Content Distribution Network:

A Content Delivery Network or Content Distribution Network (CDN) is a system of computers networked together across the Internet that cooperate transparently to deliver content most often for the purpose of improving performance, scalability, and cost efficiency, to end users.

CDN Benefits

The capacity sum of strategically placed servers can be higher than the network backbone capacity. This results in a impressive increase in the number of concurrent users.

Strategically placed edge servers decrease the load on interconnects, public peers, private peers and backbones, freeing up capacity and lowering delivery costs.

Assets are delivered via fewer hops, resulting in lower latency and increased delivery speed. End users should experience less jitter, fewer network peaks and surges, and improved stream quality - especially in remote areas.

The increased reliability allows a CDN operator to deliver HD quality content with high QoS, low costs and low network load.

Assets are dynamically, strategically placed on redundant core, fallback and edge servers. CDN's have automatic server availability sensing with instant user redirection. A CDN can offer 100% availability, even with large power, network or hardware outages.

A CDN gives more control on asset delivery and network load. Optimize capacity per customer, view realtime load and statistics, see popular assets, see active regions and report exact viewing details to your customers.

ASP versus on-net

Most CDN's are operated as a ASP on the Internet, although an increasing number of internet network owners are building their own CDN to improve on-net content delivery and to generate revenues from content customers. Some develop internal CDN software, others use commercially available software.

Current Market Overview

As of 2008, the top 3 global Content Delivery ASP networks are Akamai, Limelight and CDNetworks,^[1] forecasted by Tier1 Research to account for roughly 79% of the U.S. market revenue for the year.^[2]

CDN Service Providers are not likely to share details. Although some are moving a lot of data, they are not profitable. Others refuse to share the number of assets or streams they have delivered. A limited number of smaller or regional operating CDN's do publish rates, volumes and are profitable.

The CDN market is getting crowded. In 2008 Akamai won a lawsuit against Limelight Networks, claiming it copied it's patented dynamic DNS redirection technologies. Level3 has bought many CDN related patents and also started a lawsuit against Limelight Networks.

Technology

Traditional CDN's focus on web acceleration. That is still their main revenue service. New CDN's have integrated media delivery services so they are optimized for live video streaming, high definition video and large asset delivery.

CDN nodes are deployed in multiple locations, often over multiple backbones. These nodes cooperate with each other to satisfy requests for content by end users, transparently moving content to optimize the delivery process. Optimization can take the form of reducing bandwidth costs, improving end-user performance, 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 PoPs. Others build a global network and have a small number of geographical PoPs.

Requests for content are 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 or fewest number of network seconds away from the requesting client, so as to optimize delivery across local networks. When optimizing for cost, locations that are least expensive may be chosen instead.

In a optimal scenario, these two goals tend to align, as servers that are close to the end user may have an advantage in serving costs, perhaps because they are located within the same network as the end user. However, when streaming to Asia for instance, it is cheaper to distribute from Europe or the US than from Asian located servers. It depends on the CDN operator if he chooses the best performing or the cheapest route.

Content networking techniques

The Internet was designed according to the end-to-end principle ^[3]. 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.^[4]. These techniques are briefly described below.

Because closer is better, web caches store popular content closer to the user. 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 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 advantages 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^[5], and anycasting^[6]. Proximity—choosing the closest service node—is estimated using a variety of techniques including reactive probing, proactive probing, and connection monitoring.

Simple CDN's require manual asset copying. Earlier CDN's used active web caches and global hardware load balancers. Modern CDN's use cheap and simple edge servers and intelligent central CDN management technologies that distribute assets dynamically.

Content service protocols

Several protocols 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^[7] ^[8] 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^[9]. 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 personalization. This creates a problem for caching systems. To overcome this problem a group of companies created ESI.

P2P CDN's

Althoug Peer2Peer is not traditional CDN technology,it is increasingly used to deliver content to end users. P2P claims low cost and efficient distribution. There is however a downside to P2P usage, because the lower delivery costs only apply to the content owner. P2P actually uses much more traffic to deliver the same asset, increasing the delivery costs for the network owners. P2P can be very disruptive for internet access providers and backbone operators. There are more downsides of P2P, for instance the lack of QoS control and measurement.

Content Distribution Network

Contents

CDN Benefits

ASP versus on-net

Current Market Overview

Technology

Content networking techniques

Content service protocols

P2P CDN's

See also

Related Technologies

Commercial CDNs

Commercial CDN solutions

Academic CDNs

Source(s):

Notes

External links