# A Literature Review on Caching Transient Contents in Vehicular Named Data Networking
## 3 Transient Contents in VANETs: An Overview
### Transient Information for Non-Safety Applications and Routing Protocols  
Pushed by the increasing diffusion of IoT applications, vehicles are turning into mobile sensing platforms that capture transient information feeding a variety of smart city services, some of them with just infotainment purposes. Therefore, the interest in transient content is steadily growing also in the presence of non-safety applications, although they usually have less time critical requirements than safety applications. In addition to traditional Internet services and media sharing, they include air pollution monitoring, urban surveillance, waste management and many other location-based services [33]. Therefore, non-safety transient vehicular traffic can be roughly distinguished into two main classes: (i) small sized IoT data, e.g., captured by sensors deployed on-board of vehicles, and (ii) multimedia contents randomly or periodically updated, e.g., audio and video information about vehicular traffic captured by cameras.  
The work in Reference [34] considers generic transient contents that are updated at an origin Internet server at Poisson-distributed time instants and then distributed in the VANET through RSUs. Vehicles aim at maximizing the freshness of the retrieved content and the efficiency in the utilization of radio resources. To reach the targeted objectives, the proposal defines an optimal scheduling of data traffic over the existing links and selects a set of parked vehicles as optimal relays. Freshness information can be also included in the request/query packets [35]. If a newly received request is too old compared to other ones previously received, then the source or cache node can simply decide to discard it.  
In addition, multiple signaling packets originated and/or processed by vehicular routing protocols are transient information. In particular, geographic routing protocols leverage the (changing) geographic position of sender and receiver (usually obtained from GPS devices) to create path(s) between the endpoints [36]. Each sender employs a location-based service to discover the position of the destination and this information must be extremely fresh to ensure a reliable routing decision. Road-based routing protocols belong to this category: they create network routes as successions of road intersections where vehicles transit with high probability and provide multi-hop connectivity. The work in Reference [37] leverages real-time vehicular traffic information to create road-based paths in a proactive or reactive fashion. In the first case, vehicles generate connectivity packets (CPs) periodically, by considering the estimated current number of vehicles in the networks, historical data about vehicle density, and the time elapsed from the last received CP. The routing packets create a graph of connected road segments and are used to compute the shortest paths to destinations. Conversely, in the second case, routes are discovered on demand and reported in the packet headers to implement a source-routing scheme. In both cases, packets carry a timestamp to intrinsically reflect the freshness of the information. Synchronization among vehicles is reliably guaranteed by the GPS technology.