Network Working Group                                          M. MurataRequest for Comments: 3023                 IBM Tokyo Research LaboratoryObsoletes: 2376                                            S. St.LaurentUpdates: 2048                                               simonstl.comCategory: Standards Track                                        D. Kohn                                                        Skymoon Ventures                                                            January 2001                            XML Media TypesStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2001).  All Rights Reserved.Abstract   This document standardizes five new media types -- text/xml,   application/xml, text/xml-external-parsed-entity, application/xml-   external-parsed-entity, and application/xml-dtd -- for use in   exchanging network entities that are related to the Extensible Markup   Language (XML).  This document also standardizes a convention (using   the suffix '+xml') for naming media types outside of these five types   when those media types represent XML MIME (Multipurpose Internet Mail   Extensions) entities.  XML MIME entities are currently exchanged via   the HyperText Transfer Protocol on the World Wide Web, are an   integral part of the WebDAV protocol for remote web authoring, and   are expected to have utility in many domains.   Major differences from RFC 2376 are (1) the addition of text/xml-   external-parsed-entity, application/xml-external-parsed-entity, and   application/xml-dtd, (2) the '+xml' suffix convention (which also   updates the RFC 2048 registration process), and (3) the discussion of   "utf-16le" and "utf-16be".Murata, et al.              Standards Track                     [Page 1]RFC 3023                    XML Media Types                 January 2001Table of Contents   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3   2.   Notational Conventions . . . . . . . . . . . . . . . . . . .   4   3.   XML Media Types  . . . . . . . . . . . . . . . . . . . . . .   5   3.1  Text/xml Registration  . . . . . . . . . . . . . . . . . . .   7   3.2  Application/xml Registration . . . . . . . . . . . . . . . .   9   3.3  Text/xml-external-parsed-entity Registration . . . . . . . .  11   3.4  Application/xml-external-parsed-entity Registration  . . . .  12   3.5  Application/xml-dtd Registration . . . . . . . . . . . . . .  13   3.6  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . .  14   4.   The Byte Order Mark (BOM) and Conversions to/from the UTF-16        Charset  . . . . . . . . . . . . . . . . . . . . . . . . . .  15   5.   Fragment Identifiers . . . . . . . . . . . . . . . . . . . .  15   6.   The Base URI . . . . . . . . . . . . . . . . . . . . . . . .  15   7.   A Naming Convention for XML-Based Media Types  . . . . . . .  16   7.1  Referencing  . . . . . . . . . . . . . . . . . . . . . . . .  18   8.   Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  18   8.1  Text/xml with UTF-8 Charset  . . . . . . . . . . . . . . . .  19   8.2  Text/xml with UTF-16 Charset . . . . . . . . . . . . . . . .  19   8.3  Text/xml with UTF-16BE Charset . . . . . . . . . . . . . . .  19   8.4  Text/xml with ISO-2022-KR Charset  . . . . . . . . . . . . .  20   8.5  Text/xml with Omitted Charset  . . . . . . . . . . . . . . .  20   8.6  Application/xml with UTF-16 Charset  . . . . . . . . . . . .  20   8.7  Application/xml with UTF-16BE Charset  . . . . . . . . . . .  21   8.8  Application/xml with ISO-2022-KR Charset . . . . . . . . . .  21   8.9  Application/xml with Omitted Charset and UTF-16 XML MIME        Entity . . . . . . . . . . . . . . . . . . . . . . . . . . .  21   8.10 Application/xml with Omitted Charset and UTF-8 Entity  . . .  22   8.11 Application/xml with Omitted Charset and Internal Encoding        Declaration  . . . . . . . . . . . . . . . . . . . . . . . .  22   8.12 Text/xml-external-parsed-entity with UTF-8 Charset . . . . .  22   8.13 Application/xml-external-parsed-entity with UTF-16 Charset .  23   8.14 Application/xml-external-parsed-entity with UTF-16BE Charset  23   8.15 Application/xml-dtd  . . . . . . . . . . . . . . . . . . . .  23   8.16 Application/mathml+xml . . . . . . . . . . . . . . . . . . .  24   8.17 Application/xslt+xml . . . . . . . . . . . . . . . . . . . .  24   8.18 Application/rdf+xml  . . . . . . . . . . . . . . . . . . . .  24   8.19 Image/svg+xml  . . . . . . . . . . . . . . . . . . . . . . .  24   8.20 INCONSISTENT EXAMPLE: Text/xml with UTF-8 Charset  . . . . .  25   9.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  25   10.  Security Considerations  . . . . . . . . . . . . . . . . . .  25        References . . . . . . . . . . . . . . . . . . . . . . . . .  27        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  31   A.   Why Use the '+xml' Suffix for XML-Based MIME Types?  . . . .  32   A.1  Why not just use text/xml or application/xml and let the XML        processor dispatch to the correct application based on the        referenced DTD?  . . . . . . . . . . . . . . . . . . . . . .  32Murata, et al.              Standards Track                     [Page 2]RFC 3023                    XML Media Types                 January 2001   A.2  Why not create a new subtree (e.g., image/xml.svg) to        represent XML MIME types?  . . . . . . . . . . . . . . . . .  32   A.3  Why not create a new top-level MIME type for XML-based media        types? . . . . . . . . . . . . . . . . . . . . . . . . . . .  32   A.4  Why not just have the MIME processor 'sniff' the content to        determine whether it is XML? . . . . . . . . . . . . . . . .  33   A.5  Why not use a MIME parameter to specify that a media type        uses XML syntax? . . . . . . . . . . . . . . . . . . . . . .  33   A.6  How about labeling with parameters in the other direction        (e.g., application/xml; Content-Feature=iotp)? . . . . . . .  34   A.7  How about a new superclass MIME parameter that is defined to        apply to all MIME types (e.g., Content-Type:        application/iotp; $superclass=xml)?  . . . . . . . . . . . .  34   A.8  What about adding a new parameter to the Content-Disposition        header or creating a new Content-Structure header to        indicate XML syntax? . . . . . . . . . . . . . . . . . . . .  35   A.9  How about a new Alternative-Content-Type header? . . . . . .  35   A.10 How about using a conneg tag instead (e.g., accept-features:        (syntax=xml))? . . . . . . . . . . . . . . . . . . . . . . .  35   A.11 How about a third-level content-type, such as text/xml/rdf?   35   A.12 Why use the plus ('+') character for the suffix '+xml'?  . .  36   A.13 What is the semantic difference between application/foo and        application/foo+xml? . . . . . . . . . . . . . . . . . . . .  36   A.14 What happens when an even better markup language (e.g.,        EBML) is defined, or a new category of data? . . . . . . . .  36   A.15 Why must I use the '+xml' suffix for my new XML-based media        type?  . . . . . . . . . . . . . . . . . . . . . . . . . . .  37   B.   Changes from RFC 2376  . . . . . . . . . . . . . . . . . . .  37   C.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  38        Full Copyright Statement . . . . . . . . . . . . . . . . . .  391. Introduction   The World Wide Web Consortium has issued Extensible Markup Language   (XML) 1.0 (Second Edition)[XML].  To enable the exchange of XML   network entities, this document standardizes five new media types --   text/xml, application/xml, text/xml-external-parsed-entity,   application/xml-external-parsed-entity, and application/xml-dtd -- as   well as a naming convention for identifying XML-based MIME media   types.   XML entities are currently exchanged on the World Wide Web, and XML   is also used for property values and parameter marshalling by the   WebDAV[RFC2518] protocol for remote web authoring.  Thus, there is a   need for a media type to properly label the exchange of XML network   entities.Murata, et al.              Standards Track                     [Page 3]RFC 3023                    XML Media Types                 January 2001   Although XML is a subset of the Standard Generalized Markup Language   (SGML) ISO 8879[SGML], which has been assigned the media types   text/sgml and application/sgml, there are several reasons why use of   text/sgml or application/sgml to label XML is inappropriate.  First,   there exist many applications that can process XML, but that cannot   process SGML, due to SGML's larger feature set.  Second, SGML   applications cannot always process XML entities, because XML uses   features of recent technical corrigenda to SGML.  Third, the   definition of text/sgml and application/sgml in [RFC1874] includes   parameters for SGML bit combination transformation format (SGML-   bctf), and SGML boot attribute (SGML-boot).  Since XML does not use   these parameters, it would be ambiguous if such parameters were given   for an XML MIME entity.  For these reasons, the best approach for   labeling XML network entities is to provide new media types for XML.   Since XML is an integral part of the WebDAV Distributed Authoring   Protocol, and since World Wide Web Consortium Recommendations have   conventionally been assigned IETF tree media types, and since similar   media types (HTML, SGML) have been assigned IETF tree media types,   the XML media types also belong in the IETF media types tree.   Similarly, XML will be used as a foundation for other media types,   including types in every branch of the IETF media types tree.  To   facilitate the processing of such types, media types based on XML,   but that are not identified using text/xml or application/xml, SHOULD   be named using a suffix of '+xml' as described in Section 7.  This   will allow XML-based tools -- browsers, editors, search engines, and   other processors -- to work with all XML-based media types.2. Notational Conventions   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].   As defined in [RFC2781], the three charsets "utf-16", "utf-16le", and   "utf-16be" are used to label UTF-16 text.  In this document, "the   UTF-16 family" refers to those three charsets.  By contrast, the   phrases "utf-16" or UTF-16 in this document refer specifically to the   single charset "utf-16".   As sometimes happens between two communities, both MIME and XML have   defined the term entity, with different meanings.  Section 2.4 of   [RFC2045] says:      "The term 'entity' refers specifically to the MIME-defined header      fields and contents of either a message or one of the parts in the      body of a multipart entity."Murata, et al.              Standards Track                     [Page 4]RFC 3023                    XML Media Types                 January 2001   Section 4 of [XML] says:      "An XML document may consist of one or many storage units" called      entities that "have content" and are normally "identified by      name".   In this document, "XML MIME entity" is defined as the latter (an XML   entity) encapsulated in the former (a MIME entity).3. XML Media Types   This document standardizes five media types related to XML MIME   entities: text/xml, application/xml, text/xml-external-parsed-entity,   application/xml-external-parsed-entity, and application/xml-dtd.   Registration information for these media types is described in the   sections below.   Within the XML specification, XML MIME entities can be classified   into four types.  In the XML terminology, they are called "document   entities", "external DTD subsets", "external parsed entities", and   "external parameter entities".  The media types text/xml and   application/xml MAY be used for "document entities", while text/xml-   external-parsed-entity or application/xml-external-parsed-entity   SHOULD be used for "external parsed entities".  The media type   application/xml-dtd SHOULD be used for "external DTD subsets" or   "external parameter entities".  application/xml and text/xml MUST NOT   be used for "external parameter entities" or "external DTD subsets",   and MUST NOT be used for "external parsed entities" unless they are   also well-formed "document entities" and are referenced as such.   Note that [RFC2376] (which this document obsoletes) allowed such   usage, although in practice it is likely to have been rare.   Neither external DTD subsets nor external parameter entities parse as   XML documents, and while some XML document entities may be used as   external parsed entities and vice versa, there are many cases where   the two are not interchangeable.  XML also has unparsed entities,   internal parsed entities, and internal parameter entities, but they   are not XML MIME entities.   If an XML document -- that is, the unprocessed, source XML document   -- is readable by casual users, text/xml is preferable to   application/xml.  MIME user agents (and web user agents) that do not   have explicit support for text/xml will treat it as text/plain, for   example, by displaying the XML MIME entity as plain text.   Application/xml is preferable when the XML MIME entity is unreadable   by casual users.  Similarly, text/xml-external-parsed-entity isMurata, et al.              Standards Track                     [Page 5]RFC 3023                    XML Media Types                 January 2001   preferable when an external parsed entity is readable by casual   users, but application/xml-external-parsed-entity is preferable when   a plain text display is inappropriate.      NOTE: Users are in general not used to text containing tags such      as <price>, and often find such tags quite disorienting or      annoying.  If one is not sure, the conservative principle would      suggest using application/* instead of text/* so as not to put      information in front of users that they will quite likely not      understand.   The top-level media type "text" has some restrictions on MIME   entities and they are described in [RFC2045] and [RFC2046].  In   particular, the UTF-16 family, UCS-4, and UTF-32 are not allowed   (except over HTTP[RFC2616], which uses a MIME-like mechanism).  Thus,   if an XML document or external parsed entity is encoded in such   character encoding schemes, it cannot be labeled as text/xml or   text/xml-external-parsed-entity (except for HTTP).   Text/xml and application/xml behave differently when the charset   parameter is not explicitly specified.  If the default charset (i.e.,   US-ASCII) for text/xml is inconvenient for some reason (e.g., bad web   servers), application/xml provides an alternative (see "Optional   parameters" of application/xml registration in Section 3.2).  The   same rules apply to the distinction between text/xml-external-   parsed-entity and application/xml-external-parsed-entity.   XML provides a general framework for defining sequences of structured   data.  In some cases, it may be desirable to define new media types   that use XML but define a specific application of XML, perhaps due to   domain-specific security considerations or runtime information.   Furthermore, such media types may allow UTF-8 or UTF-16 only and   prohibit other charsets.  This document does not prohibit such media   types and in fact expects them to proliferate.  However, developers