What is a Content Delivery Network (CDN)

A content delivery network (CDN) is a large system of servers which are deployed in multiple data centers throughout the Internet. CDN’s excel at serving content to users with a focus on speed and availability.

CDNs serve a large collection of the Internet content currently, 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 networks.

In addition to better performance and availability, CDNs also offload the traffic served directly

Content Delivery Network

CDN’s excel at delivering content to users with speed and availability.

from the content provider’s origin infrastructure. This results in cost savings for the content provider. CDNs also provide the content provider with an amount of protection from DoS attacks by using their large distributed server infrastructure to absorb the attack traffic.  While several early CDNs served content using dedicated servers owned and operated by the CDN, there is a recent trend to use a hybrid model that uses P2P technology. In the hybrid model, content is served using both dedicated servers and other peer-user-owned computers as applicable.

Most CDNs are operated as an application service provider (ASP) on the Internet (also known as on-demand software or software as a service). Currently, an increasing number of Internet network owners have built their own CDNs along with their own products. A couple of examples of these are, Windows Azure CDN and Amazon CloudFront.

On these content (potentially multiple copies) might exist on several servers. When a user makes a request to a CDN hostname, DNS will resolve to an optimized server (based on location, availability, cost and other metrics) and that server will handle the request.

CDN nodes are usually deployed in multiple locations, often over multiple backbones. The benefits to this 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.

The Internet was designed according to the end-to-end principle. This principle keeps the core network relatively simple and moves the intelligence as much as possible to the network end-points, the host and clients. As a result the core network is specialized, simplified and optimized to only forward data packets.

Content Delivery Networks

Content Delivery Networks extend 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 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.

Server-load balancing uses one or more techniques including service-based (global load balancing) or hardware-based, also known as a web switch, content switch or multilayer switch to share traffic among a number of servers or web caches.

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.

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 serving capacity of peer-to-peer networks can actually increase as more users begin to access the content (especially with protocols such as Bittorrent that require users to share).

Telco CDNs

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

To come up with a solution to this, telecommunications service providers (TSPs) have begun to launch their own content delivery networks as a way to lower the demands on the network backbone and to reduce infrastructure investments.

Advantages to Telco CDN

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.

Federated CDNs

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.

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