## 3 Problem Statement
### Problem Analysis  
The homogeneous cache allocation scheme ignores the differences in the importance of nodes in network topology and content distribution. While this default scheme is simple and easy, it does not make full use of the limited total cache budget, which seriously affects the performance of the cache system. Figure 3 compares homogeneous and heterogeneous cache allocations using a portion of the network model. Figure 2(a) uses a homogeneous allocation scheme, with four nodes of the same size of storage space, all set up as two storage units. The request from INa for content C3 was \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline \(CS\) & Content storage on nodes \\ \(C_{i}\) & Content \(i\) in the network or request for content \(i\) \\ \(CDW_{i}\) & Weight of content domain \(i\) \\ \(DPW_{i}\) & Weight of data path \(i\) \\ \(NW_{i}\) & Weight of node \(i\) \\ \(NG_{n}\) & Global weight of node \(n\) \\ \(CD\) & Collection of content domains in a network \\ \(DP\) & Collection of data paths in a content domain \\ \(N\) & Collection of nodes in a data path \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experiment parameters. Figure 2: Comparison of Two Caching Schemes. forwarded to SNa for further processing and forwarding because it missed on INa. SNa also does not store C3, so the request is forwarded to the server. After the server responds to the request, C3 in the storage units of INa and SNa replaces the original C2, which causes subsequent ENa requests for C2 to also be sent to the content server for processing. This makes C1 requests forwarded by INf have to be sent to the server in response. The cache loss on SNa is caused by frequent replacements of content due to the insufficient number of storage units. In the heterogeneous allocation scenario, as shown in Figure 3b, a storage unit of INf is allocated to SNa, where C1, C2, and C3 can all be responded to in SNa, and the cache hit ratio is significantly improved. Therefore, heterogeneous allocation is necessary to improve the performance of the cache system. Nodes that play a high role in content distribution should be allocated more cache sizes. In the example above, INa, ENa, and INf forward their own missed requests to SNa for subsequent processing and forwarding. SNa plays a significantly higher role in content distribution than other nodes. Allocating more cache space to SNa can significantly improve the cache hit ratio. The actual network structure is very complex, and it is unrealistic to analyze each node like the example above. Then, the key to the problem is to assign the nodes in a heterogeneous way. Current schemes are often based on the topological structure of the network, such as degree-centricity which represents the local importance of nodes, and betweenness centrality which represents the global importance of nodes. Network topology is bound to have an important impact on content dissemination, but in many scenarios, the importance of nodes in the topology is not exactly the same as that in content dissemination. The betweenness centrality of nodes in the model is shown in Figure 4.  
The betweenness centrality of each node is the number of times the shortest path between any two points in the graph passes through the node. In Figure 4, the values of the betweenness centrality from blue, green, yellow, orange to red gradually increase. Nodes with higher betweenness centrality values are mainly located in the center of the network. INc, SNa, and SNb have a high betweenness centrality, but in the actual content distribution process, these nodes are not the nodes with the most paths, which limits the efficiency of the cache size allocation method proportional to the betweenness centrality under cache size conditions. Other schemes based on a network topology that ignore the content distribution process also have these shortcomings.