The QName URN Namespace

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This specification defines a Uniform Resource Name namespace for XML namespace-qualified names, QNames. As long as the URN is encoded in the same character set as the document containing the original QName, the Qname URN provides enough information to maintain the semantics, and optionally the exact syntax, of the original name.¶

This specification defines a Uniform Resource Name namespace for XML namespace-qualified names, QNames. As long as the URN is encoded in the same character set as the document containing the original QName, the Qname URN provides enough information to maintain the semantics, and optionally the exact syntax, of the original name.¶

There are a variety of situations when a QName may need to be mapped to a URI. For example, when exchanging (or referencing) an identifier for an XML element contained within a document, and the medium of exchange prefers URIs to QNames, such as an XML Schema anyURI data type. Another scenario is for comparing the identifiers, which can be simpler by comparing just a string without having to also compare the context setting XML namespace attribute that may be declared arbitrarily earlier in the document.¶

The XML Namespaces specification [3] does not provide a canonical mapping between QNames and URIs. Any XML specification that wants to enable identifier exchanges must define a language specific QName to URI mapping. There have emerged a variety of different algorithms and solutions for the mapping. To date, there have been no standardized algorithms available that they can re-use, which has increased their efforts. A standardized mapping, such as this, should provide increased productivity.¶

Almost all of the algorithms for Qname to URI mappings are based upon concatenation of the URI and the name with variations based upon prefix inclusion, namespace name and name separator, etc. These are typically problematic because it is difficult to recover the QName from the URI as the namespace name and name separator may have already been used in the namespace name. Having the namespace name at the end of the identifier string avoids these and other problems.¶

The following paragraphs contain the URN namespace registration data, as defined in [5].¶

Namespace ID:

qname

Registration Information:

Version number: 1
Registration date: 2004-11-30

Declared registrant of the namespace:

The W3C Technical Architecture Group (TAG), reachable at tag@w3.org.

Declaration of syntactic structure:

The QName URN is structured as four colon-separated fields. Note that colons within the fourth field, the URI part, are not significant; the entire fourth field is treated as a single opaque entity by this URN scheme.

The first field identifies the naming scheme. The second contains the QName prefix, or an empty string if the QName comes from the default namespace, or an asterisk if the prefix is not significant.

A QName URN is defined by the following ABNF [6]:

qnameURN               = "qname" ":" prefix ":" localname ":" uri
prefix                 = ncname / "" / "*"
localname              = ncname
uri                    = <any valid URI>
ncname                 = <see production 4 of [2]>

Here are three examples of a QName URN:

urn:qname:foo:OK:http://example.com/ws/foo.xsd
urn:qname::OK:http://example.com/ws/foo.xsd
urn:qname:*:Reject:http://w3.org/2002/xkms#

The first correspond to the following element content QNames (the element name is not significant):

<foo xmlns:foo="http://example.com/ws/foo.xsd">foo:OK</foo>
<foo xmlns="http://example.com/ws/foo.xsd">foo:OK</foo>

The third example would match both of the others, as well as an inifinite number of QNames, since the namespace prefix is explicitly marked as "don't-care."

Relevant ancillary documentation:

[1] [3]

Identifier uniqueness considerations:

An XML QName is semantically defined as a (namespace-uri, localname) pair; the namespace prefix is not significant. For some applications, such as signature functions, the prefix is important and must be preserved.

The QName URN provides both a one-to-one mapping, that preserves the uniquess of the underlying QName, and an explicit many-to-one mapping, that does not preserve the uniquess when it is not important to do so.

Identifier persistence considerations:

QName URN's have the same persistance as the underlying XML QName from which they are derived.

Process of identifier assignment:

Assignment of identifiers depends on the original XML QName, typically deferring to the namespace URI. Anyone with access to an XML QName can create an equivalent QName URN; no registration is required.

Process for identifier resolution:

Inherited from the QName resolution rules (typically the namespace URI) from which the QName URN is created.

Rules for Lexical Equivalence:

If necessary, convert each QName URN to the same encoding. The encoding of a QName URN is determined by context, and depends on the encoding of the document in which it appears.

To be lexically equivalent the resultant QName URN's must be identical when compared byte-for-byte. To be semantically equivalent, ignore the prefix field when comparing bytes.

Conformance with URN Syntax:

Fully conformant.

Validation mechanism:

Inherited from the namespace URI of the original QName.

Scope:

Inherited from the original QName.

This scheme can also support the XML 1.1 [2] and XML namespaces 1.1 [3] standards.¶

In the 1.1 Namespace specification, namespace URI's (universal resource identifiers; see ...xref...) become IRI's (internationalized resource identifiers, see ...xref...). Although currently an Internet-Draft, and therefore not feasible to use as a normative reference here, it is expected that the final RFC will continue to define a way to map IRI's to URI's. To apply this specification to IRI's, then, first apply that mapping before using the syntax here.¶

QName URN's provide a way to transcribe XML QName's into and out of URN syntax. Any security considerations are inherited from the original QName.¶

5. Normative References

[1]

Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, “Extensible Markup Language (XML) 1.0 (Third Edition)”, W3C Recommendation, February 2004, <http://www.w3.org/TR/2004/REC-xml-20040204>.

[2]

Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., Yergeau, F., and J. Cowan, “Extensible Markup Language (XML) 1.1”, W3C Recommendation, February 2004, <http://www.w3.org/TR/2004/REC-xml-20040204>.

[3]

Bray, T., Hollander, D., and A. Layman, “Namespaces in XML”, W3C Recommendation, January 1990, <http://www.w3.org/TR/1999/REC-xml-names-19990114>.

[4]

Bray, T., Hollander, D., Layman, A., and R. Tobin, “Namespaces in XML”, W3C Recommendation, February 2004, <http://www.w3.org/TR/1999/REC-xml-names-19990114>.

[5]

Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom, “URN Namespace Definition Mechanisms”, BCP 33, RFC 2611, June 1999.

[6]

Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, RFC 2234, November 1997.

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