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Versions: (draft-melnikov-mime-default-charset)
00 01 02 03 04 RFC 6657
Applications Area Working Group A. Melnikov
Internet-Draft Isode Limited
Updates: 2046 (if approved) J. Reschke
Intended status: Standards Track greenbytes
Expires: August 4, 2012 February 1, 2012
Update to MIME regarding Charset Parameter Handling in
Textual Media Types
draft-ietf-appsawg-mime-default-charset-00
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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 4, 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
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described in the Simplified BSD License.
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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 . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . 4
7.1. Normative References . . . . . . . . . . . . . . . . . 4
7.2. Informative References . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . 5
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1. Introduction and Overview
[RFC2046] specified that the default charset parameter (i.e. the
value used when it is not specified) is "US-ASCII". [RFC2616]
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
rule in HTTP adds confusion about which specification ([RFC2046] or
[RFC2616]) is authoritative in regards to the default charset for
text/* media types. [[anchor2: At the time of writing of this
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
[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.
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 absence of any default.
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In order to improve interoperability with deployed agents, "text/*"
media type definitions SHOULD either a) specify that the "charset"
parameter is not used for the defined subtype, because the charset
information is transported inside the payload (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, "text/*" media types that can transport
charset information inside the corresponding payloads, specifically
including "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.
Specifications of how to specify 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
TBD. Guessing of default charset is a security problem. Conflicting
information in-band vs out-of-band is also a security problem.
6. IANA Considerations
This document asks IANA to update the "text" subregistry of the Media
Types registry to additionally point to this document.
7. References
7.1. Normative References
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[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 Barry Leiba for suggested
text.
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
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Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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