Figure 8: Delivery latency as a function of node density, protocol, and geocast group.  
speaking, in this simulation scenario, the V-GRADIENT achieves considerably better delay-related results than the other protocols, apart from the GeoEpidemic Constrained protocol.