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Versions: 00 01 draft-ietf-appsawg-mime-default-charset

Network Working Group                                        A. Melnikov
Internet-Draft                                             Isode Limited
Updates: 2046 (if approved)                                   J. Reschke
Intended status: Standards Track                              greenbytes
Expires: December 16, 2011                                 June 14, 2011


  Update to MIME regarding Charset Parameter Handling in Textual Media
                                 Types
                 draft-melnikov-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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 16, 2011.

Copyright Notice

   Copyright (c) 2011 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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   the Trust Legal Provisions and are provided without warranty as
   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 and text/xml 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) recommend no default charset
   parameter value (i.e. the charset information is transport inside the
   payload, for example as in "text/xml") or b) require explicit
   unconditional inclusion of the charset parameter with the default
   value. "text/*" media types that can transport charset information
   inside the corresponding payloads SHOULD NOT specify any default, in
   order to avoid conflicting instructions if the charset parameter
   value and the value specified in the payload don't agree.

   New subtypes of the "text" media type that do define a default
   charset SHOULD use the "UTF-8" [RFC3629] charset as the default.

   Protocols using MIME MUST NOT override default charset values for
   "text/*" media types to be different for their specific protocol.


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

   [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.



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7.2.  Informative References

   [HTML]     Le Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01
              Specification", W3C Recommendation REC-html401-19991224,
              December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

              Latest version available at
              <http://www.w3.org/TR/html401>.

   [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.

   [XML]      Sperberg-McQueen, C., Maler, E., Yergeau, F., Paoli, J.,
              and T. Bray, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", W3C Recommendation REC-xml-20081126,
              November 2008,
              <http://www.w3.org/TR/2008/REC-xml-20081126/>.

              Latest version available at <http://www.w3.org/TR/xml>.


Appendix A.  Acknowledgements

   Many thanks to Ned Freed and John Klensin for comments and ideas that
   motivated creation of this document.


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



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