Request for Comments: 7940
Category: Standards Track
ISSN: 2070-1721
ICANN
A. Freytag
ASMUS, Inc.
August 2016
Representing Label Generation Rulesets Using XML
Abstract
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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
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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 7841.
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
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Copyright © 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
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1. Introduction ....................................................4 2. Design Goals ....................................................5 3. Normative Language ..............................................6 4. LGR Format ......................................................6 4.1. Namespace ..................................................7 4.2. Basic Structure ............................................7 4.3. Metadata ...................................................8 4.3.1. The "version" Element ...............................8 4.3.2. The "date" Element ..................................9 4.3.3. The "language" Element ..............................9 4.3.4. The "scope" Element ................................10 4.3.5. The "description" Element ..........................10 4.3.6. The "validity-start" and "validity-end" Elements ...11 4.3.7. The "unicode-version" Element ......................11 4.3.8. The "references" Element ...........................12 5. Code Points and Variants .......................................13 5.1. Sequences .................................................14 5.2. Conditional Contexts ......................................15 5.3. Variants ..................................................16 5.3.1. Basic Variants .....................................16 5.3.2. The "type" Attribute ...............................17 5.3.3. Null Variants ......................................18 5.3.4. Variants with Reflexive Mapping ....................19 5.3.5. Conditional Variants ...............................20 5.4. Annotations ...............................................22 5.4.1. The "ref" Attribute ................................22 5.4.2. The "comment" Attribute ............................23 5.5. Code Point Tagging ........................................23 6. Whole Label and Context Evaluation .............................23 6.1. Basic Concepts ............................................23 6.2. Character Classes .........................................25 6.2.1. Declaring and Invoking Named Classes ...............25 6.2.2. Tag-Based Classes ..................................26 6.2.3. Unicode Property-Based Classes .....................26 6.2.4. Explicitly Declared Classes ........................28 6.2.5. Combined Classes ...................................29 6.3. Whole Label and Context Rules .............................30 6.3.1. The "rule" Element .................................31 6.3.2. The Match Operators ................................32 6.3.3. The "count" Attribute ..............................33 6.3.4. The "name" and "by-ref" Attributes .................34 6.3.5. The "choice" Element ...............................34 6.3.6. Literal Code Point Sequences .......................35 6.3.7. The "any" Element ..................................35 6.3.8. The "start" and "end" Elements .....................35 6.3.9. Example Context Rule from IDNA Specification .......36 6.4. Parameterized Context or When Rules .......................37 6.4.1. The "anchor" Element ...............................37 6.4.2. The "look-behind" and "look-ahead" Elements ........38 6.4.3. Omitting the "anchor" Element ......................40 7. The "action" Element ...........................................40 7.1. The "match" and "not-match" Attributes ....................41 7.2. Actions with Variant Type Triggers ........................41 7.2.1. The "any-variant", "all-variants", and "only-variants" Attributes .........................41 7.2.2. Example from Tables in the Style of RFC 3743 .......44 7.3. Recommended Disposition Values ............................45 7.4. Precedence ................................................45 7.5. Implied Actions ...........................................45 7.6. Default Actions ...........................................46 8. Processing a Label against an LGR ..............................47 8.1. Determining Eligibility for a Label .......................47 8.1.1. Determining Eligibility Using Reflexive Variant Mappings ...................................47 8.2. Determining Variants for a Label ..........................48 8.3. Determining a Disposition for a Label or Variant Label ....49 8.4. Duplicate Variant Labels ..................................50 8.5. Checking Labels for Collision .............................50 9. Conversion to and from Other Formats ...........................51 10. Media Type ....................................................51 11. IANA Considerations ...........................................52 11.1. Media Type Registration ..................................52 11.2. URN Registration .........................................53 11.3. Disposition Registry .....................................53 12. Security Considerations .......................................54 12.1. LGRs Are Only a Partial Remedy for Problem Space .........54 12.2. Computational Expense of Complex Tables ..................54 13. References ....................................................55 13.1. Normative References .....................................55 13.2. Informative References ...................................56 Appendix A. Example Tables ........................................58 Appendix B. How to Translate Tables Based on RFC 3743 into the XML Format ............................................63 Appendix C. Indic Syllable Structure Example ......................68 C.1. Reducing Complexity .......................................70 Appendix D. RELAX NG Compact Schema ...............................71 Acknowledgements ..................................................82 Authors' Addresses ................................................82
1. Introduction
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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
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The following goals informed the design of this format:
- The format needs to be implementable in a reasonably straightforward manner in software.
- The format should be able to be automatically checked for formatting errors, so that common mistakes can be caught.
- 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.
- 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".
- 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).
- 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.
- 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.
- 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.
- 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.
- 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
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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
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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
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The XML Namespace URI is "urn:ietf:params:xml:ns:lgr-1.0".
See Section 11.2 for more information.
4.2. Basic Structure
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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 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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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):
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<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.
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5.2. Conditional Contexts
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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:
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<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.
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5.3. Variants
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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:
- literal code points or code point sequences
- character classes, which define sets of code points to be used for context comparisons
- context operators, which define when character classes and literals may appear
- 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
- are incorrectly defined or nested,
2. have invalid attributes, or
- 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
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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:
- 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;
- by referencing a value of one of the Unicode character properties defined in the Unicode Character Database;
- by explicitly listing all the code points in the class; or
- by defining the class as a set combination of any number of other classes.
6.2.1. Declaring and Invoking Named Classes
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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
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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:
- General Category (gc)
- Script (sc)
- Canonical Combining Class (ccc)
- Bidi Class (bc)
- Arabic Joining Type (jt)
- Indic Syllabic Category (InSC)
- 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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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:
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<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).
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7.2.2. Example from Tables in the Style of RFC 3743
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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
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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
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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
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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 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
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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.
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<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.
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8. Processing a Label against an LGR
8.1. Determining Eligibility for a Label
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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
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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
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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:
- 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).
- Apply variant mappings with "when" or "not-when" attributes only if the conditions are satisfied; otherwise, they are not defined.
- 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).
- Determine the disposition for each variant label per Section 8.3.
- If the disposition is "invalid", remove the label from the set.
- 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
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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.
- 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".
- 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:
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- 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
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- the action does not contain any "match", "not-match", "any-variant", "all-variants", or "only-variants" attributes: catch-all.
- 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.
- For any remaining label, set the disposition to "valid".
8.4. Duplicate Variant Labels
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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
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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
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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
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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
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IANA has completed the following actions:
11.1. Media Type Registration
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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:
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Deprecated alias names for this type: N/A
Magic number(s): N/A File extension(s): .lgr Macintosh file type code(s): N/APerson & email address to contact for further information:
Kim Davies <kim.davies@icann.org> Asmus Freytag <asmus@unicode.org> Intended usage: COMMONRestrictions on usage: N/A
Author:
Kim Davies <kim.davies@icann.org> Asmus Freytag <asmus@unicode.org> Change controller: IESGProvisional registration? (standards tree only): No
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11.2. URN Registration
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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:
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None.
11.3. Disposition Registry
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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:
- 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.
- 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 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:
- The original (requested) label is checked to make sure that all the code points are a subset of the repertoire.
- If it passes the check, the original label is allocatable.
- 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+6F27The 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+4E7EHowever, 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:
- Block all variant labels containing at least one blocked variant.
- Allocate all labels that consist entirely of variants that are "simp" or "both".
- Also allocate all labels that are entirely "trad" or "both".
- 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.
- 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+62E0The 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 presentBy 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
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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
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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