This is the sixth article in a short series looking at the issues made topical by the proposed merger between Comcast and Time Warner Cable. In a previous article, we explained the role of Content Delivery Networks (CDNs) in delivering high-capacity content, such as streaming video. CDNs serve a large proportion of the content on the Internet today by placing it on servers at the edge of the network. This article looks at how they work in greater detail.
Most CDNs are operated as Software as a Service (SaaS) and shared between a number of clients. However, an increasing number of big content owners and telcos have built their own CDNs to improve content delivery and reduce demand on their own network infrastructures. In addition, a group of telcos have gotten together to create a federated CDN using all of their networks. Telcos have an advantage because they can cache content deep in their networks and bring it much closer to the end users than other providers.
A CDN may have tens of thousands of servers deployed in thousands of locations often using multiple backbones, although some CDN owners focus on building global high-performance networks and have a smaller number of sites. In peer-to-peer CDNs, the clients' servers are used to store content; the more clients there are, the more capacity there is to serve end users.
CDNs create web-caches that store copies of popular content on servers where they are most in demand. When a user makes a request for content hosted on a CDN, the DNS (Domain Name Server) chooses the optimal server using an algorithm that takes into account location, availability, cost and other variables. The site chosen may be the fewest hops away, the least number of network seconds, or possess the highest availability in terms of server performance. Some content owners choose to reduce costs rather than deliver the best possible performance, but the server closest to the end-user may well optimize both. Server load balancing, similar to that in cloud computing, is often used to create content clusters in which a layer 4-7 switch (see post) or content switch receives traffic to a virtual IP address and then allocates it across a number of real servers.
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