Background and motivation
The amount of heavy content, especially video, is increasing in the Internet. According to Cisco (2015), 80% of all consumer Internet traffic will be video in 2019. Global IP traffic is estimated to grow at a compound annual rate of 23% between 2014 and 2019. At the same time, mobile data volume is estimated to grow 61% annually between 2014 and 2019. As a consequence, both Internet service providers (ISPs) and mobile network operators (MNOs) face heavy investments in network capacity. One solution to efficiently cope with the increasing traffic volume is to employ caching, which enhances the end-user perceived quality of experience (QoE), improves network scalability and reduces the traffic volume leaving the ISP or MNO networks. Other prominent traffic optimization solutions include multipath technologies (Wischik et al., 2008) that typically require the involvement of the end users and are, thus, outside the scope of this research. Caching first emerged in the form of web caching (Barish and Obraczka, 2000), which temporarily stores web pages in web proxies to reduce load on the origin servers and to improve the response time. Web caching preserves the client-server model, whereas in peer-to-peer (P2P) networks (Schollmeier, 2002), established for file sharing, each peer acts both as a client and a cache server. The commercial content providers (CPs) are increasingly outsourcing their content delivery to third-party content delivery networks (CDNs) (Dilley et al., 2002), which place content servers in different ISP or MNO networks to better serve the end users. For example, already in 2014, 57% of all Internet video traffic passed through CDNs (Cisco, 2015). Recent developments (e.g. Krishnan et al., 2000; Chen et al., 2002; Xu et al., 2002; Tang and Chanson, 2002; Jia et al., 2003; Sourlas et al., 2011) focus on cache placement at different parts of the network, such as router and access networks, which is called in-network caching. In-network caching can be enabled by different technologies, such as information-centric networking (ICN) (Jacobson et al., 2012) or software-defined networking (SDN) (Raghavan et al., 2012) Technically, ICN (Jacobson et al., 2012) introduces flexible routing by content names instead of locations. In addition, the economic incentives of caching with ICN are discussed in Agyapong and Sirbu (2012) and Wang et al. (2014). However, clean-slate ICN implementations require changes in the network topology and the routing principles, and the overlay implementations complicate the network structure as well as add overhead to the network (Ahlgren et al., 2012). At Introduction 2 the same time, ICN caching is limited to fixed networks and provides minor benefits in mobile networks, because mobile data is typically enclosed in the GPRS tunneling protocol (GTP) within the mobile network. Increasing digitalization, on the other hand, shifts the network equipment market from hardware business to software business in a way similar to the handheld device and computer markets. For example, network function virtualization (NFV) and SDN in the context of mobile networking have been widely discussed in recent literature (e.g. Aleksic and Miladinovic, 2014; Salsano et al., 2014; Taleb, 2014; Haleplidis et al., 2015a; Taleb et al., 2015). Thus, SDN has been proposed as a solution to enable in-network caching in a mobile environment by removing the GTP tunneling from the mobile networks (Costa-Requena, 2014). SDN benefits arise from the decoupling of the control and user planes (Raghavan et al., 2012), and include increased flexibility and dynamicity of the network (Kreutz et al., 2015), as well as faster service and network software update cycles to the ISPs and MNOs (Pentikousis et al., 2013). In addition, SDN promotes new business models, such as virtual CDNs (Veitch et al., 2015), and reduces cost through dynamic service chains (Quinn and Guichard, 2014), where less popular content can bypass the caches to prevent cache throttling. On the other hand, SDN increases signaling traffic in the network as well as adds new points of failure by centralizing the control plane into the data centers. Thus, the net benefits of both ICN and SDN should be quantified.
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