Content delivery network

A content delivery network or content distribution network (CDN) is a geographically distributed network of proxy servers and their data centers. The goal is to provide high availability and high performance by distributing the service spatially relative to end-users. CDNs serve a large portion of the Internet content today, including web objects (text, graphics and scripts), downloadable objects (media files, software, documents), applications (e-commerce, portals), live streaming media, on-demand streaming media, and social media sites.[1]

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 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, with DDoS protection and web application firewalls (WAF), and WAN optimization.

(Left) Single server distribution
(Right) CDN scheme of distribution


CDN nodes are usually deployed in multiple locations, often over multiple backbones. Benefits include reducing bandwidth costs, improving page load times, or increasing 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 (PoPs). Others build a global network and have a small number of geographical PoPs.[2]

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, the least number of network seconds away from the requesting client, or the highest availability in terms of server performance (both current and historical), so as to optimize delivery across local networks. When optimizing for cost, locations that are 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" or "edge networks" as they would be the closest edge of CDN assets to the end user.[3]

The CDN's Edge Network grows outward from the origins through further acquisitions (via purchase, peering, or exchange) of co-locations facilities, bandwidth, and servers.

Content networking techniques

The Internet was designed according to the end-to-end principle.[4] 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.[5] These techniques are briefly described below.

Web caches 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).[6]

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. 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,[7] and anycasting.[8] Proximity—choosing the closest service node—is estimated using a variety of techniques including reactive probing, proactive probing, and connection monitoring.[5]

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[9][10] 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.[11] 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 (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 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.[12][13]

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,[14] reverse proxies or application delivery controllers.[15] It can be as simple as two caching servers,[14] or large enough to serve petabytes of content.[16]

Large content distribution networks may even build and set up their own private network to distribute copies of content across cache locations.[17][18] Such private networks are usually used in conjunction with public networks as a backup option in case the capacity of 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 multicasting 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.[19][20]

CDN trends

Emergence of telco CDNs

The rapid growth of streaming video traffic[21] uses large capital expenditures by broadband providers[22] in order to meet this demand and to retain subscribers by delivering a sufficiently good quality of experience.

To address this, telecommunications service providers (TSPs) have begun to launch their own content delivery networks as a means to lessen the demands on the network backbone and to 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 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 a 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 which have a limited sphere of actions in face of the impact of these operations on the utilization of their resources.

In contrast, the deployment of telco-CDNs allow operators to implement their own content management operations,[23][24] which enables them to have better control over the utilization of their resources and, as such, provide better quality of service and experience to their end users.

Federated CDNs

In June 2011, reported that a group of TSPs had founded an Operator Carrier Exchange (OCX)[25] 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.

edns-client-subnet EDNS0 option

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,[26] which is intended to accurately localize DNS resolution responses. The initiative involves a limited number of leading DNS and CDN service providers. With the edns-client-subnet EDNS0 option, the recursive DNS servers of CDNs will utilize the IP address of the requesting client subnet when resolving DNS requests. If a CDN relies on the IP address of the DNS resolver instead of the client when resolving DNS requests, it can incorrectly geo-locate a client if the client is using Google anycast addresses for their DNS resolver, which can create latency problems. Initially, Google's DNS addresses geo-located to California, potentially far from the location of the requesting client, but now the Google Public DNS servers are available worldwide.[27]

Notable content delivery service providers

Free CDNs

Traditional commercial CDNs

Telco CDNs

Commercial CDNs using P2P for delivery

Multi CDNs

Generally speaking, all Internet service providers can provide a content delivery network.

See also


  1. ^ Evi,, Nemeth, (2018). "Chapter 19, Web hosting, Content delivery networks". UNIX and Linux system administration handbook (Fifth ed.). Boston: Pearson Education. p. 690. ISBN 9780134277554. OCLC 1005898086.
  2. ^ "How Content Delivery Networks Work". CDNetworks. Retrieved 22 September 2015.
  3. ^ "How content delivery networks (CDNs) work". NCZOnline. Retrieved 22 September 2015.
  4. ^ Saltzer, J. H., Reed, D. P., Clark, D. D.: “End-to-End Arguments in System Design,” ACM Transactions on Communications, 2(4), 1984
  5. ^ a b Hofmann, Markus; Leland R. Beaumont (2005). Content Networking: Architecture, Protocols, and Practice. Morgan Kaufmann Publisher. ISBN 1-55860-834-6.
  6. ^ Bestavros, Azer (March 1996). "Speculative Data Dissemination and Service to Reduce Server Load, Network Traffic and Service Time for Distributed Information Systems" (PDF). Proceedings of ICDE'96: The 1996 International Conference on Data Engineering. 1996: 180–189.
  7. ^ RFC 3568 Barbir, A., Cain, B., Nair, R., Spatscheck, O.: "Known Content Network (CN) Request-Routing Mechanisms," July 2003
  8. ^ RFC 1546 Partridge, C., Mendez, T., Milliken, W.: "Host Anycasting Services," November 1993.
  9. ^ RFC 3507 Elson, J., Cerpa, A.: "Internet Content Adaptation Protocol (ICAP)," April 2003.
  10. ^ ICAP Forum
  11. ^ RFC 3835 Barbir, A., Penno, R., Chen, R., Hofmann, M., and Orman, H.: "An Architecture for Open Pluggable Edge Services (OPES)," August 2004.
  12. ^ Li, Jin. "On peer-to-peer (P2P) content delivery" (PDF). Peer-to-Peer Networking and Applications. 1 (1): 45–63. doi:10.1007/s12083-007-0003-1.
  13. ^ Stutzbach, Daniel; et al. (2005). "The scalability of swarming peer-to-peer content delivery". In Boutaba, Raouf; et al. NETWORKING 2005 -- Networking Technologies, Services, and Protocols; Performance of Computer and Communication Networks; Mobile and Wireless Communications Systems (PDF). Springer. pp. 15–26. ISBN 978-3-540-25809-4.
  14. ^ a b "How to build your own CDN using BIND, GeoIP, Nginx, Varnish - UNIXy".
  15. ^ "How to Create Your Content Delivery Network With aiScaler".
  16. ^ "Netflix Shifts Traffic To Its Own CDN; Akamai, Limelight Shrs Hit". Forbes. 5 June 2012.
  17. ^ Mikel Jimenez; et al. (May 1, 2017). "Building Express Backbone: Facebook's new long-haul network". Retrieved October 27, 2017.
  18. ^ "Inter-Datacenter WAN with centralized TE using SDN and OpenFlow" (PDF). 2012. Retrieved October 27, 2017.
  19. ^ M. Noormohammadpour; et al. (July 10, 2017). "DCCast: Efficient Point to Multipoint Transfers Across Datacenters". USENIX. Retrieved July 26, 2017.
  20. ^ M. Noormohammadpour; et al. (2018). "QuickCast: Fast and Efficient Inter-Datacenter Transfers using Forwarding Tree Cohorts". Retrieved January 23, 2018.
  21. ^ "Online Video Sees Tremendous Growth, Spurs some Major Updates". SiliconANGLE.
  22. ^ "Overall Telecom CAPEX to Rise in 2011 Due to Video, 3G, LTE Investments". cellular-news.
  23. ^ 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.
  24. ^ 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.
  25. ^ "Telcos And Carriers Forming New Federated CDN Group Called OCX (Operator Carrier Exchange)". Dan Rayburn -
  26. ^ "Client Subnet in DNS Requests".
  27. ^ "Where are your servers currently located?".
  28. ^ a b c d e f "How CDN and International Servers Networking Facilitate Globalization". The Huffington Post. Delarno Delvix. Retrieved 9 September 2016.

Further reading

Akamai Technologies

Akamai Technologies, Inc. is an American content delivery network (CDN) and cloud service provider headquartered in Cambridge, Massachusetts, in the United States. Akamai's content delivery network is one of the world's largest distributed computing platforms, responsible for serving between 15% and 30% of all web traffic.

The company operates a network of servers around the world and rents out capacity on these servers to customers who want their websites to work faster by distributing content from locations close to the user. When a user navigates to the URL of an Akamai customer, their browser is redirected to one of Akamai's copies of the website.

The company was founded in 1998 by Daniel M. Lewin (then a graduate student at MIT) and MIT applied mathematics professor Tom Leighton. Lewin was killed aboard American Airlines Flight 11, which crashed in the September 11 attacks of 2001. Leighton currently serves as Akamai's CEO.

Akamai is a Hawaiian word meaning "intelligent" or "clever".

Amazon CloudFront

Amazon CloudFront is a content delivery network (CDN) offered by Amazon Web Services. Content delivery networks provide a globally-distributed network of proxy servers which cache content, such as web videos or other bulky media, more locally to consumers, thus improving access speed for downloading the content.

CloudFront has servers located in Europe (United Kingdom, Ireland, The Netherlands, Germany, Spain), Asia (Hong Kong, Singapore, Japan, Taiwan and India), Australia, South America, as well as in several major cities in the United States. The service operates from (as of December 2017) 107 edge locations on five continents.CloudFront operates on a pay-as-you-go basis.

CloudFront competes with larger content delivery networks such as Akamai and Limelight Networks. Upon launch, Larry Dignan of ZDNet News stated that CloudFront could cause price and margin reductions from competing CDNs.


BootstrapCDN is a public content delivery network. Users of BootstrapCDN can load CSS, JavaScript and images remotely, from its servers, and it's been used by more than 7.9 million websites worldwide (including 30% of the top-10k websites) and delivers more than 70 billion requests a month


Bootstrap uses StackPath's global content delivery network, which makes websites using its service resilient to unexpected surges in web traffic.


Founded in 2000, CDNetworks is a full-service content delivery network (CDN) which provides technology, network infrastructure, and customer services for the delivery of Internet content and applications. The company is positioning itself as a multinational provider of content delivery services, with a particular emphasis on emerging Internet markets, including South America, India and China. The company's content delivery network consists of 169 Point of Presence (PoPs) on six continents, with over 30 locations in Russia and China. Services include CDN, video acceleration, DDoS protection, cloud storage, cloud access security broker (CASB), web application firewall (WAF) and managed DNS with cloud load balancing. Key differentiators include a large number of global PoPs, good network presence in China and Russia, and high-profile clients such as Forbes, Samsung and Hyundai. CDNetworks has offices in the U.S., South Korea, China, Japan, UK and Singapore.


Cloudflare, Inc. is a U.S. company that provides content delivery network services, DDoS mitigation, Internet security and distributed domain name server services. Cloudflare's services sit between the visitor and the Cloudflare user's hosting provider, acting as a reverse proxy for websites. Cloudflare's headquarters are in San Francisco, California, with additional offices in London, Singapore, Champaign, Austin, Boston and Washington, D.C.


Cotendo, Inc. was a content delivery network

and an application delivery network

service provider. The company's headquarters are in Sunnyvale, California, with research and development based in Netanya, Israel. On March, 2012, Akamai acquired Cotendo for over USD $250 million.


Fastly, Inc. is an American cloud computing services provider. Fastly's edge cloud platform provides a content delivery network, Internet security services, load balancing, and video & streaming services. Fastly's headquarters are in San Francisco, California, with additional offices in Denver, New York, Portland, London, and Tokyo.

Highwinds Network Group

Highwinds Network Group, Inc. (Highwinds) is a company that was founded in 2002 and offers IP services including CDN, cloud storage, IP transit, transport and colocation. The company headquarters are located in Winter Park, Florida, United States. Highwinds maintains Network Operations Centers (NOCs) in Winter Park, FL, Phoenix, AZ, and Amsterdam, Netherlands as well as offices in Costa Mesa, CA, São Paulo, Brazil and London, England. The Highwinds network, called RollingThunder, consists of more than 70 points of presence throughout North America, South America, Europe, Asia and Australia. Highwinds provides video streaming services to media companies including Blip.TV and Hudl, delivers online games for publishers such as Valve Corporation and CCP Games and distributes advertising assets for leading platforms including Facebook's LiveRail.


Imperva Incapsula is a cloud-based application delivery platform. It uses a global content delivery network to provide web application security, DDoS mitigation, content caching, application delivery, load balancing and failover services.

Limelight Networks

Limelight Networks is an American company that provides a content delivery network (CDN) service, used for delivery of digital media content and software. As of March 2018, the company's network has more than 80 points-of-presence and delivers between 40-80 petabytes of data daily, with 31+ Terabits per second of egress capacity across the globe.


OnApp is a London, UK-based software company. Its software enables service providers to build, operate and sell IaaS public cloud, private cloud and Content Delivery Network services. OnApp also operates the OnApp Federation, a wholesale cloud infrastructure marketplace, which enables service providers to buy and sell cloud infrastructure managed by OnApp software; and enables enterprises to adopt a hybrid cloud model by combining their on-premises cloud infrastructure with public cloud resources. OnApp was founded in 2010.


Savvis, formerly SVVS on Nasdaq and formerly known as Savvis Communications Corporation, and, later, Savvis Inc., is a subsidiary of CenturyLink, a company headquartered in Monroe, Louisiana. The company sells managed hosting and colocation services with more than 50 data centers (over 2 million square feet) in North America, Europe, and Asia, automated management and provisioning systems, and information technology consulting. Savvis has approximately 2,500 unique business and government customers.


StreamZilla is a streaming media hosting and content delivery network (CDN) in Europe. The company is located in the Mediacentrale in the city Groningen, Netherlands.

Subresource Integrity

Subresource Integrity or SRI is a W3C recommendation to provide a method to protect website delivery. Specifically, it validates assets served by a third party, such as a content delivery network (CDN). This ensures these assets have not been compromised for hostile purposes.

To use SRI, a website author wishing to include a resource from a third party can specify a cryptographic hash of the resource in addition to the location of the resource. Browsers fetching the resource can then compare the hash provided by the website author with the hash computed from the resource. If the hashes don't match, the resource is discarded.As of May 2018, SRI is supported by Microsoft Edge, Firefox, Safari, Google Chrome, and Opera.Sample link element with integrity attribute used by SRI:


Tata Communications

Tata Communications formerly called VSNL is a global provider of telecommunications solutions and services. It is part of the Tata Group.

Its telecommunications network spans the globe and includes more than 500,000 km of subsea fibre and more than 210,000 km of terrestrial fibre. The company has invested $1.19 billion in its global subsea fibre network.Tata Communications uses its network to deliver network services and software-defined network platforms, such as Ethernet, SD-WAN, Content delivery network (CDN), Internet, Multiprotocol Label Switching (MPLS) and Private Line. It has more than 400 Points of Presence (POP) globally with data centre and colocation in 44 sites.In 2016, Tata Communications had a revenue of $3.2 billion, with 77% of its revenue being generated outside India. The company has over 8,000 employees from 40 nationalities.Tata Communications is the Official Connectivity Provider for Formula One and MotoGP. Since 2012, the company has provided Web Hosting and been the Content Delivery Network Provider for Communications Limited is headquartered in Mumbai and is listed on the Bombay Stock Exchange and the National Stock Exchange of India. The company itself also holds a stake in Tata Communications Lanka Limited in Sri Lanka and Tata Teleservices Limited, one of the largest mobile service providers in India.

The company is listed as one of AON Hewitt's Best Employers in India for 2017.

UK2 Group

UK2 Group is a global provider of Internet services based in the United Kingdom, with data centers in the UK and US; it is currently owned by The Hut Group.

Its services include web hosting, virtual private servers, domain name registration and management, dedicated servers, web design and a content delivery network.

It provides services to individuals, small businesses, blue-chip enterprises and resellers, through a family of brands it has created or acquired, among them,, Midphase, WestHost,

VI.Net, and 100Tb.

Verizon Digital Media Services

Verizon Digital Media Services (VDMS) is an American company known for their content delivery network (CDN).

The company was founded in 2006 as EdgeCast Networks, Inc and was funded by the venture arm of The Walt Disney Company, Steamboat Ventures. The company was headquartered in Santa Monica, California.

Edgecast is notable for being a self-provisioning CDN technology used by the telecommunication and hosting industries. Edgecast was rated the third in the CDN industry by the Yankee Group in August 2009, and turned EBITA positive in Q2 of 2009. EdgeCast Networks was ranked 13th on the Deloitte Fast 500 list for North America 2012.

Web cache

A web cache (or HTTP cache) is an information technology for the temporary storage (caching) of web documents, such as HTML pages and images, to reduce server lag. A web cache system stores copies of documents passing through it; subsequent requests may be satisfied from the cache if certain conditions are met. A web cache system can refer either to an appliance, or to a computer program.


Xunlei Limited (Chinese: 迅雷; pinyin: Xùn Léi; literally: 'thunderbolt") is a Chinese multinational technology company and an online service provider founded in 2003. The subsidiary of Xunlei Limited, Shenzhen Xunlei Networking Technologies, Co., Ltd. (Chinese: 迅雷网络技术有限公司) was formerly known as Sandai Technologies (Shenzhen) Inc. and changed its name to Shenzhen Xunlei Networking Technologies, Co., Ltd. in May 2005. In April 2014, Xunlei received an investment from a Chinese electronics company Xiaomi of $200 million. On 24 June 2014, it went public on the Nasdaq Stock Exchange, selling 7.315 million American depositary shares (ADS) at $12 and raising just shy of $88 million. According to the annual ranking of China's top 100 internet companies released by Ministry of Industry and Information Technology of the Chinese government, Xunlei occupied 42nd place in 2017's ranking.The main products developed by Xunlei Limited is the Xunlei download manager and Peer-to-peer software, supporting HTTP, FTP, eDonkey, and BitTorrent protocols. As of 2010, it was the most commonly used BitTorrent client in the world. In October 2017, the company announced that it will transform itself into a blockchain company, and release a blockchain-based product named OneThing Cloud. OneThing Cloud users get LinkToken(one kind of virtual token) for contributing their bandwidth to the Xunlei's Content Delivery Network.Xunlei Ltd. announced that its board of directors has appointed Mr. Lei Chen, who is a former Tencent cloud computing unit leader, as its Chief Executive Officer of the Company and Director of the Board on June 29, 2017. Mr. Sean Zou, founder of the Company, resigned as Chief Executive Officer and continues to serve as the Chairman of the Board, with specific focus on Xunlei's growth strategy and initiatives related to products and technological innovation in cloud computing including AI, as well as strategic investments.

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.