How To Model The Content Delivery Network (Cdn) From A Content Bridge To A Cloud (Cloud)

Transcription

1 Kingston University London In network content caching contributing to the Future Internet Architecture Dissertation submitted for the Degree of Master of Science in Networking and Data Communications By DOUNIS CHARALAMPOS SUPERVISOR PAPADAKIS ANDREAS KINGSTON UNIVERSITY, FACULTY OF SCIENCE, ENGINEERING AND COMPUTING ΤEI OF PIRAEUS, DEPARTMENTS OF ELECTRONICS AND AUTOMATION MARCH 2013

2

3 Table of Contents Table of Figures... 5 Abstract Introduction Background Information... Error! Bookmark not defined Thesis Scope and research goals Technologies and Resources... Error! Bookmark not defined. 2. Content Distribution Methods Clustering and Mirroring Web Caching Hierarchical Caching Content Distribution Networks Distribution services and content delivery Content Distribution Network Benefits CDN Functionality and components User Redirection Mechanisms Policy rules for content delivery networks Introduction to network policies Consistency Models Alternative CDN policies Edge Services Application Content Distribution Networks Introduction to ACDN ACDN Requirements Content Distribution Business Models... 27

4 3.1. Business Models Content Distribution Business Chain Content Distribution Business Models Content-centric model Access-centric model Alternative business models Peer-To-Peer (P2P) model Pay-per-view model The Cloud Cloud computing features Cloud Computing Services Cloud Deployment Models The Cloud and CDN Examples of Cloud CDN services Conclusions References... 47

5 Table of Figures Figure 1 Proxy server (source: Wikipedia.org) Figure 2 Reverse proxy (source: Wikipedia.org) Figure 3 Cache hierarchy Figure 4 Content distribution chain Figure 5 Content-Centric CDN Model Figure 6 Access-centric model Figure 7 Content Bridge model Figure 8 P2P CDN model Figure 9 The stack of the Cloud Figure 10 Cloud deployment models Figure 11 CDN cloud model... 43

6 Abstract CDN architectures perform content replication caching (mainly at the edges of the network). Content Distribution Networks consist of groups of intermediate servers (Proxy-servers) placed in key positions on the Internet. The key idea is so that ensure that content required by an application is retrieved form a "nearby" server. These networks are essentially an intermediate level between the servers and customers, a middleware that uses caching techniques, load balancing and replication of information. The Internet of the Future is tightly coupled with the Cloud. The Cloud comprises of distributed data centers, which offer economies of scale, and cheaper computing resources. Cloud computing provides computation, software, data access, and storage resources without requiring cloud users to know the location and other details of the computing infrastructure. It is reasonable to believe that the Cloud may have an important role in multimedia content delivery, since in-network content replication schemes have to be implemented. In the current thesis present a comparative analysis of business models in the area of content distribution, namely models used by content distributors, who are responsible for the operation of the CDN. The models are categorized based on the CDN client, that can be either the content provider or the Internet Service Provider (ISP). Typically, the CDN customer selects the content to be copied to the CDN nodes and is charged by the content distributor accordingly. Then the content distributor pays a share of the revenue to other Internet business entities that contribute to the delivery of content, such as data centers and backbone providers. We then focus on the economic dimension of the problem of allocating the storage of a cache to multiple nodes. We will present and examine the financial mechanisms that have been proposed to solve the problem of allocating the space of a cache. These models are compared to existing business models of Cloud providers.

7 First, we examine the case of hierarchical caching in typical CDNs and elaborate on the effects caused by the requests and content distribution between caches. Secondly, the analysis is extended to include Cloud providers that may be used as intermediates. The Cloud providers may be included in the model in two ways: By providing Software as a service (SaaS). SaaS is a software/application delivery model in which software and associated data are centrally hosted on the cloud. SaaS is typically accessed by users using a thin client via a web browser. In this case, the Cloud can be used is an intermediate between the CDN and the end user. By providing Infrastructure as a service (IaaS). In the most basic cloud-service model, providers of IaaS offer computers - physical or (more often) virtual machines - and other resources. In that case, content distributors may deploy their applications as cloud users and install operating-system images and their application software on the cloud infrastructure. The goal is to study how robust and effective is the solution of using the Cloud for content replication and distribution. For the aforementioned aims and goals of this Thesis, the research that we will conduct will be hopefully greatly beneficiary to network and content providers.

8 1. Introduction In recent years the popularity of the Internet and especially the World Wide Web as sources of information and entertainment for people all over the world has grown exponentially. The phenomenal growth is largely due both to the ease of Internet access and use of browsers, and in addition to the delivery of more attractive content such as audio and video. As end users gain faster access to the Internet from their home via DSL and fiber to the home technologies, the demand for high quality content will grow steadily in coming years. Also, through the new advanced wireless devices such as mobile phones and handheld computers (PDAs), Internet access is possible from anywhere and at any time. This ever increasing demand for content, however, places a heavy burden on the existing Internet infrastructure. The servers and network connections must be able at any moment to meet the demand, which can be highly variable and unpredictable. The biggest problem is hot-spots. These are created when some content becomes extremely highly popular, typically for a limited period. Examples of such hot spots are the broadcast of major sporting and other events, or the public release of popular software via the Internet. All these incidents produce a heavy burden on the source server's content. Furthermore, an intractable problem is the accurate prediction of the required network capacity. In addition, upgrading the servers or network connections to satisfy the demand is not always a feasible solution as millions of new users every time wish to access the same content. The Internet is a complex grid of interconnected networks. Congestion and failures (ie packet drops) may occur in many places, such as Error! Reference source not found.: The "first mile that connects the server in a data center to the Internet. The Backbone network of a provider. Peering points between the providers of network services. The "last mile that connects the user to the internet.

9 The World Web is based on a client-server architecture. The content is hosted on Web servers, while the user requests are served using HTTP (HyperText Transfer Protocol) Error! Reference source not found.. Users use browsers as the client software to communicate with the server software. Objects are uniquely identified on the Web by the (URLs - Uniform Resource Locators), which specify the server name that is the subject and also the location in the filesystem where the object is located. Websites are written in HTML (HyperText Markup Language) and may contain text, pictures, other multimedia content (audio/video) and links to other content. The problem of content distribution stems from the shortcomings of client-server architecture that fails to handle the increased demands of the multimedia content that requires high bandwidth Thesis Scope and research goals In the context of the Thesis we will investigate in network content replication scenarios, evaluating their performance under variable traffic conditions and user requests schemes. Different network topologies will be investigated, under various network load and network link conditions. The goal is to study how robust and effective is the solution of in-network content replication, and as a result examine their feasibility in the Internet. For the aforementioned aims and goals of this Thesis, the research that we will conduct will be hopefully greatly beneficiary to network and content providers. This thesis will involve the following: A discussion about Content Delivery Networks (CDN). CDN architectures perform content replication caching (mainly at the edges of the network). Content Distribution Networks consist of groups of intermediate servers (Proxy-servers) placed in key positions on the Internet. The key idea is so that ensure that content required by an application is retrieved form a "nearby" server. These networks are essentially an intermediate level between the servers and customers, a middleware that uses caching techniques, load balancing and replication of information. Moreover, the new tendency in Internet Services, the Cloud, will be presented. The Internet of the Future is tightly coupled with the Cloud. The Cloud comprises of

10 distributed data centers, which offer economies of scale, and cheaper computing resources. Cloud computing provides computation, software, data access, and storage resources without requiring cloud users to know the location and other details of the computing infrastructure. It is reasonable to believe that the Cloud may have an important role in multimedia content delivery, since in-network content replication schemes have to be implemented We will then proceed with the presentation and comparative analysis of business models in the area of content distribution, namely models used by content distributors, who are responsible for the operation of the CDN. The models are categorized based on the CDN client, that can be either the content provider or the Internet Service Provider (ISP). First, we will examine the case of hierarchical caching in typical CDNs and elaborate on the effects caused by the requests and content distribution between caches. Secondly, the analysis can be extended to include Cloud providers that may be used as intermediates. The goal is to study how robust and effective is the solution of using the Cloud for content replication and distribution. For the aforementioned aims and goals of this Thesis, the research that we will conduct will be hopefully greatly beneficiary to network and content providers. The structure of the current thesis is as follows: In chapter 2 we will present a thorough survey on current CDN architectures. In chapter 3, will proceed with the presentation and comparative analysis of business models in the area of content distribution. In chapter 4, cloud Computing will be presented, and we will examine how it can be used in content distribution. Finally, chapter 5 will conclude the thesis.

11 2. Content Distribution Methods Researchers have been studying several approaches to the delivery of multimedia content in a way that offers both the desired reliability and scalability Error! Reference source not found Clustering and Mirroring One of the approaches that can help solve the problems of fault tolerance and scalability is local clustering of servers in a data center. However, if the network connection of the data center fails, then the entire server complex is inaccessible to users. To solve the problem of single point failure, content delivery sites may perform content distribution, such as mirroring (the usage of back-end servers that host the same replicated content at several different sites) and multihoming (the use of multiple ISPs for connecting to the Internet). In the content mirroring approach, the content of the origin server is copied entirely to another server, called mirror or replica. The contents of a mirror must be identical to the stored content in the origin server so users may access any of the two machines interchangeably. Mirroring and multihoming are popular methods for support sites with strict requirements on reliability and scalability. However, these methods do not solve all the problems of connectivity and often introduce additional difficulties in the content distribution system, such as the following: Escalating mirroring to thousands of servers is a tedious process and has large demands in administration and management. In the case of multihoming, routing protocols must converge fast enough to support continuous content delivery in case of failure of the initial links.

12 Mirroring requires continuous synchronization between the mirrors and the origin server. Each of these solutions introduce significant economic costs, which may exceed multiple times the initial infrastructure costs and operating costs of a website. In the case of clustering, there must be several servers at each location to process the workload at peak times (which can be orders of magnitude higher than the average load). Multihoming is also expensive, since the backup connections are not utilized in normal conditions. The main disadvantage of mirroring is how to inform users about the existence of multiple mirrors. Usually, the origin server maintains a list of available mirrors, which is supplied to the user, who then may choose an available mirror whenever they visit the site. Users, however, are not always aware of the identity of these mirrors and usually do not know which is the most close or less loaded server. Also, the management of multiple mirrors is demanding since it may not be possible to become fully automated Web Caching Caching is a technique similar to mirroring, for copying content to multiple locations closer to the consumers of content, but with two key differences. A mirror typically hosts all the content of the content provider (a website for example),whereas in caching content is copied to the cache on demand. Furthermore, caching is a more dynamic than mirroring, as it operates object level and is based on a dynamic process of replacement of the cache contents in response to changes in demand. When a requested object is not in the cache, the proxy server is forced to load the content from the origin server, store it locally and deliver it to the user. The use of a proxy cache requires configuring the web browser of the user, or setting up a transparent proxy Error! Reference source not found.error! Reference source not found. which intercepts user requests and redirects them to the cache.

13 Caching exploits the phenomenon of temporal locality observed in content delivery applications in the Internet. Specifically, requests for an object tend to be concentrated in a relatively narrow time period. After that time period, the popularity of the object deteriorates over time. So, by copying the object in a cache, all future requests will be served immediately until the object is replaced by another object. There are several places online where a cache may be placed: the browser cache, an ISP network, and near the origin server. So we have the following levels of hierarchy in the placement of caches on the Internet: Browser Caches: Located at the lower end of the hierarchy of caches on the Internet. A user-specified portion of the hard disk of the computer used to store objects already "downloaded" to the machine, so that the next visit to a website may be served by its own local cache. It is widely used for web images since they are usually the largest objects in an HTML page. Proxy Caches: Proxies operate under the same principle, but on a larger scale, serving hundreds or thousands of users. Proxy caches are a type of shared cache as opposed to individual browser caches. Most proxy caches are installed in large corporations and ISPs to reduce bandwidth consumption, as there are many common requests for the same objects by multiple users. Schematically, a proxy cache is illustrated in figure 1. Figure 1 Proxy server (source: Wikipedia.org) Reverse proxy caches: Placed close to the source server and operate to its advantage. This way a content provider improved its content availability, but all benefits are limited this provider, as opposed to proxy caches that accommodate

14 different content providers. In figure 2, a reverse proxy is illustrated taking requests from the Internet and forwarding them to servers in an internal network. Those making requests connect to the proxy and may not be aware of the internal network. Figure 2 Reverse proxy (source: Wikipedia.org) The benefit of using caches to distribute Internet content is threefold. Specifically, the caches can help reduce three sizes: Reduction of the number of requests for content served by content source servers, with a consequent reduction of the CPU burden and costs for the required infrastructure of the site. Reduction of network traffic: As each content item is taken only once from the origin server and subsequently it is retrieved from the cache, the amount of bandwidth used is reduced. In particular, the amount packets to the backbone network due to requests for the cached content is reduced, while shifting the network traffic in the region (usually as close to the users as possible) where the caches reside. Delay reduction of the response to the end user: If the user request is served by the cache that is closest to the customer rather than the origin server, less time is required for retrieving a content item. Typically the performance of a cache is estimated using the following metrics: Hit Rate: The rate of requests served by the cache, divided by the total number of user requests

15 Byte Hit Rate: Same as above, only this time network traffic is estimated rather that requests Hierarchical Caching Although proxy caches are useful and efficient for serving users of an organization or an ISP, they are suitable for covering only a relatively small user population. The hierarchies of caches try to solve scalability problems, by enabling intercommunication between different levels of caches Error! Reference source not found.. In hierarchical caching, caches create a mesh and are logically organized in a hierarchy. Each node has a parent, siblings (nodes at the same level) and children. The term "adjacent" refers to the parent or fraternal nodes located a cache-hop away. The protocol used to communicate with caches in a hierarchy is the Internet Cache Protocol (ICP) Error! Reference source not found.. Figure 3 Cache hierarchy

16 There are two main objectives in the design of hierarchical caches. The first is to ensure that popular items are stored at lower levels of the hierarchy, while the less popular items at higher levels. The second objective is to ensure that when there is a failure in a cache, it is possible to serve the request using an adjacent cache, minimizing the possibility to access the source server. These two objectives reduce the delay perceived by the user and the overall network traffic. ICP in hierarchical caching works as follows: Suppose there is an end user served by a cache at the organization level, requesting for an object. The user request is sent to the local cache. When the cache receives the request, it checks if it already has the requested object. If it does, content delivery is achieved immediately. In case the object is not in the local cache, the cache broadcasts an ICP request to sibling nodes. If one or more siblings have the object, then it is retrieved from the sibling with the minimum delay recorded, it is sent to the user and stored in the cache. If no sibling node has the object, the request is forwarded either to the parent cache or directly to the origin server. Forwarding the request to a parent simply repeats the above process. If the request fails on every level of the hierarchy, then the object is retrieved from the source server, and then passed downwards in the hierarchy to the leaf node that created the initial request. All nodes in the tree of the request update their cache with the requested object. A main problem with the hierarchy of caches is that they may introduce a significant additional delay to requests. If a client asks a unpopular object that is not stored in any cache, the request must still be forwarded through the hierarchy to higher levels before the higher level cache sends the request to the source server. Also there is no guarantee that the caches higher in the hierarchy are actually closest to the source server. This problem is usually solved by filtering the requests sent to the top levels of the hierarchy. For example, a national-wide cache may accept applications only for objects whose country of origin (as distinguished from the URL) is abroad. Moreover, the usage of caching (either in stand-alone proxies or in hierarchies) has been associated with a series of problems Error! Reference source not found.. The main one is that the content provider loses control over its content once it is retrieved from the source server and placed in a cache. Thus, content providers often prefer to

17 characterize the content as non-cacheable so as to prevent it from being copied to intermediate caches between the origin server and the client. Moreover, dynamic content providers avoid caching due to the risk of delivery to end user content not updated with the latest changes made to the origin server. There are some simple mechanisms to inform caches on changes in the content, like header "Expires" or the control mechanisms of the HTTP/1.1 cache but have not received widespread development. Also, simple caching leads to loss of business knowledge about the visitors of a content provider website. The accuracy of reports about visitor access and navigation to the site has a crucial role in marketing and sales promotion. Caching can result in loss of statistical accuracy since only a small portion of user requests are delivered to the origin server. In addition, a variety of e-marketing techniques are based on personalization of content using cookies. Google for example uses cookies in order to display user-specific advertisements Error! Reference source not found.. Finally, caching services may create new challenges to security, such as breaches of confidentiality, integrity and authentication for sensitive data such as medical records when they are not under the direct control of their providers Content Distribution Networks Distribution services and content delivery Content distribution and delivery services provide a higher level of service that complements and extends the Internet by proactively putting premium content as near as possible to the end users and by forwarding each user request to the best available server. Trying to make a distinction between the terms of content distribution and content delivery, we could define content delivery as the service provided directly to end users, while the distribution of content as the transmission of content from a central server to multiple regional servers installed at the ends of network.

18 The distribution and content delivery networks have emerged as an added value over the existing Internet infrastructure, offering new possibilities for processing content flows through the identification of content type, the routing of requests to the optimal server and the dynamic creation of content. The goal of the research community was to create content-aware networking, where the network elements network elements have the necessary intelligence to recognize the specific content requested, allowing this way optimal request routing and content delivery content Error! Reference source not found.error! Reference source not found.. A Content Distribution Network is a collection of intermediate caching servers that remove workload from the source servers by delivering web content on their behalf. The servers belonging to a CDN can be placed in the same physical location with the source server, or in different locations in the network, close to the end user. There are two types of CDN depending on the percentage of the contents of the source server that is copied to CDN caches. If all the contents of the origin are copied then the CDN uses full site replication scheme, but when only a part of the contents of the origin web site is copied the CDN uses a partial site replication scheme. In the partial site replication scheme mostly static and large-size objects such as images, graphics, etc. are copied to the CDN caches Content Distribution Network Benefits The main benefits from using the Content Distribution Networks in comparison with traditional best-effort transmission services can be grouped into the following categories: Increasing the bandwidth: CDNs preserve network bandwidth since requests are served by local cache servers. Buying and selling bandwidth: For ISPs and network service providers to shift traffic from backbone to the local loop is a critical success factor. With local storage and playback of content, ISPs can use local bandwidth, which has a lower cost than the bandwidth of backbone network and effectively manage the cost of content delivery with high bandwidth requirements.

19 Improve efficiency: By caching popular content closer to the end user delay and jitter are reduced. Flash-crowds: When web pages or event broadcasts are simultaneously accessed by large crowds, proactive replication and caching of content in regional caches distributes the workload across the Internet and prevents crowding in the origin servers. Of course, CDN quality of service depends on the size of the CDN, i.e. the number of sites where caches are installed by the service provider Error! Reference source not found.. The existence of more regional caches in the CDN implies a smaller average distance from the end user to the cache. Since CDNs allow access to information to be made locally rather than through backbone networks, they are in direct competition with Backbone Service Providers. This competition is strongly influenced by the relative prices of storage and bandwidth. Relatively larger prices in bandwidth in comparison with disk storage prices favor the usage of CDNs CDN Functionality and components The major functions of a CDN are: Copying, distributing, storing content on cache servers at the edge of the network, ensuring awareness of the data, and redirecting users to the most suitable servers. In detail: The first step is to copy, distribute and store content on local servers. Content delivery systems update, store and deliver to end users copies of the original content. There are several ways to update the content. Some systems forward content proactively to cache servers. This procedure is controlled by a central system that is aware of the CDN topology. Alternatively, the central system directs requests to specific servers and they recover the content on demand. Finally, the CDNs route incoming requests to the appropriate cache servers. The routing optimizes access times and reduces the cost of content delivery.

20 Mechanisms used for routing requests HTTP, IP, or DNS redirection. HTTP allows servers to redirect a client request to a different location. Anycast is a network addressing and routing methodology in which datagrams from a single sender are routed to the topologically nearest node in a group of potential receivers all identified by the same destination address [1][63]. DNS redirection and HTTP redirection (URL Rewriting) are the easiest and most popular methods for implementing load balancing. DNs redirection is completely transparent to the user, as opposed to URL Rewriting. Finally, a rather new method for redirecting user requests that is widely used in Mobile telephony is location-based service (LBS). LBS include services to identify a location of a person or object, such as discovering the nearest banking cash machine or the whereabouts of a friend or employee [64][65]. To perform these functions effectively an integrated CDN must include the following fundamental components Error! Reference source not found.error! Reference source not found.: A content distribution and management mechanism, which distributes content to nodes at the edge of the network. The central management determines the provision of content and policy settings for all nodes in the CDN and is responsible for synchronizing the content between the origin server and the cache servers. A content routing mechanism, which redirects user requests to the most suitable nodes of the CDN, based on measurements that include real-time delay, the network topology, the load of servers and policies. A content switching mechanism performs load balancing. The switching elements determine how redirect user request balancing availability of content and servers workload. A content edge delivery mechanism for delivering content from the network edge to the end user. Intelligent network services that include functions such as quality of service (QOS), virtual private networks (VPNs) and multicasting.

21 A management framework that allows providers of distribution services to monitor and control procedures as required User Redirection Mechanisms One of the main problems in a distributed content delivery network is to ensure that end-users will be able to retrieve the copied content from a regional cache. This is achieved with the usage of client redirection mechanisms that redirect user requests to the most suitable/optimal server in the network. The optimality is a policy decision that is mainly based on the proximity of the replica (the proxy server) to the client. Other criteria that may be used are the workload of the servers and the network congestion. There are several ways to direct the client to a replica: manual selection of the replica by the user, HTTP Redirection, DNS redirection which is the most widely used method, and Anycast. Using DNS, the DNS server is the source server redirects user requests matching the nominal address of the source server to the IP address of a server content CDN. This matching is carried out based on factors such as the availability of resources and the network status. Anycast is a network addressing and routing methodology in which datagrams from a single sender are routed to the topologically nearest node in a group of potential receivers, though it may be sent to several nodes, all identified by the same destination address. On the Internet, anycast is usually implemented by using BGP to simultaneously announce the same destination IP address range from many different places on the Internet. As a result, packets addressed to destination addresses in this range are being routed to the "nearest" point on the net announcing the given destination IP address. Content delivery networks may use anycast for actual HTTP connections to their distribution centers, or for DNS. Because most HTTP connections to such networks request static content such as images and style sheets, they are short-lived and stateless. The general stability of routes and statelessness of connections makes anycast suitable for this application, even though it uses TCP.

22 Regarding DNS Redirection, there are two distinct mechanisms, depending on whether the entire website is replicated or just a part of it. In the first case full redirection is supported, whereas in the second case selective redirection is supported. When full redirection is used, all user requests are redirected via DNS to a replica in the CDN. The main benefit of this mechanism is that all client requests are first sent to replicas and not to the origin servers. Another advantage of this mechanism is that it dynamically adapts to the creation of new hot-spots, since all client requests are redirected to geographically dispersed replicas. When selective redirection is used, the source server converts the URLs embedded in the replicated objects (web pages) so that the host names in the URLs are resolved to IP addresses that belong to the CDN. The benefit of the mechanism of selective redirection is that it reduces capacity requirements (only a part of the data in the source server is copied to the replicas usually the most popular objects) Policy rules for content delivery networks Introduction to network policies In general, network policies is a set of rules for the administration, management, and access control to network resources. Network policies provide a way to consistently manage multiple devices that implement complex technologies from a central point. The basic rule for defining a policy is an expression of the form "If condition is valid, then do action Y". In a content distribution network, the resource that must be efficiently managed is mainly the available storage in the regional network caches. Therefore, appropriate policies [9] must be established to control the distribution of content between the various nodes of the overlay network. A policy in a content distribution network would be a statement that defines what type of software / data may be

23 transferred to a CDN node, and what type of software / data must be running on the origin server. With this definition of policies there are only two types of possible actions. Specifically a file / software is either storable (where a program can be executed on a CDN node or where content can be copied and stored in a CDN node) or non-storable and must be placed only on the origin server. However each storage type is accompanied by assumptions about how to preserve the consistency of data stored. That is, when an action specifies that an object can be stored must also specify the consistency model that must be used for the stored object Consistency Models Different applications have different requirements on the consistency of the data stored in caches. In the case of static web page storing, there are three different consistency models. The first model that is called strong consistency requires each access to the cached page to return the most up to date copy of the page (ie the page that is saved in the origin server). This type of consistency is usually guaranteed by the origin server that pushes all modifications to the tree of replicas. In this case, a modification must be forwarded to all network nodes before it is considered completed. A second model of consistency, the time-limited consistency, requires updating a saved page within a specified period of time. This period of time is called TTL (Time to Live). After the expiration of this period, the replica must retrieve and renew the object from the origin server. A third model of consistency, loose consistency, requires objects stored in the cache to be updated as soon as the network is able to. This can be achieved by having the origin server creating batched updates or invalidations and transmitting the batches based on a best effort policy. Such a protocol may help to protect the network from increased load and high bandwidth consumption while it is sufficient for the needs of some applications (applications with rare content renewal).

24 Alternative CDN policies Apart from the model of consistency, there are other policy decisions to be taken. It is not valid to assume that all data is storable in all locations. The CDN nodes are scattered in different locations and may be subject to restrictions that do not allow them to store specific types of content. Therefore, there must be a policy that restricts the operation of storing in specific locations. In many cases the decision to copy and store applications or content to nodes of a CDN is determined by the current network status. A node of the CDN may only be needed if there is bottleneck in the network that affect content delivery to the end use. On the other hand, in a non-congested network the utilization of the CDN node may be unnecessary. Similarly, some origin servers may require the usage of the CDN only when they are overloaded. In these cases the content provider may define a threshold that determines the request rate beyond which the copying of content to the CDN nodes is allowed. For this reason, there should be a separate mechanism for monitoring the load on the system, both at the origin server and at the nodes of the CDN. For example, a performance monitoring system can use the information on the server load and register the server as empty, moderately loaded and overloaded. Copying and storage of content or applications to the nodes of the CDN would be allowed only when the server load is considered high enough Edge Services The emergence of content delivery networks not only accelerates the process of content delivery on the Internet and in corporate networks, but also enables the delivery of specialized services by the network edge. These services are called edge services. The edge services exploit the infrastructure of the CDN and overlap to some extent with the services of the CDN. They can range from content adaptation based on

25 the profile of an independent client, to compiling the content and providing protection using virus-like software. For providing edge services, it is necessary to develop the appropriate technology and obtain the necessary intelligence to produce dynamic content pages. The traditional architecture of websites/ web servers requires the use of the same infrastructure for the creation and delivery of content to users. For static pages and static content this architecture is satisfactory. However, creating dynamic content increases the load of a website. We therefore need a way to separate the content delivery process from content creation/compilation. Towards this direction several attempts have been made in recent years, either from companies operating in the content industry or by independent organizations that create Internet Standards. The creation of the Edge Side Includes (ESI) [14] is a result of these efforts. The goal of ESI is to to solve the performance problems inherent in caching content, by accelerating the execution of dynamic web applications. ESI is a simple markup language (mark-up language), based on XML, which is intended to describe cacheable and non-cacheable components of a website. ESI is not intended to replace HTML or other languages used to create dynamic content. In contrast, it coexists with them for the separation of the static from the dynamic part of a page. The static part of a page, a template in essence, can then be stored in the cache, and the cache may only request the dynamic content from the origin server. The assembly of the page is done in the processing nodes at the network edge, where the cache is located Application Content Distribution Networks Introduction to ACDN An Application Content Delivery Network - ACDN [10] is a CDN which improves access to dynamic content that can be stored in caches. An ACDN initially allows the installation of the application only on a single node anywhere on the

26 network and then copies and transfers the application where it is needed in accordance with the observed demand. Unlike a traditional CDN that servers only static content from either a local cache server or directly from the origin server, an ACDN must have the required the computing environment ( the specific application, including executable files and data) in order to process a request. The transfer and installation of the application upon request is not a practical solution to the problem. For this reason an ACDN may only distribute requests only between servers that are currently updated with a copy of the application ACDN Requirements Beyond the classic requirements of a traditional CDN, an ACDN must provide solutions to the following problems: Application distribution framework: An ACDN needs a mechanism to dynamically install a copy of the required application on a server, and maintain consistency in terms of the original copy. The latter issue is complicated due to the fact that an application consists of multiple components that may have different versions. All components of the application must be updated in order to function properly. Content Placement Algorithm: An ACDN must decide which applications to install on what servers and when. Request distribution algorithm: In addition to the load and proximity factor that are taken into account in traditional CDNs, the delivery mechanisms of a ACDN should also take into account in which nodes is the requested application available. The distribution of an application on a ACDN may be achieved by using a metafile of the application consisting of two parts: a list of all files included in the application along with the dates of last modification for each file, and an initialization script which includes all actions to be performed by the CDN node before accepting any requests. The metafile has its own URL and receives treatment similar to any other static content. Thus, using the application metafile, the problem of maintaining

27 the consistency of the application is reduced to the corresponding problem of maintaining consistency of an independent static object, the metafile.

28 3. Content Distribution Business Models 3.1. Business Models Business models are in general one of the most debated and least understood aspects of the Web. The changes brought about by the Internet in the traditional business models have been the subject of lengthy debates and recriminations [24]. A business model defines the process by which a company conducts its functions and produces revenue. The business model clarifies the way in which a company generates revenue by identifying its position in the value chain. [28] : More specifically, there are three key elements that define a business model An architecture for flows of goods, services and information, including a description of the various business players and their roles, A description of the potential benefits for the various business players, and A description of the sources of revenue. In the distribution and delivery of content in the Internet, perhaps the bestknown business model is one followed by Akamai [41], the market leader company. According to this model, one content provider pays the content distributor, a company that operates Content Distribution Networks to ensure the storage of copies of the content on servers located in data centers that are closer to end users than the origin servers. Noteworthy is that content distributors do not possess their own network infrastructure for data transfer and rent the network infrastructure of third parties, in essence creating an overlay network. Apart from this model, but there are a variety of business models which are distinguished based on the following key points: the identity of the entity that benefits from the content distribution service the way content is selected and copied to the replicas and the direction of cash flow.

29 3.2. Content Distribution Business Chain The business chain of content distribution in the Internet comprises of five different categories of entities [43]: Content Providers wish to put their content closer to end users. Hosting Providers operate data centers. They provide a secure and reliable infrastructure that guarantees high availability of servers and connection to a high-speed backbone network. Backbone Providers: Entities that offer high speed Internet access. They operate core networks that are interconnected at Network Access Points. Internet Access Providers - ISPs: They provide Internet connectivity to end users. Content Distributors provide content delivery services to content providers. End Users consume Internet services. Figure 4 Content distribution chain

30 These five groups of players comprise the content distribution chain. The purpose of this chain is to connect consumers with content providers. Despite possible overlaps that may exist between Internet access providers, hosting providers and Backbone operators, their roles as described above,are distinguishable. Schematically, the interrelationship between these entities, is illustrated in Figure 6. The arrows illustrate the direction of data flow. Notably, the content distributor has to cooperate with almost all entities in the chain Content Distribution Business Models Two primary business models have been developed in the area of CDNs ([25], [26]). The first CDN model focuses on meeting the needs of content providers and is known in the literature as Content-Centric CDN. In the second model, the emphasis is given on meeting the needs of ISPs, and is therefore referred to as Access - Centric CDN. The similarity in both models is that the entity that is charged requires the satisfaction of content consumers. Their differences lie in the identity of the entity that is charged for the service, the identity of the entity selecting the content to be distributed through the CDN, and the way of distribution (what percentage will be copied to regional servers, etc.). In the first model (content-centric), the content provider pays the content distributor for delivering content to its customers in order to reduce costs for purchasing bandwidth and hosting in data centers. In the second model, the access provider is charged by the content distributor for saving bandwidth and providing better and faster service to the end users. It should be noted that whereas the data flow direction is from the content providers to the end users, the flow of money is not always the same. In the contentcentric model, cash flows from content providers to content distributors whereas in the access-centric model the cash flows from Internet access providers to content distributors.

31 Content centric model This is the model that has been adopted by the majority of companies involved in the distribution of content, such as Akamai and Speedera. In this model, the profit of content distributors comes from content providers who are charged for the use of CDNs in order to facilitate the distribution of their content. The selection of content to be copied to CDN servers is done by the providers. The customers of the service, that is the content providers, shall be charged according to the amount of content delivered via the CDN, without ruling out the existence of a minimum monthly fee. In the contract signed between the content provider and the operator of CDN, the service provider guarantees uninterrupted and continuous delivery of content to end users through regional caches of the CDN. CDN servers are installed in data centers belonging to third party entities, such as Internet service providers, hosting providers or backbone network operators. Usually content distributors pay rent for the space occupied by their servers, the required power supply and network access. However, sometimes the above features are free of charge [44]. More specifically, by installing CDN servers in the premises of an ISP both parties are mutually benefited. The ISP acquires direct access to popular Internet content carried over the CDN and therefore needs less backbone bandwidth, since it is no longer required to retrieve the content from the original source. So through peering agreements, the content distributor offers free use of the its servers to the ISP and in return the ISP provides a very low price or even completely free of charge hosting to the CDN servers. Content is served to the end users through the ISP. Apart from the costs of placing CDN servers to a data center, content distributors are also charged by core network operators for interconnecting the CDN overlay. This charge is based on the amount of content moved through the core network. Moreover, in some cases, core network operators resell CDN services to their own customers through their own network, taking a share of the content distributors revenue.

32 Figure 5 Content-Centric CDN Model Access centric model In this business model, the revenue of content distributors comes from ISPs that serve their subscribers with popular content that is copied to regional CDN caches. The selection of content is now independent of the identity of its owner, and is based on its popularity as recorded through demand. This model is based on the idea that the cost of storage is cheaper than the cost of bandwidth. By copying frequently accessible content on servers closer to end users, there is a double gain: the distribution process is accelerated and secondly ISPs lower their biggest expense, the utilization of network bandwidth. Moreover, the positive effects of the access-centric model are increased as the scale increases. More specifically, as the number of users increases, the greater the probability of common user requests served by regional servers. However, the access-

33 centric model proved to be not viable. The factors that contributed to this outcome are: The poor economic situation of ISPs, the result of intense competition between them pushing prices down and leaving little margin for profit. In such a business environment, the market for content delivery is deemed unnecessary, as its cost cannot be easily passed to the end user. Content-centric CDNs, provide popular content distribution with the same benefits for free, in return for placing their servers to the premises of Internet Service Providers. In some cases ISPs use their own cache servers for caching popular content. Figure 6 Access-centric model