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Internet Engineering Task Force (IETF)                       A. MelnikovRequest for Comments: 6657                                 Isode LimitedUpdates:2046                                                 J. ReschkeCategory: Standards Track                                     greenbytesISSN: 2070-1721                                                July 2012Update to MIME regarding "charset" Parameter Handlingin Textual Media TypesAbstract   This document changesRFC 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 2012Table of Contents1. Introduction and Overview .......................................22. Conventions Used in This Document ...............................2   3. New Rules for Default "charset" Parameter Values for      "text/*" Media Types ............................................34. Default "charset" Parameter Value for "text/plain" Media Type ...45. Security Considerations .........................................46. IANA Considerations .............................................47. References ......................................................47.1. Normative References .......................................47.2. Informative References .....................................5Appendix A.  Acknowledgements ......................................61.  Introduction and OverviewRFC 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" (Section3.7.1 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 changesRFC 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 20123.  New Rules for Default "charset" Parameter Values for "text/*" Media    TypesSection 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 inSection 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 charsetMelnikov & Reschke           Standards Track                    [Page 3]

RFC 6657               MIME Charset Default Update             July 2012   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.  References7.1.  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.Melnikov & Reschke           Standards Track                    [Page 4]

RFC 6657               MIME Charset Default Update             July 20127.2.  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.Melnikov & Reschke           Standards Track                    [Page 5]

RFC 6657               MIME Charset Default Update             July 2012Appendix 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: 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/Melnikov & Reschke           Standards Track                    [Page 6]