Content delivery network
A content delivery network or content distribution network (CDN) is a large distributed system of servers deployed in multiple data centers or "nodes" across the Internet. The goal of a CDN is to serve content to end-users with the intended benefit of reducing bandwidth costs, improving page load times, and/or increasing global availability of content. This is done by hosting the content on several servers, and when a user makes a request to CDN-hosted content, the domain name server (DNS) will resolve to an optimized server based on location, availability, cost, and other metrics.
Content providers such as media companies and e-commerce vendors pay CDN operators to deliver their content to their audience of end-users. In turn, a CDN pays Internet service providers (ISPs), carriers, and network operators for hosting its servers in their data centers. Besides better performance and availability, CDNs also offload the traffic served directly from the content provider's origin infrastructure, resulting in possible cost savings for the content provider. In addition, CDNs provide the content provider a degree of protection from denial-of-service (DoS) attacks by using their large distributed server infrastructure to absorb the traffic.
An increasing number of ISPs have built their own CDNs to improve on-net content delivery, reduce demand on their own infrastructure, and generate revenues from content customers. Additionally, some companies such as Microsoft, Amazon, and Netflix have built their own CDNs to tie in with their own products.
CDNs serve a large fraction of the Internet content today, including web objects (text, graphics, and scripts), downloadable objects (media files, software, documents), applications (e-commerce and portals), streaming media, and social networks. IT company Cisco Systems reported in June 2014 that 36 percent of global Internet traffic was delivered via CDNs in 2013, with that number expected to reach 55 percent by 2018.
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 hosts and clients. As a result, the core network is specialized, simplified, and optimized to only forward data packets. However, CDNs alter the 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 to optimize the data stream.
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 offer the fewest hops, the least 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 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 point-of-presences (PoPs) or locations, depending on the geographic coverage desired, such as United States, Asia-Pacific, international, global, etc. These sets of PoPs can be called "edges" or "edge networks" as they would be the closest edge of CDN assets to the end user. Those edge networks can later grow outward from their origin(s) through further acquisitions (via purchase, peering, or exchange) of co-location facilities, bandwidth, and servers.
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.A more recently defined and robust solution is provided by the Open Pluggable Edge Services (OPES) protocol. This architecture defines OPES service applications that can reside on the OPES processor itself or be executed remotely on a callout server.
In August 2011, a global consortium of leading Internet service providers led by Google announced their official implementation of "edns-client-subnet," which is intended to accurately localize DNS resolution responses. The initiative involves a limited number of leading DNS and CDN service providers. The general idea involves the situation where a CDN relies on the IP address of the DNS resolver, which can lead to incorrectly geo-locating a client if the client is using Google anycast addresses for their DNS resolver. This creates latency problems. With the edns-client-subnet extension mechanism for DNS (EDNS0), the recursive DNS servers of CDNs can instead utilize the IP address of the requesting client subnet when resolving DNS requests. However, not all CDN providers allow anycast for HTTP and/or support edns-client-subnet.
While most early CDNs served content using dedicated servers owned and operated by the CDN, some began to serve content via a hybrid model that uses peer-to-peer (P2P) technology. In the hybrid peer-to-peer model, content is served using both dedicated servers and other peer-user-owned computers. Unlike client-server systems, the content serving capacity of a peer-to-peer-capable CDN can actually increase as more users begin to access the content.
The rapid growth of streaming video traffic forces broadband providers to make large capital expenditures in order to meet this demand and to retain subscribers by delivering a sufficiently good quality of experience.
To address this, some telecommunications service providers (TSPs) have launched their own content delivery networks as a means to lessen the demands on the network backbone and to reduce infrastructure investments. As of December 2013, many of these CDNs, however, tend to be used primarily for internal use.
In May 2011, Cisco Systems senior director Chris Osika stated "the future of CDN models is a federated structure" and would likely run under a hybrid of bilateral agreements, bilateral hierarchical, and CDN exchange service models. A month later, a group of TSPs had founded the Operator Carrier Exchange (OCX) to interconnect their CDN networks and compete more directly against large traditional CDNs like Akamai and Limelight Networks, which have extensive PoPs worldwide.
Another example of a federated CDN arrived in the form of infrastructure as a service provider (IaaS) OnApp's CDNify in 2013, which was touted as being able to "tap into the spare capacity of service providers around the world, with more than 150 points of presence (PoPs) theoretically being at their disposal." However, a year later, the product was revamped, moving away from the federated CDN model due to limitations in what it could achieve.
This article reuses numerous content elements from the Wikipedia article.
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