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Network Working Group                                         N. FreedRequest for Comments: 2184                                    InnosoftUpdates:2045,2047,2183                                     K. MooreCategory: Standards Track                      University of Tennessee                                                           August 1997MIME Parameter Value and Encoded Word Extensions:Character Sets, Languages, and ContinuationsStatus 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 theRFC 2045 media type andRFC 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 inRFC 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, andFreed & 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          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 toRFC 822 folding rules.  This makes long parameter values          problematic.    (2)   MIME headers, like theRFC 822 headers they often appear in,          are limited to 7bit US-ASCII, and the encoded-word mechanisms          ofRFC 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 byRFC 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;      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 withRFC 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;      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.Freed & Moore               Standards Track                     [Page 5]

RFC 2184    MIME Parameter Value and Encoded Word Extensions August 19975.  Language specification in Encoded WordsRFC 2047 provides support for non-US-ASCII character sets inRFC 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 inRFC 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-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 inRFC 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.Freed & Moore               Standards Track                     [Page 7]

RFC 2184    MIME Parameter Value and Encoded Word Extensions August 19979.  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",RFC1766, 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",RFC2048, 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.Freed & Moore               Standards Track                     [Page 8]

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.eduFreed & Moore               Standards Track                     [Page 9]