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

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
    (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.

   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.

   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.

   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.

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"
   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.

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.

   [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