Access and Retrieval

Figure 2 illustrates how files are accessed or retrieved in RCDS.

  
Figure 2: Accessing a file via RCDS.

1. A user acquires a URN of a resource that seems to suit his needs from a search service, hypertext link, or other means. This URN is resolved using DNS (see below) to find the network addresses of one or more resource catalog servers. One of those servers is selected by the client, perhaps based on network proximity estimates.
2. The resource catalog server is queried for the description of the resource named by the URN. This description may itself contain sub-descriptions for each of several versions of a resource (which might vary according to time created, medium, language, etc.). Each of these sub-descriptions will contain a LIFN for that particular version. The client selects a particular LIFN from those available.
3. The client resolves the LIFN using DNS to find the network addresses of one or more location servers. One of those location servers is then queried for locations of the file named by that LIFN.
4. The location server returns one or more URLs at which the file can be obtained.
5. The client chooses one of those file servers (again, perhaps based on network proximity estimates) and fetches the file from that server.

The interaction with RCDS may be accomplished either directly by a client, or via a proxy server which communicates with the client via HTTP. The proxy arrangement is shown in Figure 3.

  
Figure 3: Accessing a file via RCDS, using a proxy server.