Internet DRAFT - draft-hoffman-i18n-terms
draft-hoffman-i18n-terms
Internet Draft Paul Hoffman
draft-hoffman-i18n-terms-11.txt IMC & VPNC
December 3, 2002
Expires in six months
Terminology Used in Internationalization in the IETF
Status of this memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering Task
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Abstract
This document provides a glossary of terms used in the IETF when
discussing internationalization. The purpose is to help frame
discussions of internationalization in the various areas of the IETF and
to help introduce the main concepts to IETF participants.
Table of Contents
1. Introduction
1.1 Purpose of this document
1.2 Format of the definitions in this document
2. Fundamental Terms
3. Standards Bodies and Standards
3.1 Standards bodies
3.2 Encodings and transformation formats of ISO/IEC 10646
3.3 Native CCSs and charsets
4. Character Issues
4.1 Types of characters
5. User interface for text
6. Text in current IETF protocols
7. Other Common Terms In Internationalization
8. Security Considerations
9. References
9.1 Normative References
9.2 Non-normative References
10. Additional Interesting Reading
11. Index
A. Acknowledgements
B. Author Contact Information
1. Introduction
As [RFC2277] summarizes: "Internationalization is for humans. This means
that protocols are not subject to internationalization; text strings
are." Many protocols throughout the IETF use text strings that are
entered by, or are visible to, humans. It should be possible for anyone
to enter or read these text strings, which means that Internet users
must be able to be enter text in typical input methods and displayed in
any human language. Further, text containing any character should be
able to be passed between Internet applications easily. This is the
challenge of internationalization.
1.1 Purpose of this document
This document provides a glossary of terms used in the IETF when
discussing internationalization. The purpose is to help frame
discussions of internationalization in the various areas of the IETF and
to help introduce the main concepts to IETF participants.
Internationalization is discussed in many working groups of the IETF.
However, few working groups have internationalization experts. When
designing or updating protocols, the question often comes up "should we
internationalize this" (or, more likely, "do we have to internationalize
this").
This document gives an overview of internationalization as it applies to
IETF standards work by lightly covering the many aspects of
internationalization and the vocabulary associated with those topics. It
is not meant to be a complete description of internationalization. The
definitions in this document are not normative for IETF standards;
however, they are useful and standards may make non-normative reference
to this document after it becomes an RFC. Some of the definitions in
this document come from many earlier IETF documents and books.
As in many fields, there is disagreement in the internationalization
community on definitions for many words. The topic of language brings up
particularly passionate opinions for experts and non-experts alike. This
document attempts to define terms in a way that will be most useful to
the IETF audience.
This document uses definitions from many documents that have been
developed outside the IETF. The primary documents used are:
- ISO/IEC 10646 [ISOIEC10646]
- The Unicode Standard [UNICODE]
- W3C Character Model [CHARMOD]
- IETF RFCs, including [RFC2277]
1.2 Format of the definitions in this document
In the body of this document, the source for the definition is shown in
angle brackets, such as "<ISOIEC10646>". Many definitions are
shown as "<NONE>", which means that the definitions were crafted
originally for this document. The angle bracket notation for the source
of definitions is different than the square bracket notation used
for references to documents, such as in the paragraph above; these
references are given in Section 9.
For some terms, there are commentary and examples after the
definitions. In those cases, the part before the angle brackets is the
definition that comes from the original source, and the part after the
angle brackets is commentary that is not a definition (such as
examples or further exposition).
Examples in this document use the notation for code points and names
from the Unicode Standard [UNICODE] and ISO/IEC 10646 [ISOIEC10646].
For example, the letter "a" may be represented as either "U+0061" or
"LATIN SMALL LETTER A".
2. Fundamental Terms
This section covers basic topics that are needed for almost anyone who
is involved with making IETF protocols more friendly to non-ASCII
text and with other aspects of internationalization.
language
A language is a way that humans interact. The use of language occurs
in many forms, the most common of which are speech, writing, and
signing. <NONE>
Some languages have a close relationship between the written and
spoken forms, while others have a looser relationship. [RFC3066]
discusses languages in more detail and provides identifiers for
languages for use in Internet protocols. Note that computer
languages are explicitly excluded from this definition.
script
A set of graphic characters used for the written form of one or more
languages. <ISOIEC10646>
Examples of scripts are Latin, Cyrillic, Greek, Arabic, and Han (the
ideographs used in writing Chinese, Japanese, and Korean). [RFC2277]
discusses scripts in detail.
It is common for internationalization novices to mix up the terms
"language" and "script". This can be a problem in protocols that
differentiate the two. Almost all protocols that are designed
(or were re-designed) to handle non-ASCII text deal
with scripts (the written systems) or characters, while fewer
actually deal with languages.
A single name can mean either a language or a script; for example,
"Arabic" is both the name of a language and the name of a script. In
fact, many scripts borrow their names from the names of languages.
Further, many scripts are used for many languages; for example, the
Russian and Bulgarian languages are written in the Cyrillic script.
Some languages can be expressed using different scripts; the
Mongolian language can be written in either the Mongolian and
Cyrillic scripts, and the Serbo-Croatian language is written using
both the Latin and Cyrillic scripts. Further, some languages are
normally expressed with more than one script at the same time; for
example, the Japanese language is normally expressed in the Kanji
(Han), Katakana, and Hiragana scripts in a single string of text.
character
A member of a set of elements used for the organization, control, or
representation of data. <ISOIEC10646>
There are at least three common definitions of the word "character":
- a general description of a text entity
- a unit of a writing system, often synonymous with "letter" or similar
terms
- the encoded entity itself
When people talk about characters, they are mostly using one of the
first two definitions.
A particular character is identified by its name, not by its shape.
A name may suggest a meaning, but the character may be used for
representing other meanings as well. A name may suggest a shape,
but that does not imply that only that shape is commonly used in
print, nor that the particular shape is associated only with that
name.
coded character
A character together with its coded representation. <ISOIEC10646>
coded character set
A coded character set (CCS) is a set of unambiguous rules that
establishes a character set and the relationship between the
characters of the set and their coded representation. <ISOIEC10646>
character encoding form
A character encoding form is a mapping from a character set
definition to the actual code units used to represent the data.
<UNICODE>
repertoire
The collection of characters included in a character set. Also
called a character repertoire. <UNICODE>
glyph
A glyph is an abstract form that represents one or more glyph
images. The term "glyph" is often a synonym for glyph image, which
is the actual, concrete image of a glyph representation having been
rasterized or otherwise imaged onto some display surface. In
displaying character data, one or more glyphs may be selected to
depict a particular character. These glyphs are selected by a
rendering engine during composition and layout processing.
<UNICODE>
glyph code
A glyph code is a numeric code that refers to a glyph. Usually, the
glyphs contained in a font are referenced by their glyph code.
Glyph codes are local to a particular font; that is, a different
font containing the same glyphs may use different codes. <UNICODE>
transcoding
Transcoding is the process of converting text data from one
character encoding form to another. Transcoders work only at the
level of character encoding and do not parse the text. Note:
Transcoding may involve one-to-one, many-to-one, one-to-many or
many-to-many mappings. Because some legacy mappings are glyphic,
they may not only be many-to-many, but also discontinuous: thus XYZ
may map to yxz. <CHARMOD>
In this definition, "many-to-one" means a sequence of characters
mapped to a single character. The "many" does not mean alternative
characters that map to the single character.
character encoding scheme
A character encoding scheme (CES) is a character encoding form plus
byte serialization. There are many character encoding schemes in
Unicode, such as UTF-8 and UTF-16. <UNICODE>
Some CESs are associated with a single CCS; for example, UTF-8
[RFC2279] applies only to ISO/IEC 10646. Other CESs, such as ISO
2022, are associated with many CCSs.
charset
A charset is a method of mapping a sequence of octets to a sequence
of abstract characters. A charset is, in effect, a combination of
one or more CCSs with a CES. Charset names are registered by the
IANA according to procedures documented in [RFC2278]. <NONE>
Many protocol definitions use the term "character set" in their
descriptions. The terms "charset" or "character encoding scheme"
are strongly preferred over the term "character set" because
"character set" has other definitions in other contexts and this can
be confusing.
internationalization
In the IETF, "internationalization" means to add or improve the
handling of non-ASCII text in a protocol. <NONE>
Many protocols that handle text only handle one script (often, the
one that contains the letters used in English text), or leave the
question of what character set is used up to local guesswork (which
leads, of course, to interoperability problems). Adding non-ASCII
text to such a protocol allows the protocol to handle more scripts,
hopefully all of the ones useful in the world.
localization
The process of adapting an internationalized application platform or
application to a specific cultural environment. In localization, the
same semantics are preserved while the syntax may be changed.
[FRAMEWORK]
Localization is the act of tailoring an application for a different
language or script or culture. Some internationalized applications
can handle a wide variety of languages. Typical users only
understand a small number of languages, so the program must be
tailored to interact with users in just the languages they know.
The major work of localization is translating the user interface and
documentation. Localization involves not only changing the language
interaction, but also other relevant changes such as display of
numbers, dates, currency, and so on. The better internationalized an
application is, the easier it is to localize it for a particular
language and character encoding scheme.
Localization is rarely an IETF matter, and protocols that are merely
localized, even if they are serially localized for several
locations, are generally considered unsatisfactory for the global
Internet.
Do not confuse "localization" with "locale", which is described in
Section 7 of this document.
i18n, l10n
These are abbreviations for "internationalization" and
"localization". <NONE>
"18" is the number of characters between the "i" and the "n" in
"internationalization", and "10" is the number of characters between
the "l" and the "n" in "localization".
multilingual
The term "multilingual" has many widely-varying definitions and thus
is not recommended for use in standards. Some of the definitions
relate to the ability to handle international characters; other
definitions relate to the ability to handle multiple charsets; and
still others relate to the ability to handle multiple languages.
<NONE>
displaying and rendering text
To display text, a system puts characters on a visual display device
such as a screen or a printer. To render text, a system analyzes the
character input to determine how to display the text. The terms
"display" and "render" are sometimes used interchangeably. Note,
however, that text might be rendered as audio and/or tactile
output, such as in systems that have been designed for people with
visual disabilities. <NONE>
Combining characters modify the display of the character (or, in
some cases, characters) that precede them. When rendering such text,
the display engine must either find the glyph in the font that
represents the base character and all of the combining characters,
or it must render the combination itself. Such rendering can be
straight-forward, but it is sometimes complicated when the combining
marks interact with each other, such as when there are two combining
marks that would appear above the same character. Formatting
characters can also change the way that a renderer would display
text. Rendering can also be difficult for some scripts that have
complex display rules for base characters, such as Arabic and Indic
scripts.
3. Standards Bodies and Standards
This section describes some of the standards bodies and standards that
appear in discussions of internationalization in the IETF. This is an
incomplete and possibly over-full list; listing too few bodies or
standards can be just as politically dangerous as listing too many. Note
that there are many other bodies that deal with internationalization;
however, few if any of them appear commonly in IETF standards work.
3.1 Standards bodies
ISO
The International Organization for Standardization has been involved
with standards for characters since before the IETF was started.
ISO is a non-governmental group made up of national bodies. ISO has
many diverse standards in the international characters area; the one
that is most used in the IETF is commonly referred to as "ISO/IEC
10646", although its official name has more qualifications. (The
IEC is International Electrotechnical Commission). ISO/IEC 10646
describes a CCS that covers almost all known written characters in
use today.
ISO/IEC 10646 is controlled by the group known as "ISO/IEC JTC 1/SC
2 WG2", often called "WG2" for short. ISO standards go through many
steps before being finished, and years often go by between changes
to ISO/IEC 10646. Information on WG2, and its work products, can be
found at <http://www.dkuug.dk/JTC1/SC2/WG2/>.
The standard, which comes in multiple parts, can be purchased in
both print and CD-ROM versions. One example of how to cite the
standard is given in [RFC2279]. Any standard that cites ISO/IEC
10646 needs to evaluate how to handle the versioning problem that is
relevant to the protocol's needs.
ISO is responsible for other standards that might be of interest to
protocol developers. [ISO 639] specifies the names of languages, and
[ISO 3166] specifies the abbreviations of countries. Character work
is done in the group known as ISO/IEC JTC1/SC22 and ISO TC46, as
well as other ISO groups.
Another relevant ISO group is JTC 1/SC22/WG20, which is responsible
for internationalization in JTC1, such as for international string
ordering. Information on WG20, and its work products, can be found
at <http://www.dkuug.dk/jtc1/sc22/wg20/>
Unicode Consortium
The second important group for international character standards is
the Unicode Consortium. The Unicode Consortium is a trade
association of companies, governments, and other groups interested
in promoting the Unicode Standard [UNICODE]. The Unicode Standard is
a CCS whose repertoire and code points are identical to ISO/IEC
10646. The Unicode Consortium has added features to the base CCS
which make it more useful in protocols, such as defining attributes
for each character. Examples of these attributes include case
conversion and numeric properties.
The Unicode Consortium publishes addenda to the Unicode Standard as
Unicode Technical Reports. There are many types of technical reports
at various stages of maturity. The Unicode Standard and affiliated
technical reports can be found at <http://www.unicode.org/>.
World Wide Web Consortium (W3C)
This group created and maintains the standard for XML, the markup
language for text that has become very popular. XML has always been
fully internationalized so that there is no need for a new version
to handle international text.
local and regional standards organizations
Just as there are many native CCSs and charsets, there are many
local and regional standards organizations to create and support
them. Common examples of these are ANSI (United States), and
CEN/ISSS (Europe).
3.2 Encodings and transformation formats of ISO/IEC 10646
Characters in the ISO/IEC 10646 CCS can be expressed in many ways.
Encoding forms are direct addressing methods, while transformation
formats are methods for expressing encoding forms as bits on the wire.
Basic Multilingual Plane (BMP)
The BMP is composed of the first 2^16 code points in ISO/IEC 10646.
The BMP is also called "plane 0".
UCS-2 and UCS-4
UCS-2 and UCS-4 are the two encoding forms defined for ISO/IEC
10646. UCS-2 addresses only the BMP. Because many useful characters
(such as many Han characters) have been defined outside of the BMP,
many people would consider UCS-2 to be dead. Theoretically, UCS-4
addresses the entire range of 2^31 code points from ISO/IEC 10646 as
32-bit values. However, for interoperability with UTF-16, ISO 10646
restricts the range of characters that will actually be allocated to
the values 0..0x10FFFF.
UTF-8
UTF-8, a transformation format specified in [RFC2279], is the
preferred encoding for IETF protocols. Characters in the BMP are
encoded as one, two, or three octets. Characters outside the BMP are
encoded as four octets. Characters from the US-ASCII repertoire have
the same on-the-wire representation in UTF-8 as they do in US-ASCII.
UTF-16, UTF-16BE, and UTF-16LE
UTF-16, UTF-16BE, and UTF-16LE, three transformation formats defined
in [RFC2781], are not required by any IETF standards, and are thus
used much less often than UTF-8. Characters in the BMP are always
encoded as two octets, and characters outside the BMP are encoded as
four octets. The three formats differ based on the order of the
octets and the presence of a special lead-in mark called the "byte
order mark" or "BOM".
UTF-32
The Unicode Consortium has defined UTF-32 as a transformation format
for UCS-4 in [UTR19].
SCSU and BOCU-1
The Unicode Consortium has defined an encoding, SCSU, which is
designed to offer good compression for typical text. SCSU is
described in [UTR6]. A different encoding that is meant to be
MIME-friendly, BOCU-1, is described in [UTN6]. Although compression
is attractive, as opposed to UTF-8 , neither of these (at the time
of this writing) has attracted much interest in the IETF.
3.3 Native CCSs and charsets
Before ISO/IEC 10646 was developed, many countries developed their own
CCSs and charsets. Many dozen of these are in common use on the Internet
today. Examples include ISO 8859-5 for Cyrillic and Shift-JIS for
Japanese scripts.
The official list of the registered charset names for use with IETF
protocols is maintained by IANA and can be found at
<http://www.iana.org/assignments/character-sets>. The list contains
preferred names and aliases. Note that this list has historically
contained many errors, such as names that are in fact not charsets
or references that do not give enough detail to reliably map names
to charsets.
Probably the most well-known native CCS is ASCII [US-ASCII]. This CCS is
used as the basis for keywords and parameter names in many IETF
protocols, and as the sole CCS in numerous IETF protocols that have not
yet been internationalized.
[UTR22] describes issues involved in mapping character data between
charsets, and an XML format for mapping table data.
4. Character Issues
This section contains terms and topics that are commonly used in
character handling and therefore are of concern to people adding
non-ASCII text handling to protocols. These topics are standardized
outside the IETF.
combining character
A member of an identified subset of the coded character set of
ISO/IEC 10646 intended for combination with the preceding
non-combining graphic character, or with a sequence of combining
characters preceded by a non-combining character. <ISOIEC10646>
composite sequence
A sequence of graphic characters consisting of a non-combining
character followed by one or more combining characters. A graphic
symbol for a composite sequence generally consists of the
combination of the graphic symbols of each character in the
sequence. A composite sequence is not a character and therefore is
not a member of the repertoire of ISO/IEC 10646. <ISOIEC10646>
In some CCSs, some characters consist of combinations of other
characters. For example, the letter "a with acute" might be a
combination of the two characters "a" and "combining acute", or it
might be a combination of the three characters "a", a
non-destructive backspace, and an acute. The rules for combining
two or more characters are called "composition rules", and the rules
for taking apart a character into other characters is called
"decomposition rules". The results of composition is called a
"precomposed character"; the results of decomposition is called a
"decomposed character".
normalization
Normalization is the transformation of data to a normal form, for
example, to unify spelling. <UNICODE>
Note that the phrase "unify spelling" in the definition above does
not mean unifying different words with the same meaning (such as
"color" and "colour"). Instead, it means unifying different
character sequences that are intended to form the same composite
characters (such as "<a><n><combining tilde><o>" and "<a><n with
tilde><o>").
The purpose of normalization is to allow two strings to be compared
for equivalence. The strings "<a><n><combining tilde><o>" and "<a><n
with tilde><o>" would be shown identically on a text display device.
If a protocol designer wants those two strings to be considered
equivalent during comparison, the protocol must define where
normalization occurs.
The terms "normalization" and "canonicalization" are often used
interchangeably. Generally, they both mean to convert a string of
one or more characters into another string based on standardized
rules. Some CCSs allow multiple equivalent representations for a
written string; normalization selects one among multiple equivalent
representations as a base for reference purposes in comparing
strings. In strings of text, these rules are usually based on
decomposing combined characters or composing characters with
combining characters. [UTR15] describes the process and many forms
of normalization in detail. Normalization is important when
comparing strings to see if they are the same.
case
Case is the feature of certain alphabets where the letters have two
distinct forms. These variants, which may differ markedly in shape
and size, are called the uppercase letter (also known as capital or
majuscule) and the lowercase letter (also known as small or
minuscule). Case mapping is the association of the uppercase and
lowercase forms of a letter. <UNICODE>
There is usually (but not always) a one-to-one mapping between the
same letter in the two cases. However, there are many examples of
characters which exist in one case but for which there is no
corresponding character in the other case or for which there is a
special mapping rule, such as the Turkish dotless "i" and some Greek
characters with modifiers. Case mapping can even be dependent on
locale. Converting text to have only one case is called "case
folding".
sorting and collation
Collating is the process of ordering units of textual information.
Collation is usually specific to a particular language. It is
sometimes known as alphabetizing, although alphabetization is just a
special case of sorting and collation. <UNICODE>
Collation is concerned with the determination of the relative order
of any particular pair of strings, and algorithms concerned with
collation focus on the problem of providing appropriate weighted
keys for string values, to enable binary comparison of the key
values to determine the relative ordering of the strings.
Sorting is the process of actually putting data records into
specified orders, according to criteria for comparison between the
records. Sorting can apply to any kind of data (including textual
data) for which an ordering criterion can be defined. Algorithms
concerned with sorting focus on the problem of performance (in terms
of time, memory, or other resources) in actually putting the data
records into a specified order.
A sorting algorithm for string data can be internationalized by
providing it with the appropriate collation-weighted keys
corresponding to the strings to be ordered.
Many processes have a need to order strings in a consistent sequence
(sorted). For only a few CCS/CES combinations, there is an obvious
sort order that can be done without reference to the linguistic
meaning of the characters: the codepoint order is sufficient for
sorting. That is, the codepoint order is also the order that a
person would use in sorting the characters. For many CCS/CES
combinations, the codepoint order would make no sense to a person
and therefore is not useful for sorting if the results will be
displayed to a person.
Codepoint order is usually not how any human educated by a local
school system expects to see strings ordered; if one orders to the
expectations of a human, one has a language-specific sort. Sorting
to codepoint order will seem inconsistent if the strings are not
normalized before sorting because different representations of the
same character will sort differently. This problem may be smaller
with a language-specific sort.
code table
A code table is a table showing the characters allocated to the
octets in a code. <ISOIEC10646>
Code tables are also commonly called "code charts".
4.1 Types of characters
The following definitions of types of characters do not clearly
delineate each character into one type, nor do they allow someone to
accurately predict what types would apply to a particular character. The
definitions are intended for application designers to help them think
about the many (sometimes confusing) properties of text.
alphabetic
An informative Unicode property. Characters that are the primary
units of alphabets and/or syllabaries, whether combining or
noncombining. This includes composite characters that are canonical
equivalents to a combining character sequence of an alphabetic base
character plus one or more combining characters: letter digraphs;
contextual variant of alphabetic characters; ligatures of alphabetic
characters; contextual variants of ligatures; modifier letters;
letterlike symbols that are compatibility equivalents of single
alphabetic letters; and miscellaneous letter elements. <UNICODE>
ideographic
Any symbol that primarily denotes an idea (or meaning) in contrast
to a sound (or pronunciation), for example, a symbol showing a
telephone or the Han characters used in Chinese, Japanese, and
Korean. <UNICODE>
punctuation
Characters that separate units of text, such as sentences and
phrases, thus clarifying the meaning of the text. The use of
punctuation marks is not limited to prose; they are also used in
mathematical and scientific formulae, for example. <UNICODE>
symbol
One of a set of characters other than those used for letters,
digits, or punctuation, and representing various concepts generally
not connected to written language use per se. Examples include
symbols for mathematical operators, symbols for OCR, symbols for
box-drawing or graphics, and symbols for dingbats. <NONE>
Examples of symbols include characters for arrows, faces, and
geometric shapes. [UNICODE] has a property that defines characters
as symbols.
nonspacing character
A combining character whose positioning in presentation is dependent
on its base character. It generally does not consume space along the
visual baseline in and of itself. <UNICODE>
A combining acute accent (U+0301) is an example of a nonspacing
character.
diacritic
A mark applied or attached to a symbol to create a new symbol that
represents a modified or new value. They can also be marks applied
to a symbol irrespective of whether it changes the value of that
symbol. In the latter case, the diacritic usually represents an
independent value (for example, an accent, tone, or some other
linguistic information). Also called diacritical mark or
diacritical. <UNICODE>
control character
The 65 characters in the ranges U+0000..U+001F and U+007F..U+009F.
They are also known as control codes. <UNICODE>
formatting character
Characters that are inherently invisible but that have an effect on
the surrounding characters. <UNICODE>
Examples of formatting characters include characters for specifying
the direction of text and characters that specify how to join
multiple characters.
compatibility character
A graphic character included as a coded character of ISO/IEC 10646
primarily for compatibility with existing coded character sets.
<ISOIEC10646>
For example, U+FF01 (FULLWIDTH EXCLAMATION MARK) was included for
compatibility with Asian character sets that include full-width and
half-width ASCII characters.
5. User interface for text
Although the IETF does not standardize user interfaces, many protocols
make assumptions about how a user will enter or see text that is used in
the protocol. Internationalization challenges assumptions about the type
and limitations of the input and output devices that may be used with
applications that use various protocols. It is therefore useful to
consider how users typically interact with text that might contain
one or more non-ASCII characters.
input methods
An input method is a mechanism for a person to enter text into an
application. <NONE>
Text can be entered into a computer in many ways. Keyboards are by
far the most common device used, but many characters cannot be
entered on typical computer keyboards in a single stroke. Many
operating systems come with system software that lets users input
characters outside the range of what is allowed by keyboards.
For example, there are dozens of different input methods for Han
characters in Chinese, Japanese, and Korean. Some start with
phonetic input through the keyboard, while others use the number of
strokes in the character. Input methods are also needed for scripts
that have many diacritics, such as European characters that have two
or three diacritics on a single alphabetic character.
rendering rules
A rendering rule is an algorithm that a system uses to decide how to
display a string of text. <NONE>
Some scripts can be directly displayed with fonts, where each
character from an input stream can simply be copied from a glyph
system and put on the screen or printed page. Other scripts need
rules that are based on the context of the characters in order to
render text for display.
Some examples of these rendering rules include:
- Scripts such as Arabic (and many others), where the form of the
letter changes depending on the adjacent letters, whether the
letter is standing alone, at the beginning of a word, in the
middle of a word, or at the end of a word. The rendering rules
must choose between two or more glyphs.
- Scripts such as the Indic scripts, where consonants may change
their form if they are adjacent to certain other consonants or may
be displayed in an order different from the way they are stored
and pronounced. The rendering rules must choose between two or
more glyphs.
- Arabic and Hebrew scripts, where the order of the characters
displayed are changed by the bidirectional properties of the
alphabetic characters and with right-to-left and left-to-right
ordering marks. The rendering rules must choose the order that
characters are displayed.
graphic symbol
A graphic symbol is the visual representation of a graphic character
or of a composite sequence. <ISOIEC10646>
font
A font is a collection of glyphs used for the visual depiction of
character data. A font is often associated with a set of parameters
(for example, size, posture, weight, and serifness), which, when set
to particular values, generate a collection of imagable glyphs.
<UNICODE>
bidirectional display
The process or result of mixing left-to-right oriented text and
right-to-left oriented text in a single line is called bidirectional
display. <UNICODE>
Most of the world's written languages are displayed left-to-right.
However, many widely-used written languages such as ones based on
the Hebrew or Arabic scripts are displayed right-to-left.
Right-to-left text often confuses protocol writers because they have
to keep thinking in terms of the order of characters in a string in
memory, and that order might be different than what they see on the
screen. (Note that some languages are written both horizontally and
vertically.)
Further, bidirectional text can cause confusion because there are
formatting characters in ISO/IEC 10646 which cause the order of
display of text to change. These explicit formatting characters
change the display regardless of the implicit left-to-right or
right-to-left properties of characters.
It is common to see strings with text in both directions, such as
strings that include both text and numbers, or strings that contain
a mixture of scripts.
[UNICODE] has a long and incredibly detailed algorithm for
displaying bidirectional text.
undisplayable character
A character that has no displayable form. <NONE>
For instance, the zero-width space (U+200B) cannot be displayed
because it takes up no horizontal space. Formatting characters such
as those for setting the direction of text are also undisplayable.
Note, however, that every character in [UNICODE] has a glyph
associated with it, and that the glyphs for undisplayable characters
are enclosed in a dashed square as an indication that the actual
character is undisplayable.
6. Text in current IETF protocols
Many IETF protocols started off being fully internationalized, while
others have been internationalized as they were revised. In this
process, IETF members have seen patterns in the way that many protocols
use text. This section describes some specific protocol interactions
with text.
protocol elements
Protocol elements are uniquely-named parts of a protocol. <NONE>
Almost every protocol has named elements, such as "source port" in
TCP. In some protocols, the names of the elements (or text tokens
for the names) are transmitted within the protocol. For example, in
SMTP and numerous other IETF protocols, the names of the verbs are
part of the command stream. The names are thus part of the protocol
standard. The names of protocol elements are not normally seen by
end users.
name spaces
A name space is the set of valid names for a particular item, or the
syntactic rules for generating these valid names. <NONE>
Many items in Internet protocols use names to identify specific
instances or values. The names may be generated (by some prescribed
rules), registered centrally (e.g., such as with IANA), or have a
distributed registration and control mechanism, such as the names in
the DNS.
on-the-wire encoding
The encoding and decoding used before and after transmission over
the network is often called the "on-the-wire" (or sometimes just
"wire") format. <NONE>
Characters are identified by codepoints. Before being transmitted in
a protocol, they must first be encoded as bits and octets.
Similarly, when characters are received in a transmission, they have
been encoded, and a protocol that needs to process the individual
characters needs to decode them before processing.
parsed text
Text strings that is analyzed for subparts. <NONE>
In some protocols, free text in text fields might be parsed. For
example, many mail user agents will parse the words in the text of
the Subject: field to attempt to thread based on what appears after
the "Re:" prefix.
charset identification
Specification of the charset used for a string of text. <NONE>
Protocols that allow more than one charset to be used in the same
place should require that the text be identified with the
appropriate charset. Without this identification, a program looking
at the text cannot definitively discern the charset of the text.
Charset identification is also called "charset tagging".
language identification
Specification of the human language used for a string of text.
<NONE>
Some protocols (such as MIME and HTTP) allow text that is meant for
machine processing to be identified with the language used in the
text. Such identification is important for machine-processing of the
text, such as by systems that render the text by speaking it.
Language identification is also called "language tagging".
MIME
MIME (Multipurpose Internet Mail Extensions) is a message format
that allows for textual message bodies and headers in character sets
other than US-ASCII in formats that require ASCII (most notably,
[RFC2822], the standard for Internet mail headers). MIME is
described in RFCs 2045 through 2049, as well as more recent RFCs.
<NONE>
transfer encoding syntax
A transfer encoding syntax (TES) (sometimes called a transfer
encoding scheme) is a reversible transform of already-encoded data
that is represented in one or more character encoding schemes.
<NONE>
TESs are useful for encoding types of character data into an another
format, usually for allowing new types of data to be transmitted
over legacy protocols. The main examples of TESs used in the IETF
include Base64 and quoted-printable.
Base64
Base64 is a transfer encoding syntax that allows binary data to be
represented by the ASCII characters A through Z, a through z, 0
through 9, +, /, and =. It is defined in [RFC2045]. <NONE>
quoted printable
Quoted printable is a transfer encoding syntax that allows strings
that have non-ASCII characters mixed in with mostly ASCII printable
characters to be somewhat human readable. It is described in
[RFC2047]. <NONE>
The quoted printable syntax is generally considered to be a failure
at being readable. It is jokingly referred to as "quoted
unreadable".
XML
XML (which is an approximate abbreviation for Extensible Markup
Language) is a popular method for structuring text. XML text is
explicitly tagged with charsets. The specification for XML can be
found at <http://www.w3.org/XML/>. <NONE>
ASN.1 text formats
The ASN.1 data description language has many formats for text data.
The formats allow for different repertoires and different encodings.
Some of the formats that appear in IETF standards based on ASN.1
include IA5String (all ASCII characters), PrintableString (most
ASCII characters, but missing many punctuation characters),
BMPString (characters from ISO/IEC 10646 plane 0 in UTF-16BE
format), UTF8String (just as the name implies), and TeletexString
(also called T61String; the repertoire changes over time).
ASCII-compatible encoding (ACE)
Starting in 1996, many ASCII-compatible encoding schemes (which are
actually transfer encoding syntaxes) have been proposed as possible
solutions for internationalizing host names. Their goal is to be
able to encode any string of ISO/IEC 10646 characters as legal DNS
host names (as described in STD 13). At the time of this writing, no
ACE has become an IETF standard.
7. Other Common Terms In Internationalization
This is a hodge-podge of other terms that have appeared in
internationalization discussions in the IETF. It is likely that
additional terms will be added as this document matures.
locale
Locale is the user-specific location and cultural information
managed by a computer. <NONE>
Because languages differ from country to country (and even region to
region within a country), the locale of the user can often be an
important factor. Typically, the locale information for a user
includes the language(s) used.
Locale issues go beyond character use, and can include things such
as the display format for currency, dates, and times. Some locales
(especially the popular "C" and "POSIX" locales) do not include
language information.
It should be noted that there are many thorny, unsolved issues with
locale. For example, should text be viewed using the locale
information of the person who wrote the text or the person viewing
it? What if the person viewing it is travelling to different
locations? Should only some of the locale information affect
creation and editing of text?
Latin characters
"Latin characters" is a not-precise term for characters historically
related to ancient Greek script and currently used throughout the
world. <NONE>
The base Latin characters make up the ASCII repertoire and have been
augmented by many single and multiple diacritics and quite a few
other characters. ISO/IEC 10646 encodes the Latin characters in the
ranges U+0020..U+024F, U+1E00..U+1EFF, and other ranges.
romanization
The transliteration of a non-Latin script into Latin characters.
<NONE>
Because of the widespread use of Latin characters, people have tried
to represent many languages that are not based on a Latin repertoire
in Latin. For example, there are two popular romanizations of
Chinese: Wade-Giles and Pinyin, the latter of which is by far more
common today. Many romanization systems are inexact and do not give
perfect round trip mappings between the native script and the Latin
characters.
CJK characters and Han characters
The ideographic characters used in Chinese, Japanese, Korean, and
traditional Vietnamese writing systems are often called 'CJK
characters' after the initial letters of the language names in
English. They are also called "Han characters", after the term in
Chinese that is often used for these characters. <NONE>
Note that CJK and Han characters do not include the phonetic
characters used in the Japanese and Korean languages.
In ISO/IEC 10646, the Han characters were "unified", meaning that
each set of Han characters from Japanese, Chinese, and/or Korean
that had the same origin was assigned a single code point. The
positive result of this was that many fewer code points were needed
to represent Han; the negative result of this was that characters
that people who write the three languages think are different have
the same code point. There is a great deal of disagreement on the
nature, the origin, and the severity of the problems caused by Han
unification.
translation
The process of conveying the meaning of some passage of text in one
language, so that it can be expressed equivalently in another
language. <NONE>
Many language translation systems are inexact and cannot be applied
repeatedly to go from one language to another to another.
transliteration
The process of representing the characters of an alphabetical or
syllabic system of writing by the characters of a conversion
alphabet. <NONE>
Many script transliterations are exact, and many have perfect
round-trip mappings. The notable exception to this is romanization,
described above. Transliteration involves converting text expressed
in one script into another script, generally on a letter-by-letter
basis.
transcription
The process of systematically writing the sounds of some passage of
spoken language, generally with the use of a technical phonetic
alphabet (usually Latin-based) or other systematic transcriptional
orthography. Transcription also sometimes refers to the conversion
of written text into a transcribed (usually Latin-based) form, based
on the sound of the text as if it had been spoken. <NONE>
Unlike transliterations, which are generally designed to be
round-trip convertible, transcriptions of written material are
almost never round-trip convertible to their original form.
regular expressions
Regular expressions provide a mechanism to select specific strings
from a set of character strings. Regular expressions are a language
used to search for text within strings, and possibly modify the text
found with other text. <NONE>
Pattern matching for text involves being able to represent one or
more code points in an abstract notation, such as searching for all
capital Latin letters or all punctuation. The most common mechanism
in IETF protocols for naming such patterns is the use of regular
expressions. There is no single regular expression language, but
there are numerous very similar dialects.
The Unicode Consortium has a good discussion about how to adapt
regular expression engines to use Unicode. [UTR18]
private use
ISO/IEC 10646 code points from U+E000 to U+F8FF, U+F0000 to U+FFFFD,
and U+100000 to U+10FFFD are available for private use. This refers
to code points of the standard whose interpretation is not specified
by the standard and whose use may be determined by private agreement
among cooperating users. <UNICODE>
The use of these "private use" characters is defined by the parties
who transmit and receive them, and is thus not appropriate for
standardization. (The IETF has a long history of private use names
for things such as "x-" names in MIME types, charsets, and
languages. The experience with these has been quite negative, with
many implementors assuming that private use names are in fact public
and long-lived.)
8. Security Considerations
Security is not discussed in this document.
9. References
9.1 Normative References
[ISOIEC10646] ISO/IEC 10646-1:2000. International Standard --
Information technology -- Universal Multiple-Octet Coded Character Set
(UCS) -- Part 1: Architecture and Basic Multilingual Plane, 2000.
[UNICODE] The Unicode Standard, Version 3.2.0 is defined by The Unicode
Standard, Version 3.0 (Reading, MA, Addison-Wesley, 2000. ISBN
0-201-61633-5), as amended by the Unicode Standard Annex #27: Unicode
3.1 (http://www.unicode.org/reports/tr27/) and by the Unicode Standard
Annex #28: Unicode 3.2 (http://www.unicode.org/reports/tr28/), The
Unicode Consortium, 2002.
9.2 Non-normative References
[CHARMOD] Character Model for the World Wide Web 1.0, W3C,
<http://www.w3.org/TR/charmod/>.
[FRAMEWORK] ISO/IEC TR 11017:1997(E). Information technology - Framework
for internationalization, prepared by ISO/IEC JTC 1/SC 22/WG 20, 1997.
[ISO 639] ISO 639:2000 (E/F) - Code for the representation of names of
languages, 2000.
[ISO 3166] ISO 3166:1988 (E/F) - Codes for the representation of names
of countries, 2000.
[RFC2045] "MIME Part One: Format of Internet Message Bodies", N. Freed
and N. Borenstein, RFC 2045, November 1996.
[RFC2047] "MIME Part Three: Message Header Extensions for Non-ASCII
Text", K. Moore, RFC 2047, November 1996.
[RFC2277] "IETF Policy on Character Sets and Languages", H. Alvestrand,
RFC 2277 (also BCP 18), January 1998.
[RFC2279] "UTF-8, a transformation format of ISO 10646", F. Yergeau, RFC
2279, January 1998.
[RFC2781] "UTF-16, an encoding of ISO 10646", P. Hoffman and F. Yergeau,
RFC 2781, February 2000.
[RFC2822] "Internet Message Format", P. Resnick, RFC 2822, April 2001.
[RFC3066] "Tags for the Identification of Languages", H. Alvestrand, RFC
3066, January 2001.
[US-ASCII] Coded Character Set -- 7-bit American Standard Code for
Information Interchange, ANSI X3.4-1986, 1986.
[UTN6] "BOCU-1: MIME-Compatible Unicode Compression", M. Scherer & M.
Davis, Unicode Technical Note #6.
[UTR6] "A Standard Compression Scheme for Unicode", M. Wolf, et. al.,
Unicode Technical Report #6.
[UTR15] "Unicode Normalization Forms", M. Davis & M. Duerst, Unicode
Technical Report #15.
[UTR18] "Unicode Regular Expression Guidelines", M. Davis, Unicode
Technical Report #18.
[UTR19] "UTF-32", M. Davis, Unicode Technical Report #19.
[UTR22] "Character Mapping Markup Language", M. Davis, Unicode Technical
Report #22.
10. Additional Interesting Reading
ALA-LC Romanization Tables, Randall Barry (ed.), U.S. Library
of Congress, 1997, ISBN 0844409405
Blackwell Encyclopedia of Writing Systems, Florian Coulmas, Blackwell
Publishers, 1999, ISBN 063121481X
The World's Writing Systems, Peter Daniels and William Bright, Oxford
University Press, 1996, ISBN 0195079930
Writing Systems of the World, Akira Nakanishi, Charles E. Tuttle
Company, 1980, ISBN 0804816549
11. Index
alphabetic -- 4.1
ASCII-compatible encoding (ACE) -- 6
ASN.1 text formats -- 6
Base64 -- 6
Basic Multilingual Plane (BMP) -- 3.2
bidirectional display -- 5
BOCU-1 -- 3.2
case -- 4
character -- 2
character encoding form -- 2
character encoding scheme -- 2
charset -- 2
charset identification -- 6
CJK characters and Han characters -- 7
code chart -- 4
code table -- 4
coded character -- 2
coded character set -- 2
combining character -- 4
compatibility character -- 4.1
composite sequence -- 4
control character -- 4.1
diacritic -- 4.1
displaying and rendering text -- 2
font -- 5
formatting character -- 4.1
glyph -- 2
glyph code -- 2
graphic symbol -- 5
i18n, l10n -- 2
ideographic -- 4.1
input methods -- 5
internationalization -- 2
ISO -- 3.1
language -- 2
language identification -- 6
Latin characters -- 7
local and regional standards organizations -- 3.1
locale -- 7
localization -- 2
MIME -- 6
multilingual -- 2
name spaces -- 6
nonspacing character -- 4.1
normalization -- 4
on-the-wire encoding -- 6
parsed text -- 6
private use -- 7
protocol elements -- 6
punctuation -- 4.1
quoted printable -- 6
regular expressions -- 7
rendering rules -- 5
romanization -- 7
script -- 2
SCSU -- 3.2
sorting and collation -- 4
symbol -- 4.1
transcoding -- 2
transcription -- 7
transfer encoding syntax -- 6
translation -- 7
transliteration -- 7
UCS-2 and UCS-4 -- 3.2
undisplayable character -- 5
Unicode Consortium -- 3.1
UTF-32 -- 3.2
UTF-16, UTF-16BE, and UTF-16LE -- 3.2
UTF-8 -- 3.2
World Wide Web Consortium -- 3.1
XML -- 6
A. Acknowledgements
The definitions in this document come from many sources, including a
wide variety of IETF documents.
James Seng contributed to the initial outline of this document. Harald
Alvestrand and Martin Duerst made extensive useful comments on early
versions. Others who contributed to the development include:
Dan Kohn
Jacob Palme
Johan van Wingen
Peter Constable
Yuri Demchenko
Susan Harris
Zita Wenzel
John Klensin
Henning Schulzrinne
Leslie Daigle
Markus Scherer
Ken Whistler
B. Author Contact Information
Paul Hoffman
Internet Mail Consortium and VPN Consortium
127 Segre Place
Santa Cruz, CA 95060 USA
paul.hoffman@imc.org and paul.hoffman@vpnc.org
