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Internet-Draft                           Editor: Kurt D. Zeilenga
Intended Category: Informational                 OpenLDAP Foundation
Expires in six months                            3 March 2003



            Internationalized String Matching Rules for X.500
                 <draft-zeilenga-ldapbis-strmatch-02.txt>


Status of this Memo

  This document is an Internet-Draft and is in full conformance with all
  provisions of Section 10 of RFC2026.

  This document is intended to be submitted to the ITU for publication
  as an amendment to X.520 and published as an Informational RFC.
  Distribution of this memo is unlimited.  Technical discussion of this
  document will take place on the IETF LDAP Revision Working Group
  mailing list <ietf-ldapbis@openldap.org>.  Please send editorial
  comments directly to the author <Kurt@OpenLDAP.org>.

  Internet-Drafts are working documents of the Internet Engineering Task
  Force (IETF), its areas, and its working groups.  Note that other
  groups may also distribute working documents as Internet-Drafts.
  Internet-Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
  time.  It is inappropriate to use Internet-Drafts as reference
  material or to cite them other than as ``work in progress.''

  The list of current Internet-Drafts can be accessed at
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  Copyright 2003, The Internet Society.  All Rights Reserved.

  Please see the Copyright section near the end of this document for
  more information.


Abstract

  The existing X.500 Directory Service technical specifications do not
  precisely define how string matching is to be performed.  This has
  lead to a number of interoperability problems.  This document provides
  string preparation profiles for standard syntaxes and matching rules
  defined in X.520.



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  This document is intended to be submitted to the ITU-T for publication
  as an amendment to X.520 and published as an Informational RFC.


Conventions

  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 BCP 14 [RFC2119].

  Character names in this document use the notation for code points and
  names from the Unicode Standard [UNICODE] and ISO/IEC 10646-1
  [ISO10646].  For example, the letter "a" may be represented as either
  <U+0061> or <LATIN SMALL LETTER A>.  In the lists of mappings and the
  prohibited characters, the "U+" is left off to make the lists easier
  to read.  The comments for character ranges are shown in square
  brackets (such as "[CONTROL CHARACTERS]") and do not come from the
  standards.

  Note: a glossary of terms used in Unicode and ISO/IEC 10646 can be
  found in [GLOSSARY].  Information on the ISO/IEC 10646/Unicode
  character encoding model can be found in [UTR17].



1. Introduction

1.1. Background

  An X.500 matching rule [X.501] defines an algorithm for determining
  whether a presented value matches an attribute value in accordance
  with the criteria defined for the rule.  The proposition may be
  evaluated to True, False, or Undefined.

      True      - the attribute contains a matching value,

      False     - the attribute contains no matching value,

      Undefined - it cannot be determined whether the attribute contains
                  a matching value or not.

  For instance, the caseIgnoreMatch matching rule may be used to compare
  whether the commonName attribute contains a particular value without
  regard for case and insignificant spaces.


1.2. X.500 String Matching Rules




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  "X.520: Selected attribute types" [X.520] provides (amongst other
  things) value syntaxes and matching rules for comparing values
  commonly used in the Directory [X.500].  These specifications are
  inadequate for strings composed of characters from the Universal
  Character Set (UCS) [ISO10646], a superset of Unicode [UNICODE].

  The CaseIgnoreMatch matching rule, for example, is simply defined as
  being a case insensitive comparison where insignificant spaces are
  ignored.  For printableString, there is only one space character and
  case mapping is bijective, hence this definition is sufficient.
  However, for UCS-based string types such as universalString, this is
  not sufficient.  For example, a case insensitive matching
  implementation which folded lower case characters to upper case would
  yield different different results than an implementation which used
  upper case to lower case folding.  Or one implementation may view
  space as referring to only SPACE (U+0020), a second implementation may
  view any character with the space separator (Zs) property as a space,
  and another implementation may view any character with the whitespace
  (WS) category as a space.

  The lack of precise specification for string matching has led to
  significant interoperability problems.  When used in certificate chain
  validation, security vulnerabilities can arise.  To address these
  problems, this document updates X.520 [X.520] with a detailed
  specification of string syntax and matching rule requirements.


1.3. Relationship to "stringprep"

  The matching rule algorithms described in this document are based upon
  the "stringprep" approach [RFC3454].  In "stringprep", presented and
  stored values are first prepared for comparison and so that a
  character-by-character comparison yields the "correct" result.

  The algorithm used here is a refinement of the "stringprep" [RFC3454]
  approach.  The algorithm involves two additional preparation steps.

  a) prior to applying the Unicode string preparation steps outlined in
     "stringprep", the string is transcoded to Unicode;

  b) after applying the Unicode string preparation steps outlined in
     "stringprep", characters insignificant to the matching rules are
     removed.

  Hence, preparation of strings for X.500 matching involves the
  following steps:

      1) Transcode



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      2) Map
      3) Normalize
      4) Prohibit
      5) Check Bidi (Bidirectional)
      6) Insignificant Character Removal

  These steps are described in Section 3.  Section 2 details design
  considerations.


1.4. Relationship to X.500

  This document updates X.520 [X.520] with additional normative and
  informative information.  Sections 3, 4, and 5 are normative parts of
  this update.  Other sections are informative.

  Section 3 provides a specification for X.500 string preparation.  It
  is intended to be added as a new section in X.520.

  Section 4 replaces section 6.1 of X.520 [X.520].  It updates select
  string matching rules.

  Section 5 replaces portions of section 6.2 of X.520 [X.520].  It
  updates select syntax-based matching rules.


2. Design Considerations

  The X.500 string matching rule specification provided in Section 3 is
  designed to leverage the "stringprep" framework [RFC3454] for
  comparing of strings.  As noted above, transcoding and space removal
  steps have been added.

  This section describes the rationale for these and other design
  decisions.


2.1. Transcode

  In the past, transcoding only occurred when all of the input strings
  were not encoded in the same character set.  If all were encoded in
  the same character set, no transcoding was to be performed.
  Otherwise, all of the strings would be transcoded to one of character
  sets used.

  As mappings between character sets, such as T.61 and UCS, are not
  bijective, this specification requires transliteration of all strings
  to a common character encoding set.  UCS was the logical choice as all



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  other character sets (used in X.500) can be transcoded to it without
  information loss.  None of the other character sets (used in X.500)
  offer this property.


2.2. Map

  Code points which have no semantic meaning in normal text are mapped
  to nothing.  Code points which are semantically equivalent in normal
  text are mapped to a single code point.

  "Normal text", in this context, is viewed as text commonly held in
  attributes of Directory String syntax, such as identifiers, common
  names, and short descriptive text.


2.3. Normalize

  Normalization is performed to ensure that comparison is always done
  between canonical-equivalent strings.  As directory strings are often
  used as identifiers, we selected Form KC (compatibility composed) as
  it allows a greater number of strings to be treated as equivalent.

  Unfortunately, this choice is not best for all applications.
  Additional matching rules which use different string preparation
  algorithms may be introduced in the future to better support these
  applications.  In particular, matching rules which use Form C
  (composed) normalization instead of Form KC would also be generally
  useful.  It may be desirable to add additional matching rules to X.500
  which use Form C normalization.


2.4. Prohibit

  TBD


2.5. Check bidi

  TBD


2.6. Insignificant Character Removal

  This step is used to remove insignificant characters from the string.
  Unlike the map step, which supports mapping of characters to nothing,
  this step allows removal of characters based upon their location in
  the string, surrounding characters in the string, and other factors.



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3. String Preparation

  The following six-step process SHALL be applied to each presented and
  attribute value in preparation for string match rule evaluation.

      1) Transcode
      2) Map
      3) Normalize
      4) Prohibit
      5) Check bidi
      6) Insignificant Character Removal

  Failure in any step is be cause the assertion to be Undefined.

  The character repertoire of this process is Unicode 3.2 [UNICODE].


3.1. Transcode

  Each non-Unicode string value is transcoded to Unicode.

  TeletexString values are transcoded to Unicode as described in
  [T61-UCS].

  PrintableString value are transcoded directly to Unicode.

  UniversalString, UTF8String, and bmpString values need not be
  transcoded as they are Unicode-based strings (in the case of
  bmpString, restricted to a subset of Unicode).

  If the implementation is unable or unwilling to perform the
  transcoding as described above, or the transcoding fails, this step
  fails and the assertion is evaluated to Undefined.

  The transcoded string is the output string.


3.2. Map

  SOFT HYPHEN (U+00AD) and MONGOLIAN TODO SOFT HYPHEN (U+1806) code
  points are mapped to nothing.  COMBINING GRAPHEME JOINER (U+034F) and
  VARIATION SELECTORs (U+180B-180D,FF00-FE0F) code points are also
  mapped to nothing.  The OBJECT REPLACEMENT CHARACTER (U+FFFC) is
  mapped to nothing.

  CHARACTER TABULATION (U+0009), LINE FEED (LF) (U+000A), LINE
  TABULATION (U+000B), FORM FEED (FF) (U+000C), CARRIAGE RETURN (CR)
  (U+000D), and NEXT LINE (NEL) (U+0085) are mapped to SPACE (U+0020).



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  All other control code points (e.g., Cc) or code points with a control
  function (e.g., Cf) are mapped to nothing.

  ZERO WIDTH SPACE (U+200B) is mapped to nothing.  All other code points
  with Separator (space, line, or paragraph) property (e.g, Zs, Zl, or
  Zp) are mapped to SPACE (U+0020).

  For case ignore, numeric, and stored prefix string matching rules,
  characters are case folded per B.2 of [RFC3454].


3.3. Normalize

  The input string is be normalized to Unicode Form KC (compatibility
  composed) as described in [UAX15].


3.4. Prohibit

  All Unassigned, Private Use, and non-character code points are
  prohibited.  Surrogate codes (U+D800-DFFFF) are prohibited.

  The REPLACEMENT CHARACTER (U+FFFD) code point is prohibited.

  The first code point of a string is probibited from being a combining
  character.

  Empty strings are prohibited.

  The step fails and the assertion is evaluated to Undefined if the
  input string contains any prohibited code point.  The output string is
  the input string.


3.5. Check bidi

  There are no bidirectional restrictions.  The output string is the
  input string.


3.6. Insignificant Character Removal

  In this step, characters insignificant to the matching rule are to be
  removed.  The characters to be removed differ from matching rule to
  matching rule.

  Section 3.6.1 applies to case ignore and exact string matching.
  Section 3.6.2 applies to numericString matching.



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  Section 3.6.3 applies to telephoneNumber matching


3.6.1. Insignificant Space Removal

  For the purposes of this section, a space is defined to be the SPACE
  (U+0020) code point followed by no combining marks.

         NOTE - The previous steps ensure that the string cannot contain
         any code points in the separator class, other than SPACE
         (U+0020).

  The following spaces are regarded as not significant and are to be
  removed:
    - leading spaces (i.e. those preceding the first character that is
      not a space);
    - trailing spaces (i.e. those following the last character that is
      not a space);
    - multiple consecutive spaces (these are taken as equivalent to a
      single space character).

  (A string consisting entirely of spaces is equivalent to a string
  containing exactly one space.)

  For example, removal of spaces from the Form KC string:
      "<SPACE><SPACE>foo<SPACE><SPACE>bar<SPACE><SPACE>" would result in
  the output string:
      "<SPACE>foo<SPACE>bar<SPACE>".

  and the Form KC string:
      "<SPACE><SPACE><SPACE>" would result in the output string:
      "<SPACE>".


3.6.2. NumericString Insignificant Character Removal

  For the purposes of this section, a space is defined to be the SPACE
  (U+0020) code point followed by no combining marks.

  All spaces are regarded as not significant and are to be removed.

  For example, removal of spaces from the Form KC string:
      "<SPACE><SPACE>123<SPACE><SPACE>456<SPACE><SPACE>" would result in
  the output string:
      "123456".

  and the Form KC string:
      "<SPACE><SPACE><SPACE>" would result in an empty output string.



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3.6.3. TelephoneNumber Insignificant Character Removal

  For the purposes of this section, a hyphen is defined to be
  HYPHEN-MINUS (U+002D), ARMENIAN HYPHEN (U+058A), HYPHEN (U+2010),
  NON-BREAKING HYPHEN (U+2011), MINUS SIGN (U+2212), SMALL HYPHEN-MINUS
  (U+FE63), or FULLWIDTH HYPHEN-MINUS (U+FF0D) code point followed by no
  combining marks and a space is defined to be the SPACE (U+0020) code
  point followed by no combining marks.

  All hyphens and spaces are regarded as not significant and are to be
  removed.


4. String Matching Rules

  In the matching rules specified in this section, all presented and
  stored string values are be prepared for matching as described in
  Section 3.  String preparation produces strings suitable for
  character-by-character matching.


4.1. Case Exact / Ignore Match

  The Case Exact Match rule compares for equality a presented string
  with an attribute value of type DirectoryString or one of the data
  types appearing in the choice type DirectoryString, e.g. UTF8String,
  without regards to insignificant spaces (3.4.1).

      caseExactMatch MATCHING-RULE ::= {
          SYNTAX DirectoryString {ub-match}
          ID id-mr-caseExactMatch }

  The Case Ignore Match rule compares for equality a presented string
  with an attribute value of type DirectoryString or one of the data
  types appearing in the choice type DirectoryString, e.g. UTF8String,
  without regard to the case (upper or lower) of the strings (e.g.
  "Dundee" and "DUNDEE" match) and insignificant spaces (3.4.1).  The
  rule is identical to the caseExactMatch rule except upper case
  characters are folded to lower case during string preparation as
  discussed in 3.2.

      caseIgnoreMatch MATCHING-RULE ::= {
          SYNTAX DirectoryString {ub-match}
          ID id-mr-caseIgnoreMatch }

  Both rules return TRUE if the prepared strings are the same length and
  corresponding characters are identical.




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4.2. Case Exact / Ignore Ordering Match

  The Case Exact Ordering Match rule compares the collation order of a
  presented string with an attribute value of type DirectoryString or
  one of the data types appearing in the choice type DirectoryString,
  e.g. UTF8String, without regard to insignificant spaces (3.4.1).

      caseExactOrderingMatch MATCHING-RULE ::= {
          SYNTAX DirectoryString {ub-match}
          ID id-mr-caseExactOrderingMatch }

  The Case Ignore Ordering Match rule compares the collation order of a
  presented string an attribute value of type DirectoryString or one of
  the data types appearing in the choice type DirectoryString, e.g.
  UTF8String, without regard to the case (upper or lower) of the strings
  and insignificant spaces (3.4.1).  The rule is identical to the
  caseExactOrderingMatch rule except upper case characters are folded to
  lower case during string preparation as discussed in 3.2.

      caseIgnoreOrderingMatch MATCHING-RULE ::= {
          SYNTAX DirectoryString {ub-match}
          ID id-mr-caseIgnoreOrderingMatch }

  Both rules return TRUE if the attribute value is "less" or appears
  earlier than the presented value, when the prepared strings are
  compared using Unicode code point collation order.


4.3. Case Exact / Ignore Substrings Match

  The Case Exact Substrings Match rule determines whether a presented
  value is a substring of an attribute value of type DirectoryString or
  one of the data types appearing in the choice type DirectoryString,
  e.g. UTF8String, without regard to insignficant spaces (3.4.1).

      caseExactSubstringsMatch MATCHING-RULE ::= {
          SYNTAX SubstringAssertion
          ID id-mr-caseExactSubstringsMatch }
      SubstringAssertion ::= SEQUENCE OF CHOICE {
          initial [0] DirectoryString {ub-match},
          any [1] DirectoryString {ub-match},
          final [2] DirectoryString {ub-match},
      control Attribute }
      -- Used to specify interpretation of the following items
      -- at most one initial and one final component

  The Case Ignore Substrings Match rule determines whether a presented
  value is a substring of an attribute value of type DirectoryString or



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  one of the data types appearing in the choice type DirectoryString,
  e.g. UTF8String, without regard to the case (upper or lower) of the
  strings and insignificant spaces (3.4.1).  The rule is identical to
  the caseExactSubstringsMatch rule except upper case characters are
  folded to lower case during string preparation as discussed in 3.2.

      caseIgnoreSubstringsMatch MATCHING-RULE ::= {
          SYNTAX SubstringAssertion
          ID id-mr-caseIgnoreSubstringsMatch }

  Both rules return TRUE if there is a partitioning of the prepared
  attribute value (into portions) such that:
    - the specified substrings (initial, any, final) match different
      portions of the value in the order of the strings sequence.
    - initial, if present, matches the first portion of the value;
    - any, if present, matches some arbitrary portion of the value;
    - final, if present, matches the last portion of the value.
    - control is not used for the caseExactSubstringsMatch,
      caseIgnoreSubstringsMatch, telephoneNumberSubstringsMatch, or any
      other form of substring match for which only initial, any, or
      final elements are used in the matching algorithm; if a control
      element is encountered, it is ignored.  The control element is
      only used for matching rules that explicitly specify its use in
      the matching algorithm. Such a matching rule may also redefine the
      semantics of the initial, any and final substrings.
        NOTE - The generalWordMatch matching rule is an example of such
               a matching rule.

  There shall be at most one initial, and at most one final in the
  SubstringAssertion.  If initial is present, it shall be the first
  element.  If final is present, it shall be the last element. There
  shall be zero or more any elements.

  For a component of substrings to match a portion of the attribute
  value, corresponding characters must be identical (including all
  combining characters in the combining character sequences).


4.4. Numeric String Match

  The Numeric String Match rule compares for equality a presented
  numeric string with an attribute value of type NumericString.

      numericStringMatch MATCHING-RULE ::= {
          SYNTAX NumericString
          ID id-mr-numericStringMatch }

  The rule is identical to the caseIgnoreMatch rule (case is irrelevant



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  as characters are numeric) except that all space characters are
  removed during string preparation as detailed in Section 3.6.2.


4.5. Numeric String Ordering Match

  The Numeric String Ordering Match rule compares the collation order of
  a presented string with an attribute value of type NumericString.

      numericStringOrderingMatch MATCHING-RULE ::= {
          SYNTAX NumericString
          ID id-mr-numericStringOrderingMatch }

  The rule is identical to the caseIgnoreOrderingMatch rule (case is
  irrelevant as characters are numeric) except that all space characters
  are removed during string preparation as detailed in Section 3.6.


4.6. Numeric String Substrings Match

  The Numeric String Substrings Match rule determines whether a
  presented value is a substring of an attribute value of type
  NumericString.

      numericStringSubstringsMatch MATCHING-RULE ::= {
          SYNTAX SubstringAssertion
          ID id-mr-numericStringSubstringsMatch }

  The rule is identical to the caseIgnoreSubstringsMatch rule (case is
  irrelevant as characters are numeric) except that all space characters
  are removed during string preparation as detailed in Section 3.6.


4.7. Case Ignore List Match

  The Case Ignore List Match rule compares for equality a presented
  sequence of strings with an attribute value which is a sequence of
  DirectoryStrings, without regard to the case (upper or lower) of the
  strings and insignificant spaces (3.6.1).

      caseIgnoreListMatch MATCHING-RULE ::= {
          SYNTAX CaseIgnoreList
          ID id-mr-caseIgnoreListMatch }
      CaseIgnoreList ::= SEQUENCE OF DirectoryString {ub-match}

  The rule returns TRUE if and only if the number of strings in each is
  the same, and corresponding strings match. The latter matching is as
  for the caseIgnoreMatch matching rule.



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4.8. Case Ignore List Substrings Match

  The Case Ignore List Substring rule compares a presented substring
  with an attribute value which is a sequence of DirectoryStrings, but
  without regard for the case (upper or lower) of the strings and
  insignificant spaces (3.6.1).

      caseIgnoreListSubstringsMatch MATCHING-RULE ::= {
          SYNTAX SubstringAssertion
          ID id-mr-caseIgnoreListSubstringsMatch }

  A presented value matches a stored value if and only if the presented
  value matches the string formed by concatenating the strings of the
  stored value. This matching is done according to the
  caseIgnoreSubstringsMatch rule; however, none of the initial, any, or
  final values of the presented value are considered to match a
  substring of the concatenated string which spans more than one of the
  strings of the stored value.


4.9. Stored Prefix Match

  The Stored Prefix Match rule determines whether an attribute value,
  whose syntax is DirectoryString, is a prefix (i.e.  initial substring)
  of the presented value, without regard to the case (upper or lower) of
  the strings and insignficant spaces (3.6.1).

             NOTE - It can be used, for example, to compare values in
             the Directory which are telephone area codes with a value
             which is a purported telephone number.

      storedPrefixMatch MATCHING-RULE ::= {
          SYNTAX DirectoryString {ub-match}
          ID id-mr-storedPrefixMatch }

  The rule returns TRUE if the attribute value is an initial substring
  of the presented value with corresponding characters identical except
  with regard to case.


5. Other changes to X.520

  This document makes the following changes to X.520:

  The section 6.2.8 (Telephone Number Match) sentence:
      The rules for matching are identical to those for caseIgnoreMatch,
      except that all space and "-" characters are skipped during the
      comparison.



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  is replaced with:
      The rules for matching are identical to those for caseIgnoreMatch,
      except that all hyphens and spaces are insignficant (3.6.3) and
      removed during the insignificant character removal step.

  The section 6.2.9 (Telephone Number Substrings Match) sentence:
      The rules for matching are identical to those for
      caseExactSubstringsMatch, except that all space and "-" characters
      are skipped during the comparison.

  is replaced with:
      The rules for matching are identical to those for
      caseExactSubstringsMatch, except that all hyphens and spaces are
      insignficant (3.6.3) and removed during the insignificant
      character removal step.


6. Security Considerations

  See [RFC3454].


7. Acknowledgments

  The approach used in this document is based upon design principles and
  algorithms described in "Preparation of Internationalized Strings
  ('stringprep')" [RFC3454] by Paul Hoffman and Marc Blanchet.  Some
  additional guidance was drawn from Unicode Technical Standards,
  Technical Reports, and Notes.

  Sections 3.3 and 4 of this document are derived from Section 6.1 of
  [X.520].   Additionally, some text was borrowed from [RFC3454].

  This document is the product of IETF and ITU-T collaboration [IETF-
  ITU].


8. Editor's Address

  Kurt Zeilenga
  E-mail: <kurt@openldap.org>


9. References

9.1. Normative References

  [RFC2119]  S. Bradner, "Key words for use in RFCs to Indicate



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             Requirement Levels", BCP 14 (also RFC 2119), March 1997.

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

  [X.501]    International Telephone Union, "The Directory: The Models",
             X.501, 2000.

  [X.520]    International Telephone Union, "The Directory: Selected
             Attribute Types", X.520, 2000.

  [ISO10646]   Universal Multiple-Octet Coded Character Set (UCS) -
             Architecture and Basic Multilingual Plane, ISO/IEC 10646-1
             : 1993.

  [UNICODE]  The Unicode Consortium, "The Unicode Standard, Version
             3.2.0" is defined by "The Unicode Standard, Version 3.0"
             (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5), as
             amended by the "Unicode Standard Annex #27: Unicode 3.1"
             (http://www.unicode.org/reports/tr27/) and by the "Unicode
             Standard Annex #28: Unicode 3.2"
             (http://www.unicode.org/reports/tr28/).

  [UAX15]    M. Davis, M. Duerst, "Unicode Standard Annex #15: Unicode
             Normalization Forms, Version 3.2.0".
             <http://www.unicode.org/unicode/reports/tr15/tr15-22.html>,
             March 2002.

  [T61-UCS]  TBD


9.2. Informative References

  [X.500]    International Telephone Union, "The Directory: Overview of
             Concepts, Models and Service", X.500, 2000.

  [IETF-ITU] G. Fishman, S. Bradner, "Internet Engineering Task Force
             and International Telecommunication Union -
             Telecommunications Standardization Sector Collaboration
             Guidelines", TSAG A-Series Supplement 3, November 2001
             (also RFC 3356, published August 2002).

             [GLOSSARY] The Unicode Consortium, "Unicode Glossary",
             <http://www.unicode.org/glossary/>.

             [UTR17]    K. Whistler, M. Davis, "Unicode Technical Report
             #17, Character Encoding Model", UTR17,
             <http://www.unicode.org/unicode/reports/tr17/>, August



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             2000.



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