# Priority-Based Content Delivery in the Internet of Vehicles through Named Data Networking +
## 3 Background and Motivations
### NDN Forwarding in VANETs
#### 3.2.2 Multi-Face Communications  
The previously-scanned literature considers NDN to exchange Interest/Data packets over a single wireless interface, i.e., 802.11. However, in the near future, vehicles will be equipped with a variety of wireless communication interfaces.  
Thanks to name-based communication, the NDN forwarding strategy is natively able to deal with multiple network interfaces. Indeed, there is no need to set up a connection for data retrieval; IP addressing and port numbers are not used since forwarding depends only on a label, i.e., the content name. Therefore, Interest can be sent unmodified on any available interface.  
NDN can decide, for each incoming Interest, which outgoing face(s) to use, e.g., the best-performing face, a subset of possible faces or all available faces. The decision depends on the implemented forwarding strategy, which may take as inputs the face cost, determined by the routing protocol, and the face rank, maintained by the forwarding strategy [30]. Indeed, unlike traditional IP-based networks where forwarding is a \"dumb\" operation driven by routing, in NDN, each node maintains a soft-state for every pending Interest in the PIT, and when the Data packets are received from the face(s), the node can know how the downstream faces actually work. The performance of each face, e.g., its Smoothed Round Trip Time (SRTT), can be therefore recorded to improve future forwarding decisions, without waiting for the routing protocol's updates.  
Standard NDN forwarding mechanisms include: (i) the broadcast (also known as parallel) strategy, where the Interest is forwarded over each available interface; (ii) the best route strategy, where the Interest is forwarded upstream with the lowest routing cost; (iii) the CCNx 0.7.2 strategy, where the Interest is first forwarded on the best route and, if a matching Data does not come in the predicted time, the Interest is forwarded to the other possible faces. How to properly estimate the Data arrival time and decide if/when transmitting redundant Interests over the other faces in the CCNx 0.7.2 strategy is however an open issue, which deserves proper investigation under different network environments.  
In [12], the broadcast strategy is used on vehicles equipped with 3G/LTE/Wi-Fi connectivity, regardless of the content requirements. Results show that the solution is robust and resilient to connectivity disruption, but it causes high packet redundancy that should be eliminated.  
In [7], heterogeneous technologies are used for different purposes: the cellular network is used to carry the NDN Interests over long distances and counteract possible connectivity gaps over the V2V links, while short-range V2V communications are exploited for content delivery.  
In this paper, we consider vehicles equipped with two wireless access technologies: a cellular 3G/LTE face and an IEEE 802.11 OCB face. Although the cellular network offers global V2I coverage, V2V and V2R communications via IEEE 802.11 OCB are a good alternative connectivity solution that offloads the cellular infrastructure and takes advantage of high-speed opportunistic connectivity and data muling. Our designed forwarding strategy will take the best of the two faces in order to satisfy the priority requirements of the consumer application.