Network Working Group Y. YONEYA
Internet-Draft JPRS
Intended status: Informational T. NEMOTO
Expires: June 29, 2013 Keio University
December 26, 2012

Mapping characters for precis classes


Preparation and comparison of internationalized strings ("precis") framework [I-D.ietf-precis-framework] is defining several classes of strings for preparation and comparison. In the document, case mapping is defined because many of protocols handle case sensitive or case insensitive string comparison and therefore preparation of string is mandatory. As described in IDNA mapping [RFC5895] and precis problem statement [I-D.ietf-precis-problem-statement], mappings in internationalized strings are not limited to case, but also width, delimiters and/or other specials are taken into consideration. This document is a guideline for authors of protocol profiles of precis framework and describes the mappings, that may be performed before precis framework, that must be considered between receiving user input and passing permitted code points to internationalized protocols.

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Table of Contents

1. Introduction

In many cases, user input of internationalized strings is generated by input method editor ("IME") or copy-and-paste from free text. Usually users do not care case and/or width of input characters because they are identical for users' eyes. Further, users rarely switch IME state to input special characters such as protocol elements. For Internationalized Domain Names ("IDNs"), IDNA Mapping [RFC5895] describes methods to treat these issues. For precis strings, case mapping is defined as a process in precis framework [I-D.ietf-precis-framework], but width mapping, delimiter mapping, special mapping and language dependent mapping are not defined. Handling of mappings other than case is also important to increase chance of strings match as users expect. This document is a guideline for authors of protocol profiles of precis framework and describes the mappings that must be considered between receiving user input and passing permitted code points to internationalized protocols. Mappings that this document describes are expected to perform before precis framework as well as Mapping Characters for IDNA2008 [RFC5895].

2. Types of mapping

This document defines two types of mapping. One is protocol independent mapping that doesn't depend on protocol rules and the other is protocol dependent mapping that depend on protocol rules. This document defines some mappings in these mapping types. Authors of protocol profiles of precis framework should need to give careful consideration to choice of mappings.

Each mapping type is described in following sections.

3. Protocol independent mapping

Protocol independent mapping is a mapping that doesn't depend on protocol rules.

3.1. Width mapping

Fullwidth and halfwidth characters (those defined with Decomposition Types <wide> and <narrow>) are mapped to their decomposition mappings as shown in the Unicode character database [Unicode].

Width mapping will increase backward compatibility with Stringprep [RFC3454] and precis framework [I-D.ietf-precis-framework]. Because in a Stringprep profile which specifies Unicode normalization form KC (NFKC) for normalization method, fullwidth/halfwidth characters are mapped into its compatible form. If a precis framework profile specified NFKC (which is not recommended), width mapping might not be useful.

4. Protocol dependent mapping

Protocol dependent mapping is a mapping that depend on protocol rules.

4.1. Delimiter mapping

Definitions of delimiters in certain protocols are differ from each other. Therefore, delimiter mapping table should be based on well defined mapping table for each protocol.

One of the most useful case of delimiter mapping is when FULL STOP character (U+002E) is a delimiter as well as domain name. Some of IME generates FULL STOP compatible characters such as IDEOGRAPHIC FULL STOP (U+3002) when users type FULL STOP on the keyboard.

4.2. Special mapping

Certain protocols have characters which need to map different character from precis framework defined mapping rule other than delimiter characters. In this document, these mappings are named special mapping. They are differ from each protocol. Therefore, special mapping table should be based on well defined mapping table for each protocol. Examples of special mapping are following; [RFC4518] defines the rule that some codepoints(Appendix B.4) are mapped to SPACE (U+0020).


4.3. Local case mapping

Local case mapping is case folding that depend on language context. For example, given there is upper case I in a user ID strings, you should care what's language context that this user ID depend on when this character is mapped into lower case character. And if this depends on Turkish, the character should be mapped into LATIN SMALL LETTER DOTLESS I (U+0131) as this character's lower case.

This document defines characters that need local case mapping based on the Specialcasing.txt [Specialcasing] in section 3.13 of The Unicode Standerd [Unicode] to solve such a problem for precis framework. Local case mapping targets only characters that get two different results to perfom just casefolding that is defined in the Casefolding.txt [Casefolding] and perfom special casefolding that is defined in the Specialcasing.txt then casefolding, because precis framework have casefolding.

There are two types casefoldings defined as Unconditional Mappings and Conditional Mappings in the Specialcasing.txt. Conditional mappings have Language-Insensitive Mappings that targets characters whose full case mappings do not depend on language, but do depend on context and Language-Sensitive Mappings that these are characters whose full case mappings depend on language and perhaps also context.

Of these mappings, characters that Unconditional Mappings and Language-Insensitive Mappings in Conditional Mappings target are mapped into same codepoint(s) with just casefolding and special casefolding then casefolding. But characters that Language-Sensitive Mappings in Conditional Mappings targets are mapped into different codepoint with them. Therefore this document defined characters that are a part of characters of Lithuanian(lt), Turkish(tr) and Azerbaijanian(az) that Language-Sensitive Mappings targets as targets for local case mapping.

Followings are the method to calculate codepoints that local case mapping targets. Here Casefolding() means casefolding described in the Casefolding.txt [Casefolding] and Specialcasing() means specialcasing described in the Specialcasing.txt [Specialcasing].

If Casefolding(Specialcasing(cp)) != Casefolding(cp)
Then cp is a target
Else cp is not a target;

Appendix C "Code points list for local case mapping" lists codepoints are calculated by this method.

5. Applying order of mapping

Basically, applying order of mapping that this document describes aren't sensitive. This section defines applying order of mapping to minimize effect of codepoint change by mappings. This mapping order is very general and was designed to be acceptable to the widest user community.

  1. Width mapping
  2. Delimiter mapping
  3. Special mapping
  4. Local case mapping

Mappings that this document describes are expected to perform before precis framework.

6. Open issues

Followings are cullent open issues for this document.

  1. Whether is local case mapping belong in additional mappings in precis framework?
  2. If local case mapping belong in precis framework, it's necessary to specify mapping order as local case mapping then case mapping. Because it makes no sense to perform local case mapping after case mapping.
  3. Handling order of precis framework and precis mappings is ambiguous. It's necessary to define the order in precis framework or in this document or in both documents.

7. IANA Considerations

This document does not define any IANA-related things.

8. Security Considerations

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

9. Acknowledgment

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

Joe Hildebrand, John Klensin, Marc Blanchet, Pete Resnick and Peter Saint-Andre, et al. gave important suggestion for this document during at WG meeting.

10. References

[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.
[RFC3722] Bakke, M., "String Profile for Internet Small Computer Systems Interface (iSCSI) Names", RFC 3722, April 2004.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H. Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, February 2005.
[RFC4314] Melnikov, A., "IMAP4 Access Control List (ACL) Extension", RFC 4314, December 2005.
[RFC4518] Zeilenga, K., "Lightweight Directory Access Protocol (LDAP): Internationalized String Preparation", RFC 4518, June 2006.
[RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for Internationalized Domain Names in Applications (IDNA) 2008", RFC 5895, September 2010.
[RFC6122] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Address Format", RFC 6122, March 2011.
[I-D.ietf-precis-framework] Saint-Andre, P and M Blanchet, "PRECIS Framework: Preparation and Comparison of Internationalized Strings in Application Protocols", Internet-Draft draft-ietf-precis-framework-06, September 2012.
[I-D.ietf-precis-problem-statement] Blanchet, M and A Sullivan, "Stringprep Revision and PRECIS Problem Statement", Internet-Draft draft-ietf-precis-problem-statement-08, September 2012.
[Unicode] The Unicode Consortium, , "The Unicode Standard, Version 6.2.0", <>, 2012.
[Casefolding] , , "CaseFolding-6.2.0.txt", Unicode Character Database, July 2011, <>, .
[Specialcasing] , , "SpecialCasing-6.2.0.txt", Unicode Character Database, July 2011, <>, .
[ISO.3166-1] International Organization for Standardization, "Codes for the representation of names of countries and their subdivisions - Part 1: Country codes", ISO Standard 3166- 1:1997, 1997.

Appendix A. Mapping type list each protocol

A.1. Mapping type list for each protocol

This table is the mapping type list for each protocol. Values marked "o" indicate that the protocol use the type of mapping. Values marked "-" indicate that the protocol doesn't use the type of mapping.

|    \ Type of mapping |    Width    | Delimiter | Case | Special |
| RFC \                |    (NFKC)   |           |      |         |
|         3490         |      -      |     o     |   -  |    -    |
|         3491         |      o      |     -     |   o  |    -    |
|         3722         |      o      |     -     |   o  |    -    |
|         3748         |      o      |     -     |   -  |    o    |
|         4013         |      o      |     -     |   -  |    o    |
|         4314         |      o      |     -     |   -  |    o    |
|         4518         |      o      |     -     |   o  |    o    |
|         6120         |      -      |     -     |   o  |    -    |

Appendix B. Codepoints which need special mapping

B.1. RFC3748

Non-ASCII space characters [StringPrep, C.1.2] that can be mapped to SPACE (U+0020).

B.2. RFC4013

Non-ASCII space characters [StringPrep, C.1.2] that can be mapped to SPACE (U+0020).

B.3. RFC4314

Non-ASCII space characters [StringPrep, C.1.2] that can be mapped to SPACE (U+0020).

B.4. RFC4518

Codepoints mapped to SPACE (U+0020) are following;

U+0085 (NEXT LINE (NEL))
U+0020 (SPACE)
U+2000 (EN QUAD)
U+2001 (EM QUAD)
U+2002 (EN SPACE)
U+2003 (EM SPACE)
U+2028 (Line Separator)
U+2029 (Paragraph Separator)

All other control code (e.g., Cc) points or code points with a control function (e.g., Cf) are mapped to nothing. Codepoints mapped to nothing that aren't specified by Stringprep are following;


Appendix C. Code points list for local case mapping

Followings are a list of characters that need Local case mapping. Format:
<Language>; <Codepoint>; <Lowercase>; <Comments>
<Language> means the alpha-2 codes in [ISO.3166-1].

C.1. Unidode 6.2

Appendix D. Change Log

D.1. Changes since -00

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:
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: