# IMU: A Content Replacement Policy for CCN, Based on Immutance Content Selection
## 4 Performance Evaluation
### Cache Hit Ratio  
The cache hit ratio is an essential metric for evaluating the performance of CCN cache. It identifies the response to network cache storage, in which content is cached locally within a specific time frame. Two terms are important in the cache hit ratio. The first is the cache hit (requested content is found from the cache), and the second is the cache miss (unlike cache hit). When content is available in the cache, the content request does not forward to the publisher. Therefore, a higher hit ratio indicates good cache performance and represents low bandwidth utilization, reduction in latency, and low server load. The cache hit ratio is defined as follows:  
\[Hit\ Ratio=\frac{Cache_{\mathit{hits}}}{Cache_{\mathit{hits}}+Server_{ \mathit{hits}}} \tag{1}\]  
Our proposed strategy, IMU, compared to existing well-known replacement strategies in terms of the cache hit ratio. We have extracted the results from low to high popularity and different cache sizes. We first comment that content eviction policies behave the same under different caching strategies. Regardless of the content eviction policy, we observe in Figure 2 that the opt_cache performs best and the LCE performs the worst in terms of the cache hit ratio. Moreover, different eviction policies affect the performance of the cache hit ratio. Figure 2 illustrates that the IMU's performance is better than the existing replacement strategies; this is because the IMU not only considers the time \(\mathbb{T}_{\mathit{cr},\mathit{s}_{i}}\) but also the frequency \(\mathbb{F}_{\mathit{c},\mathit{s}_{i}}\) of the requested content within the specific period \(W_{T}\). When the \(W_{T}\)  is expired, then all the \(\mathbb{F}_{\mathit{c},\mathit{s}_{i}}\) initialize to their starting frequency (\(\mathbb{F}_{\mathit{c},\mathit{s}_{i}}\)= 1). Moreover, it helps to evict content from the cache space whose popularity increases for a while and decreases shortly. When the cache is full, it is evicted from the cache after selecting the least value of the maturity index \(\mathbb{M}_{\mathit{cl},\mathit{s}_{i}}\). The advantage of immature content eviction from the cache is that most of the content is mature, which leads to a higher cache hit ratio. Figure 2: _Cont._We observed that FIFO underperformed because contents are removed from the cache in the same order in which they were cached, regardless of how many times they were previously accessed. Besides, increasing cache space and similar content requests improve FIFO's performance because the content stays in the cache for a longer period, which increases the chances of increasing the cache hit ratio. LFU performs better than LRU when the cache size is large and the content is repeatedly requested because LFU considers the frequency of the requested content, while LRU does not. Moreover, LFU caches popular content and evicts unpopular content from the cache. Besides, contents are often evicted from the cache when the cache size is small. However, LFU displays low performance in small cache sizes. LFRU has better performance due to the coupling of LRU and LFU; however, when the content request rate is minimum from the maximum normalized request rate, the content is evicted from the unprivileged partition. Therefore, the new content is cached in the unprivileged partition. Besides, if the content request rate is higher than the maximum normalized request rate, it chooses the least recent content from the privileged partition and pushes that content into the unprivileged partition. Hence, new content is cached in the privileged partition and hit counter associated with each partition. However, content that loses popularity stays in the unprivileged partition for a long time due to its  
Figure 2: Caching hit ratio with different cache sizes and \(\alpha\) by using different placement policies. (**a**) Cache hit ratio with LCE. (**b**) Cache hit ratio with CL4M. (**c**) Cache hit ratio with ProbCache. (**d**) Cache hit ratio with LCD. (**e**) Cache hit ratio with opt-Cache. high frequency.