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RFC 3536 - Terminology Used in Internationalization in the IETF

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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.


   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

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]

      - 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

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.


      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.


      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

      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.


      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

         - 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.

   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.


      The collection of characters included in a character set.  Also
      called a character repertoire.  <UNICODE>


      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.


      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.


      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.


      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.


      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

      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

      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".


      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


      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

      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

   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

   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


      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".


      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

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

   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 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

      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 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.


      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>


      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>


      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>


      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.

      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


      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.

      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

   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

   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>


      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.

      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 (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.

      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 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


      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 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.


      The transliteration of a non-Latin script into Latin characters.

      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.


      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.


      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.


      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 Informative References

   [CHARMOD]     Character Model for the World Wide Web 1.0, W3C,

   [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

   [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.

   [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'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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