This approach is designed to interoperate with the trivial URC scenario described above. An SGML Document Type Definition (DTD)[40] specifies valid attributes which may be associated with a resource. These are based on the Dublin Metadata Core Element Set[41]. This defines the following set of metadata elements for document-like objects:
Using this DTD, the URC from the previous examples could be written as follows:
<urc> <urn>urn:x-dns-2:shells.unix.computing.subjects.int:zsh</urn> <author>Coleman, Richard</author> <author type="email">zsh@math.gatech.edu</author> <title>The Z-shell</title> <subject scheme="abstract"> A command interpreter for many UNIX systems which is freely available to anyone with FTP access. Zsh is more powerful than every other common shell (sh, ksh, csh, tcsh and bash) put together. </subject> <instance> <coverage>Canonical distribution site</coverage> <url>ftp://ftp.math.gatech.edu/pub/zsh</url> </instance> <instance> <coverage>A mirror site in the US</coverage> <url>ftp://ftp.sterling.com/zsh</url> </instance> <instance> <coverage>A mirror site in France</coverage> <url>ftp://ftp.cenatls.cena.dgac.fr/pub/shells/zsh</url> </instance> <instance> <coverage>A mirror site in the UK</coverage> <url>ftp://mrrl.lut.ac.uk/zsh</url> </instance> </urc>
As is to be expected from the use of SGML, this provides the opportunity to be far more precise about the information contained within the URC - and in a form which is readily suited to automatic processing.
It should be noted that whilst this form of the URC may be expressed as an SGML DTD, this does not imply that a full-blown SGML parser is necessary in order to make use of it. Since the number of elements has purposely been kept small, implementing a URC parser would be a task with the same order of complexity as found in existing HTML parser implementations.