Network Working Group S. St.Laurent Internet-Draft O'Reilly & Associates Expires: April 28, 2003 October 28, 2002 The XPointer content-type() Scheme draft-stlaurent-content-type-frag-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 28, 2003. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document specifies a content-type() scheme for use in XPointer- based fragment identifiers. This scheme, like other XPointer Framework [12] schemes, is designed primarily for use with the XML Media Types defined in RFC 3023 [5], to identify locations within a given XML representation of a resource, though it may potentially be used to mix schemes for XML and non-XML representations. The content-type() scheme notifies an XPointer processor whether the creator of the XPointer intended for a particular pointer part to apply to a resource representation which uses a particular MIME content type identifier. St.Laurent Expires April 28, 2003 [Page 1] Internet-Draft content-type() scheme October 2002 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Justification . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Relation to MIME Media Types . . . . . . . . . . . . . . . . . 7 6. Other MIME issues . . . . . . . . . . . . . . . . . . . . . . 8 7. Conformance . . . . . . . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13 A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14 B. Revision History . . . . . . . . . . . . . . . . . . . . . . . 15 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 16 St.Laurent Expires April 28, 2003 [Page 2] Internet-Draft content-type() scheme October 2002 1. Introduction The content-type() scheme is intended to be used within the XPointer Framework [12] to support the addressing of nodes within XML (and potentially other) documents. Pointer parts using the content-type() scheme will always fail, as they do not identify portions of an XML document, but they can communicate (though not enforce) expectations for processing of later parts of an XPointer. St.Laurent Expires April 28, 2003 [Page 3] Internet-Draft content-type() scheme October 2002 2. Justification Even if all of the possible representations of a resource are themselves XML, they may contain very different structures and labels. The impact of that variability will vary with the nature of the representations, but it may cause various schemes used within the XPointer Framework [12] (including xpointer() [15], element() [14], and xpath1() [16]) to return different answers if the representation of the resource is of an unexpected type. While content-type() pointer parts can never provide more than a suggestion, they may be helpful in coordinating program behavior or at least giving a human reader a clue as to why fragment identifiers are behaving differently than expected. St.Laurent Expires April 28, 2003 [Page 4] Internet-Draft content-type() scheme October 2002 3. Syntax The scheme name is "content-type". The scheme data syntax is as follows; if scheme data in a pointer part with the content-type() scheme does not conform to the syntax defined in this section, it is an error and the pointer part fails. content-type() Scheme Syntax: ptrpart ::= content-type( content-typeschemedata ) content-typeschemedata ::= type type must be a MIME Content-Type identifier conforming to RFC 2046 [1], and should preferably be a registered type, as registration provides a medium in which to describe the different kinds of fragment identifiers which may apply to a particular type. St.Laurent Expires April 28, 2003 [Page 5] Internet-Draft content-type() scheme October 2002 4. Processing The XPointer Framework provides limited support for specifying XML processing context. The xmlns() [13] scheme provides support for identifying mappings between prefixes used in pointer parts and namespace URIs, changing the processing context of later pointer parts. By definition, the xmlns() scheme "never identifies a subresource and thus always fails". The content-type() scheme relies on a similar mechanism, but because of the XPointer framework's consumption of failures inside an XPointer expression, there is no way for content-type() generally to signal an error. A typical use for content-type would just signal to which representation a given XPointer is likely to apply: #content-type(application/xhtml+xml) element(framework/1/7) In this case, the XPointer may fail if fed the wrong representation, but at least a developer might have a chance of figuring out why the failure is taking place. In the event that an XPointer processor provides explicit support for the content-type() scheme, it should do its best to find a representation specified by each content-type() part, conceivably making multiple attempts to retrieve an acceptable representation. For example, if an XPointer using content-type() looked like: #content-type(application/xhtml+xml) element(/1/7) content-type(image/svg+xml) element(/1/4) content-type(application/mathml+xml) element(/1/3) then the processor should first attempt to retrieve an XHTML representation of the resource, then find the seventh child of the first element in the document. If that didn't return a node, it would attempt to retrieve an SVG representation, then find the fourth child of the first element in the document. If that didn't return a node, it would attempt to retrieve a MathML representation, and return the the third child of the first element in the document. This will not likely work in most XPointer processors, as the representation of the resource will likely have been set at some point earlier in the process without fragment identifier consultation, and the XPointer processor will likely grab the seventh child element of the first element in the document regardless of whether it was represented as XHTML, SVG, or MathML. It does, however, raise the question of how XPointer and content-negotiation interact (or don't.) St.Laurent Expires April 28, 2003 [Page 6] Internet-Draft content-type() scheme October 2002 5. Relation to MIME Media Types This scheme is somewhat separate from association with any particular MIME type, but it could conceivably be used with any registration made in accordance with RFC 2046 [1] and RFC 2048 [2]. St.Laurent Expires April 28, 2003 [Page 7] Internet-Draft content-type() scheme October 2002 6. Other MIME issues There may be other aspects of MIME content-negotiation worth considering in this context. Content-features, charset, and other parameters may in some circumstances affect XPointer processing. Consideration of those issues is left for other drafts. St.Laurent Expires April 28, 2003 [Page 8] Internet-Draft content-type() scheme October 2002 7. Conformance This specification normatively depends on the XPointer Framework [12], except insofar as it rejects the claim in Section 3.3 that "this specification reserves all scheme names for definition in additional W3C XPointer scheme specifications". While XPointer processors may simply ignore content-type() pointer parts with only occasional damage, those explicitly claiming support for the content-type() scheme should support content-negotiation and attempt to supply the context requested by the content-type() pointer part or parts. St.Laurent Expires April 28, 2003 [Page 9] Internet-Draft content-type() scheme October 2002 8. Security Considerations There may be security considerations involved in content-negotiation, but they are outside of the scope of this document. St.Laurent Expires April 28, 2003 [Page 10] Internet-Draft content-type() scheme October 2002 9. IANA Considerations None. St.Laurent Expires April 28, 2003 [Page 11] Internet-Draft content-type() scheme October 2002 References [1] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996. [2] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures", RFC 2048, November 1996. [3] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998. [4] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, June 1999. [5] Murata, M., St.Laurent, S. and D. Kohn, "XML Media Types", RFC 3023, January 2001. [6] Bray, T., Paoli, J. and C. Sperberg-McQueen, "Extensible Markup Language (XML) 1.0", World Wide Web Consortium Recommendation REC-xml, February 1998, . [7] DeRose, S. and J. Clark, "XML Path Language (XPath) Version 1.0", World Wide Web Consortium Recommendation REC-xpath- 19991116, November 1999, . [8] Berglund, A., Boag, S., Chamberlin, D., Fernandez, M., Kay, M., Robie, J. and J. Simeon, "XML Path Language (XPath) 2.0", World Wide Web Consortium Working Draft WD-xpath20-20020816, November 1999, . [9] Bray, T., Hollander, D. and A. Layman, "Namespaces in XML", World Wide Web Consortium Recommendation REC-xml-names, January 1999, . [10] DeRose, S., Maler, E. and D. Orchard, "XML Linking Language (XLink)", World Wide Web Consortium Recommendation XLink, June 2001, . [11] Marsh, J. and D. Orchard, "XML Inclusions (XInclude) Version 1.0", World Wide Web Consortium Candidate Recommendation CR- xinclude-20020221/, September 2002, . [12] Grosso, P., Maler, E., Marsh, J. and N. Walsh, "XPointer St.Laurent Expires April 28, 2003 [Page 12] Internet-Draft content-type() scheme October 2002 Framework", World Wide Web Consortium Working Draft XPointer Framework, July 2002, . [13] DeRose, S., Daniel Jr., R. and E. Maler, "XPointer xmlns() Scheme", World Wide Web Consortium Working Draft XPointer xmlns() Scheme, July 2002, . [14] Grosso, P., Maler, E., Marsh, J. and N. Walsh, "XPointer element() Scheme", World Wide Web Consortium Working Draft XPointer element() Scheme, July 2002, . [15] DeRose, S., Daniel Jr., R. and E. Maler, "XPointer xpointer() Scheme", World Wide Web Consortium Working Draft XPointer xpointer() Scheme, July 2002, . [16] St.Laurent, S., "The XPointer xpath1() Scheme", I-D draft- stlaurent-xpath-frag-00.txt, October 2002. [17] St.Laurent, S., "The XPointer xinclude1() Scheme", I-D draft- stlaurent-xinclude-frag-00.txt, October 2002, . Author's Address Simon St.Laurent O'Reilly & Associates 1259 Dryden Road Ithaca, New York 14850 USA EMail: simonstl@simonstl.com URI: http://www.simonstl.com/ St.Laurent Expires April 28, 2003 [Page 13] Internet-Draft content-type() scheme October 2002 Appendix A. Acknowledgements Thanks to Uche Ogbuji and Eric van der Vlist for inspiration. St.Laurent Expires April 28, 2003 [Page 14] Internet-Draft content-type() scheme October 2002 Appendix B. Revision History 00 - First version. [To be deleted before publication.] St.Laurent Expires April 28, 2003 [Page 15] Internet-Draft content-type() scheme October 2002 Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. St.Laurent Expires April 28, 2003 [Page 16]