    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
              US-ASCII,

        (2)   non-textual message bodies,

        (3)   multi-part message bodies, and

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    RFC 2184    MIME Parameter Value and Encoded Word Extensions August 1997

        (4)   textual header information in character sets other than
              US-ASCII.

       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 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.

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       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.

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       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;
          URL*0="ftp://";
          URL*1="cs.utk.edu/pub/moore/bulk-mailer/bulk-mailer.tar"

       is semantically identical to

         Content-Type: message/external-body; access-type=URL;
          URL="ftp://cs.utk.edu/pub/moore/bulk-mailer/bulk-mailer.tar"

       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.

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       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;
          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 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*1*=us-ascii'en'This%20is%20even%20more%20
        title*2*=%2A%2A%2Afun%2A%2A%2A%20
        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
              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.

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    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?= <moore@cs.utk.edu>

    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"

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       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-other-values)

       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] "?" 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
       usage.

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    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 Borenstein, N., "Multipurpose Internet Mail
          Extensions (MIME) Part One: Format of Internet Message Bodies",
          RFC 2045, Innosoft, First Virtual Holdings, December 1996.

       [RFC-2046]
          Freed, N. and Borenstein, N., "Multipurpose Internet Mail
          Extensions (MIME) Part Two: Media Types", RFC 2046, Innosoft,
          First Virtual Holdings, December 1996.

       [RFC-2047]
          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.

       [RFC-2048]
          Freed, N., Klensin, J., Postel, J., "Multipurpose Internet Mail
          Extensions (MIME) Part Four: MIME Registration Procedures", RFC
          2048, Innosoft, MCI, ISI, December 1996.

       [RFC-2049]
          Freed, N. and Borenstein, N., "Multipurpose Internet Mail
          Extensions (MIME) Part Five: Conformance Criteria and Examples",
          RFC 2049, Innosoft, FIrst Virtual Holdings, 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.

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    RFC 2184    MIME Parameter Value and Encoded Word Extensions August 1997

       [RFC-2130]
          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.

       [RFC-2183]
          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
       USA
        tel: +1 818 919 3600           fax: +1 818 919 3614
        email: ned@innosoft.com

       Keith Moore
       Computer Science Dept.
       University of Tennessee
       107 Ayres Hall
       Knoxville, TN 37996-1301
       USA
        email: moore@cs.utk.edu

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