# Congestion-Aware Geocast Routing in Vehicular Delay-Tolerant Networks
## 1 Introduction  
Delay Tolerant Networks (DTNs) are characterized by long or variable delays, intermittent connectivity, asymmetric data rates, and high error rates. In DTNs, routing strategies rely on the store-carry-and-forward paradigm, i.e., on the dissemination of messages to intermediate nodes that can retain, transport, and deliver them either to the final destination or to other intermediate nodes. The complexity of the routing decisions is, for that reason, closely related to the selection of a proper message replication mechanism.  
The geocast concept refers to a delivery scheme that filters the eligibility of the network's target nodes using their location as a criterion. With this in mind, the diversity of geocast environments encompassed by DTNs suggests the demarcation of strategies according to generic properties such as the definition of regions of interest (ROI) or the replication mechanisms applied. While unicast routing in DTNs has been extensively studied [1], geocast routing has been studied mainly in Mobile Ad Hoc Network (MANET) scenarios [2], where permanent end-to-end connectivity is assumed, which does not apply to DTNs.  
The coupling between the advances in wireless communications technologies and the car industry makes Vehicle-to-Vehicle (V2V) connectivity an enabler for a broad range of applications that can be grouped into two categories: safety-related and commercial-related. For real-time short-range safety communications, several mechanisms have been designed, like using beacons for vehicles todisseminate information to their direct neighbors [3] with high periodicity (e.g., 10 Hz) or at two hops distance [4]. For non-real-time geocast dissemination of messages through ROIs larger than the wireless communication range, the DTN paradigm of store-carry-and-forward opportunistic dissemination of messages is particularly useful. Therefore, in light of the advances mentioned, the DTN communication paradigm has received a lot of attention in research and has motivated the design of routing strategies for Vehicular Delay Tolerant Networks (VDTNs) [5] that can properly handle the challenges of such distinguishable environments.  
In this sense, the design of strategies in VDTNs that can efficiently deliver messages to a group of recipients, which would particularly benefit from the reception of such messages, has become a widespread topic of discussion. To that extent, the implementation of algorithms that exploit information from the vehicles' navigation systems to route messages within specific ROIs motivates the development of the V-GRADIENT geocast routing protocol for VDTN environments.  
The proposed routing protocol is the basis for a message dissemination service within an ROI in highly dynamic or sparse vehicular networks. Applications include V2V commercial or safety-related message dissemination such as disseminating maps, software updates, fuel prices, current weather, incident warnings, or others [6]. The proposed protocol provides the following contributions: mechanisms to monitor node density and node's buffer occupancy; mechanisms to control message replication avoiding network congestion and buffer exhaustion; efficient message delivery within the ROI. This paper extends the preliminary version of the V-GRADIENT protocol proposed in [7] by including new dropping policies and replication mechanisms, an overhead analysis, and an extensive scenario analysis.