## 4 Hierarchical Cache Management (HCM) Policy
### SSD Cache Replica Management
In this experiment, we used traffic group B from Table 2 and set the cache capacity to 5% of the total number of contents. The following is an introduction to these cache placement strategies. * LCE: a copy of the content is cached at every hop node along the content delivery path;
* LCD: content is cached only at the downstream one-hop node of the current service node;
* CL4M: among all the nodes along the content delivery path, the node with the highest betweenness centrality is selected to cache the content;
* Random: a node along the content delivery path is randomly selected to cache the content;
* ProbCache: content is cached probabilistically at each node along the content delivery path. The caching probability at each node is determined based on two key parameters: _TimesIn_ (which approximates the caching capability of the path based on traffic load) and _CacheWeight_ (which reflects the router's distance from the user). These factors guide the content caching decision to optimize cache resource utilization and reduce redundancy. As shown in Figure 5(a), there are significant differences in cache hit rates for different replacement policies under various placement strategies. For example, under the LCE strategy, LFU outperforms LRU, while under the LCD and CL4M strategies, LFU's performance significantly declines, and LRU performs the best. Nevertheless, HCM achieves satisfactory results under all placement strategies, indicating that it can operate stably across different placement strategies. Notably, HCM combined with Random achieves the best cache hit rate across all combinations, suggesting that good cache performance can be obtained even with simple placement decisions. This reduces the complexity of caching decisions and helps optimize the forwarding performance of ICN routers. Figure 5(b) shows that HCM consistently achieves the lowest network link load across different scenarios, indicating that HCM can effectively reduce network traffic costs through closer cache hit distances. Notably, under the LCD and CL4M strategies, although HCM's hit rate is slightly lower than LRU's, it still results in lower network link load. This suggests that HCM, through SSD cache filtering and effective SSD cache management, can prioritize retaining replicas with higher cache utility, further reducing link load. Figure 5(c) presents the experimental results for SSD write load. Since LRU and LFU lack SSD write filtering, their write load is mainly determined by the placement strategy, resulting in similar write load levels for LCD, CL4M, and Random. In contrast, HCM, uCache, and PI maintain very low write loads across different placement strategies. #### 5.2.4 Results Analysis Under Different Cache Capacities

As the number of cache nodes increases or storage capacity expands, the cache capacity will increase accordingly. Therefore, the cache management policy should have good scalability to ensure that, as cache capacity grows, both the number of cached replicas and the cache hit rate can gradually improve. In this section, we evaluate the performance of different replacement policies under various cache capacity settings. The \(x\)-axis in the figures represents the proportion of cache capacity relative to the total number of contents. Considering that the trends observed under different traffic loads and cache placement strategies are roughly the same, we selected traffic groups B and C from Table 2 as input workloads and set the cache placement strategy to LCE. As shown in Figure 7, with the increase in cache capacity, the cache hit rate of all replacement policies improves, primarily due to the increased diversity of replicas in the cache. Under both traffic loads, HCM consistently achieves the highest hit rate across different cache capacities, demonstrating its broad applicability and excellent scalability in various application scenarios. In contrast, LFU shows significant differences in performance under different traffic loads.