# A Literature Review on Caching Transient Contents in Vehicular Named Data Networking
## 7 Conclusions  
In this paper we provided a review of research works focusing on caching transient contents in VNDN. Our analysis also considered the role of transient contents in traditional (non-NDN-based) VANETs, when deploying safety and non-safety applications. Since vehicular applications are usually interested in timely and updated information, freshness is the most crucial parameter to be considered in caching decision and replacement strategies, but it is not the only one. If the contents exhibit a skewed popularity distribution, e.g., Zipf-like, popularity also becomes an essential decision metric.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Metric** & **Target** & **Reference Paper (s)** \\ \hline Residual Lifetime & \(\bullet\) & To evict stale contents thus ensuring \\  & \(\bullet\) & To cache consistency \\  & \(\bullet\) & To cache with higher probability \\  & \(\bullet\) & long-lasting contents \\  & \(\bullet\) & To replace with higher priority items \\  & \(\bullet\) & that are ready to expire \\ \hline Popularity & \(\bullet\) & To cache with higher probability \\  & \(\bullet\) & Popular contents \\  & \(\bullet\) & To replace with higher priority less \\  & \(\bullet\) & popular contents \\ \hline Distance & \(\bullet\) & To replace with higher priority contents \\  & \(\bullet\) & cached in areas that are far away from \\  & \(\bullet\) & the current vehicle’s position \\ \hline Availability & \(\bullet\) & To cache with higher probability the \\  & \(\bullet\) & contents that are not available in the \\  & \(\bullet\) & local neighborhood \\ \hline Vehicular Content Class & \(\bullet\) & To differentiate the caching probability \\  & \(\bullet\) & and the local storage space depending \\  & \(\bullet\) & on the content class, which is \\  & \(\bullet\) & determined by size and lifetime \\  & \(\bullet\) & attributes \\ \hline \hline \end{tabular}
\end{table}
Table 2: Main decision parameters for caching and replacing transient contents in VNDN.  
Research in this field is still at its infancy, but we expect that the interest will grow fast in the next few years, thanks to increasing diffusion of IoT/IoV traffic and intelligent driving applications.  
This research received no external funding.  
The author declares no conflict of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} AI & Artificial Intelligence \\ AoI & Age of Information \\ CAM & Cooperative Awareness Messages \\ CEE & Cache Everything Everywhere \\ CS & Content Store \\ ETSI & European Telecommunications Standards Institute \\ FIB & Forwarding Information Base \\ FIFO & First In First Out \\ GPS & Global Positioning System \\ ICN & Information Centric Networking \\ IoT & Internet of Things \\ IoV & Internet of Vehicles \\ ITS & Intelligent Transportation System \\ LFU & Least Frequently Used \\ LRU & Least Recently Used \\ MAC & Medium Access Control \\ NDN & Named Data Networking \\ PIT & Pending Interest Table \\ RSU & Road Side Unit \\ SDN & Software Defined Networking \\ VANET & Vehicular Ad Hoc Network \\ VCC & Vehicular Cloud Computing \\ V2I & Vehicle To Infrastructure \\ V2V & Vehicle To Vehicle \\ VNDN & Vehicular NDN \\ \end{tabular}