











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:











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    (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













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          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











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    (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 accommodate 8

   bit information in parameter values.

















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   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 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!"



   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.











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    (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











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   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-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 labeling. Such facilities are

   inherently more flexible than those defined here as they allow for

   language switching in the middle of a string.















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   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.











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   [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: ned.freed@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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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.

















































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