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Figure 2 illustrates how files are accessed or retrieved in RCDS.
Figure 2: Accessing a file via RCDS.
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.
- 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.
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.
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.
The location server returns one or more URLs at which the file can be
The client chooses one of those file servers (again, perhaps based on
network proximity estimates) and fetches the file from that server.
Figure 3: Accessing a file via RCDS, using a proxy server.
Fri Feb 7 11:53:58 EST 1997