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+Network Working Group                                         M. Crispin
+Request for Comments: 5051                      University of Washington
+Category: Standards Track                                   October 2007
+
+
+         i;unicode-casemap - Simple Unicode Collation Algorithm
+
+Status of This Memo
+
+   This document specifies an Internet standards track protocol for the
+   Internet community, and requests discussion and suggestions for
+   improvements.  Please refer to the current edition of the "Internet
+   Official Protocol Standards" (STD 1) for the standardization state
+   and status of this protocol.  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.
+
+1.  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].
+
+2.  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.
+
+
+
+
+
+Crispin                     Standards Track                     [Page 1]
+
+RFC 5051                   i;unicode-casemap                October 2007
+
+
+   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 cannot 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
+
+
+
+Crispin                     Standards Track                     [Page 2]
+
+RFC 5051                   i;unicode-casemap                October 2007
+
+
+          (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.
+
+
+
+Crispin                     Standards Track                     [Page 3]
+
+RFC 5051                   i;unicode-casemap                October 2007
+
+
+3.  Unicode Casemap Collation Registration
+
+   <?xml version='1.0'?>
+   <!DOCTYPE collation SYSTEM 'collationreg.dtd'>
+   <collation rfc="5051" scope="global" intendedUse="common">
+   <identifier>i;unicode-casemap</identifier>
+   <title>Unicode Casemap</title>
+   <operations>equality order substring</operations>
+   <specification>RFC 5051</specification>
+   <owner>IETF</owner>
+   <submitter>mrc@cac.washington.edu</submitter>
+   </collation>
+
+4.  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 (NFKD) 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).
+
+
+
+
+
+
+Crispin                     Standards Track                     [Page 4]
+
+RFC 5051                   i;unicode-casemap                October 2007
+
+
+   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 causing a
+   non-match of strings which visually appear the same and/or causing
+   the string to appear elsewhere in a sort.
+
+5.  IANA Considerations
+
+   The i;unicode-casemap collation defined in section 2 has been added
+   to the registry of collations defined in [COMPARATOR].
+
+6.  Normative References
+
+   [COMPARATOR]          Newman, C., Duerst, M., and A. Gulbrandsen,
+                         "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/>.
+
+
+
+
+
+
+
+
+Crispin                     Standards Track                     [Page 5]
+
+RFC 5051                   i;unicode-casemap                October 2007
+
+
+7.  Informative References
+
+   [BASIC]               Newman, C., Duerst, M., and A. Gulbrandsen,
+                         "i;basic - the Unicode Collation Algorithm",
+                         Work in Progress, March 2007.
+
+   [IMAP-SORT]           Crispin, M. and K. Murchison, "Internet Message
+                         Access Protocol - SORT and THREAD Extensions",
+                         Work in Progress, September 2007.
+
+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
+
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+Crispin                     Standards Track                     [Page 6]
+
+RFC 5051                   i;unicode-casemap                October 2007
+
+
+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
+
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+   pertain to the implementation or use of the technology described in
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+Crispin                     Standards Track                     [Page 7]
+