Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 6657                                 Isode Limited
Updates: 2046                                                 J. Reschke
Category: Standards Track                                     greenbytes
ISSN: 2070-1721                                                July 2012


        Update to MIME regarding "charset" Parameter Handling
                        in Textual Media Types

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 is an Internet Standards Track document.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Further information on
  Internet Standards is available in Section 2 of RFC 5741.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  http://www.rfc-editor.org/info/rfc6657.

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
  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.







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Table of Contents

  1. Introduction and Overview .......................................2
  2. Conventions Used in This Document ...............................2
  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 ......................................6

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].










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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
  outdated, so this document replaces it with the following set of
  rules:

  Each subtype of the "text" media type that 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

  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.

  Thus, new subtypes of the "text" media type 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




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  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
  charset information that is more likely to be correct (for example,
  in-band over out-of-band).

6.  IANA Considerations

  IANA has updated the "text" subregistry of the Media Types registry
  (<http://www.iana.org/assignments/media-types/text/>) to add the
  following preamble: "See [RFC6657] for information about 'charset'
  parameter handling for text media types."

  Also, IANA has added this RFC to the list of references at the
  beginning of the Application for Media Type
  (<http://www.iana.org/form/media-types>).

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

  [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.








































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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
  suggestions.

Authors' Addresses

  Alexey Melnikov
  Isode Limited
  5 Castle Business Village
  36 Station Road
  Hampton, Middlesex  TW12 2BX
  UK

  EMail: [email protected]


  Julian F. Reschke
  greenbytes GmbH
  Hafenweg 16
  Muenster, NW  48155
  Germany

  EMail: [email protected]
  URI:   http://greenbytes.de/tech/webdav/
























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