# Popularity-Aware Closeness Based Caching in NDN Edge Networks
###### Abstract  
By enabling name-based routing and ubiquitous in-network caching, Named Data Networking (NDN) is a promising network architecture for sixth generation (6G) edge network infrastructures. However, the performance of content retrieval largely depends on the selected caching strategy, which is implemented in a distributed fashion by each NDN node. Previous research showed the effectiveness of caching decisions based on content popularity and network topology information. This paper presents a new distributed caching strategy for NDN edge networks based on a metric called popularity-aware closeness (PaC), which measures the proximity of the potential cache to the majority of requesters of a certain content. After identifying the most popular contents, the strategy caches them in the available edge nodes that guarantee the higher PaC. Achieved simulation results show that the proposed strategy outperforms other benchmark schemes, in terms of reduced content retrieval delay and exchanged data traffic.  
Named Data Networking; information centric networking; caching; edge networks; 6G +
Footnote †: journal: Physics Letters B  
definition  
One of the most important caching decision metrics is the content popularity. Typically, Internet contents show a skewed popularity distribution [3]: only a few contents are highly requested and deserve to be cached. Another crucial caching decision metric is the centrality of the nodes. The work in [4] focuses on the betweenness centrality metric and demonstrates that caching at the most central nodes can increase the cache hit probability and decrease the cache eviction rate, as these nodes are traversed by the majority of content requests. However, from the perspective of content retrieval, the node centrality should be re-defined by taking into account the capability of the node of satisfying the content requests, in addition to its topological feature [5; 6].  
So far, existing work [5; 6; 7] defined a popularity-aware betweenness centrality metric to enable coordinated caching decisions between distributed caches. Basically, these strategies weight the betweenness centrality of a node with the popularity of the contents that can be transmitted through it. By introducing a relevant signalling among the nodes, collaborative decisions are deployed, where the most popular contents are cached in the most central nodes. However, in edge topologies, typically characterized by hierarchical topologies [6; 8], the nodes with the highest betweenness centrality are also far away from the end-clients, i.e., the content consumers. Therefore, they cannot guarantee a low retrieval delay and a small network traffic. Instead, it would be crucial to cache the content as close as possible to the consumers to meet the proximity requirements of upcoming 6G applications.  
In this paper, we define a new caching scheme based on a popularity-aware closeness (_PaC_) metric, which allows to cache the most popular contents in the edge nodes according to their proximity to the majority of requesters. Thanks to lightweight signalling piggybacked in Interest and Data packets, the potential caches are ranked according to the _PaC_ metric, and the best available cache per path is selected. Performance evaluation shows that the conceived strategy is able to limit the retrieval delay while maintaining low exchanged traffic and the signalling overhead, compared to related literature based on the betweenness centrality metric [6].  
The remainder of this paper is organized as follows. Section 2 provides background material on caching strategies in NDN. The proposal is discussed in Section 3 and evaluated in Section 4. Final remarks are reported in Section 5.