As per Relevance of the word parameter, we have this rfc below:
Network Working Group N.
Request for Comments: 2231
Updates: 2045, 2047, 2183 K.
Obsoletes: 2184 University of
Category: Standards Track November 1997
MIME Parameter Value and Encoded Word Extensions
Character Sets, Languages, and
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (1997). All Rights Reserved
1.
This memo defines extensions to the RFC 2045 media type and RFC 2183
disposition parameter value mechanisms to
(1) a means to specify parameter values in character
other than US-ASCII
(2) to specify the language to be used should the value
displayed,
(3) a continuation mechanism for long parameter values
avoid problems with header line wrapping
This memo also defines an extension to the encoded words defined
RFC 2047 to allow the specification of the language to be used
display as well as the character set
2.
The Multipurpose Internet Mail Extensions, or MIME [RFC-2045, RFC
2046, RFC-2047, RFC-2048, RFC-2049], define a message format
allows for
Freed & Moore Standards Track [Page 1]
RFC 2231 MIME Value and Encoded Word Extensions November 1997
(1) textual message bodies in character sets other
US-ASCII
(2) non-textual message bodies
(3) multi-part message bodies,
(4) textual header information in character sets other
US-ASCII
MIME is now widely deployed and is used by a variety of
protocols, including, of course, Internet email. However, MIME'
success has resulted in the need for additional mechanisms that
not provided in the original protocol specification
In particular, existing MIME mechanisms provide for named media
(content-type field) parameters as well as named
(content-disposition field). A MIME media type may specify
number of parameters associated with all of its subtypes, and
specific subtype may specify additional parameters for its own use.
MIME disposition value may specify any number of
parameters, the most important of which is probably the
disposition's filename parameter
These parameter names and values end up appearing in the content-
and content-disposition header fields in Internet email.
inherently imposes three crucial limitations
(1) Lines in Internet email header fields are
according to RFC 822 folding rules. This makes
parameter values problematic
(2) MIME headers, like the RFC 822 headers they
appear in, are limited to 7bit US-ASCII, and
encoded-word mechanisms of RFC 2047 are not
to parameter values. This makes it impossible to
parameter values in character sets other than US-
without specifying some sort of private per-
encoding
(3) It has recently become clear that character
information is not sufficient to properly display
sorts of information -- language information is
needed [RFC-2130]. For example, support
handicapped users may require reading text
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RFC 2231 MIME Value and Encoded Word Extensions November 1997
aloud. The language the text is written in is
for this to be done correctly. Some parameter
may need to be displayed, hence there is a need
allow for the inclusion of language information
The last problem on this list is also an issue for the encoded
defined by RFC 2047, as encoded words are intended primarily
display purposes
This document defines extensions that address all of
limitations. All of these extensions are implemented in a
that is completely compatible at a syntactic level with existing
implementations. In addition, the extensions are designed to have
little impact as possible on existing uses of MIME
IMPORTANT NOTE: These mechanisms end up being somewhat gibbous
they actually are used. As such, these mechanisms should not be
lightly; they should be reserved for situations where a real need
them exists
2.1. Requirements
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
requirements of this specification. A discussion of the meanings
these terms appears in [RFC- 2119].
3. Parameter Value
Long MIME media type or disposition parameter values do not
well with header line wrapping conventions. In particular,
header line wrapping depends on there being places where
whitespace (LWSP) is allowed, which may or may not be present in
parameter value, and even if present may not be recognizable as
since specific knowledge of parameter value syntax may not
available to the agent doing the line wrapping. The result is
long parameter values may end up getting truncated or
damaged by incorrect line wrapping implementations
A mechanism is therefore needed to break up parameter values
smaller units that are amenable to line wrapping. Any such
MUST be compatible with existing MIME processors. This means
(1) the mechanism MUST NOT change the syntax of MIME
type and disposition lines,
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RFC 2231 MIME Value and Encoded Word Extensions November 1997
(2) the mechanism MUST NOT depend on parameter
since MIME states that parameters are not
sensitive. Note that while MIME does
modification of MIME headers during transport, it
still possible that parameters will be reordered
user agent level processing is done
The obvious solution, then, is to use multiple parameters to
a single parameter value and to use some kind of distinguished
to indicate when this is being done. And this obvious solution
exactly what is specified here: The asterisk character ("*")
by a decimal count is employed to indicate that multiple
are being used to encapsulate a single parameter value. The
starts at 0 and increments by 1 for each subsequent section of
parameter value. Decimal values are used and neither leading
nor gaps in the sequence are allowed
The original parameter value is recovered by concatenating
various sections of the parameter, in order. For example,
content-type
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
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
syntax; they are NOT part of the value itself. Furthermore, it
explicitly permitted to have a mixture of quoted and
continuation fields
4. Parameter Value Character Set and Language
Some parameter values may need to be qualified with character set
language information. It is clear that a distinguished
name is needed to identify when this information is present
with a specific syntax for the information in the value itself.
addition, a lightweight encoding mechanism is needed to accommodate 8
bit information in parameter values
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RFC 2231 MIME Value and Encoded Word Extensions November 1997
Asterisks ("*") are reused to provide the indicator that language
character set information is present and encoding is being used.
single quote ("'") is used to delimit the character set and
information at the beginning of the parameter value. Percent
("%") are used as the encoding flag, which agrees with RFC 2047.
Specifically, an asterisk at the end of a parameter name acts as
indicator that character set and language information may appear
the beginning of the parameter value. A single quote is used
separate the character set, language, and actual value information
the parameter value string, and an percent sign is used to
octets encoded in hexadecimal. For example
Content-Type: application/x-stuff
title*=us-ascii'en-us'This%20is%20%2A%2A%2Afun%2A%2A%2
Note that it is perfectly permissible to leave either the
set or language field blank. Note also that the single
delimiters MUST be present even when one of the field values
omitted. This is done when either character set, language, or
are not relevant to the parameter value at hand. This MUST NOT
done in order to indicate a default character set or language --
parameter field definitions MUST NOT assign a default character
or language
4.1. Combining Character Set, Language, and Parameter
Character set and language information may be combined with
parameter continuation mechanism. For example
Content-Type: application/x-
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
the beginning of a given parameter value
(2) Continuations do not provide a facility for using
than one character set or language in the
parameter value
(3) A value presented using multiple continuations
contain a mixture of encoded and unencoded segments
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RFC 2231 MIME Value and Encoded Word Extensions November 1997
(4) The first segment of a continuation MUST be encoded
language and character set information are given
(5) If the first segment of a continued parameter value
encoded the language and character set field
MUST be present even when the fields are left blank
5. Language specification in Encoded
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
in any of these places. Given that these are fields intended
display, it is sometimes necessary to associate language
with encoded words as well as just the character set.
specification extends the definition of an encoded word to allow
inclusion of such information. This is simply done by suffixing
character set specification with an asterisk followed by the
tag. For example
From: =?US-ASCII*EN?Q?Keith_Moore?=
6. IMAP4 Handling of Parameter
IMAP4 [RFC-2060] servers SHOULD decode parameter value
when generating the BODY and BODYSTRUCTURE fetch attributes
7. Modifications to MIME
The ABNF for MIME parameter values given in RFC 2045 is
parameter := attribute "="
attribute :=
; Matching of
; is ALWAYS case-insensitive
This specification changes this ABNF to
parameter := regular-parameter / extended-
regular-parameter := regular-parameter-name "="
regular-parameter-name := attribute [section
attribute := 1*attribute-
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attribute-char :=
"*", "'", "%", or tspecials
section := initial-section / other-
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-
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
In the future it is likely that some character sets will
facilities for inline language labeling. Such facilities
inherently more flexible than those defined here as they allow
language switching in the middle of a string
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RFC 2231 MIME Value and Encoded Word Extensions November 1997
If and when such facilities are developed they SHOULD be used
preference to the language labeling facilities specified here.
that all the mechanisms defined here allow for the omission
language labels so as to be able to accommodate this possible
usage
9. Security
This RFC does not discuss security issues and is not believed
raise any security issues not already endemic in electronic mail
present in fully conforming implementations of MIME
10.
[RFC-822]
Crocker, D., "Standard for the Format of ARPA
Text Messages", STD 11, RFC 822 August 1982.
[RFC-1766]
Alvestrand, H., "Tags for the Identification
Languages", RFC 1766, March 1995.
[RFC-2045]
Freed, N., and N. Borenstein, "Multipurpose Internet
Extensions (MIME) Part One: Format of Internet
Bodies", RFC 2045, December 1996.
[RFC-2046]
Freed, N. and N. Borenstein, "Multipurpose Internet
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
Message Headers", RFC 2047, December 1996.
[RFC-2048]
Freed, N., Klensin, J. and J. Postel, "
Internet Mail Extensions (MIME) Part Four:
Registration Procedures", RFC 2048, December 1996.
[RFC-2049]
Freed, N. and N. Borenstein, "Multipurpose Internet
Extensions (MIME) Part Five: Conformance Criteria
Examples", RFC 2049, December 1996.
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RFC 2231 MIME Value and Encoded Word Extensions November 1997
[RFC-2060]
Crispin, M., "Internet Message Access Protocol -
4rev1", RFC 2060, December 1996.
[RFC-2119]
Bradner, S., "Key words for use in RFCs to
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
IAB Character Set Workshop", RFC 2130, April 1997.
[RFC-2183]
Troost, R., Dorner, S. and K. Moore, "
Presentation Information in Internet Messages:
Content-Disposition Header", RFC 2183, August 1997.
11. Authors'
Ned
Innosoft International, Inc
1050 Lakes
West Covina, CA 91790
Phone: +1 626 919 3600
Fax: +1 626 919 3614
EMail: ned.freed@innosoft.
Keith
Computer Science Dept
University of
107 Ayres
Knoxville, TN 37996-1301
EMail: moore@cs.utk.
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RFC 2231 MIME Value and Encoded Word Extensions November 1997
12. Full Copyright
Copyright (C) The Internet Society (1997). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Freed & Moore Standards Track [Page 10]
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