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rfc:rfc7790

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.

Yoneya & Nemoto Informational [Page 1] RFC 7790 PRECIS Mapping February 2016

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.

Yoneya & Nemoto Informational [Page 2] RFC 7790 PRECIS Mapping February 2016

 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.

Yoneya & Nemoto Informational [Page 3] RFC 7790 PRECIS Mapping February 2016

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

Yoneya & Nemoto Informational [Page 4] RFC 7790 PRECIS Mapping February 2016

 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.

Yoneya & Nemoto Informational [Page 5] RFC 7790 PRECIS Mapping February 2016

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>.

Yoneya & Nemoto Informational [Page 6] RFC 7790 PRECIS Mapping February 2016

 [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>.

Yoneya & Nemoto Informational [Page 7] RFC 7790 PRECIS Mapping February 2016

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.

Yoneya & Nemoto Informational [Page 8] RFC 7790 PRECIS Mapping February 2016

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.

Yoneya & Nemoto Informational [Page 9] RFC 7790 PRECIS Mapping February 2016

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

Yoneya & Nemoto Informational [Page 10]

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