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Network Working Group N. Freed Request for Comments: 2184 Innosoft Updates: 2045, 2047, 2183 K. Moore Category: Standards Track University of Tennessee

                                                         August 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.

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

Freed & Moore Standards Track [Page 1] RFC 2184 MIME Parameter Value and Encoded Word Extensions August 1997

  (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
  (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 encoding.
  (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.

Freed & Moore Standards Track [Page 2] RFC 2184 MIME Parameter Value and Encoded Word Extensions August 1997

 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, use of 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.

Freed & Moore Standards Track [Page 3] RFC 2184 MIME Parameter Value and Encoded Word Extensions August 1997

 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 accomodate 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.

Freed & Moore Standards Track [Page 4] RFC 2184 MIME Parameter Value and Encoded Word Extensions August 1997

 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 an percent sign is used to flag
 octets encoded in hexadecimal.  For example:
   Content-Type: application/x-stuff;
 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 lanugage.

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*3="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
  (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.

Freed & Moore Standards Track [Page 5] RFC 2184 MIME Parameter Value and Encoded Word Extensions August 1997

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 := "*1"

Freed & Moore Standards Track [Page 6] RFC 2184 MIME Parameter Value and Encoded Word Extensions August 1997

 other-sections := "*" (("2" / "3" / "4" / "5" /
                         "6" / "7" / "8" / "9") *DIGIT) /
                        ("1" 1*DIGIT))
 extended-parameter := (extended-initial-name "="
                        extended-value) /
                       (extended-other-names "="
 extended-initial-name := attribute [initial-section] "*"
 extended-other-names := attribute other-sections "*"
 extended-initial-value := [charset] "'" [language] "'"
 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] "?" 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 labelling. 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 labelling facilities specified here. Note
 that all the mechanisms defined here allow for the omission of
 language labels so as to be able to accomodate this possible future

Freed & Moore Standards Track [Page 7] RFC 2184 MIME Parameter Value and Encoded Word Extensions August 1997

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

    Crocker, D., "Standard for the Format of ARPA Internet Text
    Messages", STD 11, RFC 822, August 1982.
    Alvestrand, H., "Tags for the Identification of Languages", RFC
    1766, March 1995.
    Freed, N. and Borenstein, N., "Multipurpose Internet Mail
    Extensions (MIME) Part One: Format of Internet Message Bodies",
    RFC 2045, Innosoft, First Virtual Holdings, December 1996.
    Freed, N. and Borenstein, N., "Multipurpose Internet Mail
    Extensions (MIME) Part Two: Media Types", RFC 2046, Innosoft,
    First Virtual Holdings, December 1996.
    Moore, K., "Multipurpose Internet Mail Extensions (MIME) Part
    Three: Representation of Non-ASCII Text in Internet Message
    Headers", RFC 2047, University of Tennessee, December 1996.
    Freed, N., Klensin, J., Postel, J., "Multipurpose Internet Mail
    Extensions (MIME) Part Four: MIME Registration Procedures", RFC
    2048, Innosoft, MCI, ISI, December 1996.
    Freed, N. and Borenstein, N., "Multipurpose Internet Mail
    Extensions (MIME) Part Five: Conformance Criteria and Examples",
    RFC 2049, Innosoft, FIrst Virtual Holdings, December 1996.
    Crispin, M., "Internet Message Access Protocol - Version 4rev1",
    RFC 2060, December 1996.
    Bradner, S., "Key words for use in RFCs to Indicate Requirement
    Levels", RFC 2119, March 1997.

Freed & Moore Standards Track [Page 8] RFC 2184 MIME Parameter Value and Encoded Word Extensions August 1997

    Weider, C., Preston, C., Simonsen, K., Alvestrand, H., Atkinson,
    R., Crispin, M., Svanberg, P., "Report from the IAB Character Set
    Workshop", RFC 2130, April 1997.
    Troost, R., Dorner, S., and Moore, K., "Communicating Presentation
    Information in Internet Messages:  The Content-Disposition
    Header", RFC 2183, August 1997.

11. Authors' Addresses

 Ned Freed
 Innosoft International, Inc.
 1050 East Garvey Avenue South
 West Covina, CA 91790
  tel: +1 818 919 3600           fax: +1 818 919 3614
 Keith Moore
 Computer Science Dept.
 University of Tennessee
 107 Ayres Hall
 Knoxville, TN 37996-1301

Freed & Moore Standards Track [Page 9]

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