GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc8228

Internet Engineering Task Force (IETF) A. Freytag Request for Comments: 8228 August 2017 Category: Informational ISSN: 2070-1721

 Guidance on Designing Label Generation Rulesets (LGRs) Supporting
                           Variant Labels

Abstract

 Rules for validating identifier labels and alternate representations
 of those labels (variants) are known as Label Generation Rulesets
 (LGRs); they are used for the implementation of identifier systems
 such as Internationalized Domain Names (IDNs).  This document
 describes ways to design LGRs to support variant labels.  In
 designing LGRs, it is important to ensure that the label generation
 rules are consistent and well behaved in the presence of variants.
 The design decisions can then be expressed using the XML
 representation of LGRs that is defined in RFC 7940.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It has been approved for publication by the Internet
 Engineering Steering Group (IESG).  Not all documents approved by the
 IESG are a candidate for any level of Internet Standard; see
 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/rfc8228.

Freytag Informational [Page 1] RFC 8228 Variant Rules August 2017

Copyright Notice

 Copyright (c) 2017 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Variant Relations . . . . . . . . . . . . . . . . . . . . . .   4
 3.  Symmetry and Transitivity . . . . . . . . . . . . . . . . . .   5
 4.  A Word on Notation  . . . . . . . . . . . . . . . . . . . . .   5
 5.  Variant Mappings  . . . . . . . . . . . . . . . . . . . . . .   6
 6.  Variant Labels  . . . . . . . . . . . . . . . . . . . . . . .   7
 7.  Variant Types and Label Dispositions  . . . . . . . . . . . .   7
 8.  Allocatable Variants  . . . . . . . . . . . . . . . . . . . .   8
 9.  Blocked Variants  . . . . . . . . . . . . . . . . . . . . . .   9
 10. Pure Variant Labels . . . . . . . . . . . . . . . . . . . . .  10
 11. Reflexive Variants  . . . . . . . . . . . . . . . . . . . . .  11
 12. Limiting Allocatable Variants by Subtyping  . . . . . . . . .  12
 13. Allowing Mixed Originals  . . . . . . . . . . . . . . . . . .  14
 14. Handling Out-of-Repertoire Variants . . . . . . . . . . . . .  15
 15. Conditional Variants  . . . . . . . . . . . . . . . . . . . .  16
 16. Making Conditional Variants Well Behaved  . . . . . . . . . .  18
 17. Variants for Sequences  . . . . . . . . . . . . . . . . . . .  19
 18. Corresponding XML Notation  . . . . . . . . . . . . . . . . .  21
 19. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
 20. Security Considerations . . . . . . . . . . . . . . . . . . .  23
 21. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
   21.1.  Normative References . . . . . . . . . . . . . . . . . .  23
   21.2.  Informative References . . . . . . . . . . . . . . . . .  23
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  24
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  24

Freytag Informational [Page 2] RFC 8228 Variant Rules August 2017

1. Introduction

 Label Generation Rulesets (LGRs) that define the set of permissible
 labels may be applied to identifier systems that rely on labels, such
 as the Domain Name System (DNS) [RFC1034] [RFC1035].  To date, LGRs
 have mostly been used to define policies for implementing
 Internationalized Domain Names (IDNs) using IDNA2008 [RFC5890]
 [RFC5891] [RFC5892] [RFC5893] [RFC5894] in the DNS.  This document
 aims to discuss the generation of LGRs for such circumstances, but
 the techniques and considerations here are almost certainly
 applicable to a wider range of internationalized identifiers.
 In addition to determining whether a given label is eligible, LGRs
 may also define the condition under which alternate representations
 of these labels, so-called "variant labels", may exist and their
 status (disposition).  In the most general sense, variant labels are
 typically labels that are either visually or semantically
 indistinguishable from another label in the context of the writing
 system or script supported by the LGR.  Unlike merely similar labels,
 where there may be a measurable degree of similarity, variant labels
 considered here represent a form of equivalence in meaning or
 appearance.  What constitutes an appropriate variant in any writing
 system or given context, particularly in the DNS, is assumed to have
 been determined ahead of time and therefore is not a subject of this
 document.
 Once identified, variant labels are typically delegated to some
 entity together with the applied-for label, or permanently reserved,
 based on the disposition derived from the LGR.  Correctly defined,
 variant labels can improve the security of an LGR, yet successfully
 defining variant rules for an LGR so that the result is well behaved
 is not always trivial.  This document describes the basic
 considerations and constraints that must be taken into account and
 gives examples of what might be use cases for different types of
 variant specifications in an LGR.
 This document does not address whether variants are an appropriate
 means to solve any given issue or the basis on which they should be
 defined.  It is intended to explain in more detail the effects of
 various declarations and the trade-offs in making design choices.  It
 implicitly assumes that any LGR will be expressed using the XML
 representation defined in [RFC7940] and therefore conforms to any
 requirements stated therein.  Purely for clarity of exposition,
 examples in this document use a more compact notation than the XML
 syntax defined in [RFC7940].  However, the reader is expected to have
 some familiarity with the concepts described in that RFC (see
 Section 4).

Freytag Informational [Page 3] RFC 8228 Variant Rules August 2017

 The user of any identifier system, such as the DNS, interacts with it
 in the context of labels; variants are experienced as variant labels,
 i.e., two (or more) labels that are functionally "same as" under the
 conventions of the writing system used, even though their code point
 sequences are different.  An LGR specification, on the other hand,
 defines variant mappings between code points and, only in a secondary
 step, derives the variant labels from these mappings.  For a
 discussion of this process, see [RFC7940].
 The designer of an LGR can control whether some or all of the variant
 labels created from an original label should be allocatable, i.e.,
 available for allocation (to the original applicant), or whether some
 or all of these labels should be blocked instead, i.e., remain not
 allocatable (to anyone).  This document describes how this choice of
 label disposition is accomplished (see Section 7).
 The choice of desired label disposition would be based on the
 expectations of the users of the particular zone; it is not the
 subject of this document.  Likewise, this document does not address
 the possibility of an LGR defining custom label dispositions.
 Instead, this document suggests ways of designing an LGR to achieve
 the selected design choice for handling variants in the context of
 the two standard label dispositions: "allocatable" and "blocked".
 The information in this document is based on operational experience
 gained in developing LGRs for a wide number of languages and scripts
 using RFC 7940.  This information is provided here as a benefit to
 the wider community.  It does not alter or change the specification
 found in RFC 7940 in any way.

2. Variant Relations

 A variant relation is fundamentally a "same as" relation; in other
 words, it is an equivalence relation.  Now, the strictest sense of
 "same as" would be equality, and for any equality, we have both
 symmetry
   A = B => B = A
 and transitivity
   A = B and B = C => A = C

Freytag Informational [Page 4] RFC 8228 Variant Rules August 2017

 The variant relation with its functional sense of "same as" must
 really satisfy the same constraint.  Once we say A is the "same as"
 B, we also assert that B is the "same as" A.  In this document, the
 symbol "~" means "has a variant relation with".  Thus, we get
   A ~ B => B ~ A
 Likewise, if we make the same claim for B and C (B ~ C), then we get
 A ~ C, because if B is the "same as" both A and C, then A must be the
 "same as" C:
   A ~ B and B ~ C => A ~ C

3. Symmetry and Transitivity

 Not all potential relations between labels constitute equivalence,
 and those that do not are not transitive and may not be symmetric.
 For example, the degree to which labels are confusable is not
 transitive: two labels can be confusingly similar to a third without
 necessarily being confusable with each other, such as when the third
 one has a shape that is "in between" the other two.  In contrast, a
 relation based on identical or effectively identical appearance would
 meet the criterion of transitivity, and we would consider it a
 variant relation.  Examples of variant relations include other forms
 of equivalence, such as semantic equivalence.
 Using [RFC7940], a set of mappings could be defined that is neither
 symmetric nor transitive; such a specification would be formally
 valid.  However, a symmetric and transitive set of mappings is
 strongly preferred as a basis for an LGR, not least because of the
 benefits from an implementation point of view; for example, if all
 mappings are symmetric and transitive, it greatly simplifies the
 check for collisions between labels with variants.  For this reason,
 we will limit the discussion in this document to those relations that
 are symmetric and transitive.  Incidentally, it is often
 straightforward to verify mechanically whether an LGR is symmetric
 and/or transitive and to compute any mappings required to make it so
 (but see Section 15).

4. A Word on Notation

 [RFC7940] defines an XML schema for Label Generation Rulesets in
 general and variant code points and sequences in particular (see
 Section 18).  That notation is rather verbose and can easily obscure
 salient features to anyone not trained to read XML.  For this reason,
 this document uses a symbolic shorthand notation in presenting the
 examples for discussion.  This shorthand is merely a didactic tool

Freytag Informational [Page 5] RFC 8228 Variant Rules August 2017

 for presentation and is not intended as an alternative to or
 replacement for the XML syntax that is used in formally specifying an
 LGR under [RFC7940].
 When it comes time to capture the LGR in a formal definition, the
 notation used for any of the examples in this document can be
 converted to the XML format as described in Section 18.

5. Variant Mappings

 So far, we have treated variant relations as simple "same as"
 relations, ignoring that each relation representing equivalence would
 consist of a symmetric pair of reciprocal mappings.  In this
 document, the symbol "-->" means "maps to".
 A ~ B => A --> B, B --> A
 In an LGR, these mappings are not defined directly between labels but
 between code points (or code point sequences; see Section 17).  In
 the transitive case, given
 A ~ B => A --> B, B --> A
 A ~ C => A --> C, C --> A
 we also get
 B ~ C => B --> C, C --> B
 for a total of six possible mappings.  Conventionally, these are
 listed in tables in order of the source code point, like so:
   A --> B
   A --> C
   B --> A
   B --> C
   C --> A
   C --> B
 As we can see, A, B, and C can each be mapped two ways.

Freytag Informational [Page 6] RFC 8228 Variant Rules August 2017

6. Variant Labels

 To create a variant label, each code point in the original label is
 successively replaced by all variant code points defined by a mapping
 from the original code point.  For a label AAA (the letter "A" three
 times), the variant labels (given the mappings from the transitive
 example above) would be
   AAB
   ABA
   ABB
   BAA
   BAB
   BBA
   BBB
   AAC
   ...
   CCC
 So far, we have merely defined what the variant labels are, but we
 have not considered their possible dispositions.  In the next
 section, we discuss how to set up the variant mappings so that some
 variant labels are mutually exclusive (blocked), but some may be
 allocated to the same applicant as the original label (allocatable).

7. Variant Types and Label Dispositions

 Assume we wanted to allow a variant relation between code points O
 and A, and perhaps between O and B or O and C as well.  Assuming
 transitivity, this would give us:
   O ~ A ~ B ~ C
 Now, further assume that we would like to distinguish the case where
 someone applies for OOO from the case where someone applies for the
 label ABC.  In this case, we would like to allocate only the applied-
 for label OOO, but in the latter case, we would like to also allow
 the allocation of either the label OOO or the variant label ABC, or
 both, but not of any of the other possible variant labels, like OAO,
 BCO, or the like.  (A real-world example might be the case where O
 represents an unaccented letter, while A, B, and C might represent
 various accented forms of the same letter.  Because unaccented
 letters are a common fallback, there might be a desire to allocate an
 unaccented label as a variant, but not the other way around.)
 How would we specify such a distinction?

Freytag Informational [Page 7] RFC 8228 Variant Rules August 2017

 The answer lies in labeling the mappings A --> O, B --> O, and C -->
 O with the type "allocatable" and the mappings O --> A, O --> B, and
 O --> C with the type "blocked".  In this document, the symbol "x-->"
 means "maps with type blocked", and the symbol "a-->" means "maps
 with type allocatable".  Thus:
   O  x--> A
   O  x--> B
   O  x--> C
   A  a--> O
   B  a--> O
   C  a--> O
 When we generate all permutations of labels, we use mappings with
 different types depending on which code points we start from.  The
 set of all permuted variant labels would be the same, but the
 disposition of the variant label depends on which label we start from
 (we call that label the "original" or "applied-for" label).
 In creating an LGR with variants, all variant mappings should always
 be labeled with a type ([RFC7940] does not formally require a type,
 but any well-behaved LGR would be fully typed).  By default, these
 types correspond directly to the dispositions for variant labels,
 with the most restrictive type determining the disposition of the
 variant label.  However, as we shall see later, it is sometimes
 useful to assign types from a wider array of values than the final
 dispositions for the labels and then define explicitly how to derive
 label dispositions from them.

8. Allocatable Variants

 If we start with AAA and use the mappings from Section 7, the
 permutation OOO will be the result of applying the mapping A a--> O
 at each code point.  That is, only mappings with type "a"
 (allocatable) were used.  To know whether we can allocate both the
 label OOO and the original label AAA, we track the types of the
 mappings used in generating the label.
 We record the variant types for each of the variant mappings used in
 creating the permutation in an ordered list.  Such an ordered list of
 variant types is called a "variant type list".  In running text, we
 often show it enclosed in square brackets.  For example, [a x -]
 means the variant label was derived from a variant mapping with the
 "a" variant type in the first code point position, "x" in the second
 code point position, and the original code point in the third
 position ("-" means "no variant mapping").

Freytag Informational [Page 8] RFC 8228 Variant Rules August 2017

 For our example permutation, we get the following variant type list
 (brackets dropped):
   AAA --> OOO : a a a
 From the variant type list, we derive a "variant type set", denoted
 by curly braces, that contains an unordered set of unique variant
 types in the variant type list.  For the variant type list for the
 given permutation, [a a a], the variant type set is { a }, which has
 a single element "a".
 Deciding whether to allow the allocation of a variant label then
 amounts to deriving a disposition for the variant label from the
 variant type set created from the variant mappings that were used to
 create the label.  For example, the derivation
   if "all variants" = "a" => set label disposition to "allocatable"
 would allow OOO to be allocated, because the types of all variant
 mappings used to create that variant label from AAA are "a".
 The "all-variants" condition is tolerant of an extra "-" in the
 variant set (unlike the "only-variants" condition described in
 Section 10).  So, had we started with AOA, OAA, or AAO, the variant
 set for the permuted variant OOO would have been { a - } because in
 each case one of the code points remains the same code point as the
 original.  The "-" means that because of the absence of a mapping O
 --> O, there is no variant type for the O in each of these labels.
 The "all-variants" = "a" condition ignores the "-", so using the
 derivation from above, we find that OOO is an allocatable variant for
 each of the labels AOA, OAA, or AAO.
 Allocatable variant labels, especially large numbers of allocatable
 variants per label, incur a certain cost to users of the LGR.  A
 well-behaved LGR will minimize the number of allocatable variants.

9. Blocked Variants

 Blocked variants are not available to another registrant.  They
 therefore protect the applicant of the original label from someone
 else registering a label that is the "same as" under some user-
 perceived metric.  Blocked variants can be a useful tool even for
 scripts for which no allocatable labels are ever defined.

Freytag Informational [Page 9] RFC 8228 Variant Rules August 2017

 If we start with OOO and use the mappings from Section 7, the
 permutation AAA will have been the result of applying only mappings
 with type "blocked", and we cannot allocate the label AAA, only the
 original label OOO.  This corresponds to the following derivation:
   if "any variants" = "x" => set label disposition to "blocked"
 Additionally, to prevent allocating ABO as a variant label for AAA,
 we need to make sure that the mapping A --> B has been defined with
 type "blocked", as in
   A  x--> B
 so that
   AAA --> ABO: - x a.
 Thus, the set {x a} contains at least one "x" and satisfies the
 derivation of a blocked disposition for ABO when AAA is applied for.
 If an LGR results in a symmetric and transitive set of variant
 labels, then the task of determining whether a label or its variants
 collide with another label or its variants can be implemented very
 efficiently.  Symmetry and transitivity imply that sets of labels
 that are mutual variants of each other are disjoint from all other
 such sets.  Only labels within the same set can be variants of each
 other.  Identifying the variant set can be an O(1) operation, and
 enumerating all variants is not necessary.

10. Pure Variant Labels

 Now, if we wanted to prevent allocation of AOA when we start from
 AAA, we would need a rule disallowing a mix of original code points
 and variant code points; this is easily accomplished by use of the
 "only-variants" qualifier, which requires that the label consist
 entirely of variants and that all the variants are from the same set
 of types.
   if "only-variants" = "a" => set label disposition to "allocatable"
 The two code points A in AOA are not arrived at by variant mappings,
 because the code points are unchanged and no variant mappings are
 defined for A --> A.  So, in our example, the set of variant mapping
 types is
   AAA --> AOA:  - a -

Freytag Informational [Page 10] RFC 8228 Variant Rules August 2017

 but unlike the "all-variants" condition, "only-variants" requires a
 variant type set { a } corresponding to a variant type list [a a a]
 (no - allowed).  By adding a final derivation
   else if "any-variants" = "a" => set label disposition to "blocked"
 and executing that derivation only on any remaining labels, we
 disallow AOA when starting from AAA but still allow OOO.
 Derivation conditions are always applied in order, with later
 derivations only applying to labels that did not match any earlier
 conditions, as indicated by the use of "else" in the last example.
 In other words, they form a cascade.

11. Reflexive Variants

 But what if we started from AOA?  We would expect the original label
 OOO to be allocatable, but, using the mappings from Section 7, the
 variant type set would be
   AOA --> OOO:  a - a
 because the middle O is unchanged from the original code point.  Here
 is where we use a reflexive mapping.  Realizing that O is the "same
 as" O, we can map it to itself.  This is normally redundant, but
 adding an explicit reflexive mapping allows us to specify a
 disposition on that mapping:
   O  a--> O
 With that, the variant type list for AOA --> OOO becomes:
   AOA --> OOO: a a a
 and the label OOO again passes the derivation condition
   if "only-variants" = "a" => set label disposition to "allocatable"
 as desired.  This use of reflexive variants is typical whenever
 derivations with the "only-variants" qualifier are used.  If any code
 point uses a reflexive variant, a well-behaved LGR would specify an
 appropriate reflexive variant for all code points.

Freytag Informational [Page 11] RFC 8228 Variant Rules August 2017

12. Limiting Allocatable Variants by Subtyping

 As we have seen, the number of variant labels can potentially be
 large, due to combinatorics.  Sometimes it is possible to divide
 variants into categories and to stipulate that only variant labels
 with variants from the same category should be allocatable.  For some
 LGRs, this constraint can be implemented by a rule that disallows
 code points from different categories to occur in the same
 allocatable label.  For other LGRs, the appropriate mechanism may be
 dividing the allocatable variants into subtypes.
 To recap, in the standard case, a code point C can have (up to) two
 types of variant mappings
   C  x--> X
   C  a--> A
 where a--> means a variant mapping with type "allocatable" and x-->
 means "blocked".  For the purpose of the following discussion, we
 name the target code point with the corresponding uppercase letter.
 Subtyping allows us to distinguish among different types of
 allocatable variants.  For example, we can define three new types:
 "s", "t", and "b".  Of these, "s" and "t" are mutually incompatible,
 but "b" is compatible with either "s" or "t" (in this case, "b"
 stands for "both").  A real-world example for this might be variant
 mappings appropriate for "simplified" or "traditional" Chinese
 variants, or appropriate for both.
 With subtypes defined as above, a code point C might have (up to)
 four types of variant mappings
   C  x--> X
   C  s--> S
   C  t--> T
   C  b--> B
 and explicit reflexive mappings of one of these types
   C  s--> C
   C  t--> C
   C  b--> C
 As before, all mappings must have one and only one type, but each
 code point may map to any number of other code points.

Freytag Informational [Page 12] RFC 8228 Variant Rules August 2017

 We define the compatibility of "b" with "t" or "s" by our choice of
 derivation conditions as follows
   if "any-variants" = "x" =>  blocked
   else if "only-variants" = "s" or "b" =>  allocatable
   else if "only-variants" = "t" or "b" =>  allocatable
   else if "any-variants" = "s" or "t" or "b" =>  blocked
 An original label of four code points
   CCCC
 may have many variant labels, such as this example listed with its
 corresponding variant type list:
   CCCC --> XSTB : x s t b
 This variant label is blocked because to get from C to B required
 x-->.  (Because variant mappings are defined for specific source code
 points, we need to show the starting label for each of these
 examples, not merely the code points in the variant label.)  The
 variant label
   CCCC --> SSBB : s s b b
 is allocatable, because the variant type list contains only
 allocatable mappings of subtype "s" or "b", which we have defined as
 being compatible by our choice of derivations.  The actual set of
 variant types {s, b} has only two members, but the examples are
 easier to follow if we list each type.  The label
   CCCC --> TTBB : t t b b
 is again allocatable, because the variant type set {t, b} contains
 only allocatable mappings of the mutually compatible allocatable
 subtypes "t" or "b".  In contrast,
   CCCC --> SSTT : s s t t
 is not allocatable, because the type set contains incompatible
 subtypes "t" and "s" and thus would be blocked by the final
 derivation.

Freytag Informational [Page 13] RFC 8228 Variant Rules August 2017

 The variant labels
   CCCC --> CSBB : c s b b
   CCCC --> CTBB : c t b b
 are only allocatable based on the subtype for the C --> C mapping,
 which is denoted here by "c" and (depending on what was chosen for
 the type of the reflexive mapping) could correspond to "s", "t", or
 "b".
 If the subtype is "s", the first of these two labels is allocatable;
 if it is "t", the second of these two labels is allocatable; if it is
 "b", both labels are allocatable.
 So far, the scheme does not seem to have brought any huge reduction
 in allocatable variant labels, but that is because we tacitly assumed
 that C could have all three types of allocatable variants "s", "t",
 and "b" at the same time.
 In a real-world example, the types "s", "t", and "b" are assigned so
 that each code point C normally has, at most, one non-reflexive
 variant mapping labeled with one of these subtypes, and all other
 mappings would be assigned type "x" (blocked).  This holds true for
 most code points in existing tables (such as those used in current
 IDN Top-Level Domains (TLDs)), although certain code points have
 exceptionally complex variant relations and may have an extra
 mapping.

13. Allowing Mixed Originals

 If the desire is to allow original labels (but not variant labels)
 that are s/t mixed, then the scheme needs to be slightly refined to
 distinguish between reflexive and non-reflexive variants.  In this
 document, the symbol "r-n" means "a reflexive (identity) mapping of
 type 'n'".  The reflexive mappings of the preceding section thus
 become:
 C  r-s--> C
 C  r-t--> C
 C  r-b--> C
 With this convention, and redefining the derivations
 if "any-variants" = "x" =>  blocked
 else if "only-variants" = "s" or "r-s" or "b" or "r-b" => allocatable
 else if "only-variants" = "t" or "r-t" or "b" or "r-b" => allocatable
 else if "any-variants" = "s" or "t" or "b"  => blocked
 else => allocatable

Freytag Informational [Page 14] RFC 8228 Variant Rules August 2017

 any labels that contain only reflexive mappings of otherwise mixed
 type (in other words, any mixed original label) now fall through, and
 their disposition is set to "allocatable" in the final derivation.
 In a well-behaved LGR, it is preferable to explicitly define the
 derivation for allocatable labels instead of using a fall through.
 In the derivation above, code points without any variant mappings
 fall through and become allocatable by default if they are part of an
 original label.  Especially in a large repertoire, it can be
 difficult to identify which code points are affected.  Instead, it is
 preferable to mark them with their own reflexive mapping type
 "neither" or "r-n".
   C  r-n--> C
 With that, we can change
   else =>  allocatable
 to
   else if "only-variants" = "r-s" or "r-t" or "r-b" or "r-n"
        =>  allocatable
   else => invalid
 This makes the intent more explicit, and by ensuring that all code
 points in the LGR have a reflexive mapping of some kind, it is easier
 to verify the correct assignment of their types.

14. Handling Out-of-Repertoire Variants

 At first, it may seem counterintuitive to define variants that map to
 code points that are not part of the repertoire.  However, for zones
 for which multiple LGRs are defined, there may be situations where
 labels valid under one LGR should be blocked if a label under another
 LGR is already delegated.  This situation can arise whether or not
 the repertoires of the affected LGRs overlap and, where repertoires
 overlap, whether or not the labels are both restricted to the common
 subset.
 In order to handle this exclusion relation through definition of
 variants, it is necessary to be able to specify variant mappings to
 some code point X that is outside an LGR's repertoire, R:
   C  x--> X : where C = elementOf(R) and X != elementOf(R)

Freytag Informational [Page 15] RFC 8228 Variant Rules August 2017

 Because of symmetry, it is necessary to also specify the inverse
 mapping in the LGR:
   X  x--> C : where X != elementOf(R) and C = elementOf(R)
 This makes X a source of variant mappings, and it becomes necessary
 to identify X as being outside the repertoire, so that any attempt to
 apply for a label containing X will lead to a disposition of
 "invalid", just as if X had never been listed in the LGR.  The
 mechanism to do this uses reflexive variants but with a new type of
 reflexive mapping of "out-of-repertoire-var", shown as "r-o-->":
   X  r-o--> X
 This indicates X != elementOf(R), as long as the LGR is provided with
 a suitable derivation, so that any label containing "r-o-->" is
 assigned a disposition of "invalid", just as if X was any other code
 point not part of the repertoire.  The derivation used is:
   if "any-variant" = "out-of-repertoire-var" => invalid
 It is inserted ahead of any other derivation of the "any-variant"
 kind in the chain of derivations.  As a result, instead of the
 minimum two symmetric variants, for any out-of-repertoire variants,
 there are a minimum of three variant mappings defined:
   C  x--> X
   X  x--> C
   X  r-o--> X
 where C = elementOf(R) and X != elementOf(R).
 Because no variant label with any code point outside the repertoire
 could ever be allocated, the only logical choice for the non-
 reflexive mappings to out-of-repertoire code points is "blocked".

15. Conditional Variants

 Variant mappings are based on whether code points are "same as" to
 the user.  In some writing systems, code points change shape based on
 where they occur in the word (positional forms).  Some code points
 have matching shapes in some positions but not in others.  In such
 cases, the variant mapping exists only for some possible positions
 or, more generally, only for some contexts.  For other contexts, the
 variant mapping does not exist.

Freytag Informational [Page 16] RFC 8228 Variant Rules August 2017

 For example, take two code points that have the same shape at the end
 of a label (or in final position) but not in any other position.  In
 that case, they are variants only when they occur in the final
 position, something we indicate like this:
   final: C --> D
 In cursively connected scripts, like Arabic, a code point may take
 its final form when next to any following code point that interrupts
 the cursive connection, not just at the end of a label.  (We ignore
 the isolated form to keep the discussion simple; if included, "final"
 might be "final-or-isolate", for example).
 From symmetry, we expect that the mapping D --> C should also exist
 only when the code point D is in final position.  (Similar
 considerations apply to transitivity.)
 Sometimes a code point has a final form that is practically the same
 as that of some other code point while sharing initial and medial
 forms with another.
   final: C --> D
   !final: C --> E
 Here, the case where the condition is the opposite of final is shown
 as "!final".
 Because shapes differ by position, when a context is applied to a
 variant mapping, it is treated independently from the same mapping in
 other contexts.  This extends to the assignment of types.  For
 example, the mapping C --> F may be "allocatable" in final position
 but "blocked" in any other context:
   final:  C  a--> F
   !final: C  x--> F
 Now, the type assigned to the forward mapping is independent of the
 reverse symmetric mapping or any transitive mappings.  Imagine a
 situation where the symmetric mapping is defined as F a--> C, that
 is, all mappings from F to C are "allocatable":
   final: F  a--> C
   !final: F  a-->C
 Why not simply write F a--> C?  Because the forward mapping is
 divided by context.  Adding a context makes the two forward variant
 mappings distinct, and that needs to be accounted for explicitly in
 the reverse mappings so that human and machine readers can easily

Freytag Informational [Page 17] RFC 8228 Variant Rules August 2017

 verify symmetry and transitivity of the variant mappings in the LGR.
 (This is true even though the two opposite contexts of "final" and
 "!final" should together cover all possible cases.)

16. Making Conditional Variants Well Behaved

 To ensure that LGR with contextual variants is well behaved, it is
 best to always use "fully qualified" variant mappings that always
 agree in the names of the context rules for forward and reverse
 mappings.  It is also necessary to ensure that no label can match
 more than one context for the same mapping.  Using mutually exclusive
 contexts, such as "final" and "!final", is an easy way to ensure
 that.
 However, it is not always necessary to define dual or multiple
 contexts that together cover all possible cases.  For example, here
 are two contexts that do not cover all possible positional contexts:
   final: C --> D
   initial: C --> D.
 A well-behaved LGR using these two contexts would define all
 symmetric and transitive mappings involving C, D, and their variants
 consistently in terms of the two conditions "final" and "initial" and
 ensure that both cannot be satisfied at the same time by some label.
 In addition to never defining the same mapping with two contexts that
 may be satisfied by the same label, a well-behaved LGR never combines
 a variant mapping with a context with the same variant mapping
 without a context:
   context: C --> D
   C --> D
 Inadvertent mixing of conditional and unconditional variants can be
 detected and flagged by a parser, but verifying that two formally
 distinct contexts are never satisfied by the same label would depend
 on the interaction between labels and context rules, which means that
 it will be up to the LGR designer to ensure that the LGR is well
 behaved.
 A well-behaved LGR never assigns conditions on a reflexive variant,
 as that is effectively no different from having a context on the code
 point itself; the latter is preferred.

Freytag Informational [Page 18] RFC 8228 Variant Rules August 2017

 Finally, for symmetry to work as expected, the context must be
 defined such that it is satisfied for both the original code point in
 the context of the original label and for the variant code point in
 the variant label.  In other words, the context should be "stable
 under variant substitution" anywhere in the label.
 Positional contexts usually satisfy this last condition; for example,
 a code point that interrupts a cursive connection would likely share
 this property with any of its variants.  However, as it is possible
 in principle to define other kinds of contexts, it is necessary to
 make sure that the LGR is well behaved in this aspect at the time the
 LGR is designed.
 Due to the difficulty in verifying these constraints mechanically, it
 is essential that an LGR designer document the reasons why the LGR
 can be expected to meet them and the details of the techniques used
 to ensure that outcome.  This information should be found in the
 description element of the LGR.
 In summary, conditional contexts can be useful for some cases, but
 additional care must be taken to ensure that an LGR containing
 conditional contexts is well behaved.  LGR designers would be well
 advised to avoid using conditional contexts and to prefer
 unconditional rules whenever practical, even though it will
 doubtlessly reduce the number of labels practically available.

17. Variants for Sequences

 Variant mappings can be defined between sequences or between a code
 point and a sequence.  For example, one might define a "blocked"
 variant between the sequence "rn" and the code point "m" because they
 are practically indistinguishable in common UI fonts.
 Such variants are no different from variants defined between single
 code points, except if a sequence is defined such that there is a
 code point or shorter sequence that is a prefix (initial subsequence)
 and both it and the remainder are also part of the repertoire.  In
 that case, it is possible to create duplicate variants with
 conflicting dispositions.

Freytag Informational [Page 19] RFC 8228 Variant Rules August 2017

 The following shows such an example resulting in conflicting
 reflexive variants:
   A  a--> C
   AB  x--> CD
 where AB is a sequence with an initial subsequence of A.  For
 example, B might be a combining code point used in sequence AB.  If B
 only occurs in the sequence, there is no issue, but if B also occurs
 by itself, for example:
   B  a--> D
 then a label "AB" might correspond to either {A}{B}, that is, the two
 code points, or {AB}, the sequence, where the curly braces show the
 sequence boundaries as they would be applied during label validation
 and variant mapping.
 A label AB would then generate the "allocatable" variant label {C}{D}
 and the "blocked" variant label {CD}, thus creating two variant
 labels with conflicting dispositions.
 For the example of a blocked variant between "m" and "rn" (and vice
 versa), there is no issue as long as "r" and "n" do not have variant
 mappings of their own, so that there cannot be multiple variant
 labels for the same input.  However, it is preferable to avoid
 ambiguities altogether where possible.
 The easiest way to avoid an ambiguous segmentation into sequences is
 by never allowing both a sequence and all of its constituent parts
 simultaneously as independent parts of the repertoire, for example,
 by not defining B by itself as a member of the repertoire.
 Sequences are often used for combining sequences that consist of a
 base character B followed by one or more combining marks C.  By
 enumerating all sequences in which a certain combining mark is
 expected and by not listing the combining mark by itself in the LGR,
 the mark cannot occur outside of these specifically enumerated
 contexts.  In cases where enumeration is not possible or practicable,
 other techniques can be used to prevent ambiguous segmentation, for
 example, a context rule on code points that disallows B preceding C
 in any label except as part of a predefined sequence or class of
 sequences.  The details of such techniques are outside the scope of
 this document (see [RFC7940] for information on context rules for
 code points).

Freytag Informational [Page 20] RFC 8228 Variant Rules August 2017

18. Corresponding XML Notation

 The XML format defined in [RFC7940] corresponds fairly directly to
 the notation used for variant mappings in this document.  (There is
 no notation in the RFC for variant type sets).  In an LGR document, a
 simple member of a repertoire that does not have any variants is
 listed as:
 <char cp="nnnn" />
 where nnnn is the [UNICODE] code point value in the standard
 uppercase hexadecimal notation padded to at least 4 digits and
 without leading "U+".  For a code point sequence of length 2, the XML
 notation becomes:
 <char cp="uuuu vvvvv" />
 Variant mappings are defined by nesting <var> elements inside the
 <char> element.  For example, a variant relation of type "blocked"
   C  x--> X
 is expressed as
   <char cp="nnnn">
     <var cp="mmmm" type="blocked" />
   </char>
 where "x-->" identifies a "blocked" type.  (Other types include
 "a-->" for "allocatable", for example.  Here, nnnn and mmmm are the
 [UNICODE] code point values for C and X, respectively.  Either C or X
 could be a code point sequence or a single code point.
 A reflexive mapping is specified the same way, except that it always
 uses the same code point value for both the <char> and <var> element,
 for example:
   X  r-o--> X
 would correspond to
 <char cp="nnnn"><var cp="nnnn" type="out-of-repertoire-var" /></char>
 Multiple <var> elements may be nested inside a single <char> element,
 but their "cp" values must be distinct (unless attributes for context
 rules are present and the combination of "cp" value and context
 attributes are distinct).

Freytag Informational [Page 21] RFC 8228 Variant Rules August 2017

   <char cp="nnnn">
     <var cp="kkkk" type="allocatable" />
     <var cp="mmmm" type="blocked" />
   </char>
 A set of conditional variants like
   final: C  a--> K
   !final: C  x--> K
 would correspond to
   <var cp="kkkk" when="final" type="allocatable" />
   <var cp="kkkk" not-when="final" type="blocked" />
 where the string "final" references a name of a context rule.
 Context rules are defined in [RFC7940]; they conceptually correspond
 to regular expressions.  The details of how to create and define
 these rules are outside the scope of this document.  If the label
 matches the context defined in the rule, the variant mapping is valid
 and takes part in further processing.  Otherwise, it is invalid and
 ignored.  Using the "not-when" attribute inverts the sense of the
 match.  The two attributes are mutually exclusive.
 A derivation of a variant label disposition
   if "only-variants" = "s" or "b" => allocatable
 is expressed as
   <action disp="allocatable" only-variants= "s b" />
 Instead of using "if" and "else if", the <action> elements implicitly
 form a cascade, where the first action triggered defines the
 disposition of the label.  The order of action elements is thus
 significant.
 For the full specification of the XML format, see [RFC7940].

19. IANA Considerations

 This document does not require any IANA actions.

Freytag Informational [Page 22] RFC 8228 Variant Rules August 2017

20. Security Considerations

 As described in [RFC7940], variants may be used as a tool to reduce
 certain avenues of attack in security-relevant identifiers by
 allowing certain labels to be "mutually exclusive or registered only
 to the same user".  However, if indiscriminately designed, variants
 may themselves contribute to risks to the security or usability of
 the identifiers, whether resulting from an ambiguous definition or
 from allowing too many allocatable variants per label.
 The information in this document is intended to allow the reader to
 design a specification of an LGR that is "well behaved" with respect
 to variants; as used here, this term refers to an LGR that is
 predictable in its effects to the LGR author (and reviewer) and more
 reliable in its implementation.
 A well-behaved LGR is not merely one that can be expressed in
 [RFC7940], but, in addition, it actively avoids certain edge cases
 not prevented by the schema, such as those that would result in
 ambiguities in the specification of the intended disposition for some
 variant labels.  By applying the additional considerations introduced
 in this document, including adding certain declarations that are
 optional under the schema and may not alter the results of processing
 a label, such an LGR becomes easier to review and its implementations
 easier to verify.
 It should be noted that variants are an important part, but only a
 part, of an LGR design.  There are many other features of an LGR that
 this document does not touch upon.  Also, the question of whether to
 define variants at all, or what labels are to be considered variants
 of each other, is not addressed here.

21. References

21.1. Normative References

 [RFC7940]  Davies, K. and A. Freytag, "Representing Label Generation
            Rulesets Using XML", RFC 7940, DOI 10.17487/RFC7940,
            August 2016, <https://www.rfc-editor.org/info/rfc7940>.

21.2. Informative References

 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
            <https://www.rfc-editor.org/info/rfc1034>.

Freytag Informational [Page 23] RFC 8228 Variant Rules August 2017

 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
            November 1987, <https://www.rfc-editor.org/info/rfc1035>.
 [RFC5890]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Definitions and Document Framework",
            RFC 5890, DOI 10.17487/RFC5890, August 2010,
            <https://www.rfc-editor.org/info/rfc5890>.
 [RFC5891]  Klensin, J., "Internationalized Domain Names in
            Applications (IDNA): Protocol", RFC 5891,
            DOI 10.17487/RFC5891, August 2010,
            <https://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,
            <https://www.rfc-editor.org/info/rfc5892>.
 [RFC5893]  Alvestrand, H., Ed. and C. Karp, "Right-to-Left Scripts
            for Internationalized Domain Names for Applications
            (IDNA)", RFC 5893, DOI 10.17487/RFC5893, August 2010,
            <https://www.rfc-editor.org/info/rfc5893>.
 [RFC5894]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Background, Explanation, and
            Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,
            <https://www.rfc-editor.org/info/rfc5894>.
 [UNICODE]  The Unicode Consortium, "The Unicode Standard",
            <http://www.unicode.org/versions/latest/>.

Acknowledgments

 Contributions that have shaped this document have been provided by
 Marc Blanchet, Ben Campbell, Patrik Faltstrom, Scott Hollenbeck,
 Mirja Kuehlewind, Sarmad Hussain, John Klensin, Alexey Melnikov,
 Nicholas Ostler, Michel Suignard, Andrew Sullivan, Wil Tan, and
 Suzanne Woolf.

Author's Address

 Asmus Freytag
 Email: asmus@unicode.org

Freytag Informational [Page 24]

/data/webs/external/dokuwiki/data/pages/rfc/rfc8228.txt · Last modified: 2017/08/22 21:34 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki