[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 RFC 5051

Network Working Group                                         M. Crispin
Internet-Draft                                  University of Washington
Intended status: Proposed Standard                       August 30, 2007
Expires: February 30, 2008
Document: internet-drafts/draft-crispin-collation-unicasemap-07.txt

             i;unicode-casemap - Simple Unicode Collation Algorithm

Status of this Memo

    By submitting this Internet-Draft, each author represents that
    any applicable patent or other IPR claims of which he or she is
    aware have been or will be disclosed, and any of which he or she
    becomes aware will be disclosed, in accordance with Section 6 of
    BCP 79.

    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
    http://www.ietf.org/ietf/1id-abstracts.txt

    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html.

    A revised version of this document will be submitted to the RFC
    editor as an Informational Document for the Internet Community.

    A revised version of this draft document will be submitted to the RFC
    editor as a Proposed Standard for the Internet Community.  Discussion
    and suggestions for improvement are requested, and should be sent to
    ietf-imapext@IMC.ORG.

    Distribution of this memo is unlimited.


Abstract

    This document describes "i;unicode-casemap", a simple
    case-insensitive collation for Unicode strings.  It provides
    equality, substring and ordering operations.


Introduction

    The "i;ascii-casemap" collation described in [COMPARATOR] is quite
    simple to implement and provides case-independent comparisons for the
    26 Latin alphabetics.  It is specified as the default and/or baseline
    comparator in some application protocols, e.g., [IMAP-SORT].

    However, the "i;ascii-casemap" collation does not produce
    satisfactory results with non-ASCII characters.  It is possible, with
    a modest extension, to provide a more sophisticated collation with
    greater multilingual applicability than "i;ascii-casemap".  This
    extension provides case-independent comparisons for a much greater
    number of characters.  It also collates characters with diacriticals
    with the non-diacritical character forms.

    This collation, "i;unicode-casemap", is intended to be an alternative
    to, and preferred over, "i;ascii-casemap".  It does not replace the
    "i;basic" collation described in [BASIC].


1. Unicode Casemap Collation Description

    The "i;unicode-casemap" collation is a simple collation which is
    case-insensitive in its treatment of characters.  It provides
    equality, substring and ordering operations.  The validity test
    operation returns "valid" for any input.

    This collation allows strings in arbitrary (and mixed) character
    sets, as long as the character set for each string is identified and
    it is possible to convert the string to Unicode.  Strings which have
    an unidentified character set and/or can not be converted to Unicode
    are not rejected, but are treated as binary.

    Each input string is prepared by converting it to a "titlecased
    canonicalized UTF-8" string according to the following steps, using
    UnicodeData.txt ([UNICODE-DATA]):

       (1) A Unicode codepoint is obtained from the input string.

           (a) If the input string is in a known charset that can be
               converted to Unicode, a sequence in the string's charset
               is read and checked for validity according to the rules of
               that charset.  If the sequence is valid, it is converted
               to a Unicode codepoint.  Note that for input strings in
               UTF-8, the UTF-8 sequence must be valid according to the
               rules of [UTF-8]; e.g., overlong UTF-8 sequences are
               invalid.

           (b) If the input string is in an unknown charset, or an
               invalid sequence occurs in step (1)(a), conversion ceases.
               No further preparation is performed, and any partial
               preparation results are discarded.  The original string is
               used unchanged with the i;octet comparator.

       (2) The following steps, using UnicodeData.txt ([UNICODE-DATA]),
           are performed on the resulting codepoint from step (1)(a).

           (a) If the codepoint has a titlecase property in
               UnicodeData.txt (this is normally the same as the
               uppercase property), the codepoint is converted to the
               codepoints in the titlecase property.

           (b) If the resulting codepoint from (2)(a) has a decomposition
               property of any type in UnicodeData.txt, the codepoint is
               converted to the codepoints in the decomposition property.
               This step is recursively applied to each of the resulting
               codepoints until no more decomposition is possible
               (effectively Normalization Form KD).

           Example: codepoint U+01C4 (LATIN CAPITAL LETTER DZ WITH CARON)
           has a titlecase property of U+01C5 (LATIN CAPITAL LETTER D
           WITH SMALL LETTER Z WITH CARON).  Codepoint U+01C5 has a
           decomposition property of U+0044 (LATIN CAPITAL LETTER D)
           U+017E (LATIN SMALL LETTER Z WITH CARON).  U+017E has a
           decomposition property of U+007A (LATIN SMALL LETTER Z) U+030c
           (COMBINING CARON).  Neither U+0044, U+007A, nor U+030C have
           any decomposition properties.  Therefore, U+01C4 is converted
           to U+0044 U+007A U+030C by this step.

       (3) The resulting codepoint(s) from step (2) is/are appended, in
           UTF-8 format, to the "titlecased canonicalized UTF-8" string.

       (4) Repeat from step (1) until there is no more data in the input
           string.

    Following the above preparation process on each string, the equality,
    ordering and substring operations are as for i;octet.

    It is permitted to use an alternative implementation of the above
    preparation process if it produces the same results.  For example, it
    may be more convenient for an implementation to convert all input
    strings to a sequence of UTF-16 or UTF-32 values prior to performing
    any of the step (2) actions.  Similarly, if all input strings are (or
    are convertible to) Unicode, it may be possible to use UTF-32 as an
    alternative to UTF-8 in step (3).

       Note: UTF-16 is unsuitable as an alternative to UTF-8 in step (3),
       because UTF-16 surrogates will cause i;octet to collate codepoints
       U+E0000 through U+FFFF after non-BMP codepoints.

    This collation is not locale sensitive.  Consequently, care should be
    taken when using OS-supplied functions to implement this collation.
    Functions such as strcasecmp and toupper are sometimes locale
    sensitive and may inconsistently casemap letters.

    The i;unicode-casemap collation is well suited to use with many
    Internet protocols and computer languages.  Use with natural language
    is often inappropriate; even though the collation apparently supports
    languages such as Swahili and English, in real-world use it tends to
    mis-sort a number of types of string:

    o  people and place names containing scripts that are not collated
       according to "alphabetical order".
    o  words with characters that have diacriticals.  However,
       i;unicode-casemap generally does a better job than i;ascii-casemap
       for most (but not all) languages.  For example, German umlaut
       letters will sort correctly, but some Scandinavian letters will
       not.
    o  names such as "Lloyd" (which in Welsh sorts after "Lyon", unlike
       in English),
    o  strings containing other non-letter symbols; e.g., euro and pound
       sterling symbols, quotation marks other than '"', dashes/hyphens,
       etc.

2. Unicode Casemap Collation Registration

    <?xml version='1.0'?>
    <!DOCTYPE collation SYSTEM 'collationreg.dtd'>
    <collation rfc="XXXX" scope="global" intendedUse="common">
      <identifier>i;unicode-casemap</identifier>
      <title>Unicode Casemap</title>
      <operations>equality order substring</operations>
      <specification>RFC XXXX</specification>
      <owner>IETF</owner>
      <submitter>mrc@cac.washington.edu</submitter>
    </collation>

3. Security Considerations

    The security considerations for [UTF-8], [STRINGPREP] and
    [UNICODE-SECURITY] apply and are normative to this specification.

    The results from this comparator will vary depending upon the
    implementation for several reasons.  Implementations MUST consider
    whether these possibilities are a problem for their use case:

     1) New characters added in Unicode may have decomposition or
        titlecase properties that will not be known to an implementation
        based upon an older revision of Unicode.  This impacts Step (2).

     2) Step (2)(b) defines a subset of Normalization Form KD that does
        not require normalization of out-of-order diacriticals.  However,
        an implementation MAY use an NFKD library routine that does such
        normalization.  This impacts step (2)(b) and possibly also step
        (1)(a), and is an issue only with ill-formed UTF-8 input.

     3) The set of charsets handled in step (1)(a) is open-ended.  UTF-8
        (and, by extension, US-ASCII) are the only mandatory-to-implement
        charsets.  This impacts step (1)(a).

        Implementations SHOULD, as far as feasible, support all the
        charsets they are likely to encounter in the input data, in order
        to avoid poor collation caused by the fall through to the (1)(b)
        rule.

     4) Other charsets may have revisions which add new characters that
        are not known to an implementation based upon an older revision.
        This impacts step (1)(a) and possibly also step (1)(b).

    An attacker may create input that is ill-formed or in an unknown
    charset, with the intention of impacting the results of this
    comparator or exploiting other parts of the system which process this
    input in different ways.  Note, however, that even well-formed data
    in a known charset can impact the result of this comparator in
    unexpected ways.  For example, an attacker can substitute U+0041
    (LATIN CAPITAL LETTER A) with U+0391 (GREEK CAPITAL LETTER ALPHA) or
    U+0410 (CYRILLIC CAPITAL LETTER A) in the intention of a non-match of
    strings which visually appear the same and/or to cause the string to
    appear elsewhere in a sort.


4. IANA Considerations

    The i;unicode-casemap collation defined in section 2 should be added
    to the registry of collations defined in [COMPARATOR].


5. Normative References

    The following documents are normative to this document:

    [COMPARATOR]          Newman, C., "Internet Application Protocol
                          Collation Registry", RFC 4790, February 2007.

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

    [UTF-8]               Yergeau, F., "UTF-8, a transformation format
                          of ISO 10646", STD 63, RFC 3629, November 2003.

    [UNICODE-DATA]        <http://www.unicode.org/Public/UNIDATA/
                          UnicodeData.txt>

                          Although the UnicodeData.txt file referenced
                          here is part of the Unicode standard, it is
                          subject to change as new characters are added
                          to Unicode and errors are corrected in Unicode
                          revisions.  As a result, it may be less stable
                          than might otherwise be implied by the
                          standards status of this specification.

    [UNICODE-SECURITY]    Davis, M. and M. Suignard, "Unicode Security
                          Considerations", February 2006,
                          <http://www.unicode.org/reports/tr36/>.


6. Informative References:

    [BASIC]               Newman, C., Duerst, M., and Gulbrandsen, A.,
                          "i;basic - the Unicode Collation Algorithm",
                          draft-gulbrandsen-collation-basic, Work in
                          Progress.

    [IMAP-SORT]           Crispin, M. "Internet Message Access Protocol -
                          SORT and THREAD Extensions",
                          draft-ietf-imapext-sort, Work in Progress (in
                          RFC Editor queue).


Appendices

Author's Address

    Mark R. Crispin
    Networks and Distributed Computing
    University of Washington
    4545 15th Avenue NE
    Seattle, WA  98105-4527

    Phone: +1 (206) 543-5762

    EMail: MRC@CAC.Washington.EDU


Full Copyright Statement

    Copyright (C) The IETF Trust (2007).

    This document is subject to the rights, licenses and restrictions
    contained in BCP 78, and except as set forth therein, the authors
    retain all their rights.

    This document and the information contained herein are provided on an
    "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
    OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
    THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
    OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
    THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
    WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Intellectual Property

    The IETF takes no position regarding the validity or scope of any
    Intellectual Property Rights or other rights that might be claimed to
    pertain to the implementation or use of the technology described in
    this document or the extent to which any license under such rights
    might or might not be available; nor does it represent that it has
    made any independent effort to identify any such rights.  Information
    on the procedures with respect to rights in RFC documents can be
    found in BCP 78 and BCP 79.

    Copies of IPR disclosures made to the IETF Secretariat and any
    assurances of licenses to be made available, or the result of an
    attempt made to obtain a general license or permission for the use of
    such proprietary rights by implementers or users of this
    specification can be obtained from the IETF on-line IPR repository at
    http://www.ietf.org/ipr.

    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights that may cover technology that may be required to implement
    this standard.  Please address the information to the IETF at ietf-
    ipr@ietf.org.


Acknowledgement

    Funding for the RFC Editor function is currently provided by the
    Internet Society.


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/