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Network Working Group P. Hoffman
Request for Comments: 3536 IMC & VPNC
Category: Informational May 2003
Terminology Used in Internationalization in the IETF
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
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................................................... 2
1.1 Purpose of this document.................................... 2
1.2 Format of the definitions in this document.................. 3
2. Fundamental Terms.............................................. 3
3. Standards Bodies and Standards................................. 8
3.1 Standards bodies............................................ 8
3.2 Encodings and transformation formats of ISO/IEC 10646....... 10
3.3 Native CCSs and charsets.................................... 11
4. Character Issues............................................... 12
4.1 Types of characters......................................... 15
5. User interface for text........................................ 17
6. Text in current IETF protocols................................. 19
7. Other Common Terms In Internationalization..................... 22
8. Security Considerations........................................ 25
9. References..................................................... 25
9.1 Normative References........................................ 25
9.2 Informative References...................................... 26
10. Additional Interesting Reading................................ 27
11. Index......................................................... 27
A. Acknowledgements............................................... 29
B. Author's Address............................................... 29
Full Copyright Statement.......................................... 30
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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
informative 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]
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- 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>
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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
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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>
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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
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(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
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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
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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/>.
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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.
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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.
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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>").
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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>
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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".
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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.
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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.
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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.
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- 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.
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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>
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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".
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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>
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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?
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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.
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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.
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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.
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9.2 Informative 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] Freed, N. and N. Borenstein, "MIME Part One: Format of
Internet Message Bodies", November 1996.
[RFC2047] Moore, K., "MIME Part Three: Message Header Extensions
for Non-ASCII Text", RFC 2047, November 1996.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
[RFC2781] Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO
10646", RFC 2781, February 2000.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, 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.
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[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
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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
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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's Address
Paul Hoffman
Internet Mail Consortium and VPN Consortium
127 Segre Place
Santa Cruz, CA 95060 USA
EMail: paul.hoffman@imc.org and paul.hoffman@vpnc.org
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Full Copyright Statement
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This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
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English.
The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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