Network Working Group Ned Freed, Innosoft Internet Draft Keith Moore, Univ. of Tenn MIME Parameter Value and Encoded Words: Character Sets, Language, and Continuations October 1996 Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet-Drafts as reference material or to cite them other than as a "working draft" or "work in progress". To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet- Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). 1. Abstract This memo defines extensions to the RFC MIME-IMB media type and RFC 1806 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 Internet Draft MIME Parameters and Encoded Words October 1996 (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 MIME-HEADERS 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-MIME- IMB, RFC-MIME-IMT, RFC-MIME-HEADERS, RFC-MIME-REG, RFC-MIME- CONF], 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 paramters, 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: Expires April 1997 [Page 2] Internet Draft MIME Parameters and Encoded Words October 1996 (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 inherently limited to 7bit US-ASCII, and the encoded-word mechanisms of RFC MIME-HEADERS 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-IAB-CHARSETS]. 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 MIME-HEADERS, 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. 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 Expires April 1997 [Page 3] Internet Draft MIME Parameters and Encoded Words October 1996 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. Expires April 1997 [Page 4] Internet Draft MIME Parameters and Encoded Words October 1996 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. A single quote ("'") is used to delimit the character set and language information. Percent signs ("%") are used as the encoding flag, which agrees with RFC 1522. 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'This%20is%20%42%42%42fun%42%42%42 Note that it is perfectly permissible to leave either the character set or language field blank. 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 fields may not assign a default character set or lanugage. 5. Language specification in Encoded Words RFC MIME-HEADERS 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: Expires April 1997 [Page 5] Internet Draft MIME Parameters and Encoded Words October 1996 From: =?US-ASCII*EN?Q?Keith_Moore?= 6. Modifications to the ABNF for MIME Parameter Values and Encoded Words The ABNF for MIME parameter values given in RFC MIME-IMB 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 := section := "*" ("1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9") *DIGIT extended-parameter := extended-parameter-name "=" extended-value extended-parameter-name := attribute [section] "*" extended-value := [charset] "'" [language] "'" *(ext-octet / attribute-char) ext-octet := "%" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F") charset := language := The ABNF given in RFC MIME-HEADERS for encoded-words is: Expires April 1997 [Page 6] Internet Draft MIME Parameters and Encoded Words October 1996 encoded-word := "=?" charset "?" encoding "?" encoded-text "?=" This specification changes this ABNF to: encoded-word := "=?" charset ["*" language] "?" encoded-text "?=" 7. 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. 8. References [RFC-822] Crocker, D., "Standard for the Format of ARPA Internet Text Messages", RFC 822 August, 1982. [RFC-1766] Alvestrand, H., "Tags for the Identification of Languages", RFC 1766, March, 1995. [RFC-1806] Troost, R. and Dorner, S., "Communicating Presentation Information in Internet Messages: The Content- Disposition Header", RFC 1806, June 1995. [RFC-MIME-IMB] Freed, N. and Borenstein, N., "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC MIME-IMB, Innosoft, FIrst Virtual Holdings, June 1996. [RFC-MIME-IMT] Freed, N. and Borenstein, N., "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC MIME-IMT, Innosoft, First Virtual Holdings, June 1996. Expires April 1997 [Page 7] Internet Draft MIME Parameters and Encoded Words October 1996 [RFC-MIME-HEADERS] Moore, K., "Multipurpose Internet Mail Extensions (MIME) Part Three: Representation of Non-ASCII Text in Internet Message Headers", RFC MIME-HEADERS, University of Tennessee, June 1996. [RFC-MIME-REG] Freed, N., Klensin, J., Postel, J., "Multipurpose Internet Mail Extensions (MIME) Part Four: MIME Registration Procedures", RFC MIME-REG, Innosoft, MCI, ISI, June 1996. [RFC-MIME-CONF] Freed, N. and Borenstein, N., "Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples", RFC MIME-CONF, Innosoft, FIrst Virtual Holdings, June 1996. [RFC-CHARSET-REG] Freed, N. and Postel, J., "IANA Character Set Registration Procedures, Innosoft, ISI, June 1996. [RFC-IAB-CHARSETS] Weider, C., Preston, C., Simonsen, K., Alvestrand, H., Atkinson, R., Crispin, M., Svanberg, P., "Report from the IAB Character Set Workshop", Version 2.0, June 1996. 9. 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 Expires April 1997 [Page 8]