Content Delivery Networks (CDNs)

- CDNs for Content Delivery Services

A content delivery network (CDN), or content distribution network (CDN), is a geographically distributed network of proxy servers and their data centers. The goal is to provide high availability and performance by distributing the service spatially relative to end users. 

A CDN is a globally distributed network of web servers or Points of Presence (PoP) whose purpose is to provide faster content delivery. The content is replicated and stored throughout the CDN so the user can access the data that is stored at a location that is geographically closest to the user. This is different (and more efficient) than the traditional method of storing content on just one, central server. A client accesses a copy of the data near to the client, as opposed to all clients accessing the same central server, in order to avoid bottlenecks near that server. 

CDNs are a layer 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 Internet service providers (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: video streaming, software downloads, web and mobile content acceleration, licensed/managed CDN, transparent caching, and services to measure CDN performance, load balancing, Multi CDN switching and analytics and cloud intelligence. CDN vendors may cross over into other industries like security, with DDoS protection and web application firewalls (WAF), and WAN optimization.

- History Short Description

CDNs came into existence in the late 1990s as a means for alleviating the performance bottlenecks of the Internet, even as the Internet was starting to become a mission-critical medium for people and enterprises. Since then, CDNs have grown to serve a large portion of the Internet content today, 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 media sites. 

[(Left) Single Server Distribution and (Right) CDN Scheme of Distribution - Wikipedia]

- Remote Points of Presence (PoPs)

High content loading speed = positive User Experience. If all data is located on a central server, the User Experience is negatively affected by limited loading speed. The greater the distance between two objects in communication (in this case, the user and the server), the longer it takes for the content to reach any of these objects.

CDN nodes are usually deployed in multiple locations, often over multiple Internet backbones. Benefits 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. 

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 hops, the lowest number of network seconds away from the requesting client, or the highest availability in terms of server performance (both current and historical), so as to optimize delivery across local networks. When optimizing for cost, locations that are 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 or cost. 

Most CDN providers will provide their services over a varying, defined, set of PoPs, depending on the coverage desired, such as United States, International or Global, Asia-Pacific, etc. These sets of PoPs can be called "edges", "edge nodes" or "edge networks" as they would be the closest edge of CDN assets to the end user.[5]

- Private Content Delivery Networks 

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, reverse proxies or application delivery controllers. It can be as simple as two caching servers, or large enough to serve petabytes of content. 

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 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 multicasting 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.

Many large companies are evaluating private content delivery networks (CDNs) to securely distribute customized content so that only specific business units, user groups or individuals can gain access to it. From streaming Internal corporate communications videos to distributing subscription-based content, the applications supported by private CDNs require consistent, reliable and secure on-demand performance. 

- The Future of Content Delivery Networks

Even though it is hard to accurately predict what the future of CDNs will be, one thing is certain: this industry will grow exponentially, which means that the technologies used to run them will develop even further. The main goals for the future are performance, availability and security. 

[More to come ...]