draft-ietf-appsawg-mime-default-charset-01.txt   draft-ietf-appsawg-mime-default-charset-02.txt 
Applications Area Working Group A. Melnikov Applications Area Working Group A. Melnikov
Internet-Draft Isode Limited Internet-Draft Isode Limited
Updates: 2046 (if approved) J. Reschke Updates: 2046 (if approved) J. Reschke
Intended status: Standards Track greenbytes Intended status: Standards Track greenbytes
Expires: October 1, 2012 March 30, 2012 Expires: October 23, 2012 April 21, 2012
Update to MIME regarding Charset Parameter Handling in Update to MIME regarding Charset Parameter Handling in
Textual Media Types Textual Media Types
draft-ietf-appsawg-mime-default-charset-01 draft-ietf-appsawg-mime-default-charset-02
Abstract Abstract
This document changes RFC 2046 rules regarding default charset This document changes RFC 2046 rules regarding default charset
parameter values for text/* media types to better align with common parameter values for text/* media types to better align with common
usage by existing clients and servers. usage by existing clients and servers.
Editorial Note (To be removed by RFC Editor) Editorial Note (To be removed by RFC Editor)
Discussion of this draft should take place on the Apps Area Working Discussion of this draft should take place on the Apps Area Working
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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 October 1, 2012. This Internet-Draft will expire on October 23, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 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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Table of Contents Table of Contents
1. Introduction and Overview . . . . . . . . . . . . . . . 3 1. Introduction and Overview . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . 3
3. New rules for default charset parameter values for 3. New rules for default charset parameter values for
text/* media types . . . . . . . . . . . . . . . . . . 3 text/* media types . . . . . . . . . . . . . . . . . . 3
4. Default charset parameter value for text/plain 4. Default charset parameter value for text/plain
media type . . . . . . . . . . . . . . . . . . . . . . 4 media type . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . 4 5. Security Considerations . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . 5 6. IANA Considerations . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . 5 7. References . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . 5 7.1. Normative References . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . 5 7.2. Informative References . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 5 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . 5
1. Introduction and Overview 1. Introduction and Overview
[RFC2046] specified that the default charset parameter (i.e. the [RFC2046] specified that the default charset parameter (i.e. the
value used when it is not specified) is "US-ASCII". [RFC2616] value used when it is not specified) is "US-ASCII". [RFC2616]
changed the default for use by HTTP to be "ISO-8859-1". This changed the default for use by HTTP to be "ISO-8859-1". This
encoding is not very common for new text/* media types and a special encoding is not very common for new text/* media types and a special
rule in HTTP adds confusion about which specification ([RFC2046] or rule in HTTP adds confusion about which specification ([RFC2046] or
[RFC2616]) is authoritative in regards to the default charset for [RFC2616]) is authoritative in regards to the default charset for
text/* media types. [[anchor2: At the time of writing of this text/* media types.
document the IETF HTTPBIS WG is working on an update to RFC 2616
which removes the default charset of "ISO-8859-1" for "text/*" media
types. It is expected that the set of HTTPBIs documents will
reference this document in order to use the updated rules of default
charset in "text/*" media types.]]
Many complex text subtypes such as text/html [RFC2854] and text/xml Many complex text subtypes such as text/html [RFC2854] and text/xml
[RFC3023] have internal (to their format) means of describing the [RFC3023] have internal (to their format) means of describing the
charset. Many existing User Agents ignore the default of "US-ASCII" charset. Many existing User Agents ignore the default of "US-ASCII"
rule for at least text/html and text/xml. rule for at least text/html and text/xml.
This document changes RFC 2046 rules regarding default charset This document changes RFC 2046 rules regarding default charset
parameter values for text/* media types to better align with common parameter values for text/* media types to better align with common
usage by existing clients and servers. It does not change the usage by existing clients and servers. It does not change the
defaults for any currently registered media type. defaults for any currently registered media type.
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The default character set, which must be assumed in the absence of The default character set, which must be assumed in the absence of
a charset parameter, is US-ASCII. a charset parameter, is US-ASCII.
As explained in the Introduction section this rule is considered to As explained in the Introduction section this rule is considered to
be outdated, so this document replaces it with the following set of be outdated, so this document replaces it with the following set of
rules: rules:
Each subtype of the "text" media type which uses the "charset" Each subtype of the "text" media type which uses the "charset"
parameter can define its own default value for the "charset" parameter can define its own default value for the "charset"
parameter, including absence of any default. parameter, including the absence of any default.
In order to improve interoperability with deployed agents, "text/*" In order to improve interoperability with deployed agents, "text/*"
media type registrations SHOULD either media type registrations SHOULD either
a. specify that the "charset" parameter is not used for the defined a. specify that the "charset" parameter is not used for the defined
subtype, because the charset information is transported inside subtype, because the charset information is transported inside
the payload (such as in "text/xml"), or the payload (such as in "text/xml"), or
b. require explicit unconditional inclusion of the "charset" b. require explicit unconditional inclusion of the "charset"
parameter eliminating the need for a default value. parameter eliminating the need for a default value.
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corresponding payloads (such as "text/html" and "text/xml") SHOULD corresponding payloads (such as "text/html" and "text/xml") SHOULD
NOT specify the use of a "charset" parameter, nor any default value, NOT specify the use of a "charset" parameter, nor any default value,
in order to avoid conflicting interpretations should the charset in order to avoid conflicting interpretations should the charset
parameter value and the value specified in the payload disagree. parameter value and the value specified in the payload disagree.
New subtypes of the "text" media type, thus, SHOULD NOT define a New subtypes of the "text" media type, thus, SHOULD NOT define a
default "charset" value. If there is a strong reason to do so default "charset" value. If there is a strong reason to do so
despite this advice, they SHOULD use the "UTF-8" [RFC3629] charset as despite this advice, they SHOULD use the "UTF-8" [RFC3629] charset as
the default. the default.
Specifications of how to specify the "charset" parameter, and what Specifications covering the "charset" parameter, and what default
default value, if any, is used, are subtype-specific, NOT protocol- value, if any, is used, are subtype-specific, NOT protocol-specific.
specific. Protocols that use MIME, therefore, MUST NOT override Protocols that use MIME, therefore, MUST NOT override default charset
default charset values for "text/*" media types to be different for values for "text/*" media types to be different for their specific
their specific protocol. The protocol definitions MUST leave that to protocol. The protocol definitions MUST leave that to the subtype
the subtype definitions. definitions.
4. Default charset parameter value for text/plain media type 4. Default charset parameter value for text/plain media type
The default charset parameter value for text/plain is unchanged from The default charset parameter value for text/plain is unchanged from
[RFC2046] and remains as "US-ASCII". [RFC2046] and remains as "US-ASCII".
5. Security Considerations 5. Security Considerations
Guessing of the charset parameter can lead to security issues such as Guessing of the charset parameter can lead to security issues such as
content buffer overflows, denial of services or bypass of filtering content buffer overflows, denial of services or bypass of filtering
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[RFC2854] Connolly, D. and L. Masinter, "The 'text/html' Media [RFC2854] Connolly, D. and L. Masinter, "The 'text/html' Media
Type", RFC 2854, June 2000. Type", RFC 2854, June 2000.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media [RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001. Types", RFC 3023, January 2001.
Appendix A. Acknowledgements Appendix A. Acknowledgements
Many thanks to Ned Freed and John Klensin for comments and ideas that Many thanks to Ned Freed and John Klensin for comments and ideas that
motivated creation of this document, and to Barry Leiba for suggested motivated creation of this document, and to Carsten Bormann, Murray
text. S. Kucherawy, Barry Leiba, and Henri Sivonen for feedback and text
suggestions.
Authors' Addresses Authors' Addresses
Alexey Melnikov Alexey Melnikov
Isode Limited Isode Limited
5 Castle Business Village 5 Castle Business Village
36 Station Road 36 Station Road
Hampton, Middlesex TW12 2BX Hampton, Middlesex TW12 2BX
UK UK
Email: Alexey.Melnikov@isode.com
Email: Alexey.Melnikov@isode.com
Julian F. Reschke Julian F. Reschke
greenbytes GmbH greenbytes GmbH
Hafenweg 16 Hafenweg 16
Muenster, NW 48155 Muenster, NW 48155
Germany Germany
Email: julian.reschke@greenbytes.de Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/ URI: http://greenbytes.de/tech/webdav/
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