page_title: Content delivery network interconnection
section_title: Exemplary operation of CDNI interfaces in content delivery
hierarchical_section_title: Content delivery network interconnection / Exemplary operation of CDNI interfaces in content delivery
caption_attribution_description: An example of iterative DNS redirection of content request in CDNI.
context_page_description: Content delivery network interconnection is a set of interfaces and mechanisms required for interconnecting two independent content delivery networks that enables one to deliver content on behalf of the other. Interconnected CDNs offer many benefits, such as footprint extension, reduced infrastructure costs, higher availability, etc., for content service providers, CDNs, and end users. Among its many use cases, it allows small CDNs to interconnect and provides services for CSPs that allows them to compete against the CDNs of global CSPs.
context_section_description: The sequence diagram presented in the figure below provides some details on CDNI and the iterative DNS redirection operation. In the depicted example, a UE downloads content from the address cdn.csp.com/foo, which is primarily delivered by the CDN-A on behalf of a CSP with the address csp.com.
  Before any request redirection, the CDN-B (dCDN) announces information on supported footprint and capabilities.
  The UE performs a DNS lookup for a server cdn.csp.com in the domain of the CSP from which it is going to download the content.
  A request router in CDN-A (uCDN) servicing the domain cdn.csp.com processes the request and recognises, based on the source IP address of the request, that the end user could be better served by the dCDN. Therefore, it performs an inquiry in dCDN to determine if it is willing and able to serve this request.
  If the dCDN is able to handle the request, the request router in uCDN returns a DNS CNAME response. This response contains a new domain, e.g. b.cdn.csp.com, indicating dCDN and the original domain and an NS record that maps this new domain to a request router in dCDN.
  The UE does a DNS lookup using the new domain (b.cdn.csp.com). A request router in dCDN responds to this request with the IP address of a suitable delivery node.
  The UE requests the content /foo from the delivery node in dCDN. At this point, the delivery node receives the real IP address of the UE and the information on the requested content. If the redirections in previous steps were incorrect, the delivery node could perform a HTTP redirection.
  If the metadata for content /foo is not available in dCDN, the metadata interface is used to request it from the uCDN.
  If the request is going to be served, i.e. metadata restrictions were met and a cache miss occurs, the delivery node in dCDN must start the acquisition process. The delivery node does a DNS lookup for an internal domain address op-b-acq.op-a.net. The uCDN recognises that the request is from a dCDN, rather than from a UE, and returns an IP address of a delivery node in the uCDN.
  The content /foo is delivered to the delivery node in dCDN from the delivery node in uCDN.
  The content /foo is delivered to the UE from the delivery node in dCDN.
  After some time, the uCDN may instruct the dCDN to purge the content /foo to ensure that it is not delivered again.
  After the content is delivered a log of delivery actions is provided to the uCDN.
caption: An example of iterative DNS redirection of content request in CDNI.
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