Internet DRAFT - draft-ietf-appsawg-mime-default-charset
draft-ietf-appsawg-mime-default-charset
Applications Area Working Group A. Melnikov
Internet-Draft Isode Limited
Updates: 2046 (if approved) J. Reschke
Intended status: Standards Track greenbytes
Expires: November 10, 2012 May 9, 2012
Update to MIME regarding Charset Parameter Handling in
Textual Media Types
draft-ietf-appsawg-mime-default-charset-04
Abstract
This document changes RFC 2046 rules regarding default charset
parameter values for text/* media types to better align with common
usage by existing clients and servers.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft should take place on the Apps Area Working
Group mailing list (apps-discuss@ietf.org), which is archived at
<http://www.ietf.org/mail-archive/web/apps-discuss>.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 10, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
Table of Contents
1. Introduction and Overview . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . 3
3. New rules for default charset parameter values for
text/* media types . . . . . . . . . . . . . . . . . . 3
4. Default charset parameter value for text/plain
media type . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . 6
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1. Introduction and Overview
RFC 2046 specified that the default charset parameter (i.e. the value
used when the parameter is not specified) is "US-ASCII" (Section
4.1.2 of [RFC2046]). RFC 2616 changed the default for use by HTTP
(Hypertext Transfer Protocol) to be "ISO-8859-1" (Section 3.7.1 of
[RFC2616]). This encoding is not very common for new text/* media
types and a special rule in the HTTP specification adds confusion
about which specification ([RFC2046] or [RFC2616]) is authoritative
in regards to the default charset for text/* media types.
Many complex text subtypes such as text/html [RFC2854] and text/xml
[RFC3023] have internal (to their format) means of describing the
charset. Many existing User Agents ignore the default of "US-ASCII"
rule for at least text/html and text/xml.
This document changes RFC 2046 rules regarding default charset
parameter values for text/* media types to better align with common
usage by existing clients and servers. It does not change the
defaults for any currently registered media type.
2. Conventions Used in This Document
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].
3. New rules for default charset parameter values for text/* media
types
Section 4.1.2 of [RFC2046] says:
The default character set, which must be assumed in the absence of
a charset parameter, is US-ASCII.
As explained in the Introduction section this rule is considered to
be outdated, so this document replaces it with the following set of
rules:
Each subtype of the "text" media type which uses the "charset"
parameter can define its own default value for the "charset"
parameter, including the absence of any default.
In order to improve interoperability with deployed agents, "text/*"
media type registrations SHOULD either
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a. specify that the "charset" parameter is not used for the defined
subtype, because the charset information is transported inside
the payload (such as in "text/xml"), or
b. require explicit unconditional inclusion of the "charset"
parameter eliminating the need for a default value.
In accordance with option (a), above, registrations for "text/*"
media types that can transport charset information inside the
corresponding payloads (such as "text/html" and "text/xml") SHOULD
NOT specify the use of a "charset" parameter, nor any default value,
in order to avoid conflicting interpretations should the charset
parameter value and the value specified in the payload disagree.
New subtypes of the "text" media type, thus, SHOULD NOT define a
default "charset" value. If there is a strong reason to do so
despite this advice, they SHOULD use the "UTF-8" [RFC3629] charset as
the default.
Regardless of what approach is chosen, all new text/* registrations
MUST clearly specify how the charset is determined; relying on the
default defined in Section 4.1.2 of [RFC2046] is no longer permitted.
However, existing text/* registrations that fail to specify how the
charset is determined still default to US-ASCII.
Specifications covering the "charset" parameter, and what default
value, if any, is used, are subtype-specific, NOT protocol-specific.
Protocols that use MIME, therefore, MUST NOT override default charset
values for "text/*" media types to be different for their specific
protocol. The protocol definitions MUST leave that to the subtype
definitions.
4. Default charset parameter value for text/plain media type
The default charset parameter value for text/plain is unchanged from
[RFC2046] and remains as "US-ASCII".
5. Security Considerations
Guessing of the charset parameter can lead to security issues such as
content buffer overflows, denial of services or bypass of filtering
mechanisms. However, this document does not promote guessing, but
encourages use of charset information that is specified by the
sender.
Conflicting information in-band vs out-of-band can also lead to
similar security problems, and this document recommends the use of
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charset information which is more likely to be correct (for example,
in-band over out-of-band).
6. IANA Considerations
This document asks IANA to update the "text" subregistry of the Media
Types registry (<http://www.iana.org/assignments/media-types/text/>),
to add the following preamble: "See [this RFC] for information about
'charset' parameter handling for text media types."
IANA is also asked to add this RFC to the list of references at the
beginning of the Application for Media Type
(<http://www.iana.org/cgi-bin/mediatypes.pl>).
7. References
7.1. Normative References
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
7.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2854] Connolly, D. and L. Masinter, "The 'text/html' Media
Type", RFC 2854, June 2000.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
Appendix A. Acknowledgements
Many thanks to Ned Freed and John Klensin for comments and ideas that
motivated creation of this document, and to Carsten Bormann, Murray
S. Kucherawy, Barry Leiba, and Henri Sivonen for feedback and text
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suggestions.
Authors' Addresses
Alexey Melnikov
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
Email: Alexey.Melnikov@isode.com
Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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