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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                                     H. S. Thompson
Internet-Draft                                   University of Edinburgh
Obsoletes: 3023 (if approved)                                  C. Lilley
Updates: 6839 (if approved)                                          W3C
Intended status: Standards Track                       December 05, 2013
Expires: June 08, 2014


                            XML Media Types
                  draft-ietf-appsawg-xml-mediatypes-06

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 the '+xml' suffix for
   naming media types outside of these five types when those media types
   represent XML MIME entities.

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
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on June 08, 2014.

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



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   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Conformance Keywords  . . . . . . . . . . . . . . . . . .   3
     2.2.  Characters, Encodings, Charsets . . . . . . . . . . . . .   4
     2.3.  MIME Entities, XML Entities . . . . . . . . . . . . . . .   4
   3.  Encoding Considerations . . . . . . . . . . . . . . . . . . .   5
     3.1.  XML MIME producers  . . . . . . . . . . . . . . . . . . .   5
     3.2.  XML MIME consumers  . . . . . . . . . . . . . . . . . . .   6
     3.3.  The Byte Order Mark (BOM) and Encoding Conversions  . . .   7
   4.  XML Media Types . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  XML MIME Entities . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Application/xml Registration  . . . . . . . . . . . . . .  10
     4.3.  Text/xml Registration . . . . . . . . . . . . . . . . . .  11
     4.4.  Application/xml-external-parsed-entity Registration . . .  11
     4.5.  Text/xml-external-parsed-entity Registration  . . . . . .  12
     4.6.  Application/xml-dtd Registration  . . . . . . . . . . . .  12
   5.  Fragment Identifiers  . . . . . . . . . . . . . . . . . . . .  13
   6.  The Base URI  . . . . . . . . . . . . . . . . . . . . . . . .  14
   7.  XML Versions  . . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  The '+xml' Naming Convention for XML-Based Media Types  . . .  15
     8.1.  XML-based Media Types . . . . . . . . . . . . . . . . . .  15
     8.2.  +xml Structured Syntax Suffix Registration  . . . . . . .  16
     8.3.  Registration guidelines for XML-based media types not
           using '+xml'  . . . . . . . . . . . . . . . . . . . . . .  18
   9.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  UTF-8 Charset . . . . . . . . . . . . . . . . . . . . . .  19
     9.2.  UTF-16 Charset  . . . . . . . . . . . . . . . . . . . . .  19
     9.3.  Omitted Charset and 8-bit MIME Entity . . . . . . . . . .  20



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     9.4.  Omitted Charset and 16-bit MIME Entity  . . . . . . . . .  20
     9.5.  Omitted Charset, no Internal Encoding Declaration . . . .  21
     9.6.  UTF-16BE Charset  . . . . . . . . . . . . . . . . . . . .  21
     9.7.  Non-UTF Charset . . . . . . . . . . . . . . . . . . . . .  21
     9.8.  INCONSISTENT EXAMPLE: Conflicting Charset and Internal
           Encoding Declaration  . . . . . . . . . . . . . . . . . .  22
     9.9.  INCONSISTENT EXAMPLE: Conflicting Charset and BOM . . . .  22
   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?   29
   Appendix B.  Core XML specifications  . . . . . . . . . . . . . .  29
   Appendix C.  Changes from RFC 3023  . . . . . . . . . . . . . . .  30
   Appendix D.  Acknowledgements . . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

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.

   This specification replaces [RFC3023].  Major differences are in the
   areas of alignment of text/xml and text/xml-external-parsed-entity
   with application/xml and application/xml-external-parsed-entity
   respectively, the addition of XPointer and XML Base as fragment
   identifiers and base URIs, respectively, integration of the XPointer
   Registry and updating of many references.

2.  Notational Conventions

2.1.  Conformance Keywords




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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].

2.2.  Characters, Encodings, Charsets

   Both XML (in an XML or Text declaration using the encoding pseudo-
   attribute) and MIME (in a Content-Type header field using the charset
   parameter) use a common set of labels [IANA-charsets] to identify the
   MIME charset (mapping from byte stream to character sequence
   [RFC2978]).

   In this specification we will use the phrases "charset parameter" and
   "encoding declaration" to refer to whatever MIME charset is specified
   by a MIME charset parameter or XML encoding declaration respectively.
   We reserve the phrase "character encoding" (or, when the context
   makes the intention clear, simply "encoding") for the MIME charset
   actually used in a particular XML MIME entity.

   [UNICODE] defines three "encoding forms", which are independent of
   serialization, namely UTF-8, UTF-16 and UTF-32.  This specification
   follows this precedent.  Furthermore, note that UTF-16 XML documents
   may be serialised into MIME entities in one of two ways: either big-
   endian, labelled (optionally) "utf-16" or "utf-16be", or little-
   endian, labelled (optionally) "utf-16" or "utf-16le".  As UTF-8 can
   only be serialized in one way, the only possible label for
   UTF-8-encoded documents when serialised into MIME entities is
   "utf-8".

2.3.  MIME Entities, XML Entities

   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).



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   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.  Encoding Considerations

   The registrations below all address issues around character encoding
   in the same way, by referencing this section.

   As many as three distinct sources of information about character
   encoding may be present for an XML MIME entity: a charset parameter,
   a Byte Order Mark (BOM -- see Section 3.3 below) and an XML encoding
   declaration (see Section 4.3.3 of [XML]).  Ensuring consistency among
   these sources requires coordination between entity authors and MIME
   agents (that is, processes which package, transfer, deliver and/or
   receive MIME entities).  Some MIME agents will be what we will call
   "XML-aware", that is, capable of processing XML MIME entities and
   detecting the XML encoding declaration (or its absence).  Others will
   not be XML-aware, and thus cannot know anything about the XML
   encoding declaration.  Some MIME agents, such as proxies and
   transcoders, both consume and produce MIME entities.

3.1.  XML MIME producers

   XML-aware MIME producers SHOULD supply a charset parameter and/or an
   appropriate BOM with non-UTF-8-encoded XML MIME entities which lack
   an encoding declaration, and SHOULD remove or correct an encoding
   declaration which is known to be incorrect (for example, as a result
   of transcoding).

   XML-aware MIME producers MUST supply an XML text declaration at the
   beginning of non-UNICODE XML external parsed entities which would
   otherwise begin with the hexadecimal octet sequences 0xFE 0xFF, 0xFF
   0xFE or 0xEF 0xBB 0xBF, in order to avoid the mistaken detection of a
   BOM.

   XML-unaware MIME producers MUST NOT supply a charset parameter with
   an XML MIME entity unless the entity's character encoding is reliably
   known.

   XML MIME producers are RECOMMENDED to provide means for XML MIME
   entity authors to determine what value, if any, is given to charset
   parameters for their entities, for example by enabling user-level
   configuration of filename-to-Content-Type-header mappings on a file-
   by-file or suffix basis.

   The use of UTF-32 is NOT RECOMMENDED for XML MIME entities.



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3.2.  XML MIME consumers

   For XML MIME consumers, the question of priority arises in cases when
   the available character encoding information is not consistent.
   Again, we must distinguish betweeen XML-aware and XML-unaware agents.

   When a charset parameter is specified for an XML MIME entity, the
   normative component of the [XML] specification leaves the question
   open as to how to determine the encoding with which to attempt to
   process the entity.  This is true independently of whether or not the
   entity contains in-band encoding information, that is, either a BOM
   (Section 3.3) or an XML encoding declaration, or both, or neither.
   In particular, in the case where there is in-band information and it
   conflicts with the charset parameter, the [XML] specification does
   not specify which is authoritative.  In its (non-normative)
   Appendix F it defers to this specification:

      [T]he preferred method of handling conflict should be specified as
      part of the higher-level protocol used to deliver XML.  In
      particular, please refer to [IETF RFC 3023] or its successor...

   Accordingly, to conform with deployed processors and content and to
   avoid conflicting with this or other normative specifications, this
   specification sets the priority as follows:

      All consumers SHOULD treat a BOM (Section 3.3) as authoritative if
      it is present in an XML MIME entity.  In the absence of a BOM
      (Section 3.3), all consumers SHOULD treat the charset parameter as
      authoritative if it is present.  For XML-aware consumers, note
      that Section 4.3.3 of [XML] does _not_ make it an error for the
      charset parameter and the XML encoding declaration (or the UTF-8
      default in the absence of encoding declaration and BOM) to be
      inconsistent, although such consumers might choose to issue a
      warning in this case.

   When MIME producers conform to the requirements stated above
   (Section 3.1), such inconsistencies will not arise---this statement
   of priorities only has practical impact in the case of non-conforming
   XML MIME entities.

   If an XML MIME entity is received where the charset parameter is
   omitted, no information is being provided about the character
   encoding by the MIME Content-Type header.  XML-aware consumers MUST
   follow the requirements in section 4.3.3 of [XML] that directly
   address this case.  XML-unaware MIME consumers SHOULD NOT assume a
   default encoding in this case.





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3.3.  The Byte Order Mark (BOM) and Encoding Conversions

   Section 4.3.3 of [XML] specifies thatUTF-16 XML MIME entitiesnot
   labelled as "utf-16le" or "utf16-be" MUST begin with a byte order
   mark (BOM), U+FEFF, which appears as the hexadecimal octet sequence
   0xFE 0xFF (big-endian) or 0xFF 0xFE (little-endian).  [XML] 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 an encoding other than UTF-8 MUST strip the BOM before
   conversion.  Similarly, when converting from another encoding into
   UTF-16, either without a charset parameter, or labelled "utf-16", the
   BOM MUST be added unless the original encoding was UTF-8 and a BOM
   was already present, in which case it MUST be transcoded into the
   appropriate UTF-16 BOM.

   Section 4.3.3 of [XML] also allows for UTF-8 XML MIME entities to
   begin with a BOM, which appears as the hexadecimal octet sequence
   0xEF 0xBB 0xBF.  This is likewise defined to be an encoding
   signature, and not part of either the markup or the character data of
   the XML document.

   Applications that convert XML from UTF-8 to an encoding other than
   UTF-16 MUST strip the BOM, if present, before conversion.
   Applications which convert XML into UTF-8 MAY add a BOM.

   In addition to the MIME charset "utf-16", [RFC2781] introduces "utf-
   16le" (little endian) and "utf-16be" (big endian).  The BOM is
   prohibited in MIME entities with these labels.  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-8 or UTF-16 (unlabelled, or labelled with
   "utf-16") to "utf-16be" or "utf-16le" MUST strip a BOM if present,
   and conversion in the other direction MUST (for UTF-16) or MAY (for
   UTF-8) add the appropriate BOM.

   Appendix F of [XML] also implies the a UTF-32 BOM may be used in
   conjunction with UTF-32-encoded documents.  As noted above, this
   specification recommends against the use of UTF-32, but if it is
   used, the same considerations apply with respect to its being a
   signature, not part of the document, with respect to transcoding into
   or out of it and with respect to the MIME charsets "utf-32le" and
   "utf-32be", as for UTF-16.  Consumers which do not support UTF-32
   SHOULD none-the-less recognise UTF-32 signatures in order to give
   helpful error messages (instead of treating them as invalid UTF-16).





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4.  XML Media Types

   Registration information for media types for use with XML MIME
   entities is described in the sections below, after some relevant
   background information about XML itself.

4.1.  XML MIME Entities

   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 the types
   registered below is as follows:

   document entities:  The media types application/xml or text/xml, or a
      more specific media type (see Section 8), SHOULD 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:  The media types 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".

   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
   handling of character encoding which differed from common
   implementation practice.  These media types are still commonly used,
   and this specification aligns the handling of character encoding 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.









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   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.

   Compared to [RFC2376] or [RFC3023], this specification alters the
   handling of character encoding of text/xml and text/xml-external-
   parsed-entity, treating them no differently from the respective
   application/ types.  However application/xml and application/xml-
   external-parsed-entity are still RECOMMENDED, to avoid possible
   confusion based on the earlier distinction.  The former confusion
   around the question of default character sets for the two text/ types
   no longer arises because

      [HTTPbis] changes [RFC2616] by removing the ISO-8859-1 default and
      not defining any default at all;

      [RFC6657] updates [RFC2046] to remove the US-ASCII default.

   See Section 3 for the now-unified approach to the charset parameter
   which results.

   XML provides a general framework for defining sequences of structured
   data.  It is often appropriate to define new media types that use XML
   but define a specific application of XML, due to domain-specific
   display, editing, security considerations or runtime information.
   Furthermore, such media types may allow only UTF-8 and/or UTF-16 and
   prohibit other character sets.  This specification does not prohibit
   such media types and in fact expects them to proliferate.  However,
   developers of such media types are RECOMMENDED to use this
   specification as a basis for their registration.  See Section 8 for
   more detailed recommendations on using the '+xml' suffix for
   registration of such media types.

   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.  Appendix B lists the core
   XML specifications which, taken together with [XML] itself, show how
   to determine an XML document's semantics at both the language level
   and the application level.




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4.2.  Application/xml Registration

   Type name:  application

   Subtype name:  xml

   Required parameters:  none

   Optional parameters:  charset

      See Section 3.

   Encoding considerations:  Depending on the character encoding used,
      XML MIME entities can consist of 7bit, 8bit or binary data
      [RFC6838].  For 7-bit transports, 7bit data, for example US-ASCII-
      encoded data, does not require content-transfer-encoding, but 8bit
      or binary data, for example UTF-8 or UTF-16 data, MUST be content-
      transfer-encoded in quoted-printable or base64.  For 8-bit clean
      transport (e.g.  8BITMIME ESMTP [RFC6152] or NNTP [RFC3977]), 7bit
      or 8bit data, for example US-ASCII or UTF-8 data, does not require
      content-transfer-encoding, but binary data, for example data with
      a UTF-16 encoding, MUST be content-transfer-encoded in base64.
      For binary clean transports (e.g.  BINARY ESMTP [RFC3030] or HTTP
      [HTTPbis]), no content-transfer-encoding is necessary (or even
      possible, in the case of HTTP) for 7bit, 8bit or binary data.

   Security considerations:  See Section 11.

   Interoperability considerations:  XML has proven to be interoperable
      across both generic and task-specific applications and for import
      and export from multiple XML authoring and editing tools.
      Validating processors provide maximum interoperability.  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] or subsequent editions or versions thereof.

   Applications that use this media type:  XML is device-, platform-,
      and vendor-neutral and is supported by generic and task-specific
      applications and a wide range of generic XML tools (editors,
      parsers, Web agents, ...).

   Additional information:

      Magic number(s):  None.



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         Although no byte sequences can be counted on to always be
         present, XML MIME entities in ASCII-compatible character sets
         (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"

      Base URI:  See Section 6

   Person and email address for further information:  See Authors'
      Addresses section

   Intended usage:  COMMON

   Author:  See Authors' Addresses section

   Change controller:  The XML specification is a work product of the
      World Wide Web Consortium's XML Core Working Group.  The W3C has
      change control over this specification.

4.3.  Text/xml Registration

   The registration information for text/xml is in all respects the same
   as that given for application/xml above (Section 4.2).

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

   Type name:  application

   Subtype name:  xml-external-parsed-entity

   Required parameters:  none

   Optional parameters:  charset

      See Section 3.

   Encoding considerations:  Same as for application/xml (Section 4.2).

   Security considerations:  See Section 11.

   Interoperability considerations:  XML external parsed entities are as
      interoperable as XML documents, though they have a less tightly



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      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 enhances
      interoperability of both XML documents and XML external parsed
      entities.

   Published specification:  Same as for application/xml (Section 4.2).

   Applications which use this media type:  Same as for application/xml
      (Section 4.2).

   Additional information:

      Magic number(s):  Same as for application/xml (Section 4.2).

      File extension(s):  .xml or .ent

      Macintosh File Type Code(s):  "TEXT"

      Base URI:  See Section 6

   Person and email address for further information:  See Authors'
      Addresses section.

   Intended usage:  COMMON

   Author:  See Authors' Addresses section.

   Change controller:  The XML specification is a work product of the
      World Wide Web Consortium's XML Core Working Group.  The W3C has
      change control over this specification.

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

   The registration information for text/xml-external-parsed-entity is
   in all respects the same as that given for application/xml-external-
   parsed-entity above (Section 4.4).

4.6.  Application/xml-dtd Registration

   Type name:  application

   Subtype name:  xml-dtd

   Required parameters:  none



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   Optional parameters:  charset

      See Section 3.

   Encoding considerations:  Same as for application/xml (Section 4.2).

   Security considerations:  See Section 11.

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

   Published specification:  Same as for application/xml (Section 4.2).

   Applications which use this media type:  DTD authoring tools handle
      external DTD subsets as well as external parameter entities.  XML
      validators may also access external DTD subsets and external
      parameter entities.

   Additional information:

      Magic number(s):  Same as for application/xml (Section 4.2).

      File extension(s):  .dtd or .mod

      Macintosh File Type Code(s):  "TEXT"

   Person and email address for further information:  See Authors'
      Addresses section.

   Intended usage:  COMMON

   Author:  See Authors' Addresses section.

   Change controller:  The XML specification is a work product of the
      World Wide Web Consortium's XML Core Working Group.  The W3C has
      change control over this specification.

5.  Fragment Identifiers

   Uniform Resource Identifiers (URIs) can contain fragment identifiers
   (see Section 3.5 of [RFC3986]).  Specifying the syntax and semantics
   of fragment identifiers is devolved by [RFC3986] to the appropriate
   media type registration.

   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



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   more complex constructions based on named schemes.  When the syntax
   of a fragment identifier part of any URI or IRI ([RFC3987]) with a
   retrieved media type governed by this specification conforms to the
   syntax specified in [XPointerFramework], conforming applications MUST
   interpret such fragment identifiers as designating whatever is
   specified by the [XPointerFramework] together with any other
   specifications governing the XPointer schemes used in those
   identifiers which the applications support.  Conforming 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 ([XPtrRegPolicy] describes requirements
   and procedures for doing so).

   See Section 8.3 for additional requirements which apply when an XML-
   based 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 characters, see
   [XPointerFramework] for details..

6.  The Base URI

   An XML MIME entity of type application/xml, text/xml, application/
   xml-external-parsed-entity or text/xml-external-parsed-entity MAY use
   the xml:base attribute, as described in [XMLBase], to embed a base
   URI in that entity for use in resolving relative URI references (see
   Section 5.1 of [RFC3986]).

   Note that the base URI itself might be embedded in a different MIME
   entity, since the default value for the xml:base attribute can be
   specified in an external DTD subset or external parameter entity.
   Since conforming XML processors need not always read and process
   external entities, the effect of such an external default is
   uncertain and therefore its use is NOT RECOMMENDED.

7.  XML Versions





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   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" might also appear,
   providing the content does indeed conform to [XML1.1].

   The normative requirement of this specification upon XML documents
   and processors 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 1st or 2nd edition [XML1.1].  Therefore,
   references herein to [XML] may be interpreted as referencing any
   existing version or edition of XML, or any subsequent edition or
   version which makes no incompatible changes to that section.

   Specifications and recommendations based on or referring to this RFC
   SHOULD indicate any limitations on the particular versions or
   editions of XML to be used.

8.  The '+xml' Naming Convention for XML-Based Media Types

   This section supersedes the earlier registration of the '+xml' suffix
   [RFC6839].

   This specification recommends the use of the '+xml' naming convention
   for identifying XML-based media types, 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.

8.1.  XML-based Media Types

   When a new media type is introduced for an XML-based format, the name
   of the media type SHOULD end with '+xml' unless generic XML
   processing is in some way inappropriate for documents of the new
   type.  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 check
   for types that represent XML MIME entities by comparing the last four
   characters of the subtype to the string '+xml'.  (However note that 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 ending with '+xml'.)





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      NOTE: Section 5.3.2HTTPbis [HTTPbis] does not support any form of
      Accept header which will match only '+xml' types.  In particular,
      Accept headers of the form "Accept: */*+xml" are not allowed, and
      so this header MUST NOT be used for this purpose.

   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 is the
      default for 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 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 -- the '+xml' convention is to be preferred unless
   there is some particularly compelling reason not to.

   The registration process for specific '+xml' media types is described
   in [RFC6838].  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.  Media subtypes that do not represent XML MIME entities
   MUST NOT be allowed to register with a '+xml' suffix.

   In addition to the changes described above, the change controller has
   been changed to be the World Wide Web Consortium (W3C).

8.2.  +xml Structured Syntax Suffix Registration

   Name:  Extensible Markup Language (XML)

   +suffix:  +xml

   Reference:  This specification




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   Encoding considerations:  Same as Section 4.2.

   Fragment identifier considerations:  Registrations which use this
      '+xml' convention MUST also make reference to RFC XXXX,
      specifically Section 5, 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.



         In practice these constraints imply that for a fragment
         identifier addressed to an instance of a specific "xxx/yyy+xml"
         type, there are three cases:

            For fragment identifiers matching the syntax defined in
            [XPointerFramework], where the fragment identifier resolves
            per the rules specified there, then process as specified
            there;

            For fragment identifiers matching the syntax defined in
            [XPointerFramework], where the fragment identifier does
            _not_ resolve per the rules specified there, then process as
            specified in "xxx/yyy+xml";

            For fragment identifiers _not_ matching the syntax defined
            in [XPointerFramework], then process as specified in "xxx/
            yyy+xml".  A fragment identifier of the form
            "xywh=160,120,320,240", as defined in [MediaFrags], which
            might be used in a URI for an XML-encoded image, would fall
            in this category.

   Interoperability considerations:  Same as Section 4.2.  See above,
      and also Section 3, for guidelines on the use of the 'charset'
      parameter.

   Security considerations:  See Section 11.

   Contact:  See Authors' Addresses section.

   Author:  See Authors' Addresses section.





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   Change controller:  The XML specification is a work product of the
      World Wide Web Consortium's XML Core Working Group.  The W3C has
      change control over this specification.

8.3.  Registration guidelines for XML-based media types not using '+xml'

   Registrations for new XML-based media types which do _not_ use the
   '+xml' suffix 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."

   Enabling the charset parameter is RECOMMENDED, since this information
   can be used by XML processors to determine authoritatively the
   character encoding of the XML MIME entity in the absence of a BOM.
   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.

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

9.  Examples

   This section is non-normative.  In particular, note that all
   [RFC2119] language herein reproduces or summarizes the consequences
   of normative statements already made above, and has no independent
   normative force, and accordingly does not appear in uppercase.













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

   Although they show a content type of 'application/xml', all the
   examples below apply to all five media types declared above in
   Section 4, as well as to any media types declared using the '+xml'
   convention (with the exception of the examples involving the charset
   parameter for any such media types which do not enable its use).  See
   the XML MIME entities table (Section 4.1, Paragraph 1) for discussion
   of which types are appropriate for which varieties of XML MIME
   entity.

9.1.  UTF-8 Charset

   Content-Type: application/xml; charset=utf-8

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

   or

   <?xml version="1.0"?>

   UTF-8 is the recommended encoding for use with all the media types
   defined in this specification.  Since the charset parameter is
   provided and there is no overriding BOM, 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]), in general, a
   UTF-8 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.  BINARY
   ESMTP or HTTP), no content-transfer-encoding is necessary (or even
   possible, in the case of HTTP).

9.2.  UTF-16 Charset

   Content-Type: application/xml; charset=utf-16

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




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   or

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

   For the three application/ media types defined above, 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.
   BINARY ESMTP or HTTP), no content-transfer-encoding is necessary (or
   even possible, in the case of HTTP).

   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 A.2 of HTTP [HTTPbis] for details).  Hence one of the
   two text/ media types defined above can be used with this exampleonly
   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.  Since HTTP
   is binary clean, no content-transfer-encoding is necessary (or even
   possible).

9.3.  Omitted Charset and 8-bit MIME Entity

   Content-Type: application/xml

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

   Since the charset parameter is not provided in the Content-Type
   header and there is no overriding BOM, XML processors must treat the
   "iso-8859-1" encoding as authoritative.  XML-unaware MIME processors
   should make no assumptions about the character encoding of the XML
   MIME entity.

9.4.  Omitted Charset and 16-bit MIME Entity

   Content-Type: application/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.
   However since there is a BOM all processors must treat the entity as
   UTF-16-encoded.





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   Omitting the charset parameter is not recommended in conjunction with
   media types under the top-level type "application" when used with
   transports other than HTTP or HTTPS.  Media types under the top-level
   type "text" should not be used for 16-bit MIME with transports other
   than HTTP or HTTPS (see discussion above (Section 9.2, Paragraph 7)).

9.5.  Omitted Charset, no Internal Encoding Declaration

   Content-Type: application/xml

   <?xml version='1.0'?>

   In this example, the charset parameter has been omitted, there is no
   internal encoding declaration, and there is no BOM.  Since there is
   no BOM or charset parameter, 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 an encoding of UTF-8.  Although the XML MIME entity does
   not contain an encoding declaration, provided the encoding actually
   _is_ UTF-8, this is a conforming XML MIME entity.

   An XML-unaware MIME processor should make no assumptions about the
   character encoding of the XML MIME entity.

   See Section 9.1 for transport-related issues for UTF-8 XML MIME
   entities.

9.6.  UTF-16BE Charset

   Content-Type: application/xml; charset=utf-16be

   <?xml version='1.0' encoding='utf-16be'?>

   Observe that, as required for this encoding, there is no BOM.  Since
   the charset parameter is provided and there is no overriding BOM,
   MIME and XML processors must treat the enclosed entity as UTF-16BE
   encoded.

   See also the additional considerations in the UTF-16 example
   (Section 9.2) above.

9.7.  Non-UTF Charset

   Content-Type: application/xml; charset=iso-2022-kr

   <?xml version="1.0" encoding="iso-2022-kr"?>





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   This example shows the use of a non-UTF character encoding (in this
   case Hangul, but this example is intended to cover all non-UTF-family
   character encodings).  Since the charset parameter is provided and
   there is no overriding BOM, all processors must treat the enclosed
   entity as encoded per RFC 1557.

   Since ISO-2022-KR [RFC1557] has been defined to use only 7 bits of
   data, no content-transfer-encoding is necessary with any transport:
   for character sets needing 8 or more bits, considerations such as
   those discussed above (Section 9.1, Section 9.2) would apply.

9.8.  INCONSISTENT EXAMPLE: Conflicting Charset and Internal Encoding
      Declaration

   Content-Type: application/xml; charset=iso-8859-1

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

   Although the charset parameter is provided in the Content-Type header
   and there is no BOM and the charset parameter differs from the XML
   encoding declaration, MIME and XML processors will interoperate.
   Since the charset parameter is authoritative in the absence of a BOM,
   all processors will treat the enclosed entity as iso-8859-1 encoded.
   That is, the "UTF-8" encoding declaration will be ignored.

   Processors generating XML MIME entities must not label conflicting
   character encoding information between the MIME Content-Type and the
   XML declaration unless they have definitive information about the
   actual encoding, for example as a result of systematic transcoding.
   In particular, the addition by servers of an explicit, site-wide
   charset parameter default has frequently lead to interoperability
   problems for XML documents.

9.9.  INCONSISTENT EXAMPLE: Conflicting Charset and BOM

   Content-Type: application/xml; charset=iso-8859-1

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

   Although the charset parameter is provided in the Content-Type
   header, there is a BOM, so MIME and XML processors may not
   interoperate.  Since the BOM parameter is authoritative for XML
   processors, they will treat the enclosed entity as UTF-16-encoded.
   That is, the "iso-8859-1" charset parameter will be ignored.  XML-
   unaware MIME processors on the other hand may be unaware of the BOM
   and so treat the entity as encoded in iso-8859-1.





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   Processors generating XML MIME entities must not label conflicting
   character encoding information between the MIME Content-Type and an
   entity-initial BOM.

10.  IANA Considerations

   As described in Section 8, this specification updates the [RFC6839]
   registration for XML-based MIME types (the "+xml" types).

11.  Security Considerations

   XML MIME entities contain information which may be parsed and further
   processed by the recipient.  These entities may contain, and
   recipients may permit, explicit system level commands to be executed
   while processing the data.  To the extent that a recipient
   application executes arbitrary command strings from within XML MIME
   entities, they may be at risk.

   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
   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 attackers, when external DTD
   subsets or external parameter entities or other externally-specified
   defaulting is involved, 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.




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   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 is possible to insert malicious content where an XML-
   entity reference is used, such as by inserting the full text of
   Winnie the Pooh in place of 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.

   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.  For
   example, consider the case where 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.





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12.  References

12.1.  Normative References

   [HTTPbis]  Fielding, R., Ed. and J. F. Reschke, Ed., "Hypertext
              Transfer Protocol (HTTP/1.1): Message Syntax and Routing",
              draft-ietf-httpbis-p1-messaging-25 (work in progress),
              November 2013.

   [IANA-charsets]
              IANA, "Character Sets Registry", 2013, <http://
              www.iana.org/assignments/character-sets/character-
              sets.xhtml>.

   [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.

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

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", RFC 2978, October 2000.

   [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.

   [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.





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

   [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
              6.3.0", 2013,
              <http://www.unicode.org/versions/Unicode6.3.0/>.

              Defined by: The Unicode Standard, Version 6.3 (Mountain
              View, CA: The Unicode Consortium, 2013.  ISBN
              978-1-936213-08-5)

   [XML1.1]   Bray, T., Paoli, J., Sperberg-McQueen, C.M., Maler, E.,
              Yergeau, F., and J. Cowan, "Extensible Markup Language
              (XML) 1.1 (Second Edition)", W3C Recommendation REC-xml,
              September 2006,
              <http://www.w3.org/TR/2006/REC-xml11-20060816/>.

              Latest version available at

   [XMLBase]  Marsh, J. and R. Tobin, "XML Base (Second Edition)", W3C
              Recommendation REC-xmlbase-20090128, January 2009,
              <http://www.w3.org/TR/2009/REC-xmlbase-20090128/>.

              Latest version available at

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C.M., Maler, E.,
              and F. Yergeau, "Extensible Markup Language (XML) 1.0
              (Fifth Edition)", W3C Recommendation REC-xml, November
              2008, <http://www.w3.org/TR/2008/REC-xml-20081126/>.

              Latest version available at

   [XPointerElement]
              Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
              element() Scheme", W3C Recommendation REC-XPointer-
              Element, March 2003,
              <http://www.w3.org/TR/2003/REC-xptr-element-20030325/>.

              Latest version available at

   [XPointerFramework]
              Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
              Framework", W3C Recommendation REC-XPointer-Framework,
              March 2003,
              <http://www.w3.org/TR/2003/REC-xptr-framework-20030325/>.

              Latest version available at




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   [XPtrRegPolicy]
              Hazael-Massieux, D., "XPointer Scheme Name Registry
              Policy", 2005,
              <http://www.w3.org/2005/04/xpointer-policy.html>.

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

12.2.  Informative References

   [ASCII]    American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

   [AWWW]     Jacobs, I. and N. Walsh, "Architecture of the World Wide
              Web, Volume One", W3C Recommendation REC-webarch-20041215,
              December 2004,
              <http://www.w3.org/TR/2004/REC-webarch-20041215/>.

              Latest version available at

   [FYN]      Mendelsohn, N., "The Self-Describing Web", W3C TAG Finding
              selfDescribingDocuments-2009-02-07, February 2009, <http:/
              /www.w3.org/2001/tag/doc/
              selfDescribingDocuments-2009-02-07.html>.

              Latest version available at

   [Infoset]  Cowan, J. and R. Tobin, "XML Information Set (Second
              Edition)", W3C Recommendation REC-xml-infoset-20040204,
              Febuary 2004,
              <http://www.w3.org/TR/2005/REC-xml-id-20050909/>.

              Latest version available at

   [MediaFrags]
              Troncy, R., Mannens, E., Pfeiffer, S., and D. Van Deursen,
              "Media Fragments URI 1.0 (basic)", W3C Recommendation
              media-frags, September 2012,
              <http://www.w3.org/TR/2012/REC-media-frags-20120925/>.

              Latest version available at

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

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



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   [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.

   [RFC3023]  Murata, M., St.Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [RFC3030]  Vaudreuil, G., "SMTP Service Extensions for Transmission
              of Large and Binary MIME Messages", RFC 3030, 2000.

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

   [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.

   [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", W3C Recommendation xhtml1,
              December 1999,
              <http://www.w3.org/TR/2000/REC-xhtml1-20000126/>.

              Latest version available at

   [XMLModel]
              Grosso, P. and J. Kosek, "Associating Schemas with XML
              documents 1.0 (Third Edition)", W3C Group Note NOTE-xml-
              model-20121009, October 2012,
              <http://www.w3.org/TR/2012/NOTE-xml-model-20121009/>.

              Latest version available at

   [XMLNS10]  Bray, T., Hollander, D., Layman, A., Tobin, R., and H.
              Thompson, "Namespaces in XML 1.0 (Third Edition)", W3C
              Recommendation REC-xml-names-20091208, December 2009,
              <http://www.w3.org/TR/2009/REC-xml-names-20091208/>.

              Latest version available at

   [XMLNS11]  Bray, T., Hollander, D., Layman, A., and R. Tobin,
              "Namespaces in XML 1.1 (Second Edition)", W3C



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              Recommendation REC-xml-names11-20060816, August 2006,
              <http://www.w3.org/TR/2006/REC-xml-names11-20060816/>.

              Latest version available at

   [XMLSS]    Clark, J., Pieters, S., and H. Thompson, "Associating
              Style Sheets with XML documents 1.0 (Second Edition)", W3C
              Recommendation REC-xml-stylesheet-20101028, October 2010,
              <http://www.w3.org/TR/2010/REC-xml-stylesheet-20101028/>.

              Latest version available at

   [XMLid]    Marsh, J., Veillard, D., and N. Walsh, "xml:id Version
              1.0", W3C Recommendation REC-xml-id-20050909, September
              2005, <http://www.w3.org/TR/2005/REC-xml-id-20050909/>.

              Latest version available at

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.

   The registration process for new '+xml' media types is described in
   [RFC6838]

Appendix B.  Core XML specifications

   The following specifications each articulate key aspects of XML
   document semantics:

      Namespaces in XML 1.0 [XMLNS10]/Namespaces in XML 1.1 [XMLNS11]

      XML Information Set [Infoset]

      xml:id [XMLid]

      XML Base [XMLBase]

      Associating Style Sheets with XML documents [XMLSS]

      Associating Schemas with XML documents [XMLModel]

   The W3C Technical Architecture group has produced two documents which
   are also relevant:





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      The Self-Describing Web [FYN] discusses the overall principles of
      how document semantics are determined on the Web.

      Architecture of the World Wide Web, Volume One [AWWW], section
      4.5.4, discusses the specific role of XML Namespace documents in
      this process.

Appendix C.  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.

      XPointer ([XPointerFramework] and [XPointerElement]) has been
      added as fragment identifier syntax for all the XML media types,
      and the XPointer Registry ([XPtrReg]) mentioned

      [XMLBase] has been added as a mechanism for specifying base URIs

      The language regarding character sets was updated to correspond to
      the W3C TAG finding Internet Media Type registration, consistency
      of use [TAGMIME]

      Priority is now given to a Byte Order Mark (BOM) if present

      Many references are updated, and the existence of XML 1.1 and
      relevance of this specification to it acknowledged

      A number of justifications and contextualizations which were
      appropriate when XML was new have been removed, including the
      whole of the original Appendix A

   Making BOMs authoritative is in principle a backwards-
   incompatibility.  In practice serious interoperability issues already
   exist when BOMs are used.  Making BOMs authoritative, in conjunction
   with the deprecation of the UTF-32 encoding form and the requirement
   to include an XML encoding declaration in certain cases
   (Section 3.1), is intended to improve in-practice interoperability as
   much as possible.

Appendix D.  Acknowledgements

   MURATA Makoto (FAMILY Given) and Alexey Melnikov made early and
   important contributions to the effort to revise [RFC3023].







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   This specification reflects the input of numerous participants to the
   ietf-xml-mime@imc.org, xml-mime@ietf.org and apps-discuss@ietf.org
   mailing lists, though any errors are the responsibility of the
   authors.  Special thanks to:

   Mark Baker, James Clark, Dan Connolly, Martin Duerst, Ned Freed,
   Yaron Goland, Bjoern Hoehrmann, Rick Jelliffe, Murray S.  Kucherawy,
   Larry Masinter, David Megginson, S.  Moonesamy, Keith Moore, Chris
   Newman, Gavin Nicol, Julian Reschke, Marshall Rose, Jim Whitehead,
   Erik Wilde and participants of the XML activity and the TAG at the
   W3C.

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

Authors' Addresses

   Henry S. Thompson
   University of Edinburgh

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


   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/


















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