RFC2184 - MIME Parameter Value and Encoded Word Extensions: Character Sets, Lang

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　　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 RFC2045 media type and RFC2183
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
RFC2047 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
RFC822 folding rules. This makes long parameter values
problematic.

(2) MIME headers, like the RFC822 headers they often appear in,
are limited to 7bit US-ASCII, and the encoded-word mechanisms
of RFC2047 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 RFC2047, 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 RFC2047.

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

RFC2047 provides support for non-US-ASCII character sets in RFC822
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 RFC2045 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 RFC2047 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 RFCdoes 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, RFC822, 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",
RFC2045, Innosoft, First Virtual Holdings, December 1996.

[RFC-2046]
Freed, N. and Borenstein, N., "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC2046, 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", RFC2047, 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",
RFC2049, Innosoft, FIrst Virtual Holdings, December 1996.

[RFC-2060]
Crispin, M., "Internet Message Access Protocol - Version 4rev1",
RFC2060, December 1996.

[RFC-2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC2119, 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", RFC2130, April 1997.

[RFC-2183]
Troost, R., Dorner, S., and Moore, K., "Communicating Presentation
Information in Internet Messages: The Content-Disposition
Header", RFC2183, 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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