A Survey on Content Placement Algorithms for Cloud-Based Content Delivery Networks <s> V. OVERVIEW OF CCDN INFRASTRUCTURE <s> In this paper we present a complete measurement study that compares YouTube traffic generated by mobile devices (smart-phones,tablets) with traffic generated by common PCs (desktops, notebooks, netbooks). We investigate the users' behavior and correlate it with the system performance. Our measurements are performed using unique data sets which are collected from vantage points in nation-wide ISPs and University campuses from two countries in Europe and the U.S. Our results show that the user access patterns are similar across a wide range of user locations, access technologies and user devices. Users stick with default player configurations, e.g., not changing video resolution or rarely enabling full screen playback. Furthermore it is very common that users abort video playback, with 60% of videos watched for no more than 20% of their duration. We show that the YouTube system is highly optimized for PC access and leverages aggressive buffering policies to guarantee excellent video playback. This however causes 25%-39% of data to be unnecessarily transferred, since users abort the playback very early. This waste of data transferred is even higher when mobile devices are considered. The limited storage offered by those devices makes the video download more complicated and overall less efficient, so that clients typically download more data than the actual video size. Overall, this result calls for better system optimization for both, PC and mobile accesses. <s> BIB001 </s> A Survey on Content Placement Algorithms for Cloud-Based Content Delivery Networks <s> V. OVERVIEW OF CCDN INFRASTRUCTURE <s> Since user demand for a Video-on-demand (VoD) service varies with time in one-day period, provisioning self-owned servers for the peak load it must sustain a few hours per day leads to bandwidth underutilization at other times. Content clouds, e.g. Amazon Cloud Front and Azure CDN, let VoD providers pay by bytes for bandwidth resources, potentially leading to cost savings even if the unit rate to rent a machine from a cloud provider is higher than the rate to own one. In this paper, based on long-term traces from two large-scale VoD systems and temporal development model of content clouds, we tackle challenges, design and potential benefits in migrating VoD services into the hybrid cloud-assisted deployment, where the user requests are partly served by the self-owned servers and partly served by the cloud. Our measurements show that the popularity of the most popular videos decays so quickly, for example, by 11% after one hour that it poses large challenges on updating videos in the cloud. However, the trace-driven evaluations show that our proposed migration strategies (active, reactive and smart strategies), although simply based on the current information, can make the hybrid cloud-assisted VoD deployment save up to 30% bandwidth expense compared with the Clients/Server mode. They can also handle unpredicted the flash crowd traffic with little cost. It also shows that the cloud price and server bandwidth chosen play the most important roles in saving cost, while the cloud storage size and cloud content update strategy play the key roles in the user experience improvement. <s> BIB002 </s> A Survey on Content Placement Algorithms for Cloud-Based Content Delivery Networks <s> V. OVERVIEW OF CCDN INFRASTRUCTURE <s> One of the most striking properties of the Internet is its flexibility to accommodate features it was not conceived for. Among the most significant examples, in this survey we consider the transition of the Internet from a reliable fault-tolerant network for host-to-host communication to a content-centric network, i.e. a network mostly devoted to support efficient generation, sharing and access to content. We survey this research area according to a top-down approach. We present a conceptual framework that encompasses the key building blocks required to support content-centric networking in the Internet. Then we describe in detail the two most important types of content-centric Internet technologies, i.e., Content-Delivery Networks (CDNs) and P2P systems. For each of them, we show how they cover the key building blocks. We then identify the functional components of CDN and P2P content management solutions, and discuss the main solutions proposed in the literature for each of them. We consider different types of content (both real time and non real time), and different networking environments (fixed, mobile, ...). Finally, we also discuss the main recent research trends focused on how to design the Future Internet as a native content-centric network. <s> BIB003 </s> A Survey on Content Placement Algorithms for Cloud-Based Content Delivery Networks <s> V. OVERVIEW OF CCDN INFRASTRUCTURE <s> Content distribution networks (CDNs) using storage clouds have recently started to emerge. Compared to traditional CDNs, storage cloud-based CDNs have the advantage of cost effectively offering hosting services to Web content providers without owning infrastructure. However, existing work on replica placement in CDNs does not readily apply in the cloud. In this paper, we investigated the joint problem of building distribution paths and placing Web server replicas in cloud CDNs to minimize the cost incurred on the CDN providers while satisfying QoS requirements for user requests. We formulate the cost optimization problem with accurate cost models and QoS requirements and show that the monthly cost can be as low as 2.62 US Dollars for a small Web site. We develop a suite of offline, online-static and online-dynamic heuristic algorithms that take as input network topology and work load information such as user location and request rates. We then evaluate the heuristics via Web trace-based simulation, and show that our heuristics behave very close to optimal under various network conditions. <s> BIB004 </s> A Survey on Content Placement Algorithms for Cloud-Based Content Delivery Networks <s> V. OVERVIEW OF CCDN INFRASTRUCTURE <s> Delivering on-demand web content to end-users in order to carry out strict QoS metrics is not a trivial task for globally distributed network providers. This task becomes still harder when content popularity varies over the time and the SLA definitions have to include both transfer rate and latency metrics. Current worldwide content delivery approaches and datacenter infrastructures rely on cumbersome replication schemes that are agnostic to edge-network resources, and damage content provision. In this work we present AREN, an novel replication scheme for cloud storage on edge networks. AREN relies on a collaborative cache strategy and bandwidth reservation to adapt the replication degree according to strict SLA contracts and content popularity growth. We have evaluated the performances of replication schemes on edge networks using Caju, a content distribution system for edge networks. Compared to a non-collaborative caching, evaluations show that AREN prevents nearly 99.8% of all SLA violations when the storage system is heavily loaded. We also show that AREN provides a sevenfold decrease in the amount of storage usage for replicas, and it increases by roughly 20% the aggregate bandwidth, hence accelerating content delivery. <s> BIB005 </s> A Survey on Content Placement Algorithms for Cloud-Based Content Delivery Networks <s> V. OVERVIEW OF CCDN INFRASTRUCTURE <s> The exploding HD video streaming traffic calls for deploying content servers deeper inside network operators' infrastructures. Telco-CDN are new content distribution services that are managed by Internet Service Providers (ISP). Since the network operator controls both the infrastructure and the content delivery overlay, it is in a position to engineer telco-CDN so that networking resources are optimally utilized. In this paper, we show the following two findings: 1. it is possible to implement an efficient algorithm for the placement of video chunks into a telco-CDN. We present an algorithm, which is based on a genetic algorithm implemented on the MapReduce framework. We show that, for a national VoD service, computing a quasi-optimal placement is possible. 2. such push strategy makes sense because it allows to actually take into account fine-grain traffic management strategies on the underlying infrastructure. Our proposal re-opens the debate about the relevance of such "push" approach (where the manager of telco-CDN proactively pushes video content into servers) versus the traditional caching approach (where the content is pulled to the servers from requests of clients). Our proposal of a quasi-optimal tracker enables fair comparisons between both approaches for most traffic engineering policies. We illustrate the interest of our proposal in the context of a major European Telco-CDN with real traces from a popular Video-on-Demand (VoD) service. Our experimental results show that, given a perfect algorithm for predicting user preferences, our placement algorithm is able to keep aligned with LRU caching in terms of the traditional hit-ratio, but the workload on some troubled links (e.g., over-used links) in a push-based strategy is significantly alleviated. <s> BIB006 </s> A Survey on Content Placement Algorithms for Cloud-Based Content Delivery Networks <s> V. OVERVIEW OF CCDN INFRASTRUCTURE <s> YouTube is the most popular service in today's Internet. Google relies on its massive content delivery network (CDN) to push YouTube videos as close as possible to the end-users, both to improve their watching experience as well as to reduce the load on the core of the network, using dynamic server selection strategies. However, we show that such a dynamic approach can actually have negative effects on the end-user quality of experience (QoE). Through the comprehensive analysis of one month of YouTube flow traces collected at the network of a large European ISP, we report a real case study in which YouTube QoE-relevant degradation affecting a large number of users occurs as a result of Google's server selection strategies. We present an iterative and structured process to detect, characterize, and diagnose QoE-relevant anomalies in CDN distributed services such as YouTube. The overall process uses statistical analysis methodologies to unveil the root causes behind automatically detected problems linked to the dynamics of CDNs' server selection strategies. <s> BIB007 </s> A Survey on Content Placement Algorithms for Cloud-Based Content Delivery Networks <s> V. OVERVIEW OF CCDN INFRASTRUCTURE <s> The collaboration of Internet service providers (ISPs) and content distribution network (CDN) providers was shown to be beneficial for both parties in a number of recent works. Influencing CDN edge server (surrogate) selection allows the ISP to manage the rising amount of traffic emanating from CDNs to reduce the operational expenditures (OPEX) of his infrastructure, e.g., by preventing peered traffic. At the same time, including the ISP's hidden network knowledge in the surrogate selection process influences the quality of service a CDN provider can deliver positively. As a large amount of CDN traffic is video-on-demand traffic, this paper investigates the topic of CDN/ISP collaboration from a perspective of high-volume long-living flows. These types of flows are hardly manageable with state-of-the-art Dynamic Name Service (DNS)-based redirection, as a reassignment of flows during the session is difficult to achieve. Consequently, varying load of surrogates caused by flash crowds and congestion events in the ISP's network are hard to compensate. This paper presents a novel approach promoting ISP and CDN collaboration based on a minimal deployment of software-defined networking switches in the ISP's network. The approach complements standard DNS-based redirection by allowing for a migration of high-volume flows between surrogates in the backend even if the communication has state information, such as Hyper Text Transfer Protocol sessions. In addition to a proof-of-concept, the evaluation identifies factors influencing performance and shows large performance increases when compared to standard DNS-based redirection. <s> BIB008 </s> A Survey on Content Placement Algorithms for Cloud-Based Content Delivery Networks <s> V. OVERVIEW OF CCDN INFRASTRUCTURE <s> A software defined virtual CDN (SDvCDN) is a virtual cache network deployed fully in software over a programmable cloud network infrastructure that can be elastically consumed and optimized using global information about network conditions and service requirements. We formulate the joint content-resource allocation problem in SDvCDNs as a minimum cost mixed-cast flow problem with resource activation decisions. Our solution jointly optimizes the placement and routing of content objects along with the allocation of the required virtual storage and transport resources, is applicable to arbitrary network topologies, and captures activation and operational costs, content popularity, unicast and multicast delivery, as well as capacity and latency constraints. We present preliminary results for the general mixed integer linear programming solution, as well as conditions for polynomial solvability. The value of our approach goes beyond the optimization of SDvCDNs, as it provides an efficient methodology to jointly address placement (facility location), routing (network flow) and resource allocation (network design/embedding) problems. <s> BIB009 </s> A Survey on Content Placement Algorithms for Cloud-Based Content Delivery Networks <s> V. OVERVIEW OF CCDN INFRASTRUCTURE <s> One of the key goals of Network Functions Virtualization (NFV) is achieving energy efficiency through workload consolidation. A good example for maximizing energy savings is the Virtualization of Content Delivery Networks (vCDNs) NFV use case where the video streaming workloads exhibit significant difference between prime-time and non-prime-time usage of the infrastructure. This paper examines the practical challenges in maximizing energy efficiency for vCDN workloads. This paper proposes an open NFV architectural framework for conveying content virality information from Cloud applications such as YouTube, Twitter and mechanisms for leveraging it to maximize the energy efficiency for vCDN workloads. This paper also proposes a more general architecture for any Cloud/NFV application that may experience virality. <s> BIB010
In this section, we provide a brief overview of CCDN infrastructure, with respect to cost and elasticity in the resource provisioning of the underlying cloud model. Though traditional CDNs resemble other classical data sharing distributed systems, they are intrinsically different (  , BIB003 ). For example, similar to CDNs, mesh networks increase connectivity, data grids provide data storage, distributed systems have replication groups, Peer-to-Peer (P2P) networks deliver content for distribution. Yet, they are fundamentally different. Furthermore, today's cloud-based solutions for data sharing have increased resiliency, reliability, accessibility and security. The infrastructure of traditional CDNs consists of high bandwidth links connecting geographically distributed network elements, such as routers and switches with clusters of surrogate servers. CDNs can be built based on different approaches, depending on the way network elements interact to interpret end-user requests. Software Defined Networking (SDN) ( BIB008 - BIB009 ) and, Network Functions Virtualization (NFV) BIB010 paradigms are used to build CDNs. Meanwhile, CDNs can be built via the in-network caching and routing of the request to the appropriate surrogate servers. On the other hand, Telco-CDN BIB006 has been devised for network operators to optimize resources for content delivery, in an Internet Service Provider (ISP) managed CDN infrastructure. In contrast, overlay CDNs are built by strategically placing content on the surrogate servers in the network and they use request redirection mechanisms that route content requests to the appropriate surrogate servers. In this overlay approach, network elements only perform traditional routing duties. There are major drawbacks of traditional CDNs, including a high cost of content hosting BIB004 , complexity of hosting dynamic content BIB003 , dealing with the paramount growth in the size of content and the lack of dynamic scalability of resources BIB003 . CCDNs can be private or public. In private CCDNs, the content owner owns the cloud infrastructure to store and deliver content to end-users. However, in the case of public CCDNs, content providers can lease cloud resources to build their own CCDN or utilize services provided by the public CCDN infrastructure providers (e.g., Amazon CloudFront  and Google Cloud CDN [69]). Irrespective of the type of CCDN, it will incorporate content delivery through an Autonomous System (AS) ( BIB001 , BIB007 ) that can include routers, core and access networks, accounting for virtualization and software-defined technologies. Besides, it should be noted that it is not necessary to migrate all content to the cloud or to CCDNs. There are hybrid approaches ( BIB002 , BIB005 ), where content can be uploaded to the cloud and distributed via CCDNs when end-user demands for content exceed the content provider's bandwidth capacities. These possibilities can be easily leveraged by CDN providers that move to the cloud and the content providers that do not anticipate extreme scenarios such as flash crowds. Let us consider a CCDN built by content providers by leasing the cloud resources from a public cloud infrastructure provider. The cloud resources include storage, computation and bandwidth. It consists of regions including various zones, with high capacity datacenters. They provide low latency, high bandwidth links between zones and intra-region communication over the Internet. CCDN providers pay for storage and bandwidth leased on the cloud. The storage cost is based on a flat rate depending on the content catalogue size and the bandwidth cost is decomposed into traffic coming into and going out of zones and regions in the cloud. The inter-region and intra-region communication links have different bandwidth capacities and costs. Typically, inter-region bandwidth costs are higher and bandwidth capacity is lower, in comparison to intra-region bandwidth costs and capacity. However, leading cloud infrastructure providers (e.g., Google) do not charge for intra-region bandwidth usage or traffic coming into regions. In Fig. 13 , we illustrate a cost function for bandwidth usage, with respect to the traffic going out of the regions. The non-increasing cost function is inspired by Google's network rates  that decrease per unit cost as the number of consumed units increases. Note that these rates vary from region to region; however, they follow a similar trend. This implies that cost-effective content placement can be achieved by increased resource utilization. Dynamic resource provisioning is a severe limitation of traditional CDNs, which causes over, under or at capacity utilization of resources. As illustrated in Fig. 12 , content providers estimate an average demand for their content and contract CDN providers for the respective resources, say d 0 , with Service Layer Agreement (SLA) guaranteed QoS. Static resource allocation is perilous and when demands need resources, say d 1 , they exceed the maximum available resources of CDN and the content provider's service is rendered unresponsive while end-users are dissatisfied. This results in the loss of reputation and financial revenue for the content providers. Meanwhile, when demand only utilizes resources, d 2 , it is underutilization and results in idle resources that are the eventual loss of revenue. CCDNs overcome these shortcomings of CDNs BIB004 with the advent of the cloud and utility computing. Cloud computing infrastructure consists of large-scale datacenters with hundreds and thousands of machines  , spread across the globe  , inter-connected with high-bandwidth links, offering low latency. The cloud infrastructure providers offer payas-you-go cloud resources, such as storage and bandwidth, which content providers can lease to build an overlay CDN in the cloud, i.e. a cloud-based CDN. CCDN operators can dynamically allocate and de-allocate resources across geographically distributed datacenters to cater to continuously changing end-user demands and popularity of content. The intrinsic differences in the infrastructure of CCDNs and CDNs limit various operational subsystems to be simply used ''as-is'' in CCDNs. Therefore, though, CP algorithms in traditional CDNs are mature and a logical predecessor to CP for CCDNs, they cannot be directly applied for CP in CCDNs.