# A Literature Review on Caching Transient Contents in Vehicular Named Data Networking
## 3 Transient Contents in VANETs: An Overview
### Main Insights  
As summarized in Table 1, our review shows that transient contents were considered in VANETs literature although less attention was devoted to the design of caching strategies. Without loss of generality and despite the application domain (i.e., safety or not safety), we can broadly classify vehicular transient contents into two main categories:  
* **Contents with a fixed predefined lifetime. ** In this case, contents are characterized by a fixed lifetime that is usually application-specific [41]. Traditional beaconing approaches with a fixed frequency of message generation and monitoring applications that capture parameters at regular intervals belong to this category, e.g., environmental data, like humidity and temperature periodically captured by sensors deployed in RSUs or on-board of vehicles. Timestamp and lifetime (or refresh period) information can be set by the source and advertised directly in the content packet. Therefore it is extremely simple to maintain cache consistency and evict stale data. Thanks to GPS receivers, synchronization can be easily maintained among vehicles and infrastructure elements. \begin{table}
\begin{tabular}{c c c c} \hline
**Topic** & **Challenges** & **Main Findings** \\ \hline AoI Assessment & \(\bullet\) & Vehicular applications, especially safety ones, are interested in the freshest content & \(\bullet\) & AoI increases with the distance between the endpoints and the vehicle density [26, 27] \\ \hline AoI Minimization & \(\bullet\) & To reduce AoI when dealing with transient safety and non-safety traffic & \(\bullet\) & Smart queuing mechanisms at the MAC layer, e.g., LCFS [23, 29] \\  & & & \(\bullet\) & Timestamp in both query and content packets to recognize and discard the older ones [37] \\  & & & \(\bullet\) & To adapt the sending frequency to the density of nodes [28, 31] \\ \hline Cache Consistency & \(\bullet\) & To recognize if the data is fresh or stale & \(\bullet\) & Lifetime and timestamp/sequence number information (if available) avoid the issue [37, 38] \\  & \(\bullet\) & To align cached contents with the updates at the source & \(\bullet\) & Source-initiated pushing or cache-initiated pulling to, respectively, disseminate or receive the new content version [38, 39] \\ \hline Caching Strategy & \(\bullet\) & To decide what to cache and for how long, according to the freshness information, among other parameters & \(\bullet\) & To reduce the caching probability of contents with high AoI [35, 40] \\  & & & \(\bullet\) & If more versions of the content are available, older ones should be discarded [35] \\ \hline \end{tabular}
\end{table} 
Table 1: Transient Contents in (non-Named Data Networking (NDN)) vehicular ad hoc networks (VANETs): main topics and findings. * **Contents with a variable lifetime. ** In this case, the lifetime is a-priori unknown: contents are updated by their producers randomly or on an event basis. Sometimes, the generation sampling rate is the result of policies that trade off between the timeliness of the information and other network parameters [31]. When different versions of the content are available in the network, a timestamp or a sequence number set by the source are crucial to recognize the freshest versions. Related literature works have also identified some intrinsic features of vehicular communications that affect information freshness, namely the distance between the communication endpoints and the vehicle density. Queue management at the MAC layer has been recognized as a fundamental feature to reduce the age of information and different scheduling techniques have been explored. Vice versa, we could not find many solutions about freshness-aware caching. This is mainly due to the fact that the interest in transient contents is relatively new and mainly related to the advent of IoT/IoV applications and NDN.