draft-ietf-appsawg-xml-mediatypes-09.txt   draft-ietf-appsawg-xml-mediatypes-10.txt 
Network Working Group H. Thompson Network Working Group H. Thompson
Internet-Draft University of Edinburgh Internet-Draft University of Edinburgh
Obsoletes: 3023 (if approved) C. Lilley Obsoletes: 3023 (if approved) C. Lilley
Updates: 6839 (if approved) W3C Updates: 6839 (if approved) W3C
Intended status: Standards Track March 02, 2014 Intended status: Standards Track April 07, 2014
Expires: September 3, 2014 Expires: October 9, 2014
XML Media Types XML Media Types
draft-ietf-appsawg-xml-mediatypes-09 draft-ietf-appsawg-xml-mediatypes-10
Abstract Abstract
This specification standardizes three media types -- application/xml, This specification standardizes three media types -- application/xml,
application/xml-external-parsed-entity, and application/xml-dtd -- application/xml-external-parsed-entity, and application/xml-dtd --
for use in exchanging network entities that are related to the for use in exchanging network entities that are related to the
Extensible Markup Language (XML) while defining text/xml and text/ Extensible Markup Language (XML) while defining text/xml and text/
xml-external-parsed-entity as aliases for the respective application/ xml-external-parsed-entity as aliases for the respective application/
types. This specification also standardizes the '+xml' suffix for types. This specification also standardizes the '+xml' suffix for
naming media types outside of these five types when those media types naming media types outside of these five types when those media types
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 3, 2014. This Internet-Draft will expire on October 9, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 4 2. Notational Conventions . . . . . . . . . . . . . . . . . . . 4
2.1. Conformance Keywords . . . . . . . . . . . . . . . . . . 4 2.1. Conformance Keywords . . . . . . . . . . . . . . . . . . 4
2.2. Characters, Encodings, Charsets . . . . . . . . . . . . . 4 2.2. Characters, Encodings, Charsets . . . . . . . . . . . . . 4
2.3. MIME Entities, XML Entities . . . . . . . . . . . . . . . 4 2.3. MIME Entities, XML Entities . . . . . . . . . . . . . . . 4
3. Encoding Considerations . . . . . . . . . . . . . . . . . . . 5 3. Encoding Considerations . . . . . . . . . . . . . . . . . . . 5
3.1. XML MIME producers . . . . . . . . . . . . . . . . . . . 6 3.1. XML MIME producers . . . . . . . . . . . . . . . . . . . 6
3.2. XML MIME consumers . . . . . . . . . . . . . . . . . . . 6 3.2. XML MIME consumers . . . . . . . . . . . . . . . . . . . 6
3.3. The Byte Order Mark (BOM) and Encoding Conversions . . . 7 3.3. The Byte Order Mark (BOM) and Encoding Conversions . . . 7
4. XML Media Types . . . . . . . . . . . . . . . . . . . . . . . 8 4. XML Media Types . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. XML MIME Entities . . . . . . . . . . . . . . . . . . . . 8 4.1. XML MIME Entities . . . . . . . . . . . . . . . . . . . . 9
4.2. Using '+xml' when Registering XML-based Media Types . . . 10 4.2. Using '+xml' when Registering XML-based Media Types . . . 10
4.3. Registration Guidelines for XML-based Media Types Not 4.3. Registration Guidelines for XML-based Media Types Not
Using '+xml' . . . . . . . . . . . . . . . . . . . . . . 11 Using '+xml' . . . . . . . . . . . . . . . . . . . . . . 12
5. Fragment Identifiers . . . . . . . . . . . . . . . . . . . . 12 5. Fragment Identifiers . . . . . . . . . . . . . . . . . . . . 12
6. The Base URI . . . . . . . . . . . . . . . . . . . . . . . . 13 6. The Base URI . . . . . . . . . . . . . . . . . . . . . . . . 13
7. XML Versions . . . . . . . . . . . . . . . . . . . . . . . . 13 7. XML Versions . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14 8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.1. UTF-8 Charset . . . . . . . . . . . . . . . . . . . . . . 14 8.1. UTF-8 Charset . . . . . . . . . . . . . . . . . . . . . . 14
8.2. UTF-16 Charset . . . . . . . . . . . . . . . . . . . . . 15 8.2. UTF-16 Charset . . . . . . . . . . . . . . . . . . . . . 15
8.3. Omitted Charset and 8-bit MIME Entity . . . . . . . . . . 15 8.3. Omitted Charset and 8-bit MIME Entity . . . . . . . . . . 15
8.4. Omitted Charset and 16-bit MIME Entity . . . . . . . . . 15 8.4. Omitted Charset and 16-bit MIME Entity . . . . . . . . . 16
8.5. Omitted Charset, no Internal Encoding Declaration . . . . 16 8.5. Omitted Charset, no Internal Encoding Declaration . . . . 16
8.6. UTF-16BE Charset . . . . . . . . . . . . . . . . . . . . 16 8.6. UTF-16BE Charset . . . . . . . . . . . . . . . . . . . . 17
8.7. Non-UTF Charset . . . . . . . . . . . . . . . . . . . . . 17 8.7. Non-UTF Charset . . . . . . . . . . . . . . . . . . . . . 17
8.8. INCONSISTENT EXAMPLE: Conflicting Charset and Internal 8.8. INCONSISTENT EXAMPLE: Conflicting Charset and Internal
Encoding Declaration . . . . . . . . . . . . . . . . . . 17 Encoding Declaration . . . . . . . . . . . . . . . . . . 17
8.9. INCONSISTENT EXAMPLE: Conflicting Charset and BOM . . . . 17 8.9. INCONSISTENT EXAMPLE: Conflicting Charset and BOM . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
9.1. Application/xml Registration . . . . . . . . . . . . . . 18 9.1. Application/xml Registration . . . . . . . . . . . . . . 18
9.2. Text/xml Registration . . . . . . . . . . . . . . . . . . 19 9.2. Text/xml Registration . . . . . . . . . . . . . . . . . . 20
9.3. Application/xml-external-parsed-entity Registration . . . 20 9.3. Application/xml-external-parsed-entity Registration . . . 20
9.4. Text/xml-external-parsed-entity Registration . . . . . . 21 9.4. Text/xml-external-parsed-entity Registration . . . . . . 21
9.5. Application/xml-dtd Registration . . . . . . . . . . . . 21 9.5. Application/xml-dtd Registration . . . . . . . . . . . . 21
9.6. The '+xml' Naming Convention for XML-Based Media Types . 22 9.6. The '+xml' Naming Convention for XML-Based Media Types . 22
9.6.1. +xml Structured Syntax Suffix Registration . . . . . 22 9.6.1. +xml Structured Syntax Suffix Registration . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 23 10. Security Considerations . . . . . . . . . . . . . . . . . . . 24
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
11.1. Normative References . . . . . . . . . . . . . . . . . . 25 11.1. Normative References . . . . . . . . . . . . . . . . . . 26
11.2. Informative References . . . . . . . . . . . . . . . . . 27 11.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Why Use the '+xml' Suffix for XML-Based MIME Types? 30 Appendix A. Why Use the '+xml' Suffix for XML-Based MIME Types? 30
Appendix B. Core XML specifications . . . . . . . . . . . . . . 30 Appendix B. Core XML specifications . . . . . . . . . . . . . . 30
Appendix C. Changes from RFC 3023 . . . . . . . . . . . . . . . 30 Appendix C. Operational considerations . . . . . . . . . . . . . 31
Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 31 C.1. General considerations . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31 C.2. Considerations for producers . . . . . . . . . . . . . . 31
C.3. Considerations for consumers . . . . . . . . . . . . . . 32
Appendix D. Changes from RFC 3023 . . . . . . . . . . . . . . . 32
Appendix E. Acknowledgements . . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction 1. Introduction
The World Wide Web Consortium has issued the Extensible Markup The World Wide Web Consortium has issued the Extensible Markup
Language (XML) 1.0 [XML] and Extensible Markup Language (XML) 1.1 Language (XML) 1.0 [XML] and Extensible Markup Language (XML) 1.1
[XML1.1] specifications. To enable the exchange of XML network [XML1.1] specifications. To enable the exchange of XML network
entities, this specification standardizes three media types -- entities, this specification standardizes three media types --
application/xml, application/xml-external-parsed-entity, and application/xml, application/xml-external-parsed-entity, and
application/xml-dtd and two aliases -- text/xml and text/xml- application/xml-dtd and two aliases -- text/xml and text/xml-
external-parsed-entity, as well as a naming convention for external-parsed-entity, as well as a naming convention for
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with application/xml and application/xml-external-parsed-entity with application/xml and application/xml-external-parsed-entity
respectively, the addition of XPointer and XML Base as fragment respectively, the addition of XPointer and XML Base as fragment
identifiers and base URIs, respectively, integration of the XPointer identifiers and base URIs, respectively, integration of the XPointer
Registry and updating of many references. Registry and updating of many references.
2. Notational Conventions 2. Notational Conventions
2.1. Conformance Keywords 2.1. Conformance Keywords
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
specification are to be interpreted as described in [RFC2119]. "OPTIONAL" in this specification are to be interpreted as described
in [RFC2119].
2.2. Characters, Encodings, Charsets 2.2. Characters, Encodings, Charsets
Both XML (in an XML or Text declaration using the encoding pseudo- Both XML (in an XML or Text declaration using the encoding pseudo-
attribute) and MIME (in a Content-Type header field using the charset attribute) and MIME (in a Content-Type header field using the charset
parameter) use a common set of labels [IANA-charsets] to identify the parameter) use a common set of labels [IANA-charsets] to identify the
MIME charset (mapping from byte stream to character sequence MIME charset (mapping from byte stream to character sequence
[RFC2978]). [RFC2978]).
In this specification we will use the phrases "charset parameter" and In this specification we will use the phrases "charset parameter" and
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We reserve the phrase "character encoding" (or, when the context We reserve the phrase "character encoding" (or, when the context
makes the intention clear, simply "encoding") for the MIME charset makes the intention clear, simply "encoding") for the MIME charset
actually used in a particular XML MIME entity. actually used in a particular XML MIME entity.
[UNICODE] defines three "encoding forms", namely UTF-8, UTF-16, and [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 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 possible label for UTF-8-encoded documents when serialised into MIME
entities is "utf-8". UTF-16 XML documents, however, can be entities is "utf-8". UTF-16 XML documents, however, can be
serialised into MIME entities in one of two ways: either big- endian, 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-16be", or little- endian,
labelled (optionally) "utf-16" or "utf-16le". 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 UTF-32 has four potential serializations, of which only two (UTF-32BE
and UTF-32LE) are given names in in [UNICODE]. Support for the and UTF-32LE) are given names in [UNICODE]. Support for the various
various serializations varies widely, and security concerns about serializations varies widely, and security concerns about their use
their use have been raised. The use of UTF-32 is NOT RECOMMENDED for have been raised (see for example [Sivonen]). The use of UTF-32 is
XML MIME entities. NOT RECOMMENDED for XML MIME entities.
2.3. MIME Entities, XML Entities 2.3. MIME Entities, XML Entities
As sometimes happens between two communities, both MIME and XML have As sometimes happens between two communities, both MIME and XML have
defined the term entity, with different meanings. Section 2.4 of defined the term entity, with different meanings. Section 2.4 of
[RFC2045] says: [RFC2045] says:
"The term 'entity' refers specifically to the MIME-defined header "The term 'entity' refers specifically to the MIME-defined header
fields and contents of either a message or one of the parts in the fields and contents of either a message or one of the parts in the
body of a multipart entity." body of a multipart entity."
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recommendations given below are intended to maximise interoperability recommendations given below are intended to maximise interoperability
in the face of this, by on the one hand mandating consistent in the face of this, by on the one hand mandating consistent
production and encouraging maximally robust forms of production, and production and encouraging maximally robust forms of production, and
on the other specifying recovery strategies to maximize the on the other specifying recovery strategies to maximize the
interoperability of consumers when the production rules are broken. interoperability of consumers when the production rules are broken.
3.1. XML MIME producers 3.1. XML MIME producers
XML-aware MIME producers SHOULD supply a charset parameter and/or an XML-aware MIME producers SHOULD supply a charset parameter and/or an
appropriate BOM with non-UTF-8-encoded XML MIME entities which lack appropriate BOM with non-UTF-8-encoded XML MIME entities which lack
an encoding declaration, and SHOULD remove or correct an encoding an encoding declaration. Such producers SHOULD remove or correct an
declaration which is known to be incorrect (for example, as a result encoding declaration which is known to be incorrect (for example, as
of transcoding). a result of transcoding).
XML-aware MIME producers MUST supply an XML text declaration at the XML-aware MIME producers MUST supply an XML text declaration at the
beginning of non-UNICODE XML external parsed entities which would beginning of non-UNICODE XML external parsed entities which would
otherwise begin with the hexadecimal octet sequences 0xFE 0xFF, 0xFF otherwise begin with the hexadecimal octet sequences 0xFE 0xFF, 0xFF
0xFE or 0xEF 0xBB 0xBF, in order to avoid the mistaken detection of a 0xFE or 0xEF 0xBB 0xBF, in order to avoid the mistaken detection of a
BOM. BOM.
XML-unaware MIME producers MUST NOT supply a charset parameter with XML-unaware MIME producers MUST NOT supply a charset parameter with
an XML MIME entity unless the entity's character encoding is reliably an XML MIME entity unless the entity's character encoding is reliably
known. Note that this is particularly relevant for central known. Note that this is particularly relevant for central
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If an XML MIME entity is received where the charset parameter is If an XML MIME entity is received where the charset parameter is
omitted, no information is being provided about the character omitted, no information is being provided about the character
encoding by the MIME Content-Type header. XML-aware consumers MUST encoding by the MIME Content-Type header. XML-aware consumers MUST
follow the requirements in section 4.3.3 of [XML] that directly follow the requirements in section 4.3.3 of [XML] that directly
address this case. XML-unaware MIME consumers SHOULD NOT assume a address this case. XML-unaware MIME consumers SHOULD NOT assume a
default encoding in this case. default encoding in this case.
3.3. The Byte Order Mark (BOM) and Encoding Conversions 3.3. The Byte Order Mark (BOM) and Encoding Conversions
Section 4.3.3 of [XML] specifies that UTF-16 XML MIME entities not Section 4.3.3 of [XML] specifies that UTF-16 XML MIME entities not
labelled as "utf-16le" or "utf16-be" MUST begin with a byte order labelled as "utf-16le" or "utf-16be" MUST begin with a byte order
mark (BOM), U+FEFF, which appears as the hexadecimal octet sequence mark (BOM), U+FEFF, which appears as the hexadecimal octet sequence
0xFE 0xFF (big-endian) or 0xFF 0xFE (little-endian). [XML] further 0xFE 0xFF (big-endian) or 0xFF 0xFE (little-endian). [XML] further
states that the BOM is an encoding signature, and is not part of states that the BOM is an encoding signature, and is not part of
either the markup or the character data of the XML document. either the markup or the character data of the XML document.
Due to the presence of the BOM, applications that convert XML from 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 UTF-16 to an encoding other than UTF-8 MUST strip the BOM before
conversion. Similarly, when converting from another encoding into conversion. Similarly, when converting from another encoding into
UTF-16, either without a charset parameter, or labelled "utf-16", the 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 BOM MUST be added unless the original encoding was UTF-8 and a BOM
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begin with a BOM, which appears as the hexadecimal octet sequence begin with a BOM, which appears as the hexadecimal octet sequence
0xEF 0xBB 0xBF. This is likewise defined to be an encoding 0xEF 0xBB 0xBF. This is likewise defined to be an encoding
signature, and not part of either the markup or the character data of signature, and not part of either the markup or the character data of
the XML document. the XML document.
Applications that convert XML from UTF-8 to an encoding other than Applications that convert XML from UTF-8 to an encoding other than
UTF-16 MUST strip the BOM, if present, before conversion. UTF-16 MUST strip the BOM, if present, before conversion.
Applications which convert XML into UTF-8 MAY add a BOM. Applications which convert XML into UTF-8 MAY add a BOM.
In addition to the MIME charset "utf-16", [RFC2781] introduces "utf- In addition to the MIME charset "utf-16", [RFC2781] introduces "utf-
16le" (little endian) and "utf-16be" (big endian). The BOM is 16le" (little endian) and "utf-16be" (big endian). When an XML MIME
prohibited in MIME entities with these labels. When an XML MIME
entity is encoded in "utf-16le" or "utf-16be", it MUST NOT begin with 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. the BOM but SHOULD contain an in-band XML encoding declaration.
Conversion from UTF-8 or UTF-16 (unlabelled, or labelled with 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, "utf-16") to "utf-16be" or "utf-16le" MUST strip a BOM if present.
and conversion in the other direction MUST (for UTF-16) or MAY (for Conversion from UTF-16 labelled "utf-16le" or "utf-16be" to UTF-16
UTF-8) add the appropriate BOM. 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 the a UTF-32 BOM may be used in Appendix F of [XML] also implies the a UTF-32 BOM may be used in
conjunction with UTF-32-encoded documents. As noted above, this conjunction with UTF-32-encoded documents. As noted above, this
specification recommends against the use of UTF-32, but if it is specification recommends against the use of UTF-32, but if it is
used, the same considerations apply with respect to its being a used, the same considerations apply with respect to its being a
signature, not part of the document, with respect to transcoding into signature, not part of the document, with respect to transcoding into
or out of it and with respect to the MIME charsets "utf-32le" and or out of it and with respect to the MIME charsets "utf-32le" and
"utf-32be", as for UTF-16. Consumers which do not support UTF-32 "utf-32be", as for UTF-16. Consumers which do not support UTF-32
SHOULD none-the-less recognise UTF-32 signatures in order to give SHOULD none-the-less recognise UTF-32 signatures in order to give
helpful error messages (instead of treating them as invalid UTF-16). helpful error messages (instead of treating them as invalid UTF-16).
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type. This convention will allow applications that can process XML type. This convention will allow applications that can process XML
generically to detect that the MIME entity is supposed to be an XML generically to detect that the MIME entity is supposed to be an XML
document, verify this assumption by invoking some XML processor, and document, verify this assumption by invoking some XML processor, and
then process the XML document accordingly. Applications may check then process the XML document accordingly. Applications may check
for types that represent XML MIME entities by comparing the last four for types that represent XML MIME entities by comparing the last four
characters of the subtype to the string '+xml'. (However note that 4 characters of the subtype to the string '+xml'. (However note that 4
of the 5 media types defined in this specification -- text/xml, of the 5 media types defined in this specification -- text/xml,
application/xml, text/xml-external-parsed-entity, and application/ application/xml, text/xml-external-parsed-entity, and application/
xml-external-parsed-entity -- also represent XML MIME entities while xml-external-parsed-entity -- also represent XML MIME entities while
not ending with '+xml'.) not ending with '+xml'.)
NOTE: Section 5.3.2 of HTTPbis [HTTPbis] does not support any form NOTE: Section 5.3.2 of HTTPbis [HTTPbis] does not support any form
of Accept header which will match only '+xml' types. In of Accept header which will match only '+xml' types. In
particular, Accept headers of the form "Accept: */*+xml" are not particular, Accept headers of the form "Accept: */*+xml" are not
allowed, and so this header MUST NOT be used for this purpose. allowed, and will not work for this purpose.
Media types following the naming convention '+xml' SHOULD introduce Media types following the naming convention '+xml' SHOULD define the
the charset parameter for consistency, since XML-generic processing charset parameter for consistency, since XML-generic processing by
applies the same program for any such media type. However, there are definition treats treats all XML MIME entities uniformly as regards
some cases that the charset parameter need not be introduced. For character encoding information. However, there are some cases that
example: the charset parameter need not be defined. For example:
When an XML-based media type is restricted to UTF-8, it is not When an XML-based media type is restricted to UTF-8, it is not
necessary to introduce the charset parameter. UTF-8 is the necessary to define the charset parameter. UTF-8 is the default
default for XML. for XML.
When an XML-based media type is restricted to UTF-8 and UTF-16, it 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 might not be unreasonable to omit the charset parameter. Neither
UTF-8 nor UTF-16 require XML encoding declarations. UTF-8 nor UTF-16 require XML encoding declarations.
XML generic processing is not always appropriate for XML-based media XML generic processing is not always appropriate for XML-based media
types. For example, authors of some such media types may wish that types. For example, authors of some such media types may wish that
the types remain entirely opaque except to applications that are the types remain entirely opaque except to applications that are
specifically designed to deal with that media type. By NOT following specifically designed to deal with that media type. By NOT following
the naming convention '+xml', such media types can avoid XML-generic the naming convention '+xml', such media types can avoid XML-generic
processing. Since generic processing will be useful in many cases, processing. Since generic processing will be useful in many cases,
however -- including in some situations that are difficult to predict however -- including in some situations that are difficult to predict
ahead of time -- the '+xml' convention is to be preferred unless ahead of time -- the '+xml' convention is to be preferred unless
there is some particularly compelling reason not to. there is some particularly compelling reason not to.
The registration process for specific '+xml' media types is described The registration process for specific '+xml' media types is described
in [RFC6838]. The registrar for the IETF tree will encourage new in [RFC6838]. New XML-based media type registrations in the IETF
XML-based media type registrations in the IETF tree to follow this must follow these guidelines. When other organisations register XML-
guideline. Registrars for other trees SHOULD follow this convention based media types via the "Specification Required" IANA registration
in order to ensure maximum interoperability of their XML-based policy, the relevant Media Reviewer should ensure that they use the
documents. Only media subtypes that represent XML MIME entities are '+xml' convention, in order to ensure maximum interoperability of
allowed to register with a '+xml' suffix. 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 In addition to the changes described above, the change controller has
been changed to be the World Wide Web Consortium (W3C). been changed to be the World Wide Web Consortium (W3C).
4.3. Registration Guidelines for XML-based Media Types Not Using '+xml' 4.3. Registration Guidelines for XML-based Media Types Not Using '+xml'
Registrations for new XML-based media types which do _not_ use the Registrations for new XML-based media types which do _not_ use the
'+xml' suffix SHOULD, in specifying the charset parameter and '+xml' suffix SHOULD, in specifying the charset parameter and
encoding considerations, define them as: "Same as [charset parameter encoding considerations, define them as: "Same as [charset parameter
/ encoding considerations] of application/xml as specified in RFC / encoding considerations] of application/xml as specified in RFC
XXXX." XXXX."
Enabling the charset parameter is RECOMMENDED, since this information
Defining the charset parameter is RECOMMENDED, since this information
can be used by XML processors to determine authoritatively the can be used by XML processors to determine authoritatively the
character encoding of the XML MIME entity in the absence of a BOM. 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 If there are some reasons not to follow this advice, they SHOULD be
included as part of the registration. As shown above, two such included as part of the registration. As shown above, two such
reasons are "UTF-8 only" or "UTF-8 or UTF-16 only". reasons are "UTF-8 only" or "UTF-8 or UTF-16 only".
These registrations SHOULD specify that the XML-based media type These registrations SHOULD specify that the XML-based media type
being registered has all of the security considerations described in being registered has all of the security considerations described in
RFC XXXX plus any additional considerations specific to that media RFC XXXX plus any additional considerations specific to that media
type. type.
skipping to change at page 12, line 49 skipping to change at page 13, line 12
specified by the [XPointerFramework] together with any other specified by the [XPointerFramework] together with any other
specifications governing the XPointer schemes used in those specifications governing the XPointer schemes used in those
identifiers which the applications support. Conforming applications identifiers which the applications support. Conforming applications
MUST support the 'element' scheme as defined in [XPointerElement], MUST support the 'element' scheme as defined in [XPointerElement],
but need not support other schemes. but need not support other schemes.
If an XPointer error is reported in the attempt to process the part, If an XPointer error is reported in the attempt to process the part,
this specification does not define an interpretation for the part. this specification does not define an interpretation for the part.
A registry of XPointer schemes [XPtrReg] is maintained at the W3C. A registry of XPointer schemes [XPtrReg] is maintained at the W3C.
Document authors SHOULD NOT use unregistered schemes. Scheme authors Generic processors of XML MIME entities SHOULD NOT implement
SHOULD register their schemes ([XPtrRegPolicy] describes requirements unregistered XPointer schemes ([XPtrRegPolicy] describes requirements
and procedures for doing so). and procedures for registering schemes).
See Section 4.2 for additional requirements which apply when an XML- See Section 4.2 for additional requirements which apply when an XML-
based media type follows the naming convention '+xml'. based media type follows the naming convention '+xml'.
If [XPointerFramework] and [XPointerElement] are inappropriate for If [XPointerFramework] and [XPointerElement] are inappropriate for
some XML-based media type, it SHOULD NOT follow the naming convention some XML-based media type, it SHOULD NOT follow the naming convention
'+xml'. '+xml'.
When a URI has a fragment identifier, it is encoded by a limited When a URI has a fragment identifier, it is encoded by a limited
subset of the repertoire of US-ASCII characters, see subset of the repertoire of US-ASCII characters, see
skipping to change at page 29, line 5 skipping to change at page 29, line 25
[RFC3977] Feather, B., "Network News Transfer Protocol", RFC 3977, [RFC3977] Feather, B., "Network News Transfer Protocol", RFC 3977,
October 2006. October 2006.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008. October 2008.
[RFC6152] Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP [RFC6152] Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP
Service Extension for 8-bit MIME Transport", RFC 6152, Service Extension for 8-bit MIME Transport", RFC 6152,
March 2011. 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, [TAGMIME] Bray, T., Ed., "Internet Media Type registration,
consistency of use", April 2004, consistency of use", April 2004,
<http://www.w3.org/2001/tag/2004/0430-mime>. <http://www.w3.org/2001/tag/2004/0430-mime>.
[XHTML] Pemberton, S. and et al, "XHTML 1.0: The Extensible [XHTML] Pemberton, S. and et al, "XHTML 1.0: The Extensible
HyperText Markup Language", W3C Recommendation xhtml1, HyperText Markup Language", W3C Recommendation xhtml1,
December 1999, December 1999,
<http://www.w3.org/TR/2000/REC-xhtml1-20000126/>. <http://www.w3.org/TR/2000/REC-xhtml1-20000126/>.
Latest version available at [7]. Latest version available at [7].
skipping to change at page 30, line 42 skipping to change at page 31, line 15
The W3C Technical Architecture group has produced two documents which The W3C Technical Architecture group has produced two documents which
are also relevant: are also relevant:
The Self-Describing Web [FYN] discusses the overall principles of The Self-Describing Web [FYN] discusses the overall principles of
how document semantics are determined on the Web. how document semantics are determined on the Web.
Architecture of the World Wide Web, Volume One [AWWW], section Architecture of the World Wide Web, Volume One [AWWW], section
4.5.4, discusses the specific role of XML Namespace documents in 4.5.4, discusses the specific role of XML Namespace documents in
this process. this process.
Appendix C. Changes from RFC 3023 Appendix C. Operational considerations
This section provides an informal summary of the major operational
considerations which arise when exchanging XML MIME entities over a
network.
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 which 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 which supply default charset parameters
risk misrepresenting XML MIME entities. Allowing users to control
the value of charset parameters improves interoperability.
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 which 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 There are numerous and significant differences between this
specification and [RFC3023], which it obsoletes. This appendix specification and [RFC3023], which it obsoletes. This appendix
summarizes the major differences only. summarizes the major differences only.
XPointer ([XPointerFramework] and [XPointerElement]) has been XPointer ([XPointerFramework] and [XPointerElement]) has been
added as fragment identifier syntax for all the XML media types, added as fragment identifier syntax for all the XML media types,
and the XPointer Registry ([XPtrReg]) mentioned and the XPointer Registry ([XPtrReg]) mentioned
[XMLBase] has been added as a mechanism for specifying base URIs [XMLBase] has been added as a mechanism for specifying base URIs
skipping to change at page 31, line 17 skipping to change at page 33, line 17
Priority is now given to a Byte Order Mark (BOM) if present Priority is now given to a Byte Order Mark (BOM) if present
Many references are updated, and the existence of XML 1.1 and Many references are updated, and the existence of XML 1.1 and
relevance of this specification to it acknowledged relevance of this specification to it acknowledged
A number of justifications and contextualizations which were A number of justifications and contextualizations which were
appropriate when XML was new have been removed, including the appropriate when XML was new have been removed, including the
whole of the original Appendix A whole of the original Appendix A
Making BOMs authoritative is in principle a backwards- Appendix E. Acknowledgements
incompatibility. In practice serious interoperability issues already
exist when BOMs are used. Making BOMs authoritative, in conjunction
with the deprecation of the UTF-32 encoding form and the requirement
to include an XML encoding declaration in certain cases
(Section 3.1), is intended to improve in-practice interoperability as
much as possible.
Appendix D. Acknowledgements
MURATA Makoto (FAMILY Given) and Alexey Melnikov made early and MURATA Makoto (FAMILY Given) and Alexey Melnikov made early and
important contributions to the effort to revise [RFC3023]. important contributions to the effort to revise [RFC3023].
This specification reflects the input of numerous participants to the This specification reflects the input of numerous participants to the
ietf-xml-mime@imc.org, xml-mime@ietf.org and apps-discuss@ietf.org ietf-xml-mime@imc.org, xml-mime@ietf.org and apps-discuss@ietf.org
mailing lists, though any errors are the responsibility of the mailing lists, though any errors are the responsibility of the
authors. Special thanks to: authors. Special thanks to:
Mark Baker, James Clark, Dan Connolly, Martin Duerst, Ned Freed, Mark Baker, James Clark, Dan Connolly, Martin Duerst, Ned Freed,
Yaron Goland, Bjoern Hoehrmann, Rick Jelliffe, Murray S. Kucherawy, Yaron Goland, Bjoern Hoehrmann, Rick Jelliffe, Murray S. Kucherawy,
Larry Masinter, David Megginson, S. Moonesamy, Keith Moore, Chris Larry Masinter, David Megginson, S. Moonesamy, Keith Moore, Chris
Newman, Gavin Nicol, Julian Reschke, Marshall Rose, Jim Whitehead, Newman, Gavin Nicol, Julian Reschke, Marshall Rose, Jim Whitehead,
Erik Wilde and participants of the XML activity and the TAG at the Erik Wilde and participants of the XML activity and the TAG at the
W3C. W3C.
Jim Whitehead and Simon St.Laurent were editors of [RFC2376] and Jim Whitehead and Simon St. Laurent were editors of [RFC2376] and
[RFC3023], respectively. [RFC3023], respectively.
Authors' Addresses Authors' Addresses
Henry S. Thompson Henry S. Thompson
University of Edinburgh University of Edinburgh
Email: ht@inf.ed.ac.uk Email: ht@inf.ed.ac.uk
URI: http://www.ltg.ed.ac.uk/~ht/ URI: http://www.ltg.ed.ac.uk/~ht/
Chris Lilley Chris Lilley
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