# NDN Content Store and Caching Policies: Performance Evaluation
## 1 Introduction  
Named data networking (NDN) [1, 2] is a content-centric and name-based network architecture. The content identification is based on names, which must be global in the case of a global network. Identifying content through names provides their dissociation from their location (addresses). This characteristically results in the advantage of being able to store (cache) and easily share content among several consumers. The caching performance in NDN depends on a combination of several factors, such as (a) content routing; (b) decision for content placement (caching decision strategy); (c) decision for content replacement (caching replacement policy) [3, 4]; and (d) the network topology. The networking research community is working on the development of these topics. The present work aims at evaluating the performance of caching replacement policies in a network topology with a variable number of _content stores_ (CS). Specifically, the performance of _least recently used_ (LRU), _least frequently used_ (LFU), _first in first out_ (FIFO), and random policy are evaluated. Seven network scenarios are taken into consideration. Each scenario corresponds to a network, where the CS are deployed on a varying number of routers corresponding to 5%, 20%, 30%, 40%, 50%, 80% and 100% of the total nodes. Although this work does not perform an exhaustive survey on similar studies, some representative proposals can be found in [5, 6] for performance evaluation on a caching system with a variable number of CS, or in [7, 8, 9, 10, 11, 12, 13, 14, 15, 16] for evaluation with a variable size of afixed number of CS, as presented in Section 4. A survey on replacement schemes, which can complement this Section, is performed in [17]. However, none of the aforementioned studies evaluate and compare the four replacement policies considering different scenarios, or present all the commonly used metrics for performance evaluation. The used metrics are cache hit ratio (CHR), retrieval delay, number of upstream hops to cached content, number of re-transmissions on cache miss, and the network traffic in terms of number of transmitted messages. A comprehensive evaluation of the existing caching replacement policies and caching decision strategies is necessary to better decide on the adequate combination of these components for a specific network. The evaluation is also important because its results can likely expose possible issues still to be solved. The present work only focuses on the main four caching replacement policies and only one caching decision strategy. Future work may extend the present study to include other existing policies, such as the ones presented in Section 3.2, and combine them with different caching decision strategies, such as the ones presented in Section 3.1. Our main contributions can be summarized as in (1) complementing the existing studies that evaluate the performance of caching policies in CCN/NDN; (2) extending the analysis to cover more caching policies simultaneously, with relatively much more traffic per node (100 packets/s, in contrast with 5 packets/s from previous studies); (3) highlighting where discordant results exists from previous studies; and (4) differently from existing studies, we additionally evaluate the caching performance under a relatively complex network composed of a higher number of content sources (producers). The contributions of this work are decisive on advising a better caching system configuration for a specific network in terms of (a) the metrics to consider; (b) the percentage of caching nodes in the network; (c) the adequate caching policy to apply; and (d) the complexity of the network in terms of traffic and varying number of content sources. The remainder of this work is organized as follows: Section 2 presents the overall structure of the NDN architecture. Section 3 presents the in-network caching in NDN. Section 4 presents related work. In Section 5, the simulation environment is described.