Network Working Group M. Blanchet
Internet-Draft Viagenie
Obsoletes: 3454 (if approved) P. Saint-Andre
Intended status: Standards Track Cisco
Expires: November 18, 2011 May 17, 2011
PRECIS Framework: Handling Internationalized Strings in Protocols
draft-blanchet-precis-framework-01
Abstract
Application protocols that make use of Unicode code points in
protocol strings need to prepare such strings in order to perform
comparison operations (e.g., for purposes of authentication or
authorization). In general, this problem has been labeled the
"preparation and comparison of internationalized strings" or
"PRECIS". This document defines a framework that enables application
protocols to prepare various classes of strings in a way that depends
on the properties of Unicode code points. Because this framework
does not depend on large tables of Unicode code points as in
stringprep (RFC 3454), it is more agile with regard to changes in the
underlying Unicode database and thus provides improved flexibility to
application protocols. A specification that reuses this framework
either can directly use the base string classes defined in this
document or can subclass the base string classes as needed. This
framework uses an approach similar to that of the revised
internationalized domain names in applications (IDNA) technology (RFC
5890, RFC 5891, RFC 5892, RFC 5893, RFC 5894) and thus adheres to the
high-level design goals described in RFC 4690, albeit for non-IDNA
technologies. This document obsoletes RFC 3454.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on November 18, 2011.
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Copyright Notice
Copyright (c) 2011 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
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Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. String Classes . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Nameything . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Valid . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.2. Disallowed . . . . . . . . . . . . . . . . . . . . . . 7
3.1.3. Unassigned . . . . . . . . . . . . . . . . . . . . . . 8
3.1.4. Directionality . . . . . . . . . . . . . . . . . . . . 8
3.1.5. Case Mapping . . . . . . . . . . . . . . . . . . . . . 8
3.1.6. Normalization . . . . . . . . . . . . . . . . . . . . 8
3.2. Wordything . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Valid . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.2. Disallowed . . . . . . . . . . . . . . . . . . . . . . 9
3.2.3. Unassigned . . . . . . . . . . . . . . . . . . . . . . 9
3.2.4. Directionality . . . . . . . . . . . . . . . . . . . . 9
3.2.5. Case Mapping . . . . . . . . . . . . . . . . . . . . . 9
3.2.6. Normalization . . . . . . . . . . . . . . . . . . . . 9
3.3. Stringything . . . . . . . . . . . . . . . . . . . . . . . 10
3.3.1. Valid . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3.2. Disallowed . . . . . . . . . . . . . . . . . . . . . . 10
3.3.3. Unassigned . . . . . . . . . . . . . . . . . . . . . . 10
3.3.4. Directionality . . . . . . . . . . . . . . . . . . . . 10
3.3.5. Case Mapping . . . . . . . . . . . . . . . . . . . . . 10
3.3.6. Normalization . . . . . . . . . . . . . . . . . . . . 10
4. Reuse of PRECIS String Classes . . . . . . . . . . . . . . . . 11
4.1. Principles . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Subclassing . . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Template . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.4. Registration . . . . . . . . . . . . . . . . . . . . . . . 11
5. Code Point Properties . . . . . . . . . . . . . . . . . . . . 12
6. Category Definitions Used to Calculate Derived Property
Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. LetterDigits (A) . . . . . . . . . . . . . . . . . . . . . 14
6.2. Unstable (B) . . . . . . . . . . . . . . . . . . . . . . . 14
6.3. IgnorableProperties (C) . . . . . . . . . . . . . . . . . 14
6.4. IgnorableBlocks (D) . . . . . . . . . . . . . . . . . . . 15
6.5. LDH (E) . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.6. Exceptions (F) . . . . . . . . . . . . . . . . . . . . . . 15
6.7. BackwardCompatible (G) . . . . . . . . . . . . . . . . . . 16
6.8. JoinControl (H) . . . . . . . . . . . . . . . . . . . . . 17
6.9. OldHangulJamo (I) . . . . . . . . . . . . . . . . . . . . 17
6.10. Unassigned (J) . . . . . . . . . . . . . . . . . . . . . . 17
6.11. ASCII7 (K) . . . . . . . . . . . . . . . . . . . . . . . . 17
6.12. Controls (L) . . . . . . . . . . . . . . . . . . . . . . . 18
6.13. PrecisIgnorableProperties (M) . . . . . . . . . . . . . . 18
6.14. Spaces (N) . . . . . . . . . . . . . . . . . . . . . . . . 18
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6.15. Symbols (O) . . . . . . . . . . . . . . . . . . . . . . . 18
6.16. Punctuation (P) . . . . . . . . . . . . . . . . . . . . . 18
6.17. HasCompat (Q) . . . . . . . . . . . . . . . . . . . . . . 19
7. Calculation of the Derived Property . . . . . . . . . . . . . 19
8. Code Points . . . . . . . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
9.1. PRECIS Derived Property Value Registry . . . . . . . . . . 20
9.2. PRECIS Contextual Rules Registry . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
12. Codepoints 0x0000 - 0x10FFFF . . . . . . . . . . . . . . . . . 21
12.1. Codepoints in Unicode Character Database (UCD) format . . 22
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
13.1. Normative References . . . . . . . . . . . . . . . . . . . 22
13.2. Informative References . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
A number of IETF application technologies use stringprep [RFC3454] as
the basis for comparing protocol strings that contain Unicode
characters or "code points" [UNICODE]. Since the publication of
[RFC3454] in 2002, the Internet community has gained much more
experience with internationalization, some of it reflected in
[RFC4690]. In particular, the IETF's technology for
internationalized domain names (IDNs) has changed significantly:
IDNA2003 [RFC3490], which was based on stringprep, has been
superseded by IDNA2008 ([RFC5890], [RFC5891], [RFC5892], [RFC5893],
[RFC5894]), which does not use stringprep. This migration away from
stringprep for internationalized domain names has prompted other
"customers" of stringprep to consider new approaches to the
preparation and comparison of internationalized strings ("PRECIS"),
as described in [PROBLEM].
This document proposes a technical framework for a post-stringprep
approach to the preparation and comparison of internationalized
strings in application protocols. The framework is based on several
principles:
1. Define a small set of base string classes appropriate for common
application protocol constructs such as usernames, passwords, and
free-form identifiers.
2. Define each base string class in terms of Unicode code points and
their properties, specifying whether each code point or category
of code points is valid, disallowed, or unassigned.
3. Enable application protocols to subclass the base string classes.
4. Mapping operations (e.g., case preservation or lowercasing,
Unicode normalization, right-to-left characters) are the
responsibility of application protocols, as was done for IDNA2008
via [RFC5895].
It is expected that this framework will yield the following benefits:
o Application protocols will be more version-agile with regard to
the Unicode database.
o Implementers will be able to share code point tables and software
code across application protocols, most likely by means of code
libraries.
o End users will be able to build more accurate expectations about
the code points that are acceptable in various contexts. Given
this more uniform set of string classes, it is also expected that
copy/paste operations between software implementing different
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application protocols will be more predictable and coherent.
Although this framework is similar to IDNA2008, it defines additional
string classes to meet the needs of common application protocols.
2. Terminology
Many important terms used in this document are defined in [PROBLEM],
[I18N-TERMS], [RFC5890], and [UNICODE].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. String Classes
IDNA2008 essentially defines a base string class of internationalized
domain name, although it does not use the term "string class". (This
document does not redefine a string class for domain names, and
application protocols are strongly encouraged to IDNA2008 as the
appropriate method to prepare domain names and hostnames.)
We propose the following additional base string classes for use in
application protocols:
Nameything: a word or set of words that is used to identify or
address a network entity such as a user, an account, a venue
(e.g., a chatroom), an information source (e.g., a data feed), or
a collection of data (e.g., a file).
Wordything: a sequence of letters, numbers, and symbols that is used
as a secret for access to some resource on a network (e.g., a
password or passphrase).
Stringything: a sequence of letters, numbers, symbols, spaces, and
other code points that is used for more expressive purposes in an
application protocol (e.g., a nickname in a chatroom).
The following subsections discuss these string classes in more
detail, with reference to the dimensions described in Section 3 of
[PROBLEM].
Each string class is defined by the following behavioral rules:
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Valid: defines which code points and code point categories are
treated as valid input to preparation of the string.
Disallowed: defines which code points and code point categories are
treated as disallowed during preparation of the string.
Unassigned: defines application behavior in the presence of code
points that are unassigned, i.e. unknown for the version of
Unicode the application is built upon.
Directionality: defines application behavior in the presence of code
points that have directionality, in particular right-to-left code
points as defined in the Unicode database (see [UAX9].
Casemapping: defines if case mapping is used for this class, and how
the mapping is done.
Normalization: defines which Unicode normalization form (D, KD, C,
or KC) is to be applied (see [UAX15]).
This document defines the valid, disallowed, and unassigned rules.
Application protocols that use the PRECIS string classes MUST define
the directionality, casemapping, and normalization rules, as further
described under Section 4.
3.1. Nameything
Most application technologies need a special class of strings that
can be used to refer to, include, or communicate things like
usernames, chatroom names, file names, and data feed names. We group
such things into a bucket called "nameythings" having the following
features.
3.1.1. Valid
o Letters and numbers, i.e., the LetterDigits ("A") category first
defined in [RFC5892] and listed here under Section 6.1.
o Code points in the range U+0021 through U+007E, i.e., the ASCII7
("K") rule defined under Section 6.11. These code points are
valid even if they would otherwise be disallowed according to the
property-based rules specified in the next section.
3.1.2. Disallowed
o Control characters, i.e., the Controls ("L") category defined
under Section 6.12.
o Space characters, i.e., the Spaces ("N") category defined under
Section 6.14.
o Symbol characters, i.e., the Symbols ("O") category defined under
Section 6.15.
o Punctuation characters, i.e., the Punctuation ("P") category
defined under Section 6.16.
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o Any character that has a compatibility equivalent, i.e., the
HasCompat ("Q") category defined under Section 6.17. These code
points are disallowed even if they would otherwise be valid
according to the property-based rules specified in the previous
section.
3.1.3. Unassigned
Any code points that are not yet assigned in the Unicode character
set shall be considered Unassigned for purposes of PRECIS string
classes.
3.1.4. Directionality
To be defined by application protocols.
3.1.5. Case Mapping
To be defined by application protocols.
3.1.6. Normalization
To be defined by application protocols.
### OPEN ISSUE: Perhaps recommend that application protocols use NFC,
in accordance with [RFC5198], normalization form C (NFC) is
RECOMMENDED. ###
3.2. Wordything
Many application technologies need a special class of strings that
can be used to communicate secrets of the kind that are typically
used as passwords or passphrases. We group such things into a bucket
called "wordythings" having the following features.
NOTE: Some application protocols use passwords and passphrases
directly, whereas others reuse technologies that process passwords
(e.g., the Simple Authentication and Security Layer [RFC4422]).
Moreover, passwords are often carried by a sequence of protocols with
backends such as RADIUS or LDAP. Developers of application protocols
are encouraged to look into reusing these profiles instead of
defining new ones, so that expectations for passwords by end-users
are consistent no matter what application protocol is used.
3.2.1. Valid
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o Letters and numbers, i.e., the LetterDigits ("A") category first
defined in [RFC5892] and listed here under Section 6.1.
o Code points in the range U+0021 through U+007E, i.e., the ASCII7
("K") rule defined under Section 6.11. These code points are
valid even if they would otherwise be disallowed according to the
property-based rules specified in the next section.
o Any character that has a compatibility equivalent, i.e., the
HasCompat ("Q") category defined under Section 6.17.
3.2.2. Disallowed
o Control characters, i.e., the Controls ("L") category defined
under Section 6.12.
o Space characters, i.e., the Spaces ("N") category defined under
Section 6.14.
o Symbol characters, i.e., the Symbols ("O") category defined under
Section 6.15.
o Punctuation characters, i.e., the Punctuation ("P") category
defined under Section 6.16.
### OPEN ISSUE: Why not allow symbols and punctuation characters in
wordythings? ###
3.2.3. Unassigned
Any code points that are not yet assigned in the Unicode character
set shall be considered Unassigned for purposes of PRECIS string
classes.
3.2.4. Directionality
To be defined by application protocols.
3.2.5. Case Mapping
To be defined by application protocols.
### OPEN ISSUE: Perhaps recommend that application protocols map
uppercase and titlecase code points to their lowercase equivalents,
in order to maximize the entropy of passwords and passphrases. ###
3.2.6. Normalization
To be defined by application protocols.
### OPEN ISSUE: Perhaps recommend that application protocols use NFC,
in accordance with [RFC5198], normalization form C (NFC) is
RECOMMENDED. ###
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3.3. Stringything
Some application technologies need a special class of strings that
can be used in a free-form way (e.g., a nickname in a chatroom). We
group such things into a bucket called "stringythings" having the
following features.
3.3.1. Valid
o Letters and numbers, i.e., the LetterDigits ("A") category first
defined in [RFC5892] and listed here under Section 6.1.
o Code points in the range U+0021 through U+007E, i.e., the ASCII7
("K") rule defined under Section 6.11.
o Any character that has a compatibility equivalent, i.e., the
HasCompat ("Q") category defined under Section 6.17.
o Space characters, i.e., the Spaces ("N") category defined under
Section 6.14.
o Symbol characters, i.e., the Symbols ("O") category defined under
Section 6.15.
o Punctuation characters, i.e., the Punctuation ("P") category
defined under Section 6.16.
3.3.2. Disallowed
o Control characters, i.e., the Controls ("L") category defined
under Section 6.12.
3.3.3. Unassigned
Any code points that are not yet assigned in the Unicode character
set shall be considered Unassigned for purposes of PRECIS string
classes.
3.3.4. Directionality
To be defined by application protocols.
3.3.5. Case Mapping
To be defined by application protocols.
3.3.6. Normalization
To be defined by application protocols.
### OPEN ISSUE: Perhaps recommend that application protocols use NFC,
in accordance with [RFC5198], normalization form C (NFC) is
RECOMMENDED. ###
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4. Reuse of PRECIS String Classes
4.1. Principles
This document defines the valid, disallowed, and unassigned rules.
Application protocols that use the PRECIS string classes MUST define
the directionality, casemapping, and normalization rules. Such
definitions MUST at a minimum specify the following:
Directionality: Whether any instance of the class that contains a
right-to-left code point is to be considered a right-to-left
string, or whether a more complex rule is to be applied (e.g., the
"Bidi Rule" from [RFC5893]).
Casemapping: Whether uppercase and titlecase code points are to be
(a) preserved or (b) mapped to lowercase.
Normalization: Which Unicode normalization form (D, KD, C, or KC) is
to be applied (see [UAX15] for background information and
[RFC5198] for relevant considerations).
4.2. Subclassing
Application protocols are allowed to subclass the base string classes
specified in this document. As the word "subclass" implies, a
subclass MUST NOT add as valid any code points or code point
categories that are disallowed by the base string class. However, a
subclass MAY do either of the following:
1. Exclude specific code points that are included in the base string
class.
2. exclude characters matching certain Unicode properties (e.g.,
math symbols) that are included in the base string class.
4.3. Template
The following template can be used by application protocols that
reuse the PRECIS string classes.
Base Class: [which base class is being reused]
Subclassing: [whether the base class is being subclassed]
Directionality: [the behavioral rule for handling of right-to-left
code points]
Casemapping: [the behavioral rule for handling of case]
Normalization: [which Unicode normalization form is applied]
4.4. Registration
### OPEN ISSUE: Perhaps define an IANA registry for application
protocols that reuse the PRECIS string classes.
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5. Code Point Properties
In order to implement the string classes described above, this
document does the following:
1. Reviews and classifies the collections of code points in the
Unicode character set by examining various properties of the code
points.
2. Defines an algorithm for determining a derived property value,
which can vary depending on the string class being used by the
relevant application protocol.
NOTE: This document specifies a procedure that can be applied to code
points, and not a table of code points, so that the algorithm can be
used to determine code point sets independent of the version of
Unicode that is in use.
This document is not intended to specify precisely how derived
property values are to be applied in protocol strings. That
information should be defined in the protocol specification that uses
or subclasses a base string class from this document.
The value of the property is to be interpreted as follows.
o PROTOCOL VALID: Those that are allowed to be used in any PRECIS
string class (nameything, wordything, and stringything). Code
points with this property value are permitted for general use in
any string class. The abbreviated term PVALID is used to refer to
this value in the rest of this document.
o SPECIFIC CLASS PROTOCOL VALID: Those that are allowed to be used
in specific string classes. Code points with this property value
are permitted for use in specific string classes. In the rest of
this document, the abbreviated term *_PVALID is used, where * =
(NAMEY, WORDY, STRINGY).
o CONTEXTUAL RULE REQUIRED: Some characteristics of the character,
such as it being invisible in certain contexts or problematic in
others, requires that it not be used in labels unless specific
other characters or properties are present. The abbreviated term
CONTEXT is used to refer to this value in the rest of this
document. There are two subdivisions of CONTEXTUAL RULE REQUIRED,
the first for Join_controls (called CONTEXTJ) and the second for
other characters (called CONTEXTO).
o DISALLOWED: Those that must not be included in any string class.
Code points with this property value are not permitted in any
string class.
o SPECIFIC CLASS DISALLOWED: Those that are not to be included in a
specific string class. Code points with this property value are
not permitted in one of the string classes but might be permitted
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in others. In the rest of this document, the abbreviated term
*_DISALLOWED is used, where * = (NAMEY, WORDY, STRINGY).
o UNASSIGNED: Those code points that are not designated (i.e. are
unassigned) in the Unicode Standard.
The mechanisms described here allow determination of the value of the
property for future versions of Unicode (including characters added
after Unicode 5.2 or 6.0 depending on the category, since some
categories in this document are re-used from IDNA2008). Changes in
Unicode properties that do not affect the outcome of this process do
not affect this framework. For example, a character can have its
Unicode General_Category value [UNICODE] change from So to Sm, or
from Lo to Ll, without affecting the algorithm results. Moreover,
even if such changes were to result, the BackwardCompatible list
(Section 6.7) can be adjusted to ensure the stability of the results.
### OPEN ISSUE: How to handle a backward compatible list on the level
of PRECIS string classes or sub-classes. ###
Some code points need to be allowed in exceptional circumstances, but
should be excluded in all other cases; these rules are also described
in other documents. The most notable of these are the Join Control
characters, U+200D ZERO WIDTH JOINER and U+200C ZERO WIDTH NON-
JOINER. Both of them have the derived property value CONTEXTJ. A
character with the derived property value CONTEXTJ or CONTEXTO
(CONTEXTUAL RULE REQUIRED) is not to be used unless an appropriate
rule has been established and the context of the character is
consistent with that rule. It is invalid to generate a string
containing these characters unless such a contextual rule is found
and satisfied. Please see Appendix A of [RFC5892] for more
information.
6. Category Definitions Used to Calculate Derived Property Value
The derived property obtains its value based on a two-step procedure:
1. Characters are placed in one or more character categories based
on either core properties defined by the Unicode Standard or by
treating the code point as an exception and addressing the code
point by its code point value. These categories are not mutually
exclusive.
2. Set operations are used with these categories to determine the
values for an string class specific property. These operations
are specified under Section 7.
(NOTE: Unicode property names and property value names might have
short abbreviations, such as gc for the General_Category property,
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and Ll for the Lowercase_Letter property value of the gc property.)
In the following specification of categories, the operation which
returns the value of a particular Unicode character property for a
code point is designated by using the formal name of that property
(from PropertyAliases.txt) followed by '(cp)'. For example, the
value of the General_Category property for a code point is indicated
by General_Category(cp).
The first ten categories (A-J) shown below were previously defined
for IDNA2008 and are copied directly from [RFC5892]. Some of these
categories are re-used in PRECIS and some of them are not; however,
the lettering of categories is retained to prevent overlap and to
ease implementation of both IDNA2008 and PRECIS in a single software
application. The next seven categories (K-Q) are specific to PRECIS.
6.1. LetterDigits (A)
NOTE: This category is defined in [RFC5892] and copied here for use
in PRECIS.
A: General_Category(cp) is in {Ll, Lu, Lo, Nd, Lm, Mn, Mc}
These rules identify characters commonly used in mnemonics and often
informally described as "language characters".
For more information, see section 4.5 of [UNICODE].
The categories used in this rule are:
o Ll - Lowercase_Letter
o Lu - Uppercase_Letter
o Lo - Other_Letter
o Nd - Decimal_Number
o Lm - Modifier_Letter
o Mn - Nonspacing_Mark
o Mc - Spacing_Mark
6.2. Unstable (B)
NOTE: This category is defined in [RFC5892] but not used in PRECIS.
6.3. IgnorableProperties (C)
NOTE: This category is defined in [RFC5892] but not used in PRECIS.
See the "PrecisIgnorableProperties (M)" category below for a more
inclusive category used in non-IDN identifiers.
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6.4. IgnorableBlocks (D)
NOTE: This category is defined in [RFC5892] but not used in PRECIS.
6.5. LDH (E)
NOTE: This category is defined in [RFC5892] but not used in PRECIS.
See the "ASCII7 (K)" category below for a more inclusive category
used in non-IDN identifiers.
6.6. Exceptions (F)
NOTE: This category is defined in [RFC5892] and might be used in a
future version of this specification.
F: cp is in {00B7, 00DF, 0375, 03C2, 05F3, 05F4, 0640, 0660,
0661, 0662, 0663, 0664, 0665, 0666, 0667, 0668,
0669, 06F0, 06F1, 06F2, 06F3, 06F4, 06F5, 06F6,
06F7, 06F8, 06F9, 06FD, 06FE, 07FA, 0F0B, 3007,
302E, 302F, 3031, 3032, 3033, 3034, 3035, 303B,
30FB}
This category explicitly lists code points for which the category
cannot be assigned using only the core property values that exist in
the Unicode standard. The values are according to the table below:
PVALID -- Would otherwise have been DISALLOWED
00DF; PVALID # LATIN SMALL LETTER SHARP S
03C2; PVALID # GREEK SMALL LETTER FINAL SIGMA
06FD; PVALID # ARABIC SIGN SINDHI AMPERSAND
06FE; PVALID # ARABIC SIGN SINDHI POSTPOSITION MEN
0F0B; PVALID # TIBETAN MARK INTERSYLLABIC TSHEG
3007; PVALID # IDEOGRAPHIC NUMBER ZERO
CONTEXTO -- Would otherwise have been DISALLOWED
00B7; CONTEXTO # MIDDLE DOT
0375; CONTEXTO # GREEK LOWER NUMERAL SIGN (KERAIA)
05F3; CONTEXTO # HEBREW PUNCTUATION GERESH
05F4; CONTEXTO # HEBREW PUNCTUATION GERSHAYIM
30FB; CONTEXTO # KATAKANA MIDDLE DOT
CONTEXTO -- Would otherwise have been PVALID
0660; CONTEXTO # ARABIC-INDIC DIGIT ZERO
0661; CONTEXTO # ARABIC-INDIC DIGIT ONE
0662; CONTEXTO # ARABIC-INDIC DIGIT TWO
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0663; CONTEXTO # ARABIC-INDIC DIGIT THREE
0664; CONTEXTO # ARABIC-INDIC DIGIT FOUR
0665; CONTEXTO # ARABIC-INDIC DIGIT FIVE
0666; CONTEXTO # ARABIC-INDIC DIGIT SIX
0667; CONTEXTO # ARABIC-INDIC DIGIT SEVEN
0668; CONTEXTO # ARABIC-INDIC DIGIT EIGHT
0669; CONTEXTO # ARABIC-INDIC DIGIT NINE
06F0; CONTEXTO # EXTENDED ARABIC-INDIC DIGIT ZERO
06F1; CONTEXTO # EXTENDED ARABIC-INDIC DIGIT ONE
06F2; CONTEXTO # EXTENDED ARABIC-INDIC DIGIT TWO
06F3; CONTEXTO # EXTENDED ARABIC-INDIC DIGIT THREE
06F4; CONTEXTO # EXTENDED ARABIC-INDIC DIGIT FOUR
06F5; CONTEXTO # EXTENDED ARABIC-INDIC DIGIT FIVE
06F6; CONTEXTO # EXTENDED ARABIC-INDIC DIGIT SIX
06F7; CONTEXTO # EXTENDED ARABIC-INDIC DIGIT SEVEN
06F8; CONTEXTO # EXTENDED ARABIC-INDIC DIGIT EIGHT
06F9; CONTEXTO # EXTENDED ARABIC-INDIC DIGIT NINE
DISALLOWED -- Would otherwise have been PVALID
0640; DISALLOWED # ARABIC TATWEEL
07FA; DISALLOWED # NKO LAJANYALAN
302E; DISALLOWED # HANGUL SINGLE DOT TONE MARK
302F; DISALLOWED # HANGUL DOUBLE DOT TONE MARK
3031; DISALLOWED # VERTICAL KANA REPEAT MARK
3032; DISALLOWED # VERTICAL KANA REPEAT WITH VOICED SOUND MARK
3033; DISALLOWED # VERTICAL KANA REPEAT MARK UPPER HALF
3034; DISALLOWED # VERTICAL KANA REPEAT WITH VOICED SOUND MARK
UPPER HA
3035; DISALLOWED # VERTICAL KANA REPEAT MARK LOWER HALF
303B; DISALLOWED # VERTICAL IDEOGRAPHIC ITERATION MARK
6.7. BackwardCompatible (G)
NOTE: This category is defined in [RFC5892] and copied here for use
in PRECIS.
G: cp is in {}
This category includes the code points that property values in
versions of Unicode after 5.2 have changed in such a way that the
derived property value would no longer be PVALID or DISALLOWED. If
changes are made to future versions of Unicode so that code points
might change property value from PVALID or DISALLOWED, then this
table can be updated and keep special exception values so that the
property values for code points stay stable.
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6.8. JoinControl (H)
NOTE: This category is defined in [RFC5892] and copied here for use
in PRECIS.
H: Join_Control(cp) = True
This category consists of Join Control characters (i.e., they are not
in LetterDigits (Section 6.1)) but are still required in strings
under some circumstances.
6.9. OldHangulJamo (I)
NOTE: This category is defined in [RFC5892] and copied here for use
in PRECIS.
I: Hangul_Syllable_Type(cp) is in {L, V, T}
This category consists of all conjoining Hangul Jamo (Leading Jamo,
Vowel Jamo, and Trailing Jamo).
Elimination of conjoining Hangul Jamos from the set of PVALID
characters results in restricting the set of Korean PVALID characters
just to preformed, modern Hangul syllable characters. Old Hangul
syllables, which must be spelled with sequences of conjoining Hangul
Jamos, are not PVALID for string classes.
6.10. Unassigned (J)
NOTE: This category is defined in [RFC5892] and copied here for use
in PRECIS.
J: General_Category(cp) is in {Cn} and
Noncharacter_Code_Point(cp) = False
This category consists of code points in the Unicode character set
that are not (yet) assigned. It should be noted that Unicode
distinguishes between 'unassigned code points' and 'unassigned
characters'. The unassigned code points are all but (Cn -
Noncharacters), while the unassigned *characters* are all but (Cn +
Cs).
6.11. ASCII7 (K)
This category exempts most "ASCII7" characters from other rules that
might be applied during PRECIS processing, on the assumption that
these code points are in such wide use that disallowing them would be
counter-productive.
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K: cp is in {0021..007E}
6.12. Controls (L)
L: Control(cp) = True
6.13. PrecisIgnorableProperties (M)
This category is used to group code points that are not recommended
for use in PRECIS string classes.
M: Default_Ignorable_Code_Point(cp) = True or
Noncharacter_Code_Point(cp) = True
The definition for Default_Ignorable_Code_Point can be found in
DerivedCoreProperties.txt [1] and at the time of Unicode 6.0 is as
follows:
Other_Default_Ignorable_Code_Point
+ Cf (Format characters)
+ Variation_Selector
- White_Space
- FFF9..FFFB (Annotation Characters)
- 0600..0603, 06DD, 070F (exceptional Cf characters
that should be visible)
6.14. Spaces (N)
This category is used to group code points that are space characters.
N: General_Category(cp) is in {Zs}
6.15. Symbols (O)
This category is used to group code points that are symbols.
O: General_Category(cp) is in {Sc}
6.16. Punctuation (P)
This category is used to group code points that are punctuation
marks.
P: General_Category(cp) is in {Pi}
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6.17. HasCompat (Q)
This category is used to group code points that have compatibility
equivalents as explained in Chapter 2 and Chapter 3 of [UNICODE].
Q: toNFKC(cp) != cp
The toNFKC() operation returns the code point in normalization form
KC. For more information, see Section 5 of [UAX15].
7. Calculation of the Derived Property
Possible values of the derived property are:
o PVALID
o NAMEY_VALID
o WORDY_VALID
o STRINGY_VALID
o CONTEXTJ
o CONTEXTO
o DISALLOWED
o NAMEY_DISALLOWED
o WORDY_DISALLOWED
o STRINGY_DISALLOWED
o UNASSIGNED
In some instances, the value of the derived property calculated
depends on the string class.
The algorithm to calculate the value of the derived property is as
follows. (NOTE: If the name of a rule (such as Exception) is used,
that implies the set of code points that the rule define, while the
same name as a function call (such as Exception(cp)) imply the value
cp has in the Exceptions table.)
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If .cp. .in. Exceptions Then Exceptions(cp);
Else If .cp. .in. BackwardCompatible Then BackwardCompatible(cp);
Else If .cp. .in. Unassigned Then UNASSIGNED;
Else If .cp. .in. ASCII7 Then PVALID;
Else If .cp. .in. JoinControl Then CONTEXTJ;
Else If .cp. .in. PrecisIgnorableProperties Then DISALLOWED;
Else If .cp. .in. Controls Then DISALLOWED;
Else If .cp. .in. OldHangulJamo Then DISALLOWED;
Else If .cp. .in. LetterDigits Then PVALID;
Else If .cp. .in. Spaces Then NAMEY_DISALLOWED
or WORDY_DISALLOWED
or STRINGY_VALID;
Else If .cp. .in. Symbols Then NAMEY_DISALLOWED
or WORDY_DISALLOWED
or STRINGY_VALID;
Else If .cp. .in. Punctuation Then NAMEY_DISALLOWED
or WORDY_DISALLOWED
or STRINGY_VALID;
Else If .cp. .in. HasCompat Then NAMEY_DISALLOWED
or WORDY_VALID
or STRINGY_VALID;
Else DISALLOWED;
8. Code Points
The Categories and Rules defined in Section 6 and Section 7 apply to
all Unicode code points. The table in Section 12 shows, for
illustrative purposes, the consequences of the categories and
classification rules, and the resulting property values.
The list of code points that can be found in Section 12 is non-
normative. Section 6 and Section 7 are normative.
9. IANA Considerations
9.1. PRECIS Derived Property Value Registry
IANA is to create a PRECIS-specific registry with the Derived
Properties for the versions of Unicode that are released after (and
including) version 6.0. The derived property value is to be
calculated in cooperation with a designated expert [RFC5226]
according to the specifications in Section 6 and Section 7, and not
by copying the non-normative table found in Section 12.
If during this process (creation of the table of derived property
values) followed by a designated expert review, either non-backward
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compatible changes to the table of derived properties are discovered,
or otherwise problems during the creation of the table arises, that
is to be flagged to the IESG. Changes to the rules (as specified in
Section 6 and Section 7) require IETF Review, as described in
[RFC5226].
9.2. PRECIS Contextual Rules Registry
For characters that are defined in the IDNA derived property value
registry as CONTEXTO or CONTEXTJ and that therefore require a
contextual rule, IANA will create and maintain a list of approved
contextual rules. Additions or changes to these rules require IETF
Review, as described in [RFC5226].
Appendix A (TBD) contains further discussion and a table from which
that registry can be initialized.
10. Security Considerations
This section has yet to be defined. However, the security
considerations provided in [RFC5890] provide a good starting point.
11. Acknowledgements
The authors would like to acknowledge the comments and contributions
of the following people: David Black, Mark Davis, Alan DeKok, Martin
Duerst, Patrik Faltstrom, Ted Hardie, Joe Hildebrand, Paul Hoffman,
Jeffrey Hutzelman, Simon Josefsson, John Klensin, Alexey Melnikov,
Pete Resnick, Andrew Sullivan, and Dave Thaler.
Some algorithms and textual descriptions have been borrowed from
[RFC5892].
12. Codepoints 0x0000 - 0x10FFFF
If one applies the rules (Section 7) to the code points 0x0000 to
0x10FFFF to Unicode 6.0, the result is as follows.
NOTE: This list is non-normative, and only included for illustrative
purposes. Specifically, what is displayed in the third column is not
the formal name of the code point (as defined in [UNICODE]). The
differences exists for example for the code points that have the code
point value as part of the name (example: CJK UNIFIED IDEOGRAPH-4E00)
and the naming of Hangul syllables. For many code points, what you
see is the official name.
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12.1. Codepoints in Unicode Character Database (UCD) format
0000..10FFFF; TBD!
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008.
[UNICODE] The Unicode Consortium, "The Unicode Standard, Version
6.0", 2010,
.
13.2. Informative References
[I18N-TERMS]
Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF",
draft-hoffman-rfc3536bis-02 (work in progress),
April 2011.
[PROBLEM] Blanchet, M. and A. Sullivan, "Stringprep Revision Problem
Statement", draft-ietf-precis-problem-statement-02 (work
in progress), March 2011.
[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.
[RFC4422] Melnikov, A. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
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May 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC5892] Faltstrom, P., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, August 2010.
[RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
Internationalized Domain Names for Applications (IDNA)",
RFC 5893, August 2010.
[RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, August 2010.
[RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for
Internationalized Domain Names in Applications (IDNA)
2008", RFC 5895, September 2010.
[UAX15] The Unicode Consortium, "Unicode Standard Annex #15:
Unicode Normalization Forms", September 2010,
.
[UAX9] The Unicode Consortium, "Unicode Standard Annex #9:
Unicode Bidirectional Algorithm", September 2010,
.
URIs
[1]
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Authors' Addresses
Marc Blanchet
Viagenie
2600 boul. Laurier, suite 625
Quebec, QC G1V 4W1
Canada
Email: Marc.Blanchet@viagenie.ca
URI: http://www.viagenie.ca
Peter Saint-Andre
Cisco
1899 Wyknoop Street, Suite 600
Denver, CO 80202
USA
Phone: +1-303-308-3282
Email: psaintan@cisco.com
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