[1] RFC3023 と RFC2679 の差分, そしてその和訳。 詳しくは ..// 参照。
The World Wide Web Consortium
(W3C)has issued a Recommendation [REC-XML] which defines the Extensible Markup Language (XML), version 11.0 (Second Edition)[XML] . To enable the exchange of XML network entities, this documentproposes twostandardizes five new media types,-- text/xml,andapplication/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.
(Note that, as sometimes happens between two communities, both MIME and XML have defined the term entity, with different meanings.)Although XML is a subset of the Standard Generalized Markup Language (SGML)
[ISO-8897], and currently isISO 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 applicationswhichthat can process XML, but which 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 DEL[-1874] 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.
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 DEL[-2119].
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).
This document
introduces two newstandardizes five media typesforrelated to XMLMIMEentities,: text/xml,andapplication/xml, text/xml-external-parsed-entity, application/xml-external-parsed-entity, and application/xml-dtd. Registration information for these media typesareis described in the sections below.
Every XML entity is suitable for use with the application/xml media type without modification. But this does not exploit the fact that XML can be treated as plain text in many cases. MIME user agents (and web user agents) that do not have explicit support for application/xml will treat it as application/octet-stream, for example, by offering to save it to a file.
To indicate that an XML entity should be treated as plain text by default, use the text/xml media type. This restricts the encoding used in the XML entity to those that are compatible with the requirements for text media types as described in [RFC-2045] and [RFC-2046], e.g., UTF-8, but not UTF-16 (except for HTTP).
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.
XML 仕様書のによれば、 XML MIME 実体は4種類に分類できます。
XML の用語では、文書実体, 外部解析実体,
外部DTD部分集合, 外部引数集合と呼びます。
媒体型 text/xml 及び
application/xml を、
文書実体に使っても構いません。
text/xml-external-parsed-entity
又は application/xml-external-parsed-entity
を外部解析実体に使うべきです。
application/xml-dtd
を外部 DTD 部分集合及び外部引数実体に使うべきです。
application/xml 及び text/xml
は、外部引数実体や外部 DTD 部分集合には使ってはならず、
外部解析実体には、これが文書実体として整形式であって、そう参照される場合を除いて使ってはなりません。
(この文書が廃止する) RFC2376 はそのような使い方を認めていますが、
実際のところこれは稀でしょう。
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
whichthat 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 prohibitfuturesuch media typesdedicated to such XML applicationsand in fact expects them to proliferate. However, developers of such media types arerecommendedSTRONGLY RECOMMENDED to use this document as a basis for their registration. In particular, the charset parametershouldSHOULD 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.
Within the XML specification, XML 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 can be used for any of these four types.
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" DEL[-2279] is the
recommended value, representing the UTF-8 charset. UTF-8 is
supported by all conforming XML processors [REC-XML] 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 HTTP 1.1 [RFC2616]), "UTF-16" "utf-16" (Appendix C.3 of [UNICODE] and Amendment 1 of [ISO-10646])
[RFC2781]) (訳注: ママ) is also recommended. UTF-16 is supported by all
conforming XML processors [REC-XML] 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 a text/xml; charset="utf-16" to
application/xml; charset="utf-16". Conformant with DEL[-2046], 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 both UTF-8 and UTF-16 is MUST be
encoded in quoted-printable or base64. For 8-bit clean transport
(e.g., ESMTP, 8BITMIME[RFC1652] ESMTP, or NNTP[RFC0977] ), UTF-8 is does not
need to be encoded. For binary clean transports (e.g., HTTP) Over HTTP[RFC2616] , no content-transfer-encoding is necessary and UTF-16 may also be used. 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]. 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 DEL[REC-XML].File extension(s): .xml, .dtd
Macintosh File Type Code(s): "TEXT"
Person&and email address for further information:
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:The W3C, and the W3C XML Core
wWorkinggGroup, have change control over the XML specification.
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" [RFC-2279] and "UTF-16" (Appendix C.3 of [UNICODE] and Amendment 1 of [ISO-10646])"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 conformingXMLprocessors of DEL[REC-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 DEL[REC-XML]
which that directly address this contingency. However, MIME processors
which that are not XML processors should not 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 both either UTF-8 and or
UTF-16 is MUST be encoded in quoted-printable or base64. For 8-bit
clean transport (e.g., ESMTP, 8BITMIME 8BITMIME[RFC1652] ESMTP or NNTP[RFC0977] ),
UTF-8 is not encoded, but the UTF-16 is base64 family MUST be encoded in base64. For binary clean transports (e.g., HTTP[RFC2616] ), no
content-transfer-encoding is necessary.Security considerations:See section 4 below.See Section 10.
Interoperability considerations: Same as Section 3.1.
XML has proven to be interoperable for import and export from multiple XML authoring tools.
XML is device-, platform-, and vendor-neutral and is supported by a wide range of Web user agents and XML authoring tools.
Although no byte sequences can be counted on to always be present, XML 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 or FF FE 3C 00 3F 00 78 00 6D 00 (the Byte Order Mark (BOM) followed by "<?xml"). For more information, see Annex F of [REC-XML].
- File extension(s)
- .xml, .dtd
The XML specification is a work product of the World Wide Web Consortium's XML Working Group, and was edited by:
The charset parameter of text/xml-external-parsed-entity is
handled the same as that of text/xml as described in Section 3.1. 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.Magic number(s): Same as Section 3.1.
File extension(s): .xml or .ent
Macintosh File Type Code(s): "TEXT"
The charset parameter of application/xml-external-parsed-entity is
handled the same as that of application/xml as described in
Section 3.2.Magic number(s): Same as Section 3.1.
File extension(s): .xml or .ent
Macintosh File Type Code(s): "TEXT"
The charset parameter of application/xml-dtd is handled the same
as that of application/xml as described in Section 3.2. Interoperability considerations: XML DTDs have proven to be
interoperable by DTD authoring tools and XML browsers, among
others. 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.Magic number(s): Same as Section 3.1.
File extension(s): .dtd or .mod
Macintosh File Type Code(s): "TEXT"
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: