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RFC 7790
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
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