# A Literature Review on Caching Transient Contents in Vehicular Named Data Networking
###### Abstract  
Vehicular Named Data Networking (VNDN) is a revolutionary information-centric architecture specifically conceived for vehicular networks and characterized by name-based forwarding and in-network caching. So far, a variety of caching schemes have been proposed for VNDN that work in presence of static Data packets, like traditional Internet contents. However, with the advent of Internet of Things (IoT) and Internet of Vehicles (IoV) applications, large sets of vehicular contents are expected to be transient, i.e., they are characterized by a limited lifetime and become invalid after the latter expires. This is the case of information related to road traffic or parking lot availability, which can change after a few minutes--or even after a few seconds--it has been generated at the source. The transiency of contents may highly influence the network performance, including the gain of in-network caching. Therefore, in this paper, we consider the dissemination of transient contents in vehicular networks and its effects on VNDN caching. By providing a detailed review of related work, we identify the main challenges and objectives when caching transient contents, e.g., to avoid cache inconsistency, to minimize the Age of Information (AoI) and the retrieval latency, and the main strategies to fulfill them. We scan the existing caching and replacement policies specifically designed for transient contents in VNDN and, finally, we outline interesting research perspectives. Vehicular Named Data Networking; vehicular networks; Named Data Networking; Information Centric Networking; Internet of Things +
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coupled with Least Recently Used (LRU) replacement, NDN is open to the implementation of other strategies. So far, a large body of research has been devoted to the development of forwarding strategies that reduce potential broadcast storm issues and speed up the data retrieval in VNDN [5, 6, 7]. In parallel, several caching strategies have been proposed to improve the availability of contents [8, 9]. However, these works have considered the dissemination of static contents, like traditional Internet multimedia files, which do not change over time. Vice versa, due to the increasing popularity of Internet of Things (IoT) and Internet of Vehicles (IoV) applications, a large number of vehicular contents are expected to be transient, i.e., they can change over time to reflect the current status of vehicles, streets, and nearby point of interests [10]. NDN nodes recognize transient contents, thanks to the so-called FreshnessPeriod, a field in the header of the Data packet that can be set by the source to specify the lifetime of the content, in milliseconds ([[https://named-data.net/doc/NDN-packet-spec/current/data.html](https://named-data.net/doc/NDN-packet-spec/current/data.html)]([https://named-data.net/doc/NDN-packet-spec/current/data.html](https://named-data.net/doc/NDN-packet-spec/current/data.html)), accessed on 23 December 2020). If a received Data packet carries a FreshnessPeriod greater than zero, a node considers it fresh. The FreshnessPeriod is treated as a timeout by the CS: when the latter expires, the packet is marked as non-fresh. Clearly, the transiency of contents highly impacts on the performance of in-network caching and the research community is questioning if it would be still useful to fill the limited content store of NDN nodes with packets that are ready to expire. Especially in vehicular contexts, consumer applications are interested in fresh data. For instance, processing outdated information about road traffic conditions could compromise the reliability of intelligent driving applications. Receiving outdated information about parking lots availability may affect route and trip planning. In this paper, we explore challenges and benefits of caching transient information in VNDN.