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rfc:rfc3023

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".

Murata, et al. Standards Track [Page 1] RFC 3023 XML Media Types January 2001

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

Murata, 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 . . . . . . . . . . . . . . . . . .  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.

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 is

Murata, 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
 of such media types are STRONGLY RECOMMENDED to use this document as
 a basis for their registration.  In particular, the charset parameter
 SHOULD be used in the same manner, as described in Section 7.1, in
 order to enhance interoperability.
 An XML document labeled as text/xml or application/xml might contain
 namespace declarations, stylesheet-linking processing instructions
 (PIs), schema information, or other declarations that might be used
 to suggest how the document is to be processed.  For example, a
 document might have the XHTML namespace and a reference to a CSS
 stylesheet.  Such a document might be handled by applications that
 would use this information to dispatch the document for appropriate
 processing.

Murata, et al. Standards Track [Page 6] RFC 3023 XML Media Types January 2001

3.1 Text/xml Registration

 MIME media type name: text
 MIME subtype name: xml
 Mandatory parameters: none
 Optional parameters: charset
    Although listed as an optional parameter, the use of the charset
    parameter is STRONGLY RECOMMENDED, since this information can be
    used by XML processors to determine authoritatively the character
    encoding of the XML MIME entity.  The charset parameter can also
    be used to provide protocol-specific operations, such as charset-
    based content negotiation in HTTP.  "utf-8" [RFC2279] is the
    recommended value, representing the UTF-8 charset.  UTF-8 is
    supported by all conforming processors of [XML].
    If the XML MIME entity is transmitted via HTTP, which uses a
    MIME-like mechanism that is exempt from the restrictions on the
    text top-level type (see section 19.4.1 of [RFC2616]), "utf-16"
    [RFC2781]) is also recommended.  UTF-16 is supported by all
    conforming processors of [XML].  Since the handling of CR, LF and
    NUL for text types in most MIME applications would cause undesired
    transformations of individual octets in UTF-16 multi-octet
    characters, gateways from HTTP to these MIME applications MUST
    transform the XML MIME entity from text/xml; charset="utf-16" to
    application/xml; charset="utf-16".
    Conformant with [RFC2046], if a text/xml entity is received with
    the charset parameter omitted, MIME processors and XML processors
    MUST use the default charset value of "us-ascii"[ASCII].  In cases
    where the XML MIME entity is transmitted via HTTP, the default
    charset value is still "us-ascii".  (Note: There is an
    inconsistency between this specification and HTTP/1.1, which uses
    ISO-8859-1[ISO8859] as the default for a historical reason.  Since
    XML is a new format, a new default should be chosen for better
    I18N.  US-ASCII was chosen, since it is the intersection of UTF-8
    and ISO-8859-1 and since it is already used by MIME.)
    There are several reasons that the charset parameter is
    authoritative.  First, some MIME processing engines do transcoding
    of MIME bodies of the top-level media type "text" without
    reference to any of the internal content.  Thus, it is possible
    that some agent might change text/xml; charset="iso-2022-jp" to
    text/xml; charset="utf-8" without modifying the encoding
    declaration of an XML document.  Second, text/xml must be

Murata, et al. Standards Track [Page 7] RFC 3023 XML Media Types January 2001

    compatible with text/plain, since MIME agents that do not
    understand text/xml will fallback to handling it as text/plain.
    If the charset parameter for text/xml were not authoritative, such
    fallback would cause data corruption.  Third, recent web servers
    have been improved so that users can specify the charset
    parameter.  Fourth, [RFC2130] specifies that the recommended
    specification scheme is the "charset" parameter.
    Since the charset parameter is authoritative, the charset is not
    always declared within an XML encoding declaration.  Thus, special
    care is needed when the recipient strips the MIME header and
    provides persistent storage of the received XML MIME entity (e.g.,
    in a file system).  Unless the charset is UTF-8 or UTF-16, the
    recipient SHOULD also persistently store information about the
    charset, perhaps by embedding a correct XML encoding declaration
    within the XML MIME entity.
 Encoding considerations: This media type MAY be encoded as
    appropriate for the charset and the capabilities of the underlying
    MIME transport.  For 7-bit transports, data in UTF-8 MUST be
    encoded in quoted-printable or base64.  For 8-bit clean transport
    (e.g., 8BITMIME[RFC1652] ESMTP or NNTP[RFC0977]), UTF-8 does not
    need to be encoded.  Over HTTP[RFC2616], no content-transfer-
    encoding is necessary and UTF-16 may also be used.
 Security considerations: See Section 10.
 Interoperability considerations: XML has proven to be interoperable
    across WebDAV clients and servers, and for import and export from
    multiple XML authoring tools.  For maximum interoperability,
    validating processors are recommended.  Although non-validating
    processors may be more efficient, they are not required to handle
    all features of XML.  For further information, see sub-section 2.9
    "Standalone Document Declaration" and section 5 "Conformance" of
    [XML].
 Published specification: Extensible Markup Language (XML) 1.0 (Second
    Edition)[XML].
 Applications which use this media type: XML is device-, platform-,
    and vendor-neutral and is supported by a wide range of Web user
    agents, WebDAV[RFC2518] clients and servers, as well as XML
    authoring tools.
 Additional information:

Murata, et al. Standards Track [Page 8] RFC 3023 XML Media Types January 2001

    Magic number(s): None.
       Although no byte sequences can be counted on to always be
       present, XML MIME entities in ASCII-compatible charsets
       (including UTF-8) often begin with hexadecimal 3C 3F 78 6D 6C
       ("<?xml"), and those in UTF-16 often begin with hexadecimal FE
       FF 00 3C 00 3F 00 78 00 6D 00 6C or FF FE 3C 00 3F 00 78 00 6D
       00 6C 00 (the Byte Order Mark (BOM) followed by "<?xml").  For
       more information, see Appendix F of [XML].
    File extension(s): .xml
    Macintosh File Type Code(s): "TEXT"
 Person and email address for further information:
    MURATA Makoto (FAMILY Given) <mmurata@trl.ibm.co.jp>
    Simon St.Laurent <simonstl@simonstl.com>
    Daniel Kohn <dan@dankohn.com>
 Intended usage: COMMON
 Author/Change controller: The XML specification is a work product of
    the World Wide Web Consortium's XML Working Group, and was edited
    by:
    Tim Bray <tbray@textuality.com>
    Jean Paoli <jeanpa@microsoft.com>
    C. M. Sperberg-McQueen <cmsmcq@uic.edu>
    Eve Maler <eve.maler@east.sun.com>
    The W3C, and the W3C XML Core Working Group, have change control
    over the XML specification.

3.2 Application/xml Registration

 MIME media type name: application
 MIME subtype name: xml
 Mandatory parameters: none

Murata, et al. Standards Track [Page 9] RFC 3023 XML Media Types January 2001

 Optional parameters: charset
    Although listed as an optional parameter, the use of the charset
    parameter is STRONGLY RECOMMENDED, since this information can be
    used by XML processors to determine authoritatively the charset of
    the XML MIME entity.  The charset parameter can also be used to
    provide protocol-specific operations, such as charset-based
    content negotiation in HTTP.
    "utf-8" [RFC2279] and "utf-16" [RFC2781] are the recommended
    values, representing the UTF-8 and UTF-16 charsets, respectively.
    These charsets are preferred since they are supported by all
    conforming processors of [XML].
    If an application/xml entity is received where the charset
    parameter is omitted, no information is being provided about the
    charset by the MIME Content-Type header.  Conforming XML
    processors MUST follow the requirements in section 4.3.3 of [XML]
    that directly address this contingency.  However, MIME processors
    that are not XML processors SHOULD NOT assume a default charset if
    the charset parameter is omitted from an application/xml entity.
    There are several reasons that the charset parameter is
    authoritative.  First, recent web servers have been improved so
    that users can specify the charset parameter.  Second, [RFC2130]
    specifies that the recommended specification scheme is the
    "charset" parameter.
    On the other hand, it has been argued that the charset parameter
    should be omitted and the mechanism described in Appendix F of
    [XML] (which is non-normative) should be solely relied on.  This
    approach would allow users to avoid configuration of the charset
    parameter; an XML document stored in a file is likely to contain a
    correct encoding declaration or BOM (if necessary), since the
    operating system does not typically provide charset information
    for files.  If users would like to rely on the encoding
    declaration or BOM and to hide charset information from protocols,
    they may determine not to use the parameter.
    Since the charset parameter is authoritative, the charset is not
    always declared within an XML encoding declaration.  Thus, special
    care is needed when the recipient strips the MIME header and
    provides persistent storage of the received XML MIME entity (e.g.,
    in a file system).  Unless the charset is UTF-8 or UTF-16, the
    recipient SHOULD also persistently store information about the
    charset, perhaps by embedding a correct XML encoding declaration
    within the XML MIME entity.

Murata, et al. Standards Track [Page 10] RFC 3023 XML Media Types January 2001

 Encoding considerations: This media type MAY be encoded as
    appropriate for the charset and the capabilities of the underlying
    MIME transport.  For 7-bit transports, data in either UTF-8 or
    UTF-16 MUST be encoded in quoted-printable or base64.  For 8-bit
    clean transport (e.g., 8BITMIME[RFC1652] ESMTP or NNTP[RFC0977]),
    UTF-8 is not encoded, but the UTF-16 family MUST be encoded in
    base64.  For binary clean transports (e.g., HTTP[RFC2616]), no
    content-transfer-encoding is necessary.
 Security considerations: See Section 10.
 Interoperability considerations: Same as Section 3.1.
 Published specification: Same as Section 3.1.
 Applications which use this media type: Same as Section 3.1.
 Additional information: Same as Section 3.1.
 Person and email address for further information: Same as Section
    3.1.
 Intended usage: COMMON
 Author/Change controller: Same as Section 3.1.

3.3 Text/xml-external-parsed-entity Registration

 MIME media type name: text
 MIME subtype name: xml-external-parsed-entity
 Mandatory parameters: none
 Optional parameters: charset
    The charset parameter of text/xml-external-parsed-entity is
    handled the same as that of text/xml as described in Section 3.1.
 Encoding considerations: Same as Section 3.1.
 Security considerations: See Section 10.
 Interoperability considerations: XML external parsed entities are as
    interoperable as XML documents, though they have a less tightly
    constrained structure and therefore need to be referenced by XML
    documents for proper handling by XML processors.  Similarly, XML
    documents cannot be reliably used as external parsed entities

Murata, et al. Standards Track [Page 11] RFC 3023 XML Media Types January 2001

    because external parsed entities are prohibited from having
    standalone document declarations or DTDs.  Identifying XML
    external parsed entities with their own content type should
    enhance interoperability of both XML documents and XML external
    parsed entities.
 Published specification: Same as Section 3.1.
 Applications which use this media type: Same as Section 3.1.
 Additional information:
    Magic number(s): Same as Section 3.1.
    File extension(s): .xml or .ent
    Macintosh File Type Code(s): "TEXT"
 Person and email address for further information: Same as Section
    3.1.
 Intended usage: COMMON
 Author/Change controller: Same as Section 3.1.

3.4 Application/xml-external-parsed-entity Registration

 MIME media type name: application
 MIME subtype name: xml-external-parsed-entity
 Mandatory parameters: none
 Optional parameters: charset
    The charset parameter of application/xml-external-parsed-entity is
    handled the same as that of application/xml as described in
    Section 3.2.
 Encoding considerations: Same as Section 3.2.
 Security considerations: See Section 10.
 Interoperability considerations: Same as those for text/xml-
    external-parsed-entity as described in Section 3.3.
 Published specification: Same as text/xml as described in Section
    3.1.

Murata, et al. Standards Track [Page 12] RFC 3023 XML Media Types January 2001

 Applications which use this media type: Same as Section 3.1.
 Additional information:
    Magic number(s): Same as Section 3.1.
    File extension(s): .xml or .ent
    Macintosh File Type Code(s): "TEXT"
 Person and email address for further information: Same as Section
    3.1.
 Intended usage: COMMON
 Author/Change controller: Same as Section 3.1.

3.5 Application/xml-dtd Registration

 MIME media type name: application
 MIME subtype name: xml-dtd
 Mandatory parameters: none
 Optional parameters: charset
    The charset parameter of application/xml-dtd is handled the same
    as that of application/xml as described in Section 3.2.
 Encoding considerations: Same as Section 3.2.
 Security considerations: See Section 10.
 Interoperability considerations: XML DTDs have proven to be
    interoperable by DTD authoring tools and XML browsers, among
    others.
 Published specification: Same as text/xml as described in Section
    3.1.
 Applications which use this media type: DTD authoring tools handle
    external DTD subsets as well as external parameter entities.  XML
    browsers may also access external DTD subsets and external
    parameter entities.

Murata, et al. Standards Track [Page 13] RFC 3023 XML Media Types January 2001

 Additional information:
    Magic number(s): Same as Section 3.1.
    File extension(s): .dtd or .mod
    Macintosh File Type Code(s): "TEXT"
 Person and email address for further information: Same as Section
    3.1.
 Intended usage: COMMON
 Author/Change controller: Same as Section 3.1.

3.6 Summary

 The following list applies to text/xml, text/xml-external-parsed-
 entity, and XML-based media types under the top-level type "text"
 that define the charset parameter according to this specification:
 o  Charset parameter is strongly recommended.
 o  If the charset parameter is not specified, the default is "us-
    ascii".  The default of "iso-8859-1" in HTTP is explicitly
    overridden.
 o  No error handling provisions.
 o  An encoding declaration, if present, is irrelevant, but when
    saving a received resource as a file, the correct encoding
    declaration SHOULD be inserted.
 The next list applies to application/xml, application/xml-external-
 parsed-entity, application/xml-dtd, and XML-based media types under
 top-level types other than "text" that define the charset parameter
 according to this specification:
 o  Charset parameter is strongly recommended, and if present, it
    takes precedence.
 o  If the charset parameter is omitted, conforming XML processors
    MUST follow the requirements in section 4.3.3 of [XML].

Murata, et al. Standards Track [Page 14] RFC 3023 XML Media Types January 2001

4. The Byte Order Mark (BOM) and Conversions to/from the UTF-16 Charset

 Section 4.3.3 of [XML] specifies that XML MIME entities in the
 charset "utf-16" MUST begin with a byte order mark (BOM), which is a
 hexadecimal octet sequence 0xFE 0xFF (or 0xFF 0xFE, depending on
 endian).  The XML Recommendation further states that the BOM is an
 encoding signature, and is not part of either the markup or the
 character data of the XML document.
 Due to the presence of the BOM, applications that convert XML from
 "utf-16" to a non-Unicode encoding MUST strip the BOM before
 conversion.  Similarly, when converting from another encoding into
 "utf-16", the BOM MUST be added after conversion is complete.
 In addition to the charset "utf-16", [RFC2781] introduces "utf-16le"
 (little endian) and "utf-16be" (big endian) as well.  The BOM is
 prohibited for these charsets.  When an XML MIME entity is encoded in
 "utf-16le" or "utf-16be", it MUST NOT begin with the BOM but SHOULD
 contain an encoding declaration.  Conversion from "utf-16" to "utf-
 16be" or "utf-16le" and conversion in the other direction MUST strip
 or add the BOM, respectively.

5. Fragment Identifiers

 Section 4.1 of [RFC2396] notes that the semantics of a fragment
 identifier (the part of a URI after a "#") is a property of the data
 resulting from a retrieval action, and that the format and
 interpretation of fragment identifiers is dependent on the media type
 of the retrieval result.
 As of today, no established specifications define identifiers for XML
 media types.  However, a working draft published by W3C, namely "XML
 Pointer Language (XPointer)", attempts to define fragment identifiers
 for text/xml and application/xml.  The current specification for
 XPointer is available at http://www.w3.org/TR/xptr.

6. The Base URI

 Section 5.1 of [RFC2396] specifies that the semantics of a relative
 URI reference embedded in a MIME entity is dependent on the base URI.
 The base URI is either (1) the base URI embedded in the MIME entity,
 (2) the base URI of the encapsulating MIME entity, (3) the URI used
 to retrieve the MIME entity, or (4) the application-dependent default
 base URI, where (1) has the highest precedence.  [RFC2396] further
 specifies that the mechanism for embedding the base URI is dependent
 on the media type.

Murata, et al. Standards Track [Page 15] RFC 3023 XML Media Types January 2001

 As of today, no established specifications define mechanisms for
 embedding the base URI in XML MIME entities.  However, a Proposed
 Recommendation published by W3C, namely "XML Base", attempts to
 define such a mechanism for text/xml, application/xml, text/xml-
 external-parsed-entity, and application/xml-external-parsed-entity.
 The current specification for XML Base is available at
 http://www.w3.org/TR/xmlbase.

7. A Naming Convention for XML-Based Media Types

 This document recommends the use of a naming convention (a suffix of
 '+xml') for identifying XML-based MIME media types, whatever their
 particular content may represent.  This allows the use of generic XML
 processors and technologies on a wide variety of different XML
 document types at a minimum cost, using existing frameworks for media
 type registration.
 Although the use of a suffix was not considered as part of the
 original MIME architecture, this choice is considered to provide the
 most functionality with the least potential for interoperability
 problems or lack of future extensibility.  The alternatives to the '
 +xml' suffix and the reason for its selection are described in
 Appendix A.
 As XML development continues, new XML document types are appearing
 rapidly.  Many of these XML document types would benefit from the
 identification possibilities of a more specific MIME media type than
 text/xml or application/xml can provide, and it is likely that many
 new media types for XML-based document types will be registered in
 the near and ongoing future.
 While the benefits of specific MIME types for particular types of XML
 documents are significant, all XML documents share common structures
 and syntax that make possible common processing.
 Some areas where 'generic' processing is useful include:
 o  Browsing - An XML browser can display any XML document with a
    provided [CSS] or [XSLT] style sheet, whatever the vocabulary of
    that document.
 o  Editing - Any XML editor can read, modify, and save any XML
    document.
 o  Fragment identification - XPointers (work in progress) can work
    with any XML document, whatever vocabulary it uses and whether or
    not it uses XPointer for its own fragment identification.

Murata, et al. Standards Track [Page 16] RFC 3023 XML Media Types January 2001

 o  Hypertext linking - XLink (work in progress) hypertext linking is
    designed to connect any XML documents, regardless of vocabulary.
 o  Searching - XML-oriented search engines, web crawlers, agents, and
    query tools should be able to read XML documents and extract the
    names and content of elements and attributes even if the tools are
    ignorant of the particular vocabulary used for elements and
    attributes.
 o  Storage - XML-oriented storage systems, which keep XML documents
    internally in a parsed form, should similarly be able to process,
    store, and recreate any XML document.
 o  Well-formedness and validity checking - An XML processor can
    confirm that any XML document is well-formed and that it is valid
    (i.e., conforms to its declared DTD or Schema).
 When a new media type is introduced for an XML-based format, the name
 of the media type SHOULD end with '+xml'.  This convention will allow
 applications that can process XML generically to detect that the MIME
 entity is supposed to be an XML document, verify this assumption by
 invoking some XML processor, and then process the XML document
 accordingly.  Applications may match for types that represent XML
 MIME entities by comparing the subtype to the pattern '*/*+xml'.  (Of
 course, 4 of the 5 media types defined in this document -- text/xml,
 application/xml, text/xml-external-parsed-entity, and
 application/xml-external-parsed-entity -- also represent XML MIME
 entities while not conforming to the '*/*+xml' pattern.)
    NOTE: Section 14.1 of HTTP[RFC2616] does not support Accept
    headers of the form "Accept: */*+xml" and so this header MUST NOT
    be used in this way.  Instead, content negotiation[RFC2703] could
    potentially be used if an XML-based MIME type were needed.
 XML generic processing is not always appropriate for XML-based media
 types.  For example, authors of some such media types may wish that
 the types remain entirely opaque except to applications that are
 specifically designed to deal with that media type.  By NOT following
 the naming convention '+xml', such media types can avoid XML-generic
 processing.  Since generic processing will be useful in many cases,
 however -- including in some situations that are difficult to predict
 ahead of time -- those registering media types SHOULD use the '+xml'
 convention unless they have a particularly compelling reason not to.
 The registration process for these media types is described in
 [RFC2048].  The registrar for the IETF tree will encourage new XML-
 based media type registrations in the IETF tree to follow this
 guideline.  Registrars for other trees SHOULD follow this convention

Murata, et al. Standards Track [Page 17] RFC 3023 XML Media Types January 2001

 in order to ensure maximum interoperability of their XML-based
 documents.  Similarly, media subtypes that do not represent XML MIME
 entities MUST NOT be allowed to register with a '+xml' suffix.

7.1 Referencing

 Registrations for new XML-based media types under the top-level type
 "text" SHOULD, in specifying the charset parameter and encoding
 considerations, define them as: "Same as [charset parameter /
 encoding considerations] of text/xml as specified in RFC 3023."
 Registrations for new XML-based media types under top-level types
 other than "text" SHOULD, in specifying the charset parameter and
 encoding considerations, define them as: "Same as [charset parameter
 / encoding considerations] of application/xml as specified in RFC
 3023."
 The use of the charset parameter is STRONGLY RECOMMENDED, since this
 information can be used by XML processors to determine
 authoritatively the charset of the XML MIME entity.
 These registrations SHOULD specify that the XML-based media type
 being registered has all of the security considerations described in
 RFC 3023 plus any additional considerations specific to that media
 type.
 These registrations SHOULD also make reference to RFC 3023 in
 specifying magic numbers, fragment identifiers, base URIs, and use of
 the BOM.
 These registrations MAY reference the text/xml registration in RFC
 3023 in specifying interoperability considerations, if these
 considerations are not overridden by issues specific to that media
 type.

8. Examples

 The examples below give the value of the MIME Content-type header and
 the XML declaration (which includes the encoding declaration) inside
 the XML MIME entity.  For UTF-16 examples, the Byte Order Mark
 character is denoted as "{BOM}", and the XML declaration is assumed
 to come at the beginning of the XML MIME entity, immediately
 following the BOM.  Note that other MIME headers may be present, and
 the XML MIME entity may contain other data in addition to the XML
 declaration; the examples focus on the Content-type header and the
 encoding declaration for clarity.

Murata, et al. Standards Track [Page 18] RFC 3023 XML Media Types January 2001

8.1 Text/xml with UTF-8 Charset

 Content-type: text/xml; charset="utf-8"
 <?xml version="1.0" encoding="utf-8"?>
 This is the recommended charset value for use with text/xml.  Since
 the charset parameter is provided, MIME and XML processors MUST treat
 the enclosed entity as UTF-8 encoded.
 If sent using a 7-bit transport (e.g., SMTP[RFC0821]), the XML MIME
 entity MUST use a content-transfer-encoding of either quoted-
 printable or base64.  For an 8-bit clean transport (e.g., 8BITMIME
 ESMTP or NNTP), or a binary clean transport (e.g., HTTP), no
 content-transfer-encoding is necessary.

8.2 Text/xml with UTF-16 Charset

 Content-type: text/xml; charset="utf-16"
 {BOM}<?xml version='1.0' encoding='utf-16'?>
 or
 {BOM}<?xml version='1.0'?>
 This is possible only when the XML MIME entity is transmitted via
 HTTP, which uses a MIME-like mechanism and is a binary-clean
 protocol, hence does not perform CR and LF transformations and allows
 NUL octets.  As described in [RFC2781], the UTF-16 family MUST NOT be
 used with media types under the top-level type "text" except over
 HTTP (see section 19.4.1 of [RFC2616] for details).
 Since HTTP is binary clean, no content-transfer-encoding is
 necessary.

8.3 Text/xml with UTF-16BE Charset

 Content-type: text/xml; charset="utf-16be"
 <?xml version='1.0' encoding='utf-16be'?>
 Observe that the BOM does not exist.  This is again possible only
 when the XML MIME entity is transmitted via HTTP.

Murata, et al. Standards Track [Page 19] RFC 3023 XML Media Types January 2001

8.4 Text/xml with ISO-2022-KR Charset

 Content-type: text/xml; charset="iso-2022-kr"
 <?xml version="1.0" encoding='iso-2022-kr'?>
 This example shows text/xml with a Korean charset (e.g., Hangul)
 encoded following the specification in [RFC1557].  Since the charset
 parameter is provided, MIME and XML processors MUST treat the
 enclosed entity as encoded per RFC 1557.
 Since ISO-2022-KR has been defined to use only 7 bits of data, no
 content-transfer-encoding is necessary with any transport.

8.5 Text/xml with Omitted Charset

 Content-type: text/xml
 {BOM}<?xml version="1.0" encoding="utf-16"?>
 or
 {BOM}<?xml version="1.0"?>
 This example shows text/xml with the charset parameter omitted.  In
 this case, MIME and XML processors MUST assume the charset is "us-
 ascii", the default charset value for text media types specified in
 [RFC2046].  The default of "us-ascii" holds even if the text/xml
 entity is transported using HTTP.
 Omitting the charset parameter is NOT RECOMMENDED for text/xml.  For
 example, even if the contents of the XML MIME entity are UTF-16 or
 UTF-8, or the XML MIME entity has an explicit encoding declaration,
 XML and MIME processors MUST assume the charset is "us-ascii".

8.6 Application/xml with UTF-16 Charset

 Content-type: application/xml; charset="utf-16"
 {BOM}<?xml version="1.0" encoding="utf-16"?>
 or
 {BOM}<?xml version="1.0"?>
 This is a recommended charset value for use with application/xml.
 Since the charset parameter is provided, MIME and XML processors MUST
 treat the enclosed entity as UTF-16 encoded.

Murata, et al. Standards Track [Page 20] RFC 3023 XML Media Types January 2001

 If sent using a 7-bit transport (e.g., SMTP) or an 8-bit clean
 transport (e.g., 8BITMIME ESMTP or NNTP), the XML MIME entity MUST be
 encoded in quoted-printable or base64.  For a binary clean transport
 (e.g., HTTP), no content-transfer-encoding is necessary.

8.7 Application/xml with UTF-16BE Charset

 Content-type: application/xml; charset="utf-16be"
 <?xml version='1.0' encoding='utf-16be'?>
 Observe that the BOM does not exist.  Since the charset parameter is
 provided, MIME and XML processors MUST treat the enclosed entity as
 UTF-16BE encoded.

8.8 Application/xml with ISO-2022-KR Charset

 Content-type: application/xml; charset="iso-2022-kr"
 <?xml version="1.0" encoding="iso-2022-kr"?>
 This example shows application/xml with a Korean charset (e.g.,
 Hangul) encoded following the specification in [RFC1557].  Since the
 charset parameter is provided, MIME and XML processors MUST treat the
 enclosed entity as encoded per RFC 1557, independent of whether the
 XML MIME entity has an internal encoding declaration (this example
 does show such a declaration, which agrees with the charset
 parameter).
 Since ISO-2022-KR has been defined to use only 7 bits of data, no
 content-transfer-encoding is necessary with any transport.

8.9 Application/xml with Omitted Charset and UTF-16 XML MIME Entity

 Content-type: application/xml
 {BOM}<?xml version='1.0' encoding="utf-16"?>
 or
 {BOM}<?xml version='1.0'?>
 For this example, the XML MIME entity begins with a BOM.  Since the
 charset has been omitted, a conforming XML processor follows the
 requirements of [XML], section 4.3.3.  Specifically, the XML
 processor reads the BOM, and thus knows deterministically that the
 charset is UTF-16.

Murata, et al. Standards Track [Page 21] RFC 3023 XML Media Types January 2001

 An XML-unaware MIME processor SHOULD make no assumptions about the
 charset of the XML MIME entity.

8.10 Application/xml with Omitted Charset and UTF-8 Entity

 Content-type: application/xml
 <?xml version='1.0'?>
 In this example, the charset parameter has been omitted, and there is
 no BOM.  Since there is no BOM, the XML processor follows the
 requirements in section 4.3.3 of [XML], and optionally applies the
 mechanism described in Appendix F (which is non-normative) of [XML]
 to determine the charset encoding of UTF-8.  The XML MIME entity does
 not contain an encoding declaration, but since the encoding is UTF-8,
 this is still a conforming XML MIME entity.
 An XML-unaware MIME processor SHOULD make no assumptions about the
 charset of the XML MIME entity.

8.11 Application/xml with Omitted Charset and Internal Encoding

   Declaration
 Content-type: application/xml
 <?xml version='1.0' encoding="iso-10646-ucs-4"?>
 In this example, the charset parameter has been omitted, and there is
 no BOM.  However, the XML MIME entity does have an encoding
 declaration inside the XML MIME entity that specifies the entity's
 charset.  Following the requirements in section 4.3.3 of [XML], and
 optionally applying the mechanism described in Appendix F (non-
 normative) of [XML], the XML processor determines the charset of the
 XML MIME entity (in this example, UCS-4).
 An XML-unaware MIME processor SHOULD make no assumptions about the
 charset of the XML MIME entity.

8.12 Text/xml-external-parsed-entity with UTF-8 Charset

 Content-type: text/xml-external-parsed-entity; charset="utf-8"
 <?xml encoding="utf-8"?>
 This is the recommended charset value for use with text/xml-
 external-parsed-entity.  Since the charset parameter is provided,
 MIME and XML processors MUST treat the enclosed entity as UTF-8
 encoded.

Murata, et al. Standards Track [Page 22] RFC 3023 XML Media Types January 2001

 If sent using a 7-bit transport (e.g., SMTP), the XML MIME entity
 MUST use a content-transfer-encoding of either quoted-printable or
 base64.  For an 8-bit clean transport (e.g., 8BITMIME ESMTP or NNTP),
 or a binary clean transport (e.g., HTTP) no content-transfer-encoding
 is necessary.

8.13 Application/xml-external-parsed-entity with UTF-16 Charset

 Content-type: application/xml-external-parsed-entity;
  charset="utf-16"
 {BOM}<?xml encoding="utf-16"?>
 or
 {BOM}<?xml?>
 This is a recommended charset value for use with application/xml-
 external-parsed-entity.  Since the charset parameter is provided,
 MIME and XML processors MUST treat the enclosed entity as UTF-16
 encoded.
 If sent using a 7-bit transport (e.g., SMTP) or an 8-bit clean
 transport (e.g., 8BITMIME ESMTP or NNTP), the XML MIME entity MUST be
 encoded in quoted-printable or base64.  For a binary clean transport
 (e.g., HTTP), no content-transfer-encoding is necessary.

8.14 Application/xml-external-parsed-entity with UTF-16BE Charset

 Content-type: application/xml-external-parsed-entity;
  charset="utf-16be"
 <?xml encoding="utf-16be"?>
 Since the charset parameter is provided, MIME and XML processors MUST
 treat the enclosed entity as UTF-16BE encoded.

8.15 Application/xml-dtd

 Content-type: application/xml-dtd; charset="utf-8"
 <?xml encoding="utf-8"?>
 Charset "utf-8" is a recommended charset value for use with
 application/xml-dtd.  Since the charset parameter is provided, MIME
 and XML processors MUST treat the enclosed entity as UTF-8 encoded.

Murata, et al. Standards Track [Page 23] RFC 3023 XML Media Types January 2001

8.16 Application/mathml+xml

 Content-type: application/mathml+xml
 <?xml version="1.0" ?>
 MathML documents are XML documents whose content describes
 mathematical information, as defined by [MathML].  As a format based
 on XML, MathML documents SHOULD use the '+xml' suffix convention in
 their MIME content-type identifier.  However, no content type has yet
 been registered for MathML and so this media type should not be used
 until such registration has been completed.

8.17 Application/xslt+xml

 Content-type: application/xslt+xml
 <?xml version="1.0" ?>
 Extensible Stylesheet Language (XSLT) documents are XML documents
 whose content describes stylesheets for other XML documents, as
 defined by [XSLT].  As a format based on XML, XSLT documents SHOULD
 use the '+xml' suffix convention in their MIME content-type
 identifier.  However, no content type has yet been registered for
 XSLT and so this media type should not be used until such
 registration has been completed.

8.18 Application/rdf+xml

 Content-type: application/rdf+xml
 <?xml version="1.0" ?>
 RDF documents identified using this MIME type are XML documents whose
 content describes metadata, as defined by [RDF].  As a format based
 on XML, RDF documents SHOULD use the '+xml' suffix convention in
 their MIME content-type identifier.  However, no content type has yet
 been registered for RDF and so this media type should not be used
 until such registration has been completed.

8.19 Image/svg+xml

 Content-type: image/svg+xml
 <?xml version="1.0" ?>

Murata, et al. Standards Track [Page 24] RFC 3023 XML Media Types January 2001

 Scalable Vector Graphics (SVG) documents are XML documents whose
 content describes graphical information, as defined by [SVG].  As a
 format based on XML, SVG documents SHOULD use the '+xml' suffix
 convention in their MIME content-type identifier.  However, no
 content type has yet been registered for SVG and so this media type
 should not be used until such registration has been completed.

8.20 INCONSISTENT EXAMPLE: Text/xml with UTF-8 Charset

 Content-type: text/xml; charset="utf-8"
 <?xml version="1.0" encoding="iso-8859-1"?>
 Since the charset parameter is provided in the Content-Type header,
 MIME and XML processors MUST treat the enclosed entity as UTF-8
 encoded.  That is, the "iso-8859-1" encoding MUST be ignored.
 Processors generating XML MIME entities MUST NOT label conflicting
 charset information between the MIME Content-Type and the XML
 declaration.

9. IANA Considerations

 As described in Section 7, this document updates the [RFC2048]
 registration process for XML-based MIME types.

10. Security Considerations

 XML, as a subset of SGML, has all of the same security considerations
 as specified in [RFC1874], and likely more, due to its expected
 ubiquitous deployment.
 To paraphrase section 3 of RFC 1874, XML MIME entities contain
 information to be parsed and processed by the recipient's XML system.
 These entities may contain and such systems may permit explicit
 system level commands to be executed while processing the data.  To
 the extent that an XML system will execute arbitrary command strings,
 recipients of XML MIME entities may be a risk.  In general, it may be
 possible to specify commands that perform unauthorized file
 operations or make changes to the display processor's environment
 that affect subsequent operations.
 In general, any information stored outside of the direct control of
 the user -- including CSS style sheets, XSL transformations, entity
 declarations, and DTDs -- can be a source of insecurity, by either
 obvious or subtle means.  For example, a tiny "whiteout attack"
 modification made to a "master" style sheet could make words in
 critical locations disappear in user documents, without directly

Murata, et al. Standards Track [Page 25] RFC 3023 XML Media Types January 2001

 modifying the user document or the stylesheet it references.  Thus,
 the security of any XML document is vitally dependent on all of the
 documents recursively referenced by that document.
 The entity lists and DTDs for XHTML 1.0[XHTML], for instance, are
 likely to be a commonly used set of information.  Many developers
 will use and trust them, few of whom will know much about the level
 of security on the W3C's servers, or on any similarly trusted
 repository.
 The simplest attack involves adding declarations that break
 validation.  Adding extraneous declarations to a list of character
 entities can effectively "break the contract" used by documents.  A
 tiny change that produces a fatal error in a DTD could halt XML
 processing on a large scale.  Extraneous declarations are fairly
 obvious, but more sophisticated tricks, like changing attributes from
 being optional to required, can be difficult to track down.  Perhaps
 the most dangerous option available to crackers is redefining default
 values for attributes: e.g., if developers have relied on defaulted
 attributes for security, a relatively small change might expose
 enormous quantities of information.
 Apart from the structural possibilities, another option, "entity
 spoofing," can be used to insert text into documents, vandalizing and
 perhaps conveying an unintended message.  Because XML 1.0 permits
 multiple entity declarations, and the first declaration takes
 precedence, it's possible to insert malicious content where an entity
 is used, such as by inserting the full text of Winnie the Pooh in
 every occurrence of &mdash;.
 Use of the digital signatures work currently underway by the xmldsig
 working group may eventually ameliorate the dangers of referencing
 external documents not under one's own control.
 Use of XML is expected to be varied, and widespread.  XML is under
 scrutiny by a wide range of communities for use as a common syntax
 for community-specific metadata.  For example, the Dublin
 Core[RFC2413] group is using XML for document metadata, and a new
 effort has begun that is considering use of XML for medical
 information.  Other groups view XML as a mechanism for marshalling
 parameters for remote procedure calls.  More uses of XML will
 undoubtedly arise.
 Security considerations will vary by domain of use.  For example, XML
 medical records will have much more stringent privacy and security
 considerations than XML library metadata.  Similarly, use of XML as a
 parameter marshalling syntax necessitates a case by case security
 review.

Murata, et al. Standards Track [Page 26] RFC 3023 XML Media Types January 2001

 XML may also have some of the same security concerns as plain text.
 Like plain text, XML can contain escape sequences that, when
 displayed, have the potential to change the display processor
 environment in ways that adversely affect subsequent operations.
 Possible effects include, but are not limited to, locking the
 keyboard, changing display parameters so subsequent displayed text is
 unreadable, or even changing display parameters to deliberately
 obscure or distort subsequent displayed material so that its meaning
 is lost or altered.  Display processors SHOULD either filter such
 material from displayed text or else make sure to reset all important
 settings after a given display operation is complete.
 Some terminal devices have keys whose output, when pressed, can be
 changed by sending the display processor a character sequence.  If
 this is possible the display of a text object containing such
 character sequences could reprogram keys to perform some illicit or
 dangerous action when the key is subsequently pressed by the user.
 In some cases not only can keys be programmed, they can be triggered
 remotely, making it possible for a text display operation to directly
 perform some unwanted action.  As such, the ability to program keys
 SHOULD be blocked either by filtering or by disabling the ability to
 program keys entirely.
 Note that it is also possible to construct XML documents that make
 use of what XML terms "entity references" (using the XML meaning of
 the term "entity" as described in Section 2), to construct repeated
 expansions of text.  Recursive expansions are prohibited by [XML] and
 XML processors are required to detect them.  However, even non-
 recursive expansions may cause problems with the finite computing
 resources of computers, if they are performed many times.

References

 [ASCII]    "US-ASCII. Coded Character Set -- 7-Bit American Standard
            Code for Information Interchange", ANSI X3.4-1986, 1986.
 [CSS]      Bos, B., Lie, H.W., Lilley, C. and I. Jacobs, "Cascading
            Style Sheets, level 2 (CSS2) Specification", World Wide
            Web Consortium Recommendation REC-CSS2, May 1998,
            <http://www.w3.org/TR/REC-CSS2/>.
 [ISO8859]  "ISO-8859. International Standard -- Information
            Processing -- 8-bit Single-Byte Coded Graphic Character
            Sets -- Part 1: Latin alphabet No. 1, ISO-8859-1:1987",
            1987.

Murata, et al. Standards Track [Page 27] RFC 3023 XML Media Types January 2001

 [MathML]   Ion, P. and R. Miner, "Mathematical Markup Language
            (MathML) 1.01", World Wide Web Consortium Recommendation
            REC-MathML, July 1999, <http://www.w3.org/TR/REC-MathML/>.
 [PNG]      Boutell, T., "PNG (Portable Network Graphics)
            Specification", World Wide Web Consortium Recommendation
            REC-png, October 1996, <http://www.w3.org/TR/REC-png>.
 [RDF]      Lassila, O. and R.R. Swick, "Resource Description
            Framework (RDF) Model and Syntax Specification", World
            Wide Web Consortium Recommendation REC-rdf-syntax,
            February 1999, <http://www.w3.org/TR/REC-rdf-syntax/>.
 [RFC0821]  Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
            821, August 1982.
 [RFC0977]  Kantor, B. and P. Lapsley, "Network News Transfer
            Protocol", RFC 977, February 1986.
 [RFC1557]  Choi, U., Chon, K. and H. Park, "Korean Character Encoding
            for Internet Messages", RFC 1557, December 1993.
 [RFC1652]  Klensin, J., Freed, N., Rose, M., Stefferud, E. and D.
            Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
            RFC 1652, July 1994.
 [RFC1874]  Levinson, E., "SGML Media Types", RFC 1874, December 1995.
 [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part One: Format of Internet Message
            Bodies", RFC 2045, November 1996.
 [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part Two: Media Types", RFC 2046,
            November 1996.
 [RFC2048]  Freed, N., Klensin, J. and J. Postel, "Multipurpose
            Internet Mail Extensions (MIME) Part Four: Registration
            Procedures", RFC 2048, November 1996.
 [RFC2060]  Crispin, M., "Internet Message Access Protocol - Version
            4rev1", RFC 2060, December 1996.
 [RFC2077]  Nelson, S., Parks, C. and Mitra, "The Model Primary
            Content Type for Multipurpose Internet Mail Extensions",
            RFC 2077, January 1997.

Murata, et al. Standards Track [Page 28] RFC 3023 XML Media Types January 2001

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2130]  Weider, C., Preston, C., Simonsen, K., Alvestrand, H.,
            Atkinson, R., Crispin, M. and P. Svanberg, "The Report of
            the IAB Character Set Workshop held 29 February - 1 March,
            1996", RFC 2130, April 1997.
 [RFC2279]  Yergeau, F., "UTF-8, a transformation format of ISO
            10646", RFC 2279, January 1998.
 [RFC2376]  Whitehead, E. and M. Murata, "XML Media Types", RFC 2376,
            July 1998.
 [RFC2396]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
            Resource Identifiers (URI): Generic Syntax.", RFC 2396,
            August 1998.
 [RFC2413]  Weibel, S., Kunze, J., Lagoze, C. and M. Wolf, "Dublin
            Core Metadata for Resource Discovery", RFC 2413, September
            1998.
 [RFC2445]  Dawson, F. and D. Stenerson, "Internet Calendaring and
            Scheduling Core Object Specification (iCalendar)", RFC
            2445, November 1998.
 [RFC2518]  Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D.
            Jensen, "HTTP Extensions for Distributed Authoring --
            WEBDAV", RFC 2518, February 1999.
 [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
            Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext
            Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
 [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
            June 1999.
 [RFC2703]  Klyne, G., "Protocol-independent Content Negotiation
            Framework", RFC 2703, September 1999.
 [RFC2781]  Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO
            10646", RFC 2781, Februrary 2000.
 [RFC2801]  Burdett, D., "Internet Open Trading Protocol - IOTP
            Version 1.0", RFC 2801, April 2000.

Murata, et al. Standards Track [Page 29] RFC 3023 XML Media Types January 2001

 [SGML]     International Standard Organization, "Information
            Processing -- Text and Office Systems -- Standard
            Generalized Markup Language (SGML)", ISO 8879, October
            1986.
 [SVG]      Ferraiolo, J., "Scalable Vector Graphics (SVG)", World
            Wide Web Consortium Candidate Recommendation SVG, November
            2000, <http://www.w3.org/TR/SVG>.
 [XHTML]    Pemberton, S. and  et al, "XHTML 1.0: The Extensible
            HyperText Markup Language", World Wide Web Consortium
            Recommendation xhtml1, January 2000,
            <http://www.w3.org/TR/xhtml1>.
 [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C.M. and E. Maler,
            "Extensible Markup Language (XML) 1.0 (Second Edition)",
            World Wide Web Consortium Recommendation REC-xml, October
            2000, <http://www.w3.org/TR/REC-xml>.
 [XSLT]     Clark, J., "XSL Transformations (XSLT) Version 1.0", World
            Wide Web Consortium Recommendation xslt, November 1999,
            <http://www.w3.org/TR/xslt>.

Murata, et al. Standards Track [Page 30] RFC 3023 XML Media Types January 2001

Authors' Addresses

 MURATA Makoto (FAMILY Given)
 IBM Tokyo Research Laboratory
 1623-14, Shimotsuruma
 Yamato-shi, Kanagawa-ken  242-8502
 Japan
 Phone: +81-46-215-4678
 EMail: mmurata@trl.ibm.co.jp
 Simon St.Laurent
 simonstl.com
 1259 Dryden Road
 Ithaca, New York  14850
 USA
 EMail: simonstl@simonstl.com
 URI:   http://www.simonstl.com/
 Dan Kohn
 Skymoon Ventures
 3045 Park Boulevard
 Palo Alto, California  94306
 USA
 Phone: +1-650-327-2600
 EMail: dan@dankohn.com
 URI:   http://www.dankohn.com/

Murata, et al. Standards Track [Page 31] RFC 3023 XML Media Types January 2001

Appendix A. Why Use the '+xml' Suffix for XML-Based MIME Types?

 Although the use of a suffix was not considered as part of the
 original MIME architecture, this choice is considered to provide the
 most functionality with the least potential for interoperability
 problems or lack of future extensibility.  The alternatives to the
 '+xml' suffix and the reason for its selection are described below.

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?
 text/xml and application/xml remain useful in many situations,
 especially for document-oriented applications that involve combining
 XML with a stylesheet in order to present the data.  However, XML is
 also used to define entirely new data types, and an XML-based format
 such as image/svg+xml fits the definition of a MIME media type
 exactly as well as image/png[PNG] does.  (Note that image/svg+xml is
 not yet registered.) Although extra functionality is available for
 MIME processors that are also XML processors, XML-based media types
 -- even when treated as opaque, non-XML media types -- are just as
 useful as any other media type and should be treated as such.
 Since MIME dispatchers work off of the MIME type, use of text/xml or
 application/xml to label discrete media types will hinder correct
 dispatching and general interoperability.  Finally, many XML
 documents use neither DTDs nor namespaces, yet are perfectly legal
 XML.

A.2 Why not create a new subtree (e.g., image/xml.svg) to represent XML

  MIME types?
 The subtree under which a media type is registered -- IETF, vendor
 (*/vnd.*), or personal (*/prs.*); see [RFC2048] for details -- is
 completely orthogonal from whether the media type uses XML syntax or
 not.  The suffix approach allows XML document types to be identified
 within any subtree.  The vendor subtree, for example, is likely to
 include a large number of XML-based document types.  By using a
 suffix, rather than setting up a separate subtree, those types may
 remain in the same location in the tree of MIME types that they would
 have occupied had they not been based on XML.

A.3 Why not create a new top-level MIME type for XML-based media types?

 The top-level MIME type (e.g., model/*[RFC2077]) determines what kind
 of content the type is, not what syntax it uses.  For example, agents
 using image/* to signal acceptance of any image format should
 certainly be given access to media type image/svg+xml, which is in

Murata, et al. Standards Track [Page 32] RFC 3023 XML Media Types January 2001

 all respects a standard image subtype.  It just happens to use XML to
 describe its syntax.  The two aspects of the media type are
 completely orthogonal.
 XML-based data types will most likely be registered in ALL top-level
 categories.  Potential, though currently unregistered, examples could
 include application/mathml+xml[MathML] and image/svg+xml[SVG].

A.4 Why not just have the MIME processor 'sniff' the content to

  determine whether it is XML?
 Rather than explicitly labeling XML-based media types, the processor
 could look inside each type and see whether or not it is XML.  The
 processor could also cache a list of XML-based media types.
 Although this method might work acceptably for some mail
 applications, it would fail completely in many other uses of MIME.
 For instance, an XML-based web crawler would have no way of
 determining whether a file is XML except to fetch it and check.  The
 same issue applies in some IMAP4[RFC2060] mail applications, where
 the client first fetches the MIME type as part of the message
 structure and then decides whether to fetch the MIME entity.
 Requiring these fetches just to determine whether the MIME type is
 XML could have significant bandwidth and latency disadvantages in
 many situations.
 Sniffing XML also isn't as simple as it might seem.  DOCTYPE
 declarations aren't required, and they can appear fairly deep into a
 document under certain unpreventable circumstances.  (E.g., the XML
 declaration, comments, and processing instructions can occupy space
 before the DOCTYPE declaration.) Even sniffing the DOCTYPE isn't
 completely reliable, thanks to a variety of issues involving default
 values for namespaces within external DTDs and overrides inside the
 internal DTD.  Finally, the variety in potential character encodings
 (something XML provides tools to deal with), also makes reliable
 sniffing less likely.

A.5 Why not use a MIME parameter to specify that a media type uses XML

  syntax?
 For example, one could use "Content-Type: application/iotp;
 alternate-type=text/xml" or "Content-Type: application/iotp;
 syntax=xml".
 Section 5 of [RFC2045] says that "Parameters are modifiers of the
 media subtype, and as such do not fundamentally affect the nature of
 the content".  However, all XML-based media types are by their nature

Murata, et al. Standards Track [Page 33] RFC 3023 XML Media Types January 2001

 always XML.  Parameters, as they have been defined in the MIME
 architecture, are never invariant across all instantiations of a
 media type.
 More practically, very few if any MIME dispatchers and other MIME
 agents support dispatching off of a parameter.  While MIME agents on
 the receiving side will need to be updated in either case to support
 (or fall back to) generic XML processing, it has been suggested that
 it is easier to implement this functionality when acting off of the
 media type rather than a parameter.  More important, sending agents
 require no update to properly tag an image as "image/svg+xml", but
 few if any sending agents currently support always tagging certain
 content types with a parameter.

A.6 How about labeling with parameters in the other direction (e.g.,

  application/xml; Content-Feature=iotp)?
 This proposal fails under the simplest case, of a user with neither
 knowledge of XML nor an XML-capable MIME dispatcher.  In that case,
 the user's MIME dispatcher is likely to dispatch the content to an
 XML processing application when the correct default behavior should
 be to dispatch the content to the application responsible for the
 content type (e.g., an ecommerce engine for
 application/iotp+xml[RFC2801], once this media type is registered).
 Note that even if the user had already installed the appropriate
 application (e.g., the ecommerce engine), and that installation had
 updated the MIME registry, many operating system level MIME
 registries such as .mailcap in Unix and HKEY_CLASSES_ROOT in Windows
 do not currently support dispatching off a parameter, and cannot
 easily be upgraded to do so.  And, even if the operating system were
 upgraded to support this, each MIME dispatcher would also separately
 need to be upgraded.

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)?
 This combines the problems of Appendix A.5 and Appendix A.6.
 If the sender attaches an image/svg+xml file to a message and
 includes the instructions "Please copy the French text on the road
 sign", someone with an XML-aware MIME client and an XML browser but
 no support for SVG can still probably open the file and copy the
 text.  By contrast, with superclasses, the sender must add superclass
 support to her existing mailer AND the receiver must add superclass
 support to his before this transaction can work correctly.

Murata, et al. Standards Track [Page 34] RFC 3023 XML Media Types January 2001

 If the receiver comes to rely on the superclass tag being present and
 applications are deployed relying on that tag (as always seems to
 happen), then only upgraded senders will be able to interoperate with
 those receiving applications.

A.8 What about adding a new parameter to the Content-Disposition header

  or creating a new Content-Structure header to indicate XML syntax?
 This has nearly identical problems to Appendix A.7, in that it
 requires both senders and receivers to be upgraded, and few if any
 operating systems and MIME dispatchers support working off of
 anything other than the MIME type.

A.9 How about a new Alternative-Content-Type header?

 This is better than Appendix A.8, in that no extra functionality
 needs to be added to a MIME registry to support dispatching of
 information other than standard content types.  However, it still
 requires both sender and receiver to be upgraded, and it will also
 fail in many cases (e.g., web hosting to an outsourced server), where
 the user can set MIME types (often through implicit mapping to file
 extensions), but has no way of adding arbitrary HTTP headers.

A.10 How about using a conneg tag instead (e.g., accept-features:

   (syntax=xml))?
 When the conneg protocol is fully defined, this may potentially be a
 reasonable thing to do.  But given the limited current state of
 conneg[RFC2703] development, it is not a credible replacement for a
 MIME-based solution.

A.11 How about a third-level content-type, such as text/xml/rdf?

 MIME explicitly defines two levels of content type, the top-level for
 the kind of content and the second-level for the specific media type.
 [RFC2048] extends this in an interoperable way by using prefixes to
 specify separate trees for IETF, vendor, and personal registrations.
 This specification also extends the two-level type by using the '
 +xml' suffix.  In both cases, processors that are unaware of these
 later specifications treat them as opaque and continue to
 interoperate.  By contrast, adding a third-level type would break the
 current MIME architecture and cause numerous interoperability
 failures.

Murata, et al. Standards Track [Page 35] RFC 3023 XML Media Types January 2001

A.12 Why use the plus ('+') character for the suffix '+xml'?

 As specified in Section 5.1 of [RFC2045], a tspecial can't be used:
    tspecials :=
    "(" / ")" / "<" / ">" / "@" /
    "," / ";" / ":" / "\" / <">
    "/" / "[" / "]" / "?" / "="
 It was thought that "." would not be a good choice since it is
 already used as an additional hierarchy delimiter.  Also, "*" has a
 common wildcard meaning, and "-" and "_" are common word separators
 and easily confused.  The characters %'`#& are frequently used for
 quoting or comments and so are not ideal.
 That leaves: ~!$^+{}|
 Note that "-" is used heavily in the current registry.  "$" and "_"
 are used once each.  The others are currently unused.
 It was thought that '+' expressed the semantics that a MIME type can
 be treated (for example) as both scalable vector graphics AND ALSO as
 XML; it is both simultaneously.

A.13 What is the semantic difference between application/foo and

   application/foo+xml?
 MIME processors that are unaware of XML will treat the '+xml' suffix
 as completely opaque, so it is essential that no extra semantics be
 assigned to its presence.  Therefore, application/foo and
 application/foo+xml SHOULD be treated as completely independent media
 types.  Although, for example, text/calendar+xml could be an XML
 version of text/calendar[RFC2445], it is possible that this
 (hypothetical) new media type would include new semantics as well as
 new syntax, and in any case, there would be many applications that
 support text/calendar but had not yet been upgraded to support
 text/calendar+xml.

A.14 What happens when an even better markup language (e.g., EBML) is

   defined, or a new category of data?
 In the ten years that MIME has existed, XML is the first generic data
 format that has seemed to justify special treatment, so it is hoped
 that no further suffixes will be necessary.  However, if some are
 later defined, and these documents were also XML, they would need to
 specify that the '+xml' suffix is always the outermost suffix (e.g.,

Murata, et al. Standards Track [Page 36] RFC 3023 XML Media Types January 2001

 application/foo+ebml+xml not application/foo+xml+ebml).  If they were
 not XML, then they would use a regular suffix (e.g.,
 application/foo+ebml).

A.15 Why must I use the '+xml' suffix for my new XML-based media type?

 You don't have to, but unless you have a good reason to explicitly
 disallow generic XML processing, you should use the suffix so as not
 to curtail the options of future users and developers.
 Whether the inventors of a media type, today, design it for dispatch
 to generic XML processing machinery (and most won't) is not the
 critical issue.  The core notion is that the knowledge that some
 media type happens to use XML syntax opens the door to unanticipated
 kinds of processing beyond those envisioned by its inventors, and on
 this basis identifying such encoding is a good and useful thing.
 Developers of new media types are often tightly focused on a
 particular type of processing that meets current needs.  But there is
 no need to rule out generic processing as well, which could make your
 media type more valuable over time.  It is believed that registering
 with the '+xml' suffix will cause no interoperability problems
 whatsoever, while it may enable significant new functionality and
 interoperability now and in the future.  So, the conservative
 approach is to include the '+xml' suffix.

Appendix B. Changes from RFC 2376

 There are numerous and significant differences between this
 specification and [RFC2376], which it obsoletes.  This appendix
 summarizes the major differences only.
 First, text/xml-external-parsed-entity and application/xml-external-
 parsed-entity are added as media types for external parsed entities,
 and text/xml and application/xml are now prohibited.
 Second, application/xml-dtd is added as a media type for external DTD
 subsets and external parameter entities, and text/xml and
 application/xml are now prohibited.
 Third, "utf-16le" and "utf-16be" are added.  RFC 2781 has introduced
 these BOM-less variations of the UTF-16 family.
 Fourth, a naming convention ('+xml') for XML-based media types has
 been added, which also updates [RFC2048] as described in Section 7.
 By following this convention, an XML-based media type can be easily
 recognized as such.

Murata, et al. Standards Track [Page 37] RFC 3023 XML Media Types January 2001

Appendix C. Acknowledgements

 This document reflects the input of numerous participants to the
 ietf-xml-mime@imc.org mailing list, though any errors are the
 responsibility of the authors.  Special thanks to:
 Mark Baker, James Clark, Dan Connolly, Martin Duerst, Ned Freed,
 Yaron Goland, Rick Jelliffe, Larry Masinter, David Megginson, Keith
 Moore, Chris Newman, Gavin Nicol, Marshall Rose, Jim Whitehead and
 participants of the XML activity at the W3C.

Murata, et al. Standards Track [Page 38] RFC 3023 XML Media Types January 2001

Full Copyright Statement

 Copyright (C) The Internet Society (2001).  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.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Murata, et al. Standards Track [Page 39]

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