Content delivery network

(Right) CDN scheme of distribution
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 performance ("speed") by distributing the service spatially relative to end users. CDNs came into existence in the late 1990s as a means for alleviating the performance bottlenecks of the Internet[1][2] 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 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.[3]
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 providers (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:
Notable content delivery service providers include Akamai Technologies, Edgio, Cloudflare, Amazon CloudFront, Fastly, and Google Cloud CDN.
Technology
CDN nodes are usually deployed in multiple locations, often over multiple
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 lowest number of network seconds away from the requesting client, or the highest availability in terms of server performance (both current and historical), 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.[5]
Security and privacy
CDN providers profit either from direct fees paid by
In particular, a website using a CDN may violate the EU's General Data Protection Regulation (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.[8]
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.[9]
Content networking techniques
The Internet was designed according to the end-to-end principle.[10] 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.[11]
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).[12]
Server-load balancing uses one or more techniques including service-based (global load balancing) or hardware-based (i.e.
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,[13] and anycasting.[14] Proximity—choosing the closest service node—is estimated using a variety of techniques including reactive probing, proactive probing, and connection monitoring.[11]
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[15][16] 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.[17] 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
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,
Large content distribution networks may even build and set up their own private network to distribute copies of content across cache locations.[23][24] 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 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.[25][26]
CDN trends
Emergence of telco CDNs
The rapid growth of
To address this,
Telco CDN advantages
Because they own the networks over which video content is transmitted,
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,[29][30] 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
This section needs additional citations for verification. (June 2021) |
In June 2011, StreamingMedia.com reported that a group of TSPs had founded an Operator Carrier Exchange (OCX)
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.[citation needed]
The Open Caching specification by
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
Virtual CDN (vCDN)
Virtualization technologies are being used to deploy virtual CDNs (vCDNs) with the goal to reduce
Image Optimization and Delivery (Image CDNs)
In 2017, Addy Osmani of
Arguably, the Image CDN term was originally a misnomer, as neither
While providing a universally agreed-on definition of what an Image CDN is may not be possible, generally speaking, an Image CDN supports the following three components:[41]
- A Content Delivery Network (CDN) for the fast serving of images.
- Image manipulation and optimization, either on-the-fly through URL directives, in batch mode (through manual upload of images) or fully automatic (or a combination of these).
- Device Detection (also known as Device Intelligence), i.e. the ability to determine the properties of the requesting browser and/or device through analysis of the
The following table summarizes the current situation with the main software CDNs in this space:[42]
Name | CDN | Image Optimization | Device Detection |
---|---|---|---|
Akamai ImageManager | Y | Batch mode | based on HTTP Accept header |
Cloudflare Polish | Y | fully-automatic | based on HTTP Accept header |
Cloudinary | Through Akamai | Batch, URL directives | Accept header, Client-Hints |
Fastly IO | Y | URL directives | based on HTTP Accept header |
ImageEngine | Y | fully-automatic | WURFL, Client-Hints, Accept header |
Imgix | Through Fastly | fully-automatic | Accept header / Client-Hints |
PageCDN | Y | URL directives | based on HTTP Accept header |
Tinify CDN | Multiple | fully-automatic | based on HTTP Accept header |
Notable content delivery service providers
![]() | This article is in prose. is available. (June 2024) |
Free
Traditional commercial
- Akamai Technologies[45]
- Amazon CloudFront[45]
- Aryaka
- Ateme CDN
- Azure CDN
- CacheFly
- CDNetworks[45]
- CenterServ[45]
- ChinaCache
- Cloudflare[46]
- Cotendo
- Edgio
- Fastly
- Gcore
- Google Cloud CDN
- HP Cloud Services
- Incapsula
- Instart
- Internap
- LeaseWeb
- Lumen Technologies
- MetaCDN
- NACEVI
- OnApp
- GoDaddy
- OVHcloud
- Rackspace Cloud Files
- Speedera Networks
- StreamZilla
- Wangsu Science & Technology
- Yottaa
Telco CDNs
- AT&T Inc.
- Bharti Airtel
- Bell Canada
- BT Group
- China Telecom
- Chunghwa Telecom
- Deutsche Telekom
- KT
- KPN
- Lumen Technologies
- Megafon
- NTT
- Pacnet
- PCCW
- Singtel
- SK Broadband
- Tata Communications
- Telecom Argentina
- Telefonica
- Telenor
- TeliaSonera
- Telin
- Telstra
- Telus
- TIM
- Türk Telekom
- Verizon
Commercial using P2P for delivery
- BitTorrent, Inc.
- Internap
- Pando Networks
- Rawflow
Multi
In-house
See also
- Application software
- Bel Air Circuit
- Comparison of streaming media systems
- Comparison of video services
- Content delivery network interconnection
- Content delivery platform
- Data center
- Digital television
- Dynamic site acceleration
- Edge computing
- Internet radio
- Internet television
- InterPlanetary File System
- IPTV
- List of music streaming services
- List of streaming media systems
- Multicast
- NetMind
- Open Music Model
- Over-the-top content
- P2PTV
- Protection of Broadcasts and Broadcasting Organizations Treaty
- Push technology
- Software as a service
- Streaming media
- Webcast
- Web syndication
- Web television
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{{cite web}}
: CS1 maint: numeric names: authors list (link) - ^ How Netflix works: the (hugely simplified) complex stuff that happens every time you hit Play
Further reading
- Buyya, R.; Pathan, M.; ISBN 9783540778868. Archived from the originalon 2017-09-27. Retrieved 2008-07-07.
- Hau, T.; Burghardt, D.; Brenner, W. (2011). "Multihoming, Content Delivery Networks, and the Market for Internet Connectivity". Telecommunications Policy. 35 (6): 532–542. .
- Majumdar, S.; Kulkarni, D.; Ravishankar, C. (2007). "Addressing Click Fraud in Content Delivery Systems" (PDF). Infocom. IEEE. .
- Nygren., E.; Sitaraman R. K.; Sun, J. (2010). "The Akamai Network: A Platform for High-Performance Internet Applications" (PDF). ACM SIGOPS Operating Systems Review. 44 (3): 2–19. S2CID 207181702. Retrieved November 19, 2012.
- S2CID 2861167.