Request for Comments: 3023 IBM Tokyo Research Laboratory
Obsoletes: 2376 S. St.Laurent
Updates: 2048 simonstl.com
Category: Standards Track D. Kohn
Skymoon Ventures
January 2001
XML Media Types
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document standardizes five new media types -- text/xml,
application/xml, text/xml-external-parsed-entity, application/xml-
external-parsed-entity, and application/xml-dtd -- for use in
exchanging network entities that are related to the Extensible Markup
Language (XML). This document also standardizes a convention (using
the suffix '+xml') for naming media types outside of these five types
when those media types represent XML MIME (Multipurpose Internet Mail
Extensions) entities. XML MIME entities are currently exchanged via
the HyperText Transfer Protocol on the World Wide Web, are an
integral part of the WebDAV protocol for remote web authoring, and
are expected to have utility in many domains.
Major differences from RFC2376 are (1) the addition of text/xml-
external-parsed-entity, application/xml-external-parsed-entity, and
application/xml-dtd, (2) the '+xml' suffix convention (which also
updates the RFC2048 registration process), and (3) the discussion of
"utf-16le" and "utf-16be".
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 4
3. XML Media Types . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Text/xml Registration . . . . . . . . . . . . . . . . . . . 7
3.2 Application/xml Registration . . . . . . . . . . . . . . . . 9
3.3 Text/xml-external-parsed-entity Registration . . . . . . . . 11
3.4 Application/xml-external-parsed-entity Registration . . . . 12
3.5 Application/xml-dtd Registration . . . . . . . . . . . . . . 13
3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4. The Byte Order Mark (BOM) and Conversions to/from the UTF-16
Charset . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5. Fragment Identifiers . . . . . . . . . . . . . . . . . . . . 15
6. The Base URI . . . . . . . . . . . . . . . . . . . . . . . . 15
7. A Naming Convention for XML-Based Media Types . . . . . . . 16
7.1 Referencing . . . . . . . . . . . . . . . . . . . . . . . . 18
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1 Text/xml with UTF-8 Charset . . . . . . . . . . . . . . . . 19
8.2 Text/xml with UTF-16 Charset . . . . . . . . . . . . . . . . 19
8.3 Text/xml with UTF-16BE Charset . . . . . . . . . . . . . . . 19
8.4 Text/xml with ISO-2022-KR Charset . . . . . . . . . . . . . 20
8.5 Text/xml with Omitted Charset . . . . . . . . . . . . . . . 20
8.6 Application/xml with UTF-16 Charset . . . . . . . . . . . . 20
8.7 Application/xml with UTF-16BE Charset . . . . . . . . . . . 21
8.8 Application/xml with ISO-2022-KR Charset . . . . . . . . . . 21
8.9 Application/xml with Omitted Charset and UTF-16 XML MIME
Entity . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.10 Application/xml with Omitted Charset and UTF-8 Entity . . . 22
8.11 Application/xml with Omitted Charset and Internal Encoding
Declaration . . . . . . . . . . . . . . . . . . . . . . . . 22
8.12 Text/xml-external-parsed-entity with UTF-8 Charset . . . . . 22
8.13 Application/xml-external-parsed-entity with UTF-16 Charset . 23
8.14 Application/xml-external-parsed-entity with UTF-16BE Charset 23
8.15 Application/xml-dtd . . . . . . . . . . . . . . . . . . . . 23
8.16 Application/mathml+xml . . . . . . . . . . . . . . . . . . . 24
8.17 Application/xslt+xml . . . . . . . . . . . . . . . . . . . . 24
8.18 Application/rdf+xml . . . . . . . . . . . . . . . . . . . . 24
8.19 Image/svg+xml . . . . . . . . . . . . . . . . . . . . . . . 24
8.20 INCONSISTENT EXAMPLE: Text/xml with UTF-8 Charset . . . . . 25
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 25
10. Security Considerations . . . . . . . . . . . . . . . . . . 25
References . . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 31
A. Why Use the '+xml' Suffix for XML-Based MIME Types? . . . . 32
A.1 Why not just use text/xml or application/xml and let the XML
processor dispatch to the correct application based on the
referenced DTD? . . . . . . . . . . . . . . . . . . . . . . 32
A.2 Why not create a new subtree (e.g., image/xml.svg) to
represent XML MIME types? . . . . . . . . . . . . . . . . . 32
A.3 Why not create a new top-level MIME type for XML-based media
types? . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
A.4 Why not just have the MIME processor 'sniff' the content to
determine whether it is XML? . . . . . . . . . . . . . . . . 33
A.5 Why not use a MIME parameter to specify that a media type
uses XML syntax? . . . . . . . . . . . . . . . . . . . . . . 33
A.6 How about labeling with parameters in the other direction
(e.g., application/xml; Content-Feature=iotp)? . . . . . . . 34
A.7 How about a new superclass MIME parameter that is defined to
apply to all MIME types (e.g., Content-Type:
application/iotp; $superclass=xml)? . . . . . . . . . . . . 34
A.8 What about adding a new parameter to the Content-Disposition
header or creating a new Content-Structure header to
indicate XML syntax? . . . . . . . . . . . . . . . . . . . . 35
A.9 How about a new Alternative-Content-Type header? . . . . . . 35
A.10 How about using a conneg tag instead (e.g., accept-features:
(syntax=xml))? . . . . . . . . . . . . . . . . . . . . . . . 35
A.11 How about a third-level content-type, such as text/xml/rdf? 35
A.12 Why use the plus ('+') character for the suffix '+xml'? . . 36
A.13 What is the semantic difference between application/foo and
application/foo+xml? . . . . . . . . . . . . . . . . . . . . 36
A.14 What happens when an even better markup language (e.g.,
EBML) is defined, or a new category of data? . . . . . . . . 36
A.15 Why must I use the '+xml' suffix for my new XML-based media
type? . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
B. Changes from RFC2376 . . . . . . . . . . . . . . . . . . . 37
C. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 38
Full Copyright Statement . . . . . . . . . . . . . . . . . . 39
1. Introduction
The World Wide Web Consortium has issued Extensible Markup Language
(XML) 1.0 (Second Edition)[XML]. To enable the exchange of XML
network entities, this document standardizes five new media types --
text/xml, application/xml, text/xml-external-parsed-entity,
application/xml-external-parsed-entity, and application/xml-dtd -- as
well as a naming convention for identifying XML-based MIME media
types.
XML entities are currently exchanged on the World Wide Web, and XML
is also used for property values and parameter marshalling by the
WebDAV[RFC2518] protocol for remote web authoring. Thus, there is a
need for a media type to properly label the exchange of XML network
entities.
Although XML is a subset of the Standard Generalized Markup Language
(SGML) ISO 8879[SGML], which has been assigned the media types
text/sgml and application/sgml, there are several reasons why use of
text/sgml or application/sgml to label XML is inappropriate. First,
there exist many applications that can process XML, but that cannot
process SGML, due to SGML's larger feature set. Second, SGML
applications cannot always process XML entities, because XML uses
features of recent technical corrigenda to SGML. Third, the
definition of text/sgml and application/sgml in [RFC1874] includes
parameters for SGML bit combination transformation format (SGML-
bctf), and SGML boot attribute (SGML-boot). Since XML does not use
these parameters, it would be ambiguous if such parameters were given
for an XML MIME entity. For these reasons, the best approach for
labeling XML network entities is to provide new media types for XML.
Since XML is an integral part of the WebDAV Distributed Authoring
Protocol, and since World Wide Web Consortium Recommendations have
conventionally been assigned IETF tree media types, and since similar
media types (HTML, SGML) have been assigned IETF tree media types,
the XML media types also belong in the IETF media types tree.
Similarly, XML will be used as a foundation for other media types,
including types in every branch of the IETF media types tree. To
facilitate the processing of such types, media types based on XML,
but that are not identified using text/xml or application/xml, SHOULD
be named using a suffix of '+xml' as described in Section 7. This
will allow XML-based tools -- browsers, editors, search engines, and
other processors -- to work with all XML-based media types.
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
As defined in [RFC2781], the three charsets "utf-16", "utf-16le", and
"utf-16be" are used to label UTF-16 text. In this document, "the
UTF-16 family" refers to those three charsets. By contrast, the
phrases "utf-16" or UTF-16 in this document refer specifically to the
single charset "utf-16".
As sometimes happens between two communities, both MIME and XML have
defined the term entity, with different meanings. Section 2.4 of
[RFC2045] says:
"The term 'entity' refers specifically to the MIME-defined header
fields and contents of either a message or one of the parts in the
body of a multipart entity."
Section 4 of [XML] says:
"An XML document may consist of one or many storage units" called
entities that "have content" and are normally "identified by
name".
In this document, "XML MIME entity" is defined as the latter (an XML
entity) encapsulated in the former (a MIME entity).
3. XML Media Types
This document standardizes five media types related to XML MIME
entities: text/xml, application/xml, text/xml-external-parsed-entity,
application/xml-external-parsed-entity, and application/xml-dtd.
Registration information for these media types is described in the
sections below.
Within the XML specification, XML MIME entities can be classified
into four types. In the XML terminology, they are called "document
entities", "external DTD subsets", "external parsed entities", and
"external parameter entities". The media types text/xml and
application/xml MAY be used for "document entities", while text/xml-
external-parsed-entity or application/xml-external-parsed-entity
SHOULD be used for "external parsed entities". The media type
application/xml-dtd SHOULD be used for "external DTD subsets" or
"external parameter entities". application/xml and text/xml MUST NOT
be used for "external parameter entities" or "external DTD subsets",
and MUST NOT be used for "external parsed entities" unless they are
also well-formed "document entities" and are referenced as such.
Note that [RFC2376] (which this document obsoletes) allowed such
usage, although in practice it is likely to have been rare.
Neither external DTD subsets nor external parameter entities parse as
XML documents, and while some XML document entities may be used as
external parsed entities and vice versa, there are many cases where
the two are not interchangeable. XML also has unparsed entities,
internal parsed entities, and internal parameter entities, but they
are not XML MIME entities.
If an XML document -- that is, the unprocessed, source XML document
-- is readable by casual users, text/xml is preferable to
application/xml. MIME user agents (and web user agents) that do not
have explicit support for text/xml will treat it as text/plain, for
example, by displaying the XML MIME entity as plain text.
Application/xml is preferable when the XML MIME entity is unreadable
by casual users. Similarly, text/xml-external-parsed-entity is
preferable when an external parsed entity is readable by casual
users, but application/xml-external-parsed-entity is preferable when
a plain text display is inappropriate.
NOTE: Users are in general not used to text containing tags such
as <price>, and often find such tags quite disorienting or
annoying. If one is not sure, the conservative principle would
suggest using application/* instead of text/* so as not to put
information in front of users that they will quite likely not
understand.
The top-level media type "text" has some restrictions on MIME
entities and they are described in [RFC2045] and [RFC2046]. In
particular, the UTF-16 family, UCS-4, and UTF-32 are not allowed
(except over HTTP[RFC2616], which uses a MIME-like mechanism). Thus,
if an XML document or external parsed entity is encoded in such
character encoding schemes, it cannot be labeled as text/xml or
text/xml-external-parsed-entity (except for HTTP).
Text/xml and application/xml behave differently when the charset
parameter is not explicitly specified. If the default charset (i.e.,
US-ASCII) for text/xml is inconvenient for some reason (e.g., bad web
servers), application/xml provides an alternative (see "Optional
parameters" of application/xml registration in Section 3.2). The
same rules apply to the distinction between text/xml-external-
parsed-entity and application/xml-external-parsed-entity.
XML provides a general framework for defining sequences of structured
data. In some cases, it may be desirable to define new media types
that use XML but define a specific application of XML, perhaps due to
domain-specific security considerations or runtime information.
Furthermore, such media types may allow UTF-8 or UTF-16 only and
prohibit other charsets. This document does not prohibit such media
types and in fact expects them to proliferate. However, developers
of such media types are STRONGLY RECOMMENDED to use this document as
a basis for their registration. In particular, the charset parameter
SHOULD be used in the same manner, as described in Section 7.1, in
order to enhance interoperability.
An XML document labeled as text/xml or application/xml might contain
namespace declarations, stylesheet-linking processing instructions
(PIs), schema information, or other declarations that might be used
to suggest how the document is to be processed. For example, a
document might have the XHTML namespace and a reference to a CSS
stylesheet. Such a document might be handled by applications that
would use this information to dispatch the document for appropriate
processing.
3.1 Text/xml Registration
MIME media type name: text
MIME subtype name: xml
Mandatory parameters: none
Optional parameters: charset
Although listed as an optional parameter, the use of the charset
parameter is STRONGLY RECOMMENDED, since this information can be
used by XML processors to determine authoritatively the character
encoding of the XML MIME entity. The charset parameter can also
be used to provide protocol-specific operations, such as charset-
based content negotiation in HTTP. "utf-8" [RFC2279] is the
recommended value, representing the UTF-8 charset. UTF-8 is
supported by all conforming processors of [XML].
If the XML MIME entity is transmitted via HTTP, which uses a
MIME-like mechanism that is exempt from the restrictions on the
text top-level type (see section 19.4.1 of [RFC2616]), "utf-16"
[RFC2781]) is also recommended. UTF-16 is supported by all
conforming processors of [XML]. Since the handling of CR, LF and
NUL for text types in most MIME applications would cause undesired
transformations of individual octets in UTF-16 multi-octet
characters, gateways from HTTP to these MIME applications MUST
transform the XML MIME entity from text/xml; charset="utf-16" to
application/xml; charset="utf-16".
Conformant with [RFC2046], if a text/xml entity is received with
the charset parameter omitted, MIME processors and XML processors
MUST use the default charset value of "us-ascii"[ASCII]. In cases
where the XML MIME entity is transmitted via HTTP, the default
charset value is still "us-ascii". (Note: There is an
inconsistency between this specification and HTTP/1.1, which uses
ISO-8859-1[ISO8859] as the default for a historical reason. Since
XML is a new format, a new default should be chosen for better
I18N. US-ASCII was chosen, since it is the intersection of UTF-8
and ISO-8859-1 and since it is already used by MIME.)
There are several reasons that the charset parameter is
authoritative. First, some MIME processing engines do transcoding
of MIME bodies of the top-level media type "text" without
reference to any of the internal content. Thus, it is possible
that some agent might change text/xml; charset="iso-2022-jp" to
text/xml; charset="utf-8" without modifying the encoding
declaration of an XML document. Second, text/xml must be
compatible with text/plain, since MIME agents that do not
understand text/xml will fallback to handling it as text/plain.
If the charset parameter for text/xml were not authoritative, such
fallback would cause data corruption. Third, recent web servers
have been improved so that users can specify the charset
parameter. Fourth, [RFC2130] specifies that the recommended
specification scheme is the "charset" parameter.
Since the charset parameter is authoritative, the charset is not
always declared within an XML encoding declaration. Thus, special
care is needed when the recipient strips the MIME header and
provides persistent storage of the received XML MIME entity (e.g.,
in a file system). Unless the charset is UTF-8 or UTF-16, the
recipient SHOULD also persistently store information about the
charset, perhaps by embedding a correct XML encoding declaration
within the XML MIME entity.
Encoding considerations: This media type MAY be encoded as
appropriate for the charset and the capabilities of the underlying
MIME transport. For 7-bit transports, data in UTF-8 MUST be
encoded in quoted-printable or base64. For 8-bit clean transport
(e.g., 8BITMIME[RFC1652] ESMTP or NNTP[RFC0977]), UTF-8 does not
need to be encoded. Over HTTP[RFC2616], no content-transfer-
encoding is necessary and UTF-16 may also be used.
Security considerations: See Section 10.
Interoperability considerations: XML has proven to be interoperable
across WebDAV clients and servers, and for import and export from
multiple XML authoring tools. For maximum interoperability,
validating processors are recommended. Although non-validating
processors may be more efficient, they are not required to handle
all features of XML. For further information, see sub-section 2.9
"Standalone Document Declaration" and section 5 "Conformance" of
[XML].
Published specification: Extensible Markup Language (XML) 1.0 (Second
Edition)[XML].
Applications which use this media type: XML is device-, platform-,
and vendor-neutral and is supported by a wide range of Web user
agents, WebDAV[RFC2518] clients and servers, as well as XML
authoring tools.
Additional information:
Magic number(s): None.
Although no byte sequences can be counted on to always be
present, XML MIME entities in ASCII-compatible charsets
(including UTF-8) often begin with hexadecimal 3C 3F 78 6D 6C
("<?xml"), and those in UTF-16 often begin with hexadecimal FE
FF 00 3C 00 3F 00 78 00 6D 00 6C or FF FE 3C 00 3F 00 78 00 6D
00 6C 00 (the Byte Order Mark (BOM) followed by "<?xml"). For
more information, see Appendix F of [XML].
File extension(s): .xml
Macintosh File Type Code(s): "TEXT"
Person and email address for further information:
MURATA Makoto (FAMILY Given) <mmurata@trl.ibm.co.jp>
Simon St.Laurent <simonstl@simonstl.com>
Daniel Kohn <dan@dankohn.com>
Intended usage: COMMON
Author/Change controller: The XML specification is a work product of
the World Wide Web Consortium's XML Working Group, and was edited
by:
Tim Bray <tbray@textuality.com>
Jean Paoli <jeanpa@microsoft.com>
C. M. Sperberg-McQueen <cmsmcq@uic.edu>
Eve Maler <eve.maler@east.sun.com>
The W3C, and the W3C XML Core Working Group, have change control
over the XML specification.
3.2 Application/xml Registration
MIME media type name: application
MIME subtype name: xml
Mandatory parameters: none
Optional parameters: charset
Although listed as an optional parameter, the use of the charset
parameter is STRONGLY RECOMMENDED, since this information can be
used by XML processors to determine authoritatively the charset of
the XML MIME entity. The charset parameter can also be used to
provide protocol-specific operations, such as charset-based
content negotiation in HTTP.
"utf-8" [RFC2279] and "utf-16" [RFC2781] are the recommended
values, representing the UTF-8 and UTF-16 charsets, respectively.
These charsets are preferred since they are supported by all
conforming processors of [XML].
If an application/xml entity is received where the charset
parameter is omitted, no information is being provided about the
charset by the MIME Content-Type header. Conforming XML
processors MUST follow the requirements in section 4.3.3 of [XML]
that directly address this contingency. However, MIME processors
that are not XML processors SHOULD NOT assume a default charset if
the charset parameter is omitted from an application/xml entity.
There are several reasons that the charset parameter is
authoritative. First, recent web servers have been improved so
that users can specify the charset parameter. Second, [RFC2130]
specifies that the recommended specification scheme is the
"charset" parameter.
On the other hand, it has been argued that the charset parameter
should be omitted and the mechanism described in Appendix F of
[XML] (which is non-normative) should be solely relied on. This
approach would allow users to avoid configuration of the charset
parameter; an XML document stored in a file is likely to contain a
correct encoding declaration or BOM (if necessary), since the
operating system does not typically provide charset information
for files. If users would like to rely on the encoding
declaration or BOM and to hide charset information from protocols,
they may determine not to use the parameter.
Since the charset parameter is authoritative, the charset is not
always declared within an XML encoding declaration. Thus, special
care is needed when the recipient strips the MIME header and
provides persistent storage of the received XML MIME entity (e.g.,
in a file system). Unless the charset is UTF-8 or UTF-16, the
recipient SHOULD also persistently store information about the
charset, perhaps by embedding a correct XML encoding declaration
within the XML MIME entity.
Encoding considerations: This media type MAY be encoded as
appropriate for the charset and the capabilities of the underlying
MIME transport. For 7-bit transports, data in either UTF-8 or
UTF-16 MUST be encoded in quoted-printable or base64. For 8-bit
clean transport (e.g., 8BITMIME[RFC1652] ESMTP or NNTP[RFC0977]),
UTF-8 is not encoded, but the UTF-16 family MUST be encoded in
base64. For binary clean transports (e.g., HTTP[RFC2616]), no
content-transfer-encoding is necessary.
Security considerations: See Section 10.
Interoperability considerations: Same as Section 3.1.
Published specification: Same as Section 3.1.
Applications which use this media type: Same as Section 3.1.
Additional information: Same as Section 3.1.
Person and email address for further information: Same as Section
3.1.
Intended usage: COMMON
Author/Change controller: Same as Section 3.1.
3.3 Text/xml-external-parsed-entity Registration
MIME media type name: text
MIME subtype name: xml-external-parsed-entity
Mandatory parameters: none
Optional parameters: charset
The charset parameter of text/xml-external-parsed-entity is
handled the same as that of text/xml as described in Section 3.1.
Encoding considerations: Same as Section 3.1.
Security considerations: See Section 10.
Interoperability considerations: XML external parsed entities are as
interoperable as XML documents, though they have a less tightly
constrained structure and therefore need to be referenced by XML
documents for proper handling by XML processors. Similarly, XML
documents cannot be reliably used as external parsed entities
because external parsed entities are prohibited from having
standalone document declarations or DTDs. Identifying XML
external parsed entities with their own content type should
enhance interoperability of both XML documents and XML external
parsed entities.
Published specification: Same as Section 3.1.
Applications which use this media type: Same as Section 3.1.
Additional information:
Magic number(s): Same as Section 3.1.
File extension(s): .xml or .ent
Macintosh File Type Code(s): "TEXT"
Person and email address for further information: Same as Section
3.1.
Intended usage: COMMON
Author/Change controller: Same as Section 3.1.
3.4 Application/xml-external-parsed-entity Registration
MIME media type name: application
MIME subtype name: xml-external-parsed-entity
Mandatory parameters: none
Optional parameters: charset
The charset parameter of application/xml-external-parsed-entity is
handled the same as that of application/xml as described in
Section 3.2.
Encoding considerations: Same as Section 3.2.
Security considerations: See Section 10.
Interoperability considerations: Same as those for text/xml-
external-parsed-entity as described in Section 3.3.
Published specification: Same as text/xml as described in Section
3.1.
Applications which use this media type: Same as Section 3.1.
Additional information:
Magic number(s): Same as Section 3.1.
File extension(s): .xml or .ent
Macintosh File Type Code(s): "TEXT"
Person and email address for further information: Same as Section
3.1.
Intended usage: COMMON
Author/Change controller: Same as Section 3.1.
3.5 Application/xml-dtd Registration
MIME media type name: application
MIME subtype name: xml-dtd
Mandatory parameters: none
Optional parameters: charset
The charset parameter of application/xml-dtd is handled the same
as that of application/xml as described in Section 3.2.
Encoding considerations: Same as Section 3.2.
Security considerations: See Section 10.
Interoperability considerations: XML DTDs have proven to be
interoperable by DTD authoring tools and XML browsers, among
others.
Published specification: Same as text/xml as described in Section
3.1.
Applications which use this media type: DTD authoring tools handle
external DTD subsets as well as external parameter entities. XML
browsers may also access external DTD subsets and external
parameter entities.
Additional information:
Magic number(s): Same as Section 3.1.
File extension(s): .dtd or .mod
Macintosh File Type Code(s): "TEXT"
Person and email address for further information: Same as Section
3.1.
Intended usage: COMMON
Author/Change controller: Same as Section 3.1.
3.6 Summary
The following list applies to text/xml, text/xml-external-parsed-
entity, and XML-based media types under the top-level type "text"
that define the charset parameter according to this specification:
o Charset parameter is strongly recommended.
o If the charset parameter is not specified, the default is "us-
ascii". The default of "iso-8859-1" in HTTP is explicitly
overridden.
o No error handling provisions.
o An encoding declaration, if present, is irrelevant, but when
saving a received resource as a file, the correct encoding
declaration SHOULD be inserted.
The next list applies to application/xml, application/xml-external-
parsed-entity, application/xml-dtd, and XML-based media types under
top-level types other than "text" that define the charset parameter
according to this specification:
o Charset parameter is strongly recommended, and if present, it
takes precedence.
o If the charset parameter is omitted, conforming XML processors
MUST follow the requirements in section 4.3.3 of [XML].
4. The Byte Order Mark (BOM) and Conversions to/from the UTF-16 Charset
Section 4.3.3 of [XML] specifies that XML MIME entities in the
charset "utf-16" MUST begin with a byte order mark (BOM), which is a
hexadecimal octet sequence 0xFE 0xFF (or 0xFF 0xFE, depending on
endian). The XML Recommendation further states that the BOM is an
encoding signature, and is not part of either the markup or the
character data of the XML document.
Due to the presence of the BOM, applications that convert XML from
"utf-16" to a non-Unicode encoding MUST strip the BOM before
conversion. Similarly, when converting from another encoding into
"utf-16", the BOM MUST be added after conversion is complete.
In addition to the charset "utf-16", [RFC2781] introduces "utf-16le"
(little endian) and "utf-16be" (big endian) as well. The BOM is
prohibited for these charsets. When an XML MIME entity is encoded in
"utf-16le" or "utf-16be", it MUST NOT begin with the BOM but SHOULD
contain an encoding declaration. Conversion from "utf-16" to "utf-
16be" or "utf-16le" and conversion in the other direction MUST strip
or add the BOM, respectively.
5. Fragment Identifiers
Section 4.1 of [RFC2396] notes that the semantics of a fragment
identifier (the part of a URI after a "#") is a property of the data
resulting from a retrieval action, and that the format and
interpretation of fragment identifiers is dependent on the media type
of the retrieval result.
As of today, no established specifications define identifiers for XML
media types. However, a working draft published by W3C, namely "XML
Pointer Language (XPointer)", attempts to define fragment identifiers
for text/xml and application/xml. The current specification for
XPointer is available at http://www.w3.org/TR/xptr.
6. The Base URI
Section 5.1 of [RFC2396] specifies that the semantics of a relative
URI reference embedded in a MIME entity is dependent on the base URI.
The base URI is either (1) the base URI embedded in the MIME entity,
(2) the base URI of the encapsulating MIME entity, (3) the URI used
to retrieve the MIME entity, or (4) the application-dependent default
base URI, where (1) has the highest precedence. [RFC2396] further
specifies that the mechanism for embedding the base URI is dependent
on the media type.
As of today, no established specifications define mechanisms for
embedding the base URI in XML MIME entities. However, a Proposed
Recommendation published by W3C, namely "XML Base", attempts to
define such a mechanism for text/xml, application/xml, text/xml-
external-parsed-entity, and application/xml-external-parsed-entity.
The current specification for XML Base is available at
http://www.w3.org/TR/xmlbase.
7. A Naming Convention for XML-Based Media Types
This document recommends the use of a naming convention (a suffix of
'+xml') for identifying XML-based MIME media types, whatever their
particular content may represent. This allows the use of generic XML
processors and technologies on a wide variety of different XML
document types at a minimum cost, using existing frameworks for media
type registration.
Although the use of a suffix was not considered as part of the
original MIME architecture, this choice is considered to provide the
most functionality with the least potential for interoperability
problems or lack of future extensibility. The alternatives to the '
+xml' suffix and the reason for its selection are described in
Appendix A.
As XML development continues, new XML document types are appearing
rapidly. Many of these XML document types would benefit from the
identification possibilities of a more specific MIME media type than
text/xml or application/xml can provide, and it is likely that many
new media types for XML-based document types will be registered in
the near and ongoing future.
While the benefits of specific MIME types for particular types of XML
documents are significant, all XML documents share common structures
and syntax that make possible common processing.
Some areas where 'generic' processing is useful include:
o Browsing - An XML browser can display any XML document with a
provided [CSS] or [XSLT] style sheet, whatever the vocabulary of
that document.
o Editing - Any XML editor can read, modify, and save any XML
document.
o Fragment identification - XPointers (work in progress) can work
with any XML document, whatever vocabulary it uses and whether or
not it uses XPointer for its own fragment identification.
o Hypertext linking - XLink (work in progress) hypertext linking is
designed to connect any XML documents, regardless of vocabulary.
o Searching - XML-oriented search engines, web crawlers, agents, and
query tools should be able to read XML documents and extract the
names and content of elements and attributes even if the tools are
ignorant of the particular vocabulary used for elements and
attributes.
o Storage - XML-oriented storage systems, which keep XML documents
internally in a parsed form, should similarly be able to process,
store, and recreate any XML document.
o Well-formedness and validity checking - An XML processor can
confirm that any XML document is well-formed and that it is valid
(i.e., conforms to its declared DTD or Schema).
When a new media type is introduced for an XML-based format, the name
of the media type SHOULD end with '+xml'. This convention will allow
applications that can process XML generically to detect that the MIME
entity is supposed to be an XML document, verify this assumption by
invoking some XML processor, and then process the XML document
accordingly. Applications may match for types that represent XML
MIME entities by comparing the subtype to the pattern '*/*+xml'. (Of
course, 4 of the 5 media types defined in this document -- text/xml,
application/xml, text/xml-external-parsed-entity, and
application/xml-external-parsed-entity -- also represent XML MIME
entities while not conforming to the '*/*+xml' pattern.)
NOTE: Section 14.1 of HTTP[RFC2616] does not support Accept
headers of the form "Accept: */*+xml" and so this header MUST NOT
be used in this way. Instead, content negotiation[RFC2703] could
potentially be used if an XML-based MIME type were needed.
XML generic processing is not always appropriate for XML-based media
types. For example, authors of some such media types may wish that
the types remain entirely opaque except to applications that are
specifically designed to deal with that media type. By NOT following
the naming convention '+xml', such media types can avoid XML-generic
processing. Since generic processing will be useful in many cases,
however -- including in some situations that are difficult to predict
ahead of time -- those registering media types SHOULD use the '+xml'
convention unless they have a particularly compelling reason not to.
The registration process for these media types is described in
[RFC2048]. The registrar for the IETF tree will encourage new XML-
based media type registrations in the IETF tree to follow this
guideline. Registrars for other trees SHOULD follow this convention
in order to ensure maximum interoperability of their XML-based
documents. Similarly, media subtypes that do not represent XML MIME
entities MUST NOT be allowed to register with a '+xml' suffix.
7.1 Referencing
Registrations for new XML-based media types under the top-level type
"text" SHOULD, in specifying the charset parameter and encoding
considerations, define them as: "Same as [charset parameter /
encoding considerations] of text/xml as specified in RFC3023."
Registrations for new XML-based media types under top-level types
other than "text" SHOULD, in specifying the charset parameter and
encoding considerations, define them as: "Same as [charset parameter
/ encoding considerations] of application/xml as specified in RFC
3023."
The use of the charset parameter is STRONGLY RECOMMENDED, since this
information can be used by XML processors to determine
authoritatively the charset of the XML MIME entity.
These registrations SHOULD specify that the XML-based media type
being registered has all of the security considerations described in
RFC3023 plus any additional considerations specific to that media
type.
These registrations SHOULD also make reference to RFC3023 in
specifying magic numbers, fragment identifiers, base URIs, and use of
the BOM.
These registrations MAY reference the text/xml registration in RFC
3023 in specifying interoperability considerations, if these
considerations are not overridden by issues specific to that media
type.
8. Examples
The examples below give the value of the MIME Content-type header and
the XML declaration (which includes the encoding declaration) inside
the XML MIME entity. For UTF-16 examples, the Byte Order Mark
character is denoted as "{BOM}", and the XML declaration is assumed
to come at the beginning of the XML MIME entity, immediately
following the BOM. Note that other MIME headers may be present, and
the XML MIME entity may contain other data in addition to the XML
declaration; the examples focus on the Content-type header and the
encoding declaration for clarity.
8.1 Text/xml with UTF-8 Charset
Content-type: text/xml; charset="utf-8"
<?xml version="1.0" encoding="utf-8"?>
This is the recommended charset value for use with text/xml. Since
the charset parameter is provided, MIME and XML processors MUST treat
the enclosed entity as UTF-8 encoded.
If sent using a 7-bit transport (e.g., SMTP[RFC0821]), the XML MIME
entity MUST use a content-transfer-encoding of either quoted-
printable or base64. For an 8-bit clean transport (e.g., 8BITMIME
ESMTP or NNTP), or a binary clean transport (e.g., HTTP), no
content-transfer-encoding is necessary.
8.2 Text/xml with UTF-16 Charset
Content-type: text/xml; charset="utf-16"
{BOM}<?xml version='1.0' encoding='utf-16'?>
or
{BOM}<?xml version='1.0'?>
This is possible only when the XML MIME entity is transmitted via
HTTP, which uses a MIME-like mechanism and is a binary-clean
protocol, hence does not perform CR and LF transformations and allows
NUL octets. As described in [RFC2781], the UTF-16 family MUST NOT be
used with media types under the top-level type "text" except over
HTTP (see section 19.4.1 of [RFC2616] for details).
Since HTTP is binary clean, no content-transfer-encoding is
necessary.
8.3 Text/xml with UTF-16BE Charset
Content-type: text/xml; charset="utf-16be"
<?xml version='1.0' encoding='utf-16be'?>
Observe that the BOM does not exist. This is again possible only
when the XML MIME entity is transmitted via HTTP.
8.4 Text/xml with ISO-2022-KR Charset
Content-type: text/xml; charset="iso-2022-kr"
<?xml version="1.0" encoding='iso-2022-kr'?>
This example shows text/xml with a Korean charset (e.g., Hangul)
encoded following the specification in [RFC1557]. Since the charset
parameter is provided, MIME and XML processors MUST treat the
enclosed entity as encoded per RFC1557.
Since ISO-2022-KR has been defined to use only 7 bits of data, no
content-transfer-encoding is necessary with any transport.
8.5 Text/xml with Omitted Charset
Content-type: text/xml
{BOM}<?xml version="1.0" encoding="utf-16"?>
or
{BOM}<?xml version="1.0"?>
This example shows text/xml with the charset parameter omitted. In
this case, MIME and XML processors MUST assume the charset is "us-
ascii", the default charset value for text media types specified in
[RFC2046]. The default of "us-ascii" holds even if the text/xml
entity is transported using HTTP.
Omitting the charset parameter is NOT RECOMMENDED for text/xml. For
example, even if the contents of the XML MIME entity are UTF-16 or
UTF-8, or the XML MIME entity has an explicit encoding declaration,
XML and MIME processors MUST assume the charset is "us-ascii".
8.6 Application/xml with UTF-16 Charset
Content-type: application/xml; charset="utf-16"
{BOM}<?xml version="1.0" encoding="utf-16"?>
or
{BOM}<?xml version="1.0"?>
This is a recommended charset value for use with application/xml.
Since the charset parameter is provided, MIME and XML processors MUST
treat the enclosed entity as UTF-16 encoded.
If sent using a 7-bit transport (e.g., SMTP) or an 8-bit clean
transport (e.g., 8BITMIME ESMTP or NNTP), the XML MIME entity MUST be
encoded in quoted-printable or base64. For a binary clean transport
(e.g., HTTP), no content-transfer-encoding is necessary.
8.7 Application/xml with UTF-16BE Charset
Content-type: application/xml; charset="utf-16be"
<?xml version='1.0' encoding='utf-16be'?>
Observe that the BOM does not exist. Since the charset parameter is
provided, MIME and XML processors MUST treat the enclosed entity as
UTF-16BE encoded.
8.8 Application/xml with ISO-2022-KR Charset
Content-type: application/xml; charset="iso-2022-kr"
<?xml version="1.0" encoding="iso-2022-kr"?>
This example shows application/xml with a Korean charset (e.g.,
Hangul) encoded following the specification in [RFC1557]. Since the
charset parameter is provided, MIME and XML processors MUST treat the
enclosed entity as encoded per RFC1557, independent of whether the
XML MIME entity has an internal encoding declaration (this example
does show such a declaration, which agrees with the charset
parameter).
Since ISO-2022-KR has been defined to use only 7 bits of data, no
content-transfer-encoding is necessary with any transport.
8.9 Application/xml with Omitted Charset and UTF-16 XML MIME Entity
Content-type: application/xml
{BOM}<?xml version='1.0' encoding="utf-16"?>
or
{BOM}<?xml version='1.0'?>
For this example, the XML MIME entity begins with a BOM. Since the
charset has been omitted, a conforming XML processor follows the
requirements of [XML], section 4.3.3. Specifically, the XML
processor reads the BOM, and thus knows deterministically that the
charset is UTF-16.
An XML-unaware MIME processor SHOULD make no assumptions about the
charset of the XML MIME entity.
8.10 Application/xml with Omitted Charset and UTF-8 Entity
Content-type: application/xml
<?xml version='1.0'?>
In this example, the charset parameter has been omitted, and there is
no BOM. Since there is no BOM, the XML processor follows the
requirements in section 4.3.3 of [XML], and optionally applies the
mechanism described in Appendix F (which is non-normative) of [XML]
to determine the charset encoding of UTF-8. The XML MIME entity does
not contain an encoding declaration, but since the encoding is UTF-8,
this is still a conforming XML MIME entity.
An XML-unaware MIME processor SHOULD make no assumptions about the
charset of the XML MIME entity.
8.11 Application/xml with Omitted Charset and Internal Encoding
Declaration
Content-type: application/xml
<?xml version='1.0' encoding="iso-10646-ucs-4"?>
In this example, the charset parameter has been omitted, and there is
no BOM. However, the XML MIME entity does have an encoding
declaration inside the XML MIME entity that specifies the entity's