Network Working Group A. Freytag
Internet-Draft December 29, 2016
Intended status: Informational
Expires: July 2, 2017
Variant Rules
draft-freytag-lager-variant-rules-02
Abstract
This document gives guidance on designing well-behaved Label
Generation Rulesets (LGRs) that support variant labels. Typical
examples of labels and LGRs are IDNs and zone registration policies
defining permissible IDN labels. Variant labels are labels that are
either visually or semantically indistinguishable from an applied for
label and are typically delegated together with the applied-for
label, or permanently reserved. While [RFC7940] defines the
syntactical requirements for specifying the label generation rules
for variant labels, additional considerations apply that ensure that
the label generation rules are consistent and well-behaved in the
presence of variants.
Status of This Memo
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This Internet-Draft will expire on July 2, 2017.
Copyright Notice
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document authors. All rights reserved.
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Variant Relationships . . . . . . . . . . . . . . . . . . . . 3
3. Variant Mappings . . . . . . . . . . . . . . . . . . . . . . 4
4. Variant Labels . . . . . . . . . . . . . . . . . . . . . . . 5
5. Variant Types and Label Dispositions . . . . . . . . . . . . 5
6. Allocatable Variants . . . . . . . . . . . . . . . . . . . . 6
7. Blocked Variants . . . . . . . . . . . . . . . . . . . . . . 7
8. Pure Variant Labels . . . . . . . . . . . . . . . . . . . . . 8
9. Reflexive Variants . . . . . . . . . . . . . . . . . . . . . 8
10. Limiting Allocatable Variants by Subtyping . . . . . . . . . 9
11. Allowing Mixed Originals . . . . . . . . . . . . . . . . . . 11
12. Handling Out-of-Repertoire Variants . . . . . . . . . . . . . 12
13. Conditional Variants . . . . . . . . . . . . . . . . . . . . 13
14. Conditional Variants and Well-Behaved LGRs . . . . . . . . . 15
15. Variants for Sequences . . . . . . . . . . . . . . . . . . . 16
16. Corresponding XML Notation . . . . . . . . . . . . . . . . . 17
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
18. Security Considerations . . . . . . . . . . . . . . . . . . . 19
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
19.1. Normative References . . . . . . . . . . . . . . . . . . 19
19.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 19
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
Label Generation Rulesets (LGR) [RFC7940] define permissible labels,
but may also define the condition under which variant labels may
exist and their status (disposition). Variant labels are labels that
are either visually or semantically indistinguishable from an applied
for label in the context of the writing system or script supported by
the LGR. 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.
Successfully defining variant rules for an LGR is not trivial. A
number of considerations and constraints have to be taken into
account. This document describes the basic constraints and use cases
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for variant rules in an LGR by using a more readable notation than
the XML format defined in RFC 7940. When it comes time to capture
the LGR in a formal definition, the notation used in this document
can be converted to the XML format fairly directly.
From the perspective of a user of the DNS, variants are experienced
as variant labels; two (or more) labels that are functionally "the
same" 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], or as it
relates to the root zone, see [Procedure].
By assigning a "type" to the variant mappings and carefully
constructing the derivation of variant label dispositions from these
types, the designer of an LGR can control whether some or all of the
variant labels created from an original label should be available for
allocation (to the original applicant) or whether some or all of
these labels should be blocked instead and remain not allocatable (to
anyone).
The choice of desired label disposition would be based on the
expectations of the users of the particular zone, and is not the
subject of this document. Instead, this document suggests how to
best design an LGR to achieve the selected design choice for handling
variants.
2. Variant Relationships
A variant relationship is fundamentally a "same as", in other words,
it is an equivalence relationship. 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
The variant relationship 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 relationship with". Thus, we get
A ~ B => B ~ A
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Likewise, if we make the same claim for B and C (B ~ C) then we do
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
Not all relationships between labels constitute equivalence. 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. A variant relation based
on (effectively) identical appearance would pass the test, as would
other forms of equivalence (e.g., semantic).
3. Variant Mappings
So far, we have treated variant relationships as simple "same as"
ignoring that each relationship consists of a pair of reciprocal
mappings. In this document, the symbol "-->" means "maps to".
A ~ B => A --> B, B --> A
These mappings are not defined between labels, but between code
points (or code point sequences). 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, each of A, B and C can be mapped two ways.
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4. 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 transitive
example above) would be
AAB
ABA
ABB
BAA
BAB
BBA
BBB
AAC
...
CCC
5. Variant Types and Label Dispositions
Assume we wanted to allow a variant relation between some code points
O and A, and perhaps also between O and B as well as O and C. By
transitivity we would have
O ~ A ~ B ~ C
However, we would like to distinguish the case where someone applies
for OOO from the case where someone applies for the label ABC. In
the former case we would like to allocate only the label OOO, but in
the latter case, we would like to also allow the allocation of either
the original 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 do we make that distinction?
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:
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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 from which code points we start.
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.
6. Allocatable Variants
If we start with AAA, the permutation OOO will have been 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 that the third position is the original code
point ("-" means "no variant mapping").
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".
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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 variants
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 below).
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 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.
7. 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.
If we start with OOO, 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"
To additionally prevent allocating ABO as a variant label for AAA we
further 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.
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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.
8. 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, which is easily accomplished by use of the
"only-variants" qualifier, which requires that the label consist
entirely of variants and 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 -
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.
9. Reflexive Variants
But what if we started from AOA? We would expect OOO to be
allocatable, but the variant type set would be
OOO --> OOO: a - a
because the O is the original code point. Here is where we use a
reflexive mapping, by realizing that O is "the same as" O, which is
normally redundant, but allows us to specify a disposition on the
mapping
O a--> O
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with that, the variant type list for OOO --> 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.
10. 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
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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.
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
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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.
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 it 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 TLDs),
although certain code points have exceptionally complex variant
relations and may have an extra mapping.
11. 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
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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
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.
12. Handling Out-of-Repertoire Variants
At first it may seem counterintuitive to define variants that map to
code points 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.
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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)
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, again uses reflexive variants, but with a new
type of reflexive mapping of "out-of-repertoire-var", shown as
"r-o-->":
X r-o--> X
When paired with a suitable derivation, any label containing X 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 for any out-of
repertoire variants three entries are minimally required:
C x--> X : where C = elementOf( R) and X != elementOf( R)
X x--> C : where X = !elementOf( R) and C = elementOf( R)
X r-o--> X : where 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".
13. Conditional Variants
Variant mappings are based on whether code points are "the same" 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 only exists for some possible positions,
or more general, only for some contexts. For other contexts, the
variant mapping does not exist.
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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 it was 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 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
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verify symmetry and transitivity of the variant mappings in the LGR.
(This is true even though the two opposite contexts "final" and
"!final" should together cover all possible cases).
14. Conditional Variants and Well-Behaved LGRs
A well-behaved LGR with contextual variants always uses "fully
qualified" variant mappings and always agrees in the names of the
context rules for forward and reverse mappings. It also ensures 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 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 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 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.
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.
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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 in
principle possible 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 an essential tool, but some
additional care must be taken to ensure that an LGR containing
conditional contexts is well behaved.
15. Variants for Sequences
Variants 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.
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.
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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, which 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).
16. Corresponding XML Notation
The XML format defined in [RFC7940] corresponds fairly directly to
the notation used in this document. For example, a variant relation
of type "blocked"
C x--> X
is expressed as
where we assume that nnnn and mmmm are the [Unicode9] code point
values for C and X, respectively. A reflexive mapping always uses
the same code point value for and element, for example
X r-o--> X
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would correspond to
Multiple elements may be nested inside a single element,
but their "cp" values must be distinct (unless other distinguishing
attributes are present that are not discussed here).
A set of conditional variants like
final: C a--> K
!final: C b--> K
would correspond to
where the string "final" references a name of a context rule.
Context rules are defined in [RFC7940] and the details of how to
create and define them 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
Instead of using "if" and "else if" the 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].
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17. IANA Considerations
This document does not specify any IANA actions.
18. Security Considerations
There are no security considerations for this memo.
19. References
19.1. Normative References
[RFC7940] Davies, K. and A. Freytag, "Representing Label Generation
Rulesets Using XML", RFC 7940, DOI 10.17487/RFC7940,
August 2016, .
19.2. Informative References
[Procedure]
Internet Corporation for Assigned Names and Numbers,
"Procedure to Develop and Maintain the Label Generation
Rules for the Root Zone in Respect of IDNA Labels", 2013,
.
[Unicode9]
The Unicode Consortium, "The Unicode Standard, Version
9.0.0", ISBN 978-1-936213-13-9, 2016,
.
Preferred Citation: The Unicode Consortium. The Unicode
Standard, Version 9.0.0, (Mountain View, CA: The Unicode
Consortium, 2016. ISBN 978-1-936213-13-9)
Appendix A. Acknowledgements
Contributions that have shaped this document have been provided by
Marc Blanchet, Sarmad Hussain, Nicholas Ostler, Michel Suignard, and
Wil Tan.
Appendix B. Change Log
RFC Editor: Please remove this appendix before publication.
-00 Initial draft.
-01 Minor fix to references.
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-02 Some formattinga nd grammar issues as well as typos fixed.
Added a few real-world examples where required for context.
Added "r-n" to description of subtyping.
Author's Address
Asmus Freytag
Email: asmus@unicode.org
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