One feature is particularly important in a CDN's architecture and that is its approach to request routing. In the early UCL MICE days, multicasts on their MBONE network were announced using a Session Description Protocol (.sdp) file that essentially contained the multicast address and application port on which to listen for the multicast signal.
In a predominantly unicast world, and one that was evolving surrounded by the explosive World Wide Web, discovery of multimedia would typically be reasonably unstructured. Requests for the content might be generated by any number of technical devices and software. This itself presented many issues, central to which was ensuring that the end user would be directed to the CDN's proxy server.
This process is called “request routing” There are two main schisms of request routing: passive and active request routing.
Passive Request Routing
These routings typically are not specifically user, or content aware, and they much more concern the pure networking of the contents request. The main topics relating to CDNs are DNS, ANYCAST, and routing protocols such as OSPF.
A well-managed DNS is a central part of the CDN architecture. In a single DNS reference it is possible to provide a list of server IP addresses that the single DNS can refer to. Should any one of the addresses not be available, the DNS lookup will return a further IP address from the list. This is relatively First In-First Out and cycles - and hence is known as “Round Robin DNS" Round Robin DNS can provide an extremely simple way to ensure that a service is always available. However, there is no way to simply use DNS in this way to route a request to the users “nearest” proxy server.
IP ANYCAST is a networking technique where the same IP address is advertised in multiple locations, and the network processes such as Open Shortest Path First (OSPF) determine which is the closest to route the request to. ANYCAST is an excellent first step in refining the route to a regional geographical location, while purely using resources within the network.