path: root/doc/rfc/rfc6365.txt

Internet Engineering Task Force (IETF)                        P. Hoffman
Request for Comments: 6365                                VPN Consortium
BCP: 166                                                      J. Klensin
Obsoletes: 3536                                           September 2011
Category: Best Current Practice
ISSN: 2070-1721


          Terminology Used in Internationalization in the IETF

Abstract

   This document provides a list 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.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6365.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.






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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Purpose of this Document . . . . . . . . . . . . . . . . .  3
     1.2.  Format of the Definitions in This Document . . . . . . . .  4
     1.3.  Normative Terminology  . . . . . . . . . . . . . . . . . .  4
   2.  Fundamental Terms  . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Standards Bodies and Standards . . . . . . . . . . . . . . . . 10
     3.1.  Standards Bodies . . . . . . . . . . . . . . . . . . . . . 11
     3.2.  Encodings and Transformation Formats of ISO/IEC 10646  . . 13
     3.3.  Native CCSs and Charsets . . . . . . . . . . . . . . . . . 15
   4.  Character Issues . . . . . . . . . . . . . . . . . . . . . . . 16
     4.1.  Types of Characters  . . . . . . . . . . . . . . . . . . . 20
     4.2.  Differentiation of Subsets . . . . . . . . . . . . . . . . 23
   5.  User Interface for Text  . . . . . . . . . . . . . . . . . . . 24
   6.  Text in Current IETF Protocols . . . . . . . . . . . . . . . . 27
   7.  Terms Associated with Internationalized Domain Names . . . . . 31
     7.1.  IDNA Terminology . . . . . . . . . . . . . . . . . . . . . 31
     7.2.  Character Relationships and Variants . . . . . . . . . . . 32
   8.  Other Common Terms in Internationalization . . . . . . . . . . 33
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 36
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 37
     10.2. Informative References . . . . . . . . . . . . . . . . . . 37
   Appendix A.  Additional Interesting Reading  . . . . . . . . . . . 41
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 42
   Appendix C.  Significant Changes from RFC 3536 . . . . . . . . . . 42
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43























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1.  Introduction

   As the IETF Character Set Policy specification [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.  Subject only to the limitations of their
   own knowledge and facilities, it should be possible for anyone to
   enter or read these text strings, which means that Internet users
   must be able to enter text using typical input methods and have it be
   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 terminology
   as it applies to IETF standards work by lightly covering the many
   aspects of internationalization and the vocabulary associated with
   those topics.  Some of the overview is somewhat tutorial in nature.
   It is not meant to be a complete description of internationalization.
   The definitions here SHOULD be used by IETF standards.  IETF
   standards that explicitly want to create different definitions for
   the terms defined here can do so, but unless an alternate definition
   is provided the definitions of the terms in this document apply.
   IETF standards that have a requirement for different definitions are
   encouraged, for clarity's sake, to find terms different than the ones
   defined here.  Some of the definitions in this document come from
   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.





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   This document uses definitions from many documents that have been
   developed inside and outside the IETF.  The primary documents used
   are:

   o  ISO/IEC 10646 [ISOIEC10646]

   o  The Unicode Standard [UNICODE]

   o  W3C Character Model [CHARMOD]

   o  IETF RFCs, including the Character Set Policy specification
      [RFC2277] and the domain name internationalization standard
      [RFC5890]

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 "<RFC6365>", 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 the reference sections of this
   document.

   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 an example 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".  See RFC 5137 [RFC5137] for a description of
   this notation.

1.3.  Normative Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].









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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 (see Section 4.2) and with other aspects of
   internationalization.

   language

      A language is a way that humans communicate.  The use of language
      occurs in many forms, the most common of which are speech,
      writing, and signing. <RFC6365>

      Some languages have a close relationship between the written and
      spoken forms, while others have a looser relationship.  The so-
      called LTRU (Language Tag Registry Update) standards [RFC5646]
      [RFC4647] discuss languages in more detail and provide identifiers
      for languages for use in Internet protocols.  Note that computer
      languages are explicitly excluded from this definition.

   script

      A set of graphic characters used for the written form of one or
      more languages. <ISOIEC10646>

      Examples of scripts are Latin, Cyrillic, Greek, Arabic, and Han
      (the characters, often called ideographs after a subset of them,
      used in writing Chinese, Japanese, and Korean).  RFC 2277
      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 to write more than one
      language; for example, the Russian and Bulgarian languages are
      written in the Cyrillic script.  Some languages can be expressed
      using different scripts or were used with different scripts at
      different times; the Mongolian language can be written in either
      the Mongolian or Cyrillic scripts; Malay is primarily written in
      Latin script today, but the earlier, Arabic-script-based, Jawa
      form is still in use; and a number of languages were converted



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      from other scripts to Cyrillic in the first half of the last
      century, some of which have switched again more recently.
      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.

   writing system

      A set of rules for using one or more scripts to write a particular
      language.  Examples include the American English writing system,
      the British English writing system, the French writing system, and
      the Japanese writing system. <UNICODE>

   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, but generalized to include digits and symbols of
         various sorts

      *  the encoded entity itself


      When people talk about characters, they usually intend one of the
      first two definitions.  The term "character" is often abbreviated
      as "char".

      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>






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   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 coded character set
      (CCS) 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 image of a character that can be displayed after
      being imaged onto a display surface. <RFC6365>

      The Unicode Standard has a different definition that refers to an
      abstract form that may represent different images when the same
      character is rendered under different circumstances.

   glyph code

      A glyph code is a numeric code that refers to a glyph.  Usually,
      the glyphs contained in a font are referenced by their glyph code.
      Glyph codes are local to a particular font; that is, a different
      font containing the same glyphs may use different codes. <UNICODE>

   transcoding

      Transcoding is the process of converting text data from one
      character encoding form to another.  Transcoders work only at the
      level of character encoding and do not parse the text.  Note:
      Transcoding may involve one-to-one, many-to-one, one-to-many, or
      many-to-many mappings.  Because some legacy mappings are glyphic,
      they may not only be many-to-many, but also unordered: 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.




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   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-16BE. <UNICODE>

      Some CESs are associated with a single CCS; for example, UTF-8
      [RFC3629] applies only to the identical CCSs of ISO/IEC 10646 and
      Unicode.  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
      [RFC2978]. <RFC6365>

      Many protocol definitions use the term "character set" in their
      descriptions.  The terms "charset", or "character encoding scheme"
      and "coded character set", are strongly preferred over the term
      "character set" because "character set" has other definitions in
      other contexts, particularly outside the IETF.  When reading IETF
      standards that use "character set" without defining the term, they
      usually mean "a specific combination of one CCS with a CES",
      particularly when they are talking about the "US-ASCII character
      set".

   internationalization

      In the IETF, "internationalization" means to add or improve the
      handling of non-ASCII text in a protocol. <RFC6365>  A different
      perspective, more appropriate to protocols that are designed for
      global use from the beginning, is the definition used by W3C:

         "Internationalization is the design and development of a
         product, application or document content that enables easy
         localization for target audiences that vary in culture, region,
         or language."  [W3C-i18n-Def]

      Many protocols that handle text only handle one charset
      (US-ASCII), or leave the question of what CCS and encoding are
      used up to local guesswork (which leads, of course, to
      interoperability problems).  If multiple charsets are permitted,
      they must be explicitly identified [RFC2277].  Adding non-ASCII
      text to a protocol allows the protocol to handle more scripts,
      hopefully all of the ones useful in the world.  In today's world,



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      that is normally best accomplished by allowing Unicode encoded in
      UTF-8 only, thereby shifting conversion issues away from
      individual choices.

   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 8 of this document.

   i18n, l10n

      These are abbreviations for "internationalization" and
      "localization". <RFC6365>

      "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".










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   multilingual

      The term "multilingual" has many widely varying definitions and
      thus is not recommended for use in standards.  Some of the
      definitions relate to the ability to handle international
      characters; other definitions relate to the ability to handle
      multiple charsets; and still others relate to the ability to
      handle multiple languages. <RFC6365>

   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. <RFC6365>

      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 straightforward, 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.










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3.1.  Standards Bodies

   ISO and ISO/IEC JTC 1

      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.  Most of ISO's work in information technology is performed
      jointly with a similar body, the International Electrotechnical
      Commission (IEC) through a joint committee known as "JTC 1".  ISO
      and ISO/IEC JTC 1 have 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", sometimes with a specific date.
      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 "SC2/WG2" or "WG2" for short.  ISO
      standards go through many steps before being finished, and years
      often go by between changes to the base ISO/IEC 10646 standard
      although amendments are now issued to track Unicode changes.
      Information on WG2, and its work products, can be found at
      <http://www.dkuug.dk/JTC1/SC2/WG2/>.  Information on SC2, and its
      work products, can be found at <http://www.iso.org/iso/
      standards_development/technical_committees/
      list_of_iso_technical_committees/
      iso_technical_committee.htm?commid=45050>

      The standard comes as a base part and a series of attachments or
      amendments.  It is available in PDF form for downloading or in a
      CD-ROM version.  One example of how to cite the standard is given
      in [RFC3629].  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 concerned about internationalization.
      ISO 639 [ISO639] specifies the names of languages and forms part
      of the basis for the IETF's Language Tag work [RFC5646].  ISO 3166
      [ISO3166] specifies the names and code abbreviations for countries
      and territories and is used in several protocols and databases
      including names for country-code top level domain names.  The
      responsibilities of ISO TC 46 on Information and Documentation
      <http://www.iso.org/iso/standards_development/
      technical_committees/list_of_iso_technical_committees/
      iso_technical_committee.htm?commid=48750> include a series of
      standards for transliteration of various languages into Latin
      characters.



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      Another relevant ISO group was JTC 1/SC22/WG20, which was
      responsible for internationalization in JTC 1, such as for
      international string ordering.  Information on WG20, and its work
      products, can be found at <http://www.dkuug.dk/jtc1/sc22/wg20/>.
      The specific tasks of SC22/WG20 were moved from SC22 into SC2, and
      there has been little significant activity since that occurred.

   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 that make it more useful in protocols, such as defining
      attributes for each character.  Examples of these attributes
      include case conversion and numeric properties.

      The actual technical and definitional work of the Unicode
      Consortium is done in the Unicode Technical Committee (UTC).  The
      terms "UTC" and "Unicode Consortium" are often treated,
      imprecisely, as synonymous in the IETF.

      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/>.

      A reciprocal agreement between the Unicode Consortium and
      ISO/IEC JTC 1/SC 2 provides for ISO/IEC 10646 and The Unicode
      Standard to track each other for definitions of characters and
      assignments of code points.  Updates, often in the form of
      amendments, to the former sometimes lag updates to the latter for
      a short period, but the gap has rarely been significant in recent
      years.

      At the time that the IETF character set policy [RFC2277] was
      established and the first version of this terminology
      specification was published, there was a strong preference in the
      IETF community for references to ISO/IEC 10646 (rather than
      Unicode) when possible.  That preference largely reflected a more
      general IETF preference for referencing established open
      international standards over specifications from consortia.
      However, the Unicode definitions of character properties and
      classes are not part of ISO/IEC 10646.  Because IETF
      specifications are increasingly dependent on those definitions



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      (for example, see the explanation in Section 4.2) and the Unicode
      specifications are freely available online in convenient machine-
      readable form, the IETF's preference has shifted to referencing
      the Unicode Standard.  The latter is especially important when
      version consistency between code points (either standard) and
      Unicode properties (Unicode only) is required.

   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.  However, in some
      circumstances, XML files may be sensitive to differences among
      Unicode versions.

   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), CEN/ISSS
      (Europe), JIS (Japan), and SAC (China).

3.2.  Encodings and Transformation Formats of ISO/IEC 10646

   Characters in the ISO/IEC 10646 CCS can be expressed in many ways.
   Historically, "encoding forms" are both direct addressing methods,
   while "transformation formats" are methods for expressing encoding
   forms as bits on the wire.  That distinction has mostly disappeared
   in recent years.

   Documents that discuss characters in the ISO/IEC 10646 CCS often need
   to list specific characters.  RFC 5137 describes the common methods
   for doing so in IETF documents, and these practices have been adopted
   by many other communities as well.

   Basic Multilingual Plane (BMP)

      The BMP is composed of the first 2^16 code points in ISO/IEC 10646
      and contains almost all characters in contemporary use.  The BMP
      is also called "Plane 0".

   UCS-2 and UCS-4

      UCS-2 and UCS-4 are the two encoding forms historically 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 consider UCS-2 to be obsolete.



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      UCS-4 addresses the entire range of code points from ISO/IEC 10646
      (by agreement between ISO/IEC JTC 1 SC2 and the Unicode
      Consortium, a range from 0..0x10FFFF) as 32-bit values with zero
      padding to the left.  UCS-4 is identical to UTF-32BE (without use
      of a BOM (see below)); UTF-32BE is now the preferred term.

   UTF-8

      UTF-8 [RFC3629] 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.  The IETF-specific
      definition of UTF-8 in RFC 3629 is identical to that in recent
      versions of the Unicode Standard (e.g., in Section 3.9 of Version
      6.0 [UNICODE]).

   UTF-16, UTF-16BE, and UTF-16LE

      UTF-16, UTF-16BE, and UTF-16LE, three transformation formats
      described in [RFC2781] and defined in The Unicode Standard
      (Sections 3.9 and 16.8 of Version 6.0), are not required by any
      IETF standards, and are thus used much less often in protocols
      than UTF-8.  Characters in the BMP are always encoded as two
      octets, and characters outside the BMP are encoded as four octets
      using a "surrogate pair" arrangement.  The latter is not part of
      UCS-2, marking the difference between UTF-16 and UCS-2.  The three
      UTF-16 formats differ based on the order of the octets and the
      presence or absence of a special lead-in ordering identifier
      called the "byte order mark" or "BOM".

   UTF-32

      The Unicode Consortium and ISO/IEC JTC 1 have defined UTF-32 as a
      transformation format that incorporates the integer code point
      value right-justified in a 32-bit field.  As with UTF-16, the byte
      order mark (BOM) can be used and UTF-32BE and UTF-32LE are
      defined.  UTF-32 and UCS-4 are essentially equivalent and the
      terms are often used interchangeably.

   SCSU and BOCU-1

      The Unicode Consortium has defined an encoding, SCSU [UTR6], which
      is designed to offer good compression for typical text.  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.



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      The compression provided as a side effect of the Punycode
      algorithm [RFC3492] is heavily used in some contexts, especially
      IDNA [RFC5890], but imposes some restrictions.  (See also
      Section 7.)

3.3.  Native CCSs and Charsets

   Before ISO/IEC 10646 was developed, many countries developed their
   own CCSs and charsets.  Some of these were adopted into international
   standards for the relevant scripts or writing systems.  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.  ASCII became the basis for
   ISO/IEC 646 which, in turn, formed the basis for many national and
   international standards, such as the ISO 8859 series, that mix Basic
   Latin characters with characters from another script.

   It is important to note that, strictly speaking, "ASCII" is a CCS and
   repertoire, not an encoding.  The encoding used for ASCII in IETF
   protocols involves the 7-bit integer ASCII code point right-justified
   in an 8-bit field and is sometimes described as the "Network Virtual
   Terminal" or "NVT" encoding [RFC5198].  Less formally, "ASCII" and
   "NVT" are often used interchangeably.  However, "non-ASCII" refers
   only to characters outside the ASCII repertoire and is not linked to
   a specific encoding.  See Section 4.2.

   A Unicode publication describes issues involved in mapping character
   data between charsets, and an XML format for mapping table data
   [UTR22].










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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.

   code point

      A value in the codespace of a repertoire.  For all common
      repertoires developed in recent years, code point values are
      integers (code points for ASCII and its immediate descendants were
      defined in terms of column and row positions of a table).

   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.  Combining
      characters are inherently non-spacing. <ISOIEC10646>

   composite sequence or combining character 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.  The Unicode Standard often uses the term "combining
      character sequence" to refer to composite sequences.  A composite
      sequence is not a character and therefore is not a member of the
      repertoire of ISO/IEC 10646. <ISOIEC10646>  However, Unicode now
      assigns names to some such sequences especially when the names are
      required to match terminology in other standards [UAX34].

      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.  In the same or other CCSs,
      it might be available as a single code point.  The rules for
      combining two or more characters are called "composition rules",
      and the rules for taking apart a character into other characters
      are called "decomposition rules".  The result of decomposition is
      called a "decomposed character"; the result of composition is
      usually a "precomposed character".





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   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 strings with the same meaning as words
      (such as "color" and "colour").  Instead, it means unifying
      different character sequences that are intended to form the same
      composite characters, such as "<n><combining tilde>" and "<n with
      tilde>" (where "<n>" is U+006E, "<combining tilde>" is U+0303, and
      "<n with tilde>" is U+00F1).

      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.  However, in Unicode, "canonicalization" or similar terms
      are used to refer to a particular type of normalization
      equivalence ("canonical equivalence" in contrast to "compatibility
      equivalence"), so the term should be used with some care.  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.  Unicode Standard Annex #15 [UTR15]
      describes the process and many forms of normalization in detail.
      Normalization is important when comparing strings to see if they
      are the same.

      The Unicode NFC and NFD normalizations support canonical
      equivalence; NFKC and NFKD support canonical and compatibility
      equivalence.











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   case

      Case is the feature of certain alphabets where the letters have
      two (or occasionally more) distinct forms.  These forms, 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 that 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", some Greek
      characters with modifiers, and characters like the German Sharp S
      (Eszett) and Greek Final Sigma that traditionally do not have
      uppercase forms.  Case mapping can even be dependent on locale or
      language.  Converting text to have only a single case, primarily
      for comparison purposes, is called "case folding".  Because of the
      various unusual cases, case mapping can be quite controversial and
      some case folding algorithms even more so.  For example, some
      programming languages such as Java have case-folding algorithms
      that are locale-sensitive; this makes those algorithms incredibly
      resource-intensive and makes them act differently depending on the
      location of the system at the time the algorithm is used.

   sorting and collation

      Collating is the process of ordering units of textual information.
      Collation is usually specific to a particular language or even to
      a particular application or locale.  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.

      The relative orders of letters in collation sequences can differ
      widely based on the needs of the system or protocol defining the
      collation order.  For example, even within ASCII characters, there
      are two common and very different collation orders: "A, a, B,
      b,..." and "A, B, C, ..., Z, a, b,...", with additional variations
      for lowercase first and digits before and after letters.




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      In practice, it is rarely necessary to define a collation sequence
      for characters drawn from different scripts, but arranging such
      sequences so as to not surprise users is usually particularly
      problematic.

      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 the desired 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
      (sorted) sequence.  For only a few CCS/CES combinations, there is
      an obvious sort order that can be applied without reference to the
      linguistic meaning of the characters: the code point order is
      sufficient for sorting.  That is, the code point order is also the
      order that a person would use in sorting the characters.  For many
      CCS/CES combinations, the code point order would make no sense to
      a person and therefore is not useful for sorting if the results
      will be displayed to a person.

      Code point 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" or "human
      language" sort.  Sorting to code point 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 non-
      combining.  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 variants 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>

      While Unicode and many other systems use this term to refer to all
      Han characters, strictly speaking not all of those characters are
      actually ideographic.  Some are pictographic (such as the
      telephone example above), some are used phonetically, and so on.
      However, the convention is to describe the script as ideographic
      as contrasted to alphabetic.

   digit or number

      All modern writing systems use decimal digits in some form; some
      older ones use non-positional or other systems.  Different scripts
      may have their own digits.  Unicode distinguishes between numbers
      and other kinds of characters by assigning a special General
      Category value to them and subdividing that value to distinguish
      between decimal digits, letter digits, and other digits. <UNICODE>








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   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. <RFC6365>

      Examples of symbols include characters for mathematical operators,
      symbols for optical character recognition (OCR), symbols for box-
      drawing or graphics, as well as symbols for dingbats, arrows,
      faces, and geometric shapes.  Unicode has a property that
      identifies symbol characters.

   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 they change 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.
      The basic space character, U+0020, is often considered as a
      control character as well, making the total number 66.  They are
      also known as control codes.  In terminology adopted by Unicode
      from ASCII and the ISO 8859 standards, these codes are treated as
      belonging to three ranges: "C0" (for U+0000..U+001F), "C1" (for
      U+0080...U+009F), and the single control character "DEL" (U+007F).
      <UNICODE>



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      Occasionally, in other vocabularies, the term "control character"
      is used to describe any character that does not normally have an
      associated glyph; it is also sometimes used for device control
      sequences [ISO6429].  Neither of those usages is appropriate to
      internationalization terminology in the IETF.

   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 or compatibility variant

      A graphic character included as a coded character of ISO/IEC 10646
      primarily for compatibility with existing coded character sets.
      <ISOIEC10646)>

      The Unicode definition of compatibility charter also includes
      characters that have been incorporated for other reasons.  Their
      list includes several separate groups of characters included for
      compatibility purposes: halfwidth and fullwidth characters used
      with East Asian scripts, Arabic contextual forms (e.g., initial or
      final forms), some ligatures, deprecated formatting characters,
      variant forms of characters (or even copies of them) for
      particular uses (e.g., phonetic or mathematical applications),
      font variations, CJK compatibility ideographs, and so on.  For
      additional information and the separate term "compatibility
      decomposable character", see the Unicode standard.

      For example, U+FF01 (FULLWIDTH EXCLAMATION MARK) was included for
      compatibility with Asian charsets that include full-width and
      half-width ASCII characters.

      Some efforts in the IETF have concluded that it would be useful to
      support mapping of some groups of compatibility equivalents and
      not others (e.g., supporting or mapping width variations while
      preserving or rejecting mathematical variations).  See the IDNA
      Mapping document [RFC5895] for one example.









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4.2.  Differentiation of Subsets

   Especially as existing IETF standards are internationalized, it is
   necessary to describe collections of characters including especially
   various subsets of Unicode.  Because Unicode includes ways to code
   substantially all characters in contemporary use, subsets of the
   Unicode repertoire can be a useful tool for defining these
   collections as repertoires independent of specific Unicode coding.

   However specific collections are defined, it is important to remember
   that, while older CCSs such as ASCII and the ISO 8859 family are
   close-ended and fixed, Unicode is open-ended, with new character
   definitions, and often new scripts, being added every year or so.
   So, while, e.g., an ASCII subset, such as "uppercase letters", can be
   specified as a range of code points (4/1 to 5/10 for that example),
   similar definitions for Unicode either have to be specified in terms
   of Unicode properties or are very dependent on Unicode versions (and
   the relevant version must be identified in any specification).  See
   the IDNA code point specification [RFC5892] for an example of
   specification by combinations of properties.

   Some terms are commonly used in the IETF to define character ranges
   and subsets.  Some of these are imprecise and can cause confusion if
   not used carefully.

   non-ASCII

      The term "non-ASCII" strictly refers to characters other than
      those that appear in the ASCII repertoire, independent of the CCS
      or encoding used for them.  In practice, if a repertoire such as
      that of Unicode is established as context, "non-ASCII" refers to
      characters in that repertoire that do not appear in the ASCII
      repertoire.  "Outside the ASCII repertoire" and "outside the ASCII
      range" are practical, and more precise, synonyms for "non-ASCII".

   letters

      The term "letters" does not have an exact equivalent in the
      Unicode standard.  Letters are generally characters that are used
      to write words, but that means very different things in different
      languages and cultures.










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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. <RFC6365>

      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 or Vietnamese
      characters that have two or three diacritics on a single
      alphabetic character.

      The term "input method editor" (IME) is often used generically to
      describe the tools and software used to deal with input of
      characters on a particular system.

   rendering rules

      A rendering rule is an algorithm that a system uses to decide how
      to display a string of text. <RFC6365>

      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.







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      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 and other characters and with right-to-left and
         left-to-right ordering marks.  The rendering rules must choose
         the order that characters are displayed.

      *  Some writing systems cannot have their rendering rules suitably
         defined using mechanisms that are now defined in the Unicode
         Standard.  None of those languages are in active non-scholarly
         use today.

      *  Many systems use a special rendering rule when they lack a font
         or other mechanism for rendering a particular character
         correctly.  That rule typically involves substitution of a
         small open box or a question mark for the missing character.
         See "undisplayable character" below.

   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, generates a collection of
      imagable glyphs. <UNICODE>

      The term "font" is often used interchangeably with "typeface".  As
      historically used in typography, a typeface is a family of one or
      more fonts that share a common general design.  For example,
      "Times Roman" is actually a typeface, with a collection of fonts



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      such as "Times Roman Bold", "Times Roman Medium", "Times Roman
      Italic", and so on.  Some sources even consider different type
      sizes within a typeface to be different fonts.  While those
      distinctions are rarely important for internationalization
      purposes, there are exceptions.  Those writing specifications
      should be very careful about definitions in cases in which the
      exceptions might lead to ambiguity.

   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, often abbreviated as "bidi". <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 primarily right-to-left
      (numerals are a common exception in the modern scripts).  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, an order that might be different from what they see on the
      screen.  (Note that some languages are written both horizontally
      and vertically and that some historical ones use other display
      orderings.)

      Further, bidirectional text can cause confusion because there are
      formatting characters in ISO/IEC 10646 that 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.  Text that might contain
      those characters typically requires careful processing before
      being sorted or compared for equality.

      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 [UAX9].

   undisplayable character

      A character that has no displayable form. <RFC6365>

      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]



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      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.

      The property of a character that causes it to be undisplayable is
      intrinsic to its definition.  Undisplayable characters can never
      be displayed in normal text (the dashed square notation is used
      only in special circumstances).  Printable characters whose
      Unicode definitions are associated with glyphs that cannot be
      rendered on a particular system are not, in this sense,
      undisplayable.

   writing style

      Conventions of writing the same script in different styles.
      <RFC6365>

      Different communities using the script may find text in different
      writing styles difficult to read and possibly unintelligible.  For
      example, the Perso-Arabic Nastalique writing style and the Arabic
      Naskh writing style both use the Arabic script but have very
      different renderings and are not mutually comprehensible.  Writing
      styles may have significant impact on internationalization; for
      example, the Nastalique writing style requires significantly more
      line height than Naskh writing style.

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.
      <RFC6365>

      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, and it is rarely appropriate to
      internationalize protocol element names (even while the elements
      themselves can be internationalized).



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   name spaces

      A name space is the set of valid names for a particular item, or
      the syntactic rules for generating these valid names. <RFC6365>

      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. <RFC6365>

      Characters are identified by code points.  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 have been analyzed for subparts. <RFC6365>

      In some protocols, free text in text fields might be parsed.  For
      example, many mail user agents (MUAs) will parse the words in the
      text of the Subject: field to attempt to thread based on what
      appears after the "Re:" prefix.

      Such conventions are very sensitive to localization.  If, for
      example, a form like "Re:" is altered by an MUA to reflect the
      language of the sender or recipient, a system that subsequently
      does threading may not recognize the replacement term as a
      delimiter string.

   charset identification

      Specification of the charset used for a string of text. <RFC6365>

      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".



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   language identification

      Specification of the human language used for a string of text.
      <RFC6365>

      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".
      The IETF "LTRU" standards [RFC5646] and [RFC4647] provide a
      comprehensive model for language identification.

   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 RFC 5322, the standard for Internet mail headers
      [RFC5322]).  MIME is described in RFCs 2045 through 2049, as well
      as more recent RFCs. <RFC6365>

   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.
      <RFC6365>

      TESs are useful for encoding types of character data into 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.  MIME identifies the transfer
      encoding syntax for body parts as a Content-transfer-encoding,
      occasionally abbreviated C-T-E.

   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]. <RFC6365>

   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]. <RFC6365>



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      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 that
      is not encoded as UTF-8 is explicitly tagged with charsets, and
      all text in XML consists only of Unicode characters.  The
      specification for XML can be found at <http://www.w3.org/XML/>.
      <RFC6365>

   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).

   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 and some
      other purposes.  Their goal is to be able to encode any string of
      ISO/IEC 10646 characters using the preferred syntax for domain
      names (as described in STD 13).  At the time of this writing, only
      the ACE produced by Punycode [RFC3492] has become an IETF
      standard.

      The choice of ACE forms to internationalize legacy protocols must
      be made with care as it can cause some difficult side effects
      [RFC6055].

   LDH label

      The classical label form used in the DNS and most applications
      that call on it, albeit with some additional restrictions,
      reflects the early syntax of "hostnames" [RFC0952] and limits
      those names to ASCII letters, digits, and embedded hyphens.  The
      hostname syntax is identical to that described as the "preferred
      name syntax" in Section 3.5 of RFC 1034 [RFC1034] as modified by




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      RFC 1123 [RFC1123].  LDH labels are defined in a more restrictive
      and precise way for internationalization contexts as part of the
      IDNA2008 specification [RFC5890].

7.  Terms Associated with Internationalized Domain Names

7.1.  IDNA Terminology

   The current specification for Internationalized Domain Names (IDNs),
   known formally as Internationalized Domain Names for Applications or
   IDNA, is referred to in the IETF and parts of the broader community
   as "IDNA2008" and consists of several documents.  Section 2.3 of the
   first of those documents, commonly known as "IDNA2008 Definitions"
   [RFC5890] provides definitions and introduces some specialized terms
   for differentiating among types of DNS labels in an IDN context.
   Those terms are listed in the table below; see RFC 5890 for the
   specific definitions if needed.

      ACE Prefix
      A-label
      Domain Name Slot
      IDNA-valid string
      Internationalized Domain Name (IDN)
      Internationalized Label
      LDH Label
      Non-Reserved LDH label (NR-LDH label)
      U-label

   Two additional terms entered the IETF's vocabulary as part of the
   earlier IDN effort [RFC3490] (IDNA2003):

      Stringprep

         Stringprep [RFC3454] provides a model and character tables for
         preparing and handling internationalized strings.  It was used
         in the original IDN specification (IDNA2003) via a profile
         called "Nameprep" [RFC3491].  It is no longer in use in IDNA,
         but continues to be used in profiles by a number of other
         protocols. <RFC6365>

      Punycode

         This is the name of the algorithm [RFC3492] used to convert
         otherwise-valid IDN labels from native-character strings
         expressed in Unicode to an ASCII-compatible encoding (ACE).
         Strictly speaking, the term applies to the algorithm only.  In
         practice, it is widely, if erroneously, used to refer to
         strings that the algorithm encodes.



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7.2.  Character Relationships and Variants

   The term "variant" was introduced into the IETF i18n vocabulary with
   the JET recommendations [RFC3743].  As used there, it referred
   strictly to the relationship between Traditional Chinese characters
   and their Simplified equivalents.  The JET recommendations provided a
   model for identifying these pairs of characters and labels that used
   them.  Specific recommendations for variant handling for the Chinese
   language were provided in a follow-up document [RFC4713].

   In more recent years, the term has also been used to describe other
   collections of characters or strings that might be perceived as
   equivalent.  Those collections have involved one or more of several
   categories of characters and labels containing them including:

   o  "visually similar" or "visually confusable" characters.  These may
      be limited to characters in different scripts, characters in a
      single script, or both, and may be those that can appear to be
      alike even when high-distinguishability reference fonts are used
      or under various circumstances that may involve malicious choices
      of typefaces or other ways to trick user perception.  Trivial
      examples include ASCII "l" and "1" and Latin and Cyrillic "a".

   o  Characters assigned more than one Unicode code point because of
      some special property.  These characters may be considered "the
      same" for some purposes and different for others (or by other
      users).  One of the most commonly cited examples is the Arabic
      YEH, which is encoded more than once because some of its shapes
      are different across different languages.  Another example are the
      Greek lowercase sigma and final sigma: if the latter were viewed
      purely as a positional presentation variation on the former, it
      should not have been assigned a separate code point.

   o  Numerals and labels including them.  Unlike letters, the "meaning"
      of decimal digits is clear and unambiguous regardless of the
      script with which they are associated.  Some scripts are routinely
      used almost interchangeably with European digits and digits native
      to that script.  The Arabic script has two sets of digits
      (U+0660..U+0669 and U+06F0..U=06F9), written identically for zero
      through three and seven through nine but differently for four
      through six; European digits predominate in other areas.
      Substitution of digits with the same numeric value in labels may
      give rise to another type of variant.

   o  Orthographic differences within a language.  Many languages have
      alternate choices of spellings or spellings that differ by locale.
      Users of those languages generally recognize the spellings as
      equivalent, at least as much so as the variations described above.



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      Examples include "color" and "colour" in English, German words
      spelled with o-umlaut or "oe", and so on.  Some of these
      relationships may also create other types of language-specific
      perceived differences that do not exist for other languages using
      the same script.  For example, in Arabic language usage at the end
      of words, ARABIC LETTER TEH MARBUTA (U+0629) and ARABIC LETTER HEH
      (U+0647) are differently shaped (one has 2 dots in top of it), but
      they are used interchangeably in writing: they "sound" similar
      when pronounced at the end of phrase, and hence the LETTER TEH
      MARBUTA sometimes is written as LETTER HEH and the two are
      considered "confusable" in that context.

   The term "variant" as used in this section should also not be
   confused with other uses of the term in this document or in Unicode
   terminology (e.g., those in Section 4.1 above).  If the term is to be
   used at all, context should clearly distinguish among these different
   uses and, in particular, between variant characters and variant
   labels.  Local text should identify which meaning, or combination of
   meanings, are intended.

8.  Other Common Terms in Internationalization

   This is a hodge-podge of other terms that have appeared in
   internationalization discussions in the IETF.

   locale

      Locale is the user-specific location and cultural information
      managed by a computer. <RFC6365>

      Because languages and orthographic conventions 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, information that
      would apply to the location of the system storing or providing the
      text, or the person viewing it?  What if the person viewing it is
      traveling 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 as modified in the
      Roman Republic and Empire and currently used throughout the world.
      <RFC6365>

      The base Latin characters are a subset of 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 including ranges U+0020..U+024F and U+1E00..U+1EFF.

      Because "Latin characters" is used in different contexts to refer
      to the letters from the ASCII repertoire, the subset of those
      characters used late in the Roman Republic period, or the
      different subset used to write Latin in medieval times, the entire
      ASCII repertoire, all of the code points in the extended Latin
      script as defined by Unicode, and other collections, the term
      should be avoided in IETF specifications when possible.
      Similarly, "Basic Latin" should not be used as a synonym for
      "ASCII".

   romanization

      The transliteration of a non-Latin script into Latin characters.
      <RFC6365>

      Because of their widespread use, Latin characters (or graphemes
      constructed from them) are often used to try to write text in
      languages that didn't previously have writing systems or whose
      writing systems were originally based on different scripts.  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. <RFC6365>







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      Note that Han characters do not include the phonetic characters
      used in the Japanese and Korean languages.  Users of the term "CJK
      characters" may or may not assume those additional characters are
      included.

      In ISO/IEC 10646, the Han characters were "unified", meaning that
      each set of Han characters from Japanese, Chinese, and/or Korean
      that had the same origin was assigned a single code point.  The
      positive result of this was that many fewer code points were
      needed to represent Han; the negative result of this was that
      characters that people who write the three languages think are
      different have the same code point.  There is a great deal of
      disagreement on the nature, the origin, and the severity of the
      problems caused by Han unification.

   translation

      The process of conveying the meaning of some passage of text in
      one language, so that it can be expressed equivalently in another
      language. <RFC6365>

      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. <RFC6365>

      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.  There are many official
      and unofficial transliteration standards, most notably those from
      ISO TC 46 and the U.S. Library of Congress.

   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 form, based on the
      sound of the text as if it had been spoken. <RFC6365>





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      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, at least
      without some supplemental information.

   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. <RFC6365>

      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 that are
      not quite consistent with each other.

      The Unicode Consortium has a good discussion about how to adapt
      regular expression engines to use Unicode.  [UTR18]

   private use character

      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.  Most of the experience with these has been quite
      negative, with many implementors assuming that private use names
      are in fact public and long-lived.)

9.  Security Considerations

   Security is not discussed directly in this document.  While the
   definitions here have no direct effect on security, they are used in
   many security contexts.  For example, authentication usually involves
   comparing two tokens, and one or both of those tokens might be text;
   thus, some methods of comparison might involve using some of the
   internationalization concepts for which terms are defined in this
   document.



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   Having said that, other RFCs dealing with internationalization have
   security consideration descriptions that may be useful to the reader
   of this document.  In particular, the security considerations in RFC
   3454, RFC 3629, RFC 4013 [RFC4013], and RFC 5890 go into a fair
   amount of detail.

10.  References

10.1.  Normative References

   [ISOIEC10646]   ISO/IEC, "ISO/IEC 10646:2011.  International Standard
                   -- Information technology - Universal Multiple-Octet
                   Coded Character Set (UCS)", 2011.

   [RFC2047]       Moore, K., "MIME (Multipurpose Internet Mail
                   Extensions) Part Three: Message Header Extensions for
                   Non-ASCII Text", RFC 2047, November 1996.

   [UNICODE]       The Unicode Consortium, "The Unicode Standard,
                   Version 6.0", (Mountain View, CA: The Unicode
                   Consortium, 2011. ISBN 978-1-936213-01-6).
                   <http://www.unicode.org/versions/Unicode6.0.0/>.

10.2.  Informative References

   [CHARMOD]       W3C, "Character Model for the World Wide Web 1.0",
                   2005, <http://www.w3.org/TR/charmod/>.

   [FRAMEWORK]     ISO/IEC, "ISO/IEC TR 11017:1997(E).  Information
                   technology - Framework for internationalization,
                   prepared by ISO/IEC JTC 1/SC 22/WG 20", 1997.

   [ISO3166]       ISO, "ISO 3166-1:2006 - Codes for the representation
                   of names of countries and their subdivisions -- Part
                   1: Country codes", 2006.

   [ISO639]        ISO, "ISO 639-1:2002 - Code for the representation of
                   names of languages - Part 1: Alpha-2 code", 2002.

   [ISO6429]       ISO/IEC, "ISO/IEC, "ISO/IEC 6429:1992.  Information
                   technology -- Control functions for coded character
                   sets"", ISO/IEC 6429:1992, 1992.

   [RFC0952]       Harrenstien, K., Stahl, M., and E. Feinler, "DoD
                   Internet host table specification", RFC 952,
                   October 1985.





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   [RFC1034]       Mockapetris, P., "Domain names - concepts and
                   facilities", STD 13, RFC 1034, November 1987.

   [RFC1123]       Braden, R., "Requirements for Internet Hosts -
                   Application and Support", STD 3, RFC 1123,
                   October 1989.

   [RFC2045]       Freed, N. and N. Borenstein, "Multipurpose Internet
                   Mail Extensions (MIME) Part One: Format of Internet
                   Message Bodies", RFC 2045, November 1996.

   [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2277]       Alvestrand, H., "IETF Policy on Character Sets and
                   Languages", BCP 18, RFC 2277, January 1998.

   [RFC2781]       Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
                   ISO 10646", RFC 2781, February 2000.

   [RFC2978]       Freed, N. and J. Postel, "IANA Charset Registration
                   Procedures", BCP 19, RFC 2978, October 2000.

   [RFC3454]       Hoffman, P. and M. Blanchet, "Preparation of
                   Internationalized Strings ("stringprep")", RFC 3454,
                   December 2002.

   [RFC3490]       Faltstrom, P., Hoffman, P., and A. Costello,
                   "Internationalizing Domain Names in Applications
                   (IDNA)", RFC 3490, March 2003.

   [RFC3491]       Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
                   Profile for Internationalized Domain Names (IDN)",
                   RFC 3491, March 2003.

   [RFC3492]       Costello, A., "Punycode: A Bootstring encoding of
                   Unicode for Internationalized Domain Names in
                   Applications (IDNA)", RFC 3492, March 2003.

   [RFC3629]       Yergeau, F., "UTF-8, a transformation format of ISO
                   10646", STD 63, RFC 3629, November 2003.

   [RFC3743]       Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
                   Engineering Team (JET) Guidelines for
                   Internationalized Domain Names (IDN) Registration and
                   Administration for Chinese, Japanese, and Korean",
                   RFC 3743, April 2004.




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   [RFC4013]       Zeilenga, K., "SASLprep: Stringprep Profile for User
                   Names and Passwords", RFC 4013, February 2005.

   [RFC4647]       Phillips, A. and M. Davis, "Matching of Language
                   Tags", BCP 47, RFC 4647, September 2006.

   [RFC4713]       Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin,
                   "Registration and Administration Recommendations for
                   Chinese Domain Names", RFC 4713, October 2006.

   [RFC5137]       Klensin, J., "ASCII Escaping of Unicode Characters",
                   BCP 137, RFC 5137, February 2008.

   [RFC5198]       Klensin, J. and M. Padlipsky, "Unicode Format for
                   Network Interchange", RFC 5198, March 2008.

   [RFC5322]       Resnick, P., Ed., "Internet Message Format",
                   RFC 5322, October 2008.

   [RFC5646]       Phillips, A. and M. Davis, "Tags for Identifying
                   Languages", BCP 47, RFC 5646, September 2009.

   [RFC5890]       Klensin, J., "Internationalized Domain Names for
                   Applications (IDNA): Definitions and Document
                   Framework", RFC 5890, August 2010.

   [RFC5892]       Faltstrom, P., "The Unicode Code Points and
                   Internationalized Domain Names for Applications
                   (IDNA)", RFC 5892, August 2010.

   [RFC5895]       Resnick, P. and P. Hoffman, "Mapping Characters for
                   Internationalized Domain Names in Applications (IDNA)
                   2008", RFC 5895, September 2010.

   [RFC6055]       Thaler, D., Klensin, J., and S. Cheshire, "IAB
                   Thoughts on Encodings for Internationalized Domain
                   Names", RFC 6055, February 2011.

   [UAX34]         The Unicode Consortium, "Unicode Standard Annex #34:
                   Unicode Named Character Sequences", 2010,
                   <http://www.unicode.org/reports/tr34>.

   [UAX9]          The Unicode Consortium, "Unicode Standard Annex #9:
                   Unicode Bidirectional Algorithm", 2010,
                   <http://www.unicode.org/reports/tr9>.






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   [US-ASCII]      ANSI, "Coded Character Set -- 7-bit American Standard
                   Code for Information Interchange, ANSI X3.4-1986",
                   1986.

   [UTN6]          The Unicode Consortium, "Unicode Technical Note #5:
                   BOCU-1: MIME-Compatible Unicode Compression", 2006,
                   <http://www.unicode.org/notes/tn6/>.

   [UTR15]         The Unicode Consortium, "Unicode Standard Annex #15:
                   Unicode Normalization Forms", 2010,
                   <http://www.unicode.org/reports/tr15>.

   [UTR18]         The Unicode Consortium, "Unicode Standard Annex #18:
                   Unicode Regular Expressions", 2008,
                   <http://www.unicode.org/reports/tr18>.

   [UTR22]         The Unicode Consortium, "Unicode Technical Standard
                   #22: Unicode Character Mapping Markup Language",
                   2009, <http://www.unicode.org/reports/tr22>.

   [UTR6]          The Unicode Consortium, "Unicode Technical Standard
                   #6: A Standard Compression Scheme for Unicode", 2005,
                   <http://www.unicode.org/reports/tr6>.

   [W3C-i18n-Def]  W3C, "Localization vs. Internationalization",
                   September 2010, <http://www.w3.org/International/
                   questions/qa-i18n.en>.
























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Appendix A.  Additional Interesting Reading

   Barry, Randall, ed.  ALA-LC Romanization Tables.  Washington: U.S.
   Library of Congress, 1997.  ISBN 0844409405

   Coulmas, Florian.  Blackwell Encyclopedia of Writing Systems.
   Oxford: Blackwell Publishers, 1999.  ISBN 063121481X

   Dalby, Andrew.  Dictionary of Languages: The Definitive Reference to
   More than 400 Languages.  New York: Columbia University Press, 2004.
   ISBN 978-0231115698

   Daniels, Peter, and William Bright.  The World's Writing Systems.
   New York: Oxford University Press, 1996.  ISBN 0195079930

   DeFrancis, John.  The Chinese Language: Fact and Fantasy.  Honolulu:
   University of Hawaii Press, 1984.  ISBN 0-8284-085505 and
   0-8248-1058-6

   Drucker, Joanna.  The Alphabetic Labyrinth: The Letters in History
   and Imagination.  London: Thames & Hudson, 1995.  ISBN 0-500-28068-1

   Fazzioli, Edoardo.  Chinese Calligraphy.  New York: Abbeville Press,
   1986, 1987 (English translation).  ISBN 0-89659-774-1

   Hooker, J.T., et al.  Reading the Past: Ancient Writing from
   Cuneiform to the Alphabet.  London: British Museum Press, 1990.  ISBN
   0-7141-8077-7

   Lunde, Ken.  CJKV Information Processing.  Sebastopol, CA: O'Reilly &
   Assoc., 1999.  ISBN 1-56592-224-7

   Nakanishi, Akira.  Writing Systems of the World.  Rutland, VT:
   Charles E. Tuttle Company, 1980.  ISBN 0804816549

   Robinson, Andrew.  The Story of Writing: Alphabets, Hieroglyphs, &
   Pictograms.  London: Thames & Hudson, 1995, 2000.  ISBN 0-500-28156-4

   Sacks, David.  Language Visible.  New York: Broadway Books (a
   division of Random House, Inc.), 2003.  ISBN 0-7679-1172-5











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Appendix B.  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 RFC 3536.  Harald
   Alvestrand and Martin Duerst made extensive useful comments on early
   versions.  Others who contributed to the development of RFC 3536
   include Dan Kohn, Jacob Palme, Johan van Wingen, Peter Constable,
   Yuri Demchenko, Susan Harris, Zita Wenzel, John Klensin, Henning
   Schulzrinne, Leslie Daigle, Markus Scherer, and Ken Whistler.

   Abdulaziz Al-Zoman, Tim Bray, Frank Ellermann, Antonio Marko, JFC
   Morphin, Sarmad Hussain, Mykyta Yevstifeyev, Ken Whistler, and others
   identified important issues with, or made specific suggestions for,
   this new version.

Appendix C.  Significant Changes from RFC 3536

   This document mostly consists of additions to RFC 3536.  The
   following is a list of the most significant changes.

   o  Changed the document's status to BCP.

   o  Commonly used synonyms added to several descriptions and indexed.

   o  A list of terms defined and used in IDNA2008 was added, with a
      pointer to RFC 5890.  Those definitions have not been repeated in
      this document.

   o  The much-abused term "variant" is now discussed in some detail.

   o  A discussion of different subsets of the Unicode repertoire was
      added as Section 4.2 and associated definitions were included.

   o  Added a new term, "writing style".

   o  Discussions of case-folding and mapping were expanded.

   o  Minor edits were made to some section titles and a number of other
      editorial improvements were made.

   o  The discussion of control codes was updated to include additional
      information and clarify that "control code" and "control
      character" are synonyms.

   o  Many terms were clarified to reflect contemporary usage.




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   o  The index to terms by section in RFC 3536 was replaced by an index
      to pages containing considerably more terms.

   o  The acknowledgments were updated.

   o  Some of the references were updated.

   o  The supplemental reading list was expanded somewhat.

Index

   A
      A-label  31
      ACE  30, 31
      ACE Prefix  31
      alphabetic  20
      ANSI  13
      ASCII  15
      ASCII-compatible encoding  30, 31
      ASN.1 text formats  30

   B
      Base64  29
      Basic Multilingual Plane  13
      bidi  26
      bidirectional display  26
      BMP  13
      BMPString  30
      BOCU-1  14
      BOM  14
      byte order mark  14

   C
      C-T-E  29
      case  18
      CCS  7
      CEN/ISSS  13
      character  6
      character encoding form  7
      character encoding scheme  8
      character repertoire  7
      charset  8
      charset identification  28
      CJK characters  34
      code chart  19
      code point  16
      code table  19
      coded character  6



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      coded character set  7
      collation  18
      combining character  16
      combining character sequence  16
      compatibility character  22
      compatibility variant  22
      composite sequence  16
      content-transfer-encoding  29
      control character  21
      control code  21
      control sequence  22

   D
      decomposed character  16
      diacritic  21
      displaying and rendering text  10
      Domain Name Slot  31

   E
      encoding forms  13

   F
      font  25
      formatting character  22

   G
      glyph  7
      glyph code  7
      graphic symbol  25

   H
      Han characters  34

   I
      i18n  9
      IA5String  30
      ideographic  20
      IDN  31
      IDNA  31
      IDNA-valid string  31
      IDNA2003  31
      IDNA2008  31
      IME  24
      input method editor  24
      input methods  24
      internationalization  8
      Internationalized Domain Name  31
      Internationalized Label  31



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      ISO  11
      ISO 639  11
      ISO 3166  11
      ISO 8859  15
      ISO TC 46  11

   J
      JIS  13
      JTC 1  11

   L
      l10n  9
      language  5
      language identification  29
      Latin characters  34
      LDH Label  30
      letters  23
      Local and regional standards organizations  13
      locale  33
      localization  9

   M
      MIME  29
      multilingual  10

   N
      name spaces  28
      Nameprep  31
      NFC  17
      NFD  17
      NFKC  17
      NFKD  17
      non-ASCII  23
      nonspacing character  21
      normalization  17
      NR-LDH label  31
      NVT  15

   O
      on-the-wire encoding  28











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   P
      parsed text  28
      precomposed character  16
      PrintableString  30
      private use charater  36
      protocol elements  27
      punctuation  21
      Punycode  30, 31

   Q
      quoted-printable  29

   R
      regular expressions  36
      rendering rules  24
      repertoire  7
      romanization  34

   S
      SAC  13
      script  5
      SCSU  14
      sorting  18
      Stringprep  31
      surrogate pair  14
      symbol  21

   T
      T61String  30
      TeletexString  30
      TES  29
      transcoding  7
      transcription  35
      transfer encoding syntax  29
      transformation formats  13
      translation  35
      transliteration  34, 35
      typeface  25

   U
      U-label  31
      UCS-2  13
      UCS-4  13
      undisplayable character  26
      Unicode Consortium  12
      US-ASCII  15
      UTC  12
      UTF-8  14



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      UTF-16  14
      UTF-16BE  14
      UTF-16LE  14
      UTF-32  14
      UTF8String  30

   V
      variant  32

   W
      W3C  13
      World Wide Web Consortium  13
      writing style  27
      writing system  6

   X
      XML  13, 30

Authors' Addresses

   Paul Hoffman
   VPN Consortium

   EMail: paul.hoffman@vpnc.org


   John C Klensin
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA  02140
   USA

   Phone: +1 617 245 1457
   EMail: john+ietf@jck.com


















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