# IMU: A Content Replacement Policy for CCN, Based on Immutance Content Selection
## 4 Performance Evaluation
### Latency  
Latency indicates the delay in the delivery of requests and content from consumers. It is a vital metric for evaluating the performance of the CCN cache, and it is defined as follows:  
\[Latency=Request\;Travel\;Delay+Content\;Travel\;Delay \tag{3}\]  
The IMU provides low latency because it evicts the most suitable content from the cache, based on immaturity. If the cache is full, the IMU jointly considers the frequency and time and selects the content for potential eviction from the cache. Hence, more popular and mature content will be in the cache, and content that may be popular. However, most consumer requests are satisfied along the routing path, which reduces latency. Figure 5 illustrates that IMU's performance is better than other content replacement policies, regarding latency with different cache sizes and popularity.  
Figure 5 illustrates that FIFO represents a high latency because the duration of popular and unpopular content is the same. However, latency increases when popular content is evicted from the cache. LRU ignores the popularity of the content. Therefore, requests for less popular content come before eviction, and the content will remain in the cache, which causes high latency. LFU considers the frequency of the content, and contents that increase in frequency over a short period of time but are no longer popular; such contents use cache space due to their high frequency.  
Therefore, fresh contents are reduced in the cache, which increases the latency. LFRU performs better than the previous two discussed replacement techniques because LRU and LFU are used together. When the request rate is high, then the required processing should be high, because the least recently used content is evicted from the privileged partition and pushed to the unprivileged partition and associated with the access history of content. In addition, low-frequency content is evicted from unprivileged partition. However, content with a high access history that is no longer popular will spend more time in the cache space, reducing the freshness of the content. IMU outperformed FIFO, LRU, LFU, and LFRU in terms of latency by 12.32%, 9.97%, 9.08%, and 5.91%, respectively.  
When the alpha equals 0.8 with cache size is 0.04, IMU is 64.44 ms, which is 9.45% lower than LFRU (71.16 ms), 13.90% lower than LFU (74.84 ms), 13.34% lower than LRU (74.35 ms), and 15.60% lower than FIFO (76.34 ms). When the alpha equals 0.8, the cache size is 0.12, IMU is 61.03 ms, which is 5.40% lower than LFRU (64.51 ms), 9.65% lower than LFU (67.55 ms), 12.05% lower than LRU (69.39 ms), and 14.50% lower than FIFO (71.38 ms). When the alpha equals 0.8 with cache size is 0.2, IMU is 60.05 ms, which is 3.71% lower than LFRU (62.37 ms), 5.58% lower than LFU (63.60 ms), 8.93% lower than LRU (65.94 ms), and 11.59% lower than FIFO (67.92 ms). Therefore, as the cache size increases, the latency is reduced because more content in the network can be cached.  
The latency improvement performance from IMU is illustrated in Table 12, using different content placement strategies with low to high popularities and cache sizes. We have observed that, as the popularity of content increases, so performs IMU. The IMU evicts content from the cache that has been in the cache for a long time and has few requests. Furthermore, content that has been in high demand for some time but declined over time has also been evicted from the cache. Therefore, the cache contains mostly mature content. However, when a consumer requests specific content, the consumer's request does not reach the publisher because the consumer is satisfied along the routing path.  
Figure 5: Link load with different cache sizes and \(\alpha\), using different placement policies. **(a)** Link load with LCE. **(b)** Link load with CLAM. **(c)** Link load with ProbCache. **(d)** Link load with LCD. **(e)** Link load with opt-Cache.