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Content delivery networks have become a critical component in supporting today's high-volume internet traffic. While shared CDN services offered by providers, including Akamai Technologies, CenturyLink -- formerly Level 3 Communications -- and Limelight Networks, supply excellent content delivery services for a wide variety of enterprises, others find their requirements can be met only by implementing private CDNs that employ dedicated edge servers to handle the traffic load.
Content delivery networks accelerate web response by moving content closer to the end user. CDNs consist of an origin server at the site where web content is initially created and at a number of network edge servers at points of presence (POPs) throughout the internet. Servers are often in locations where networks interconnect to provide connections to multiple service provider networks. Edge servers may also be placed at POPs that offer access to a single service provider's network.
The initial user request for a page or file travels to an edge server that determines whether the requested page has been cached at the network edge. If not, the edge server then routes the user request to the origin site, which responds, sending the content back to the edge server and then on to the end user. The edge server caches commonly accessed pages and responds to subsequent user requests with cached content to reduce the load on the central site.
Who needs private CDN services?
The negative aspects of distributing content via a shared CDN are summed up in the word shared. A shared service maintains content for a wide variety of companies. Subscribers share storage, network bandwidth and CPU cycles. Cache update algorithms are a compromise across varying subscriber requirements.
Shared CDN services may not be a problem for businesses with static webpages, because storage requirements don't change rapidly, and relatively little network or processor load is required for adequate performance. Other types of web traffic place demands on server resources that can be met only by a private CDN, because its edge servers are completely dedicated to an individual company. All configured storage, compute and network bandwidth resources are available when required.
Video is a good example. Video places strict requirements on server and network performance and has become the largest factor in the growth of private CDNs. A fraction-of-a-second delay in an online video will be apparent to a viewer. Private edge servers can be implemented with processors, storage and network components chosen to meet video's requirements. Software can be configured to support the functions required by video -- and only those functions.
The need for data security is also a factor in the growth of private CDNs. Companies with strict security requirements may not feel confident placing data on a shared server. A private server can be configured with security components selected specifically to deal with the types of threats and level of security required by the firm's data.
The hybrid CDN services option
Firms choosing private CDN services can determine where to place edge servers. Shared services often place servers in major cities where the most users are located. Enterprises know if their customers are concentrated in a specific area, and multiple edge servers can be placed there to serve those customers, with minimal or no servers located in areas without customers.
A combination that uses shared CDN providers' services along with a private service can be the best choice. A shared CDN service can support users in heavily populated areas, while private edge servers can be deployed in places where shared services have no POPs. A private edge server supporting an e-commerce site can cache pages describing products that are offered, of course. But, more importantly, it can terminate encrypted connections. An encrypted link is created when a customer begins a transaction and terminates when the transaction is complete. Details of the order and responses to the end user must travel back and forth from the central site, but terminating encryption links at the edge server free the central site from the burden of creating and shutting down an encrypted link for each transaction. Each edge server creates a VPN to the central site when it comes online and remains up until either the edge or central site server is shut down.
In terms of cost, shared CDN services' prices are based on the amount of bandwidth consumed and the number of requests per second to the edge servers. For organizations generating a high volume of traffic, shared service charges can become more expensive than the cost of maintaining a private service. Also, shared service charges can vary from month to month, as traffic rises and falls. The costs of private CDNs are predictable, because hardware, support and the charges for use of POPs are relatively fixed.
Security issues are always front and center. Distributed denial-of-service attacks have become a serious concern. An attack against one subscriber on a shared CDN service can affect all the subscribers. Private CDNs can also be attacked, but the owners of private CDNs will not be affected by an attack against an unrelated firm.
How to build private CDNs
Building private CDNs requires determining where to place edge servers and choosing hardware and software adequate to support expected traffic. Enterprises need to choose POP locations and have agreements in place for their use. They also need to choose a distributed domain-name-system service that can direct end-user requests to the nearest edge server.
Companies don't have do all of the steps required to build a private CDN in-house. Vendors, including NS1, OnApp and Varnish Software, all offer help to choose and configure hardware and software and to maintain POPs throughout the network. Some network service providers and shared CDN services providers also maintain and rent space for private edge servers at their POPs.
Video, e-commerce and online financial services have been the main factors driving the growth of private CDNs. Combine the expected continued increase in these types of network traffic with the availability of products and services that reduce the complexity of building private CDN services, and the growth of private CDNs is likely to continue.