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rfc:rfc7303

Internet Engineering Task Force (IETF) H. Thompson Request for Comments: 7303 University of Edinburgh Obsoletes: 3023 C. Lilley Updates: 6839 W3C Category: Standards Track July 2014 ISSN: 2070-1721

                          XML Media Types

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 is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7303.

Thompson & Lilley Standards Track [Page 1] RFC 7303 XML Media Types July 2014

Copyright Notice

 Copyright (c) 2014 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.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
 2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   4
   2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.2.  Characters, Encodings, Charsets . . . . . . . . . . . . .   4
   2.3.  MIME Entities, XML Entities . . . . . . . . . . . . . . .   5
 3.  Encoding Considerations . . . . . . . . . . . . . . . . . . .   6
   3.1.  XML MIME Producers  . . . . . . . . . . . . . . . . . . .   6
   3.2.  XML MIME Consumers  . . . . . . . . . . . . . . . . . . .   7
   3.3.  The BOM and Encoding Conversions  . . . . . . . . . . . .   8
 4.  XML Media Types . . . . . . . . . . . . . . . . . . . . . . .   9
   4.1.  XML MIME Entities . . . . . . . . . . . . . . . . . . . .   9
   4.2.  Using '+xml' when Registering XML-Based Media Types . . .  11
   4.3.  Registration Guidelines for XML-Based Media Types Not
         Using   '+xml'  . . . . . . . . . . . . . . . . . . . . .  12
 5.  Fragment Identifiers  . . . . . . . . . . . . . . . . . . . .  13
 6.  The Base URI  . . . . . . . . . . . . . . . . . . . . . . . .  14
 7.  XML Versions  . . . . . . . . . . . . . . . . . . . . . . . .  14
 8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   8.1.  UTF-8 Charset . . . . . . . . . . . . . . . . . . . . . .  15

Thompson & Lilley Standards Track [Page 2] RFC 7303 XML Media Types July 2014

   8.2.  UTF-16 Charset  . . . . . . . . . . . . . . . . . . . . .  16
   8.3.  Omitted Charset and 8-Bit MIME Entity . . . . . . . . . .  16
   8.4.  Omitted Charset and 16-Bit MIME Entity  . . . . . . . . .  16
   8.5.  Omitted Charset, No Internal Encoding Declaration . . . .  17
   8.6.  UTF-16BE Charset  . . . . . . . . . . . . . . . . . . . .  17
   8.7.  Non-UTF Charset . . . . . . . . . . . . . . . . . . . . .  18
   8.8.  INCONSISTENT EXAMPLE: Conflicting Charset and Internal
         Encoding Declaration  . . . . . . . . . . . . . . . . . .  18
   8.9.  INCONSISTENT EXAMPLE: Conflicting Charset and BOM . . . .  18
 9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   9.1.  application/xml Registration  . . . . . . . . . . . . . .  19
   9.2.  text/xml Registration . . . . . . . . . . . . . . . . . .  21
   9.3.  application/xml-external-parsed-entity Registration . . .  21
   9.4.  text/xml-external-parsed-entity Registration  . . . . . .  22
   9.5.  application/xml-dtd Registration  . . . . . . . . . . . .  22
   9.6.  The '+xml' Naming Convention for XML-Based Media Types  .  23
     9.6.1.  The '+xml' Structured Syntax Suffix Registration  . .  23
 10. Security Considerations . . . . . . . . . . . . . . . . . . .  25
 11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
   11.1.  Normative References . . . . . . . . . . . . . . . . . .  27
   11.2.  Informative References . . . . . . . . . . . . . . . . .  29
 Appendix A.  Why Use the '+xml' Suffix for XML-Based MIME Types?   32
 Appendix B.  Core XML Specifications  . . . . . . . . . . . . . .  32
 Appendix C.  Operational Considerations . . . . . . . . . . . . .  32
   C.1.  General Considerations  . . . . . . . . . . . . . . . . .  33
   C.2.  Considerations for Producers  . . . . . . . . . . . . . .  33
   C.3.  Considerations for Consumers  . . . . . . . . . . . . . .  34
 Appendix D.  Changes from RFC 3023  . . . . . . . . . . . . . . .  34
 Appendix E.  Acknowledgements . . . . . . . . . . . . . . . . . .  35

Thompson & Lilley Standards Track [Page 3] RFC 7303 XML Media Types July 2014

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), two aliases (text/xml and text/xml-external-
 parsed-entity), and 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
 registered in Section 9.6.  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. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "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,

Thompson & Lilley Standards Track [Page 4] RFC 7303 XML Media Types July 2014

 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", namely UTF-8, UTF-16, and
 UTF-32.  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".  UTF-16 XML documents, however, can 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".  See Section 3.3 below
 for how a Byte Order Mark (BOM) is required when the "utf-16"
 serialization is used.
 UTF-32 has four potential serializations, of which only two (UTF-32BE
 and UTF-32LE) are given names in [UNICODE].  Support for the various
 serializations varies widely, and security concerns about their use
 have been raised (for example, see [Sivonen]).  The use of UTF-32 is
 NOT RECOMMENDED for XML MIME entities.

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).
 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.

Thompson & Lilley Standards Track [Page 5] RFC 7303 XML Media Types July 2014

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 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 that package, transfer, deliver, and/or receive MIME
 entities).
 The use of UTF-8, without a BOM, is RECOMMENDED for all XML MIME
 entities.
 Some MIME agents will be what we will call "XML-aware", that is,
 capable of processing XML MIME entities as XML and detecting the XML
 encoding declaration (or its absence).  All three sources of
 information about encoding are available to them, and they can be
 expected to be aware of this specification.
 Other MIME agents will not be XML-aware; thus, they cannot know
 anything about the XML encoding declaration.  Not only do they lack
 one of the three sources of information about encoding, they are also
 less likely to be aware of or responsive to this specification.
 Some MIME agents, such as proxies and transcoders, both consume and
 produce MIME entities.
 This mixture of two kinds of agents handling XML MIME entities
 increases the complexity of the coordination task.  The
 recommendations given below are intended to maximise interoperability
 in the face of this: on the one hand, by mandating consistent
 production and encouraging maximally robust forms of production and,
 on the other, by specifying recovery strategies to maximize the
 interoperability of consumers when the production rules are broken.

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 that lack an
 encoding declaration.  Such producers SHOULD remove or correct an
 encoding declaration that is known to be incorrect (for example, as a
 result of transcoding).

Thompson & Lilley Standards Track [Page 6] RFC 7303 XML Media Types July 2014

 XML-aware MIME producers MUST supply an XML text declaration at the
 beginning of non-UNICODE XML external parsed entities that 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.  Note that this is particularly relevant for central
 configuration of web servers, where configuring a default for the
 charset parameter will almost certainly violate this requirement.
 XML MIME producers are RECOMMENDED to provide means for users to
 control what value, if any, is given to charset parameters for XML
 MIME entities, for example, by giving users control of the
 configuration of Web server filename-to-Content-Type-header mappings
 on a file-by-file or suffix basis.

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 between 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, 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:
    A BOM (Section 3.3) is authoritative if it is present in an XML
    MIME entity;
    In the absence of a BOM (Section 3.3), the charset parameter is
    authoritative if it is present.

Thompson & Lilley Standards Track [Page 7] RFC 7303 XML Media Types July 2014

 Whenever the above determines a source of encoding information as
 authoritative, consumers SHOULD process XML MIME entities based on
 that information.
 When MIME producers conform to the requirements stated above
 (Section 3.1, Section 3) inconsistencies will not arise -- the above
 statement of priorities only has practical impact in the case of non-
 conforming XML MIME entities.  In the face of inconsistencies, no
 uniform strategy can deliver the 'right' answer every time: the
 purpose of specifying one here is to encourage convergence over time,
 first on the part of consumers, then on the part of producers.
 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.
 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.

3.3. The BOM and Encoding Conversions

 Section 4.3.3 of [XML] specifies that UTF-16 XML MIME entities not
 labelled as "utf-16le" or "utf-16be" MUST begin with a 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.

Thompson & Lilley Standards Track [Page 8] RFC 7303 XML Media Types July 2014

 Applications that convert XML from UTF-8 to an encoding other than
 UTF-16 MUST strip the BOM, if present, before conversion.
 Applications that 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).  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.
 Conversion from UTF-16 labelled "utf-16le" or "utf-16be" to UTF-16
 without a label or labelled "utf-16" MUST add the appropriate BOM.
 Conversion from UTF-16 labelled "utf-16le" or "utf-16be" to UTF-8 MAY
 add a UTF-8 BOM, but this is NOT RECOMMENDED.
 Appendix F of [XML] also implies that 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.  If it is used,
 the same considerations as UTF-16 apply with respect to its being a
 signature (not part of the document), transcoding into or out of it,
 and transcoding into or out of the MIME charsets "utf-32le" and "utf-
 32be".  Consumers that do not support UTF-32 SHOULD nonetheless
 recognise UTF-32 signatures in order to give helpful error messages
 (instead of treating them as invalid UTF-16).

4. XML Media Types

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 9.6), 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".

Thompson & Lilley Standards Track [Page 9] RFC 7303 XML Media Types July 2014

 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 that 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.
 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
    [RFC7231] changes [RFC2616] by removing the ISO-8859-1 default and
    not defining any default at all;
    [RFC6657] updates [RFC2046] to remove the US-ASCII [ASCII]
    default.
 See Section 3 for the now-unified approach to the charset parameter
 that 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; in fact, they are expected to proliferate.

Thompson & Lilley Standards Track [Page 10] RFC 7303 XML Media Types July 2014

 However, developers of such media types are RECOMMENDED to use this
 specification as a basis for their registration.  See Section 4.2 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 Cascading Style Sheets (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 that, taken together
 with [XML] itself, show how to determine an XML document's language-
 level semantics and suggest how information about its application-
 level semantics may be locatable.

4.2. Using '+xml' when Registering XML-Based Media Types

 In Section 9.6, this specification updates the registration in
 [RFC6839] for XML-based MIME types (the '+xml' 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
 four of the five 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'.)
    NOTE: Section 5.3.2 of [RFC7231] does not support any form of
    Accept header that will match only '+xml' types.  In particular,
    Accept headers of the form "Accept: */*+xml" are not allowed, and
    will not work for this purpose.
 Media types following the naming convention '+xml' SHOULD define the
 charset parameter for consistency, since XML-generic processing by
 definition treats all XML MIME entities uniformly as regards
 character encoding information.  However, there are some cases that
 the charset parameter need not be defined.  For example:

Thompson & Lilley Standards Track [Page 11] RFC 7303 XML Media Types July 2014

    When an XML-based media type is restricted to UTF-8, it is not
    necessary to define 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 use it.
 The registration process for specific '+xml' media types is described
 in [RFC6838].  New XML-based media type registrations in the IETF
 must follow these guidelines.  When other organisations register XML-
 based media types via the "Specification Required" IANA registration
 policy [RFC5226], the relevant Media Reviewer should ensure that they
 use the '+xml' convention, in order to ensure maximum
 interoperability of their XML-based documents.  Only media subtypes
 that represent XML MIME entities are 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).

4.3. Registration Guidelines for XML-Based Media Types Not Using '+xml'

 Registrations for new XML-based media types that 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
 7303".
 Defining 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".

Thompson & Lilley Standards Track [Page 12] RFC 7303 XML Media Types July 2014

 These registrations SHOULD specify that the XML-based media type
 being registered has all of the security considerations described in
 this specification plus any additional considerations specific to
 that media type.
 These registrations SHOULD also make reference to this specification
 in specifying magic numbers, base URIs, and use of the BOM.
 These registrations MAY reference the application/xml registration in
 this document in specifying interoperability and fragment identifier
 considerations, if these considerations are not overridden by issues
 specific to that media type.

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
 more complex constructions based on named schemes.  When the syntax
 of a fragment identifier part of any URI or Internationalized
 Resource Identifier (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 that 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.
 Generic processors of XML MIME entities SHOULD NOT implement
 unregistered XPointer schemes ([XPtrRegPolicy] describes requirements
 and procedures for registering schemes).
 See Section 4.2 for additional requirements that 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'.

Thompson & Lilley Standards Track [Page 13] RFC 7303 XML Media Types July 2014

 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; therefore, its use is NOT RECOMMENDED.

7. XML Versions

 application/xml, application/xml-external-parsed-entity, 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 first or second 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 that 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. 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.

Thompson & Lilley Standards Track [Page 14] RFC 7303 XML Media Types July 2014

 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 below in
 Section 9, 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 that 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.

8.1. 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, conformant 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).

Thompson & Lilley Standards Track [Page 15] RFC 7303 XML Media Types July 2014

8.2. UTF-16 Charset

 Content-Type: application/xml; charset=utf-16
 {BOM}<?xml version="1.0" encoding="utf-16"?>
 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 [RFC7231] for details).  Hence, one of the
 two text/media types defined above can be used with this example only
 when the XML MIME entity is transmitted via HTTP or HTTPS, which use
 a MIME-like mechanism and are binary-clean protocols and 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).

8.3. 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, conformant XML processors must
 treat the "iso-8859-1" encoding as authoritative.  Conformant XML-
 unaware MIME processors should make no assumptions about the
 character encoding of the XML MIME entity.

8.4. 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"?>

Thompson & Lilley Standards Track [Page 16] RFC 7303 XML Media Types July 2014

 This example shows a 16-bit MIME entity with no charset parameter.
 However, since there is a BOM, conformant processors must treat the
 entity as UTF-16 encoded.
 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 in
 Section 8.2, Paragraph 7).

8.5. 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.
 A conformant XML-unaware MIME processor should make no assumptions
 about the character encoding of the XML MIME entity.
 See Section 8.1 for transport-related issues for UTF-8 XML MIME
 entities.

8.6. 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,
 conformant MIME and XML processors must treat the enclosed entity as
 UTF-16BE encoded.
 See also the additional considerations in the UTF-16 example in
 Section 8.2.

Thompson & Lilley Standards Track [Page 17] RFC 7303 XML Media Types July 2014

8.7. Non-UTF Charset

 Content-Type: application/xml; charset=iso-2022-kr
 <?xml version="1.0" encoding="iso-2022-kr"?>
 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, conformant 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 (Sections 8.1 and 8.2) would apply.

8.8. 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, conformant MIME and XML processors will
 interoperate.  Since the charset parameter is authoritative in the
 absence of a BOM, conformant processors will treat the enclosed
 entity as iso-8859-1 encoded.  That is, the "UTF-8" encoding
 declaration will be ignored.
 Conformant 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.

8.9. INCONSISTENT EXAMPLE: Conflicting Charset and BOM

 Content-Type: application/xml; charset=iso-8859-1
 {BOM}<?xml version="1.0"?>

Thompson & Lilley Standards Track [Page 18] RFC 7303 XML Media Types July 2014

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

9. IANA Considerations

9.1. 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 [RFC7230]), no content-transfer-
    encoding is necessary (or even possible, in the case of HTTP) for
    7bit, 8bit, or binary data.
 Security considerations:  See Section 10.
 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, because
    they have to handle all aspects of XML.  Although a non-validating

Thompson & Lilley Standards Track [Page 19] RFC 7303 XML Media Types July 2014

    processor may be more efficient, it might not handle all aspects.
    For further information, see Section 2.9 "Standalone Document
    Declaration" and Section 5 "Conformance" of [XML] .
    In practice, character set issues have proved to be the biggest
    source of interoperability problems.  The use of UTF-8, and
    careful attention to the guidelines set out in Section 3, are the
    best ways to avoid such problems.
 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.
       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 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 RFC 7303.

Thompson & Lilley Standards Track [Page 20] RFC 7303 XML Media Types July 2014

9.2. text/xml Registration

 The registration information for text/xml is in all respects the same
 as that given for application/xml above (Section 9.1), except that
 the "Type name" is "text".

9.3. 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 9.1).
 Security considerations:  See Section 10.
 Interoperability considerations:  XML external parsed entities are as
    interoperable as XML documents, though they have a less tightly
    constrained structure and therefore need to be referenced by XML
    documents for proper handling by XML processors.  Similarly, XML
    documents cannot be reliably used as external parsed entities
    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 9.1).
 Applications which use this media type:  Same as for application/xml
    (Section 9.1).
 Additional information:
    Magic number(s):  Same as for application/xml (Section 9.1).
    File extension(s):  .xml or .ent
    Macintosh File Type Code(s):  "TEXT"
    Base URI:  See Section 6

Thompson & Lilley Standards Track [Page 21] RFC 7303 XML Media Types July 2014

 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 RFC 7303.

9.4. 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 9.3), except that the "Type name" is
 "text".

9.5. application/xml-dtd Registration

 Type name:  application
 Subtype name:  xml-dtd
 Required parameters:  none
 Optional parameters:  charset
    See Section 3.
 Encoding considerations:  Same as for application/xml (Section 9.1).
 Security considerations:  See Section 10.
 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 9.1).
 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.

Thompson & Lilley Standards Track [Page 22] RFC 7303 XML Media Types July 2014

 Additional information:
    Magic number(s):  Same as for application/xml (Section 9.1).
    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 RFC 7303.

9.6. 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.
 See Section 4.2 for guidance on when and how to register a media
 subtype that is '+xml' based, and Section 4.3 on registering a media
 subtype for XML but _not_ using '+xml'.

9.6.1. The '+xml' Structured Syntax Suffix Registration

 Name:  Extensible Markup Language (XML)
 +suffix:  +xml
 Reference:  RFC 7303
 Encoding considerations:  Same as Section 9.1.
 Fragment identifier considerations:  Registrations that use this
    '+xml' convention MUST also make reference to this document,
    specifically Section 5, in specifying fragment identifier syntax

Thompson & Lilley Standards Track [Page 23] RFC 7303 XML Media Types July 2014

    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 they MAY add additional semantics
    for barename XPointers that, as provided for in Section 5, will
    only apply when this document 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 9.1.  See above,
    and also Section 3, for guidelines on the use of the 'charset'
    parameter.
 Security considerations:  See Section 10.
 Contact:  See Authors' Addresses section.
 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 RFC 7303.

Thompson & Lilley Standards Track [Page 24] RFC 7303 XML Media Types July 2014

10. Security Considerations

 XML MIME entities contain information that 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 widely exploited set of resources.  They will be
 used and trusted by many developers, 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:
 for example, if developers have relied on defaulted attributes for
 security, a relatively small change might expose enormous quantities
 of information.
 Apart from the structural possibilities, another option, "XML-entity
 spoofing," can be used to insert text into documents, vandalizing and
 perhaps conveying an unintended message.  Because XML permits
 multiple XML-entity declarations, and the first declaration takes
 precedence, it 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;.

Thompson & Lilley Standards Track [Page 25] RFC 7303 XML Media Types July 2014

 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.
 With some terminal devices, sending particular character sequences to
 the display processor can change the output of subsequent key
 presses.  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.

Thompson & Lilley Standards Track [Page 26] RFC 7303 XML Media Types July 2014

11. References

11.1. Normative References

 [IANA-CHARSETS]
            IANA, "Character Sets Registry", 2013,
            <http://www.iana.org/assignments/character-sets/>.
 [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", BCP 19, RFC 2978, October 2000.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66, RFC
            3986, January 2005.
 [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
            Identifiers (IRIs)", RFC 3987, January 2005.
 [RFC6657]  Melnikov, A. and J. Reschke, "Update to MIME regarding
            "charset" Parameter Handling in Textual Media Types", RFC
            6657, July 2012.
 [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
            Specifications and Registration Procedures", BCP 13, RFC
            6838, January 2013.
 [RFC6839]  Hansen, T. and A. Melnikov, "Additional Media Type
            Structured Syntax Suffixes", RFC 6839, January 2013.
 [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
            (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
            2014.

Thompson & Lilley Standards Track [Page 27] RFC 7303 XML Media Types July 2014

 [RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
            (HTTP/1.1): Semantics and Content", RFC 7231, June 2014.
 [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
            7.0.0", (Mountain View, CA: The Unicode Consortium, 2014
            ISBN 978-1-936213-09-2),
            <http://www.unicode.org/versions/Unicode7.0.0/>.
 [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C., 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 <http://www.w3.org/TR/xml>.
 [XML1.1]   Bray, T., Paoli, J., Sperberg-McQueen, C., 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 <http://www.w3.org/TR/xml11>.
 [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
            <http://www.w3.org/TR/xmlbase>.
 [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
            <http://www.w3.org/TR/xptr-element>.
 [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
            <http://www.w3.org/TR/xptr-framework>.

Thompson & Lilley Standards Track [Page 28] RFC 7303 XML Media Types July 2014

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

11.2. 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
            <http://www.w3.org/TR/webarch>.
 [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
            <http://www.w3.org/2001/tag/doc/
            selfDescribingDocuments.html>
 [Infoset]  Cowan, J. and R. Tobin, "XML Information Set (Second
            Edition)", W3C Recommendation REC-xml-infoset-20040204,
            Febuary 2004,
            <http://www.w3.org/TR/2004/REC-xml-infoset-20040204/>.
            Latest version available at
            <http://www.w3.org/TR/xml-infoset/>.
 [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
            <http://www.w3.org/TR/media-frags>.

Thompson & Lilley Standards Track [Page 29] RFC 7303 XML Media Types July 2014

 [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.
 [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, 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, December
            2000.
 [RFC3977]  Feather, C., "Network News Transfer Protocol (NNTP)", RFC
            3977, October 2006.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [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", STD 71, RFC
            6152, March 2011.
 [Sivonen]  Sivonen, H. and others, "Mozilla bug: Remove support for
            UTF-32 per HTML5 spec", October 2011,
            <https://bugzilla.mozilla.org/show_bug.cgi?id=604317#c6>.
 [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 <http://www.w3.org/TR/xhtml1>.

Thompson & Lilley Standards Track [Page 30] RFC 7303 XML Media Types July 2014

 [XMLModel] Grosso, P. and J. Kosek, "Associating Schemas with XML
            documents 1.0 (Third Edition)", W3C Working Group Note
            NOTE-xml-model-20121009, October 2012,
            <http://www.w3.org/TR/2012/NOTE-xml-model-20121009/>.
            Latest version available at
            <http://www.w3.org/TR/xml-model>.
 [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
            <http://www.w3.org/TR/xml-names>.
 [XMLNS11]  Bray, T., Hollander, D., Layman, A., and R. Tobin,
            "Namespaces in XML 1.1 (Second Edition)", W3C
            Recommendation REC-xml-names11-20060816, August 2006,
            <http://www.w3.org/TR/2006/REC-xml-names11-20060816/>.
            Latest version available at
            <http://www.w3.org/TR/xml-names11>.
 [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
            <http://www.w3.org/TR/xml-stylesheet>.
 [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
            <http://www.w3.org/TR/xml-id>.

Thompson & Lilley Standards Track [Page 31] RFC 7303 XML Media Types July 2014

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 that has since become widespread.  Those
 interested in a historical perspective on this topic 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 that
 are also relevant:
    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. Operational Considerations

 This section provides an informal summary of the major operational
 considerations that arise when exchanging XML MIME entities over a
 network.

Thompson & Lilley Standards Track [Page 32] RFC 7303 XML Media Types July 2014

C.1. General Considerations

 The existence of both XML-aware and XML-unaware agents handling XML
 MIME entities can compromise introperability.  Generic transcoding
 proxies pose a particular risk in this regard.  Detailed advice about
 the handling of BOMs when transcoding can be found in Section 3.3.
 This specification requires XML consumers to treat BOMs as
 authoritative: this 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 over time.
 This specification establishes Section 5 as the basis for
 interpreting URIs for XML MIME entities that include fragment
 identifiers, mandates support only for shorthand ("simple name") and
 'element'-scheme fragments and deprecates support for unregistered
 XPointer schemes by XML MIME entity processors.  Accordingly, URIs
 will interoperate best if they use only simple names and 'element'-
 scheme fragment identifiers, with registered schemes varying widely
 in the degree of support to be found in generic tools.  XPointer
 scheme authors can only expect generic tool support if they register
 their schemes.

C.2. Considerations for Producers

 Interoperability for all XML MIME entities is maximized by the use of
 UTF-8, without a BOM.  When UTF-8 is _not_ used, a charset parameter
 and/or a BOM improve interoperability, particularly when XML-unaware
 consumers may be involved.
 In the very rare case where the substantive content of a non-UNICODE
 XML external parsed entity begins with the hexadecimal octet
 sequences 0xFE 0xFF, 0xFF 0xFE or 0xEF 0xBB 0xBF, including an XML
 text declaration will forestall the mistaken detection of a BOM.
 The use of UTF-32 for XML MIME entities puts interoperability at very
 high risk.
 Web-server configurations that supply default charset parameters risk
 misrepresenting XML MIME entities.  Allowing users to control the
 value of charset parameters improves interoperability.

Thompson & Lilley Standards Track [Page 33] RFC 7303 XML Media Types July 2014

 Supplying a mistaken charset parameter is worse than supplying none
 at all.  In particular, generic processors such as transcoders, when
 processing based on a mistaken charset parameter, if they do not fail
 altogether are likely to produce arbitrarily bogus results from which
 the original is not recoverable.

C.3. Considerations for Consumers

 Consumers of XML MIME entities can maximize interoperability by
 1.  Taking a BOM as authoritative if it is present in an XML MIME
     entity;
 2.  In the absence of a BOM, taking a charset parameter as
     authoritative if it is present.
 Assuming a default character encoding in the absence of a charset
 parameter harms interoperability.
 Although support for UTF-32 is not required by [XML] itself, and this
 specification deprecates its use, consumers that check for UTF-32
 BOMs can thereby avoid mistakenly processing UTF-32 entities as
 (invalid) UTF-16 entities.

Appendix D. 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 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 that were
    appropriate when XML was new have been removed, including the
    whole of the original Appendix A

Thompson & Lilley Standards Track [Page 34] RFC 7303 XML Media Types July 2014

Appendix E. Acknowledgements

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

Thompson & Lilley Standards Track [Page 35]

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