Update to MIME regarding "charset" Parameter Handling in Textual Media Types
RFC 6657

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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.

Melnikov & Reschke           Standards Track                    [Page 1]
RFC 6657               MIME Charset Default Update             July 2012

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].

Melnikov & Reschke           Standards Track                    [Page 2]
RFC 6657               MIME Charset Default Update             July 2012

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.