# Congestion-Aware Geocast Routing in Vehicular Delay-Tolerant Networks
## 3 Geocast Strategies Comparison  
In this work, geocast focuses on distributing messages during a specified Time-To-Live (TTL) to all nodes that come across an invariant circularly shaped ROI, defined around the node that creates the messages. Moreover, the development of the intended message dissemination algorithm is performed by understanding which features of the considered schemes yield better results under a broad variety of simulations.  
In this study, in order to drive such conclusions, the following routing strategies are taken into account, which result from the extension of the definitions regarding the aforementioned unicast routing protocols to a geocast context: Multiple Copy GeoDirectDelivery (MC-GeoDirectDelivery), GeoSpray-and-Wait (GeoS&W), GeoEpidemic, and GeoEpidemic Constrained.  
Firstly, when using the MC-GeoDirectDelivery protocol, on one hand, messages are delivered to and kept by interested nodes inside the ROI, at the delivery time instant. On the other hand, messages are retained when nodes leave the ROI, as they may return to the ROI in the future. Because messages are only dropped when their TTL expires or the nodes' buffers are filled up, all nodes that receive a message also become responsible to deliver it to the remaining recipients, enabling the presence of multiple copies of the same message in the network.  
Secondly, the GeoS&W scheme couples the principles of the Spray and Wait and the GSAF protocols. In this sense, during the spray phase, messages are replicated, even when nodes are located outside the ROI, using the spraying heuristic adopted by the GSAF protocol. Regarding the wait phase, the GeoS&W behaves the same as the MC-GeoDirectDelivery protocol.  
The Epidemic protocol nature may assist message distribution in a geocast context as it already distributes an unbounded number of copies of the same messages by multiple receivers. However, to distinguish the protocol's operation in unicast scenarios from its operation in a geocast routing paradigm, this protocol will be referred as the GeoEpidemic protocol throughout this article. Likewise, the GeoEpidemic Constrained scheme follows the Epidemic protocol's principles as well. However, in this case, the routing scheme under consideration constrains the dissemination of messages within the boundaries of the ROI, since messages are dropped as soon as the nodes carrying them leave this region.  
Additionally, it is also crucial to make explicit the following performance assessment metrics utilized: delivery ratio, delivery latency, and overhead. The delivery ratio is computed using the formulation developed in [11]. By monitoring the recipients that reside inside a given message's ROI during its TTL, it is possible to compute a ratio between the number of successful receivers and the total number of nodes belonging to the list of recipients. Obviously, this ratio only translates itself into a per-message result. To that end, to obtain an overall delivery ratio estimation, the per-message metric is extended to all created messages through an average operation. Latency is also monitored, in the first instance, taking into account a single message. However, in this case, the per-message metric is computed by summing the amount of time it takes for the message under consideration to reach each one of its successful receivers and, then, by dividing this outcome by the same number of recipients. Finally, the latency for all messages is averaged. The network overhead, in terms of transmissions, is by definition the ratio between the surplus of transmissions relative to the number of delivered messages, and the number of delivered messages. In spite of being computed similarly to unicast schemes, the outcome of this metric is expected to be much lower in a geocast context, since the existence of several recipients per message is reflected on a larger number of delivered messages.