rfc3023.txt

RFC3023:XML Media Types

Network Working Group                                          M. Murata
Request for Comments: 3023                 IBM Tokyo Research Laboratory
Obsoletes: 2376                                            S. St.Laurent
Updates: 2048                                               simonstl.com
Category: Standards Track                                        D. Kohn
                                                        Skymoon Ventures
                                                            January 2001


                            XML Media Types

Status 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".









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Table 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?  . . . . . . . . . . . . . . . . . . . . . .  32



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   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 . . . . . . . . . . . . . . . . . .  39

1. 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.





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   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."



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   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 is





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   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