# A Literature Review on Caching Transient Contents in Vehicular Named Data Networking
## 3 Transient Contents in VANETs: An Overview
### Caching  
In-network caching in VANETs was considered years before the advent of VNDN, thanks to its intrinsic benefits, like reducing retrieval delay and traffic congestion and improving data access. However only few works investigated transient content caching and focused on two main research areas, namely the support of cache consistency and the design of caching decision strategies.  
**Cache consistency.** In presence of transient contents and mobile environments, a first problem to face is the cache consistency, as preliminary analyzed in Reference [38]. The simplest way to ensure consistency is to include a lifetime in the content packet: when the latter expires, the node can drop the information since it is outdated. When the lifetime information is not available, then two approaches are possible: (i) in the source-initiated one, the source pushes to the other caches the new version of the content; and (ii) in the cache-initiated one, the caches pull the source to receive the update. To limit the signaling overhead between the endpoints, in Reference [38], some highly accessible and stable mobile nodes are selected to relay invalidation information to the caches. A cooperative cache invalidation strategy is also proposed in Reference [39], where access points or RSUs are in charge of distributing updates of Internet contents. Clearly, these schemes are not always feasible in VANETs due to the presence of frequent disconnections and opportunistic contacts and the possible lack of infrastructure elements.  
**Caching strategies.** A pioneering caching strategy for wireless ad hoc networks with data freshness awareness is Hamlet [35]. The proposal assumes that contents are fragmented in chunks tagged with a version identifier value, to distinguish the fresh ones from the stale ones. Consumers nodes cache packets according to a metric called _content presence index_, which estimates the availability of the content in the network. This information is obtained by taking into account the messages overheard in the neighborhood and the hop count distance from the origin source/cacher piggybacked in the retrieved content. In order to maximize the data diversity and limit the network traffic, the caching probability is inversely proportional to the presence index: there is basically no need to cache a content that is easily accessible in the network. In addition, if a node receives different versions of the requested content, it will only cache the most recent one and discard the others. If an older version is available in the cache, it will be replaced.  
More recently, the work in Reference [40] defined a caching strategy based on the relationship between content popularity and freshness. Potential caches track the AoI of contents and their request rate and select the content to cache in order to maximize the cache hit ratio. The rationale behind the conceived strategy is twofold: first, packets with higher AoI are less likely to be requested, and, second, popular packets have a higher chance to be requested. The strategy is autonomous and based on local measurements of content request rate and AoI.