# A Literature Review on Caching Transient Contents in Vehicular Named Data Networking
## 5 To Cache and Replace Transient Contents in VNDN
### Replacement Policies  
As discussed in Reference [55], replacement policies, like LRU or LFU, traditionally used in wired networks, could be not well suited for VANETs in certain scenarios. Indeed, the request pattern of vehicular contents can be driven by specific conditions, like vehicle mobility and geographical position or application priority and popularity. These parameters could highly influence the replacement policy. However, the majority of existing works consider the LRU scheme, due to its intrinsic simplicity; only very few ones specifically target cache replacement in VNDN and they almost unanimously do not refer to transient contents. For instance, the Popularity-based Content Caching (PopCC) policy in Reference [56] employs a Hidden Markov Model to predict the future popularity of contents and then caches the items in descending order of popularity. The last packet in the CS is the one with the lower popularity and the first candidate to be evicted. Popularity is also used in Reference [57] as a replacement metric, but in a cooperative fashion. There, vehicles are organized in clusters and the cluster-head, which maintains a central vision of request rate and hit ratio, decides which contents must be evicted.  
To the best of our knowledge, only the two following works specifically target the replacement of transient contents. In particular, in Reference [58], vehicular contents are distinguished into three main classes: (i) safety information, such as emergency warnings messages, which expires quickly; (ii) road traffic information, which has a longer lifetime compared to safety contents; and (iii) comfort information, which has the longest lifetime. The CS of caching vehicles is then divided into three sections (called sub-CS), each one for a content class, thus guaranteeing that all types of vehicular traffic receive a proper quality of service (QoS) treatment. In particular, more space is provided to comfort traffic which is usually long-lived and larger in size, while less space is provided to safety traffic since it is usually small in size and highly transient. A tag is included by the producer in the packet header to indicate the traffic class and guide the CS management. The replacement policy is based on a Content Popularity-Density metric that balances popularity and size of contents; it is computed by the producer and included in the header of the Data packets. When a vehicle receives a Data packet, it first checks the tag and caches it if there is room in the related sub-CS. Otherwise, it extracts the popularity-density metric and compares it against the lower value associated with the cached items. If the newly received value is higher, then the existing cached item is replaced in favor of the new one.  
The authors in Reference [55] present a Multi-Metric Cache Replacement policy (M\({}^{2}\)CRP) based on three peculiar factors: (i) freshness of the content, as carried in the FreshnessPeriod field; (ii) frequency of the content requests, referred to the period the item has been cached in the CS; and (iii) distance between the current location of the vehicle and its position when it has originally cached the item. Intuitively, highly requested and fresh contents should be maintained in the CS. Moreover, since vehicular contents are usually location-based, they lose their relevance when the vehicle moves far away from the area they have been cached. Therefore, if the distance factor is high, the content has a high probability to be replaced.  
When a new Data packet is received and the CS is full, M\({}^{2}\)CRP selects the candidate item to be replaced by computing a candidacy score for each content in the CS. A score function is defined that equally weights the three mentioned parameters, and the cached item with the lowest score is selected to be replaced.