# A Literature Review on Caching Transient Contents in Vehicular Named Data Networking
## 3 Transient Contents in VANETs: An Overview
### Minimizing Age of Information of Safety Information  
The first works in the context of transient contents for safety applications were devoted to minimize the average end-to-end delay observed in the vehicles' states within a certain group of nodes [23] and called into the picture the concept of AoI, a performance metric measuring information freshness at the destination node. By identifying the time elapsed since the current data was created, AoI identifies how outdated the retrieved information is. We can assume that a content \(c_{k}\) is updated over time by its source, which therefore originates a time series of content objects \(c_{k,1}\), \(c_{k,2}\), etc., each one characterized by a timestamp \(T_{c_{k,1}}\), \(T_{c_{k,2}}\), and so on. At time \(t\), the age of \(c_{k,1}\) can be computed as: \(AoI_{c_{k,1}}=t-T_{c_{k,1}}\). As shown in Figure 3, whenever a new message is received at the consumer, the AoI drops to zero, and then it grows until the next update. Ideally, each consumer should receive only the current valid version of the content [25]; information with high AoI is potentially unreliable and irrelevant. Receiving \(c_{k,1}\), when the most fresh version of the content is \(c_{k,2}\), could potentially compromise the behavior of the safety application. Figure 3: Age of Information (AoI) variation at the consumer side, when receiving subsequent updates of content \(c_{k}\). The preliminary work in Reference [26] analyzes the performance of information dissemination in urban VANETs with direct focus on AoI measures. The authors find that AoI grows almost linearly with the distance from the source and it also strongly depends on the density of vehicles. Similar considerations were reported in Reference [27], in the context of a free parking places discovery applications. In Reference [28], the authors focus on the AoI of broadcast beacons sent by vehicles and study the trade-off between the sending frequency of the messages and the congestion induced in the wireless IEEE 802.11p channel. An analytical model capturing the distributed contention in the VANET is proposed that shows how AoI changes with the beacon sending frequency and the vehicle density. When the latter parameters are high, the wireless medium tends to be congested, the channel access time increases, the message loss ratio increases due to collisions and the AoI increases, as well. On the other side, when the sending frequency reduces, the contention on the wireless channel reduces, as well, but the AoI intrinsically increases. Basically, the frequency of beacons should be adaptive and set according to the current vehicle density. Vehicles that move in a sparse zone can increase the message sending frequency, since the chances of collisions are low. Conversely, in crowded areas, they must decrease the sending frequency. As a result, it is crucial to design transmission and caching mechanisms that minimize AoI and guarantee that the data, even when retrieved from the cache of an intermediate node, is not outdated. However, deciding when to discard stale data, and possibly update the cache, is not trivial in many circumstances. Sometimes, frequent updates are not possible because there are network constraints, like limited bandwidth [25], or because the cache/recipient has lost the contact with the source or has poor and unreliable connectivity with it [26], e.g., a common condition in vehicular networks. Literature works usually model the generation process of transient contents as a queuing system in which the packets arrive at the source by following a stochastic distribution [29]. The source transmits the packets using a certain policy, like First-Come-First-Served (FCFS) or Least-Come-First-Served (LCFS), and only a single packet transmission occurs at a given time. For instance, in Reference [23], the authors assume that vehicles transmit their message updates periodically and find that AoI could be improved by controlling the MAC layer queue and, in particular, by changing the queuing policy from FCFS to LCFS.