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RFC 7790

Informational
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Mapping Characters for Classes of the Preparation, Enforcement, and Comparison of Internationalized Strings (PRECIS)

 


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Internet Engineering Task Force (IETF)                         Y. Yoneya
Request for Comments: 7790                                          JPRS
Category: Informational                                        T. Nemoto
ISSN: 2070-1721                                          Keio University
                                                           February 2016


  Mapping Characters for Classes of the Preparation, Enforcement, and
            Comparison of Internationalized Strings (PRECIS)

Abstract

   The framework for the preparation, enforcement, and comparison of
   internationalized strings (PRECIS) defines several classes of strings
   for use in application protocols.  Because many protocols perform
   case-sensitive or case-insensitive string comparison, it is necessary
   to define methods for case mapping.  In addition, both the
   Internationalized Domain Names in Applications (IDNA) and the PRECIS
   problem statement describe mappings for internationalized strings
   that are not limited to case, but include width mapping and mapping
   of delimiters and other special characters that can be taken into
   consideration.  This document provides guidelines for designers of
   PRECIS profiles and describes several mappings that can be applied
   between receiving user input and passing permitted code points to
   internationalized protocols.  In particular, this document describes
   both locale-dependent and context-depending case mappings as well as
   additional mappings for delimiters and special characters.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see 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/rfc7790.

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Copyright Notice

   Copyright (c) 2016 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Protocol-Dependent Mappings . . . . . . . . . . . . . . . . .   3
     2.1.  Delimiter Mapping . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Special Mapping . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Local Case Mapping  . . . . . . . . . . . . . . . . . . .   4
   3.  Order of Operations . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Mapping Type List  . . . . . . . . . . . . . . . . .   8
     A.1.  Mapping Type List for Each Protocol . . . . . . . . . . .   8
   Appendix B.  Why Local Case Mapping Is an Alternative to Case
                Mapping in the PRECIS Framework  . . . . . . . . . .   8
   Appendix C.  Limitations of Local Case Mapping  . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   In many cases, user input of internationalized strings is generated
   through the use of an input method editor ("IME") or through copy-
   and-paste from free text.  Users generally do not care about the case
   and/or width of input characters because they consider those
   characters to be functionally equivalent or visually identical.
   Furthermore, users rarely switch the IME state to input special
   characters such as protocol elements.

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   For Internationalized Domain Names (IDNs), the IDNA Mapping
   specification [RFC5895] describes methods for handling these issues.
   For PRECIS strings, case mapping and width mapping are defined in the
   PRECIS framework specification [RFC7564].  The case and width
   mappings defined in the PRECIS framework do not handle other mappings
   such as delimiter characters, special characters, and locale-
   dependent or context-dependent cases; these mappings are also
   important in order to increase the probability that the resulting
   strings compare as users expect.

   This document provides guidelines for authors of protocol profiles of
   the PRECIS framework and describes several mappings that can be
   applied between receiving user input and passing permitted code
   points to internationalized protocols.  The delimiter mapping and
   special mapping rules described here are applied as "additional
   mappings" beyond those defined in the PRECIS framework, whereas the
   "local case mapping" rule provides locale-dependent and context-
   dependent alternative case mappings for specific target characters.

2.  Protocol-Dependent Mappings

   The PRECIS framework defines several protocol-independent mappings.
   The additional mappings and local case mapping defined in this
   document are protocol dependent, i.e., they depend on the rules for a
   particular application protocol.

2.1.  Delimiter Mapping

   Some application protocols define delimiters for their own use,
   resulting in the fact that the delimiters are different for each
   protocol.  The delimiter mapping table should therefore be based on a
   well-defined mapping table for each protocol.

   Delimiter mapping is used to map characters that are similar to
   protocol delimiters into the canonical delimiter characters.  For
   example, there are width-compatible characters that correspond to the
   '@' in email addresses and the ':' and '/' in URIs.  The '+', '-',
   '<' and '>' characters are other common delimiters that might require
   such mapping.  For the FULL STOP character (U+002E), a delimiter in
   the visual presentation of domain names, some IMEs produce a
   character such as IDEOGRAPHIC FULL STOP (U+3002) when a user types
   FULL STOP on the keyboard.  In all these cases, the visually similar
   characters that can come from user input need to be mapped to the
   correct protocol delimiter characters before the string is passed to
   the protocol.

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2.2.  Special Mapping

   Aside from delimiter characters, certain protocols have characters
   which need to be mapped in ways that are different from the rules
   specified in the PRECIS framework (e.g., mapping non-ASCII space
   characters to ASCII space).  In this document, these mappings are
   called "special mappings".  They are different for each protocol.
   Therefore, the special mapping table should be based on a well-
   defined mapping table for each protocol.  Examples of special mapping
   are the following;

   o  White spaces such as CHARACTER TABULATION (U+0009) or IDEOGRAPHIC
      SPACE (U+3000) are mapped to SPACE (U+0020)

   o  Some characters such as control characters are mapped to nothing
      (Deletion)

   As examples, the Extensible Authentication Protocol (EAP) [RFC3748],
   IMAP4 Access Control List (ACL) [RFC4314], and LDAPprep [RFC4518]
   define the rule that some code points for the non-ASCII space are
   mapped to SPACE (U+0020).

2.3.  Local Case Mapping

   The purpose of local case mapping is to increase the probability of
   results that users expect when character case is changed (e.g., map
   uppercase to lowercase) between input and use in a protocol.  Local
   case mapping selectively affects characters whose case mapping
   depends on locale and/or context.

   (Note: The term "locale" in this document practically means
   "language" or "language and region" because the locale based on that
   language configuration of applications on POSIX is selected by
   "locale" information.  See also the "Note" in Section 2.1.1 of RFC
   5646 [RFC5646].)

   As an example of locale- and context-dependent mapping, LATIN CAPITAL
   LETTER I ("I", U+0049) is normally mapped to LATIN SMALL LETTER I
   ("i", U+0069); however, if the language is Turkish (or one of several
   other languages), unless an I is before a dot_above, the character
   should be mapped to LATIN SMALL LETTER DOTLESS I (U+0131).

   Case mapping using Unicode Default Case Folding in the PRECIS
   framework does not consider such locale or context because it is a
   common framework for internationalization.  Local case mapping
   defined in this document correspond to demands from applications that
   support users' locale and/or context.  The complete set of possible
   target characters for local case mapping are the characters specified

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   in SpecialCasing.txt [Specialcasing] in Section 3.13 of the Unicode
   Standard [Unicode], but the specific set of target characters
   selected for local case mapping depends on locale and/or context, as
   further explained in SpecialCasing.txt.

   The case-folding method for a selected target character is to map
   into lowercase as defined in SpecialCasing.txt.  The case-folding
   method for all other, non-target characters is as specified in
   Section 5.2.3 of the PRECIS framework.  When an application supports
   users' locale and/or context, use of local case mapping can increase
   the probability that string comparisons yield the results that users
   expect.

   If a PRECIS profile selects Unicode Default Case Folding as the
   preferred method of case mapping, the profile designers may consider
   whether local case mapping can be applied.  And, if it can be
   applied, it is better to add "alternatively, local case mapping might
   be applicable" after "Unicode Default Case Folding" so that
   application developers are aware of the alternative.  See Appendix B
   for a description of why local case mapping can be an alternative.

3.  Order of Operations

   Delimiter mapping and special mapping as described in this document
   are expected to be applied as the "Additional Mapping Rule" mentioned
   in Section 5.2.2 of the PRECIS framework.  Although the delimiter
   mapping and special mapping could be applied in either order, this
   document recommends the following order to minimize the effect of
   code-point changes introduced by the mappings and to be acceptable to
   the widest user community:

   1.  Delimiter mapping

   2.  Special mapping

4.  Security Considerations

   Detailed security considerations for PRECIS strings are discussed in
   the PRECIS framework specification [RFC7564].  This document inherits
   the considerations as well.

   As with Mapping Characters for IDNA2008 [RFC5895], this document
   suggests creating mappings that might cause confusion for some users
   while alleviating confusion for other users.  Such confusion is not
   covered in any depth in this document.

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5.  References

5.1.  Normative References

   [RFC7564]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols",
              RFC 7564, DOI 10.17487/RFC7564, May 2015,
              <http://www.rfc-editor.org/info/rfc7564>.

   [Unicode]  The Unicode Consortium, "The Unicode Standard, Version
              7.0.0", (Mountain View, CA: The Unicode Consortium,
              2014. ISBN 978-1-936213-09-2),
              <http://www.unicode.org/versions/Unicode7.0.0/>.

   [Casefolding]
              The Unicode Consortium, "CaseFolding-7.0.0.txt", Unicode
              Character Database, July 2011,
              <http://www.unicode.org/Public/7.0.0/ucd/CaseFolding.txt>.

   [Specialcasing]
              The Unicode Consortium, "SpecialCasing-7.0.0.txt", Unicode
              Character Database, July 2011,
              <http://www.unicode.org/Public/7.0.0/ucd/
              SpecialCasing.txt>.

5.2.  Informative References

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              DOI 10.17487/RFC3454, December 2002,
              <http://www.rfc-editor.org/info/rfc3454>.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, DOI 10.17487/RFC3490, March 2003,
              <http://www.rfc-editor.org/info/rfc3490>.

   [RFC3491]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)",
              RFC 3491, DOI 10.17487/RFC3491, March 2003,
              <http://www.rfc-editor.org/info/rfc3491>.

   [RFC3722]  Bakke, M., "String Profile for Internet Small Computer
              Systems Interface (iSCSI) Names", RFC 3722,
              DOI 10.17487/RFC3722, April 2004,
              <http://www.rfc-editor.org/info/rfc3722>.

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   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
              <http://www.rfc-editor.org/info/rfc3748>.

   [RFC4314]  Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
              RFC 4314, DOI 10.17487/RFC4314, December 2005,
              <http://www.rfc-editor.org/info/rfc4314>.

   [RFC4518]  Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Internationalized String Preparation", RFC 4518,
              DOI 10.17487/RFC4518, June 2006,
              <http://www.rfc-editor.org/info/rfc4518>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <http://www.rfc-editor.org/info/rfc5646>.

   [RFC5895]  Resnick, P. and P. Hoffman, "Mapping Characters for
              Internationalized Domain Names in Applications (IDNA)
              2008", RFC 5895, DOI 10.17487/RFC5895, September 2010,
              <http://www.rfc-editor.org/info/rfc5895>.

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Appendix A.  Mapping Type List

A.1.  Mapping Type List for Each Protocol

   This table is the mapping type list for each protocol that uses the
   Stringprep framework [RFC3454] and is a PRECIS framework customer
   candidate (as Stringprep and the related IDNA versions in the table
   below are now obsolete).  Values marked "o" indicate that the
   protocol uses the type of mapping.  Values marked "-" indicate that
   the protocol doesn't use the type of mapping.

   +---------------------+-------------+-----------+------+---------+
   |     Protocol and    |    Width    | Delimiter | Case | Special |
   |     mapping RFC     |    (NFKC)   |           |      |         |
   +---------------------+-------------+-----------+------+---------+
   |   IDNA  [RFC3490]   |      -      |     o     |   -  |    -    |
   |   IDNA  [RFC3491]   |      o      |     -     |   o  |    -    |
   |   iSCSI [RFC3722]   |      o      |     -     |   o  |    -    |
   |   EAP   [RFC3748]   |      o      |     -     |   -  |    o    |
   |   IMAP  [RFC4314]   |      o      |     -     |   -  |    o    |
   |   LDAP  [RFC4518]   |      o      |     -     |   o  |    o    |
   +---------------------+-------------+-----------+------+---------+

Appendix B.  Why Local Case Mapping Is an Alternative to Case Mapping in
             the PRECIS Framework

   Local case mapping and Unicode Default Case Folding are alternatives.
   They can't be applied simultaneously or sequentially.  One
   outstanding issue regarding full case folding for characters is that
   some lowercase characters like "LATIN SMALL LETTER SHARP S" (U+00DF)
   (hereinafter referred to as "eszett") and ligatures like "LATIN SMALL
   LIGATURE FF" (U+FB00) that are described in the "Unconditional
   mappings" section of SpecialCasing.txt become a different code point
   when the case mapping is performed using Unicode Default Case Folding
   in the PRECIS framework.
   In particular, German's eszett cannot keep the locale because eszett
   becomes two "LATIN SMALL LETTER S"s (U+0073 U+0073) when the case
   mapping is performed using Unicode Default Case Folding.  (See also
   00DF in CaseFolding.txt [Casefolding].)  On the other hand, eszett
   doesn't become a different code point when performing the case
   mapping in SpecialCasing.txt.  Therefore, if it is necessary to keep
   the locale of characters, PRECIS profile designers should select
   local case mapping as an alternative to Unicode Default Case Folding.

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Appendix C.  Limitations of Local Case Mapping

   As described in Section 2.3, the possible target characters of local
   case mapping are specified in SpecialCasing.txt.  The Unicode
   Standard (at least, up to version 7.0.0) does not define any context-
   dependent mappings between "GREEK SMALL LETTER SIGMA" (U+03C3)
   (hereinafter referred to as "small sigma") and "GREEK SMALL LETTER
   FINAL SIGMA" (U+03C2) (hereinafter referred to as "final sigma").
   Thus, local case mapping is not applicable to small sigma or final
   sigma, so case mapping in the PRECIS framework always maps final
   sigma to small sigma, independent of context, as also specified by
   Unicode Default Case Folding.  The following comments are from
   SpecialCasing.txt.  (Line breaks have been added due to line-length
   limitations.)

   # Note: the following cases are not included, since they would
     case-fold in lowercasing

   # 03C3; 03C2; 03A3; 03A3; Final_Sigma; # GREEK SMALL LETTER SIGMA
   # 03C2; 03C3; 03A3; 03A3; Not_Final_Sigma; # GREEK SMALL LETTER FINAL
     SIGMA

Acknowledgments

   Martin Duerst suggested a need for the case folding about the mapping
   (map final sigma to sigma, German sz to ss, etc.).

   Alexey Melnikov, Andrew Sullivan, Barry Leiba, David Black, Heather
   Flanagan, Joe Hildebrand, John Klensin, Marc Blanchet, Pete Resnick,
   and Peter Saint-Andre, et al., gave important suggestions for this
   document during working group discussions.

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Authors' Addresses

   Yoshiro YONEYA
   JPRS
   Chiyoda First Bldg. East 13F
   3-8-1 Nishi-Kanda
   Chiyoda-ku, Tokyo  101-0065
   Japan

   Phone: +81 3 5215 8451
   Email: yoshiro.yoneya@jprs.co.jp


   Takahiro Nemoto
   Keio University
   Graduate School of Media Design
   4-1-1 Hiyoshi, Kohoku-ku
   Yokohama, Kanagawa  223-8526
   Japan

   Phone: +81 45 564 2517
   Email: t.nemo10@kmd.keio.ac.jp