RFC6657

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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.

3. New Rules for Default "charset" Parameter Values for

4. Default "charset" Parameter Value for "text/plain" Media Type ...4

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.

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.

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

values for "text/*" media types to be different for their specific protocol. The protocol definitions MUST leave that to the subtype definitions.

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".

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).

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>).

References

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.

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 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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