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Versions: (draft-lilley-xml-mediatypes) 00 01 02 03 04 05 06 07 08 09 10 RFC 7303

Network Working Group                                          C. Lilley
Internet-Draft                                                       W3C
Obsoletes: 3023 (if approved)                                  M. Murata
Updates: 4288, 4289, 6839 (if approved)International University of Japan
Intended status: Standards Track                             A. Melnikov
Expires: November 29, 2013                                    Isode Ltd.
                                                          H. S. Thompson
                                                 University of Edinburgh
                                                            May 28, 2013


                            XML Media Types
                  draft-ietf-appsawg-xml-mediatypes-01

Abstract

   This specification standardizes three media types -- application/xml,
   application/xml-external-parsed-entity, and application/xml-dtd --
   for use in exchanging network entities that are related to the
   Extensible Markup Language (XML) while defining text/xml and text/
   xml-external-parsed-entity as aliases for the respective application/
   types.  This specification also standardizes a convention (using the
   suffix '+xml') for naming media types outside of these five types
   when those media types represent XML MIME entities.

   Major differences from [RFC3023] are alignment of charset handling
   for text/xml and text/xml-external-parsed-entity with application/
   xml, the addition of XPointer and XML Base as fragment identifiers
   and base URIs, respectively, mention of the XPointer Registry, and
   updating of many references.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 29, 2013.




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Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  XML Media Types . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Application/xml Registration  . . . . . . . . . . . . . .   7
     3.2.  Text/xml Registration . . . . . . . . . . . . . . . . . .   9
     3.3.  Application/xml-external-parsed-entity Registration . . .   9
     3.4.  Text/xml-external-parsed-entity Registration  . . . . . .  10
     3.5.  Application/xml-dtd Registration  . . . . . . . . . . . .  10
     3.6.  Charset considerations  . . . . . . . . . . . . . . . . .  11
       3.6.1.  Background  . . . . . . . . . . . . . . . . . . . . .  12
   4.  The Byte Order Mark (BOM) and Conversions to/from the UTF-16
       Charset . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   5.  Fragment Identifiers  . . . . . . . . . . . . . . . . . . . .  12
   6.  The Base URI  . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  XML Versions  . . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  A Naming Convention for XML-Based Media Types . . . . . . . .  14
     8.1.  Referencing . . . . . . . . . . . . . . . . . . . . . . .  16
   9.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  application/xml or text/xml with Omitted Charset and
           8-bit MIME entity . . . . . . . . . . . . . . . . . . . .  17
     9.2.  application/xml or text/xml with Omitted Charset and
           16-bit MIME entity  . . . . . . . . . . . . . . . . . . .  17
     9.3.  application/xml or text/xml with UTF-8 Charset  . . . . .  17
     9.4.  application/xml with UTF-16 Charset . . . . . . . . . . .  18
     9.5.  text/xml with UTF-16 Charset  . . . . . . . . . . . . . .  18
     9.6.  application/xml with UTF-16BE Charset . . . . . . . . . .  18
     9.7.  text/xml with UTF-16BE Charset  . . . . . . . . . . . . .  19
     9.8.  application/xml or text/xml with ISO-2022-KR Charset  . .  19
     9.9.  application/xml or text/xml with Omitted Charset, no
           Internal Encoding Declaration and UTF-8 Entity  . . . . .  19
     9.10. application/xml or text/xml with Omitted Charset and



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           Internal Encoding Declaration . . . . . . . . . . . . . .  20
     9.11. application/xml-external-parsed-entity or text/xml-
           external-parsed-entity with UTF-8 Charset . . . . . . . .  20
     9.12. application/xml-external-parsed-entity with UTF-16
           Charset . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.13. application/xml-external-parsed-entity with UTF-16BE
           Charset . . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.14. application/xml-dtd . . . . . . . . . . . . . . . . . . .  21
     9.15. application/mathml+xml  . . . . . . . . . . . . . . . . .  21
     9.16. application/xslt+xml  . . . . . . . . . . . . . . . . . .  21
     9.17. application/rdf+xml . . . . . . . . . . . . . . . . . . .  22
     9.18. image/svg+xml . . . . . . . . . . . . . . . . . . . . . .  22
     9.19. model/x3d+xml . . . . . . . . . . . . . . . . . . . . . .  22
     9.20. INCONSISTENT EXAMPLE: text/xml with UTF-8 Charset . . . .  22
     9.21. application/soap+xml  . . . . . . . . . . . . . . . . . .  23
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  23
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     12.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Appendix A.  Why Use the '+xml' Suffix for XML-Based MIME Types?   28
   Appendix B.  Changes from RFC 3023  . . . . . . . . . . . . . . .  28
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   The World Wide Web Consortium has issued the Extensible Markup
   Language (XML) 1.0 [XML] and Extensible Markup Language (XML) 1.1
   [XML1.1] specifications.  To enable the exchange of XML network
   entities, this specification standardizes three media types --
   application/xml, application/xml-external-parsed-entity, and
   application/xml-dtd and two aliases -- text/xml and text/xml-
   external-parsed-entity, as well as a naming convention for
   identifying XML-based MIME media types (using +xml).

   XML has been 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, and in line with the recognition in
   [RFC6838] of structured syntax name suffixes, a suffix of '+xml' is
   described in Section 8.  This will allow generic XML-based tools --
   browsers, editors, search engines, and other processors -- to work
   with all XML-based media types.

2.  Notational Conventions






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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   specification are to be interpreted as described in [RFC2119].

   As defined in [RFC2781] (informative), the three charsets "utf-16",
   "utf-16le", and "utf-16be" are used to label UTF-16 text.  In this
   specification, "the UTF-16 family" refers to those three charsets.
   By contrast, the phrases "utf-16" or UTF-16 in this specification
   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."

   Section 4 of [XML] says:

      "An XML document may consist of one or many storage units.  These
      are called entities; they all have content and are all (except for
      the document entity and the external DTD subset) identified by
      entity name".

   In this specification, "XML MIME entity" is defined as the latter (an
   XML entity) encapsulated in the former (a MIME entity).

   Furthermore, XML provides for the naming and referencing of entities
   for purposes of inclusion and/or substitution.  In this specification
   "XML-entity declaration/reference/..." is used to avoid confusion
   when referring to such cases.

3.  XML Media Types

   This specification standardizes three media types related to XML MIME
   entities: application/xml (with text/xml as an alias), application/
   xml-external-parsed-entity (with text/xml-external-parsed-entity as
   an alias), 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".  Appropriate usage for these types is
   as follows:





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   document entities  The media types application/xml or text/xml MAY be
      used

   external DTD subsets  The media type application/xml-dtd SHOULD be
      used.  The media types application/xml and text/xml MUST NOT be
      used.

   external parsed entities  application/xml-external-parsed-entity or
      text/xml-external-parsed-entity SHOULD be used.  The media types
      application/xml and text/xml MUST NOT be used unless the parsed
      entities are also well-formed "document entities" and are
      referenced as such.

   external parameter entities  The media type application/xml-dtd
      SHOULD be used.  The media types application/xml and text/xml MUST
      NOT be used.

      Note that [RFC3023] (which this specification obsoletes)
      recommended the use of text/xml and text/xml-external-parsed-
      entity for document entities and external parsed entities,
      respectively, but described charset handling which differed from
      common implementation practice.  These media types are still
      commonly used, and this specification aligns the charset handling
      with industry practice.

      Note that [RFC2376] (which is obsolete) allowed application/xml
      and text/xml to be used for any of the four types, 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.

   Application/xml and application/xml-external-parsed-entity are
   recommended.  Compared to [RFC2376] or [RFC3023], this specification
   alters the charset handling of text/xml and text/xml-external-parsed-
   entity, treating them no differently from the respective application/
   types.  The reasons are as follows:










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      Conflicting specifications regarding the character encoding have
      caused confusion.  On the one hand, [RFC2046] specifies "The
      default character set, which must be assumed in the absence of a
      charset parameter, is US-ASCII.", [RFC2616] Section 3.7.1, defines
      that "media subtypes of the 'text' type are defined to have a
      default charset value of 'ISO-8859-1'", and [RFC2376] as well as
      [RFC3023] specify the default charset is US-ASCII.

      On the other hand, implementors and users of XML parsers,
      following Appendix F of [XML], assume that the default is provided
      by the XML encoding declaration or BOM.  Note that this conflict
      did not exist for application/xml or application/xml-external-
      parsed-entity (see "Optional parameters" of application/xml
      registration in Section 3.1).

      The current situation, reflected in this specification, has been
      simplified by [RFC6657] updating [RFC2046] to remove the US-ASCII
      default.  Furthermore, in accordance with [RFC6657]'s other
      recommendations, [HTTPbis] changes [RFC2616] by removing the
      ISO-8859-1 default and not defining any default at all.

      The top-level media type "text" has some restrictions on MIME
      entities and they are described in [RFC2045] and [RFC2046].  In
      particular, for transports other than HTTP [RFC2616] or HTTPS
      (which uses a MIME-like mechanism).  the UTF-16 family, UCS-4, and
      UTF-32 are not allowed However, section 4.3.3 of [XML] says:



         "Each external parsed entity in an XML document may use a
         different encoding for its characters.  All XML processors MUST
         be able to read entities in both the UTF-8 and UTF-16
         encodings."

      Thus, although all XML processors can read entities in at least
      UTF-16, if an XML document or external parsed entity is encoded in
      such character encoding schemes, it could not be labeled as text/
      xml or text/xml-external-parsed-entity (except for HTTP).

      It is not possible to deprecate text/xml because it is widely used
      in practice, and implementations are largely interoperable,
      following the rules of Appendix F of [XML] and ignoring the
      requirements of [RFC3023].

   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 display, editing, security considerations or runtime



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   information.  Furthermore, such media types may allow UTF-8 or UTF-16
   only and prohibit other charsets.  This specification 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 specification as a basis for their registration.  In
   particular, the charset parameter, if used, MUST agree with the in-
   band XML encoding of the XML entity, as described in Section 3.6, in
   order to enhance interoperability.

   An XML document labeled as application/xml or text/xml, or with a
   +xml media type, 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.

3.1.  Application/xml Registration

   MIME media type name:  application

   MIME subtype name:  xml

   Mandatory parameters:  none

   Optional parameters:  charset

      See Section 3.6.

   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 [RFC6152] ESMTP or NNTP
      [RFC3977]), 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 11.











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   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 (Fifth
      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 [RFC4918] clients and servers, as well as XML
      authoring tools.

   Additional information:

      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) <eb2m-mrt@asahi-net.or.jp>

            Alexey Melnikov <alexey.melnikov@isode.com>

            Chris Lilley <chris@w3.org>

            Henry S.  Thompson <ht@inf.ed.ac.uk>

   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:



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            Tim Bray <tbray@textuality.com>

            Jean Paoli <jeanpa@microsoft.com>

            C.  M.  Sperberg-McQueen <cmsmcq@uic.edu>

            Eve Maler <eve.maler@east.sun.com>

            Francois Yergeau <mailto:francois@yergeau.com>

3.2.  Text/xml Registration

   text/xml is an alias for application/xml, as defined in Section 3.1
   above.

3.3.  Application/xml-external-parsed-entity Registration

   MIME media type name:  application

   MIME subtype name:  xml-external-parsed-entity

   Mandatory parameters:  none

   Optional parameters:  charset

      See Section 3.6.

   Encoding considerations:  Same as application/xml as described in
      Section 3.1.

   Security considerations:  See Section 11.

   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
      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 application/xml as described in
      Section 3.1.

   Applications which use this media type:  Same as application/xml as
      described in Section 3.1.



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   Additional information:

      Magic number(s):  Same as application/xml as described in
            Section 3.1.

      File extension(s):  .xml or .ent

      Macintosh File Type Code(s):  "TEXT"

   Person and email address for further information:  Same as
      application/xml as described in Section 3.1.

   Intended usage:  COMMON

   Author/Change controller:  Same as application/xml as described in
      Section 3.1.

3.4.  Text/xml-external-parsed-entity Registration

   text/xml-external-parsed-entity is an alias for application/xml-
   external-parsed-entity, as defined in Section 3.3 above.

3.5.  Application/xml-dtd Registration

   MIME media type name:  application

   MIME subtype name:  xml-dtd

   Mandatory parameters:  none

   Optional parameters:  charset

      See Section 3.6.

   Encoding considerations:  Same as Section 3.1.

   Security considerations:  See Section 11.

   Interoperability considerations:  XML DTDs have proven to be
      interoperable by DTD authoring tools and XML browsers, among
      others.

   Published specification:  Same as application/xml as described in
      Section 3.1.







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   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.

   Additional information:

      Magic number(s):  Same as application/xml as described in
            Section 3.1.

      File extension(s):  .dtd or .mod

      Macintosh File Type Code(s):  "TEXT"

   Person and email address for further information:  Same as
      application/xml as described in Section 3.1.

   Intended usage:  COMMON

   Author/Change controller:  Same as application/xml as described in
      Section 3.1.

3.6.  Charset considerations

   [HST: new section pulled from section 3.2]

   The charset parameter MUST only be used, when the charset is reliably
   known and agrees with the in-band XML encoding declaration.  This
   information can be used by non-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" [RFC3629] 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 entity of one of the types defined above 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 such an entity.






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   Since a receiving application can, with very high reliability,
   determine the encoding of an XML document by reading it, the in-band
   XML encoding declaration SHOULD be provided.

3.6.1.  Background

   There are several reasons that the charset parameter is optionally
   allowed.  First, recent web servers have been improved so that users
   can specify the charset parameter.  Second, [RFC2130] (informative)
   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 in-band XML encoding declaration or BOM and/or to
   conceal charset information from non-XML processors, they can omit
   the parameter.

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 in-band XML 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





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   Uniform Resource Identifiers (URIs) may contain fragment identifiers
   (see Section 3.5 of [RFC3986]).  Likewise, Internationalized Resource
   Identifiers (IRIs) [RFC3987] may contain fragment identifiers.

   The syntax and semantics of fragment identifiers for the XML media
   types defined in this specification are based on the
   [XPointerFramework] W3C Recommendation.  It allows simple names, and
   more complex constructions based on named schemes.  When the syntax
   of a fragment identifier part of any URI or IRI with a retrieved
   media type governed by this specification conforms to the syntax
   specified in [XPointerFramework], conformant applications MUST
   interpret such fragment identifiers as designating that part of the
   retrieved representation specified by [XPointerFramework] and
   whatever other specifications define any XPointer schemes used.
   Conformant applications MUST support the 'element' scheme as defined
   in [XPointerElement], but need not support other schemes.

   If an XPointer error is reported in the attempt to process the part,
   this specification does not define an interpretation for the part.

   A registry of XPointer schemes [XPtrReg] is maintained at the W3C.
   Document authors SHOULD NOT use unregistered schemes.  Scheme authors
   SHOULD register their schemes.

   See Section 8.1 for additional rquirements which apply when an XML-
   based MIME media type follows the naming convention '+xml'.

   If [XPointerFramework] and [XPointerElement] are inappropriate for
   some XML-based media type, it SHOULD NOT follow the naming convention
   '+xml'.

   When a URI has a fragment identifier, it is encoded by a limited
   subset of the repertoire of US-ASCII [ASCII] characters, as defined
   in [RFC3986].  When an IRI contains a fragment identifier, it is
   encoded by a much wider repertoire of characters.  The conversion
   between IRI fragment identifiers and URI fragment identifiers is
   presented in Section 7 of [RFC3987].

6.  The Base URI

   Section 5.1 of [RFC3986] 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 context, (2) the
   base URI from the encapsulating entity, (3) the base URI from the
   Retrieval URI, or (4) the default base URI, where (1) has the highest
   precedence.  [RFC3986] further specifies that the mechanism for
   embedding the base URI is dependent on the media type.




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   The media type dependent mechanism for embedding the base URI in a
   MIME entity of type application/xml, text/xml, application/xml-
   external-parsed-entity or text/xml-external-parsed-entity is to use
   the xml:base attribute described in detail in [XBase].

   Note that the base URI may be embedded in a different MIME entity,
   since the default value for the xml:base attribute may be specified
   in an external DTD subset or external parameter entity.

7.  XML Versions

   application/xml, application/xml-external-parsed-entity, and
   application/xml-dtd, text/xml and text/xml-external-parsed-entity are
   to be used with [XML]  In all examples herein where version="1.0" is
   shown, it is understood that version="1.1" may also be used,
   providing the content does indeed conform to [XML1.1].

   The normative requirement of this specification upon XML is to follow
   the requirements of [XML], section 4.3.3.  Except for minor
   clarifications, that section is substantially identical from the
   first edition to the current (5th) edition of XML 1.0, and for XML
   1.1.  Therefore, this specification may be used with any version or
   edition of XML 1.0 or 1.1.

   Specifications and recommendations based on or referring to this RFC
   SHOULD indicate any limitations on the particular versions of XML to
   be used.  For example, a particular specification might indicate:
   "content MUST be represented using media-type application/xml, and
   the document must either (a) carry an xml declaration specifying
   version="1.0" or (b) omit the XML declaration, in which case per the
   XML recommendation the version defaults to 1.0"

8.  A Naming Convention for XML-Based Media Types

   This specification recommends the use of a naming convention (a
   suffix of '+xml') for identifying XML-based MIME media types,
   whatever their particular content may represent, in line with the
   recognition in [RFC6838] of structured syntax name suffixes.  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.

   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



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   MIME entities by comparing the subtype to the pattern '*/*+xml'.  (Of
   course, 4 of the 5 media types defined in this specification -- 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.

   Media types following the naming convention '+xml' SHOULD introduce
   the charset parameter for consistency, since XML-generic processing
   applies the same program for any such media type.  However, there are
   some cases that the charset parameter need not be introduced.  For
   example:

      When an XML-based media type is restricted to UTF-8, it is not
      necessary to introduce the charset parameter.  "UTF-8 only" is a
      generic principle and UTF-8 is the default of XML.

      When an XML-based media type is restricted to UTF-8 and UTF-16, it
      might not be unreasonable to omit the charset parameter.  Neither
      UTF-8 nor UTF-16 require in-band XML encoding declarations.

   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
   [RFC6838] and [RFC6839].  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 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.

   *HST: What do we do about the registration of +xml in RFC6839?  I
   think we need to reproduce it with appropriate changes, as it
   currently references 3023, and can be simplified/clarified by
   including it here.  . .*




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8.1.  Referencing

   Registrations for new XML-based media types under top-level types
   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
   XXXX."

   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.  If there are
   some reasons not to follow this advice, they SHOULD be included as
   part of the registration.  As shown above, two such reasons are
   "UTF-8 only" or "UTF-8 or UTF-16 only".

   These registrations SHOULD specify that the XML-based media type
   being registered has all of the security considerations described in
   RFC XXXX plus any additional considerations specific to that media
   type.

   These registrations SHOULD also make reference to RFC XXXX in
   specifying magic numbers, base URIs, and use of the BOM.

   When these registrations use the '+xml' convention, they MUST also
   make reference to RFC XXXX in specifying fragment identifier syntax
   and semantics, and they MAY restrict the syntax to a specified subset
   of schemes, except that they MUST NOT disallow barenames or 'element'
   scheme pointers.  They MAY further require support for other
   registered schemes.  They also MAY add additional syntax (which MUST
   NOT overlap with [XPointerFramework] syntax) together with associated
   semantics, and MAY add additional semantics for barename XPointers
   which, as provided for in Section 5, will only apply when this
   specification does not define an interpretation.

   These registrations MAY reference the application/xml registration in
   RFC XXXX in specifying interoperability considerations, if these
   considerations are not overridden by issues specific to that media
   type.

9.  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



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   declaration; the examples focus on the Content-type header and the
   encoding declaration for clarity.

   This section is non-normative.  In particular, note that all "MUST"
   language herein reproduces or summarizes the consequences of
   normative statement already made above, and have no independent
   normative force.

9.1.  application/xml or text/xml with Omitted Charset and 8-bit MIME
      entity

   Content-type: application/xml or text/xml

   <?xml version="1.0" encoding="iso-8859-1"?>

   Since the charset parameter is not provided in the Content-Type
   header, XML processors MUST treat the "iso-8859-1" encoding as
   authoritative.  XML-unaware MIME processors SHOULD make no
   assumptions about the charset of the XML MIME entity.

9.2.  application/xml or text/xml with Omitted Charset and 16-bit MIME
      entity

   Content-type: application/xml or text/xml

   {BOM}<?xml version="1.0" encoding="utf-16"?>

   or

   {BOM}<?xml version="1.0"?>

   This example shows a 16-bit MIME entity with no charset parameter.
   Since the charset parameter is not provided in the Content-Type
   header, in this case XML processors MUST treat the "utf-16" encoding
   and/or the BOM as authoritative.  XML-unaware MIME processors SHOULD
   make no assumptions about the charset of the XML MIME entity.

   Omitting the charset parameter is NOT RECOMMENDED for application/xml
   when used with transports other than HTTP or HTTPS---text/xml SHOULD
   NOT be used for 16-bit MIME with transports other than HTTP or HTTPS
   (see.  Section 9.5).

9.3.  application/xml or text/xml with UTF-8 Charset

   Content-type: application/xml or text/xml; charset="utf-8"

   <?xml version="1.0" encoding="utf-8"?>




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   This is the recommended encoding for use with all the media types
   defined in this specification.  Since the charset parameter is
   provided, both MIME and XML processors MUST treat the enclosed entity
   as UTF-8 encoded.

   If sent using a 7-bit transport (e.g.  SMTP [RFC5321]), 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.

9.4.  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"?>

   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.

9.5.  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 or HTTPS, which use a MIME-like mechanism and are binary-clean
   protocols, hence do not perform CR and LF transformations and allow
   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 or HTTPS (see section 19.4.1 of [RFC2616] for details).

   Since HTTP is binary clean, no content-transfer-encoding is
   necessary.

9.6.  application/xml with UTF-16BE Charset




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   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.

9.7.  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.  As for UTF-16, this is possible
   only when the XML MIME entity is transmitted via HTTP.

9.8.  application/xml or text/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 the use of a Korean charset (e.g., Hangul) encoded
   following the specification in [RFC1557].  Since the charset
   parameter is provided, MIME processors MUST treat the enclosed entity
   as encoded per RFC 1557.  Since the XML MIME entity has an internal
   encoding declaration (this example does show such a declaration,
   which agrees with the charset parameter) XML processors MUST also
   treat the enclosed entity as encoded per RFC 1557.  Thus,
   interoperability is assured.

   Since ISO-2022-KR has been defined to use only 7 bits of data, no
   content-transfer-encoding is necessary with any transport.

9.9.  application/xml or text/xml with Omitted Charset, no Internal
      Encoding Declaration and UTF-8 Entity

   Content-type: application/xml or text/xml

   <?xml version='1.0'?>

   In this example, the charset parameter has been omitted, the is no
   internal encoding declaration, and there is no BOM.  Since there is
   no BOM, the XML processor follows the requirements in section 4.3.3,
   and optionally applies the mechanism described in Appendix F (which
   is non-normative) of [XML] to determine the charset encoding of
   UTF-8.  Although the XML MIME entity does not contain an encoding



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   declaration, the encoding actually _is_ UTF-8, so 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.

9.10.  application/xml or text/xml with Omitted Charset and Internal
       Encoding Declaration

   Content-type: application/xml or text/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, and optionally
   applying the mechanism described in Appendix F (non-normative) of
   [XML], the XML processor determines the charset encoding 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.

9.11.  application/xml-external-parsed-entity or text/xml-external-
       parsed-entity with UTF-8 Charset

   Content-type: text/xml-external-parsed-entity or application/xml-
   external-parsed-entity; charset="utf-8"

   <?xml encoding="utf-8"?>

   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), 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.

9.12.  application/xml-external-parsed-entity with UTF-16 Charset

   Content-type: application/xml-external-parsed-entity;
   charset="utf-16"

   {BOM}<?xml encoding="utf-16"?>




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   or

   {BOM}<?xml?>

   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.

9.13.  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.

9.14.  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.

9.15.  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 follow the '+xml' suffix convention
   and use 'mathml+xml' in their MIME content-type identifier.This media
   type has been registered at IANA and is fully defined in [MathML].

9.16.  application/xslt+xml

   Content-type: application/xslt+xml




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   <?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
   follow the '+xml' suffix convention and use 'xslt+xml' in their MIME
   content-type identifier.This media type has been registered at IANA
   and is fully defined in [XSLT].

9.17.  application/rdf+xml

   Content-type: application/rdf+xml

   <?xml version="1.0" ?>

   Resources identified using the application/rdf+xml media type are XML
   documents whose content describe RDF metadata.  This media type has
   been registered at IANA and is fully defined in [RFC3870].

9.18.  image/svg+xml

   Content-type: image/svg+xml

   <?xml version="1.0" ?>

   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 follow the '+xml' suffix
   convention and use 'svg+xml' in their MIME content-type
   identifier.The image/svg+xml media type has been registered at IANA
   and is fully defined in [SVG].  .

9.19.  model/x3d+xml

   Content-type: model/x3d+xml

   <?xml version="1.0" ?>

   X3D is derived from VRML and is used for 3D models.  Besides the XML
   representation, it may also be serialised in classic VRML syntax and
   using a fast infoset.  Separate, but clearly related media types are
   used for these serialisations (model/x3d+vrml and model/
   x3d+fastinfoset respectively).

9.20.  INCONSISTENT EXAMPLE: text/xml with UTF-8 Charset

   Content-type: text/xml; charset="utf-8"




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   <?xml version="1.0" encoding="iso-8859-1"?>

   Since the charset parameter is provided in the Content-Type header
   and differs from the XML encoding declaration , MIME and XML
   processors will not interoperate.  MIME processors will treat the
   enclosed entity as UTF-8 encoded.  That is, the "iso-8859-1" encoding
   will be be ignored.  XML processors on the other hand will ignore the
   charset parameter and treat the XML entity as encoded in iso-8859-1.

   Processors generating XML MIME entities MUST NOT label conflicting
   charset information between the MIME Content-Type and the XML
   declaration.  In particular, the addition of an explicit, site-wide
   charset without inspecting the XML MIME entity has frequently lead to
   interoperability problems.

9.21.  application/soap+xml

   Content-type: application/soap+xml

   <?xml version="1.0" ?>

   Resources identified using the application/soap+xml media type are
   SOAP 1.2 message envelopes that have been serialized with XML 1.0.
   This media type has been registered at IANA and is fully defined in
   [RFC3902].

10.  IANA Considerations

   As described in Section 8, this specification updates the [RFC6838]
   and [RFC6839]  registration process for XML-based MIME types.

11.  Security Considerations

   XML MIME entities contain information which may be parsed and further
   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, XML-
   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



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   in critical locations disappear in user documents, without directly
   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 XML-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
   XML-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, "XML-entity
   spoofing," can be used to insert text into documents, vandalizing and
   perhaps conveying an unintended message.  Because XML permits
   multiple XML-entity declarations, and the first declaration takes
   precedence, it's possible to insert malicious content where an XML-
   entity reference is used, such as by inserting the full text of
   Winnie the Pooh in every occurrence of &mdash;.

   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.

   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.



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   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 "[XML-]entity references" 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.  (XML-
   entity A consists of 100 copies of XML-entity B, which in turn
   consists of 100 copies of XML-entity C, and so on)

12.  References

12.1.  Normative References

   [ASCII]    , "US-ASCII. Coded Character Set -- 7-Bit American
              Standard Code for Information Interchange", ANSI
              X3.4-1986, 1986.

   [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.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

   [RFC2781]  Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO
              10646", RFC 2781, Februrary 2000.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 3629, November 2003.



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   [RFC3977]  Feather, B., "Network News Transfer Protocol", RFC 3977,
              October 2006.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax.", RFC 3986,
              January 2005.

   [RFC3987]  DUeerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, July 2005.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC6152]  Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP
              Service Extension for 8-bit MIME Transport", RFC 6152,
              March 2011.

   [RFC6657]  Melnikov, A. and J. Reschke, "Update to MIME regarding
              "charset" Parameter Handling in Textual Media Types", RFC
              6657, July 2012,
              <http://www.rfc-editor.org/rfc/rfc6657.txt>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13, RFC
              6838, January 2013.

   [RFC6839]  Hansen, T. and A. Melnikov, "Additional Media Type
              Structured Syntax Suffixes", RFC 6839, January 2013.

   [XBase]    Marsh, J. and R. Tobin, "XML Base", World Wide Web
              Consortium Recommendation xmlbase, January 2009,
              <http://www.w3.org/TR/xmlbase>.

   [XML1.1]   Bray, T., Paoli, J., Sperberg-McQueen, C.M., Maler, E.,
              Yergeau, F., and J. Cowan, "Extensible Markup Language
              (XML) 1.1", World Wide Web Consortium Recommendation REC-
              xml, September 2006, <http://www.w3.org/TR/xml11>.

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C.M., Maler, E.,
              and F. Yergeau, "Extensible Markup Language (XML) 1.0
              (Fifth Edition)", World Wide Web Consortium Recommendation
              REC-xml, November 2008, <http://www.w3.org/TR/REC-xml>.

   [XPointerElement]
              Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
              element() Scheme", World Wide Web Consortium
              Recommendation REC-XPointer-Element, March 2003,
              <http://www.w3.org/TR/xptr-element/>.



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   [XPointerFramework]
              Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
              Framework", World Wide Web Consortium Recommendation REC-
              XPointer-Framework, March 2003,
              <http://www.w3.org/TR/xptr-framework/>.

   [XPtrReg]  Hazael-Massieux, D., "XPointer Registry", 2005,
              <http://www.w3.org/2005/04/xpointer-schemes/>.

12.2.  Informative References

   [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/>.

   [HTTPbis]  Fielding, R., "Hypertext Transfer Protocol -- HTTP/1.2?",
              RFC ???, January 2013.

   [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.

   [MathML]   Carlisle, D., Ion, P., and R. Miner, "Mathematical Markup
              Language (MathML) Version 3.0", World Wide Web Consortium
              Recommendation MathML, October 2010,
              <http://www.w3.org/TR/MathML/>.

   [RFC1557]  Choi, U., Chon, K., and H. Park, "Korean Character
              Encoding for Internet Messages", RFC 1557, December 1993.

   [RFC2130]  Weider, C., Cecilia 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.

   [RFC2376]  Whitehead, E. and M. Murata, "XML Media Types", RFC 2376,
              July 1998.

   [RFC2703]  Klyne, G., "Protocol-independent Content Negotiation
              Framework", RFC 2703, September 1999.

   [RFC3023]  Murata, M., St.Laurent, S., and D. Kohn, "XML Media
              Types", January 2001.

   [RFC3870]  3870, A., "application/rdf+xml Media Type Registration",
              RFC 3870, September 2004.



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   [RFC3902]  Baker, M. and M. Nottingham, "The "application/soap+xml"
              media type", RFC 3902, September 2004.

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", RFC 4288, December 2005.

   [RFC4289]  Freed, N. and J. Klensin, "Multipurpose Internet Mail
              Extensions (MIME) Part Four: Registration Procedures", RFC
              4289, December 2005.

   [RFC4918]  Dusseault, L., "HTTP Extensions for Distributed Authoring
              -- WEBDAV", RFC 4918, June 2007.

   [SVG]      Dahlstroem, E. and others.  , "Scalable Vector Graphics
              (SVG) 1.1 Specification (Second edition)", World Wide Web
              Consortium Recommendation SVG, August 2011,
              <http://www.w3.org/TR/SVG/>.

   [TAGMIME]  Bray, T., Ed., "Internet Media Type registration,
              consistency of use", April 2004,
              <http://www.w3.org/2001/tag/2004/0430-mime>.

   [XHTML]    Pemberton, S. and et al, "XHTML 1.0: The Extensible
              HyperText Markup Language", World Wide Web Consortium
              Recommendation xhtml1, December 1999,
              <http://www.w3.org/TR/xhtml1>.

   [XSLT]     Kay, M., "XSL Transformations (XSLT) Version 2.0", World
              Wide Web Consortium Recommendation xslt20, January 2007,
              <http://www.w3.org/TR/xslt20/>.

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

   [RFC3023] contains a detailed discussion of the (at the time) novel
   use of a suffix, a practice which has since become widespread.
   Interested parties are referred to [RFC3023], Appendix A.

Appendix B.  Changes from RFC 3023

   There are numerous and significant differences between this
   specification and [RFC3023], which it obsoletes.  This appendix
   summarizes the major differences only.

   First, XPointer ([XPointerFramework] and [XPointerElement] has been
   added as fragment identifier syntax for "application/xml", and the
   XPointer Registry ([XPtrReg]) mentioned.  Second, [XBase] has been
   added as a mechanism for specifying base URIs.  Third, the language
   regarding charsets was updated to correspond to the W3C TAG finding



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   Internet Media Type registration, consistency of use [TAGMIME].
   Fourth, many references are updated, and the existence and relevance
   of XML 1.1 acknowledged.  Finally, a number of justifications and
   contextualizations which were appropriate when XML was new have been
   removed, including the whole of the original Appendix A.

Appendix C.  Acknowledgements

   This specification 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 and the TAG at the W3C.

   Jim Whitehead and Simon St.Laurent are editors of [RFC2376] and
   [RFC3023], respectively.

Authors' Addresses

   Chris Lilley
   World Wide Web Consortium
   2004, Route des Lucioles - B.P. 93 06902
   Sophia Antipolis Cedex
   France

   Email: chris@w3.org
   URI:   http://www.w3.org/People/chris/


   MURATA Makoto (FAMILY Given)
   International University of Japan

   Email: eb2m-mrt@asahi-net.or.jp


   Alexey Melnikov
   Isode Ltd.

   Email: alexey.melnikov@isode.com
   URI:   http://www.melnikov.ca/








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   Henry S. Thompson
   University of Edinburgh

   Email: ht@inf.ed.ac.uk
   URI:   http://www.ltg.ed.ac.uk/~ht/













































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