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2 Network Working Group M. Crispin
3 Internet-Draft University of Washington
4 Intended status: Proposed Standard May 2, 2007
5 Expires: November 2, 2007
6 Document: internet-drafts/draft-crispin-collation-unicasemap-04.txt
8 i;unicode-casemap - Simple Unicode Collation Algorithm
10 Status of this Memo
12 By submitting this Internet-Draft, each author represents that
13 any applicable patent or other IPR claims of which he or she is
14 aware have been or will be disclosed, and any of which he or she
15 becomes aware will be disclosed, in accordance with Section 6 of
16 BCP 79.
18 Internet-Drafts are working documents of the Internet Engineering
19 Task Force (IETF), its areas, and its working groups. Note that
20 other groups may also distribute working documents as
21 Internet-Drafts.
23 Internet-Drafts are draft documents valid for a maximum of six months
24 and may be updated, replaced, or obsoleted by other documents at any
25 time. It is inappropriate to use Internet-Drafts as reference
26 material or to cite them other than as "work in progress."
28 The list of current Internet-Drafts can be accessed at
29 http://www.ietf.org/ietf/1id-abstracts.txt
31 The list of Internet-Draft Shadow Directories can be accessed at
32 http://www.ietf.org/shadow.html.
34 A revised version of this document will be submitted to the RFC
35 editor as an Informational Document for the Internet Community.
37 A revised version of this draft document will be submitted to the RFC
38 editor as a Proposed Standard for the Internet Community. Discussion
39 and suggestions for improvement are requested, and should be sent to
40 ietf-imapext@IMC.ORG.
42 Distribution of this memo is unlimited.
44 Abstract
46 This document describes "i;unicode-casemap", a simple
47 case-insensitive collation for Unicode strings. It provides
48 equality, substring and ordering operations.
50 Introduction
52 The "i;ascii-casemap" collation described in [COMPARATOR] is quite
53 simple to implement and provides case-independent comparisons for the
54 26 Latin alphabetics. It is specified as the default and/or baseline
55 comparator in some application protocols, e.g., [IMAP-SORT].
57 It is possible, with a modest extension, to provide a more
58 sophisticated collation with greater multilingual applicability than
59 "i;ascii-casemap".
61 This collation, "i;unicode-casemap", is intended to be an alternative
62 to, and preferred over, "i;ascii-casemap". It does not replace the
63 "i;basic" collation described in [BASIC].
65 1. Unicode Casemap Collation Description
67 The "i;unicode-casemap" collation is a simple collation which
68 operates on [UNICODE] strings and is case-insensitive in its
69 treatment of characters. It provides equality, substring and
70 ordering operations. All input is valid.
72 The algorithm that describes the behavior of this collation is
73 specified for Unicode input encoded in [UTF-8]. This is for ease of
74 description only. An implementation is free to use another internal
75 storage format for Unicode strings, as long as it produces the same
76 result as produced by the algorithm specified in this document for
77 any set of Unicode strings.
79 As this collation algorithm is specified for UTF-8 strings, strings
80 in other character sets and/or encodings can not be used with this
81 collation unless they are first converted to UTF-8.
83 Any input that is already in UTF-8 must be checked for invalid UTF-8
84 sequences, such as overlong sequences. A UTF-8 string that is
85 generated from a sequence of Unicode characters according to the
86 rules in [UTF-8] will not contain such invalid sequences.
88 For the equality and ordering operations, each input UTF-8 string is
89 prepared by converting it to "titlecased canonicalized UTF-8", using
90 UnicodeData.txt distributed by [UNICODE], as follows on a
91 per-character basis:
93 (1) If the codepoint has a titlecase property in UnicodeData.txt
94 (this is normally the same as the uppercase property) the
95 codepoint is converted to the titlecased codepoint.
96 (2) If the codepoint has a decomposition property of any type in
97 UnicodeData.txt the codepoint is converted to the decomposed
98 codepoints (effectively Normalization Form KD).
99 (3) The resulting codepoint(s) is/are appended to the titlecased
100 canonicalized UTF-8 string.
102 The resulting two titlecased canonicalized UTF-8 strings are then
103 treated as in i;octet for equality and ordering.
105 Care should be taken when using OS-supplied functions to implement
106 this collation as it is not locale sensitive. Functions such as
107 strcasecmp and toupper are sometimes locale sensitive and may
108 inconsistently casemap letters.
110 The i;unicode-casemap collation is well suited to use with many
111 Internet protocols and computer languages. Use with natural language
112 is often inappropriate; even though the collation apparently supports
113 languages such as Swahili and English, in real-world use it tends to
114 mis-sort a number of types of string:
116 o people and place names containing scripts that are not collated
117 according to "alphabetical order".
118 o words with characters that have diacriticals. However,
119 i;unicode-casemap generally does a better job than i;ascii-casemap
120 for most (but not all) languages. For example, German umlaut
121 letters will sort correctly, but some Scandinavian letters will
122 not.
123 o names such as "Lloyd" (which in Welsh sorts after "Lyon", unlike
124 in English),
125 o strings containing other non-letter symbols; e.g., euro and pound
126 sterling symbols, quotation marks other than '"', dashes/hyphens,
127 etc.
129 2. Unicode Casemap Collation Registration
131
132
133
134 i;unicode-casemap
135 Unicode Casemap
136 equality order substring
137 RFC XXXX
138 IETF
139 mrc@cac.washington.edu
140
142 3. Security Considerations
144 Collations will normally be used with UTF-8 strings. Thus the
145 security considerations for [UTF-8], [STRINGPREP] and
146 [UNICODE-SECURITY] also apply and are normative to this
147 specification.
149 4. IANA Considerations
151 The i;unicode-casemap collation defined in section 2 should be added
152 to the registry of collations defined in [COMPARATOR].
154 5. Normative References
156 The following documents are normative to this document:
158 [COMPARATOR] Newman, C., "Internet Appplication Protocol
159 Collation Registry", RFC 4790, February 2007.
161 [STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of
162 Internationalized Strings ("stringprep")",
163 RFC 3454, December 2002.
165 [UTF-8] Yergeau, F., "UTF-8, a transformation format
166 of ISO 10646", STD 63, RFC 3629, November 2003.
168 [UNICODE] , UnicodeData.txt
170 Although the UnicodeData.txt file referenced
171 here is part of the Unicode standard, it is
172 subject to change as new characters are added
173 to Unicode and errors are corrected in Unicode
174 revisions. As a result, it may be less stable
175 than might otherwise be implied by the
176 standards status of this specification.
178 [UNICODE-SECURITY] Davis, M. and M. Suignard, "Unicode Security
179 Considerations", February 2006,
180 .
182 6. Informative References:
184 [BASIC] Newman, C., Duerst, M., and Gulbrandsen, A.,
185 "i;basic - the Unicode Collation Algorithm",
186 draft-gulbrandsen-collation-basic, Work in
187 Progress.
189 [IMAP-SORT] Crispin, M. "Internet Message Access Protocol -
190 SORT and THREAD Extensions",
191 draft-ietf-imapext-sort, Work in Progress (in
192 RFC Editor queue).
194 Appendices
196 Author's Address
198 Mark R. Crispin
199 Networks and Distributed Computing
200 University of Washington
201 4545 15th Avenue NE
202 Seattle, WA 98105-4527
204 Phone: +1 (206) 543-5762
206 EMail: MRC@CAC.Washington.EDU
208 Full Copyright Statement
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