💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc2130.txt captured on 2023-06-14 at 18:30:25.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Network Working Group C. Weider
Request for Comments: 2130 Microsoft
Category: Informational C. Preston
Preston & Lynch
K. Simonsen
DKUUG
H. Alvestrand
UNINETT
R. Atkinson
Cisco Systems
M. Crispin
University of Washington
P. Svanberg
KTH
April 1997
The Report of the IAB Character Set Workshop
held 29 February - 1 March, 1996
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Acknowledgments
The authors would like to sincerely thank Information Sciences
Institute (ISI), and in particular Joyce K. Reynolds for graciously
hosting this event; Joe Kemp and Jeanine Yamazaki of ISI made sure
the facilities met our needs. We also wish to thank the Internet
Society, which underwrote travel for participants who might not
otherwise have been able to attend. Of course, we also wish to thank
the many experts who participated in the workshop and on the mailing
list; a complete list of these people can be found in Appendix D.
Bunyip Information Systems was kind enough to provide mailing list
facilities for this work.
Table of Contents
Abstract
0: Executive summary.......................................... 2
1: Introduction............................................... 3
2: Character sets on the Internet -- the problem.............. 3
2.1: Character set handling in existing protocols............... 4
3: Architectural model........................................ 6
3.1: Segments defined........................................... 7
3.2: On the wire................................................ 8
Weider, et. al. Informational [Page 1]
RFC 2130 Character Set Workshop Report April 1997
3.3: Determining which values of CCS, CES, and TES are used..... 9
3.4: Recommended Defaults....................................... 10
3.5: Guidelines for conversions between coded character sets.... 13
4: Presentation issues........................................ 14
5: Open issues................................................ 14
5.1: Language tags.............................................. 15
5.2: Public identifiers......................................... 16
5.3: Bi-directionality.......................................... 16
6: Security Considerations.................................... 16
7: Conclusions................................................ 16
8: Recommendations............................................ 17
8.1: To the IAB................................................. 17
8.2: For new Internet protocols................................. 18
8.3: For registration of new character sets..................... 18
Appendix A: List of protocols affected by character set issues... 20
Appendix B: Acronyms............................................. 23
Appendix C: Glossary............................................. 24
Appendix D: References........................................... 25
Appendix E: Recommended reading.................................. 27
Appendix F: Workshop attendee list............................... 29
Appendix G: Authors' Addresses................................... 30
Abstract
This report details the conclusions of an IAB-sponsored invitational
workshop held 29 February - 1 March, 1996, to discuss the use of
character sets on the Internet. It motivates the need to have
character set handling in Internet protocols which transmit text,
provides a conceptual framework for specifying character sets,
recommends the use of MIME tagging for transmitted text, recommends a
default character set *without* stating that there is no need for
other character sets, and makes a series of recommendations to the
IAB, IANA, and the IESG for furthering the integration of the
character set framework into text transmission protocols.
0: Executive summary
The term 'Character Set' means many things to many people. Even the
MIME registry of character sets registers items that have great
differences in semantics and applicability. This workshop provides
guidance to the IAB and IETF about the use of character sets on the
Internet and provides a common framework for interoperability between
the many characters in use there.
The framework consists of four components: an architecture model,
which specifies components necessary for on-the-wire transmission of
text; recommendations for tagging transmitted (and stored) text;
recommended defaults for each level of the model; and a set of
Weider, et. al. Informational [Page 2]
RFC 2130 Character Set Workshop Report April 1997
recommendations to the IAB, IANA, and the IESG for furthering the
integration of this framework into text transmission protocols.
The architectural model specifies 7 layers, of which only three are
required for on-the-wire transmission. The Coded Character Set is a
mapping from a set of abstract characters to a set of integers. The
Character Encoding Scheme is a mapping from a Coded Character Set (or
several) to a set of octets. The Transfer Encoding Syntax is a
transformation applied to data which has been encoded using a
Character Encoding Scheme to allow it to be transmitted. These layers
should be specified in a transmitted text stream by using the MIME
encoding mechanisms.
This report recommends the use of ISO 10646 as the default Coded
Character Set, and UTF-8 as the default Character Encoding Scheme in
the creation of new protocols or new version of old protocols which
transmit text. These defaults do not deprecate the use of other
character sets when and where they are needed; they are simply
intended to provide guidance and a specification for
interoperability.
1: Introduction
This is the report of an IAB-sponsored invitational workshop on the
use of Character Sets on the Internet, held 29 February - 1 March
1996 at Information Sciences Institute (ISI) in Marina del Rey,
California. In addition, this report covers the discussion on the
mailing list up to and slightly beyond the workshop itself. The
goals of this workshop were to provide guidance to the IAB and the
IETF about the use of character sets on the Internet, and if possible
a common framework for interoperability between the many character
sets in use there. Both goals were achieved.
2: Character sets on the Internet - the problem
The term 'character set' is typically applied to the contents of a
wide variety of text transmission and display protocols used on the
Internet. Because the term is used to mean different things,
confusion has arisen. For example, the MIME registry of character
sets [MIME] contains items that may differ greatly in their
applicability and semantics in various Internet protocols.
In addition, there is a vast profusion of different text encoding
schemes in use on the Internet. This per se is not a problem; each
scheme has evolved to meet real needs. However, information
applications such as mail, directories, and the World Wide Web have
each developed different techniques for dealing with the growing
number of schemes. A robust information architecture for the
Weider, et. al. Informational [Page 3]
RFC 2130 Character Set Workshop Report April 1997
Internet requires as much interoperability between these techniques
as possible.
2.1: Related topics deemed out of scope for this workshop
Successful display of plain text transmitted over the Internet
requires a lot of information about the text itself, such as the
underlying character set, language, and so forth. An additional set
of formatting information is needed if the receiving application
wishes to use local (cultural) conventions when it presents the data
to the user. This formatting includes information, that provides the
data necessary to format certain types of textual data (dates,
times, numbers and monetary notation) into a form which is familiar
to the user. The POSIX [POSIX] notation of locale encompasses
language, coded character set and cultural conventions.
To avoid unfruitful discussion, and to make the best use of the time
available for the workshop, we declared the following issues out of
scope for the purposes of this workshop:
- glyphs
- sorting
- culture (e.g. do we present the American or British spelling?)
- user interface issues
- internal representation of textual data
- included characters (why aren't certain characters available in
any character set?)
- locale (in the POSIX sense)
- font registration
- semantics
- user input/output issues
- Han unification issues
There are some related issues which were included for discussion,
most importantly the 'locale' components necessary for transport and
identification of multilingual texts.
2.2: Character Set handling in existing protocols
One of the group's overriding concerns was that the framework
developed for character set handling not break existing protocols.
With that in mind, the way character sets are being used in existing
protocols was examined. See Appendix A for a list of those protocols
and some recommendations for change.
2.2.1: General comments
The problem areas here fall into three main categories: protocols,
Weider, et. al. Informational [Page 4]
RFC 2130 Character Set Workshop Report April 1997
identifiers, and data.
2.2.1.1: Protocols
The protocol machinery SHOULD NOT be changed; allowing, for instance,
SMTP [SMTP] to use both MAIL FROM and POST FRA is dangerous to the
protocols' stability. However, many protocols carry error messages
and other information that is intended for human consumption; it
MIGHT be an advantage to allow these to be localized into a specific
language and character set, rather than staying in English and US-
ASCII [ASCII]. If this is done, new extensions should follow the
framework outlined below.
2.2.1.2: Identifiers.
There is a strong statement of direction from the IAB, RFC 1958 [RFC
1958], which states:
4.3 Public (i.e. widely visible) names should be in case
independent ASCII. Specifically, this refers to DNS names,
and to protocol elements that are transmitted in text format.
...
5.4 Designs should be fully international, with support for
localization (adaptation to local character sets). In
particular, there should be a uniform approach to character
set tagging for information content.
In protocols that up to now have used US-ASCII only, UTF-8 [UTF-8]
forms a simple upgrade path; however, its use should be negotiated
either by negotiating a protocol version or by negotiating charset
usage, and a fallback to a US-ASCII compatible representation such as
UTF-7 [UTF-7] MUST be available.
The need for passing application data such as language on individual
identifiers varies between applications; protocols SHOULD attempt to
evaluate this need when designing mechanisms. Applying the ASCII
requirement for identifiers that are only used in a local context
(such as private mailbox folder names) is both unrealistic and
unreasonable; in such cases, methods for consistency in the handling
of character set should be considered.
2.2.1.3: Data
Data that require character set handling includes text, databases,
and HTML [HTML] pages, for example. In these the support for
multiple character sets and proper application information is
absolutely vital, and MUST be supported.
Weider, et. al. Informational [Page 5]
RFC 2130 Character Set Workshop Report April 1997
2.3: Architectural requirements
To address the issues enumerated for this work, first an
architectural model was created which establishes the components that
are required to fully specify the transmission of textual data. Many
of these components are already familiar to the users of encoding
protocols such as MIME. Not all of these are discussed in detail in
this report; we restrict ourselves primarily to those components
which are required to specify the 'on-the-wire' phase of text
transmission.
Mandating a single, all-encompassing character set would not fit well
with the IETF philosophy of planning for architectural diversity.
So, the best that can be done is to provide a common *framework* for
identifying and using the multitude of character sets available on
the Internet. It would be an advantage if the total number of Coded
Character Sets could be kept to a minimum. This framework should
meet the following requirements:
- it should not break existing protocols (because then the likelihood
of deployment is very small),
- it should allow the use of character sets currently used on the
Internet, and
- it should be relatively easy to build into new protocols.
3: Architectural model
The basic architectural model which guided our discussions is shown
in below. A distinction was made between those segments which were
necessary to successfully transmit character set data on-the-wire and
those needed to present that data to a user in a comprehensible
manner. The discussions were primarily restricted to those segments
of the model which specify the 'on-the-wire' transmission of textual
data.
User interface issues: these are briefly discussed in Section 3.1.1.
Layout
Culture
Locale
Language
On-the-wire: see section 3.2 for detailed discussion.
Transfer Syntax
Character Encoding Scheme
Coded Character Set
Weider, et. al. Informational [Page 6]
RFC 2130 Character Set Workshop Report April 1997
3.1: Segments defined
3.1:1: User interface
3.1.1.1: Layout
Layout includes the elements needed for displaying text to the user,
such as font selection, word-wrapping, etc. It is similar to the
'presentation' layer in the 7-layer ISO telecommunications model
[ISO-7498].
3.1.1.2: Culture
Culture includes information about cultural preferences, which affect
spelling, word choice, and so forth.
3.1.1.3: Locale
The locale component includes the information necessary to make
choices about text manipulation which will present the text to the
user in an expected format. This information may include the display
of date, time and monetary symbol preferences. Notice that locale
modifications are typically applied to a text stream before it is
presented to the user, although they also are used to specify input
formats.
3.1.1.4: Language
This component specifies the language of the transmitted text. At
times and in specific cases, language information may be required to
achieve a particular level of quality for the purpose of displaying a
text stream. For example, UTF-8 encoded Han may require transmission
of a language tag to select the specific glyphs to be displayed at a
particular level of quality.
Note that information other than language may be used to achieve the
required level of quality in a display process. In particular, a
font tag is sufficient to produce identical results. However, the
association of a language with a specific block of text has
usefulness far beyond its use in display. In particular, as the
amount of information available in multiple languages on the World
Wide Web grows, it becomes critical to specify which language is in
use in particular documents, to assist automatic indexing and
retrieval of relevant documents.
Weider, et. al. Informational [Page 7]
RFC 2130 Character Set Workshop Report April 1997
The term 'language tag' should be reserved for the short identifier
of RFC 1766 [RFC-1766] that only serves to identify the language.
While there may be other text attributes intimately associated with
the language of the document, such as desired font or text direction,
these should be specified with other identifiers rather than
overloading the language tag.
3.2: On the wire
There are three segments of the model which are required for
completely specifying the content of a transmitted text stream (with
the occasional exception of the Language component, mentioned above).
These components are:
1) Coded Character Set,
2) Character Encoding Scheme, and
3) Transfer Encoding Syntax.
Each of these abstract components must be explicitly specified by the
transmitter when the data is sent. There may be instances of an
implicit specification due to the protocol/standard being used (i.e.
ANSI/NISO Z39.50). Also, in MIME, the Coded Character Set and
Character Encoding Scheme are specified by the Charset parameter to
the Content-Type header field, and Transfer Encoding Syntax is
specified by the Content-Transfer-Encoding header field.
3.2.1: Coded Character Set
A Coded Character Set (CCS) is a mapping from a set of abstract
characters to a set of integers. Examples of coded character sets
are ISO 10646 [ISO-10646], US-ASCII [ASCII], and ISO-8859 series
[ISO-8859].
3.2.2: Character Encoding Scheme
A Character Encoding Scheme (CES) is a mapping from a Coded Character
Set or several coded character sets to a set of octets. Examples of
Character Encoding Schemes are ISO 2022 [ISO-2022] and UTF-8 [UTF-8].
A given CES is typically associated with a single CCS; for example,
UTF-8 applies only to ISO 10646.
Weider, et. al. Informational [Page 8]
RFC 2130 Character Set Workshop Report April 1997
3.2.3: Transfer Encoding Syntax
It is frequently necessary to transform encoded text into a format
which is transmissible by specific protocols. The Transfer Encoding
Syntax (TES) is a transformation applied to character data encoded
using a CCS and possibly a CES to allow it to be transmitted.
Examples of Transfer Encoding Syntaxes are Base64 Encoding [Base64],
gzip encoding, and so forth.
3.3: Determining which values of CCS, CES, and TES are used
To completely specify which CCS, CES, and TES are used in a specific
text transmission, there needs to be a consistent set of labels for
specifying which CCS, CES, and TES are used. Once the appropriate
mechanisms have been selected, there are six techniques for attaching
these labels to the data.
The labels themselves are named and registered, either with IANA
[IANA] or with some other registry. Ideally, their definitions are
retrievable from some registration authority.
Labels may be determined in one of the following ways:
- Determined by guessing, where the receiver of the text has to
guess the values of the CCS, CES, and TES. For example: "I got
this from Sweden so it's probably ISO-8859-1." This is
obviously not a very foolproof way to decode text.
- Determined by the standard, where the protocol used to transmit
the data has made documented choices of CCS, CES, and TES in the
standard. Thus, the encodings used are known through the
access protocol, for example HTTP [HTTP] uses (but is not
limited to) ISO-8859-1, SMTP uses US-ASCII.
- Attached to the transfer envelope, where the descriptive labels are
attached to the wrapper placed around the text for transport.
MIME headers are a good example of this technique.
- Included in the data stream, where the data stream itself has
been encoded in such a way as to signal the character set used.
For example, ISO-2022 encodes the data with escape sequences to
provide information on the character subset currently being used.
- Agreed by prior bilateral agreement, where some out-of-band
negotiation has allowed the text transmitter and receiver to
determine the CCS, CES, and TES for the transmitted text.
- Agreed to by negotiation during some phase, typically
initialization of the protocol.
Weider, et. al. Informational [Page 9]
RFC 2130 Character Set Workshop Report April 1997
3.3.1: Recommendations for value specification mechanisms
While each of these techniques (with the exception of guessing) is
useful in particular situations, interoperability requires a more
consistent set of techniques. Thus, we recommend that MIME
registered values be used for all tagging of character sets and
languages UNLESS there is an existing mechanism for determining the
required information using one of the other techniques (except
guessing). This recommendation will require a fair bit of work on
the part of protocol designers, implementors, the IETF, the IESG, and
the IAB.
However, it is important to point out that the MIME concept of
'charset' in some cases cuts across several layers of components in
our model. While this can be accepted in existing registrations, we
also recommend that the MIME registration procedure for character
sets be modified to show how a proposed character set deals with the
CCS and the CES. Most 'charsets' have a well defined CCS and CES,
they should merely be teased apart for the registration.
There are a number of other recommendations, but these will be
covered in the next sections.
3.4: Recommended Defaults
For a number of reasons, one cannot define a mandatory set of
defaults for all Internet protocols. There is a mass of current
practice, future protocols are likely to have different purposes,
which may determine their handling of text, and protocols may need
specific variation support. For example, in mail, text is a
predominant data type and coded character sets then become a major
issue for the protocol. Also, since e-mail is ubiquitous and users
expect to be able to send it to everyone, the mail protocols need to
be quite adept at handling different character set encodings. On the
other hand, if strings are seldom used in a given protocol, there is
no need to weigh the protocol down with a sophisticated apparatus for
handling multiple character sets, assuming that the predicated
character set can handle all the protocol's needs. This observation
also applies to the specification techniques for character set
parameters. If only one character set encoding is needed, it can be
made explicit in the protocol specification. Protocols with a
greater need for character set support will need a more elaborate
specification technique.
Weider, et. al. Informational [Page 10]
RFC 2130 Character Set Workshop Report April 1997
3.4.1: Clarity of specification
We recommend that each protocol clearly specify what it is using for
each of the layers of the transmission model. Users (or clients)
should never have to guess what the parameter is for a given layer.
3.4.2: Default Coded Character Set:
The default Coded Character Set is the repertoire of ISO-10646.
3.4.3: Default Character Encoding Scheme
For text-oriented protocols, new protocols should use UTF-8, and
protocols that have a backwards compatibility requirement should use
the default of the existing protocol, e.g. US-ASCII for mail, and
ISO-8859-1 for HTTP. The recommended specification scheme is the
MIME "charset" specification, using the IANA "charset"
specifications. The MIME specifications will need to be clarified to
meet this model in the future.
For other protocols, the default should be UTF-8 as this initially
allows US-ASCII to be entered as-is, and enables the full repertoire
of ISO 10646.
Some protocols, such as those descended from SGML [SGML], have other
natural notations for characters outside their "natural" repertoire;
for instance, HTML [HTML] allows the use of &#nnnn to refer to any
ISO 10646 character. Note that this, like all other encodings that
depend on "escape characters", redefines at least one character from
the base character set for use as an indicator of "foreign"
characters. Use of this approach must be weighed very carefully.
3.4.4: Default Transport Encoding Scheme
There is no recommended default for this level. For plain text
oriented protocols, the bytestream transport format should be 8-bit
clean, possibly with normalization of end-of-line indicators. Some
special cases could be made for protocols that are not 8-bit clean,
such as encoding it for transport over 7-bit connections. For binary
the same recommendation holds as above. The specification technique
should either be defined in the protocol, if only one way is
permitted, or by use of MIME content-transfer-encoding (CTE)
techniques, using IANA registered values.
Weider, et. al. Informational [Page 11]
RFC 2130 Character Set Workshop Report April 1997
3.4.5: Default Language
There is no recommended default for the language level. For human
readable text, there should always be a way to specify the natural
language. The specification technique should be a MIME identifier
with IANA registered values for languages. If headers are used, the
header should be 'Content-Language'.
3.4.6: Default Locale
The default should be the POSIX locale. The specification technique
should use the Cultural register of CEN ENV 12005 [CEN] for the
values. If headers are used, the header should be 'Content-Locale'.
3.4.7: Default Culture
There is no recommended default for the Culture level. The
specification technique should be a MIME or MIME-like identifier
(e.g. Content-Culture) and should use the Cultural register of CEN
ENV 12005 for its values.
3.4.8: Default Presentation
There is no recommended default for the Presentation level. The
specification technique should be a MIME or MIME-like identifier
(e.g. Content-Layout) and use the glyph register of ISO 10036 and
other registers for its values.
3.4.9: Multiplexing
In some cases, text transmission may require the use of a number of
different values for a given parameter; for example, English
annotation of Japanese text might well require shifting the Content-
Language parameter. The way to switch the value of parameters within
a single body of text depends on the application. For instance, the
HTML I18N [I18N] work defines a language attribute on most of its
elements, including <SPAN>, <HTML>, and <BODY>, for the purpose of
switching between different languages. When only one value is
needed, this value should be as general as possible, and specified in
the protocol standard with reference to the IANA or other registry
value. All levels should be specified explicitly.
3.4.10: Storage
Because stored text may very well be stored without any of the
additional information necessary for decoding, stored text SHOULD be
tagged in a MIME compliant fashion. This alleviates the problem of
being unable to interpret text which has been stored for a long time,
Weider, et. al. Informational [Page 12]
RFC 2130 Character Set Workshop Report April 1997
or text whose provenance is not available.
3.5: Guidelines for conversions between coded character sets
This section covers various algorithms to convert a source text S,
encoded in the coded character set CCS(S), to a target text T,
encoded in the coded character set CCS(T).
Rep(X) is the character repertoire of coded character set X, i.e. the
set of characters which can be represented with X.
3.5.1: Exact conversion
When Rep(CCS(S)) and Rep(CCS(T)) are equal or Rep(CCS(S)) is a subset
of Rep(CCS(T)), exact conversion is possible; i.e. T is equal to S.
The octets just need to be remapped. The algorithm for performing
this remapping is simple, if the IANA-registered definition tables
for CCS(S) and CCS(T) are available.
3.5.2: Approximate conversion
In all other cases, any conversion creates a text T which differs
from S. There are different principles for how this inevitable
difference should be handled. A choice between them should be made,
depending on the purpose and requirements of the conversion. Where
possible, the client application should be given mechanisms to
determine what has been done to the text.
3.5.2.1: Length-modifying conversion for human display
When the length of the target text T is allowed to differ from the
length of the source text S, one should use a conversion method in
which each source character is converted to one or several target
character(s), using a best resemblance criteria in the choice of that
target character(s).
Examples:
LATIN CAPITAL LETTER [*] -> AE
COPYRIGHT SIGN [*] -> (c)
3.5.2.2: Length-preserving conversion for human display
Where the text T must be presented and the length of T cannot differ
from the length of S, one should use a conversion method where each
source character is converted to one target character, using some
kind of best resemblance criteria in the choice of target character.
Weider, et. al. Informational [Page 13]
RFC 2130 Character Set Workshop Report April 1997
Examples:
LATIN CAPITAL LETTER [*] -> A
COPYRIGHT SIGN [*] -> C
3.5.2.3: Conversion without data loss
Where the conversion of the text S into T must be completely
reversible, apply a Character Encoding Syntax or other reversible
transformation method. This case is most frequently met in data
storage requirements.
Examples:
LATIN CAPITAL LETTER [*] -> &AE
COPYRIGHT SIGN [*] -> &(C
An alternate method, which can be used if the size of Rep(CCS(T)) >=
Rep(CCS(S)), then for each character in Rep(CCS(S)) which is not
present in Rep(CCS(T)), define a mapping into a character in
Rep(CCS(T)) which is not present in Rep(CCS(S)).
Examples:
LATIN CAPITAL LETTER [*] -> CYRILLIC CAPITAL LETTER [*]
COPYRIGHT SIGN [*] -> PARTIAL DIFFERENTIAL SIGN [*]
Note that conversion without data loss requires redefining some
member of T to indicate "the introduction of character data outside
T". This effectively adds another level of CES on top of CES(T).
4: Presentation issues
There are a number of considerations to make in selecting the base
character set. One such consideration is the protocol's convenience
to users with limited equipment (for example only ISO 8859-1 or a
keyboard without the ability to enter all the characters in ISO
10646). Alternative representation should be considered for these
users, both for input and output. Possible options for the
representation of characters that can not be displayed include
transliteration (a la CEN/TC304 or ISO TC46/SC2 ), RFC 1345 [RFC-
1345] representative icons, or the WG2 short name (u+xxxx).
5: Open issues
In addition to the issues declared out of scope and enumerated in
section 2.1, the following issues are still open and will need to be
addressed in other forums. These issues: language tags, public
identifiers such as URL names, and bi-directionality are briefly
discussed below as they repeatedly encroached the discussion.
Weider, et. al. Informational [Page 14]
RFC 2130 Character Set Workshop Report April 1997
5.1: Language tags
Although the workshop decided not to explicitly address the so-called
"CJK issue", a few members felt it was necessary to have some
mechanism to address the problem of correct Han character display in
the ISO-10646 issue, and that saying that it was a "font issue" would
not suffice.
The "CJK issue" refers to the extended discussion about "Han
unification", the use of a single ISO-10646 codepoint to represent
multiple national variants of a Chinese (Han) character. ISO-10646
can map uniquely to any single CJK national character set, but in the
absence of additional information an application can not display an
ISO-10646 text using the proper national variants for that text.
It was agreed that language tags would be sufficient to disambiguate
unified characters. There was not, in our opinion, a significant
technical difference between the use of different coded character
sets with overlapping codepoints, and a single coded character set
with language tags. Either way, the application has sufficient
information to display the text properly.
It was observed that in contemporary usage of MIME charsets, the
language is implied as well as the coded character set and the
character encoding syntax. We agreed that this is excessive
overloading of MIME charsets.
To specify the language used in a particular block of text, we
recommend that the MIME tag "Content-Language" be used. There are a
number of questions about this approach that need to be worked out,
however:
- Is Content-Language: actually suitable?
- Is there an overload between this function and the other
intended functions of Content-Language: as described in RFC
1766?
- What, precisely, does "Content-Language: zh-tw, ja, ko, zh-cn"
mean in this context? We believe it means that, in drawing a
Han character, the Taiwanese variant (presumably traditional
Han) is preferred, followed by the Japanese, Korean, and
mainland Chinese (presumably simplified Han) variants. It does
*NOT* mean "mixed text containing Taiwanese, Japanese, Korean,
and mainland Chinese text with all the national variants in
each of these".
Mixed CJK text, that simultaneously displays different variants
occupying the same codepoint, requires language tags embedded in the
data. Ohta and Handa propose in RFC 1554 [RFC-1554] a MIME charset
Weider, et. al. Informational [Page 15]
RFC 2130 Character Set Workshop Report April 1997
using ISO-2022 shifts between multiple coded character sets; in
effect this is an encoding that uses coded character sets for
displaying the appropriate glyphs.
There is some speculation that states that mixed CJK text is
relatively infrequent, and that therefore it is acceptable to require
that such text be represented using a rich text format that can
support language tags. In other words, that a simplifying assumption
can be made for TEXT/PLAIN in email using ISO-10646 that will not
require multiple display representations for the same codepoint. A
mechanism such as RFC 1554 could address this need if it was
important; although arguably RFC 1554 should really be identified as
TEXT/ISO-2022.
Note again that we recommend that support for language tagging SHOULD
be built into new protocols, as this will become a critical component
of the automated indexing and retrieval in information applications
of the future.
5.2: Public identifiers
There is a considerable demand from the user community for the
ability to use non-ASCII characters in URL names, IMAP mailbox names,
file names, and other public identifiers. This is still an open
problem.
5.3: Bi-directionality
It was realized that a consistent framework for bi-directional text
was needed but there was no attempt to work on it in this workshop.
6: Security Considerations
There are no security considerations associated with character sets.
7: Conclusions
This paper provides a conceptual framework and a set of
recommendations which, if adopted, should provide a solid foundation
for interoperability on the Internet. There are, however, a number of
open issues which will need to be addressed to provide ever better
use of text on the Internet.
Weider, et. al. Informational [Page 16]
RFC 2130 Character Set Workshop Report April 1997
8: Recommendations
8.1: To the IAB
There were a number of recommendations to the IAB about making the
standards process more aware of the need for character set
interoperability, and about the framework itself.
A: The IAB should trigger the examination of all RFCs to determine
the way they handle character sets, and obsolete or annotate the
RFCs where necessary.
B: The IESG should trigger the recommendation of procedures to the
RFC editor to encourage RFCs to specify character set handling if
they specify the transmission of text.
C: The IAB should trigger the production of a perspectives document
on the character set work that has gone on in the past and relate it
to the current framework.
D: Full ISO 10646 has a sufficiently broad repertoire, and scope for
further extension, that it is sufficient for use in Internet
Protocols (without excluding the use of existing alternatives).
There is no need for specific development of character set standards
for the Internet.
E: The IAB should encourage the IRTF to create a research group to
explore the open issues of character sets on the Internet. This group
should set its sights much higher than this workshop did.
F: The IANA (perhaps with the help of an IETF or IRTF group) should
develop procedures for the registration of new character sets for
use in the Internet.
G: Register UTF-8 as a Character Encoding Scheme for MIME.
H: The current use of the "x-*" format for distinguishing
experimental tags should be continued for private use among
consenting parties. All other namespaces should be allocated by IANA.
I: Application protocol RFCs SHOULD include a section on
"multilingual Considerations".
J: Application Protocol RFCs SHOULD indicate how to transfer 'on the
wire' all characters in the character sets they use. They SHOULD also
specify how to transfer other information that applications may need
to know about the data.
Weider, et. al. Informational [Page 17]
RFC 2130 Character Set Workshop Report April 1997
K: The IESG should trigger a set of extensions to RFC 1522 to allow
language tagging of the free text parts of message headers.
8.2: For new Internet protocols
New protocols do not suffer from the need to be compatible with old
7-bit pipes. New protocol specifications SHOULD use ISO 10646 as the
base charset unless there is an overriding need to use a different
base character set.
New protocols SHOULD use values from the IANA registries when
referring to parameter values. The way these values are carried in
the protocols is protocol dependent; if the protocol uses RFC-822-
like headers, the header names already in use SHOULD be used.
For protocols with only a single choice for each component, the
protocol should use the most general specification and should be
specified with reference to the registered value in the protocol
standard.
Protocols SHOULD tag text streams with the language of the text.
8.3: For the registration of new character sets
Ned Freed will be releasing a new MIME registration document in
conjunction with this paper.
8.3.1: A definition table for a coded character set
A definition table for a coded character set A must for each
character C that is in the repertoire of A give:
a) if C is present in ISO 10646, the code value (in hexadecimal form)
for that character.
b) If C is not present in ISO 10646, but may be constructed using ISO
10646 combining characters, the series of code values (in
hexadecimal form) used to construct that character.
c) if C is not present in ISO 10646, a textual description of the
character, and a reference to its origin.
Weider, et. al. Informational [Page 18]
RFC 2130 Character Set Workshop Report April 1997
8.3.2: A definition of a character encoding scheme
A definition of a character encoding scheme consists of:
- A description of an algorithm which transforms every possible
sequence of octets to either a sequence of pairs <CCS, code
value> or to the error state "illegal octet sequence"
- Specifications, either by reference to CCS's registered by IANA or
in text, of each CCS upon which this CES is based.
Weider, et. al. Informational [Page 19]
RFC 2130 Character Set Workshop Report April 1997
Appendix A:
A-1: IETF Protocols
The following list describes how various existing protocols handle
multiple character set information.
Email
SMTP
See 8.2. ESMTP makes it easy to negotiate the use of alternate
language and encoding if it is needed.
Headers
RFC 1522 forms an adequate framework for supporting text; UTF-8
alone is not a possible solution, because the mail pathways are
assumed to be 7-bit 'forever'. However, RFC 1522 should be
extended to allow language tagging of the free text parts of
message headers.
Bodies
Selection of charset parameters for Email text bodies is
reasonably well covered by the charset= parameter on Text/* MIME
types. Language is defined by the Content-language header of
RFC 1766. Other information will have to be added using body
part headers; due to the way MIME differentiates between body
part headers and message headers, these will all have to have
names starting with Content- .
NetNews
NNTP
See 8.2. No strong tradition for negotiation of encoding in NNTP
exists.
NetNews Messages
These should be able to leverage off the mechanisms defined for
Email. One difference is that nearly all NNTP channels are 8-
bit clean; some NNTP newsgroups have a tradition of using 8-bit
charsets in both headers and bodies. Defining character set
default on a per newsgroup basis might be a suitable approach.
RTCP
The identifiers carried as information about parties are already
defined to be in UTF-8.
Weider, et. al. Informational [Page 20]
RFC 2130 Character Set Workshop Report April 1997
FTP
Protocol
See 8.2. The common use of welcome banners in the login response
means that there might be strong reason here to allow client and
server to negotiate a language different from the default for
greetings and error messages. This should be a simple protocol
extension.
Filenames
Many fileservers now how have the capability of using non-ASCII
characters in filenames, while the "dir" and "get" commands of
are defined in terms of US-ASCII only. One possible solution
would be to define a "UTF-8" mode for the transfer of filenames
and directory information; this would need to be a negotiated
facility, with fallback to US-ASCII if not negotiated. The
important point here is consistency between all implementations;
a single charset is better here than the ability to handle
multiple charsets.
World Wide Web
HTTP
See 8.2. The single-shot stype of HTTP makes negotiation more
complex than it would otherwise be.
HTML
Internationalization of HTML [I18N] seems fairly well covered in
the current "I18N" document. It needs review to see if it needs
more specific details in order to carry application information
apart from the language.
URLs
URLs are "input identifiers", and powerful arguments should be
made if they are ever to be anything but US-ASCII.
IMAP
IMAP's information objects are MIME Email objects, and therefore
are able to use that standard's methods. However, IMAP folder
names are local identifiers; there is strong reason to allow
non-ASCII characters in these. A UTF-8 negotiation might be the
most appropriate thing, however, UTF-8 is awkward to use.
Unfortunately, UTF-7 isn't suitable because it conflicts with
popular hierarchy delimiters. The most recent IMAP work in
progress specification describes a modified UTF-7 which avoids
this problem.
Weider, et. al. Informational [Page 21]
RFC 2130 Character Set Workshop Report April 1997
DNS
DNS names are the prime example of identifiers that need to stay
in US-ASCII for global interoperability. However, some DNS
information, in particular TXT records, may represent
information (such as names) that is outside the ASCII range. A
single solution is the best; problems resulting from UTF-8
should be investigated.
WHOIS++
WHOIS++ version 1 is defined to use ISO 8859-1. The next version
will use UTF-8. The currently designed changes will also allow
the specification of individual attributes on attribute names;
these will make the passing of application information about the
values (such as language) easier. No immediate action seems
necessary.
WHOIS
This has been a stable protocol for so many years now that it
seems unwise to suggest that it be modified. Furthermore,
compatible extensions exist in RWHOIS and WHOIS++; modification
should rather be made to these protocols than to the WHOIS
protocol itself.
Telnet
This is a prime example of protocol where character set support
is necessary and nonexistent. The current work in progress on
character set negotiation in Telnet seems adequate to the task;
the question of passing other application data that might be
useful is still open.
A-2: Non-IETF protocols
For these protocols, the IETF does not have any power to change them.
However, the guidelines developed by the workshop may still be useful
as input to the further development of the protocols.
Gopher: Gopher, Gopher+
Prospero (Archie)
NFS: Filesystem
CORBA, Finger, GEDI, IRC, ISO 10160/1, Kerberos, LPR, RSTAT, RWhois,
SGML, TFTP, X11, X.500, Z39.50
Weider, et. al. Informational [Page 22]
RFC 2130 Character Set Workshop Report April 1997
Appendix B: Acronyms
ASCII American National Standard Code for Information Character
Sets
CCS Coded Character Sets
CEN ENV European Committee for Standardisation (CEN) European
pre-standard (ENV)
CES Character Encoding Scheme
CJK Chinese Japanese Korean
CORBA Common Object Request Broker Architecture
CTE Content Transfer Encoding
DNS Domain Name Service
ESMTP Extended SMTP
FTP File Transfer Protocol
HTML Hypertext Transfer Protocol
I18N Internationalization (or 18 characters between the first
(I) and last (n)character)
IAB Internet Activities Board
IANA Internet Assigned Numbers Authority
IESG Internet Engineering Steering Group
IETF Internet Engineering Task Force
IMAP Internet Message Access Protocol
IRC Internet Relay Chat
IRTF Internet Research Task Force
ISI Information Sciences Institute
ISO International Standards Organization
MIME Multipurpose Internet Mail Extensions
NFS Networked File Server
NNTP Net News Transfer Protocol
POSIX Portable Operating System Interface
RFC Request for Comments (Internet standards documents)
RPC Remote Procedure Call
RSTAT Remote Statistics
RTCP Real-Time Transport Control Protocol
Rwhois Referral Whois
SGML Standard Generalized Mark-up Language
SMTP Simple Mail Transfer Protocol
TES Transfer Encoding Syntax
TFTP Trivial File Transfer Protocol
URL Uniform Resource Locator
UTF Universal Text/Translation Format
Weider, et. al. Informational [Page 23]
RFC 2130 Character Set Workshop Report April 1997
Appendix C: Glossary
Bi-directionality - A property of some text where text written right-
to- left (Arabic or Hebrew) and text written left-to-right
(e.g. Latin) are intermixed in one and the same line.
Character - A single graphic symbol represented by sequence of one or
more bytes.
Character Encoding Scheme - The mapping from a coded character set to
an encoding which may be more suitable for specific purpose. For
example, UTF-8 is a character encoding scheme for ISO 10646.
Character Set - An enumerated group of symbols (e.g., letters, numbers
or glyphs)
Coded Character Set - The mapping from a set of integers to the
characters of a character set.
Culture - Preferences in the display of text based on cultural norms,
such as spelling and word choice.
Language - The words and combinations of words the constitute a system
of expression and communication among people with a shared
history or set of traditions.
Layout - Information needed to display text to the user, similar to
the presentation layer in the ISO telecommunications model.
Locale - The attributes of communication, such as language, character
set and cultural conventions.
On-the-wire - The data that actually gets put into packets for
transmission to other computers.
Transfer Encoding Syntax - The mapping from a coded character set
which has been encoded in a Character Encoding Scheme to an
encoding which may be more suitable for transmission using
specific protocols. For example, Base64 is a transfer encoding
syntax.
Weider, et. al. Informational [Page 24]
RFC 2130 Character Set Workshop Report April 1997
Appendix D: References
[*] Non-ASCII character
[ASCII] ANSI X3.4:1986 "Coded Character Sets - 7 Bit American
National Standard Code for Information Interchange (7-bit ASCII)"
[Base64] Freed, N., and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[CEN] see http://tobbi.iti.is/TC304/welcome.html for current status.
[HTML] Berners-Lee, T., and D. Connolly, "Hypertext Markup Language -
2.0", RFC 1866, November 1995.
[HTTP] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[I18N] Yergeau, F., et.al., "Internationalization of the Hypertext
Markup Language", RFC 2070, January 1997.
[IANA] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
1700, ISI, October 1994.
[ISO-2022] ISO/IEC 2022:1994, "Information technology -- Character
Code Structure and Extension Techniques", JTC1/SC2.
[ISO-7498] ISO/IEC 7498-1:1994, "Information technology - Open Systems
Interconnection - Basic Reference Model: The Basic Model".
[ISO-8859] Information Processing -- 8-bit Single-Byte Coded Graphic
Character Sets -- Part 1: Latin Alphabet no. 1,
ISO 8859-1:1987(E). Part 2: Latin Alphabet no. 2, ISO 8859-2
1987(E). Part 3: Latin Alphabet no. 3, ISO 8859-3:1988(E).
Part 4: Latin Alphabet no. 4, ISO 8859-4, 1988(E). Part 5:
Latin/Cyrillic Alphabet ISO 8859-5, 1988(E). Part 6:
Latin/Arabic Alphabet, ISO 8859-6, 1987(E). Part 7: Latin/Greek
Alphabet, ISO 8859-7, 1987(E). Part 8: Latin/Hebrew Alphabet, ISO
8859-8-1988(E).Part 9: Latin Alphabet no. 5, ISO 8859-9, 1990(E).
Part 10: Latin Alphabet no. 6, ISO 8859-10:1992(E).
[ISO-10646] ISO/IEC 10646-1:1993(E ), "Information technology --
Universal Multiple-Octet Coded Character Set (UCS) -- Part 1:
Architecture and Basic Multilingual Plane". JTC1/SC2, 1993
Weider, et. al. Informational [Page 25]
RFC 2130 Character Set Workshop Report April 1997
[MIME] See [Base64]
[POSIX] Institute of Electrical and Electronics Engineers. "IEEE
standard interpretations for IEEE standard portable operating
systems interface for computer environments". IEEE Std 1003.1
-1988/Int, 1992 edition. Sponsor, Technical Committee on Operating
Systems of the IEEE Computer Society. New York, NY: Institute of
Electrical and Electronic Engineers, 1992.
RFC 1340 See [IANA]
[RFC-1345] Simonsen, K., "Character Mnemonics & Character Sets",
RFC 1345, Rationel Alim Planlaegning, June 1992.
[RFC-1554] Ohta, M., and K. Handa, "ISO-2022-JP-2: Multilingual
Extension of ISO-2022-JP", Tokyo Institute of Technology, ETL,
December 1993.
RFC 1642 See [UTF-7]
[RFC-1766] Alvestrad, H., "Tags for the Identification of Languages",
RFC 1766, UNINETT, March 1995.
[RFC 1958] Carpenter, B. (ed.) "Architectural Principles of the
Internet", RFC 1958, IAB, June 1996.
[SGML] ISO 8879:1986 "Information Processing - Text and Office Systems
- Standard Generalized Markup Language (SGML)"
[SMTP] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
August, 1982.
[Unicode] "The Unicode standard, version 2.0. Unicode Consortium.
Reading, Mass.: Addison-Wesley Developers Press, 1996
[UTF-7] Goldsmith, D., and M. Davis, "UTF-7: A Mail Safe
Transformation Format of Unicode", RFC 1642, Taligent, Inc., July
1994.
[UTF-8] International Standards Organization, Joint Technical
Committee 1 (ISO/JTC1), "Amendment 2:1993, UCS Transformation
Format 8 (UTF-8)", in ISO/IEC 10646-1:1993 Information technology
- Universal Multiple-Octet Coded Character Set (UCS) -- Part 1:
Architecture and Basic Multilingual Plane. JTC1/SC2, 1993.
Weider, et. al. Informational [Page 26]
RFC 2130 Character Set Workshop Report April 1997
Appendix E: Recommended reading
Alvestrand, H., "Tags for the Identification of Languages", RFC 1766,
UNINETT, March 1995.
Alvestrand, H., "X.400 Use of Extended Character Sets", RFC 1502,
SINTEF DELAB, August 1993.
Borenstein, N., "Implications of MIME for Internet Mail Gateways",
RFC 1344, Bellcore, June 1992.
Freed, N., and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
Chernov, A., "Registration of a Cyrillic Character Set", RFC 1489,
RELCOM Development Team, July 1993.
Choi, U., and K. Chan, "Korean Character Encoding for Internet
Messages", RFC 1557, KAIST, December 1993.
Freed, N., and N. Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part Two: Media Types", RFC 2046, November 1996.
Goldsmith, D., and M. Davis, "Transformation Format for Unicode",
RFC 1642, Taligent, Inc., July 1994.
Goldsmith, D., and M. Davis, "Using Unicode with MIME", RFC 1641,
Taligent, Inc., July 1994.
Jerman-Blazic, B. "Character handling in computer communication" in
"user needs in information technology standards", Computer Weekly
Professional service, eds. C.D. Evans, B.L. Meed & R.S. Walker,
P.C. Butterworth Heineman, 1993, Oxford, Boston, p. 102-129.
Jerman-Blazic, B. "Tool supporting the internationalization of the
generic network services", Computer Networks and ISDN Systems,
No. 27 (1994), p. 429-435.
Jerman-Blazic, B., A. Gogala and D. Gabrijelcic, "Transparent language
processing: A solution for internationalization of Internet
services", The LISA Forum Newsletter, 5 (1996) p. 12-21
Lee, F., "HZ - A Data Format for Exchanging Files of Arbitrarily Mixed
Chinese and ASCII Characters", RFC 1843, Stanford University,
August 1995.
Weider, et. al. Informational [Page 27]
RFC 2130 Character Set Workshop Report April 1997
McCarthy, J., "Arbitrary Character Sets", RFC 373, Stanford
University, July 1972.
Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Two:
Message Header Extensions for Non-ASCII Text", RFC 1522,
September 1993. (Obsoleted by RFC 2047.)
Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part Three:
Message Header Extensions for Non-ASCII Text", RFC 2047,
University of Tennessee, November 1996.
Murai, J., Crispin, M., and E. von der Poel. "Japanese Character
Encoding for Internet Messages", RFC 1468, Keio University &
Panda Programming, June 1993.
Nussbacher, H., "Handling of Bi-directional Texts in MIME", Israeli
Inter-University, December 1993.
Nussbacher, H., and Y. Bourvine, "Hebrew Character Encoding for
Internet Messages", RFC 1555, Israeli Inter-University and
Hebrew University, December 1993.
Ohta, M., "Character Sets ISO-10646 and ISO-10646-J-1", RFC 1815,
Tokyo Institute of Technology, July 1995.
Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)", STD 9,
RFC 959, ISI, October 1985.
Postel, J., and J. Reynolds, "Telnet Protocol Specification", STD 8,
RFC 854, ISI, May 1983.
Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
ISI, October 1994. p.100-117.
Rose, M., "The Internet Message", Prentice Hall, 1992.
Simonsen, K., "Character Mnemonics & Character Sets", RFC 1345,
Rationel Almen Planlaegning, June 1992.
Unicode Consortium. "The Unicode standard, version 2.0. Reading,
Mass.: Addison-Wesley Developers Press, 1996
Wei, U., et.al. "ASCII Printable Characters-Based Chinese Character
Encoding for Internet Messages", RFC 1842, AsiInfo Services,
Inc., et.al. August 1995.
Yergeau, F. "UTF-8, a transformation format of Unicode and ISO 10646",
RFC 2044, ALIS Technologies, October 1996.
Weider, et. al. Informational [Page 28]
RFC 2130 Character Set Workshop Report April 1997
Zhu, H., et.al., "Chinese Character Encoding for Internet Messages",
RFC 1922, Tsinghua University, et.al., March 1996.
Appendix F: Workshop attendee list
These people were participants on the workshop mailing list.
An * indicates that the person attended the workshop in person.
Glenn Adams <glenn@spyglass.com>
* Joan Aliprand <joan@unicode.org>
* Harald Alvestrand <Harald.T.Alvestrand@uninett.no>
* Ran Atkinson <ran@cisco.com>
* Bert Bos <bert@w3.org>
* Brian Carpenter <brian@dxcoms.cern.ch>
* Mark Crispin <mrc@panda.com>
Makx Dekkers <dekkers@pica.nl>
Robert Elz <kre@munnari.oz.au>
Patrik Faltstrom <paf@paf.se>
* Zhu Haifeng <zhf@net.tsinghua.edu.cn>
Keniichi Handa<handa@etl.go.jp>
Olle Jarnefors <ojarnef@admin.kth.se>
Borka Jerman-Blazic <borka@e5.ijs.si>
John Klensin <klensin@mail1.reston.mci.net>
* Larry Masinter <masinter@parc.xerox.com>
* Rick McGowan <Rick_McGowan@next.com>
* Keith Moore <moore+charsets@cs.utk.edu>
* Lisa Moore <lisam@vnet.ibm.com>
Ruth Moulton <ruth@muswell.demon.co.uk>
* Cecilia Preston <cecilia@well.com>
* Joyce K. Reynolds <jkrey@isi.edu>
* Keld Simonsen <keld@dkuug.dk>
* Gary Smith <Gary_Smith@oclc.org>
* Peter Svanberg <psv@nada.kth.se>
* Chris Weider <cweider@microsoft.com >
Weider, et. al. Informational [Page 29]
RFC 2130 Character Set Workshop Report April 1997
Appendix G: Authors' Addresses
Chris Weider
Microsoft Corp.
1 Microsoft Way
Redmond, WA 98052
USA
EMail: cweider@microsoft.com
Cecilia Preston
Preston & Lynch
PO Box 8310
Emeryville, CA 94662
USA
EMail: cecilia@well.com
Keld Simonsen
DKUUG
Freubjergvey 3
DK-2100 Kxbenhavn X
Danmark
EMail: Keld@dkuug.dk
Harald T. Alvestrand
UNINETT
P.O.Box 6883 Elgeseter
N-7002 TRONDHEIM
NORWAY
EMail: Harald.T.Alvestrand@uninett.no
Randall Atkinson
cisco Systems
170 West Tasman Drive
San Jose, CA 95134-1706
USA
EMail: rja@cisco.com
Weider, et. al. Informational [Page 30]
RFC 2130 Character Set Workshop Report April 1997
Mark Crispin
Networks & Distributed Computing
University of Washington
4545 15th Avenue NE
Seattle, WA 98105-4527
USA
EMail: mrc@cac.washington.edu
Peter Svanberg
Dept. of Numberical Analysis and Computing Science (Nada)
Royal Institute of Technology
SE-100 44 STOCKHOLM
SWEDEN
EMail: psv@nada.kth.se
Weider, et. al. Informational [Page 31]