Definition

CDN (content delivery network)

Contributor(s): Kate Gerwig

A CDN (content delivery network), also called a content distribution network, is a group of geographically distributed and interconnected servers that provide cached internet content from a network location closest to a user to accelerate its delivery. The primary goal of a CDN is to improve web performance by reducing the time needed to transmit content and rich media to users' internet-connected devices.

Content delivery network architecture is also designed to reduce network latency, which is often caused by hauling traffic over long distances and across multiple networks. Eliminating latency has become increasingly important, as more dynamic content, video and software as a service are delivered to a growing number of mobile devices.

CDN providers house cached content in either their own network points of presence (POP) or in third-party data centers. When a user requests content from a website, if that content is cached on a content delivery network, the CDN redirects the request to the server nearest to that user and delivers the cached content from its location at the network edge. This process is generally invisible to the user.

A wide variety of organizations and enterprises use CDNs to cache their website content to meet their businesses' performance and security needs. The need for CDN services is growing, as websites offer more streaming video, e-commerce applications and cloud-based applications where high performance is key. Few CDNs have POPs in every country, which means many organizations use multiple CDN providers to make sure they can meet the needs of their business or consumer customers wherever they are located.

In addition to content caching and web delivery, CDN providers are capitalizing on their presence at the network edge by offering services that complement their core functionalities.  These include security services that encompass distributed denial-of-service (DDoS) protection, web application firewalls (WAFs) and bot mitigation; web and application performance and acceleration services; streaming video and broadcast media optimization; and even digital rights management for video. Some CDN providers also make their APIs available to developers who want to customize the CDN platform to meet their business needs, particularly as webpages become more dynamic and complex.

How does a CDN work?

The process of accessing content cached on a CDN network edge location is almost always transparent to the user. CDN management software dynamically calculates which server is located nearest to the requesting user and delivers content based on those calculations. The CDN server at the network edge communicates with the content's origin server to make sure any content that has not been cached previously is also delivered to the user. This not only eliminates the distance that content travels, but reduces the number of hops a data packet must make. The result is less packet loss, optimized bandwidth and faster performance, which minimizes timeouts, latency and jitter, and it improves the overall user experience. In the event of an internet attack or outage, content hosted on a CDN server will remain available to at least some users.

Organizations buy services from CDN providers to deliver their content to their users from the nearest location. CDN providers either host content themselves or pay network operators and internet service providers (ISPs) to host CDN servers. Beyond placing servers at the network edge, CDN providers use load balancing and solid-state hard drives to help data reach users faster. They also work to reduce file sizes using compression and special algorithms, and they are deploying machine learning and AI to enable quicker load and transmission times.

CDNscache content from the origin server on geographically distributed CDN cache servers to reach users faster.

History of CDNs

The first CDN was launched in 1998 by Akamai Technologies soon after the public internet was created. Akamai's original techniques serve as the foundation of today's content distribution networks. Because content creators realized they needed to find a way to reduce the time it took to deliver information to users, CDNs were seen as a way to improve network performance and to use bandwidth efficiently. That basic premise remains important, as the amount of online content continues to grow.

So-called first-generation CDNs specialized in e-commerce transactions, software downloads, and audio and video streaming. As cloud and mobile computing gained traction, second-generation CDN services evolved to enable the efficient delivery of more complex multimedia and web content to a wider community of users via a more diverse mix of devices. As internet use grew, the number of CDN providers multiplied, as have the services CDN companies offer.

New CDN business models also include a variety of pricing methods that range from charges per usage and volume of content delivered to a flat rate or free for basic services, with add-on fees for additional performance and optimization services. A wide variety of organizations use CDN services to accelerate static and dynamic content, online gaming and mobile content delivery, streaming video and a number of other uses.

What are the main benefits of using a CDN?

The primary benefits of traditional CDN services include the following:

  • Improved webpage load times to prevent users from abandoning a slow-loading site or e-commerce application where purchases remain in the shopping cart;
  • Improved security from a growing number of services that include DDoS mitigation, WAFs and bot mitigation;
  • Increased content availability because CDNs can handle more traffic and avoid network failures better than the origin server that may be located several networks away from the end user; and
  • A diverse mix of performance and web content optimization services that complement cached site content.
This was last updated in May 2018

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