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Versions: (draft-murata-kohn-lilley-xml) 00 draft-ietf-appsawg-xml-mediatypes

Network Working Group                                          C. Lilley
Internet-Draft                                                       W3C
Intended status: Standards Track                               M. Murata
Expires: April 02, 2013                International University of Japan
                                                             A. Melnikov
                                                              Isode Ltd.
                                                          H. S. Thompson
                                                 University of Edinburgh
                                                            October 2012


                            XML Media Types
                     draft-lilley-xml-mediatypes-00

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.  XML MIME
   entities are currently exchanged via the HyperText Transfer Protocol
   on the World Wide Web, are an integral part of the WebDAV protocol
   for remote web authoring, and are expected to have utility in many
   domains.

   Major differences from [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 April 02, 2013.


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Copyright Notice

   Copyright (c) 2012 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 . . . . . . . . . . . . . . . . . . . .  4
   3.  XML Media Types  . . . . . . . . . . . . . . . . . . . . . . .  5
     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 . . . . . . . . . . . . . 11
     3.6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   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 . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  A Naming Convention for XML-Based Media Types  . . . . . . . . 14
     8.1.  Referencing  . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     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
           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 Chars 21
     9.14. application/xml-dtd  . . . . . . . . . . . . . . . . . . . 21

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     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  . . . . . . . . . . . . . . . . . 28
   Appendix A.  Why Use the '+xml' Suffix for XML-Based MIME Types? . 29
     A.1.  Why not just use text/xml or application/xml and let the XML
           processor dispatch to the correct application based on the
           referenced DTD?  . . . . . . . . . . . . . . . . . . . . . 29
     A.2.  Why not create a new subtree (e.g., image/xml.svg) to
           represent XML MIME types?  . . . . . . . . . . . . . . . . 30
     A.3.  Why not create a new top-level MIME type for XML-based media
           types? . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     A.4.  Why not just have the MIME processor 'sniff' the content to
           determine whether it is XML? . . . . . . . . . . . . . . . 30
     A.5.  Why not use a MIME parameter to specify that a media type
           uses XML syntax? . . . . . . . . . . . . . . . . . . . . . 31
     A.6.  How about labeling with parameters in the other direction
           (e.g., application/xml; Content-Feature=iotp)? . . . . . . 31
     A.7.  How about a new superclass MIME parameter that is defined to
           apply to all MIME types (e.g., Content-Type:
           application/iotp; $superclass=xml)?  . . . . . . . . . . . 32
     A.8.  What about adding a new parameter to the Content-Disposition
           header or creating a new Content-Structure header to
           indicate XML syntax? . . . . . . . . . . . . . . . . . . . 32
     A.9.  How about a new Alternative-Content-Type header? . . . . . 32
     A.10. How about using a conneg tag instead (e.g., accept-features:
           (syntax=xml))? . . . . . . . . . . . . . . . . . . . . . . 32
     A.11. How about a third-level content-type, such as text/xml/rdf 33
     A.12. Why use the plus ('+') character for the suffix '+xml'?  . 33
     A.13. What is the semantic difference between application/foo and
           application/foo+xml? . . . . . . . . . . . . . . . . . . . 33
     A.14. What happens when an even better markup language (e.g.,
           EBML) is defined, or a new category of data? . . . . . . . 34
     A.15. Why must I use the '+xml' suffix for my new XML-based media
           type?  . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   Appendix B.  Changes from RFC 3023 . . . . . . . . . . . . . . . . 34
   Appendix C.  Acknowledgements  . . . . . . . . . . . . . . . . . . 35
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35

1.  Introduction

   The World Wide Web Consortium has issued the Extensible Markup
   Language (XML) 1.0 specification.  [XML].  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).

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   XML entities are currently exchanged on the World Wide Web, and XML
   is also used for property values and parameter marshalling by the
   WebDAV  [RFC4918] protocol for remote web authoring.  Thus, there is
   a need for a media type to properly label the exchange of XML network
   entities.

   Although XML is a subset of the Standard Generalized Markup Language
   (SGML) ISO 8879  [SGML], which has been assigned the media types text
   /sgml and application/sgml, there are several reasons why use of text
   /sgml or application/sgml to label XML is inappropriate.  First,
   there exist many applications that can process XML, but that cannot
   process SGML, due to SGML's larger feature set.  Second, SGML
   applications cannot always process XML entities, because XML uses
   features of recent technical corrigenda to SGML.  Third, the
   definition of text/sgml and application/sgml in [RFC1874] includes
   parameters for SGML bit combination transformation format (SGML-
   bctf), and SGML boot attribute (SGML-boot).  Since XML does not use
   these parameters, it would be ambiguous if such parameters were given
   for an XML MIME entity.  For these reasons, the best approach for
   labeling XML network entities has been to provide new media types for
   XML.

   Since XML is an integral part of the WebDAV Distributed Authoring
   Protocol, and since World Wide Web Consortium Recommendations are
   assigned standards tree media types, and since similar media types
   (HTML, SGML) have been assigned standards tree media types, the XML
   media types were also placed in the standards  tree [RFC3023].

   Similarly, 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, media types based on XML,
   but that are not identified using application/xml (or text/xml),
   SHOULD be named using a suffix of '+xml' as 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

   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:


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      "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).

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".  The media types application/xml or
   text/xml MAY be used for "document entities", while application/xml-
   external-parsed-entity or text/xml-external-parsed-entity SHOULD be
   used for "external parsed entities".  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.  The media type application/xml-dtd SHOULD be
   used for "external DTD subsets" or "external parameter entities".
   application/xml and text/xml MUST NOT be used for "external parameter
   entities" or "external DTD subsets", and MUST NOT be used for
   "external parsed entities" unless they are also well-formed "document
   entities" and are referenced as such.  Note that [RFC2376] (which is
   obsolete) allowed such usage, although in practice it is likely to
   have been rare.

   Neither external DTD subsets nor external parameter entities parse as
   XML documents, and while some XML document entities may be used as
   external parsed entities and vice versa, there are many cases where
   the two are not interchangeable.  XML also has unparsed entities,
   internal parsed entities, and internal parameter entities, but they
   are not XML MIME entities.





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   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:

      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).










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      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
   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
   encoding of the XML entity, as described in Section 8.1, 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

      The charset parameter MUST only be used, when the charset is
      reliably known and agrees with the 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].







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      If an application/xml entity is received where the charset
      parameter is omitted, no information is being provided about the
      charset by the MIME Content-Type header.  Conforming XML
      processors MUST follow the requirements in section 4.3.3 of [XML]
      that directly address this contingency.  However, MIME processors
      that are not XML processors SHOULD NOT assume a default charset if
      the charset parameter is omitted from an application/xml entity.

      There are several reasons that the charset parameter is 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 encoding
      declaration or BOM and to hide charset information from protocols,
      they SHOULD determine not to use the parameter.

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

   Encoding considerations: This media type MAY be encoded as
      appropriate for the charset and the capabilities of the underlying
      MIME transport.  For 7-bit transports, data in either UTF-8 or
      UTF-16 MUST be encoded in quoted-printable or base64.  For 8-bit
      clean transport (e.g., 8BITMIME  [RFC1652] ESMTP or NNTP
      [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.

   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].



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   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:

         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

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   MIME media type name: application

   MIME subtype name: xml-external-parsed-entity

   Mandatory parameters: none

   Optional parameters: charset

      The charset parameter of application/xml-external-parsed-entity is
      handled the same as that of application/xml as described in
      Section 3.1.

   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.

   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


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   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

      The charset parameter of application/xml-dtd is handled the same
      as that of application/xml as described in Section 3.1.

   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.

   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.  Summary




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   o  If the charset parameter is omitted, conforming XML processors
      MUST follow the requirements in section 4.3.3 of [XML] or [XML1.1]
      as appropriate.

   o  If provided, the charset parameter MUST agree with the xml
      encoding declaration.

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

   Section 4.3.3 of [XML] specifies that XML MIME entities in the
   charset "utf-16" MUST begin with a byte order mark (BOM), which is a
   hexadecimal octet sequence 0xFE 0xFF (or 0xFF 0xFE, depending on
   endian).  The XML Recommendation further states that the BOM is an
   encoding signature, and is not part of either the markup or the
   character data of the XML document.

   Due to the presence of the BOM, applications that convert XML from
   "utf-16" to a non-Unicode encoding MUST strip the BOM before
   conversion.  Similarly, when converting from another encoding into
   "utf-16", the BOM MUST be added after conversion is complete.

   In addition to the charset "utf-16", [RFC2781] introduces "utf-16le"
   (little endian) and "utf-16be" (big endian) as well.  The BOM is
   prohibited for these charsets.  When an XML MIME entity is encoded in
   "utf-16le" or "utf-16be", it MUST NOT begin with the BOM but SHOULD
   contain an encoding declaration.  Conversion from "utf-16" to "utf-
   16be" or "utf-16le" and conversion in the other direction MUST strip
   or add the BOM, respectively.

5.  Fragment Identifiers

   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
   attempt to 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.



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   A  registry of XPointer schemes [XPtrReg] is maintained at the W3C.
   Unregistered schemes SHOULD NOT be used.

   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.

   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.




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   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.  This allows the use
   of generic XML processors and technologies on a wide variety of
   different XML document types at a minimum cost, using existing
   frameworks for media type registration.

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

   As XML development continues, new XML document types are appearing
   rapidly.  Many of these XML document types would benefit from the
   identification possibilities of a more specific MIME media type than
   text/xml or application/xml can provide, and it is likely that many
   new media types for XML-based document types will be registered in
   the near and ongoing future.

   While the benefits of specific MIME types for particular types of XML
   documents are significant, all XML documents share common structures
   and syntax that make possible common processing.

   Some areas where 'generic' processing is useful include:

   o  Browsing - An XML browser can display any XML document with a
      provided [CSS] or [XSLT] style sheet, whatever the vocabulary of
      that document.

   o  Editing - Any XML editor can read, modify, and save any XML
      document.

   o  Fragment identification - XPointers (see Section 5) can work with
      any XML document, whatever vocabulary it uses.

   o  Hypertext linking - [XLink] hypertext linking is designed to
      connect any XML documents, regardless of vocabulary.





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   o  Searching - XML-oriented search engines, web crawlers, agents, and
      query tools should be able to read XML documents and extract the
      names and content of elements and attributes even if the tools are
      ignorant of the particular vocabulary used for elements and
      attributes.

   o  Storage - XML-oriented storage systems, which keep XML documents
      internally in a parsed form, should similarly be able to process,
      store, and recreate any XML document.

   o  Well-formedness and validity checking - An XML processor can
      confirm that any XML document is well-formed and that it is valid
      (i.e., conforms to its declared DTD or Schema).

   When a new media type is introduced for an XML-based format, the name
   of the media type SHOULD end with '+xml'. This convention will allow
   applications that can process XML generically to detect that the MIME
   entity is supposed to be an XML document, verify this assumption by
   invoking some XML processor, and then process the XML document
   accordingly.  Applications may match for types that represent XML
   MIME entities by comparing the subtype to the pattern '*/*+xml'.  (Of
   course, 4 of the 5 media types defined in this 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 encoding declarations of XML.

      Note: Some argue that XML-based media types should not introduce
      the charset parameter, although others disagree.







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   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.

   *HST: This paragraph needs updating once some pending RFCs are out
   there *The registration process for these media types is described in
   [RFC4288] and [RFC4289]  .  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.

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.


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

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).




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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"?>

   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







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   UTF-8.  Although the XML MIME entity does not contain an encoding
   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].












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9.16.  application/xslt+xml

   Content-type: application/xslt+xml

   <?xml version="1.0" ?>

   Extensible Stylesheet Language (XSLT) documents are XML documents
   whose content describes stylesheets for other XML documents, as
   defined by [XSLT].  As a format based on XML, XSLT documents SHOULD
   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






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

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

   Since the charset parameter is provided in the Content-Type header
   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 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 [RFC4288]
   and [RFC4289]  registration process for XML-based MIME types.

11.  Security Considerations

   XML, as a subset of SGML, has all of the same security considerations
   as specified in [RFC1874], and likely more, due to its ubiquitous
   deployment.

   To paraphrase section 3 of RFC 1874, XML MIME entities contain
   information to be parsed and processed by the recipient's XML system.
   These entities may contain and such systems may permit explicit
   system level commands to be executed while processing the data.  To
   the extent that an XML system will execute arbitrary command strings,











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   recipients of XML MIME entities may be a risk.  In general, it may be
   possible to specify commands that perform unauthorized file
   operations or make changes to the display processor's environment
   that affect subsequent operations.

   In general, any information stored outside of the direct control of
   the user -- including CSS style sheets, XSL transformations, entity
   declarations, and DTDs -- can be a source of insecurity, by either
   obvious or subtle means.  For example, a tiny "whiteout attack"
   modification made to a "master" style sheet could make words in
   critical locations disappear in user documents, without directly
   modifying the user document or the stylesheet it references.  Thus,
   the security of any XML document is vitally dependent on all of the
   documents recursively referenced by that document.

   The entity lists and DTDs for XHTML 1.0  [XHTML], for instance, are
   likely to be a commonly used set of information.  Many developers
   will use and trust them, few of whom will know much about the level
   of security on the W3C's servers, or on any similarly trusted
   repository.

   The simplest attack involves adding declarations that break
   validation.  Adding extraneous declarations to a list of character
   entities can effectively "break the contract" used by documents.  A
   tiny change that produces a fatal error in a DTD could halt XML
   processing on a large scale.  Extraneous declarations are fairly
   obvious, but more sophisticated tricks, like changing attributes from
   being optional to required, can be difficult to track down.  Perhaps
   the most dangerous option available to crackers is redefining default
   values for attributes: e.g., if developers have relied on defaulted
   attributes for security, a relatively small change might expose
   enormous quantities of information.

   Apart from the structural possibilities, another option, "entity
   spoofing," can be used to insert text into documents, vandalizing and
   perhaps conveying an unintended message.  Because XML 1.0 permits
   multiple entity declarations, and the first declaration takes
   precedence, it's possible to insert malicious content where an entity
   is used, such as by inserting the full text of Winnie the Pooh in
   every occurrence of &mdash;.

   Use of the digital signatures work currently underway by the xmldsig
   working group may eventually ameliorate the dangers of referencing
   external documents not under one's own control.

   Use of XML is expected to be varied, and widespread.  XML is under
   scrutiny by a wide range of communities for use as a common syntax
   for community-specific metadata.  For example, the Dublin Core
   [RFC5013] group is using XML for document metadata, and a new effort
   has begun that is considering use of XML for medical information.
   Other groups view XML as a mechanism for marshalling parameters for




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   remote procedure calls.  More uses of XML will undoubtedly arise.

   Security considerations will vary by domain of use.  For example, XML
   medical records will have much more stringent privacy and security
   considerations than XML library metadata.  Similarly, use of XML as a
   parameter marshalling syntax necessitates a case by case security
   review.

   XML may also have some of the same security concerns as plain text.
   Like plain text, XML can contain escape sequences that, when
   displayed, have the potential to change the display processor
   environment in ways that adversely affect subsequent operations.
   Possible effects include, but are not limited to, locking the
   keyboard, changing display parameters so subsequent displayed text is
   unreadable, or even changing display parameters to deliberately
   obscure or distort subsequent displayed material so that its meaning
   is lost or altered.  Display processors SHOULD either filter such
   material from displayed text or else make sure to reset all important
   settings after a given display operation is complete.

   Some terminal devices have keys whose output, when pressed, can be
   changed by sending the display processor a character sequence.  If
   this is possible the display of a text object containing such
   character sequences could reprogram keys to perform some illicit or
   dangerous action when the key is subsequently pressed by the user.
   In some cases not only can keys be programmed, they can be triggered
   remotely, making it possible for a text display operation to directly
   perform some unwanted action.  As such, the ability to program keys
   SHOULD be blocked either by filtering or by disabling the ability to
   program keys entirely.

   Note that it is also possible to construct XML documents that make
   use of what XML terms "entity references" (using the XML meaning of
   the term "entity" as described in Section 2), to construct repeated
   expansions of text.  Recursive expansions are prohibited by [XML] and
   XML processors are required to detect them.  However, even non-
   recursive expansions may cause problems with the finite computing
   resources of computers, if they are performed many times.  (Entity A
   consists of 100 copies of entity B, which in turn consists of 100
   copies of 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.

   [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/>.



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   [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.

   [PNG]      Boutell, T., "PNG (Portable Network Graphics)
              Specification", World Wide Web Consortium Recommendation
              REC-png, October 1996, <http://www.w3.org/TR/REC-png>.

   [RFC1652]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
              Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
              RFC 1652, July 1994.

   [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.

   [RFC2077]  Nelson, S.D., Parks, C., and  Mitra, "The Model Primary
              Content Type for Multipurpose Internet Mail Extensions",
              RFC 2077, January 1997.

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

   [RFC2445]  Dawson, F. and D. Stenerson, "Internet Calendaring and
              Scheduling Core Object Specification (iCalendar)", RFC
              2445, November 1998.

   [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", January 2001.

   [RFC3501]  Crispin, M., "Internet Message Access Protocol - Version
              4rev1", RFC 3501, March 2003.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO








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              10646", RFC 3629, November  2003.

   [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.

   [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.

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

   [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>.

   [SGML]     International Standard Organization, "Information
              Processing -- Text and Office Systems -- Standard
              Generalized Markup Language (SGML)", ISO 8879, October
              1986.

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

   [UML]      Object Management Group, "OMG Unified Modeling Language
              Specification, Version 1.3", OMG Specification ad/
              99-06-08, June 1999, <http://www.omg.org/uml/>.

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

   [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>.




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   [XLink]    DeRose, S., Maler, E., Orchard, D., and N. Walsh, "XML
              Linking Language (XLink) Version 1.1", World Wide Web
              Consortium Recommendation xlink11, May 2010, <http://
              www.w3.org/TR/xlink/>.

   [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/>.

   [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

   [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.

   [RFC1874]  Levinson, E., "SGML Media Types", RFC 1874, December 1995.

   [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.


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

   [RFC2801]  Burdett, D., "Internet Open Trading Protocol - IOTP
              Version 1.0", RFC 2801, April 2000.

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

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

   [RFC5013]  Kunze, J. and T. Baker, "Dublin Core Metadata for Resource
              Discovery", RFC 5013, August 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/>.

   [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?

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

A.1.  Why not just use text/xml or application/xml and let the XML
      processor dispatch to the correct application based on the
      referenced DTD?

   text/xml and application/xml remain useful in many situations,
   especially for document-oriented applications that involve combining
   XML with a stylesheet in order to present the data.  However, XML is
   also used to define entirely new data types, and an XML-based format
   such as image/svg+xml fits the definition of a MIME media type
   exactly as well as image/png  [PNG] does.  (Note that image/svg+xml
   is not yet registered.) Although extra functionality is available for
   MIME processors that are also XML processors, XML-based media types
   -- even when treated as opaque, non-XML media types -- are just as
   useful as any other media type and should be treated as such.

   Since MIME dispatchers work off of the MIME type, use of text/xml or
   application/xml to label discrete media types will hinder correct
   dispatching and general interoperability.  Finally, many XML
   documents use neither DTDs nor namespaces, yet are perfectly legal
   XML.


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

   The subtree under which a media type is registered -- IETF, vendor (*
   /vnd.*), or personal (*/prs.*); see [RFC4288] and [RFC4289]  for
   details -- is completely orthogonal from whether the media type uses
   XML syntax or not.  The suffix approach allows XML document types to
   be identified within any subtree.  The vendor subtree, for example,
   is likely to include a large number of XML-based document types.  By
   using a suffix, rather than setting up a separate subtree, those
   types may remain in the same location in the tree of MIME types that
   they would have occupied had they not been based on XML.

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

   The top-level MIME type (e.g., model/*  [RFC2077]) determines what
   kind of content the type is, not what syntax it uses.  For example,
   agents using image/* to signal acceptance of any image format should
   certainly be given access to media type image/svg+xml, which is in
   all respects a standard image subtype.  It just happens to use XML to
   describe its syntax.  The two aspects of the media type are
   completely orthogonal.

   XML-based data types will most likely be registered in ALL top-level
   categories.  Potential, though currently unregistered, examples could
   include application/mathml+xml  [MathML], model/uml+xml  [UML], and
   image/svg+xml  [SVG].

A.4.  Why not just have the MIME processor 'sniff' the content to
      determine whether it is XML?

   Rather than explicitly labeling XML-based media types, the processor
   could look inside each type and see whether or not it is XML.  The
   processor could also cache a list of XML-based media types.

   Although this method might work acceptably for some mail
   applications, it would fail completely in many other uses of MIME.
   For instance, an XML-based web crawler would have no way of
   determining whether a file is XML except to fetch it and check.  The
   same issue applies in some IMAP4  [RFC3501] mail applications, where
   the client first fetches the MIME type as part of the message
   structure and then decides whether to fetch the MIME entity.
   Requiring these fetches just to determine whether the MIME type is
   XML could have significant bandwidth and latency disadvantages in
   many situations.








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   Sniffing XML also isn't as simple as it might seem.  DOCTYPE
   declarations aren't required, and they can appear fairly deep into a
   document under certain unpreventable circumstances.  (E.g., the XML
   declaration, comments, and processing instructions can occupy space
   before the DOCTYPE declaration.) Even sniffing the DOCTYPE isn't
   completely reliable, thanks to a variety of issues involving default
   values for namespaces within external DTDs and overrides inside the
   internal DTD.  Finally, the variety in potential character encodings
   (something XML provides tools to deal with), also makes reliable
   sniffing less likely.

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

   For example, one could use "Content-Type: application/iotp;
   alternate-type=text/xml" or "Content-Type: application/iotp;
   syntax=xml".

   Section 5 of [RFC2045] says that "Parameters are modifiers of the
   media subtype, and as such do not fundamentally affect the nature of
   the content".  However, all XML-based media types are by their nature
   always XML.  Parameters, as they have been defined in the MIME
   architecture, are never invariant across all instantiations of a
   media type.

   More practically, very few if any MIME dispatchers and other MIME
   agents support dispatching off of a parameter.  While MIME agents on
   the receiving side will need to be updated in either case to support
   (or fall back to) generic XML processing, it has been suggested that
   it is easier to implement this functionality when acting off of the
   media type rather than a parameter.  More important, sending agents
   require no update to properly tag an image as "image/svg+xml", but
   few if any sending agents currently support always tagging certain
   content types with a parameter.

A.6.  How about labeling with parameters in the other direction (e.g.,
      application/xml; Content-Feature=iotp)?

   This proposal fails under the simplest case, of a user with neither
   knowledge of XML nor an XML-capable MIME dispatcher.  In that case,
   the user's MIME dispatcher is likely to dispatch the content to an
   XML processing application when the correct default behavior should
   be to dispatch the content to the application responsible for the
   content type (e.g., an ecommerce engine for application/iotp+xml
   [RFC2801], once this media type is registered).









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   Note that even if the user had already installed the appropriate
   application (e.g., the ecommerce engine), and that installation had
   updated the MIME registry, many operating system level MIME
   registries such as .mailcap in Unix and HKEY_CLASSES_ROOT in Windows
   do not currently support dispatching off a parameter, and cannot
   easily be upgraded to do so.  And, even if the operating system were
   upgraded to support this, each MIME dispatcher would also separately
   need to be upgraded.

A.7.  How about a new superclass MIME parameter that is defined to apply
      to all MIME types (e.g., Content-Type: application/iotp;
      $superclass=xml)?

   This combines the problems of Appendix A.5 and Appendix A.6.

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

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

A.8.  What about adding a new parameter to the Content-Disposition
      header or creating a new Content-Structure header to indicate XML
      syntax?

   This has nearly identical problems to Appendix A.7, in that it
   requires both senders and receivers to be upgraded, and few if any
   operating systems and MIME dispatchers support working off of
   anything other than the MIME type.

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

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

A.10.  How about using a conneg tag instead (e.g., accept-features:
       (syntax=xml))?





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

   Also, note that adding a content-type parameter doesn't work with
   conneg either, since conneg only deals with media types, not their
   parameters.  This is another illustration of the limits of parameters
   for MIME dispatchers.

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

   MIME explicitly defines two levels of content type, the top-level for
   the kind of content and the second-level for the specific media type.
   [RFC4288] and [RFC4289] extends this in an interoperable way by using
   prefixes to specify separate trees for IETF, vendor, and personal
   registrations.  This specification also extends the two-level type by
   using the '+xml' suffix.  In both cases, processors that are unaware
   of these later specifications treat them as opaque and continue to
   interoperate.  By contrast, adding a third-level type would break the
   current MIME architecture and cause numerous interoperability
   failures.

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

   As specified in Section 5.1 of [RFC2045], a tspecial can't be used:

      tspecials :=
      "(" / ")" / "<" / ">" / "@" /
      "," / ";" / ":" / "\" / <">
      "/" / "[" / "]" / "?"  / "="

   It was thought that "."  would not be a good choice since it is
   already used as an additional hierarchy delimiter.  Also, "*" has a
   common wildcard meaning, and "-" and "_" are common word separators
   and easily confused.  The characters %'`#& are frequently used for
   quoting or comments and so are not ideal.

   That leaves: ~!$^+{}|

   Note that "-" is used heavily in the current registry.  "$" and "_"
   are used once each.  The others are currently unused.

   It was thought that '+' expressed the semantics that a MIME type can
   be treated (for example) as both scalable vector graphics AND ALSO as
   XML; it is both simultaneously.

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





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

A.14.  What happens when an even better markup language (e.g., EBML) is
       defined, or a new category of data?

   In the ten years that MIME has existed, XML is the first generic data
   format that has seemed to justify special treatment, so it is hoped
   that no further suffixes will be necessary.  However, if some are
   later defined, and these documents were also XML, they would need to
   specify that the '+xml' suffix is always the outermost suffix (e.g.,
   application/foo+ebml+xml not application/foo+xml+ebml).  If they were
   not XML, then they would use a regular suffix (e.g., application/
   foo+ebml).

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

   You don't have to, but unless you have a good reason to explicitly
   disallow generic XML processing, you should use the suffix so as not
   to curtail the options of future users and developers.

   Whether the inventors of a media type, today, design it for dispatch
   to generic XML processing machinery (and most won't) is not the
   critical issue.  The core notion is that the knowledge that some
   media type happens to use XML syntax opens the door to unanticipated
   kinds of processing beyond those envisioned by its inventors, and on
   this basis identifying such encoding is a good and useful thing.

   Developers of new media types are often tightly focused on a
   particular type of processing that meets current needs.  But there is
   no need to rule out generic processing as well, which could make your
   media type more valuable over time.  It is believed that registering
   with the '+xml' suffix will cause no interoperability problems
   whatsoever, while it may enable significant new functionality and
   interoperability now and in the future.  So, the conservative
   approach is to include the '+xml' suffix.

Appendix B.  Changes from RFC 3023

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





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   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
   Internet Media Type registration, consistency of use [TAGMIME].
   Fourth, many references are updated.

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/


   Henry S. Thompson
   University of Edinburgh

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

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