idnits 2.17.1
draft-freytag-lager-variant-rules-04.txt:
Checking boilerplate required by RFC 5378 and the IETF Trust (see
https://trustee.ietf.org/license-info):
----------------------------------------------------------------------------
No issues found here.
Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt:
----------------------------------------------------------------------------
No issues found here.
Checking nits according to https://www.ietf.org/id-info/checklist :
----------------------------------------------------------------------------
No issues found here.
Miscellaneous warnings:
----------------------------------------------------------------------------
== The copyright year in the IETF Trust and authors Copyright Line does not
match the current year
-- The document date (March 13, 2017) is 2600 days in the past. Is this
intentional?
Checking references for intended status: Informational
----------------------------------------------------------------------------
No issues found here.
Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--).
Run idnits with the --verbose option for more detailed information about
the items above.
--------------------------------------------------------------------------------
2 Network Working Group A. Freytag
3 Internet-Draft March 13, 2017
4 Intended status: Informational
5 Expires: September 14, 2017
7 Variant Rules
8 draft-freytag-lager-variant-rules-04
10 Abstract
12 Rules for validating identifier labels and alternate representations
13 of those labels (variants) are known as "Label Generation Rulesets"
14 (LGRs); they are used for the implementation of identifier systems
15 such as Internationalized Domain Names (IDNs). This document
16 describes ways of designing Label Generation Rulesets (LGRs) that
17 support variant labels. In designing LGRs, it is important to ensure
18 that the label generation rules are consistent and well-behaved in
19 the presence of variants. The design decisions can then be expressed
20 using the an XML representation of LGRs that is defined in RFC7940.
22 Status of This Memo
24 This Internet-Draft is submitted in full conformance with the
25 provisions of BCP 78 and BCP 79.
27 Internet-Drafts are working documents of the Internet Engineering
28 Task Force (IETF). Note that other groups may also distribute
29 working documents as Internet-Drafts. The list of current Internet-
30 Drafts is at http://datatracker.ietf.org/drafts/current/.
32 Internet-Drafts are draft documents valid for a maximum of six months
33 and may be updated, replaced, or obsoleted by other documents at any
34 time. It is inappropriate to use Internet-Drafts as reference
35 material or to cite them other than as "work in progress."
37 This Internet-Draft will expire on September 14, 2017.
39 Copyright Notice
41 Copyright (c) 2017 IETF Trust and the persons identified as the
42 document authors. All rights reserved.
44 This document is subject to BCP 78 and the IETF Trust's Legal
45 Provisions Relating to IETF Documents
46 (http://trustee.ietf.org/license-info) in effect on the date of
47 publication of this document. Please review these documents
48 carefully, as they describe your rights and restrictions with respect
49 to this document. Code Components extracted from this document must
50 include Simplified BSD License text as described in Section 4.e of
51 the Trust Legal Provisions and are provided without warranty as
52 described in the Simplified BSD License.
54 Table of Contents
56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
57 2. Variant Relationships . . . . . . . . . . . . . . . . . . . . 4
58 3. Symmetry and Transitivity . . . . . . . . . . . . . . . . . . 4
59 4. A Word on Notation . . . . . . . . . . . . . . . . . . . . . 5
60 5. Variant Mappings . . . . . . . . . . . . . . . . . . . . . . 5
61 6. Variant Labels . . . . . . . . . . . . . . . . . . . . . . . 6
62 7. Variant Types and Label Dispositions . . . . . . . . . . . . 6
63 8. Allocatable Variants . . . . . . . . . . . . . . . . . . . . 7
64 9. Blocked Variants . . . . . . . . . . . . . . . . . . . . . . 8
65 10. Pure Variant Labels . . . . . . . . . . . . . . . . . . . . . 9
66 11. Reflexive Variants . . . . . . . . . . . . . . . . . . . . . 10
67 12. Limiting Allocatable Variants by Subtyping . . . . . . . . . 10
68 13. Allowing Mixed Originals . . . . . . . . . . . . . . . . . . 13
69 14. Handling Out-of-Repertoire Variants . . . . . . . . . . . . . 14
70 15. Conditional Variants . . . . . . . . . . . . . . . . . . . . 15
71 16. Making Conditional Variants Well-Behaved . . . . . . . . . . 16
72 17. Variants for Sequences . . . . . . . . . . . . . . . . . . . 18
73 18. Corresponding XML Notation . . . . . . . . . . . . . . . . . 19
74 19. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
75 20. Security Considerations . . . . . . . . . . . . . . . . . . . 21
76 21. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
77 21.1. Normative References . . . . . . . . . . . . . . . . . . 22
78 21.2. Informative References . . . . . . . . . . . . . . . . . 22
79 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 22
80 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 22
81 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22
83 1. Introduction
85 Label Generation Rulesets (LGRs) that define the set of permissible
86 labels, may be applied to a variety of identifier systems, although
87 to date, the most common use is to define policies for implementing
88 Internationalized Domain Names (IDNs) for some zone of the Domain
89 Name System (DNS). Without restricting any of the more general
90 applications of this document, the explanations and examples in this
91 document may be stated in terms of IDNs.
93 In addition to determining whether a given label is eligible, LGRs
94 may also define the condition under which alternate representations
95 of these labels, so called variant labels, may exist and also define
96 their status (disposition). In the most general sense, variant
97 labels are typically labels that are either visually or semantically
98 indistinguishable from an applied for label in the context of the
99 writing system or script supported by the LGR. Unlike merely similar
100 labels, where there may be a measurable degree of similarity, variant
101 labels considered here represent a form of equivalence in meaning or
102 appearance. What constitutes an appropriate variant in any writing
103 system or given context, particular in the DNS, is assumed to have
104 been determined ahead of time, and therefore is not subject of this
105 document.
107 Once identified, variant labels are typically delegated to some
108 entity together with the applied-for label, or permanently reserved,
109 based on the disposition derived from the LGR. Correctly defined,
110 variant labels can improve the security of an LGR yet successfully
111 defining variant rules for an LGR so that the result is well-behaved
112 is not always trivial. This document describes the basic
113 considerations and constraints that must be taken into account and
114 gives examples of what might be use cases for different types of
115 variant specifications in an LGR.
117 This document does not address the question whether variants are an
118 appropriate means to solve any given issue, nor on what basis they
119 should be defined. It is intended to explain in more detail the
120 effects of various declarations and the tradeoffs in making design
121 choices. It tacitly assumes that any LGR will be expressed using the
122 XML representation defined in [RFC7940] and therefore conform to any
123 requirements stated therein. Purely for clarity of exposition,
124 examples in this document are using a more compact notation than the
125 XML syntax defined in [RFC7940]. However, the reader is expected to
126 have some familiarity with the concepts described in that RFC (see
127 Section 4).
129 The user of any identifier system, such as the DNS, interacts with it
130 in the context of labels; variants are experienced as variant labels:
131 two (or more) labels that are functionally "the same" under the
132 conventions of the writing system used, even though their code point
133 sequences are different. An LGR specification, on the other hand,
134 defines variant mappings between code points, and only in a secondary
135 step, derives the variant labels from these mappings. For a
136 discussion of this process, see [RFC7940].
138 By assigning a "type" to the variant mappings and carefully
139 constructing the derivation of variant label dispositions from these
140 types, the designer of an LGR can control whether some or all of the
141 variant labels created from an original label should be allocatable,
142 that is available for allocation (to the original applicant) or
143 whether some or all of these labels should be blocked instead, that
144 is, remain not allocatable (to anyone).
146 The choice of desired label disposition would be based on the
147 expectations of the users of the particular zone, and is not the
148 subject of this document. Likewise, this document does not address
149 the possibility of an LGR defining custom label dispositions.
150 Instead, this document suggests ways of desiging an LGR to achieve
151 the selected design choice for handling variants in the context of
152 the two standard label dispositions "allocatable" and "blocked".
154 2. Variant Relationships
156 A variant relationship is fundamentally a "same as", in other words,
157 it is an equivalence relationship. Now the strictest sense of "same
158 as" would be equality, and for any equality, we have both symmetry
160 A = B => B = A
162 and transitivity
164 A = B and B = C => A = C
166 The variant relationship with its functional sense of "same as" must
167 really satisfy the same constraint. Once we say A is the "same as"
168 B, we also assert that B is the "same as" A. In this document, the
169 symbol "~" means "has a variant relationship with". Thus, we get
171 A ~ B => B ~ A
173 Likewise, if we make the same claim for B and C (B ~ C) then we do
174 get A ~ C, because if B is "the same" as both A and C then A must be
175 "the same as" C:
177 A ~ B and B ~ C => A ~ C
179 3. Symmetry and Transitivity
181 Not all relationships between labels constitute equivalence and those
182 that do not are not transitive and may not be symmetric. For
183 example, the degree to which labels are confusable is not transitive:
184 two labels can be confusingly similar to a third without necessarily
185 being confusable with each other, such as when the third one has a
186 shape that is "in between" the other two. A variant relation based
187 on identical or effectively identical appearance would meet the
188 criterion of transitivity, as would other forms of equivalence, such
189 as semantic equivalence.
191 From the perspective of [RFC7940], an LGR could be specified that is
192 neither symmetric or transitive, and such an LGR would be a valid
193 specification. However, from an implementation point of view there
194 are certain benefits from an LGR that is symmetric and transitive.
195 It greatly simplifies the check for collisions between labels with
196 variants. For this reason, we will limit the discussion of variants
197 in this document to those are symmetric and transitive.
198 Incidentally, it is often straightforward to verify mechanically
199 whether an LGR is symmetric and or transitive, and to compute any
200 mappings required to make it so (but see Section 15).
202 4. A Word on Notation
204 [RFC7940] defines an XML schema for Label Generation Rulesets in
205 general, and variant code points and sequences in particular, see
206 Section 18. That notation is rather verbose and can easily obscure
207 salient features to anyone not trained to read XML. For this reason,
208 this document uses a symbolic shorthand notation in presenting the
209 examples for discussion. This shorthand is merely a didactic tool
210 for presentation and not intended as alternative to or replacement
211 for the XML syntax that is used in formally specifying an LGR under
212 [RFC7940].
214 When it comes time to capture the LGR in a formal definition, the
215 notation used for any of examples in this document can be converted
216 to the XML format as described in Section 18.
218 5. Variant Mappings
220 So far, we have treated variant relationships as simple "same as"
221 ignoring that each relationship representing an equivalence would
222 consist of a symmetric pair of reciprocal mappings. In this
223 document, the symbol "-->" means "maps to".
225 A ~ B => A --> B, B --> A
227 In an LGR, these mappings are not defined directly between labels,
228 but between code points (or code point sequences, see Section 17).
229 In the transitive case, given
231 A ~ B => A --> B, B --> A
233 A ~ C => A --> C, C --> A
235 we also get
237 B ~ C => B --> C, C --> B
239 for a total of six possible mappings. Conventionally, these are
240 listed in tables in order of the source code point, like so
241 A --> B
242 A --> C
243 B --> A
244 B --> C
245 C --> A
246 C --> B
248 As we can see, each of A, B and C can be mapped two ways.
250 6. Variant Labels
252 To create a variant label, each code point in the original label is
253 successively replaced by all variant code points defined by a mapping
254 from the original code point. For a label AAA (the letter "A" three
255 times), the variant labels (given the mappings from transitive
256 example above) would be
258 AAB
259 ABA
260 ABB
261 BAA
262 BAB
263 BBA
264 BBB
265 AAC
266 ...
267 CCC
269 So far, we have mere defined what the variant labels are, but we have
270 not considered their possible dispositions. In the next section we
271 discuss how to set up the variant mappings so that some variant
272 labels are mutually exclusive (blocked), but some may be allocated to
273 the same applicant as the original label (allocatable).
275 7. Variant Types and Label Dispositions
277 Assume we wanted to allow a variant relation between some code points
278 O and A, and perhaps also between O and B as well as O and C. By
279 transitivity we would have
281 O ~ A ~ B ~ C
283 However, we would like to distinguish the case where someone applies
284 for OOO from the case where someone applies for the label ABC. In
285 the former case we would like to allocate only the label OOO, but in
286 the latter case, we would like to also allow the allocation of either
287 the original label OOO or the variant label ABC, or both, but not of
288 any of the other possible variant labels, like OAO, BCO or the like.
290 (A real-world example might be the case where O represents an
291 unaccented letter, while A, B and C might represent various accented
292 forms of the same letter. Because unaccented letters are a common
293 fallback, there might be a desire to allocate an unaccented label as
294 a variant, but not the other way around.)
296 How do we make that distinction?
298 The answer lies in labeling the mappings A --> O, B --> O, and C -->
299 O with the type "allocatable" and the mappings O --> A, O --> B, and
300 O --> C with the type "blocked". In this document, the symbol "x-->"
301 means "maps with type blocked" and the symbol "a-->" means "maps with
302 type allocatable". Thus:
304 O x--> A
305 O x--> B
306 O x--> C
307 A a--> O
308 B a--> O
309 C a--> O
311 When we generate all permutations of labels, we use mappings with
312 different types depending from which code points we start.
314 In creating an LGR with variants, all variant mappings should always
315 be labeled with a type ([RFC7940] does not formally require a type,
316 but any well-behaved LGR would be fully typed). By default, these
317 types correspond directly to the dispositions for variant labels,
318 with the most restrictive type determining the disposition of the
319 variant label. However, as we shall see later, it is sometimes
320 useful to assign types from a wider array of values than the final
321 dispositions for the labels and then define explicitly how to derive
322 label dispositions from them.
324 8. Allocatable Variants
326 If we start with AAA, the permutation OOO will have been the result
327 of applying the mapping A a--> O at each code point. That is, only
328 mappings with type "a" (allocatable) were used. To know whether we
329 can allocate both the label OOO and the original label AAA we track
330 the types of the mappings used in generating the label.
332 We record the variant types for each of the variant mappings used in
333 creating the permutation in an ordered list. Such an ordered list of
334 variant types is called a "variant type list". In running text we
335 often show it enclosed in square brackets. For example [a x -] means
336 the variant label was derived from a variant mapping with the "a"
337 variant type in the first code point position, "x" in the second code
338 point position, and that the third position is the original code
339 point ("-" means "no variant mapping").
341 For our example permutation we get the following variant type list
342 (brackets dropped):
344 AAA --> OOO : a a a
346 From the variant type list we derive a "variant type set", denoted by
347 curly braces, that contains an unordered set of unique variant types
348 in the variant type list. For the variant type list for the given
349 permutation, [a a a], the variant type set is { a }, which has a
350 single element "a".
352 Deciding whether to allow the allocation of a variant label then
353 amounts to deriving a disposition for the variant label from the
354 variant type set created from the variant mappings that were used to
355 create the label. For example the derivation
357 if "all variants" = "a" => set label disposition to "allocatable"
359 would allow OOO to be allocated, because the types of all variants
360 mappings used to create that variant label from AAA are "a".
362 The "all-variants" condition is tolerant of an extra "-" in the
363 variant set (unlike the "only-variants" condition described below).
364 So, had we started with AOA, OAA or AAO, the variant set for the
365 permuted variant OOO would have been { a - } because in each case one
366 of the code points remains the same as the original. The "-" means
367 that because of the absence of a mapping O --> O there is no variant
368 type for the O in each of these labels.
370 The "all-variants" = "a" condition ignores the "-", so using the
371 derivation from above, we find that OOO is an allocatable variant for
372 each of the labels AOA, OAA or AAO.
374 Allocatable variant labels, especially large numbers of allocatable
375 variants per label, incur a certain cost to users of the LGR. A
376 well-behaved LGR will minimize the number allocatable variants.
378 9. Blocked Variants
380 Blocked variants are not available to another registrant. They
381 therefore protect the applicant of the original label from someone
382 else registering a label that is "the same as" under some user-
383 perceived metric. Blocked variants can be a useful tool even for
384 scripts for which no allocatable labels are ever defined.
386 If we start with OOO, the permutation AAA will have been the result
387 of applying only mappings with type "blocked" and we cannot allocate
388 the label AAA, only the original label OOO. This corresponds to the
389 following derivation:
391 if "any variants" = "x" => set label disposition to "blocked"
393 To additionally prevent allocating ABO as a variant label for AAA we
394 further need to make sure that the mapping A --> B has been defined
395 with type "blocked" as in
397 A x--> B
399 so that
401 AAA --> ABO: - x a.
403 Thus the set {x a} contains at least one "x" and satisfies the
404 derivation of a blocked disposition for ABO when AAA is applied for.
406 If an LGR results in a symmetric and transitive set of variant
407 labels, then the task of determining whether a label or its variants
408 collide with another label or its variants can be implemented very
409 efficiently. Symmetry and transitivity implies that each set of
410 labels that are mutually variants of each other is disjoint from all
411 other such sets. Only labels within the same set can be variants of
412 each other. Identifying the variant set can be an O(1) operation,
413 and enumerating all variants is not necessary.
415 10. Pure Variant Labels
417 Now, if we wanted to prevent allocation of AOA when we start from
418 AAA, we would need a rule disallowing a mix of original code points
419 and variant code points, which is easily accomplished by use of the
420 "only-variants" qualifier, which requires that the label consist
421 entirely of variants and all the variants are from the same set of
422 types.
424 if "only-variants" = "a" => set label disposition to "allocatable"
426 The two code points A in AOA are not arrived at by variant mappings,
427 because the code points are unchanged and no variant mappings are
428 defined for A --> A. So, in our example, the set of variant mapping
429 types is
431 AAA --> AOA: - a -
433 but unlike the "all-variants" condition, "only-variants" requires a
434 variant type set { a } corresponding to a variant type list [a a a]
435 (no - allowed). By adding a final derivation
437 else if "any-variants" = "a" => set label disposition to "blocked"
439 and executing that derivation only on any remaining labels, we
440 disallow AOA when starting from AAA, but still allow OOO.
442 Derivation conditions are always applied in order, with later
443 derivations only applying to labels that did not match any earlier
444 conditions, as indicated by the use of "else" in the last example.
445 In other words, they form a cascade.
447 11. Reflexive Variants
449 But what if we started from AOA? We would expect OOO to be
450 allocatable, but the variant type set would be
452 OOO --> OOO: a - a
454 because the O is the original code point. Here is where we use a
455 reflexive mapping, by realizing that O is "the same as" O, which is
456 normally redundant, but allows us to specify a disposition on the
457 mapping
459 O a--> O
461 with that, the variant type list for OOO --> OOO becomes:
463 AOA --> OOO: a a a
465 and the label OOO again passes the derivation condition
467 if "only-variants" = "a" => set label disposition to "allocatable"
469 as desired. This use of reflexive variants is typical whenever
470 derivations with the "only-variants" qualifier are used. If any code
471 point uses a reflexive variant, a well-behaved LGR would specify an
472 appropriate reflexive variant for all code points.
474 12. Limiting Allocatable Variants by Subtyping
476 As we have seen, the number of variant labels can potentially be
477 large, due to combinatorics. Sometimes it is possible to divide
478 variants into categories and to stipulate that only variant labels
479 with variants from the same category should be allocatable. For some
480 LGRs this constraint can be implemented by a rule that disallows code
481 points from different categories to occur in the same allocatable
482 label. For other LGRs the appropriate mechanism may be dividing the
483 allocatable variants into subtypes.
485 To recap, in the standard case a code point C can have (up to) two
486 types of variant mappings
488 C x--> X
489 C a--> A
491 where a--> means a variant mapping with type "allocatable", and x-->
492 means "blocked". For the purpose of the following discussion, we
493 name the target code point with the corresponding uppercase letter.
495 Subtyping allows us to distinguish among different types of
496 allocatable variants. For example, we can define three new types:
497 "s", "t" and "b". Of these, "s" and "t" are mutually incompatible,
498 but "b" is compatible with either "s" or "t" (in this case, "b"
499 stands for "both"). A real-world example for this might be variant
500 mappings appropriate for "simplified" or "traditional" Chinese
501 variants, or appropriate for both.
503 With subtypes defined as above, a code point C might have (up to)
504 four types of variant mappings
506 C x--> X
507 C s--> S
508 C t--> T
509 C b--> B
511 and explicit reflexive mappings of one of these types
513 C s--> C
514 C t--> C
515 C b--> C
517 As before, all mappings must have one and only one type, but each
518 code point may map to any number of other code points.
520 We define the compatibility of "b" with "t" or "s" by our choice of
521 derivation conditions as follows
523 if "any-variants" = "x" => blocked
524 else if "only-variants" = "s" or "b" => allocatable
525 else if "only-variants" = "t" or "b" => allocatable
526 else if "any-variants" = "s" or "t" or "b" => blocked
528 An original label of four code points
529 CCCC
531 may have many variant labels such as this example listed with its
532 corresponding variant type list:
534 CCCC --> XSTB : x s t b
536 This variant label is blocked because to get from C to B required
537 x-->. (Because variant mappings are defined for specific source code
538 points, we need to show the starting label for each of these
539 examples, not merely the code points in the variant label.) The
540 variant label
542 CCCC --> SSBB : s s b b
544 is allocatable, because the variant type list contains only
545 allocatable mappings of subtype "s" or "b", which we have defined as
546 being compatible by our choice of derivations. The actual set of
547 variant types {s, b} has only two members, but the examples are
548 easier to follow if we list each type. The label
550 CCCC --> TTBB : t t b b
552 is again allocatable, because the variant type set {t, b} contains
553 only allocatable mappings of the mutually compatible allocatable
554 subtypes "t" or "b". In contrast,
556 CCCC --> SSTT : s s t t
558 is not allocatable, because the type set contains incompatible
559 subtypes "t" and "s" and thus would be blocked by the final
560 derivation.
562 The variant labels
564 CCCC --> CSBB : c s b b
565 CCCC --> CTBB : c t b b
567 are only allocatable based on the subtype for the C --> C mapping,
568 which is denoted here by c and (depending on what was chosen for the
569 type of the reflexive mapping) could correspond to "s", "t", or "b".
571 If it is "s", the first of these two labels is allocatable; if it is
572 "t", the second of these two labels is allocatable; if it is b, both
573 labels are allocatable.
575 So far, the scheme does not seem to have brought any huge reduction
576 in allocatable variant labels, but that is because we tacitly assumed
577 that C could have all three types of allocatable variants "s", "t",
578 and "b" at the same time.
580 In a real world example, the types "s", "t" and "b" are assigned so
581 that each code point C normally has at most one non-reflexive variant
582 mapping labeled with one of these subtypes, and all other mappings
583 would be assigned type "x" (blocked). This holds true for most code
584 points in existing tables (such as those used in current IDN TLDs),
585 although certain code points have exceptionally complex variant
586 relations and may have an extra mapping.
588 13. Allowing Mixed Originals
590 If the desire is to allow original labels (but not variant labels)
591 that are s/t mixed, then the scheme needs to be slightly refined to
592 distinguish between reflexive and non-reflexive variants. In this
593 document, the symbol "r-n" means "a reflexive (identity) mapping of
594 type 'n'". The reflexive mappings of the preceding section thus
595 become:
597 C r-s--> C
598 C r-t--> C
599 C r-b--> C
601 With this convention, and redefining the derivations
603 if "any-variants" = "x" => blocked
604 else if "only-variants" = "s" or "r-s" or "b" or "r-b" => allocatable
605 else if "only-variants" = "t" or "r-t" or "b" or "r-b" => allocatable
606 else if "any-variants" = "s" or "t" or "b" => blocked
607 else => allocatable
609 any labels that contain only reflexive mappings of otherwise mixed
610 type (in other words, any mixed original label) now fall through and
611 their disposition is set to "allocatable" in the final derivation.
613 In a well-behaved LGR, it is preferable to explicitly define the
614 derivation for allocatable labels, instead of using a fall-through.
615 In the derivation above, code points without any variant mappings
616 fall through and become allocatable by default if they are part of an
617 original label. Especially in a large repertoire it can be difficult
618 to identify which code points are affected. Instead, it is
619 preferable to mark them with their own reflexive mapping type
620 "neither" or "r-n".
622 C r-n--> C
624 With that we can change
625 else => allocatable
627 to
629 else if "only-variants" = "r-s" or "r-t" or "r-b" or "r-n"
630 => allocatable
631 else => invalid
633 This makes the intent more explicit and by ensuring that all code
634 points in the LGR have a reflexive mapping of some kind, it is easier
635 to verify the correct assignment of their types.
637 14. Handling Out-of-Repertoire Variants
639 At first it may seem counterintuitive to define variants that map to
640 code points not part of the repertoire. However, for zones for which
641 multiple LGRs are defined, there may be situations where labels valid
642 under one LGR should be blocked if a label under another LGR is
643 already delegated. This situation can arise whether or not the
644 repertoires of the affected LGRs overlap, and, where repertoires
645 overlap, whether or not the labels are both restricted to the common
646 subset.
648 In order to handle this exclusion relation through definition of
649 variants, it is necessary to be able to specify variant mappings to
650 some code point X that is outside an LGR's repertoire, R:
652 C x--> X : where C = elementOf(R) and X != elementOf(R)
654 Because of symmetry, it is necessary to also specify the inverse
655 mapping in the LGR:
657 X x--> C : where X != elementOf(R) and C = elementOf(R)
659 This makes X a source of variant mappings and it becomes necessary to
660 identify X as being outside the repertoire, so that any attempt to
661 apply for a label containing X will lead to a disposition of
662 "invalid" - just as if X had never been listed in the LGR. The
663 mechanism to do this, again uses reflexive variants, but with a new
664 type of reflexive mapping of "out-of-repertoire-var", shown as
665 "r-o-->":
667 X r-o--> X
669 This indicates X != elementOf(R), as long as the LGR is provided with
670 a suitable derivation, so that any label containing "r-o-->" is
671 assigned a disposition of "invalid", just as if X was any other code
672 point not part of the repertoire. The derivation used is:
674 if "any-variant" = "out-of-repertoire-var" => invalid
676 It is inserted ahead of any other derivation of the "any-variant"
677 kind in the chain of derivations. As a result, instead of the
678 minimum two symmetric variants, for any out-of repertoire variants
679 there are a minimum of three variant mappings defined:
681 C x--> X
682 X x--> C
683 X r-o--> X
685 where C = elementOf(R) and X != elementOf(R).
687 Because no variant label with any code point outside the repertoire
688 could ever be allocated, the only logical choice for the non-
689 reflexive mappings to out-of-repertoire code points is "blocked".
691 15. Conditional Variants
693 Variant mappings are based on whether code points are "the same" to
694 the user. In some writing systems, code points change shape based on
695 where they occur in the word (positional forms). Some code points
696 have matching shapes in some positions, but not in others. In such
697 cases, the variant mapping only exists for some possible positions,
698 or more general, only for some contexts. For other contexts, the
699 variant mapping does not exist.
701 For example, take two code points, that have the same shape at the
702 end of a label (or in final position) but not in any other position.
703 In that case, they are variants only when they occur in the final
704 position, something we indicate like this:
706 final: C --> D
708 In cursively connected scripts, like Arabic, a code point may take
709 its final form when next to any following code point that interrupts
710 the cursive connection, not just at the end of a label. (We ignore
711 the isolated form to keep the discussion simple, if it was included,
712 "final" might be "final-or-isolate", for example).
714 From symmetry, we expect that the mapping D --> C should also exist
715 only when the code point D is in final position. (Similar
716 considerations apply to transitivity).
718 Sometimes a code point has a final form that is practically the same
719 as that of some code point while sharing initial and medial forms
720 with another.
722 final: C --> D
723 !final: C --> E
725 Here the case where the condition is the opposite of final is shown
726 as "!final".
728 Because shapes differ by position, when a context is applied to a
729 variant mapping, it is treated independently from the same mapping in
730 other contexts. This extends to the assignment of types. For
731 example, the mapping C --> F may be "allocatable" in final position,
732 but "blocked" in any other context:
734 final: C a--> F
735 !final: C x--> F
737 Now, the type assigned to the forward mapping is independent of the
738 reverse symmetric mapping, or any transitive mappings. Imagine a
739 situation where the symmetric mapping is defined as F a--> C, that
740 is, all mappings from F to C are "allocatable":
742 final: F a--> C
743 !final: F a-->C
745 Why not simply write F a--> C? Because the forward mapping is
746 divided by context. Adding a context makes the two forward variant
747 mappings distinct and that needs to be accounted for explicitly in
748 the reverse mappings so that human and machine readers can easily
749 verify symmetry and transitivity of the variant mappings in the LGR.
750 (This is true even though the two opposite contexts "final" and
751 "!final" should together cover all possible cases).
753 16. Making Conditional Variants Well-Behaved
755 To ensure that LGR with contextual variants is well-behaved it is
756 best to always use "fully qualified" variant mappings that always
757 agree in the names of the context rules for forward and reverse
758 mappings. It also necessary to ensure that no label can match more
759 than one context for the same mapping. Using mutually exclusive
760 contexts, such as "final" and "!final" is an easy way to ensure that.
762 However, it is not always necessary to define dual or multiple
763 contexts that together cover all possible cases. For example, here
764 are two contexts that do not cover all possible positional contexts:
766 final: C --> D
767 initial: C --> D.
769 A well-behaved LGR using these two contexts, would define all
770 symmetric and transitive mappings involving C, D and their variants
771 consistently in terms of the two conditions "final" and "initial" and
772 ensure both cannot be satisfied at the same time by some label.
774 In addition to never defining the same mapping with two contexts that
775 may be satisfied by the same label, a well-behaved LGR never combines
776 a variant mapping with context with the same variant mapping without
777 a context:
779 context: C --> D
780 C --> D
782 Inadvertent mixing of conditional and unconditional variants can be
783 detected and flagged by a parser, but verifying that two formally
784 distinct contexts are never satisfied by the same label would depend
785 on the interaction between labels and context rules, which means that
786 it will be up to the LGR designer to ensure the LGR is well-behaved.
788 A well-behaved LGR never assigns conditions on a reflexive variant,
789 as that is effectively no different from having a context on the code
790 point itself; the latter is preferred.
792 Finally, for symmetry to work as expected, the context must be
793 defined such that it is satisfied for both the original code point in
794 the context of the original label and for the variant code point in
795 the variant label. In other words the context should be "stable
796 under variant substitution" anywhere in the label.
798 Positional contexts usually satisfy this last condition; for example,
799 a code point that interrupts a cursive connection would likely share
800 this property with any of its variants. However, as it is in
801 principle possible to define other kinds of contexts, it is necessary
802 to make sure that the LGR is well behaved in this aspect at the time
803 the LGR is designed.
805 Due to the difficulty in verifying these constraints mechanically, it
806 is essential that an LGR designer document the reasons why the LGR
807 can be expected to meet them, and the details of the techniques used
808 to ensure that outcome. This information should be found in the
809 description element of the LGR.
811 In summary, conditional contexts can be an essential tool, but some
812 additional care must be taken to ensure that an LGR containing
813 conditional contexts is well behaved.
815 17. Variants for Sequences
817 Variants mappings can be defined between sequences, or between a code
818 point and a sequence. For example one might define a "blocked"
819 variant between the sequence "rn" and the code point "m" because they
820 are practically indistinguishable in common UI fonts.
822 Such variants are no different from variants defined between single
823 code points, except if a sequence is defined such that there is a
824 code point or shorter sequence that is a prefix (initial subsequence)
825 and both it and the remainder are also part of the repertoire. In
826 that case, it is possible to create duplicate variants with
827 conflicting dispositions.
829 The following shows such an example resulting in conflicting
830 reflexive variants:
832 A a--> C
833 AB x--> CD
835 where AB is a sequence with an initial subsequence of A. For
836 example, B might be a combining code point used in sequence AB. If B
837 only occurs in the sequence, there is no issue, but if B also occurs
838 by itself, for example:
840 B a--> D
842 then a label "AB" might correspond to either {A}{B}, that is the two
843 code points, or {AB}, the sequence, where the curly braces show the
844 sequence boundaries as they would be applied during label validation
845 and variant mapping.
847 A label AB would then generate the "allocatable" variant label {C}{D}
848 and the "blocked" variant label {CD} thus creating two variant labels
849 with conflicting dispositions.
851 For the example of a blocked variant between "m" and "rn" (and vice
852 versa) there is no issue as long as "r" and "n" do not have variant
853 mappings of their own, so that there cannot be multiple variant
854 labels for the same input. However, it is preferable to avoid
855 ambiguities altogether, where possible.
857 The easiest way to avoid an ambiguous segmentation into sequences is
858 by never allowing both a sequence and all of its constituent parts
859 simultaneously as independent parts of the repertoire, for example,
860 by not defining B by itself as a member of the repertoire.
862 Sequences are often used for combining sequences, which consist of a
863 base character B followed by one or more combining marks C. By
864 enumerating all sequences in which a certain combining mark is
865 expected, and by not listing the combining mark by itself in the LGR,
866 the mark cannot occur outside of these specifically enumerated
867 contexts. In cases where enumeration is not possible or practicable,
868 other techniques can be used to prevent ambiguous segmentation, for
869 example, a context rule on code points that disallows B preceding C
870 in any label except as part of a predefined sequence or class of
871 sequences. The details of such techniques are outside the scope of
872 this document (see [RFC7940] for information on context rules for
873 code points).
875 18. Corresponding XML Notation
877 The XML format defined in [RFC7940] corresponds fairly directly to
878 the notation used for variant mappings in this document. (There is
879 no notation in the RFC for variant type sets). In an LGR document, a
880 simple member of a repertoire that does not have any variants is
881 listed as:
883
885 where nnnn is the [Unicode9] code point value in the standard
886 uppercase hexadecimal notation padded to at least 4 digits and
887 without leading "U+". For a code point sequence of length two, the
888 XML notation becomes:
890
892 Variant mappings are defined by nesting elements inside the
893 element. For example, a variant relation of type "blocked"
895 C x--> X
897 is expressed as
899
900
901
903 where "x-->" identifies a "blocked" type. (Other types include
904 "a-->" for "allocatable", for example). Here, nnnn and mmmm are the
905 [Unicode9] code point values for C and X, respectively. Either C or
906 X could be a code point sequence or a single code point.
908 A reflexive mapping is specified the same way, except that it always
909 uses the same code point value for both the and element,
910 for example
912 X r-o--> X
914 would correspond to
916
918 Multiple elements may be nested inside a single element,
919 but their "cp" values must be distinct (unless attributes for context
920 rules are present and the combination of "cp" value and context
921 attributes are distinct).
923
924
925
926
928 A set of conditional variants like
930 final: C a--> K
931 !final: C x--> K
933 would correspond to
935
936
938 where the string "final" references a name of a context rule.
939 Context rules are defined in [RFC7940] and conceptually correspond to
940 regular expressions. The details of how to create and define these
941 rules are outside the scope of this document. If the label matches
942 the context defined in the rule, the variant mapping is valid and
943 takes part in further processing. Otherwise it is invalid and
944 ignored. Using the "not-when" attribute inverts the sense of the
945 match. The two attributes are mutually exclusive.
947 A derivation of a variant label disposition
949 if "only-variants" = "s" or "b" => allocatable
951 is expressed as
953
955 Instead of using "if" and "else if" the elements implicitly
956 form a cascade, where the first action triggered defines the
957 disposition of the label. The order of action elements is thus
958 significant.
960 For the full specification of the XML format see [RFC7940].
962 19. IANA Considerations
964 This document does not specify any IANA actions.
966 20. Security Considerations
968 As described in [RFC7940], variants may be used as a tool to reduce
969 certain avenues of attack in security-relevant identifiers by
970 allowing certain labels to be "mutually exclusive or registered only
971 to the same user". However, variants, if indiscriminately designed,
972 may themselves contribute to risks to the security or usability of
973 the identifiers, whether resulting from an ambiguous definition or
974 from allowing too many allocatable variants per label.
976 The information in this document is intended to allow the reader to
977 design a specification of an LGR that is "well-behaved" with respect
978 to variants; as used here, this term refers to an LGR that is
979 predictable in its effects to the LGR-author (and reviewer) and more
980 reliable in its implementation.
982 A well-behaved LGR is not merely one that can be expressed in
983 [RFC7940] but in addition, it actively avoids certain edge cases not
984 prevented by the schema, such as those that would result in
985 ambiguities in the specification of the intended disposition for some
986 variants. By applying the additional considerations introduced in
987 this document, including adding certain declarations that are
988 optional under the schema and may not alter the results of processing
989 a label, such an LGR becomes easier to review and its implementations
990 easier to verify.
992 It should be noted, that variants are an important part, but only a
993 part of an LGR design. There are many other features of an LGR that
994 this document does not touch upon. Also, the question of whether to
995 define variants are all, or what labels are to be considered variants
996 of each other is not addressed here.
998 21. References
999 21.1. Normative References
1001 [RFC7940] Davies, K. and A. Freytag, "Representing Label Generation
1002 Rulesets Using XML", RFC 7940, DOI 10.17487/RFC7940,
1003 August 2016, .
1005 21.2. Informative References
1007 [Unicode9]
1008 The Unicode Consortium, "The Unicode Standard, Version
1009 9.0.0", ISBN 978-1-936213-13-9, 2016,
1010 .
1012 Preferred Citation: The Unicode Consortium. The Unicode
1013 Standard, Version 9.0.0, (Mountain View, CA: The Unicode
1014 Consortium, 2016. ISBN 978-1-936213-13-9)
1016 Appendix A. Acknowledgments
1018 Contributions that have shaped this document have been provided by
1019 Marc Blanchet, Patrik Faltstrom, Sarmad Hussain, John Klensin,
1020 Nicholas Ostler, Michel Suignard, Wil Tan and Suzanne Woolf.
1022 Appendix B. Change Log
1024 RFC Editor: Please remove this appendix before publication.
1026 -00 Initial draft.
1028 -01 Minor fix to references.
1030 -02 Some formatting and grammar issues as well as typos fixed.
1031 Added a few real-world examples where required for context.
1032 Added "r-n" to description of subtyping.
1034 -03 Fix ID nits and other typos. Expanded security section. Minor
1035 tweaks.
1037 -04 Additional context. Added to introduction. Introduced sections
1038 on notation and symmetry and transititivy. Expanded the section
1039 on XML notation.
1041 Author's Address
1043 Asmus Freytag
1045 Email: asmus@unicode.org