This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.

The following 'Verified' errata have been incorporated in this document: EID 476, EID 477, EID 478, EID 590
Network Working Group                                         N. Freed
Request for Comments: 2231                                    Innosoft
Updates: 2045, 2047, 2183                                     K. Moore
Obsoletes: 2184                                University of Tennessee
Category: Standards Track                                November 1997

           MIME Parameter Value and Encoded Word Extensions:
              Character Sets, Languages, and Continuations

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1997).  All Rights Reserved.

1.  Abstract

   This memo defines extensions to the RFC 2045 media type and RFC 2183
   disposition parameter value mechanisms to provide

    (1)   a means to specify parameter values in character sets
          other than US-ASCII,

    (2)   to specify the language to be used should the value be
          displayed, and

    (3)   a continuation mechanism for long parameter values to
          avoid problems with header line wrapping.

   This memo also defines an extension to the encoded words defined in
   RFC 2047 to allow the specification of the language to be used for
   display as well as the character set.

2.  Introduction

   The Multipurpose Internet Mail Extensions, or MIME [RFC-2045, RFC-
   2046, RFC-2047, RFC-2048, RFC-2049], define a message format that
   allows for:

    (1)   textual message bodies in character sets other than

    (2)   non-textual message bodies,

    (3)   multi-part message bodies, and

    (4)   textual header information in character sets other than

   MIME is now widely deployed and is used by a variety of Internet
   protocols, including, of course, Internet email.  However, MIME's
   success has resulted in the need for additional mechanisms that were
   not provided in the original protocol specification.

   In particular, existing MIME mechanisms provide for named media type
   (content-type field) parameters as well as named disposition
   (content-disposition field).  A MIME media type may specify any
   number of parameters associated with all of its subtypes, and any
   specific subtype may specify additional parameters for its own use. A
   MIME disposition value may specify any number of associated
   parameters, the most important of which is probably the attachment
   disposition's filename parameter.

   These parameter names and values end up appearing in the content-type
   and content-disposition header fields in Internet email.  This
   inherently imposes three crucial limitations:

    (1)   Lines in Internet email header fields are folded
          according to RFC 822 folding rules.  This makes long
          parameter values problematic.

    (2)   MIME headers, like the RFC 822 headers they often
          appear in, are limited to 7bit US-ASCII, and the
          encoded-word mechanisms of RFC 2047 are not available
          to parameter values.  This makes it impossible to have
          parameter values in character sets other than US-ASCII
          without specifying some sort of private per-parameter

    (3)   It has recently become clear that character set
          information is not sufficient to properly display some
          sorts of information -- language information is also
          needed [RFC-2130].  For example, support for
          handicapped users may require reading text string

          aloud. The language the text is written in is needed
          for this to be done correctly.  Some parameter values
          may need to be displayed, hence there is a need to
          allow for the inclusion of language information.

   The last problem on this list is also an issue for the encoded words
   defined by RFC 2047, as encoded words are intended primarily for
   display purposes.

   This document defines extensions that address all of these
   limitations. All of these extensions are implemented in a fashion
   that is completely compatible at a syntactic level with existing MIME
   implementations. In addition, the extensions are designed to have as
   little impact as possible on existing uses of MIME.

   IMPORTANT NOTE:  These mechanisms end up being somewhat gibbous when
   they actually are used. As such, these mechanisms should not be used
   lightly; they should be reserved for situations where a real need for
   them exists.

2.1.  Requirements notation

   This document occasionally uses terms that appear in capital letters.
   When the terms "MUST", "SHOULD", "MUST NOT", "SHOULD NOT", and "MAY"
   appear capitalized, they are being used to indicate particular
   requirements of this specification. A discussion of the meanings of
   these terms appears in [RFC- 2119].

3.  Parameter Value Continuations

   Long MIME media type or disposition parameter values do not interact
   well with header line wrapping conventions.  In particular, proper
   header line wrapping depends on there being places where linear
   whitespace (LWSP) is allowed, which may or may not be present in a
   parameter value, and even if present may not be recognizable as such
   since specific knowledge of parameter value syntax may not be
   available to the agent doing the line wrapping. The result is that
   long parameter values may end up getting truncated or otherwise
   damaged by incorrect line wrapping implementations.

   A mechanism is therefore needed to break up parameter values into
   smaller units that are amenable to line wrapping. Any such mechanism
   MUST be compatible with existing MIME processors. This means that

    (1)   the mechanism MUST NOT change the syntax of MIME media
          type and disposition lines, and

    (2)   the mechanism MUST NOT depend on parameter ordering
          since MIME states that parameters are not order
          sensitive.  Note that while MIME does prohibit
          modification of MIME headers during transport, it is
          still possible that parameters will be reordered when
          user agent level processing is done.

   The obvious solution, then, is to use multiple parameters to contain
   a single parameter value and to use some kind of distinguished name
   to indicate when this is being done.  And this obvious solution is
   exactly what is specified here: The asterisk character ("*") followed
   by a decimal count is employed to indicate that multiple parameters
   are being used to encapsulate a single parameter value.  The count
   starts at 0 and increments by 1 for each subsequent section of the
   parameter value.  Decimal values are used and neither leading zeroes
   nor gaps in the sequence are allowed.

   The original parameter value is recovered by concatenating the
   various sections of the parameter, in order.  For example, the
   content-type field

        Content-Type: message/external-body; access-type=URL;

   is semantically identical to

        Content-Type: message/external-body; access-type=URL;

   Note that quotes around parameter values are part of the value
   syntax; they are NOT part of the value itself.  Furthermore, it is
   explicitly permitted to have a mixture of quoted and unquoted
   continuation fields.

4.  Parameter Value Character Set and Language Information

   Some parameter values may need to be qualified with character set or
   language information.  It is clear that a distinguished parameter
   name is needed to identify when this information is present along
   with a specific syntax for the information in the value itself.  In
   addition, a lightweight encoding mechanism is needed to accommodate 8
   bit information in parameter values.

   Asterisks ("*") are reused to provide the indicator that language and
   character set information is present and encoding is being used. A
   single quote ("'") is used to delimit the character set and language
   information at the beginning of the parameter value. Percent signs
   ("%") are used as the encoding flag, which agrees with RFC 2047.

   Specifically, an asterisk at the end of a parameter name acts as an
   indicator that character set and language information may appear at
   the beginning of the parameter value.    A single quote is used to separate the character set, language, and 
   actual value information in the parameter value string, and a
   percent sign is used to flag octets encoded in hexadecimal.
  For example:
EID 476 (Verified) is as follows:

Section: 4

Original Text:

   A single quote is used to separate the character set, language, and
   actual value information in the parameter value string, and an
   percent sign is used to flag octets encoded in hexadecimal.

Corrected Text:

   A single quote is used to separate the character set, language, and
   actual value information in the parameter value string, and a
   percent sign is used to flag octets encoded in hexadecimal.
Content-Type: application/x-stuff; title*=us-ascii'en-us'This%20is%20%2A%2A%2Afun%2A%2A%2A Note that it is perfectly permissible to leave either the character set or language field blank. Note also that the single quote delimiters MUST be present even when one of the field values is omitted. This is done when either character set, language, or both are not relevant to the parameter value at hand. This MUST NOT be done in order to indicate a default character set or language -- parameter field definitions MUST NOT assign a default character set or language. 4.1. Combining Character Set, Language, and Parameter Continuations Character set and language information may be combined with the parameter continuation mechanism. For example: Content-Type: application/x-stuff; title*0*=us-ascii'en'This%20is%20even%20more%20; title*1*=%2A%2A%2Afun%2A%2A%2A%20; title*2="isn't it!"
EID 590 (Verified) is as follows:

Section: 4.1

Original Text:

   Content-Type: application/x-stuff
    title*2="isn't it!"

Corrected Text:

   Content-Type: application/x-stuff;
    title*2="isn't it!"
Note that: (1) Language and character set information only appear at the beginning of a given parameter value. (2) Continuations do not provide a facility for using more than one character set or language in the same parameter value. (3) A value presented using multiple continuations may contain a mixture of encoded and unencoded segments. (4) The first segment of a continuation MUST be encoded if language and character set information are given. (5) If the first segment of a continued parameter value is encoded the language and character set field delimiters MUST be present even when the fields are left blank. 5. Language specification in Encoded Words RFC 2047 provides support for non-US-ASCII character sets in RFC 822 message header comments, phrases, and any unstructured text field. This is done by defining an encoded word construct which can appear in any of these places. Given that these are fields intended for display, it is sometimes necessary to associate language information with encoded words as well as just the character set. This specification extends the definition of an encoded word to allow the inclusion of such information. This is simply done by suffixing the character set specification with an asterisk followed by the language tag. For example: From: =?US-ASCII*EN?Q?Keith_Moore?= <> 6. IMAP4 Handling of Parameter Values IMAP4 [RFC-2060] servers SHOULD decode parameter value continuations when generating the BODY and BODYSTRUCTURE fetch attributes. 7. Modifications to MIME ABNF The ABNF for MIME parameter values given in RFC 2045 is: parameter := attribute "=" value attribute := token ; Matching of attributes ; is ALWAYS case-insensitive. This specification changes this ABNF to: parameter := regular-parameter / extended-parameter regular-parameter := regular-parameter-name "=" value regular-parameter-name := attribute [section] attribute := 1*attribute-char attribute-char := <any (US-ASCII) CHAR except SPACE, CTLs, "*", "'", "%", or tspecials> section := initial-section / other-sections initial-section := "*0" other-sections := "*" ("1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9") *DIGIT) extended-parameter := (extended-initial-name "=" extended-initial-value) / (extended-other-names "=" extended-other-values)
EID 477 (Verified) is as follows:

Section: 7

Original Text:

    extended-parameter := (extended-initial-name "="
                          extended-value) /
                         (extended-other-names "="

Corrected Text:

    extended-parameter := (extended-initial-name "="
                          extended-initial-value) /
                         (extended-other-names "="

extended-initial-name := attribute [initial-section] "*" extended-other-names := attribute other-sections "*" extended-initial-value := [charset] "'" [language] "'" extended-other-values extended-other-values := *(ext-octet / attribute-char) ext-octet := "%" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F") charset := <registered character set name> language := <registered language tag [RFC-1766]> The ABNF given in RFC 2047 for encoded-words is: encoded-word := "=?" charset "?" encoding "?" encoded-text "?=" This specification changes this ABNF to: encoded-word := "=?" charset ["*" language] "?" encoding "?" encoded-text "?="
EID 478 (Verified) is as follows:

Section: 7

Original Text:

   encoded-word := "=?" charset ["*" language] "?" encoded-text "?="

Corrected Text:

   encoded-word := "=?" charset ["*" language] "?" encoding "?"
                   encoded-text "?="
8. Character sets which allow specification of language In the future it is likely that some character sets will provide facilities for inline language labeling. Such facilities are inherently more flexible than those defined here as they allow for language switching in the middle of a string. If and when such facilities are developed they SHOULD be used in preference to the language labeling facilities specified here. Note that all the mechanisms defined here allow for the omission of language labels so as to be able to accommodate this possible future usage. 9. Security Considerations This RFC does not discuss security issues and is not believed to raise any security issues not already endemic in electronic mail and present in fully conforming implementations of MIME. 10. References [RFC-822] Crocker, D., "Standard for the Format of ARPA Internet Text Messages", STD 11, RFC 822 August 1982. [RFC-1766] Alvestrand, H., "Tags for the Identification of Languages", RFC 1766, March 1995. [RFC-2045] Freed, N., and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, December 1996. [RFC-2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, December 1996. [RFC-2047] Moore, K., "Multipurpose Internet Mail Extensions (MIME) Part Three: Representation of Non-ASCII Text in Internet Message Headers", RFC 2047, December 1996. [RFC-2048] Freed, N., Klensin, J. and J. Postel, "Multipurpose Internet Mail Extensions (MIME) Part Four: MIME Registration Procedures", RFC 2048, December 1996. [RFC-2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples", RFC 2049, December 1996. [RFC-2060] Crispin, M., "Internet Message Access Protocol - Version 4rev1", RFC 2060, December 1996. [RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [RFC-2130] Weider, C., Preston, C., Simonsen, K., Alvestrand, H., Atkinson, R., Crispin, M., and P. Svanberg, "Report from the IAB Character Set Workshop", RFC 2130, April 1997. [RFC-2183] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation Information in Internet Messages: The Content-Disposition Header", RFC 2183, August 1997. 11. Authors' Addresses Ned Freed Innosoft International, Inc. 1050 Lakes Drive West Covina, CA 91790 USA Phone: +1 626 919 3600 Fax: +1 626 919 3614 EMail: Keith Moore Computer Science Dept. University of Tennessee 107 Ayres Hall Knoxville, TN 37996-1301 USA EMail: 12. Full Copyright Statement Copyright (C) The Internet Society (1997). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.