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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: 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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 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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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   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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