Internet Engineering Task Force (IETF) K. Davies
Request for Comments: 7940 ICANN
Category: Standards Track A. Freytag
ISSN: 2070-1721 ASMUS, Inc.
August 2016
Representing Label Generation Rulesets Using XML
Abstract
This document describes a method of representing rules for validating
identifier labels and alternate representations of those labels using
Extensible Markup Language (XML). These policies, known as "Label
Generation Rulesets" (LGRs), are used for the implementation of
Internationalized Domain Names (IDNs), for example. The rulesets are
used to implement and share that aspect of policy defining which
labels and Unicode code points are permitted for registrations, which
alternative code points are considered variants, and what actions may
be performed on labels containing those variants.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7940.
Copyright Notice
Copyright (c) 2016 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Normative Language . . . . . . . . . . . . . . . . . . . . . 6
4. LGR Format . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Namespace . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Basic Structure . . . . . . . . . . . . . . . . . . . . . 6
4.3. Metadata . . . . . . . . . . . . . . . . . . . . . . . . 7
4.3.1. The "version" Element . . . . . . . . . . . . . . . . 8
4.3.2. The "date" Element . . . . . . . . . . . . . . . . . 8
4.3.3. The "language" Element . . . . . . . . . . . . . . . 9 8
4.3.4. The "scope" Element . . . . . . . . . . . . . . . . . 9
4.3.5. The "description" Element . . . . . . . . . . . . . . 10
4.3.6. The "validity-start" and "validity-end" Elements . . 10
4.3.7. The "unicode-version" Element . . . . . . . . . . . . 10
4.3.8. The "references" Element . . . . . . . . . . . . . . 11
5. Code Points and Variants . . . . . . . . . . . . . . . . . . 12
5.1. Sequences . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2. Conditional Contexts . . . . . . . . . . . . . . . . . . 14
5.3. Variants . . . . . . . . . . . . . . . . . . . . . . . . 15
5.3.1. Basic Variants . . . . . . . . . . . . . . . . . . . 15
5.3.2. The "type" Attribute . . . . . . . . . . . . . . . . 17 16
5.3.3. Null Variants . . . . . . . . . . . . . . . . . . . . 18 17
5.3.4. Variants with Reflexive Mapping . . . . . . . . . . . 18
5.3.5. Conditional Variants . . . . . . . . . . . . . . . . 19
5.4. Annotations . . . . . . . . . . . . . . . . . . . . . . . 21
5.4.1. The "ref" Attribute . . . . . . . . . . . . . . . . . 21
5.4.2. The "comment" Attribute . . . . . . . . . . . . . . . 22 21
5.5. Code Point Tagging . . . . . . . . . . . . . . . . . . . 22
6. Whole Label and Context Evaluation . . . . . . . . . . . . . 23 22
6.1. Basic Concepts . . . . . . . . . . . . . . . . . . . . . 23 22
6.2. Character Classes . . . . . . . . . . . . . . . . . . . . 24 23
6.2.1. Declaring and Invoking Named Classes . . . . . . . . 24
6.2.2. Tag-Based Classes . . . . . . . . . . . . . . . . . . 25 24
6.2.3. Unicode Property-Based Classes . . . . . . . . . . . 25
6.2.4. Explicitly Declared Classes . . . . . . . . . . . . . 27 26
6.2.5. Combined Classes . . . . . . . . . . . . . . . . . . 27
6.3. Whole Label and Context Rules . . . . . . . . . . . . . . 29
6.3.1. The "rule" Element . . . . . . . . . . . . . . . . . 29
6.3.2. The Match Operators . . . . . . . . . . . . . . . . . 30
6.3.3. The "count" Attribute . . . . . . . . . . . . . . . . 31
6.3.4. The "name" and "by-ref" Attributes . . . . . . . . . 32
6.3.5. The "choice" Element . . . . . . . . . . . . . . . . 33
6.3.6. Literal Code Point Sequences . . . . . . . . . . . . 33
6.3.7. The "any" Element . . . . . . . . . . . . . . . . . . 34
6.3.8. The "start" and "end" Elements . . . . . . . . . . . 34
6.3.9. Example Context Rule from IDNA Specification . . . . 35
6.4. Parameterized Context or When Rules . . . . . . . . . . . 35
6.4.1. The "anchor" Element . . . . . . . . . . . . . . . . 36
6.4.2. The "look-behind" and "look-ahead" Elements . . . . . 37
6.4.3. Omitting the "anchor" Element . . . . . . . . . . . . 38
7. The "action" Element . . . . . . . . . . . . . . . . . . . . 38
7.1. The "match" and "not-match" Attributes . . . . . . . . . 39
7.2. Actions with Variant Type Triggers . . . . . . . . . . . 39
7.2.1. The "any-variant", "all-variants", and
"only-variants" Attributes . . . . . . . . . . . . . 40
7.2.2. Example from Tables in the Style of RFC 3743 . . . . 42
7.3. Recommended Disposition Values . . . . . . . . . . . . . 43
7.4. Precedence . . . . . . . . . . . . . . . . . . . . . . . 43
7.5. Implied Actions . . . . . . . . . . . . . . . . . . . . . 44
7.6. Default Actions . . . . . . . . . . . . . . . . . . . . . 44
8. Processing a Label against an LGR . . . . . . . . . . . . . . 45
8.1. Determining Eligibility for a Label . . . . . . . . . . . 45
8.1.1. Determining Eligibility Using Reflexive Variant
Mappings . . . . . . . . . . . . . . . . . . . . . . 46
8.2. Determining Variants for a Label . . . . . . . . . . . . 46
8.3. Determining a Disposition for a Label or Variant Label . 47
8.4. Duplicate Variant Labels . . . . . . . . . . . . . . . . 48
8.5. Checking Labels for Collision . . . . . . . . . . . . . . 49
9. Conversion to and from Other Formats . . . . . . . . . . . . 49
10. Media Type . . . . . . . . . . . . . . . . . . . . . . . . . 50
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 50
11.1. Media Type Registration . . . . . . . . . . . . . . . . 50
11.2. URN Registration . . . . . . . . . . . . . . . . . . . . 51
11.3. Disposition Registry . . . . . . . . . . . . . . . . . . 51
12. Security Considerations . . . . . . . . . . . . . . . . . . . 52
12.1. LGRs Are Only a Partial Remedy for Problem Space . . . . 52
12.2. Computational Expense of Complex Tables . . . . . . . . 53
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 53
13.1. Normative References . . . . . . . . . . . . . . . . . . 53
13.2. Informative References . . . . . . . . . . . . . . . . . 54
Appendix A. Example Tables . . . . . . . . . . . . . . . . . . . 56
Appendix B. How to Translate Tables Based on RFC 3743 into the
XML Format . . . . . . . . . . . . . . . . . . . . . 60
Appendix C. Indic Syllable Structure Example . . . . . . . . . . 65
C.1. Reducing Complexity . . . . . . . . . . . . . . . . . . . 67
Appendix D. RELAX NG Compact Schema . . . . . . . . . . . . . . 68
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 77
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 78
1. Introduction
This document specifies a method of using Extensible Markup Language
(XML) to describe Label Generation Rulesets (LGRs). LGRs are
algorithms used to determine whether, and under what conditions, a
given identifier label is permitted, based on the code points it
contains and their context. These algorithms comprise a list of
permissible code points, variant code point mappings, and a set of
rules that act on the code points and mappings. LGRs form part of an
administrator's policies. In deploying Internationalized Domain
Names (IDNs), they have also been known as IDN tables or variant
tables.
There are other kinds of policies relating to labels that are not
normally covered by LGRs and are therefore not necessarily
representable by the XML format described here. These include, but
are not limited to, policies around trademarks, or prohibition of
fraudulent or objectionable words.
Administrators of the zones for top-level domain registries have
historically published their LGRs using ASCII text or HTML. The
formatting of these documents has been loosely based on the format
used for the Language Variant Table described in [RFC3743].
[RFC4290] also provides a "model table format" that describes a
similar set of functionality. Common to these formats is that the
algorithms used to evaluate the data therein are implicit or
specified elsewhere.
Through the first decade of IDN deployment, experience has shown that
LGRs derived from these formats are difficult to consistently
implement and compare, due to their differing formats. A universal
format, such as one using a structured XML format, will assist by
improving machine readability, consistency, reusability, and
maintainability of LGRs.
When used to represent a simple list of permitted code points, the
format is quite straightforward. At the cost of some complexity in
the resulting file, it also allows for an implementation of more
sophisticated handling of conditional variants that reflects the
known requirements of current zone administrator policies.
Another feature of this format is that it allows many of the
algorithms to be made explicit and machine implementable. A
remaining small set of implicit algorithms is described in this
document to allow commonality in implementation.
While the predominant usage of this specification is to represent IDN
label policy, the format is not limited to IDN usage and may also be
used for describing ASCII domain name label rulesets, or other types
of identifier labels beyond those used for domain names.
2. Design Goals
The following goals informed the design of this format:
o The format needs to be implementable in a reasonably
straightforward manner in software.
o The format should be able to be automatically checked for
formatting errors, so that common mistakes can be caught.
o An LGR needs to be able to express the set of valid code points
that are allowed for registration under a specific administrator's
policies.
o An LGR needs to be able to express computed alternatives to a
given identifier based on mapping relationships between code
points, whether one-to-one or many-to-many. These computed
alternatives are commonly known as "variants".
o Variant code points should be able to be tagged with explicit
dispositions or categories that can be used to support registry
policy (such as whether to allocate the computed variant or to
merely block it from usage or registration).
o Variants and code points must be able to be stipulated based on
contextual information. For example, some variants may only be
applicable when they follow a certain code point or when the code
point is displayed in a specific presentation form.
o The data contained within an LGR must be able to be interpreted
unambiguously, so that independent implementations that utilize
the contents will arrive at the same results.
o To the largest extent possible, policy rules should be able to be
specified in the XML format without relying on hidden or built-in
algorithms in implementations.
o LGRs should be suitable for comparison and reuse, such that one
could easily compare the contents of two or more to see the
differences, to merge them, and so on.
o As many existing IDN tables as practicable should be able to be
migrated to the LGR format with all applicable interpretation
logic retained.
These requirements are partly derived from reviewing the existing
corpus of published IDN tables, plus the requirements of ICANN's work
to implement an LGR for the DNS root zone [LGR-PROCEDURE]. In
particular, Section B of that document identifies five specific
requirements for an LGR methodology.
The syntax and rules in [RFC5892] and [RFC3743] were also reviewed.
It is explicitly not the goal of this format to stipulate what code
points should be listed in an LGR by a zone administrator. Which
registration policies are used for a particular zone are outside the
scope of this memo.
3. Normative Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
4. LGR Format
An LGR is expressed as a well-formed XML document [XML] that conforms
to the schema defined in Appendix D.
As XML is case sensitive, an LGR must be authored with the correct
casing. For example, the XML element names MUST be in lowercase as
described in this specification, and matching of attribute values is
only performed in a case-sensitive manner.
A document that is not well-formed, is non-conforming, or violates
other constraints specified in this specification MUST be rejected.
4.1. Namespace
The XML Namespace URI is "urn:ietf:params:xml:ns:lgr-1.0".
See Section 11.2 for more information.
4.2. Basic Structure
The basic XML framework of the document is as follows:
<?xml version="1.0"?>
<lgr xmlns="urn:ietf:params:xml:ns:lgr-1.0">
...
</lgr>
The "lgr" element contains up to three sub-elements or sections.
First is an optional "meta" element that contains all metadata
associated with the LGR, such as its authorship, what it is used for,
implementation notes, and references. This is followed by a required
"data" element that contains the substantive code point data.
Finally, an optional "rules" element contains information on
contextual and whole-label evaluation rules, rules
for evaluating labels, if any, along with "action" elements providing
for the disposition of labels and computed variant labels.
<?xml version="1.0"?>
<lgr xmlns="urn:ietf:params:xml:ns:lgr-1.0">
<meta>
...
</meta>
<data>
...
</data>
<rules>
...
</rules>
</lgr>
A document MUST contain exactly one "lgr" element. Each "lgr"
element MUST contain zero or one "meta" element, exactly one "data"
element, and zero or one "rules" element; and these three elements
MUST be in that order.
Some elements that are direct or nested child elements of the "rules"
element MUST be placed in a specific relative order to other elements
for the LGR to be valid. An LGR that violates these constraints MUST
be rejected. In other cases, changing the ordering would result in a
valid, but different, specification.
In the following descriptions, required, non-repeating elements or
attributes are generally not called out explicitly, in contrast to
"OPTIONAL" ones, or those that "MAY" be repeated. For attributes
that take lists as values, the elements MUST be space-separated.
4.3. Metadata
The "meta" element expresses metadata associated with the LGR, and
the element SHOULD be included so that the associated metadata are
available as part of the LGR and cannot become disassociated. The
following subsections describe elements that may appear within the
"meta" element.
The "meta" element can be used to identify the author or relevant
contact person, explain the intended usage of the LGR, and provide
implementation notes as well as references. Detailed metadata allow
the LGR document to become self-documenting -- for example, if
rendered in a human-readable format by an appropriate tool.
Providing metadata pertaining to the date and version of the LGR is
particularly encouraged to make it easier for interoperating
consumers to ensure that they are using the correct LGR.
With the exception of the "unicode-version" element, the data
contained within is not required by software consuming the LGR in
order to calculate valid labels or to calculate variants. If
present, the "unicode-version" element MUST be used by a consumer of
the table to identify that it has the correct Unicode property data
to perform operations on the table. This ensures that possible
differences in code point properties between editions of the Unicode
Standard do not impact the product of calculations utilizing an LGR.
4.3.1. The "version" Element
The "version" element is OPTIONAL. It is used to uniquely identify
each version of the LGR. No specific format is required, but it is
RECOMMENDED that it be the decimal representation of a single
positive integer, which is incremented with each revision of the
file.
An example of a typical first edition of a document:
<version>1</version>
The "version" element may have an OPTIONAL "comment" attribute.
<version comment="draft">1</version>
4.3.2. The "date" Element
The OPTIONAL "date" element is used to identify the date the LGR was
posted. The contents of this element MUST be a valid ISO 8601
"full-date" string as described in [RFC3339].
Example of a date:
<date>2009-11-01</date>
4.3.3. The "language" Element
Each OPTIONAL "language" element identifies a language or script for
which the LGR is intended. The value of the "language" element MUST
be a valid language tag as described in [RFC5646]. The tag may refer
to a script plus undefined language if the LGR is not intended for a
specific language.
Example of an LGR for the English language:
<language>en</language>
If the LGR applies to a script rather than a specific language, the
"und" language tag SHOULD be used followed by the relevant script
subtag from [RFC5646]. For example, for a Cyrillic script LGR:
<language>und-Cyrl</language>
If the LGR covers a set of multiple languages or scripts, the
"language" element MAY be repeated. However, for cases of a script-
specific LGR exhibiting insignificant admixture of code points from
other scripts, it is RECOMMENDED to use a single "language" element
identifying the predominant script. In the exceptional case of a
multi-script LGR where no script is predominant, use Zyyy (Common):
<language>und-Zyyy</language>
4.3.4. The "scope" Element
This OPTIONAL element refers to a scope, such as a domain, to which
this policy is applied. The "type" attribute specifies the type of
scope being defined. A type of "domain" means that the scope is a
domain that represents the apex of the DNS zone to which the LGR is
applied. For that type, the content of the "scope" element MUST be a
domain name written relative to the root zone, in presentation format
with no trailing dot. However, in the unique case of the DNS root
zone, it is represented as ".".
<scope type="domain">example.com</scope>
There may be multiple "scope" tags used -- for example, to reflect a
list of domains to which the LGR is applied.
No other values of the "type" attribute are defined by this
specification; however, this specification can be used for
applications other than domain names. Implementers of LGRs for
applications other than domain names SHOULD define the scope
extension grammar in an IETF specification or use XML namespaces to
distinguish their scoping mechanism distinctly from the base LGR
namespace. An explanation of any custom usage of the scope in the
"description" element is RECOMMENDED.
<scope xmlns="http://example.com/ns/scope/1.0">
... content per alternate namespace ...
</scope>
4.3.5. The "description" Element
The "description" element is an OPTIONAL, free-form element that
contains any additional relevant description that is useful for the
user in its interpretation. Typically, this field contains
authorship information, as well as additional context on how the LGR
was formulated and how it applies, such as citations and references
that apply to the LGR as a whole.
This field should not be relied upon for providing instructions on
how to parse or utilize the data contained elsewhere in the
specification. Authors of tables should expect that software
applications that parse and use LGRs will not use the "description"
element to condition the application of the LGR's data and rules.
The element has an OPTIONAL "type" attribute, which refers to the
Internet media type [RFC2045] of the enclosed data. Typical types
would be "text/plain" or "text/html". The attribute SHOULD be a
valid media type. If supplied, it will be assumed that the contents
are of that media type. If the description lacks a "type" value, it
will be assumed to be plain text ("text/plain").
4.3.6. The "validity-start" and "validity-end" Elements
The "validity-start" and "validity-end" elements are OPTIONAL
elements that describe the time period from which the contents of the
LGR become valid (are used in registry policy) and time when the
contents of the LGR cease to be used, respectively.
The dates MUST conform to the "full-date" format described in
Section 5.6 of [RFC3339].
<validity-start>2014-03-12</validity-start>
4.3.7. The "unicode-version" Element
Whenever an LGR depends on character properties from a given version
of the Unicode Standard, the version number used in creating the LGR
MUST be listed in the form x.y.z, where x, y, and z are positive
decimal integers (see [Unicode-Versions]). If any software
processing the table does not have access to character property data
of the requisite version, it MUST NOT perform any operations relating
to whole-label evaluation relying on Unicode character properties
(Section 6.2.3).
The value of a given Unicode character property may change between
versions of the Unicode Character Database [UAX44], unless such
change has been explicitly disallowed in [Unicode-Stability]. It is
RECOMMENDED to only reference properties defined as stable or
immutable. As an alternative to referencing the property, the
information can be presented explicitly in the LGR.
<unicode-version>6.3.0</unicode-version>
It is not necessary to include a "unicode-version" element for LGRs
that do not make use of Unicode character properties; however, it is
RECOMMENDED.
4.3.8. The "references" Element
An LGR may define a list of references that are used to associate
various individual elements in the LGR to one or more normative
references. A common use for references is to annotate that code
points belong to an externally defined collection or standard or to
give normative references for rules.
References are specified in an OPTIONAL "references" element
containing one or more "reference" elements, each with a unique "id"
attribute. It is RECOMMENDED that the "id" attribute be a zero-based
integer; however, in addition to digits 0-9, it MAY contain uppercase
letters A-Z, as well as a period, hyphen, colon, or underscore. The
value of each "reference" element SHOULD be the citation of a
standard, dictionary, or other specification in any suitable format.
In addition to an "id" attribute, a "reference" element MAY have a
"comment" attribute for an optional free-form annotation.
<references>
<reference id="0">The Unicode Consortium. The Unicode
Standard, Version 8.0.0, (Mountain View, CA: The Unicode
Consortium, 2015. ISBN 978-1-936213-10-8)
http://www.unicode.org/versions/Unicode8.0.0/</reference>
<reference id="1">Big-5: Computer Chinese Glyph and Character
Code Mapping Table, Technical Report C-26, 1984</reference>
<reference id="2" comment="synchronized with Unicode 6.1">
ISO/IEC
10646:2012 3rd edition</reference>
...
</references>
...
<data>
<char cp="0620" ref="0 2" />
...
</data>
A reference is associated with an element by using its id as part of
an optional "ref" attribute (see Section 5.4.1). The "ref" attribute
may be used with many kinds of elements in the "data" or "rules"
sections of the LGR, most notably those defining code points,
variants, and rules. However, a "ref" attribute may not occur in
certain kinds of elements, including references to named character
classes or rules. See below for the description of these elements.
5. Code Points and Variants
The bulk of an LGR is a description of which set of code points is
eligible for a given label. For rulesets that perform operations
that result in potential variants, the code point-level relationships
between variants need to also be described.
The code point data is collected within the "data" element. Within
this element, a series of "char" and "range" elements describe
eligible code points or ranges of code points, respectively.
Collectively, these are known as the repertoire.
Discrete permissible code points or code point sequences (see
Section 5.1) are declared with a "char" element. Here is a minimal
example declaration for a single code point, with the code point
value given in the "cp" attribute:
<char cp="002D"/>
As described below, a full declaration for a "char" element, whether
or not it is used for a single code point or for a sequence (see
Section 5.1), may have optional child elements defining variants.
Both the "char" and "range" elements can take a number of optional
attributes for conditional inclusion, commenting, cross-referencing,
and character tagging, as described below.
Ranges of permissible code points may be declared with a "range"
element, as in this minimal example:
<range first-cp="0030" last-cp="0039"/>
The range is inclusive of the first and last code points. Any
additional attributes defined for a "range" element act as if applied
to each code point within. A "range" element has no child elements.
It is always possible to substitute a list of individually specified
code points for a "range" element. The reverse is not necessarily
the case. Whenever such a substitution is possible, it makes no
difference in processing the data. Tools reading or writing the LGR
format are free to aggregate sequences of consecutive code points of
the same properties into "range" elements.
Code points MUST be represented according to the standard Unicode
convention but without the prefix "U+": they are expressed in
uppercase hexadecimal and are zero-padded to a minimum of 4 digits.
The rationale for not allowing other encoding formats, including
native Unicode encoding in XML, is explored in [UAX42]. The XML
conventions used in this format, such as element and attribute names,
mirror this document where practical and reasonable to do so. It is
RECOMMENDED to list all "char" elements in ascending order of the
"cp" attribute. Not doing so makes it unnecessarily difficult for
authors and reviewers to check for errors, such as duplications, or
to review and compare against listing of code points in other
documents and specifications.
All "char" elements in the "data" section MUST have distinct "cp"
attributes. The "range" elements MUST NOT specify code point ranges
that overlap either another range or any single code point "char"
elements. An LGR that defines the same code point more than once by
any combination of "char" or "range" elements MUST be rejected.
5.1. Sequences
A sequence of two or more code points may be specified in an LGR --
for example, when defining the source for n:m variant mappings.
Another use of sequences would be in cases when the exact sequence of
code points is required to occur in order for the constituent
elements to be eligible, such as when some code point is only
eligible when preceded or followed by a certain code point. The
following would define the eligibility of the MIDDLE DOT (U+00B7)
only when both preceded and followed by the LATIN SMALL LETTER L
(U+006C):
<char cp="006C 00B7 006C" comment="Catalan middle dot"/>
All sequences defined this way must be distinct, but sub-sequences
may be defined. Thus, the sequence defined here may coexist with
single code point definitions such as:
<char cp="006C" />
As an alternative to using sequences to define a required context, a
"char" or "range" element may specify a conditional context using an
optional "when" attribute as described below in Section 5.2. Using a
conditional context is more flexible because a context is not limited
to a specific sequence of code points. In addition, using a context
allows the choice of specifying either a prohibited or a required
context.
5.2. Conditional Contexts
A conditional context is specified by a rule that must be satisfied
(or, alternatively, must not be satisfied) for a code point in a
given label, often at a particular location in a label.
To specify a conditional context, either a "when" or "not-when"
attribute may be used. The value of each "when" or "not-when"
attribute is a context rule as described below in Section 6.3. This
rule can be a rule evaluating the whole label or a parameterized
context rule. The context condition is met when the rule specified
in the "when" attribute is matched or when the rule in the "not-when"
attribute fails to match. It is an error to reference a rule that is
not actually defined in the "rules" element.
A parameterized context rule (see Section 6.4) defines the context
immediately surrounding a given code point; unlike a sequence, the
context is not limited to a specific fixed code point but, for
example, may designate any member of a certain character class or a
code point that has a certain Unicode character property.
Given a suitable definition of a parameterized context rule named
"follows-virama", this example specifies that a ZERO WIDTH JOINER
(U+200D) is restricted to immediately follow any of several code
points classified as virama:
<char cp="200D" when="follows-virama" />
For a complete example, see Appendix A.
In contrast, a whole label rule (see Section 6.3) specifies a
condition to be met by the entire label -- for example, that it must
contain at least one code point from a given script anywhere in the
label. In the following example, no digit from either range may
occur in a label that mixes digits from both ranges:
<data>
<range first-cp="0660" last-cp="0669" not-when="mixed-digits"
tag="arabic-indic-digits" />
<range first-cp="06F0" last-cp="06F9" not-when="mixed-digits"
tag="extended-arabic-indic-digits" />
</data>
(See Section 6.3.9 for an example of the "mixed-digits" rule.)
The OPTIONAL "when" or "not-when" attributes are mutually exclusive.
They MAY be applied to both "char" and "range" elements in the "data"
element, including "char" elements defining sequences of code points,
as well as to "var" elements (see Section 5.3.5).
If a label contains one or more code points that fail to satisfy a
conditional context, the label is invalid (see Section 7.5). For
variants, the conditional context restricts the definition of the
variant to the case where the condition is met. Outside the
specified context, a variant is not defined.
5.3. Variants
Most LGRs typically only determine simple code point eligibility, and
for them, the elements described so far would be the only ones
required for their "data" section. Others additionally specify a
mapping of code points to other code points, known as "variants".
What constitutes a variant code point is a matter of policy and
varies for each implementation. The following examples are intended
to demonstrate the syntax; they are not necessarily typical.
5.3.1. Basic Variants
Variant code points are specified using one of more "var" elements as
children of a "char" element. The target mapping is specified using
the "cp" attribute. Other, optional attributes for the "var" element
are described below.
For example, to map LATIN SMALL LETTER V (U+0076) as a variant of
LATIN SMALL LETTER U (U+0075):
<char cp="0075">
<var cp="0076"/>
</char>
A sequence of multiple code points can be specified as a variant of a
single code point. For example, the sequence of LATIN SMALL LETTER O
(U+006F) then LATIN SMALL LETTER E (U+0065) might hypothetically be
specified as a variant for a LATIN SMALL LETTER O WITH DIAERESIS
(U+00F6) as follows:
<char cp="00F6">
<var cp="006F 0065"/>
</char>
The source and target of a variant mapping may both be sequences but
not ranges.
If the source of one mapping is a prefix sequence of the source for
another, both variant mappings will be considered at the same
location in the input label when generating permuted variant labels.
If poorly designed, an LGR containing such an instance of a prefix
relation could generate multiple instances of the same variant label
for the same original label, but with potentially different
dispositions. Any duplicate variant labels encountered MUST be
treated as an error (see Section 8.4).
The "var" element specifies variant mappings in only one direction,
even though the variant relation is usually considered symmetric;
that is, if A is a variant of B, then B should also be a variant of
A. The format requires that the inverse of the variant be given
explicitly to fully specify symmetric variant relations in the LGR.
This has the beneficial side effect of making the symmetry explicit:
<char cp="006F 0065">
<var cp="00F6"/>
</char>
Variant relations are normally not only symmetric but also
transitive. If A is a variant of B and B is a variant of C, then A
is also a variant of C. As with symmetry, these transitive relations
are only part of the LGR if spelled out explicitly. Implementations
that require an LGR to be symmetric and transitive should verify this
mechanically.
All variant mappings are unique. For a given "char" element, all
"var" elements MUST have a unique combination of "cp", "when", and
"not-when" attributes. It is RECOMMENDED to list the "var" elements
in ascending order of their target code point sequence. (For "when"
and "not-when" attributes, see Section 5.3.5.)
5.3.2. The "type" Attribute
Variants may be tagged with an OPTIONAL "type" attribute. The value
of the "type" attribute may be any non-empty value not starting with
an underscore and not containing spaces. This value is used to
resolve the disposition of any variant labels created using a given
variant. (See Section 7.2.)
By default, the values of the "type" attribute directly describe the
target policy status (disposition) for a variant label that was
generated using a particular variant, with any variant label being
assigned a disposition corresponding to the most restrictive variant
type. Several conventional disposition values are predefined below
in Section 7. Whenever these values can represent the desired
policy, they SHOULD be used.
<char cp="767C">
<var cp="53D1" type="allocatable"/>
<var cp="5F42" type="blocked"/>
<var cp="9AEA" type="blocked"/>
<var cp="9AEE" type="blocked"/>
</char>
By default, if a variant label contains any instance of one of the
variants of type "blocked", the label would be blocked, but if it
contained only instances of variants to be allocated, it could be
allocated. See the discussion about implied actions in Section 7.6.
The XML format for the LGR makes the relation between the values of
the "type" attribute on variants and the resulting disposition of
variant labels fully explicit. See the discussion in Section 7.2.
Making this relation explicit allows a generalization of the "type"
attribute from directly reflecting dispositions to a more
differentiated intermediate value that is then used in the resolution
of label disposition. Instead of the default action of applying the
most restrictive disposition to the entire label, such a generalized
resolution can be used to achieve additional goals, such as limiting
the set of allocatable variant labels or implementing other policies
found in existing LGRs (see, for example, Appendix B).
Because variant mappings MUST be unique, it is not possible to define
the same variant for the same "char" element with different "type"
attributes (however, see Section 5.3.5).
5.3.3. Null Variants
A null variant is a variant string that maps to no code point. This
is used when a particular code point sequence is considered
discretionary in the context of a whole label. To specify a null
variant, use an empty "cp" attribute. For example, to mark a string
with a ZERO WIDTH NON-JOINER (U+200C) to the same string without the
ZERO WIDTH NON-JOINER:
<char cp="200C">
<var cp=""/>
</char>
This is useful in expressing the intent that some code points in a
label are to be mapped away when generating a canonical variant of
the label. However, in tables that are designed to have symmetric
variant mappings, this could lead to combinatorial explosion if not
handled carefully.
The symmetric form of a null variant is expressed as follows:
<char cp="">
<var cp="200C" type="invalid" />
</char>
A "char" element with an empty "cp" attribute MUST specify at least
one variant mapping. It is strongly RECOMMENDED to use a type of
"invalid" or equivalent when defining variant mappings from null
sequences, so that variant mappings from null sequences are removed
in variant label generation (see Section 5.3.2).
5.3.4. Variants with Reflexive Mapping
At first glance, there seems to be no call for adding variant
mappings for which source and target code points are the same -- that
is, for which the mapping is reflexive, or, in other words, an
identity mapping. Yet, such reflexive mappings occur frequently in
LGRs that follow [RFC3743].
Adding a "var" element allows both a type and a reference id to be
specified for it. While the reference id is not used in processing,
the type of the variant can be used to trigger actions. In permuting
the label to generate all possible variants, the type associated with
a reflexive variant mapping is applied to any of the permuted labels
containing the original code point.
In the following example, let's assume that the goal is to allocate
only those labels that contain a variant that is considered
"preferred" in some way. As defined in the example, the code point
U+3473 exists both as a variant of U+3447 and as a variant of itself
(reflexive mapping). Assuming an original label of "U+3473 U+3447",
the permuted variant "U+3473 U+3473" would consist of the reflexive
variant of U+3473 followed by a variant of U+3447. Given the variant
mappings as defined here, the types for both of the variant mappings
used to generate that particular permutation would have the value
"preferred":
<char cp="3447" ref="0">
<var cp="3473" type="preferred" ref="1 3" />
</char>
<char cp="3473" ref="0">
<var cp="3447" type="blocked" ref="1 3" />
<var cp="3473" type="preferred" ref="0" />
</char>
Having established the variant types in this way, a set of actions
could be defined that return a disposition of "allocatable" or
"activated" for a label consisting exclusively of variants with type
"preferred", for example. (For details on how to define actions
based on variant types, see Section 7.2.1.)
In general, using reflexive variant mappings in this manner makes it
possible to calculate disposition values using a uniform approach for
all labels, whether they consist of mapped variant code points,
original code points, or a mixture of both. In particular, the
dispositions for two otherwise identical labels may differ based on
which variant mappings were executed in order to generate each of
them. (For details on how to generate variants and evaluate
dispositions, see Section 8.)
Another useful convention that uses reflexive variants is described
below in Section 7.2.1.
5.3.5. Conditional Variants
Fundamentally, variants are mappings between two sequences of code
points. However, in some instances, for a variant relationship to
exist, some context external to the code point sequence must also be
considered. For example, a positional context may determine whether
two code point sequences are variants of each other.
An example of that are Arabic code points, which can have different
forms based on position, with some code points sharing forms, thus
making them variants in the positions corresponding to those forms.
Such positional context cannot be solely derived from the code point
by itself, as the code point would be the same for the various forms.
As described in Section 5.2, an OPTIONAL "when" or "not-when"
attribute may be given for any "var" element to specify required or
prohibited contextual conditions under which the variant is defined.
Assuming that the "rules" element contains suitably defined rules for
"arabic-isolated" and "arabic-final", the following example shows how
to mark ARABIC LETTER ALEF WITH WAVY HAMZA BELOW (U+0673) as a
variant of ARABIC LETTER ALEF WITH HAMZA BELOW (U+0625), but only
when it appears in its isolated or final forms:
<char cp="0625">
<var cp="0673" when="arabic-isolated"/>
<var cp="0673" when="arabic-final"/>
</char>
While a "var" element MUST NOT contain multiple conditions (it is
only allowed a single "when" or "not-when" attribute), multiple "var"
elements using the same mapping MAY be specified with different
"when" or "not-when" attributes. The combination of mapping and
conditional context defines a unique variant.
For each variant label, care must be taken to ensure that at most one
of the contextual conditions is met for variants with the same
mapping; otherwise, duplicate variant labels would be created for the
same input label. Any such duplicate variant labels MUST be treated
as an error; see Section 8.4.
Two contexts may be complementary, as in the following example, which
shows ARABIC LETTER TEH MARBUTA (U+0629) as a variant of ARABIC
LETTER HEH (U+0647), but with two different types.
<char cp="0647" >
<var cp="0629" not-when="arabic-final" type="blocked" />
<var cp="0629" when="arabic-final" type="allocatable" />
</char>
The intent is that a label that uses U+0629 instead of U+0647 in a
final position should be considered essentially the same label and,
therefore, allocatable to the same entity, while the same
substitution in a non-final position leads to labels that are
different, but considered confusable, so that either one, but not
both, should be delegatable.
For symmetry, the reverse mappings must exist and must agree in their
"when" or "not-when" attributes. However, symmetry does not apply to
the other attributes. For example, these are potential reverse
mappings for the above:
<char cp="0629" >
<var cp="0647" not-when="arabic-final" type="allocatable" />
<var cp="0647" when="arabic-final" type="allocatable" />
</char>
Here, both variants have the same "type" attribute. While it is
tempting to recognize that, in this instance, the "when" and "not-
when" attributes are complementary; therefore, between them they
cover every single possible context, it is strongly RECOMMENDED to
use the format shown in the example that makes the symmetry easily
verifiable by parsers and tools. (The same applies to entries
created for transitivity.)
Arabic is an example of a script for which such conditional variants
have been implemented based on the joining contexts for Arabic code
points. The mechanism defined here supports other forms of
conditional variants that may be required by other scripts.
5.4. Annotations
Two attributes, the "ref" and "comment" attributes, can be used to
annotate individual elements in the LGR. They are ignored in
machine-processing of the LGR. The "ref" attribute is intended for
formal annotations and the "comment" attribute for free-form
annotations. The latter can be applied more widely.
5.4.1. The "ref" Attribute
Reference information MAY optionally be specified by a "ref"
attribute consisting of a space-delimited sequence of reference
identifiers (see Section 4.3.8).
<char cp="5220" ref="0">
<var cp="5220" ref="5"/>
<var cp="522A" ref="2 3"/>
</char>
This facility is typically used to give source information for code
points or variant relations. This information is ignored when
machine-processing an LGR. If applied to a range, the "ref"
attribute applies to every code point in the range. All reference
identifiers MUST be from the set declared in the "references" element
(see Section 4.3.8). It is an error to repeat a reference identifier
in the same "ref" attribute. It is RECOMMENDED that identifiers be
listed in ascending order.
In addition to "char", "range", and "var" elements in the "data"
section, a "ref" attribute may be present for a number of element
types contained in the "rules" element as described below: actions
and literals ("char" inside a rule), as well as for definitions of
rules and classes, but not for references to named character classes
or rules using the "by-ref" attribute defined below. (The use of the
"by-ref" and "ref" attributes is mutually exclusive.) None of the
elements in the metadata take a "ref" attribute; to provide
additional information, use the "description" element instead.
5.4.2. The "comment" Attribute
Any "char", "range", or "variant" element in the "data" section may
contain an OPTIONAL "comment" attribute. The contents of a "comment"
attribute are free-form plain text. Comments are ignored in machine
processing of the table. "comment" attributes MAY also be placed on
all elements in the "rules" section of the document, such as actions
and match operators, as well as definitions of classes and rules, but
not on child elements of the "class" element. Finally, in the
metadata, only the "version" and "reference" elements MAY have
"comment" attributes (to match the syntax in [RFC3743]).
5.5. Code Point Tagging
Typically, LGRs are used to explicitly designate allowable code
points, where any label that contains a code point not explicitly
listed in the LGR is considered an ineligible label according to the
ruleset.
For more-complex registry rules, there may be a need to discern one
or more subsets of code points. This can be accomplished by applying
an OPTIONAL "tag" attribute to "char" or "range" elements that are
child elements of the "data" element. By collecting code points that
share the same tag value, character classes may be defined (see
Section 6.2.2) that can then be used in parameterized context or
whole label rules (see Section 6.3.2).
Each "tag" attribute MAY contain multiple values separated by
white space. A tag value is an identifier that may also include
certain punctuation marks, such as a colon. Formally, it MUST
correspond to the XML 1.0 Nmtoken (Name token) production (see [XML]
Section 2.3). It is an error to duplicate a value within the same
"tag" attribute. A "tag" attribute for a "range" element applies to
all code points in the range. Because code point sequences are not
proper members of a set of code points, a "tag" attribute MUST NOT be
present in a "char" element defining a code point sequence.
6. Whole Label and Context Evaluation
6.1. Basic Concepts
The "rules" element contains the specification of both context-based
and whole label rules. Collectively, these are known as Whole Label
Evaluation (WLE) rules (Section 6.3). The "rules" element also
contains the character classes (Section 6.2) that they depend on, and
any actions (Section 7) that assign dispositions to labels based on
rules or variant mappings.
A whole label rule is applied to the whole label. It is used to
validate both original labels and any variant labels computed from
them.
A rule implementing a conditional context as discussed in Section 5.2
does not necessarily apply to the whole label but may be specific to
the context around a single code point or code point sequence.
Certain code points in a label sometimes need to satisfy context-
based rules -- for example, for the label to be considered valid, or
to satisfy the context for a variant mapping (see the description of
the "when" attribute in Section 6.4).
For example, if a rule is referenced in the "when" attribute of a
variant mapping, it is used to describe the conditional context under
which the particular variant mapping is defined to exist.
Each rule is defined in a "rule" element. A rule may contain the
following as child elements:
o literal code points or code point sequences
o character classes, which define sets of code points to be used for
context comparisons
o context operators, which define when character classes and
literals may appear
o nested rules, whether defined in place or invoked by reference
Collectively, these are called "match operators" and are listed in
Section 6.3.2. An LGR containing rules or match operators that
1. are incorrectly defined or nested,
2. have invalid attributes, or
3. have invalid or undefined attribute values
MUST be rejected. Note that not all of the constraints defined here
are validated by the schema.
6.2. Character Classes
Character classes are sets of characters that often share a
particular property. While they function like sets in every way,
even supporting the usual set operators, they are called "character
classes" here in a nod to the use of that term in regular expression
syntax. (This also avoids confusion with the term "character set" in
the sense of character encoding.)
Character classes can be specified in several ways:
o by defining the class via matching a tag in the code point data.
All characters with the same "tag" attribute are part of the same
class;
o by referencing a value of one of the Unicode character properties
defined in the Unicode Character Database;
o by explicitly listing all the code points in the class; or
o by defining the class as a set combination of any number of other
classes.
6.2.1. Declaring and Invoking Named Classes
A character class has an OPTIONAL "name" attribute consisting of a
single identifier not containing spaces. All names for classes must
be unique. If the "name" attribute is omitted, the class is
anonymous and exists only inside the rule or combined class where it
is defined. A named character class is defined independently and can
be referenced by name from within any rules or as part of other
character class definitions.
<class name="example" comment="an example class definition">
0061 4E00
</class>
...
<rule>
<class by-ref="example" />
</rule>
An empty "class" element with a "by-ref" attribute is a reference to
an existing named class. The "by-ref" attribute MUST NOT be used in
the same "class" element with any of these attributes: "name",
"from-tag", "property", or "ref". The "name" attribute MUST be
present if and only if the class is a direct child element of the
"rules" element. It is an error to reference a named class for which
the definition has not been seen.
6.2.2. Tag-Based Classes
The "char" or "range" elements that are child elements of the "data"
element MAY contain a "tag" attribute that consists of one or more
space-separated tag values; for example:
<char cp="0061" tag="letter lower"/>
<char cp="4E00" tag="letter"/>
This defines two tags for use with code point U+0061, the tag
"letter" and the tag "lower". Use
<class name="letter" from-tag="letter" />
<class name="lower" from-tag="lower" />
to define two named character classes, "letter" and "lower",
containing all code points with the respective tags, the first with
0061 and 4E00 as elements, and the latter with 0061 but not 4E00 as
an element. The "name" attribute may be omitted for an anonymous
in-place definition of a nested, tag-based class.
Tag values are typically identifiers, with the addition of a few
punctuation symbols, such as a colon. Formally, they MUST correspond
to the XML 1.0 Nmtoken production. While a "tag" attribute may
contain a list of tag values, the "from-tag" attribute MUST always
contain a single tag value.
If the document contains no "char" or "range" elements with a
corresponding tag, the character class represents the empty set.
This is valid, to allow a common "rules" element to be shared across
files. However, it is RECOMMENDED that implementations allow for a
warning to ensure that referring to an undefined tag in this way is
intentional.
6.2.3. Unicode Property-Based Classes
A class is defined in terms of Unicode properties by giving the
Unicode property alias and the property value or property value
alias, separated by a colon.
<class name="virama" property="ccc:9" />
The example above selects all code points for which the Unicode
Canonical Combining Class (ccc) value is 9. This value of the ccc is
assigned to all code points that encode viramas.
Unicode property values MUST be designated via a composite of the
attribute name and value as defined for the property value in
[UAX42], separated by a colon. Loose matching of property values and
names as described in [UAX44] is not appropriate for an XML schema
and is not supported; it is likewise not supported in the XML
representation [UAX42] of the Unicode Character Database itself.
A property-based class MAY be anonymous, or, when defined as an
immediate child of the "rules" element, it MAY be named to relate a
formal property definition to its usage, such as the use of the value
9 for ccc to designate a virama (or halant) in various scripts.
Unicode properties may, in principle, change between versions of the
Unicode Standard. However, the values assigned for a given version
are fixed. If Unicode properties are used, a Unicode version MUST be
declared in the "unicode-version" element in the header. (Note: Some
Unicode properties are by definition stable across versions and do
not change once assigned; see [Unicode-Stability].)
All implementations processing LGR files SHOULD provide support for
the following minimal set of Unicode properties:
o General Category (gc)
o Script (sc)
o Canonical Combining Class (ccc)
o Bidi Class (bc)
o Arabic Joining Type (jt)
o Indic Syllabic Category (InSC)
o Deprecated (Dep)
The short name for each property is given in parentheses.
If a program that is using an LGR to determine the validity of a
label encounters a property that it does not support, it MUST abort
with an error.
6.2.4. Explicitly Declared Classes
A class of code points may also be declared by listing all code
points that are members of the class. This is useful when tagging
cannot be used because code points are not listed individually as
part of the eligible set of code points for the given LGR -- for
example, because they only occur in code point sequences.
To define a class in terms of an explicit list of code points, use a
space-separated list of hexadecimal code point values:
<class name="abcd">0061 0062 0063 0064</class>
This defines a class named "abcd" containing the code points for
characters "a", "b", "c", and "d". The ordering of the code points
is not material, but it is RECOMMENDED to list them in ascending
order; not doing so makes it unnecessarily difficult for users to
detect errors such as duplicates or to compare and review these
classes against other specifications.
In a class definition, ranges of code points are represented by a
hexadecimal start and end value separated by a hyphen. The following
declaration is equivalent to the preceding:
<class name="abcd">0061-0064</class>
Range and code point declarations can be freely intermixed:
<class name="abcd">0061 0062-0063 0064</class>
The contents of a class differ from a repertoire in that the latter
MAY contain sequences as elements, while the former MUST NOT.
Instead, they closely resemble character classes as found in regular
expressions.
6.2.5. Combined Classes
Classes may be combined using operators for set complement, union,
intersection, difference (elements of the first class that are not in
the second), and symmetric difference (elements in either class but
not both). Because classes fundamentally function like sets, the
union of several character classes is itself a class, for example.
+-------------------+----------------------------------------------+
| Logical Operation | Example |
+-------------------+----------------------------------------------+
| Complement | <complement><class by-ref="xxx"></complement>|
+-------------------+----------------------------------------------+
| Union | <union> |
| | <class by-ref="class-1"/> |
| | <class by-ref="class-2"/> |
| | <class by-ref="class-3"/> |
| | </union> |
+-------------------+----------------------------------------------+
| Intersection | <intersection> |
| | <class by-ref="class-1"/> |
| | <class by-ref="class-2"/> |
| | </intersection> |
+-------------------+----------------------------------------------+
| Difference | <difference> |
| | <class by-ref="class-1"/> |
| | <class by-ref="class-2"/> |
| | </difference> |
+-------------------+----------------------------------------------+
| Symmetric | <symmetric-difference> |
| Difference | <class by-ref="class-1"/> |
| | <class by-ref="class-2"/> |
| | </symmetric-difference> |
+-------------------+----------------------------------------------+
Set Operators
The elements from this table may be arbitrarily nested inside each
other, subject to the following restriction: a "complement" element
MUST contain precisely one "class" or one of the operator elements,
while an "intersection", "symmetric-difference", or "difference"
element MUST contain precisely two, and a "union" element MUST
contain two or more of these elements.
An anonymous combined class can be defined directly inside a rule or
any of the match operator elements that allow child elements (see
Section 6.3.2) by using the set combination as the outer element.
<rule>
<union>
<class by-ref="xxx"/>
<class by-ref="yyy"/>
</union>
</rule>
The example shows the definition of an anonymous combined class that
represents the union of classes "xxx" and "yyy". There is no need to
wrap this union inside another "class" element, and, in fact, set
combination elements MUST NOT be nested inside a "class" element.
Lastly, to create a named combined class that can be referenced in
other classes or in rules as <class by-ref="xxxyyy"/>, add a "name"
attribute to the set combination element -- for example,
<union name="xxxyyy" /> -- and place it at the top level immediately
below the "rules" element (see Section 6.2.1).
<rules>
<union name="xxxyyy">
<class by-ref="xxx"/>
<class by-ref="yyy"/>
</union>
...
</rules>
Because (as for ordinary sets) a combination of classes is itself a
class, no matter by what combinations of set operators a combined
class is created, a reference to it always uses the "class" element
as described in Section 6.2.1. That is, a named class is always
referenced via an empty "class" element using the "by-ref" attribute
containing the name of the class to be referenced.
6.3. Whole Label and Context Rules
Each rule comprises a series of matching operators that must be
satisfied in order to determine whether a label meets a given
condition. Rules may reference other rules or character classes
defined elsewhere in the table.
6.3.1. The "rule" Element
A matching rule is defined by a "rule" element, the child elements of
which are one of the match operators from Section 6.3.2. In
evaluating a rule, each child element is matched in order. "rule"
elements MAY be nested inside each other and inside certain match
operators.
A simple rule to match a label where all characters are members of
some class called "preferred-codepoint":
<rule name="preferred-label">
<start />
<class by-ref="preferred-codepoint" count="1+"/>
<end />
</rule>
Rules are paired with explicit and implied actions, triggering these
actions when a rule matches a label. For example, a simple explicit
action for the rule shown above would be:
<action disp="allocatable" match="preferred-label" />
The rule in this example would have the effect of setting the policy
disposition for a label made up entirely of preferred code points to
"allocatable". Explicit actions are further discussed in Section 7
and implicit actions in Section 7.5. Another use of rules is in
defining conditional contexts for code points and variants as
discussed in Sections 5.2 and 5.3.5.
A rule that is an immediate child element of the "rules" element MUST
be named using a "name" attribute containing a single identifier
string with no spaces. A named rule may be incorporated into another
rule by reference and may also be referenced by an "action" element,
"when" attribute, or "not-when" attribute. If the "name" attribute
is omitted, the rule is anonymous and MUST be nested inside another
rule or match operator.
6.3.2. The Match Operators
The child elements of a rule are a series of match operators, which
are listed here by type and name and with a basic example or two.
+------------+-------------+------------------------------------+
| Type | Operator | Examples |
+------------+-------------+------------------------------------+
| logical | any | <any /> |
| +-------------+------------------------------------+
| | choice | <choice> |
| | | <rule by-ref="alternative1"/> |
| | | <rule by-ref="alternative2"/> |
| | | </choice> |
+--------------------------+------------------------------------+
| positional | start | <start /> |
| +-------------+------------------------------------+
| | end | <end /> |
+--------------------------+------------------------------------+
| literal | char | <char cp="0061 0062 0063" /> |
+--------------------------+------------------------------------+
| set | class | <class by-ref="class1" /> |
| | | <class>0061 0064-0065</class> |
+--------------------------+------------------------------------+
| group | rule | <rule by-ref="rule1" /> |
| | | <rule><any /></rule> |
+--------------------------+------------------------------------+
| contextual | anchor | <anchor /> |
| +-------------+------------------------------------+
| | look-ahead | <look-ahead><any /></look-ahead> |
| +-------------+------------------------------------+
| | look-behind | <look-behind><any /></look-behind> |
+--------------------------+------------------------------------+
Match Operators
Any element defining an anonymous class can be used as a match
operator, including any of the set combination operators (see
Section 6.2.5) as well as references to named classes.
All match operators shown as empty elements in the Examples column of
the table above do not support child elements of their own;
otherwise, match operators MAY be nested. In particular, anonymous
"rule" elements can be used for grouping.
6.3.3. The "count" Attribute
The OPTIONAL "count" attribute, when present, specifies the minimally
required or maximal permitted number of times a match operator is
used to match input. If the "count" attribute is
n the match operator matches the input exactly n times, where n is
1 or greater.
n+ the match operator matches the input at least n times, where n
is 0 or greater.
n:m the match operator matches the input at least n times, where n
is 0 or greater, but matches the input up to m times in total,
where m > n. If m = n and n > 0, the match operator matches the
input exactly n times.
If there is no "count" attribute, the match operator matches the
input exactly once.
In matching, greedy evaluation is used in the sense defined for
regular expressions: beyond the required number or times, the input
is matched as many times as possible, but not so often as to prevent
a match of the remainder of the rule.
A "count" attribute MUST NOT be applied to any element that contains
a "name" attribute but MAY be applied to operators such as "class"
that declare anonymous classes (including combined classes) or invoke
any predefined classes by reference. The "count" attribute MUST NOT
be applied to any "class" element, or element defining a combined
class, when it is nested inside a combined class.
A "count" attribute MUST NOT be applied to match operators of type
"start", "end", "anchor", "look-ahead", or "look-behind" or to any
operators, such as "rule" or "choice", that contain a nested instance
of them. This limitation applies recursively and irrespective of
whether a "rule" element containing these nested instances is
declared in place or used by reference.
However, the "count" attribute MAY be applied to any other instances
of either an anonymous "rule" element or a "choice" element,
including those instances nested inside other match operators. It
MAY also be applied to the elements "any" and "char", when used as
match operators.
6.3.4. The "name" and "by-ref" Attributes
Like classes (see Section 6.2.1), rules declared as immediate child
elements of the "rules" element MUST be named using a unique "name"
attribute, and all other instances MUST NOT be named. Anonymous
rules and classes or references to named rules and classes can be
nested inside other match operators by reference.
To reference a named rule or class inside a rule or match operator,
use a "rule" or "class" element with an OPTIONAL "by-ref" attribute
containing the name of the referenced element. It is an error to
reference a rule or class for which the complete definition has not
been seen. In other words, it is explicitly not possible to define
recursive rules or class definitions. The "by-ref" attribute
MUST NOT appear in the same element as the "name" attribute or in an
element that has any child elements.
The example shows several named classes and a named rule referencing
some of them by name.
<class name="letter" property="gc:L"/>
<class name="combining-mark" property="gc:M"/>
<class name="digit" property="gc:Nd" />
<rule name="letter-grapheme">
<class by-ref="letter" count="1+"/>
<class by-ref="combining-mark" count="0+"/>
</rule>
6.3.5. The "choice" Element
The "choice" element is used to represent a list of two or more
alternatives:
<rule name="ldh">
<choice count="1+">
<class by-ref="letter"/>
<class by-ref="digit"/>
<char cp="002D" comment="literal HYPHEN"/>
</choice>
</rule>
Each child element of a "choice" element represents one alternative.
The first matching alternative determines the match for the "choice"
element. To express a choice where an alternative itself consists of
a sequence of elements, the sequence must be wrapped in an anonymous
rule.
6.3.6. Literal Code Point Sequences
A literal code point sequence matches a single code point or a
sequence. It is defined by a "char" element, with the code point or
sequence to be matched given by the "cp" attribute. When used as a
literal, a "char" element MAY contain a "count" attribute in addition
to the "cp" attribute and OPTIONAL "comment" or "ref" attributes. No
other attributes or child elements are permitted.
6.3.7. The "any" Element
The "any" element is an empty element that matches any single code
point. It MAY have a "count" attribute. For an example, see
Section 6.3.9.
Unlike a literal, the "any" element MUST NOT have a "ref" attribute.
6.3.8. The "start" and "end" Elements
To match the beginning or end of a label, use the "start" or "end"
element. An empty label would match this rule:
<rule name="empty-label">
<start/>
<end/>
</rule>
Conceptually, whole label rules evaluate the label as a whole, but in
practice, many rules do not actually need to be specified to match
the entire label. For example, to express a requirement of not
starting a label with a digit, a rule needs to describe only the
initial part of a label.
This example uses the previously defined rules, together with "start"
and "end" elements, to define a rule that requires that an entire
label be well-formed. For this example, that means that it must
start with a letter and that it contains no leading digits or
combining marks nor combining marks placed on digits.
<rule name="leading-letter" >
<start />
<rule by-ref="letter-grapheme" count="1"/>
<choice count="0+">
<rule by-ref="letter-grapheme" count="0+"/>
<class by-ref="digit" count="0+"/>
</choice>
<end />
</rule>
Each "start" or "end" element occurs at most once in a rule, except
if nested inside a "choice" element in such a way that in matching
each alternative at most one occurrence of each is encountered.
Otherwise, the result is an error, as is any case where a "start" or
"end" element is not encountered as the first or last element to be
matched, respectively, in matching a rule. "start" and "end"
elements are empty elements that do not have a "count" attribute or
any other attribute other than "comment". It is an error for any
match operator enclosing a nested "start" or "end" element to have a
"count" attribute.
6.3.9. Example Context Rule from IDNA Specification
This is an example of the WLE rule from [RFC5892] forbidding the
mixture of the Arabic-Indic and extended Arabic-Indic digits in the
same label. It is implemented as a whole label rule associated with
the code point ranges using the "not-when" attribute, which defines
an impermissible context. The example also demonstrates several
instances of the use of anonymous rules for grouping.
<data>
<range first-cp="0660" last-cp="0669" not-when="mixed-digits"
tag="arabic-indic-digits" />
<range first-cp="06F0" last-cp="06F9" not-when="mixed-digits"
tag="extended-arabic-indic-digits" />
</data>
<rules>
<rule name="mixed-digits">
<choice>
<rule>
<class from-tag="arabic-indic-digits"/>
<any count="0+"/>
<class from-tag="extended-arabic-indic-digits"/>
</rule>
<rule>
<class from-tag="extended-arabic-indic-digits"/>
<any count="0+"/>
<class from-tag="arabic-indic-digits"/>
</rule>
</choice>
</rule>
</rules>
As specified in the example, a label containing a code point from
either of the two digit ranges is invalid for any label matching the
"mixed-digits" rule, that is, any time that a code point from the
other range is also present. Note that invalidating the label is not
the same as invalidating the definition of the "range" elements; in
particular, the definition of the tag values does not depend on the
"when" attribute.
6.4. Parameterized Context or When Rules
To recap: When a rule is intended to provide a context for evaluating
the validity of a code point or variant mapping, it is invoked by the
"when" or "not-when" attributes described in Section 5.2. For "char"
and "range" elements, an action implied by a context rule always has
a disposition of "invalid" whenever the rule given by the "when"
attribute is not matched (see Section 7.5). Conversely, a "not-when"
attribute results in a disposition of "invalid" whenever the rule is
matched. When a rule is used in this way, it is called a context or
"when" rule.
The example in the previous section shows a whole label rule used as
a context rule, essentially making the whole label the context. The
next sections describe several match operators that can be used to
provide a more specific specification of a context, allowing a
parameterized context rule. See Section 7 for an alternative method
of defining an invalid disposition for a label not matching a whole
label rule.
6.4.1. The "anchor" Element
Such parameterized context rules are rules that contain a special
placeholder represented by an "anchor" element. As each When Rule is
evaluated, if an "anchor" element is present, it is replaced by a
literal corresponding to the "cp" attribute of the element containing
the "when" (or "not-when") attribute. The match to the "anchor"
element must be at the same position in the label as the code point
or variant mapping triggering the When Rule.
For example, the Greek lower numeral sign is invalid if not
immediately preceding a character in the Greek script. This is most
naturally addressed with a parameterized When Rule using
"look-ahead":
<char cp="0375" when="preceding-greek"/>
...
<class name="greek-script" property="sc:Grek"/>
<rule name="preceding-greek">
<anchor/>
<look-ahead>
<class by-ref="greek-script"/>
</look-ahead>
</rule>
In evaluating this rule, the "anchor" element is treated as if it was
replaced by a literal
<char cp="0375"/>
but only the instance of U+0375 at the given position is evaluated.
If a label had two instances of U+0375 with the first one matching
the rule and the second not, then evaluating the When Rule MUST
succeed for the first instance and fail for the second.
Unlike other rules, rules containing an "anchor" element MUST only be
invoked via the "when" or "not-when" attributes on code points or
variants; otherwise, their "anchor" elements cannot be evaluated.
However, it is possible to invoke rules not containing an "anchor"
element from a "when" or "not-when" attribute. (See Section 6.4.3.)
The "anchor" element is an empty element, with no attributes
permitted except "comment".
6.4.2. The "look-behind" and "look-ahead" Elements
Context rules use the "look-behind" and "look-ahead" elements to
define context before and after the code point sequence matched by
the "anchor" element. If the "anchor" element is omitted, neither
the "look-behind" nor the "look-ahead" element may be present in a
rule.
Here is an example of a rule that defines an "initial" context for an
Arabic code point:
<class name="transparent" property="jt:T"/>
<class name="right-joining" property="jt:R"/>
<class name="left-joining" property="jt:L"/>
<class name="dual-joining" property="jt:D"/>
<class name="non-joining" property="jt:U"/>
<rule name="Arabic-initial">
<look-behind>
<choice>
<start/>
<rule>
<class by-ref="transparent" count="0+"/>
<class by-ref="non-joining"/>
</rule>
</choice>
</look-behind>
<anchor/>
<look-ahead>
<class by-ref="transparent" count="0+" />
<choice>
<class by-ref="right-joining" />
<class by-ref="dual-joining" />
</choice>
</look-ahead>
</rule>
A "when" rule (or context rule) is a named rule that contains any
combination of "look-behind", "anchor", and "look-ahead" elements, in
that order. Each of these elements occurs at most once, except if
nested inside a "choice" element in such a way that in matching each
alternative at most one occurrence of each is encountered.
Otherwise, the result is undefined. None of these elements takes a
"count" attribute, nor does any enclosing match operator; otherwise,
the result is undefined. If a context rule contains a "look-ahead"
or "look-behind" element, it MUST contain an "anchor" element. If,
because of a "choice" element, a required anchor is not actually
encountered, the results are undefined.
6.4.3. Omitting the "anchor" Element
If the "anchor" element is omitted, the evaluation of the context
rule is not tied to the position of the code point or sequence
associated with the "when" attribute.
According to [RFC5892], the Katakana middle dot is invalid in any
label not containing at least one Japanese character anywhere in the
label. Because this requirement is independent of the position of
the middle dot, the rule does not require an "anchor" element.
<char cp="30FB" when="japanese-in-label"/>
<rule name="japanese-in-label">
<union>
<class property="sc:Hani"/>
<class property="sc:Kata"/>
<class property="sc:Hira"/>
</union>
</rule>
The Katakana middle dot is used only with Han, Katakana, or Hiragana.
The corresponding When Rule requires that at least one code point in
the label be in one of these scripts, but the position of that code
point is independent of the location of the middle dot; therefore, no
anchor is required. (Note that the Katakana middle dot itself is of
script Common, that is, "sc:Zyyy".)
7. The "action" Element
The purpose of an action is to assign a disposition to a label in
response to being triggered by the label meeting a specified
condition. Often, the action simply results in blocking or
invalidating a label that does not match a rule. An example of an
action invalidating a label because it does not match a rule named
"leading-letter" is as follows:
<action disp="invalid" not-match="leading-letter"/>
If an action is to be triggered on matching a rule, a "match"
attribute is used instead. Actions are evaluated in the order that
they appear in the XML file. Once an action is triggered by a label,
the disposition defined in the "disp" attribute is assigned to the
label and no other actions are evaluated for that label.
The goal of the LGR is to identify all labels and variant labels and
to assign them disposition values. These dispositions are then fed
into a further process that ultimately implements all aspects of
policy. To allow this specification to be used with the widest range
of policies, the permissible values for the "disp" attribute are
neither defined nor restricted. Nevertheless, a set of commonly used
disposition values is RECOMMENDED. (See Section 7.3.)
7.1. The "match" and "not-match" Attributes
An OPTIONAL "match" or "not-match" attribute specifies a rule that
must be matched or not matched as a condition for triggering an
action. Only a single rule may be named as the value of a "match" or
"not-match" attribute. Because rules may be composed of other rules,
this restriction to a single attribute value does not impose any
limitation on the contexts that can trigger an action.
An action MUST NOT contain both a "match" and a "not-match"
attribute, and the value of either attribute MUST be the name of a
previously defined rule; otherwise, the document MUST be rejected.
An action without any attributes is triggered by all labels
unconditionally. For a very simple LGR, the following action would
allocate all labels that match the repertoire:
<action disp="allocatable" />
Since rules are evaluated for all labels, whether they are the
original label or computed by permuting the defined and valid variant
mappings for the label's code points, actions based on matching or
not matching a rule may be triggered for both original and variant
labels, but the rules are not affected by the disposition attributes
of the variant mappings. To trigger any actions based on these
dispositions requires the use of additional optional attributes for
actions described next.
7.2. Actions with Variant Type Triggers
7.2.1. The "any-variant", "all-variants", and "only-variants"
Attributes
An action may contain one of the OPTIONAL attributes "any-variant",
"all-variants", or "only-variants" defining triggers based on variant
types. The permitted value for these attributes consists of one or
more variant type values, separated by spaces. These MAY include
type values that are not used in any "var" element in the LGR. When
a variant label is generated, these variant type values are compared
to the set of type values on the variant mappings used to generate
the particular variant label (see Section 8).
Any single match may trigger an action that contains an "any-variant"
attribute, while for an "all-variants" or "only-variants" attribute,
the variant type for all variant code points must match one or
several of the type values specified in the attribute to trigger the
action. There is no requirement that the entire list of variant type
values be matched, as long as all variant code points match at least
one of the values.
An "only-variants" attribute will trigger the action only if all code
points of the variant label have variant mappings from the original
code points. In other words, the label contains no original code
points other than those with a reflexive mapping (see Section 5.3.4).
<char cp="0078" comment="x">
<var cp="0078" type="allocatable" comment="reflexive" />
<var cp="0079" type="blocked" />
</char>
<char cp="0079" comment="y">
<var cp="0078" type="allocatable" />
</char>
...
<action disp="blocked" any-variant="blocked" />
<action disp="allocatable" only-variants="allocatable" />
<action disp="some-disp" any-variant="allocatable" />
In the example above, the label "xx" would have variant labels "xx",
"xy", "yx", and "yy". The first action would result in blocking any
variant label containing "y", because the variant mapping from "x" to
"y" is of type "blocked", triggering the "any-variant" condition.
Because in this example "x" has a reflexive variant mapping to itself
of type "allocatable", the original label "xx" has a reflexive
variant "xx" that would trigger the "only-variants" condition on the
second action.
A label "yy" would have the variants "xy", "yx", and "xx". Because
the variant mapping from "y" to "x" is of type "allocatable" and a
mapping from "y" to "y" is not defined, the labels "xy" and "yx"
trigger the "any-variant" condition on the third label. The variant
"xx", being generated using the mapping from "y" to "x" of type
"allocatable", would trigger the "only-variants" condition on the
section action. As there is no reflexive variant "yy", the original
label "yy" cannot trigger any variant type triggers. However, it
could still trigger an action defined as matching or not matching a
rule.
In each action, one variant type trigger may be present by itself or
in conjunction with an attribute matching or not matching a rule. If
variant triggers and rule-matching triggers are used together, the
label MUST "match" or respectively "not-match" the specified rule AND
satisfy the conditions on the variant type values given by the
"any-variant", "all-variants", or "only-variants" attribute.
A useful convention combines the "any-variant" trigger with reflexive
variant mappings (Section 5.3.4). This convention is used, for
example, when multiple LGRs are defined within the same registry and
for overlapping repertoire. In some cases, the delegation of a label
from one LGR must prohibit the delegation of another label in some
other LGR. This can be done using a variant of type "blocked" as in
this example from an Armenian LGR, where the Armenian, Latin, and
Cyrillic letters all look identical:
<char cp="0570" comment="ARMENIAN SMALL LETTER HO">
<var cp="0068" type="blocked" comment="LATIN SMALL LETTER H" />
<var cp="04BB" type="blocked"
comment="CYRILLIC SMALL LETTER SHHA" />
</char>
The issue is that the target code points for these two variants are
both outside the Armenian repertoire. By using a reflexive variant
with the following convention:
<char cp="0068" comment="not part of repertoire">
<var cp="0068" type="out-of-repertoire-var"
comment="reflexive mapping" />
<var cp="04BB" type="blocked" />
<var cp="0570" type="blocked" />
</char>
...
and associating this with an action of the form:
<action disp="invalid" any-variant="out-of-repertoire-var" />
it is possible to list the symmetric and transitive variant mappings
in the LGR even where they involve out-of-repertoire code points. By
associating the action shown with the special type for these
reflexive mappings, any original labels containing one or more of the
out-of-repertoire code points are filtered out, just as if these code
points had not been listed in the LGR in the first place.
Nevertheless, they do participate in the permutation of variant
labels for n-repertoire labels (Armenian in the example), and these
permuted variants can be used to detect collisions with out-of-
repertoire labels (see Section 8).
7.2.2. Example from Tables in the Style of RFC 3743
This section gives an example of using variant type triggers,
combined with variants with reflexive mappings (Section 5.3.4), to
achieve LGRs that implement tables like those defined according to
[RFC3743] where the goal is to allow as variants only labels that
consist entirely of simplified or traditional variants, in addition
to the original label.
This example assumes an LGR where all variants have been given
suitable "type" attributes of "blocked", "simplified", "traditional",
or "both", similar to the ones discussed in Appendix B. Given such
an LGR, the following example actions evaluate the disposition for
the variant label:
<action disp="blocked" any-variant="blocked" />
<action disp="allocatable" only-variants="simplified both" />
<action disp="allocatable" only-variants="traditional both" />
<action disp="blocked" all-variants="simplified traditional" />
<action disp="allocatable" />
The first action matches any variant label for which at least one of
the code point variants is of type "blocked". The second matches any
variant label for which all of the code point variants are of type
"simplified" or "both" -- in other words, an all-simplified label.
The third matches any label for which all variants are of type
"traditional" or "both" -- that is, all traditional. These two
actions are not triggered by any variant labels containing some
original code points, unless each of those code points has a variant
defined with a reflexive mapping (Section 5.3.4).
The final two actions rely on the fact that actions are evaluated in
sequence and that the first action triggered also defines the final
disposition for a variant label (see Section 7.4). They further rely
on the assumption that the only variants with type "both" are also
reflexive variants.
Given these assumptions, any remaining simplified or traditional
variants must then be part of a mixed label and so are blocked; all
labels surviving to the last action are original code points only
(that is, the original label). The example assumes that an original
label may be a mixed label; if that is not the case, the disposition
for the last action would be set to "blocked".
There are exceptions where the assumption on reflexive mappings made
above does not hold, so this basic scheme needs some refinements to
cover all cases. For a more complete example, see Appendix B.
7.3. Recommended Disposition Values
The precise nature of the policy action taken in response to a
disposition and the name of the corresponding "disp" attributes are
only partially defined here. It is strongly RECOMMENDED to use the
following dispositions only in their conventional sense.
invalid The resulting string is not a valid label. This disposition
may be assigned implicitly; see Section 7.5. No variant labels
should be generated from a variant mapping with this type.
blocked The resulting string is a valid label but should be blocked
from registration. This would typically apply for a derived
variant that is undesirable due to having no practical use or
being confusingly similar to some other label.
allocatable The resulting string should be reserved for use by the
same operator of the origin string but not automatically
allocated for use.
activated The resulting string should be activated for use. (This
is the same as a Preferred Variant [RFC3743].)
valid The resultant string is a valid label. (This is the typical
default action if no dispositions are defined.)
7.4. Precedence
Actions are applied in the order of their appearance in the file.
This defines their relative precedence. The first action triggered
by a label defines the disposition for that label. To define the
order of precedence, list the actions in the desired order. The
conventional order of precedence for the actions defined in
Section 7.3 is "invalid", "blocked", "allocatable", "activated", and
then "valid". This default precedence is used for the default
actions defined in Section 7.6.
7.5. Implied Actions
The context rules on code points ("not-when" or "when" rules) carry
an implied action with a disposition of "invalid" (not eligible) if a
"when" context is not satisfied or a "not-when" context is matched,
respectively. These rules are evaluated at the time the code points
for a label or its variant labels are checked for validity (see
Section 8). In other words, they are evaluated before any of the
whole-label evaluation rules
actions are applied, and with higher precedence. The context rules
for variant mappings are evaluated when variants are generated and/or
when variant tables are made symmetric and transitive. They have an
implied action with a disposition of "invalid", which means that a
putative variant mapping does not exist whenever the given context
matches a "not-when" rule or fails to match a "when" rule specified
for that mapping. The result of that disposition is that the variant
mapping is ignored in generating variant labels and the value is
therefore not accessible to trigger any explicit actions.
Note that such non-existing variant mapping is different from a
blocked variant, which is a variant code point mapping that exists
but results in a label that may not be allocated.
7.6. Default Actions
If a label does not trigger any of the actions defined explicitly in
the LGR, the following implicitly defined default actions are
evaluated. They are shown below in their relative order of
precedence (see Section 7.4). Default actions have a lower order of
precedence than explicit actions (see Section 8.3).
The default actions for variant labels are defined as follows. The
first set is triggered based on the standard variant type values of
"invalid", "blocked", "allocatable", and "activated":
<action disp="invalid" any-variant="invalid"/>
<action disp="blocked" any-variant="blocked"/>
<action disp="allocatable" any-variant="allocatable"/>
<action disp="activated" all-variants="activated"/>
A final default action sets the disposition to "valid" for any label
matching the repertoire for which no other action has been triggered.
This "catch-all" action also matches all remaining variant labels
from variants that do not have a type value.
<action disp="valid" comment="Catch-all if other rules not met"/>
Conceptually, the implicitly defined default actions act just like a
block of "action" elements that is added (virtually) beyond the last
of the user-supplied actions. Any label not processed by the
user-supplied actions would thus be processed by the default actions
as if they were present in the LGR. As the last default action is a
"catch-all", all processing is guaranteed to end with a definite
disposition for the label.
8. Processing a Label against an LGR
8.1. Determining Eligibility for a Label
In order to test a given label for membership in the LGR, a consumer
of the LGR must iterate through each code point within a given label
and test that each instance of a code point is a member of the LGR.
If any instance of a code point is not a member of the LGR, the label
shall be deemed invalid.
An individual instance of a code point is deemed a member of the LGR
when it is listed using a "char" element, or is part of a range
defined with a "range" element, and all necessary conditions in any
"when" or "not-when" attributes are correctly satisfied for that
instance.
Alternatively, an instance of a code point is also deemed a member of
the LGR when it forms part of a sequence that corresponds to a
sequence listed using a "char" element for which the "cp" attribute
defines a sequence, and all necessary conditions in any "when" or
"not-when" attributes are correctly satisfied for that instance of
the sequence.
In determining eligibility, at each position the longest possible
sequence of code points is evaluated first. If that sequence matches
a sequence defined in the LGR and satisfies any required context at
that position, the instances of its constituent code points are
deemed members of the LGR and evaluation proceeds with the next code
point following the sequence. If the sequence does not match a
defined sequence or does not satisfy the required context,
successively shorter sequences are evaluated until only a single code
point remains. The eligibility of that code point is determined as
described above for an individual code point instance.
A label must also not trigger any action that results in a
disposition of "invalid"; otherwise, it is deemed not eligible.
(This step may need to be deferred until variant code point
dispositions have been determined.)
8.1.1. Determining Eligibility Using Reflexive Variant Mappings
For LGRs that contain reflexive variant mappings (defined in
Section 5.3.4), the final evaluation of eligibility for the label
must be deferred until variants are generated. In essence, LGRs that
use this feature treat the original label as the (identity) variant
of itself. For such LGRs, the ordinary determination of eligibility
described here is but a first step that generally excludes only a
subset of invalid labels.
To further check the validity of a label with reflexive mappings, it
is not necessary to generate all variant labels. Only a single
variant needs to be created, where any reflexive variants are applied
for each code point, and the label disposition is evaluated (as
described in Section 8.3). A disposition of "invalid" results in the
label being not eligible. (In the exceptional case where context
rules are present on reflexive mappings, multiple reflexive variants
may be defined, but for each original label, at most one of these can
be valid at each code position. However, see Section 8.4.)
8.2. Determining Variants for a Label
For a given eligible label, the set of variant labels is deemed to
consist of each possible permutation of original code points and
substituted code points or sequences defined in "var" elements,
whereby all "when" and "not-when" attributes are correctly satisfied
for each "char" or "var" element in the given permutation and all
applicable whole label rules are satisfied as follows:
1. Create each possible permutation of a label by substituting each
code point or code point sequence in turn by any defined variant
mapping (including any reflexive mappings).
2. Apply variant mappings with "when" or "not-when" attributes only
if the conditions are satisfied; otherwise, they are not defined.
3. Record each of the "type" values on the variant mappings used in
creating a given variant label in a disposition set; for any
unmapped code point, record the "type" value of any reflexive
variant (see Section 5.3.4).
4. Determine the disposition for each variant label per Section 8.3.
5. If the disposition is "invalid", remove the label from the set.
6. If final evaluation of the disposition for the unpermuted label
per Section 8.3 results in a disposition of "invalid", remove all
associated variant labels from the set.
The number of potential permutations can be very large. In practice,
implementations would use suitable optimizations to avoid having to
actually create all permutations (see Section 8.5).
In determining the permuted set of variant labels in step (1) above,
all eligible partitions into sequences must be evaluated. A label
"ab" that matches a sequence "ab" defined in the LGR but also matches
the sequence of individual code points "a" and "b" (both defined in
the LGR) must be permuted using any defined variant mappings for both
the sequence "ab" and the code points "a" and "b" individually.
8.3. Determining a Disposition for a Label or Variant Label
For a given label (variant or original), its disposition is
determined by evaluating, in order of their appearance, all actions
for which the label or variant label satisfies the conditions.
1. For any label that contains code points or sequences not defined
in the repertoire, or does not satisfy the context rules on all
of its code points and variants, the disposition is "invalid".
2. For all other labels, the disposition is given by the value of
the "disp" attribute for the first action triggered by the label.
An action is triggered if all of the following are true:
* the label matches the whole label rule given in the "match"
attribute for that action;
* the label does not match the whole label rule given in the
"not-match" attribute for that action;
* any of the recorded variant types for a variant label match
the types given in the "any-variant" attribute for that
action;
* all of the recorded variant types for a variant label match
the types given in the "all-variants" or "only-variants"
attribute given for that action;
* in case of an "only-variants" attribute, the label contains
only code points that are the target of applied variant
mappings;
or
* the action does not contain any "match", "not-match",
"any-variant", "all-variants", or "only-variants" attributes:
catch-all.
3. For any remaining variant label, assign the variant label the
disposition using the default actions defined in Section 7.6.
For this step, variant types outside the predefined recommended
set (see Section 7.3) are ignored.
4. For any remaining label, set the disposition to "valid".
8.4. Duplicate Variant Labels
For a poorly designed LGR, it is possible to generate duplicate
variant labels from the same input label, but with different, and
potentially conflicting, dispositions. Implementations MUST treat
any duplicate variant labels encountered as an error, irrespective of
their dispositions.
This situation can arise in two ways. One is described in
Section 5.3.5 and involves defining the same variant mapping with two
context rules that are formally distinct but nevertheless overlap so
that they are not mutually exclusive for the same label.
The other case involves variants defined for sequences, where one
sequence is a prefix of another (see Section 5.3.1). The following
shows such an example resulting in conflicting reflexive variants:
<char cp="0061">
<var cp="0061" type="allocatable"/>
</char>
<char cp="0062"/>
<char cp="0061 0062">
<var cp="0061 0062" type="blocked"/>
</char>
A label "ab" would generate the variant labels "{a}{b}" and "{ab}"
where the curly braces show the sequence boundaries as they were
applied during variant mapping. The result is a duplicate variant
label "ab", one based on a variant of type "allocatable" plus an
original code point "b" that has no variant, and another one based on
a single variant of type "blocked", thus creating two variant labels
with conflicting dispositions.
In the general case, it is difficult to impossible to prove by
mechanical inspection of the LGR that duplicate variant labels will
never occur, so implementations have to be prepared to detect this
error during variant label generation. The condition is easily
avoided by careful design of context rules and special attention to
the relation among code point sequences with variants.
8.5. Checking Labels for Collision
The obvious method for checking for collision between labels is to
generate the fully permuted set of variants for one of them and see
whether it contains the other label as a member. As discussed above,
this can be prohibitive and is not necessary.
Because of symmetry and transitivity, all variant mappings form
disjoint sets. In each of these sets, the source and target of each
mapping are also variants of the sources and targets of all the other
mappings. However, members of two different sets are never variants
of each other.
If two labels have code points at the same position that are members
of two different variant mapping sets, any variant labels of one
cannot be variant labels of the other: the sets of their variant
labels are likewise disjoint. Instead of generating all permutations
to compare all possible variants, it is enough to find out whether
code points at the same position belong to the same variant set
or not.
For that, it is sufficient to substitute an "index" mapping that
identifies the set. This index mapping could be, for example, the
variant mapping for which the target code point (or sequence) comes
first in some sorting order. This index mapping would, in effect,
identify the set of variant mappings for that position.
To check for collision then means generating a single variant label
from the original by substituting the respective "index" value for
each code point. This results in an "index label". Two labels
collide whenever the index labels for them are the same.
9. Conversion to and from Other Formats
Both [RFC3743] and [RFC4290] provide different grammars for IDN
tables. The formats in those documents are unable to fully support
the increased requirements of contemporary IDN variant policies.
This specification is a superset of functionality provided by the
older IDN table formats; thus, any table expressed in those formats
can be expressed in this new format. Automated conversion can be
conducted between tables conformant with the grammar specified in
each document.
For notes on how to translate a table in the style of RFC 3743, see
Appendix B.
10. Media Type
Well-formed LGRs that comply with this specification SHOULD be
transmitted with a media type of "application/lgr+xml". This media
type will signal to an LGR-aware client that the content is designed
to be interpreted as an LGR.
11. IANA Considerations
IANA has completed the following actions:
11.1. Media Type Registration
The media type "application/lgr+xml" has been registered to denote
transmission of LGRs that are compliant with this specification, in
accordance with [RFC6838].
Type name: application
Subtype name: lgr+xml
Required parameters: N/A
Optional parameters: charset (as for application/xml per [RFC7303])
Security considerations: See the security considerations for
application/xml in [RFC7303] and the specific security
considerations for Label Generation Rulesets (LGRs) in RFC 7940
Interoperability considerations: As for application/xml per
[RFC7303]
Published specification: See RFC 7940
Applications that use this media type: Software using LGRs for
international identifiers, such as IDNs, including registry
applications and client validators.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): .lgr
Macintosh file type code(s): N/A
Person & email address to contact for further information:
Kim Davies <kim.davies@icann.org>
Asmus Freytag <asmus@unicode.org>
Intended usage: COMMON
Restrictions on usage: N/A
Author:
Kim Davies <kim.davies@icann.org>
Asmus Freytag <asmus@unicode.org>
Change controller: IESG
Provisional registration? (standards tree only): No
11.2. URN Registration
This specification uses a URN to describe the XML namespace, in
accordance with [RFC3688].
URI: urn:ietf:params:xml:ns:lgr-1.0
Registrant Contact: See the Authors of this document.
XML: None.
11.3. Disposition Registry
This document establishes a vocabulary of "Label Generation Ruleset
Dispositions", which has been reflected as a new IANA registry. This
registry is divided into two subregistries:
o Standard Dispositions - This registry lists dispositions that have
been defined in published specifications, i.e., the eligibility
for such registrations is "Specification Required" [RFC5226]. The
initial set of registrations are the five dispositions in this
document described in Section 7.3.
o Private Dispositions - This registry lists dispositions that have
been registered "First Come First Served" [RFC5226] by third
parties with the IANA. Such dispositions must take the form
"entity:disposition" where the entity is a domain name that
uniquely identifies the private user of the namespace. For
example, "example.org:reserved" could be a private extension used
by the example organization to denote a disposition relating to
reserved labels. These extensions are not intended to be
interoperable, but registration is designed to minimize potential
conflicts. It is strongly recommended that any new dispositions
that require interoperability and have applicability beyond a
single organization be defined as Standard Dispositions.
In order to distinguish them from Private Dispositions, Standard
Dispositions MUST NOT contain the ":" character. All disposition
names shall be in lowercase ASCII.
The IANA registry provides data on the name of the disposition, the
intended purposes, and the registrant or defining specification for
the disposition.
12. Security Considerations
12.1. LGRs Are Only a Partial Remedy for Problem Space
Substantially unrestricted use of non-ASCII characters in security-
relevant identifiers such as domain name labels may cause user
confusion and invite various types of attacks. In many languages, in
particular those using complex or large scripts, an attacker has an
opportunity to divert or confuse users as a result of different code
points with identical appearance or similar semantics.
The use of an LGR provides a partial remedy for these risks by
supplying a framework for prohibiting inappropriate code points or
sequences from being registered at all and for permitting "variant"
code points to be grouped together so that labels containing them may
be mutually exclusive or registered only to the same owner.
In addition, by being fully machine processable the format may enable
automated checks for known weaknesses in label generation rules.
However, the use of this format, or compliance with this
specification, by itself does not ensure that the LGRs expressed in
this format are free of risk. Additional approaches may be
considered, depending on the acceptable trade-off between flexibility
and risk for a given application. One method of managing risk may
involve a case-by-case evaluation of a proposed label in context with
already-registered labels -- for example, when reviewing labels for
their degree of visual confusability.
12.2. Computational Expense of Complex Tables
A naive implementation attempting to generate all variant labels for
a given label could lead to the possibility of exhausting the
resources on the machine running the LGR processor, potentially
causing denial-of-service consequences. For many operations,
brute-force generation can be avoided by optimization, and if needed,
the number of permuted labels can be estimated more cheaply ahead of
time.
The implementation of WLE rules, using certain backtracking
algorithms, can take exponential time for pathological rules or
labels and exhaust stack resources. This can be mitigated by proper
implementation and enforcing the restrictions on permissible label
length.
13. References
13.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<http://www.rfc-editor.org/info/rfc2045>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<http://www.rfc-editor.org/info/rfc3339>.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <http://www.rfc-editor.org/info/rfc5646>.
[UAX42] The Unicode Consortium, "Unicode Character Database in
XML", May 2016, <http://unicode.org/reports/tr42/>.
[Unicode-Stability]
The Unicode Consortium, "Unicode Encoding Stability
Policy, Property Value Stability", April 2015,
<http://www.unicode.org/policies/
stability_policy.html#Property_Value>.
[Unicode-Versions]
The Unicode Consortium, "Unicode Version Numbering", June
2016, <http://unicode.org/versions/#Version_Numbering>.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium, November 2008,
<http://www.w3.org/TR/REC-xml/>.
13.2. Informative References
[ASIA-TABLE]
DotAsia Organisation, ".ASIA ZH IDN Language Table",
February 2012,
<http://www.dot.asia/policies/ASIA-ZH-1.2.pdf>.
[LGR-PROCEDURE]
Internet Corporation for Assigned Names and Numbers,
"Procedure to Develop and Maintain the Label Generation
Rules for the Root Zone in Respect of IDNA Labels",
December 2012, <http://www.icann.org/en/resources/idn/
draft-lgr-procedure-07dec12-en.pdf>.
[RELAX-NG]
The Organization for the Advancement of Structured
Information Standards (OASIS), "RELAX NG Compact Syntax",
November 2002, <https://www.oasis-open.org/committees/
relax-ng/compact-20021121.html>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<http://www.rfc-editor.org/info/rfc3688>.
[RFC3743] Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
Engineering Team (JET) Guidelines for Internationalized
Domain Names (IDN) Registration and Administration for
Chinese, Japanese, and Korean", RFC 3743,
DOI 10.17487/RFC3743, April 2004,
<http://www.rfc-editor.org/info/rfc3743>.
[RFC4290] Klensin, J., "Suggested Practices for Registration of
Internationalized Domain Names (IDN)", RFC 4290,
DOI 10.17487/RFC4290, December 2005,
<http://www.rfc-editor.org/info/rfc4290>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5564] El-Sherbiny, A., Farah, M., Oueichek, I., and A. Al-Zoman,
"Linguistic Guidelines for the Use of the Arabic Language
in Internet Domains", RFC 5564, DOI 10.17487/RFC5564,
February 2010, <http://www.rfc-editor.org/info/rfc5564>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<http://www.rfc-editor.org/info/rfc5891>.
[RFC5892] Faltstrom, P., Ed., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, DOI 10.17487/RFC5892, August 2010,
<http://www.rfc-editor.org/info/rfc5892>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<http://www.rfc-editor.org/info/rfc6838>.
[RFC7303] Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
DOI 10.17487/RFC7303, July 2014,
<http://www.rfc-editor.org/info/rfc7303>.
[TDIL-HINDI]
Technology Development for Indian Languages (TDIL)
Programme, "Devanagari Script Behaviour for Hindi Ver2.0",
<http://tdil-dc.in/index.php?option=com_download&task=show
resourceDetails&toolid=1625&lang=en>.
[UAX44] The Unicode Consortium, "Unicode Character Database", June
2016, <http://unicode.org/reports/tr44/>.
[WLE-RULES]
Internet Corporation for Assigned Names and Numbers,
"Whole Label Evaluation (WLE) Rules", August 2015, 2016,
<https://community.icann.org/download/
attachments/43989034/WLE-Rules.pdf>.
Appendix A. Example Tables
The following presents a minimal LGR table defining the lowercase LDH
(letters, digits, hyphen) repertoire and containing no rules or
metadata elements. Many simple LGR tables will look quite similar,
except that they would contain some metadata.
<?xml version="1.0" encoding="utf-8"?>
<lgr xmlns="urn:ietf:params:xml:ns:lgr-1.0">
<data>
<char cp="002D" comment="HYPHEN (-)" />
<range first-cp="0030" last-cp="0039"
comment="DIGIT ZERO - DIGIT NINE" />
<range first-cp="0061" last-cp="007A"
comment="LATIN SMALL LETTER A - LATIN SMALL LETTER Z" />
</data>
</lgr>
In practice, any LGR that includes the hyphen might also contain
rules invalidating any labels beginning with a hyphen, ending with a
hyphen, and containing consecutive hyphens in the third and fourth
positions as required by [RFC5891].
<?xml version="1.0" encoding="utf-8"?>
<lgr xmlns="urn:ietf:params:xml:ns:lgr-1.0">
<data>
<char cp="002D"
not-when="hyphen-minus-disallowed" />
<range first-cp="0030" last-cp="0039" />
<range first-cp="0061" last-cp="007A" />
</data>
<rules>
<rule name="hyphen-minus-disallowed"
comment="RFC5891 restrictions on U+002D">
<choice>
<rule comment="no leading hyphen">
<look-behind>
<start />
</look-behind>
<anchor />
</rule>
<rule comment="no trailing hyphen">
<anchor />
<look-ahead>
<end />
</look-ahead>
</rule>
<rule comment="no consecutive hyphens
in third and fourth positions">
<look-behind>
<start />
<any />
<any />
<char cp="002D" comment="hyphen-minus" />
</look-behind>
<anchor />
</rule>
</choice>
</rule>
</rules>
</lgr>
The following sample LGR shows a more complete collection of the
elements and attributes defined in this specification in a somewhat
typical context.
<?xml version="1.0" encoding="utf-8"?>
<!-- This example uses a large subset of the features of this
specification. It does not include every set operator,
match operator element, or action trigger attribute, their
use being largely parallel to the ones demonstrated. -->
<lgr xmlns="urn:ietf:params:xml:ns:lgr-1.0">
<!-- meta element with all optional elements -->
<meta>
<version comment="initial version">1</version>
<date>2010-01-01</date>
<language>sv</language>
<scope type="domain">example.com</scope>
<validity-start>2010-01-01</validity-start>
<validity-end>2013-12-31</validity-end>
<description type="text/html">
<![CDATA[
This language table was developed with the
<a href="http://swedish.example/">Swedish
examples institute</a>.
]]>
</description>
<unicode-version>6.3.0</unicode-version>
<references>
<reference id="0" comment="the most recent" >The
Unicode Standard 9.0</reference>
<reference id="1" >RFC 5892</reference>
<reference id="2" >Big-5: Computer Chinese Glyph
and Character Code Mapping Table, Technical Report
C-26, 1984</reference>
</references>
</meta>
<!-- the "data" section describing the repertoire -->
<data>
<!-- single code point "char" element -->
<char cp="002D" ref="1" comment="HYPHEN" />
<!-- "range" elements for contiguous code points, with tags -->
<range first-cp="0030" last-cp="0039" ref="1" tag="digit" />
<range first-cp="0061" last-cp="007A" ref ="1" tag="letter" />
<!-- code point sequence -->
<char cp="006C 00B7 006C" comment="Catalan middle dot" />
<!-- alternatively, use a When Rule -->
<char cp="00B7" when="catalan-middle-dot" />
<!-- code point with context rule -->
<char cp="200D" when="joiner" ref="2" />
<!-- code points with variants -->
<char cp="4E16" tag="preferred" ref="0">
<var cp="4E17" type="blocked" ref="2" />
<var cp="534B" type="allocatable" ref="2" />
</char>
<char cp="4E17" ref="0">
<var cp="4E16" type="allocatable" ref="2" />
<var cp="534B" type="allocatable" ref="2" />
</char>
<char cp="534B" ref="0">
<var cp="4E16" type="allocatable" ref="2" />
<var cp="4E17" type="blocked" ref="2" />
</char>
</data>
<!-- Context and whole label rules -->
<rules>
<!-- Require the given code point to be between two 006C
code points -->
<rule name="catalan-middle-dot" ref="0">
<look-behind>
<char cp="006C" />
</look-behind>
<anchor />
<look-ahead>
<char cp="006C" />
</look-ahead>
</rule>
<!-- example of a context rule based on property -->
<class name="virama" property="ccc:9" />
<rule name="joiner" ref="1" >
<look-behind>
<class by-ref="virama" />
</look-behind>
<anchor />
</rule>
<!-- example of using set operators -->
<!-- Subtract vowels from letters to get
consonant, demonstrating the different
set notations and the difference operator -->
<difference name="consonants">
<class comment="all letters">0061-007A</class>
<class comment="all vowels">
0061 0065 0069 006F 0075
</class>
</difference>
<!-- by using the start and end, rule matches whole label -->
<rule name="three-or-more-consonants">
<start />
<!-- reference the class defined by the difference,
and require three or more matches -->
<class by-ref="consonants" count="3+" />
<end />
</rule>
<!-- rule for negative matching -->
<rule name="non-preferred"
comment="matches any non-preferred code point">
<complement comment="non-preferred" >
<class from-tag="preferred" />
</complement>
</rule>
<!-- actions triggered by matching rules and/or
variant types -->
<action disp="invalid"
match="three-or-more-consonants" />
<action disp="blocked" any-variant="blocked" />
<action disp="allocatable" all-variants="allocatable"
not-match="non-preferred" />
</rules>
</lgr>
Appendix B. How to Translate Tables Based on RFC 3743 into the XML
Format
As background, the rules specified in [RFC3743] work as follows:
1. The original (requested) label is checked to make sure that all
the code points are a subset of the repertoire.
2. If it passes the check, the original label is allocatable.
3. Generate the all-simplified and all-traditional variant labels
(union of all the labels generated using all the simplified
variants of the code points) for allocation.
To illustrate by example, here is one of the more complicated set of
variants:
U+4E7E
U+4E81
U+5E72
U+5E79
U+69A6
U+6F27
The following shows the relevant section of the Chinese language
table published by the .ASIA registry [ASIA-TABLE]. Its entries
read:
<codepoint>;<simpl-variant(s)>;<trad-variant(s)>;<other-variant(s)>
These are the lines corresponding to the set of variants listed
above:
U+4E7E;U+4E7E,U+5E72;U+4E7E;U+4E81,U+5E72,U+6F27,U+5E79,U+69A6
U+4E81;U+5E72;U+4E7E;U+5E72,U+6F27,U+5E79,U+69A6
U+5E72;U+5E72;U+5E72,U+4E7E,U+5E79;U+4E7E,U+4E81,U+69A6,U+6F27
U+5E79;U+5E72;U+5E79;U+69A6,U+4E7E,U+4E81,U+6F27
U+69A6;U+5E72;U+69A6;U+5E79,U+4E7E,U+4E81,U+6F27
U+6F27;U+4E7E;U+6F27;U+4E81,U+5E72,U+5E79,U+69A6
The corresponding "data" section XML format would look like this:
<data>
<char cp="4E7E">
<var cp="4E7E" type="both" comment="identity" />
<var cp="4E81" type="blocked" />
<var cp="5E72" type="simp" />
<var cp="5E79" type="blocked" />
<var cp="69A6" type="blocked" />
<var cp="6F27" type="blocked" />
</char>
<char cp="4E81">
<var cp="4E7E" type="trad" />
<var cp="5E72" type="simp" />
<var cp="5E79" type="blocked" />
<var cp="69A6" type="blocked" />
<var cp="6F27" type="blocked" />
</char>
<char cp="5E72">
<var cp="4E7E" type="trad"/>
<var cp="4E81" type="blocked"/>
<var cp="5E72" type="both" comment="identity"/>
<var cp="5E79" type="trad"/>
<var cp="69A6" type="blocked"/>
<var cp="6F27" type="blocked"/>
</char>
<char cp="5E79">
<var cp="4E7E" type="blocked"/>
<var cp="4E81" type="blocked"/>
<var cp="5E72" type="simp"/>
<var cp="5E79" type="trad" comment="identity"/>
<var cp="69A6" type="blocked"/>
<var cp="6F27" type="blocked"/>
</char>
<char cp="69A6">
<var cp="4E7E" type="blocked"/>
<var cp="4E81" type="blocked"/>
<var cp="5E72" type="simp"/>
<var cp="5E79" type="blocked"/>
<var cp="69A6" type="trad" comment="identity"/>
<var cp="6F27" type="blocked"/>
</char>
<char cp="6F27">
<var cp="4E7E" type="simp"/>
<var cp="4E81" type="blocked"/>
<var cp="5E72" type="blocked"/>
<var cp="5E79" type="blocked"/>
<var cp="69A6" type="blocked"/>
<var cp="6F27" type="trad" comment="identity"/>
</char>
</data>
Here, the simplified variants have been given a type of "simp" and
the traditional variants one of "trad", and all other ones are given
"blocked".
Because some variant mappings show in more than one column, while the
XML format allows only a single type value, they have been given the
type of "both".
Note that some variant mappings map to themselves (identity); that
is, the mapping is reflexive (see Section 5.3.4). In creating the
permutation of all variant labels, these mappings have no effect,
other than adding a value to the variant type list for the variant
label containing them.
In the example so far, all of the entries with type="both" are also
mappings where source and target are identical. That is, they are
reflexive mappings as defined in Section 5.3.4.
Given a label "U+4E7E U+4E81", the following labels would be ruled
allocatable per [RFC3743], based on how that standard is commonly
implemented in domain registries:
Original label: U+4E7E U+4E81
Simplified label 1: U+4E7E U+5E72
Simplified label 2: U+5E72 U+5E72
Traditional label: U+4E7E U+4E7E
However, if allocatable labels were generated simply by a straight
permutation of all variants with type other than type="blocked" and
without regard to the simplified and traditional variants, we would
end up with an extra allocatable label of "U+5E72 U+4E7E". This
label is composed of both a Simplified Chinese character and a
Traditional Chinese code point and therefore shouldn't be
allocatable.
To more fully resolve the dispositions requires several actions to be
defined, as described in Section 7.2.2, that will override the
default actions from Section 7.6. After blocking all labels that
contain a variant with type "blocked", these actions will set to
"allocatable" labels based on the following variant types: "simp",
"trad", and "both". Note that these variant types do not directly
relate to dispositions for the variant label, but that the actions
will resolve them to the Standard Dispositions on labels, i.e.,
"blocked" and "allocatable".
To resolve label dispositions requires five actions to be defined (in
the "rules" section of the XML document in question); these actions
apply in order, and the first one triggered defines the disposition
for the label. The actions are as follows:
1. Block all variant labels containing at least one blocked variant.
2. Allocate all labels that consist entirely of variants that are
"simp" or "both".
3. Also allocate all labels that are entirely "trad" or "both".
4. Block all surviving labels containing any one of the dispositions
"simp" or "trad" or "both", because they are now known to be part
of an undesirable mixed simplified/traditional label.
5. Allocate any remaining label; the original label would be such a
label.
The rules declarations would be represented as:
<rules>
<!--"action" elements - order defines precedence-->
<action disp="blocked" any-variant="blocked" />
<action disp="allocatable" only-variants="simp both" />
<action disp="allocatable" only-variants="trad both" />
<action disp="blocked" any-variant="simp trad" />
<action disp="allocatable" comment="catch-all" />
</rules>
Up to now, variants with type "both" have occurred only associated
with reflexive variant mappings. The "action" elements defined above
rely on the assumption that this is always the case. However,
consider the following set of variants:
U+62E0;U+636E;U+636E;U+64DA
U+636E;U+636E;U+64DA;U+62E0
U+64DA;U+636E;U+64DA;U+62E0
The corresponding XML would be:
<char cp="62E0">
<var cp="636E" type="both" comment="both, but not reflexive" />
<var cp="64DA" type="blocked" />
</char>
<char cp="636E">
<var cp="636E" type="simp" comment="reflexive, but not both" />
<var cp="64DA" type="trad" />
<var cp="62E0" type="blocked" />
</char>
<char cp="64DA">
<var cp="636E" type="simp" />
<var cp="64DA" type="trad" comment="reflexive" />
<var cp="62E0" type="blocked" />
</char>
To make such variant sets work requires a way to selectively trigger
an action based on whether a variant type is associated with an
identity or reflexive mapping, or is associated with an ordinary
variant mapping. This can be done by adding a prefix "r-" to the
"type" attribute on reflexive variant mappings. For example, the
"trad" for code point U+64DA in the preceding figure would become
"r-trad".
With the dispositions prepared in this way, only a slight
modification to the actions is needed to yield the correct set of
allocatable labels:
<action disp="blocked" any-variant="blocked" />
<action disp="allocatable" only-variants="simp r-simp both r-both" />
<action disp="allocatable" only-variants="trad r-trad both r-both" />
<action disp="blocked" all-variants="simp trad both" />
<action disp="allocatable" />
The first three actions get triggered by the same labels as before.
The fourth action blocks any label that combines an original code
point with any mix of ordinary variant mappings; however, no labels
that are a combination of only original code points (code points
having either no variant mappings or a reflexive mapping) would be
affected. These are the original labels, and they are allocated in
the last action.
Using this scheme of assigning types to ordinary and reflexive
variants, all tables in the style of RFC 3743 can be converted to
XML. By defining a set of actions as outlined above, the LGR will
yield the correct set of allocatable variants: all variants
consisting completely of variant code points preferred for simplified
or traditional, respectively, will be allocated, as will be the
original label. All other variant labels will be blocked.
Appendix C. Indic Syllable Structure Example
In LGRs for Indic scripts, it may be desirable to restrict valid
labels to sequences of valid Indic syllables, or aksharas. This
appendix gives a sample set of rules designed to enforce this
restriction.
Below is an example of BNF for an akshara, which has been published
in "Devanagari Script Behaviour for Hindi" [TDIL-HINDI]. The rules
for other languages and scripts used in India are expected to be
generally similar.
For Hindi, the BNF has the form:
V[m]|{C[N]H}C[N](H|[v][m])
Where:
V (uppercase) is any independent vowel
m is any vowel modifier (Devanagari Anusvara, Visarga, and
Candrabindu)
C is any consonant (with inherent vowel)
N is Nukta
H is a halant (or virama)
v (lowercase) is any dependent vowel sign (matra)
{} encloses items that may be repeated one or more times
[ ] encloses items that may or may not be present
| separates items, out of which only one can be present
By using the Unicode character property "InSC" or
"Indic_Syllabic_Category", which corresponds rather directly to the
classification of characters in the BNF above, we can translate the
BNF into a set of WLE rules matching the definition of an akshara.
<rules>
<!--Character class definitions go here-->
<class name="halant" property="InSC:Virama" />
<union name="vowel-modifier">
<class property="InSC:Visarga" />
<class property="InSC:Bindu" comment="includes anusvara" />
</union>
<!--Whole label evaluation and context rules go here-->
<rule name="consonant-with-optional-nukta">
<class by-ref="InSC:Consonant" />
<class by-ref="InSC:Nukta" count="0:1"/>
</rule>
<rule name="independent-vowel-with-optional-modifier">
<class by-ref="InSC:Vowel_Independent" />
<class by-ref="vowel-modifier" count="0:1" />
</rule>
<rule name="optional-dependent-vowel-with-opt-modifier" >
<class by-ref="InSC:Vowel_Dependent" count="0:1" />
<class by-ref="vowel-modifier" count="0:1" />
</rule>
<rule name="consonant-cluster">
<rule count="0+">
<rule by-ref="consonant-with-optional-nukta" />
<class by-ref="halant" />
</rule>
<rule by-ref="consonant-with-optional-nukta" />
<choice>
<class by-ref="halant" />
<rule by-ref="optional-dependent-vowel-with-opt-modifier" />
</choice>
</rule>
<rule name="akshara">
<choice>
<rule by-ref="independent-vowel-with-optional-modifier" />
<rule by-ref="consonant-cluster" />
</choice>
</rule>
<rule name="WLE-akshara-or-other" comment="series of one or
more aksharas, possibly alternating with other types of
code points such as digits">
<start />
<choice count="1+">
<class property="InSC:other" />
<rule by-ref="akshara" />
</choice>
<end />
</rule>
<!--"action" elements go here - order defines precedence-->
<action disp="invalid" not-match="WLE-akshara-or-other" />
</rules>
With the rules and classes as defined above, the final action assigns
a disposition of "invalid" to all labels that are not composed of a
sequence of well-formed aksharas, optionally interspersed with other
characters, perhaps digits, for example.
The relevant Unicode character property could be replicated by
tagging repertoire values directly in the LGR; this would remove the
dependency on any specific version of the Unicode Standard.
Generally, dependent vowels may only follow consonant expressions;
however, for some scripts, like Bengali, the Unicode Standard
supports sequences of dependent vowels or their application on
independent vowels. This makes the definition of akshara less
restrictive.
C.1. Reducing Complexity
As presented in this example, the rules are rather complex --
although useful in demonstrating the features of the XML format, such
complexity would be an undesirable feature in an actual LGR.
It is possible to reduce the complexity of the rules in this example
by defining alternate rules that simply define the permissible
pair-wise context of adjacent code points by character class, such as
a rule that a halant can only follow a (nuktated) consonant. Such
pair-wise contexts are easier to understand, implement, and verify,
and have the additional benefit of allowing tools to better pinpoint
why a label failed to validate. They also tend to correspond more
directly to the kind of well-formedness requirements that are most
relevant to DNS security, like the requirement to limit the
application of a combining mark (such as a vowel modifier) to only
selected base characters (in this case, vowels). (See the example
and discussion in [WLE-RULES].)
Appendix D. RELAX NG Compact Schema
This schema is provided in RELAX NG Compact format [RELAX-NG].
<CODE BEGINS>
#
# LGR XML Schema 1.0
#
default namespace = "urn:ietf:params:xml:ns:lgr-1.0"
#
# SIMPLE TYPES
#
# RFC 5646 language tag (e.g., "de", "und-Latn")
language-tag = xsd:token
# The scope to which the LGR applies. For the "domain" scope type,
# it should be a fully qualified domain name.
scope-value = xsd:token {
minLength = "1"
}
## a single code point
code-point = xsd:token {
pattern = "[0-9A-F]{4,6}"
}
## a space-separated sequence of code points
code-point-sequence = xsd:token {
pattern = "[0-9A-F]{4,6}( [0-9A-F]{4,6})+"
}
## single code point, or a sequence of code points, or empty string
code-point-literal = code-point | code-point-sequence | ""
## code point or sequence only
non-empty-code-point-literal = code-point | code-point-sequence
## code point sent represented in short form
code-point-set-shorthand = xsd:token {
pattern = "([0-9A-F]{4,6}|[0-9A-F]{4,6}-[0-9A-F]{4,6})"
~ "( ([0-9A-F]{4,6}|[0-9A-F]{4,6}-[0-9A-F]{4,6}))*"
}
## dates are used in information fields in the meta
## section ("YYYY-MM-DD")
date-pattern = xsd:token {
pattern = "\d{4}-\d\d-\d\d"
}
## variant type
## the variant type MUST be non-empty and MUST NOT
## start with a "_"; using xsd:NMTOKEN here because
## we need space-separated lists of them
variant-type = xsd:NMTOKEN
## variant type list for action triggers
## the list MUST NOT be empty, and entries MUST NOT
## start with a "_"
variant-type-list = xsd:NMTOKENS
## reference to a rule name (used in "when" and "not-when"
## attributes, as well as the "by-ref" attribute of the "rule"
## element).
rule-ref = xsd:IDREF
## a space-separated list of tags. Tags should generally follow
## xsd:Name syntax. However, we are using the xsd:NMTOKENS here
## because there is no native XSD datatype for space-separated
## xsd:Name
tags = xsd:NMTOKENS
## The value space of a "from-tag" attribute. Although it is closer
## to xsd:IDREF lexically and semantically, tags are not unique in
## the document. As such, we are unable to take advantage of
## facilities provided by a validator. xsd:NMTOKEN is used instead
## of the stricter xsd:Names here so as to be consistent with
## the above.
tag-ref = xsd:NMTOKEN
## an identifier type (used by "name" attributes).
identifier = xsd:ID
## used in the class "by-ref" attribute to reference another class of
## the same "name" attribute value.
class-ref = xsd:IDREF
## "count" attribute pattern ("n", "n+", or "n:m")
count-pattern = xsd:token {
pattern = "\d+(\+|:\d+)?"
}
## "ref" attribute pattern
## space-separated list of "id" attribute values for
## "reference" elements. These reference ids
## must be declared in a "reference" element
## before they can be used in a "ref" attribute
ref-pattern = xsd:token {
pattern = "[\-_.:0-9A-Z]+( [\-_.:0-9A-Z]+)*"
}
#
# STRUCTURES
#
## Representation of a single code point or a sequence of code
## points
char = element char {
attribute cp { code-point-literal },
attribute comment { text }?,
attribute when { rule-ref }?,
attribute not-when { rule-ref }?,
attribute tag { tags }?,
attribute ref { ref-pattern }?,
variant*
}
## Representation of a range of code points
range = element range {
attribute first-cp { code-point },
attribute last-cp { code-point },
attribute comment { text }?,
attribute when { rule-ref }?,
attribute not-when { rule-ref }?,
attribute tag { tags }?,
attribute ref { ref-pattern }?
}
## Representation of a variant code point or sequence
variant = element var {
attribute cp { code-point-literal },
attribute type { xsd:NMTOKEN }?,
attribute when { rule-ref }?,
attribute not-when { rule-ref }?,
attribute comment { text }?,
attribute ref { ref-pattern }?
}
#
# Classes
#
## a "class" element that references the name of another "class"
## (or set-operator like "union") defined elsewhere.
## If used as a matcher (appearing under a "rule" element),
## the "count" attribute may be present.
class-invocation = element class { class-invocation-content }
class-invocation-content =
attribute by-ref { class-ref },
attribute count { count-pattern }?,
attribute comment { text }?
## defines a new class (set of code points) using Unicode property
## or code points of the same tag value or code point literals
class-declaration = element class { class-declaration-content }
class-declaration-content =
# "name" attribute MUST be present if this is a "top-level"
# class declaration, i.e., appearing directly under the "rules"
# element. Otherwise, it MUST be absent.
attribute name { identifier }?,
# If used as a matcher (appearing in a "rule" element, but not
# when nested inside a set-operator or class), the "count"
# attribute may be present. Otherwise, it MUST be absent.
attribute count { count-pattern }?,
attribute comment { text }?,
attribute ref { ref-pattern }?,
(
# define the class by property (e.g., property="sc:Latn"), OR
attribute property { xsd:NMTOKEN }
# define the class by tagged code points, OR
| attribute from-tag { tag-ref }
# text node to allow for shorthand notation
# e.g., "0061 0062-0063"
| code-point-set-shorthand
)
class-invocation-or-declaration = element class {
class-invocation-content | class-declaration-content
}
class-or-set-operator-nested =
class-invocation-or-declaration | set-operator
class-or-set-operator-declaration =
# a "class" element or set-operator (effectively defining a class)
# directly in the "rules" element.
class-declaration | set-operator
#
# set-operators
#
complement-operator = element complement {
attribute name { identifier }?,
attribute comment { text }?,
attribute ref { ref-pattern }?,
# "count" attribute MUST only be used when this set-operator is
# used as a matcher (i.e., nested in a "rule" element but not
# inside a set-operator or class)
attribute count { count-pattern }?,
class-or-set-operator-nested
}
union-operator = element union {
attribute name { identifier }?,
attribute comment { text }?,
attribute ref { ref-pattern }?,
# "count" attribute MUST only be used when this set-operator is
# used as a matcher (i.e., nested in a "rule" element but not
# inside a set-operator or class)
attribute count { count-pattern }?,
class-or-set-operator-nested,
# needs two or more child elements
class-or-set-operator-nested+
}
intersection-operator = element intersection {
attribute name { identifier }?,
attribute comment { text }?,
attribute ref { ref-pattern }?,
# "count" attribute MUST only be used when this set-operator is
# used as a matcher (i.e., nested in a "rule" element but not
# inside a set-operator or class)
attribute count { count-pattern }?,
class-or-set-operator-nested,
class-or-set-operator-nested
}
difference-operator = element difference {
attribute name { identifier }?,
attribute comment { text }?,
attribute ref { ref-pattern }?,
# "count" attribute MUST only be used when this set-operator is
# used as a matcher (i.e., nested in a "rule" element but not
# inside a set-operator or class)
attribute count { count-pattern }?,
class-or-set-operator-nested,
class-or-set-operator-nested
}
symmetric-difference-operator = element symmetric-difference {
attribute name { identifier }?,
attribute comment { text }?,
attribute ref { ref-pattern }?,
# "count" attribute MUST only be used when this set-operator is
# used as a matcher (i.e., nested in a "rule" element but not
# inside a set-operator or class)
attribute count { count-pattern }?,
class-or-set-operator-nested,
class-or-set-operator-nested
}
## operators that transform class(es) into a new class.
set-operator = complement-operator
| union-operator
| intersection-operator
| difference-operator
| symmetric-difference-operator
#
# Match operators (matchers)
#
any-matcher = element any {
attribute count { count-pattern }?,
attribute comment { text }?
}
choice-matcher = element choice {
## "count" attribute MUST only be used when the choice-matcher
## contains no nested "start", "end", "anchor", "look-behind",
## or "look-ahead" operators and no nested rule-matchers
## containing any of these elements
attribute count { count-pattern }?,
attribute comment { text }?,
# two or more match operators
match-operator-choice,
match-operator-choice+
}
char-matcher =
# for use as a matcher - like "char" but without a "tag" attribute
element char {
attribute cp { non-empty-code-point-literal },
# If used as a matcher (appearing in a "rule" element), the
# "count" attribute may be present. Otherwise, it MUST be
# absent.
attribute count { count-pattern }?,
attribute comment { text }?,
attribute ref { ref-pattern }?
}
start-matcher = element start {
attribute comment { text }?
}
end-matcher = element end {
attribute comment { text }?
}
anchor-matcher = element anchor {
attribute comment { text }?
}
look-ahead-matcher = element look-ahead {
attribute comment { text }?,
match-operators-non-pos
}
look-behind-matcher = element look-behind {
attribute comment { text }?,
match-operators-non-pos
}
## non-positional match operator that can be used as a direct child
## element of the choice-matcher.
match-operator-choice = (
any-matcher | choice-matcher | start-matcher | end-matcher
| char-matcher | class-or-set-operator-nested | rule-matcher
)
## non-positional match operators do not contain any "anchor",
## "look-behind", or "look-ahead" elements.
match-operators-non-pos = (
start-matcher?,
(any-matcher | choice-matcher | char-matcher
| class-or-set-operator-nested | rule-matcher)*,
end-matcher?
)
## positional match operators have an "anchor" element, which may be
## preceded by a "look-behind" element, or followed by a "look-ahead"
## element, or both.
match-operators-pos =
look-behind-matcher?, anchor-matcher, look-ahead-matcher?
match-operators = match-operators-non-pos | match-operators-pos
#
# Rules
#
# top-level rule must have "name" attribute
rule-declaration-top = element rule {
attribute name { identifier },
attribute comment { text }?,
attribute ref { ref-pattern }?,
match-operators
}
## "rule" element used as a matcher (either "by-ref" or contains
## other match operators itself)
rule-matcher =
element rule {
## "count" attribute MUST only be used when the rule-matcher
## contains no nested "start", "end", "anchor", "look-behind",
## or "look-ahead" operators and no nested rule-matchers
## containing any of these elements
attribute count { count-pattern }?,
attribute comment { text }?,
attribute ref { ref-pattern }?,
(attribute by-ref { rule-ref } | match-operators)
}
#
# Actions
#
action-declaration = element action {
attribute comment { text }?,
attribute ref { ref-pattern }?,
# dispositions are often named after variant types or vice versa
attribute disp { variant-type },
( attribute match { rule-ref }
| attribute not-match { rule-ref } )?,
( attribute any-variant { variant-type-list }
| attribute all-variants { variant-type-list }
| attribute only-variants { variant-type-list } )?
}
# DOCUMENT STRUCTURE
start = lgr
lgr = element lgr {
meta-section?,
data-section,
rules-section?
}
## Meta section - information recorded with an LGR that generally
## does not affect machine processing (except for "unicode-version").
## However, if any "class-declaration" uses the "property" attribute,
## a "unicode-version" element MUST be present.
meta-section = element meta {
element version {
attribute comment { text }?,
text
}?
& element date { date-pattern }?
& element language { language-tag }*
& element scope {
# type may by "domain" or an application-defined value
attribute type { xsd:NCName },
scope-value
}*
& element validity-start { date-pattern }?
& element validity-end { date-pattern }?
& element unicode-version {
xsd:token {
pattern = "\d+\.\d+\.\d+"
}
}?
& element description {
# this SHOULD be a valid MIME type
attribute type { text }?,
text
}?
& element references {
element reference {
attribute id {
xsd:token {
# limit "id" attribute to uppercase letters,
# digits, and a few punctuation marks; use of
# integers is RECOMMENDED
pattern = "[\-_.:0-9A-Z]*"
minLength = "1"
}
},
attribute comment { text }?,
text
}*
}?
}
data-section = element data { (char | range)+ }
## Note that action declarations are strictly order dependent.
## class-or-set-operator-declaration and rule-declaration-top
## are weakly order dependent; they must precede first use of the
## identifier via "by-ref".
rules-section = element rules {
( class-or-set-operator-declaration
| rule-declaration-top
| action-declaration)*
}
<CODE ENDS>
Acknowledgements
This format builds upon the work on documenting IDN tables by many
different registry operators. Notably, a comprehensive language
table for Chinese, Japanese, and Korean was developed by the "Joint
Engineering Team" [RFC3743]; this table is the basis of many registry
policies. Also, a set of guidelines for Arabic script registrations
[RFC5564] was published by the Arabic-language community.
Contributions that have shaped this document have been provided by
Francisco Arias, Julien Bernard, Mark Davis, Martin Duerst, Paul
Hoffman, Sarmad Hussain, Barry Leiba, Alexander Mayrhofer, Alexey
Melnikov, Nicholas Ostler, Thomas Roessler, Audric Schiltknecht,
Steve Sheng, Michel Suignard, Andrew Sullivan, Wil Tan, and John
Yunker.
Authors' Addresses
Kim Davies
Internet Corporation for Assigned Names and Numbers
12025 Waterfront Drive
Los Angeles, CA 90094
United States of America
Phone: +1 310 301 5800
Email: kim.davies@icann.org
URI: http://www.icann.org/
Asmus Freytag
ASMUS, Inc.
Email: asmus@unicode.org