Content delivery network

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(Left) Single server

distribution

(Right) CDN scheme of distribution

content delivery network

(

CDN

) or

content distribution network

is a

geographically distributed network of

and corresponding

CDNs provide high

and performance ("speed") through geographical

distribution

relative to

, and arose in the late 1990s to alleviate the

performance bottlenecks of the

as it was becoming a critical medium.

Since then, CDNs have grown to serve a large portion of Internet content, including

text, graphics and

, downloadable objects (media files, software, and documents), applications (

media, on-demand streaming media, and

services.

CDNs are a

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:

, software downloads,

web and mobile content acceleration, licensed/managed CDN, transparent caching, and services to measure CDN

performance,

, Multi CDN switching and analytics and cloud intelligence. CDN vendors may cross over into

other industries like security,

protection and

(WAF), and WAN optimization.

Content delivery service providers include

, Qwilt (Cisco),

CDN77, BunnyCDN, EdgeNext, and

Technology

[

]

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

. 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

(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

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 and cost.

Most CDN providers will provide their services over a varying, defined, set of PoPs, depending on the coverage

desired, such as United States, Asia-Pacific, International or Global, 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

using their network, or profit from the user

analytics and tracking data collected as their scripts are being loaded onto customers' websites inside their

. As such these services are being pointed out as potential privacy intrusions for the purpose of

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

(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.

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

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, optimized, and simplified to only forward data packets.

Content Delivery Networks extend the 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.

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.

, 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

. Traffic arriving at the

switch is then directed to one of the real

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

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

(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

(OPES) protocol.

This architecture defines OPES service applications that can reside on the

OPES processor itself or be executed remotely on a Callout Server.

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

[

]

Further information:

(P2P)

content-delivery networks, clients provide resources as well as use them. This means that,

unlike

systems, the content-centric networks can actually perform better as more users begin to access

the content (especially with protocols such as

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,

and

technologies can be used: participating

nodes receive

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,

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

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

traffic

required large

by broadband providers

in order

to meet this demand and retain subscribers by delivering a sufficiently good

To address this,

have begun to launch their own content delivery networks as a

means to lessen the demands on the

and reduce infrastructure investments.

Telco CDN advantages

[

]

Because they own the networks over which video content is transmitted,

CDNs have advantages over traditional

CDNs. They own the

and can deliver content closer to the end-user because it can be cached deep in their

networks. This deep caching minimizes the

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,

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

Please help

in this section. Unsourced

material may be challenged and removed.

(

June 2021

)

(

)

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

interconnect their networks and compete more directly against large traditional CDNs like

and

, which have extensive PoPs worldwide. This way, telcos are building a Federated CDN offering, which is more

interesting for a

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

defines a set of

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

[

]

The latency (RTT) experienced by

clients with non-local resolvers

("high") reduced drastically when

a CDN rolled-out the EDNS0

extension in April 2014, while the

latency of clients with local

resolvers are unimpacted by the

change ("low").

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

which is intended to accurately localize DNS resolution responses. The initiative involves a

limited number of leading DNS

service providers, such as

and CDN service providers as well. With

the edns-client-subnet

, 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

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

and

Image Optimization and Delivery (Image CDNs)

[

]

In 2017, Addy Osmani of

started referring to software solutions that could integrate naturally with the

paradigm (with particular reference to the <picture> element) as

Image CDN

The expression

referred to the ability of a web architecture to serve multiple versions of the same image through HTTP, depending on

the properties of the browser requesting it, as determined by either the browser or the server-side logic. The purpose

of Image CDNs was, in Google's vision, to serve high-quality images (or, better, images perceived as high-quality by

the human eye) while preserving download speed, thus contributing to a great

(UX).

[

]

Arguably, the

Image CDN

term was originally a misnomer, as neither

nor Imgix (the examples quoted by Google

in the 2017 guide by Addy Osmani) were, at the time, a CDN in the classical sense of the term.

Shortly afterwards,

though, several companies offered solutions that allowed developers to serve different versions of their graphical

assets according to several strategies. Many of these solutions were built on top of traditional CDNs, such as

and

. At the same time, other solutions that already provided an image multi-

serving service joined the Image CDN definition by either offering CDN functionality natively (ImageEngine)

integrating with one of the existing CDNs (Cloudinary/Akamai, Imgix/Fastly).

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:

A Content Delivery Network (CDN) for the fast serving of images.

Image manipulation and optimization, either on-the-fly through

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

string,

Accept headers, Client-Hints or

The following table summarizes the current situation with the main software CDNs in this space:

Main Image CDNs on the market

Name

CDN

Image Optimization

Device Detection

Akamai ImageManager

Batch mode

based on HTTP Accept header

Cloudflare Polish

fully-automatic

based on HTTP Accept header

Cloudinary

Through Akamai

Batch, URL directives

Accept header, Client-Hints

Fastly IO

URL directives

based on HTTP Accept header

ImageEngine

fully-automatic

, Client-Hints, Accept header

Imgix

Through Fastly

fully-automatic

Accept header / Client-Hints

PageCDN

URL directives

based on HTTP Accept header

Tinify CDN

Multiple

fully-automatic

based on HTTP Accept header

Content delivery service and technology providers

[

]

This article

is in

format but may read better as

You can help by

, if appropriate.

is available.

(

June 2024

)

Commercial or free software vendors (build your own CDN)

[

]

BlazingCDN

Go-Fast CDN

Velocix (spin off Nokia)

Free-as-a-Service

[

]

Commercial-as-a-Service

[

]

CDN

BaishanCloud EdgeNext

BelugaCDN

Bunny.net

BytePlus

CDN77

(ChinaNetCenter)

CDNsun

ChinaNetCenter

(Pulse)

GlobalConnect

Cloud

Jet-Stream Cloud

KeyCDN

5centsCDN

Cloud

MainStreaming

Medianova

Microsoft

Netskrt

Ngenix

Ora Streaming (

Software Group)

ProCDN.net

Qwilt

Others

[

]

CDN

Jet-Stream

Telco CDNs

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]

CDN

Commercial using P2P for delivery

[

]

Ant Media

Milicast

Goalbit Solutions

Teltoo

Hola SparkCDN

Livepeer

Nano Cosmos

Novage

PeerFlow

Peervadoo

Phenix Real Time Solutions

Quanteec

Teleport Media

Theta EdgeCloud

Multi

[

]

Jet-Stream

NPAW CDN Balancer

Velocix

Warpcache

In-house

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]