When content changes on the origin server, the content provider can use a software utility to manually or automatically rewrite embedded URLs on the origin server. An example of application-layer request routing that uses content modification is Akamai's FreeFlow content delivery service. Akamai’s FreeFlow Akamaizer software tool can automate URL modification.
CDN Peering
A CDN can quickly deliver content to users close to the CDN’s surrogates. However, no CDN can cover all the networks across the Internet. Thus, to offer fast content delivery to geographically disparate users, a content provider must subscribe to multiple CDN services.
To expand their services globally over more networks, CDNs need to link together and deliver content for each other’s customers as well as for their own. This implementation is called CDN peering or content peering.
Content delivery across multiple CDNs isn’t simple. When a user requests content that requires the involvement of two or more CDNs, the CDNs’ request-routing systems must work together to determine the best CDN for the user request. In August 2000, several vendors formed two industry alliances—Content Alliance and Content Bridge—to help develop CDN peering. To develop content peering architecture and standards, IETF formed the Content Distribution Internetworking Group in December 2000. The group includes members from both alliances and has published several drafts about content peering architecture and standards. In January 2001, Content Bridge began delivering its content peering service, in which a content provider can access the services of any Content Bridge member by subscribing to CDN service from one vendor in the alliance.
CDN services that have a content peering agreement must share core CDN services, including content distribution and request routing. To do so, CDNs use a Content Peering Gateway (CPG) service to link services to one another. The CPG provides the peering functionality. Figure 5 shows an example CDN peering architecture that involves three CDNs.
Through content distribution peering, a CDN can redistribute the content from a content provider’s origin server to other CDNs’ CPGs. In turn, these CPGs pass the content to the necessary surrogates. To distribute content, peering CDNs can use either the push or pull method. For multiple CDNs to be able to deliver content, the peering CDNs’ request-routing systems must work together to serve the content’s namespace. For this setup to work, a content provider must delegate authority for its content URLs to an authoritative request-routing system on the CPG of the CDN to which the content provider subscribes. This authoritative request-routing system further delegates authority to the request-routing systems of all peering CDNs. For DNS request routing, the content provider delegates a subdomain to the authoritative request-routing DNS server, which in turn delegates the subdomain to the request-routing DNS servers of peering CDNs.
When a CDN’s authoritative request-routing system receives a user request, it negotiates with all peering CDN CPGs to determine which CDN should serve the request. The request-routing system then directs the user request to that CDN. The selected CDN uses its internal request-routing mechanism, which is transparent to the authoritative request-routing system and other CDNs, to determine which surrogate should deliver the content to the user. The selected surrogate then delivers the requested content to the user. At the same time, the surrogate reports the accounting information for the delivered content to the accounting system of the surrogate’s CDN. One CDN can request accounting information from or send it to another CDN through the CPG accounting peering function. A billing organization, such as a third-party clearinghouse firm, handles the charge and payment process of content delivery among peering CDNs.
Content Meets Express Delivery
CDN technology is brand new. However, the number of organizations that use CDNs is growing quickly. During an online CDN seminar in March 2001, Content Bridge conducted a survey about which techniques organizations had used to improve content delivery performance. The survey results showed that 25 percent of seminar participants had used CDNs, 15.6 percent had deployed cache servers, 34.4 percent had used mirrored servers, and the remaining 25 percent had tried all three methods.
CDNs are still mainly proprietary services. However, industry alliances and IETF are developing CDN peering architecture and standards that will eventually evolve into a public network in which multivendor products and services use Internet CDN standards to interoperate. Are you ready for your Web content to meet express Internet delivery services?
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