[XUS] An XML URN Scheme

Discussion in 'XML' started by Philippe Poulard, Feb 25, 2005.

  1. I don't know if anybody already designed an URN scheme for XML; here is
    a proposal: I call it the "XML URN Scheme", or XUS.


    URNs are logical names used to identify resources. XUS defines an URN
    Scheme for XML documents that works on a delegation model.
    As XUS (XML URN Scheme) is used exclusively for XML documents, it uses
    namespace URIs to identify this scheme and the delegate scheme.

    1 Using XUS
    An URI that intends to follow the XML URN Scheme is called a XUS URN or
    a XUS URI. XUS URNs are working on a delegation model : when an URN is
    recognized to belong to the XML URN Scheme defined here, the last part
    of the URN is interpreted with a delegate scheme.
    The XUS scheme and the delegate scheme are both identified with a
    namespace URI, and expressed in a XUS URN with a related prefix, as
    explained below.

    * XUS syntax:
    The syntax of an URI is the following :
    where the characters : and # stands for themselves.
    The syntax of a XUS URI is the following :
    where the characters : and # stands for themselves and:
    * urn is a string that stands for itslef and denotes that the URI
    is an URN
    * xus-prefix is a prefix bound to the XUS namespace URI :
    http://www.inria.fr/xml/xus/xml-urn-scheme. The usual prefix is xus, but
    any other prefix bound to the XUS namespace URI suits.
    * scheme-prefix is a prefix bound to the namespace URI of the
    delegate scheme.
    * scheme-part is the part scheme that depends on the delegate
    scheme identified by its namespace URI.

    2 Namespace binding
    XML URN Schemes are found generally within XML documents inside which
    the appropriate namespace declarations can be found. In this case,
    theses declarations must be used if the XUS URN is in the scope of the
    declarations, as explained in the relevant specification.
    However, when used outside the scope of an XML document, a XUS
    interpreter may use a given map of prefixes linked to namespace URIs, or
    with a component that resolves prefixes, usually known as "namespace

    * Precautions
    A XUS URI may be used anywhere within an XML document with no particular
    precaution, except when used in the DOCTYPE declaration, in an external
    entity, or in the DTD internal subset. In fact, each time a reference is
    made with a XUS URI before reading the XML content may cause troubles.
    In this cases, the namespace binding must be provided in addition to the
    For this reasons, it is recommended to use XUS URIs within the XML content.

    * Namespace expansion
    A XUS URI may be resolved to a raw expanded URI for comparison purpose.
    The expansion consist on replacing the prefixes by their corresponding
    namespaces ; the expansion is completed by escaping the characters of
    the namespaces as specified in RFC 2396.

    3 XUS interpretation
    A XUS interpreter is a program that decodes XUS URNs.
    A XUS interpreter used to decode a XUS URN fails to resolve such an URN
    when :
    * The xus-prefix prefix is not bound to the XUS namespace URI ; the
    XUS interpreter must report that the URN to resolve is not a XUS URN.
    * The scheme-prefix prefix is not bound to a namespace URI ; the
    XUS interpreter must report that the delegate scheme is missing.
    * The scheme-prefix prefix is bound to the XUS namespace URI ; the
    XUS interpreter must report that the delegate scheme is invalid.
    * The delegate scheme is not known by the interpreter ; the XUS
    interpreter must report that the delegate scheme is unknown.
    * The delegate scheme fails ; the XUS interpreter must report that
    the delegate scheme fails.
    A delegate scheme is known by the XUS interpreter if this scheme has
    itself an interpreter that can be invoked by the XUS interpreter.
    If the XUS interpreter succeed in decoding the URI, it returns the
    following components :
    scheme the string urn
    scheme-specific-part the string that follows urn:
    xus-prefix the prefix used for the XML URN Scheme
    xus-uri the XUS namespace URI http://www.inria.fr/xml/xus/xml-urn-scheme
    scheme-prefix the prefix used for the delegate scheme
    scheme-uri the namespace URI of the delegate scheme
    scheme-part the part scheme that follows scheme-prefix:
    scheme-components the components returned by the delegate scheme

    4 Example
    <?xml version="1.0" encoding="iso-8859-1"?>
    <p><a href='urn:xus:msps:acme-fields-here'>My document</a></p>

    (. .)
    | Philippe Poulard |
    Philippe Poulard, Feb 25, 2005
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