A content delivery network (CDN) or content distribution network is a geographically distributed network of proxy servers and their

data center A data center is a building, a dedicated space within a building, or a group of buildings used to house computer systems and associated components, such as telecommunications and storage systems. Since IT operations are crucial for busines ...

s. The goal is to provide high

availability In reliability engineering, the term availability has the following meanings: * The degree to which a system, subsystem or equipment is in a specified operable and committable state at the start of a mission, when the mission is called for at ...

and performance ("speed") by distributing the service spatially relative to

end user In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ultimately use a product. The end user stands in contrast to users who support or maintain the product, such as sysops, system administrato ...

s. CDNs came into existence in the late 1990s as a means for alleviating the performance bottlenecks of the

Internet The Internet (or internet) is the Global network, global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a internetworking, network of networks ...

as the Internet was starting to become a mission-critical medium for people and enterprises. Since then, CDNs have grown to serve a large portion of Internet content, including web objects (text, graphics and scripts), downloadable objects (media files, software, documents), applications (

e-commerce E-commerce (electronic commerce) refers to commercial activities including the electronic buying or selling products and services which are conducted on online platforms or over the Internet. E-commerce draws on technologies such as mobile co ...

, portals),

live streaming Livestreaming, live-streaming, or live streaming is the streaming media, streaming of video or Digital audio, audio in real-time communication, real time or near real time. While often referred to simply as ''streaming'', the real-time nature ...

media, on-demand streaming media, and

social media Social media are interactive technologies that facilitate the Content creation, creation, information exchange, sharing and news aggregator, aggregation of Content (media), content (such as ideas, interests, and other forms of expression) amongs ...

sites. CDNs are a layer in the internet ecosystem. Content owners such as media companies and e-commerce vendors pay CDN operators to deliver their content to their end users. In turn, a CDN pays

Internet service provider An Internet service provider (ISP) is an organization that provides a myriad of services related to accessing, using, managing, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, no ...

s (ISPs), carriers, and network operators for hosting its servers in their data centers. CDN is an umbrella term spanning different types of content delivery services: video streaming, software downloads, web and mobile content acceleration, licensed/managed CDN, transparent caching, and services to measure CDN performance, load balancing, Multi CDN switching and analytics and cloud intelligence. CDN vendors may cross over into other industries like security,

DDoS In computing, a denial-of-service attack (DoS attack) is a cyberattack in which the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host co ...

protection and

web application firewall A web application firewall (WAF) is a specific form of application firewall that filters, monitors, and blocks HTTP traffic to and from a web service. By inspecting HTTP traffic, it can prevent attacks exploiting a web application's known vulne ...

s (WAF), and WAN optimization. Content delivery service providers include

Akamai Technologies Akamai Technologies, Inc. is an American company specialized in content delivery networkJ. Dilley, B. Maggs, J. Parikh, H. Prokop, R. Sitaraman, and B. Weihl. (CDN), cybersecurity, DDoS mitigation, and cloud services. It is headquartered in ...

Cloudflare Cloudflare, Inc., is an American company that provides content delivery network services, cybersecurity, DDoS mitigation, wide area network services, reverse proxies, Domain Name Service, ICANN-accredited domain registration, and other se ...

Amazon CloudFront Amazon CloudFront is a content delivery network (CDN) operated by Amazon Web Services. The content delivery network was created to provide a globally-distributed network of proxy servers to cache content, such as web videos or other bulky media ...

, Qwilt (Cisco),

Fastly Fastly, Inc. is an American company based in San Francisco, which describes itself as a cloud computing company. Fastly provides content delivery network services, image optimization, and load balancing services. Fastly's cloud security services ...

, and Google Cloud CDN.

Technology

CDN nodes are usually deployed in multiple locations, often over multiple

Internet backbone The Internet backbone is the principal data routes between large, strategically interconnected computer networks and core routers of the Internet. These data routes are hosted by commercial, government, academic and other high-capacity network ...

s. Benefits include reducing bandwidth costs, improving page load times, and increasing the global availability of content. The number of nodes and servers making up a CDN varies, depending on the architecture, some reaching thousands of nodes with tens of thousands of servers on many remote

points of presence A point of presence (PoP) is an artificial demarcation point or network interface point between communicating entities. A common example is an ISP point of presence, the local access point that allows users to connect to the Internet with their ...

(PoPs). Others build a global network and have a small number of geographical PoPs. Requests for content are typically algorithmically directed to nodes that are optimal in some way. When optimizing for performance, locations that are best for serving content to the user may be chosen. This may be measured by choosing locations that are the fewest

hops Hops are the flowers (also called seed cones or strobiles) of the hop plant ''Humulus lupulus'', a member of the Cannabaceae family of flowering plants. They are used primarily as a bittering, flavouring, and stability agent in beer, to whic ...

or the shortest time to the requesting client, or the highest server performance, to optimize delivery across local networks. When optimizing for cost, locations that are the least expensive may be chosen instead. In an optimal scenario, these two goals tend to align, as edge servers that are close to the end user at the edge of the network may have an advantage in performance or cost. Most CDN providers will provide their services over a varying, defined, set of PoPs, depending on the coverage desired, such as United States, International or Global, Asia-Pacific, etc. These sets of PoPs can be called "edges", "edge nodes", "edge servers", or "edge networks" as they would be the closest edge of CDN assets to the end user. CDN concepts: * Content Provider Origin Server: the web server providing the source content * CDN entry point(s): the servers within the CDN that fetch the content from the origin * CDN Origin Shield: the CDN service helping to protect the origin server in case of heavy traffic * CDN Edge Servers: the CDN servers serving the content request from the clients * CDN footprint: the geographic areas where the CDN Edge Servers can effectively serve clients requests * CDN selector: in the context of multi-CDN, a decision making service to choose among multiple CDNs * CDN offloading: in the context of Peer-to-Peer CDN, a mechanism to help deliver the content between clients who are consuming it, in addition to CDN Edge Server delivery

Security and privacy

CDN providers profit either from direct fees paid by content providers using their network, or profit from the user analytics and tracking data collected as their scripts are being loaded onto customers' websites inside their browser origin. As such these services are being pointed out as potential privacy intrusions for the purpose of

behavioral targeting Targeted advertising or data-driven marketing is a form of advertising, including online advertising, that is directed towards an audience with certain traits, based on the product or person the advertiser is promoting. These traits can either ...

and solutions are being created to restore single-origin serving and caching of resources. In particular, a website using a CDN may violate the EU's

General Data Protection Regulation The General Data Protection Regulation (Regulation (EU) 2016/679), abbreviated GDPR, is a European Union regulation on information privacy in the European Union (EU) and the European Economic Area (EEA). The GDPR is an important component of ...

(GDPR). For example, in 2021 a German court forbade the use of a CDN on a university website, because this caused the transmission of the user's IP address to the CDN, which violated the GDPR. CDNs serving JavaScript have also been targeted as a way to inject malicious content into pages using them. Subresource Integrity mechanism was created in response to ensure that the page loads a script whose content is known and constrained to a hash referenced by the website author.

Content networking techniques

The Internet was designed according to the

end-to-end principle The end-to-end principle is a design principle in computer networking that requires application-specific features (such as reliability and security) to be implemented in the communicating end nodes of the network, instead of in the network itse ...

. This principle keeps the core network relatively simple and moves the intelligence as much as possible to the network end-points: the hosts and clients. As a result, the core network is specialized, simplified, and optimized to only forward data packets. Content Delivery Networks augment the end-to-end transport network by distributing on it a variety of intelligent applications employing techniques designed to optimize content delivery. The resulting tightly integrated overlay uses web caching, server-load balancing, request routing, and content services.

Web cache A web cache (or HTTP cache) is a system for optimizing the World Wide Web. It is implemented both client-side and server-side. The caching of multimedia and other files can result in less overall delay when web browser, browsing the Web. Parts o ...

s store popular content on servers that have the greatest demand for the content requested. These shared network appliances reduce bandwidth requirements, reduce server load, and improve the client response times for content stored in the cache. Web caches are populated based on requests from users (pull caching) or based on preloaded content disseminated from content servers (push caching). Server-load balancing uses one or more techniques including service-based (global load balancing) or hardware-based (i.e. layer 4–7 switches, also known as a web switch, content switch, or multilayer switch) to share traffic among a number of servers or web caches. Here the switch is assigned a single virtual

IP address An Internet Protocol address (IP address) is a numerical label such as that is assigned to a device connected to a computer network that uses the Internet Protocol for communication. IP addresses serve two main functions: network interface i ...

. Traffic arriving at the switch is then directed to one of the real web servers attached to the switch. This has the advantage of balancing load, increasing total capacity, improving scalability, and providing increased reliability by redistributing the load of a failed web server and providing server health checks. A content cluster or service node can be formed using a layer 4–7 switch to balance load across a number of servers or a number of web caches within the network. Request routing directs client requests to the content source best able to serve the request. This may involve directing a client request to the service node that is closest to the client, or to the one with the most capacity. A variety of algorithms are used to route the request. These include Global Server Load Balancing, DNS-based request routing, Dynamic metafile generation, HTML rewriting, and

anycast Anycast is a network addressing and routing methodology in which a single IP address is shared by devices (generally servers) in multiple locations. Routers direct packets addressed to this destination to the location nearest the sender, using ...

ing. Proximity—choosing the closest service node—is estimated using a variety of techniques including reactive probing, proactive probing, and connection monitoring. CDNs use a variety of methods of content delivery including, but not limited to, manual asset copying, active web caches, and global hardware load balancers.

Content service protocols

Several protocol suites are designed to provide access to a wide variety of content services distributed throughout a content network. The Internet Content Adaptation Protocol (ICAP) was developed in the late 1990s to provide an open standard for connecting application servers. A more recently defined and robust solution is provided by the Open Pluggable Edge Services (OPES) protocol. This architecture defines OPES service applications that can reside on the OPES processor itself or be executed remotely on a Callout Server. Edge Side Includes or ESI is a small markup language for edge-level dynamic web content assembly. It is fairly common for websites to have generated content. It could be because of changing content like catalogs or forums, or because of the personalization. This creates a problem for caching systems. To overcome this problem, a group of companies created ESI.

Peer-to-peer CDNs

In ''

peer-to-peer Peer-to-peer (P2P) computing or networking is a distributed application architecture that partitions tasks or workloads between peers. Peers are equally privileged, equipotent participants in the network, forming a peer-to-peer network of Node ...

(P2P)'' content-delivery networks, clients provide resources as well as use them. This means that, unlike client–server systems, the content-centric networks can actually perform better as more users begin to access the content (especially with protocols such as

Bittorrent BitTorrent is a Protocol (computing), communication protocol for peer-to-peer file sharing (P2P), which enables users to distribute data and electronic files over the Internet in a Decentralised system, decentralized manner. The protocol is d ...

that require users to share). This property is one of the major advantages of using P2P networks because it makes the setup and running costs very small for the original content distributor. To incentive peers to participate in the P2P network,

web3 Web3 (also known as Web 3.0) is an idea for a new iteration of the World Wide Web which incorporates concepts such as decentralization, blockchain technologies, and token-based economics. This is distinct from Tim Berners-Lee's concept of th ...

and

blockchain The blockchain is a distributed ledger with growing lists of Record (computer science), records (''blocks'') that are securely linked together via Cryptographic hash function, cryptographic hashes. Each block contains a cryptographic hash of th ...

technologies can be used: participating nodes receive crypto tokens in exchange of their involvement.

Private CDNs

If content owners are not satisfied with the options or costs of a commercial CDN service, they can create their own CDN. This is called a private CDN. A private CDN consists of PoPs (points of presence) that are only serving content for their owner. These PoPs can be caching servers, reverse proxies or application delivery controllers. It can be as simple as two caching servers, or large enough to serve petabytes of content. When a private CDN is deployed within a company network, it is also referred as Entreprise CDN or eCDN. Large content distribution networks may even build and set up their own private network to distribute copies of content across cache locations. Such private networks are usually used in conjunction with public networks as a backup option in case the capacity of the private network is not enough or there is a failure which leads to capacity reduction. Since the same content has to be distributed across many locations, a variety of

multicast In computer networking, multicast is a type of group communication where data transmission is addressed to a group of destination computers simultaneously. Multicast can be one-to-many or many-to-many distribution. Multicast differs from ph ...

ing techniques may be used to reduce bandwidth consumption. Over private networks, it has also been proposed to select multicast trees according to network load conditions to more efficiently utilize available network capacity.

CDN trends

Emergence of telco CDNs

The rapid growth of

streaming video Video on demand (VOD) is a media distribution system that allows users to access videos, television shows and films digitally on request. These multimedia are accessed without a traditional video playback device and a typical static broadcasting ...

traffic required large

capital expenditures Capital expenditure or capital expense (abbreviated capex, CAPEX, or CapEx) is the money an organization or corporate entity spends to buy, maintain, or improve its fixed assets, such as buildings, vehicles, equipment, or land. It is considered ...

by broadband providers in order to meet this demand and retain subscribers by delivering a sufficiently good

quality of experience Quality of experience (QoE) is a measure of the delight or annoyance of a customer's experiences with a service (e.g., web browsing, phone call, TV broadcast).Qualinet White Paper on Definitions of Quality of Experience (2012). European Network on Q ...

. To address this,

telecommunications service provider Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...

s have begun to launch their own content delivery networks as a means to lessen the demands on the network backbone and reduce infrastructure investments.

Telco CDN advantages

Because they own the networks over which video content is transmitted, telco CDNs have advantages over traditional CDNs. They own the last mile and can deliver content closer to the end-user because it can be cached deep in their networks. This deep caching minimizes the

distance Distance is a numerical or occasionally qualitative measurement of how far apart objects, points, people, or ideas are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two co ...

that video data travels over the general Internet and delivers it more quickly and reliably. Telco CDNs also have a built-in cost advantage since traditional CDNs must lease bandwidth from them and build the operator's margin into their own cost model. In addition, by operating their own content delivery infrastructure, telco operators have better control over the utilization of their resources. Content management operations performed by CDNs are usually applied without (or with very limited) information about the network (e.g., topology, utilization etc.) of the telco-operators with which they interact or have business relationships. These pose a number of challenges for the telco-operators who have a limited sphere of action in face of the impact of these operations on the utilization of their resources. In contrast, the deployment of telco-CDNs allows operators to implement their own content management operations,D. Tuncer, M. Charalambides, R. Landa, G. Pavlou, "More Control Over Network Resources: an ISP Caching Perspective", proceedings of IEEE/IFIP Conference on Network and Service Management (CNSM), Zurich, Switzerland, October 2013.M. Claeys, D. Tuncer, J. Famaey, M. Charalambides, S. Latre, F. De Turck, G. Pavlou, "Proactive Multi-tenant Cache Management for Virtualized ISP Networks", proceedings of IEEE/IFIP Conference on Network and Service Management (CNSM), Rio de Janeiro, Brazil, November 2014. which enables them to have a better control over the utilization of their resources and, as such, provide better quality of service and experience to their end users.

Federated CDNs and Open Caching

In June 2011, StreamingMedia.com reported that a group of TSPs had founded an Operator Carrier Exchange (OCX) to interconnect their networks and compete more directly against large traditional CDNs like Akamai and Limelight Networks, which have extensive PoPs worldwide. This way, telcos are building a Federated CDN offering, which is more interesting for a content provider willing to deliver its content to the aggregated audience of this federation. It is likely that in a near future, other telco CDN federations will be created. They will grow by enrollment of new telcos joining the federation and bringing network presence and their Internet subscriber bases to the existing ones. The Open Caching specification by

Streaming Video Technology Alliance Streaming media refers to multimedia delivered through a network for playback using a media player. Media is transferred in a ''stream'' of packets from a server to a client and is rendered in real-time; this contrasts with file downl ...

defines a set of APIs that allows a Content Provider to deliver its content using several CDNs in a consistent way, seeing each CDN provider the same way through these APIs.

Multi CDN and CDN selection

Combining several CDN services allow Content Providers to not rely on a single CDN service, especially concerned to deal with high peak audience during live events. There are several ways to allocate traffic to a particular CDN among the list, either client-side CDN selection, or server-side (at the Content Provider's origin) or cloud-side (in the middle, between the content origin and the audience). CDN selection criteria can be performance, availability and cost.

Improving CDN performance using Extension Mechanisms for DNS

Traditionally, CDNs have used the IP of the client's recursive DNS resolver to geo-locate the client. While this is a sound approach in many situations, this leads to poor client performance if the client uses a non-local recursive DNS resolver that is far away. For instance, a CDN may route requests from a client in India to its edge server in Singapore, if that client uses a public DNS resolver in Singapore, causing poor performance for that client. Indeed, a recent study showed that in many countries where public DNS resolvers are in popular use, the median distance between the clients and their recursive DNS resolvers can be as high as a thousand miles. In August 2011, a global consortium of leading Internet service providers led by Google announced their official implementation of the edns-client-subnet IETF Internet Draft, which is intended to accurately localize DNS resolution responses. The initiative involves a limited number of leading DNS service providers, such as Google Public DNS, and CDN service providers as well. With the edns-client-subnet EDNS0 option, CDNs can now utilize the IP address of the requesting client's subnet when resolving DNS requests. This approach, called end-user mapping, has been adopted by CDNs and it has been shown to drastically reduce the round-trip latencies and improve performance for clients who use public DNS or other non-local resolvers. However, the use of EDNS0 also has drawbacks as it decreases the effectiveness of caching resolutions at the recursive resolvers, increases the total DNS resolution traffic, and raises a privacy concern of exposing the client's subnet.

Virtual CDN (vCDN)

Virtualization technologies are being used to deploy virtual CDNs (vCDNs) (also known as a software-defined CDN or sd-CDN) with the goal to reduce content provider costs, and at the same time, increase elasticity and decrease service delay. With vCDNs, it is possible to avoid traditional CDN limitations, such as performance, reliability and availability since virtual caches are deployed dynamically (as virtual machines or containers) in physical servers distributed across the provider's geographical coverage. As the virtual cache placement is based on both the content type and server or end-user geographic location, the vCDNs have a significant impact on service delivery and network congestion.

CDN using non-HTTP delivery

To boost performance, delivery to clients from servers can use alternate non-HTTP protocols such as

WebRTC WebRTC (Web Real-Time Communication) is a free and open-source project providing web browsers and mobile applications with real-time communication (RTC) via application programming interfaces (APIs). It allows audio and video communication and ...

and WebSockets.

Image Optimization and Delivery (Image CDNs)

In 2017, Addy Osmani of

Google Google LLC (, ) is an American multinational corporation and technology company focusing on online advertising, search engine technology, cloud computing, computer software, quantum computing, e-commerce, consumer electronics, and artificial ...

started referring to software solutions that could integrate naturally with the

Responsive Web Design Responsive web design (RWD) or responsive design is an approach to web design that aims to make web pages render well on a variety of devices and window or screen sizes from minimum to maximum display size to ensure usability and satisfactio ...

paradigm (with particular reference to the

Technology

Security and privacy

Content networking techniques

Content service protocols

Peer-to-peer CDNs

Private CDNs

CDN trends

Emergence of telco CDNs

Telco CDN advantages

Federated CDNs and Open Caching

Multi CDN and CDN selection

Improving CDN performance using Extension Mechanisms for DNS

Virtual CDN (vCDN)

CDN using non-HTTP delivery

Image Optimization and Delivery (Image CDNs)

Content delivery service and technology providers

Commercial or free software vendors (build your own CDN)

Free-as-a-Service

Commercial-as-a-Service

Others

Telco CDNs

Commercial using P2P for delivery

Multi

In-house

See also

References

Further reading