5. IDNs and the Robustness Principle
The "Robustness Principle" is often stated as "Be conservative about
what you send and liberal in what you accept" (see, e.g., Section
1.2.2 of the applications-layer Host Requirements specification
[RFC1123]). This principle applies to IDNA. In applying the
principle to registries as the source ("sender") of all registered
and useful IDNs, registries are responsible for being conservative
about what they register and put out in the Internet. For IDNs to
work well, zone administrators (registries) must have and require
sensible policies about what is registered -- conservative policies
-- and implement and enforce them.
Conversely, lookup applications are expected to reject labels that
clearly violate global (protocol) rules (no one has ever seriously
claimed that being liberal in what is accepted requires being
stupid). However, once one gets past such global rules and deals
with anything sensitive to script or locale, it is necessary to
assume that garbage has not been placed into the DNS, i.e., one must
be liberal about what one is willing to look up in the DNS rather
than guessing about whether it should have been permitted to be
If a string cannot be successfully found in the DNS after the lookup
processing described here, it makes no difference whether it simply
wasn't registered or was prohibited by some rule at the registry.
Application implementers should be aware that where DNS wildcards are
used, the ability to successfully resolve a name does not guarantee
that it was actually registered.
6. Front-end and User Interface Processing for Lookup
Domain names may be identified and processed in many contexts. They
may be typed in by users themselves or embedded in an identifier such
as an email address, URI, or IRI. They may occur in running text or
be processed by one system after being provided in another. Systems
may try to normalize URLs to determine (or guess) whether a reference
is valid or if two references point to the same object without
actually looking the objects up (comparison without lookup is
necessary for URI types that are not intended to be resolved). Some
of these goals may be more easily and reliably satisfied than others.
While there are strong arguments for any domain name that is placed
"on the wire" -- transmitted between systems -- to be in the zero-
ambiguity forms of A-labels, it is inevitable that programs that
process domain names will encounter U-labels or variant forms.
An application that implements the IDNA protocol [RFC5891] will
always take any user input and convert it to a set of Unicode code
points. That user input may be acquired by any of several different
input methods, all with differing conversion processes to be taken
into consideration (e.g., typed on a keyboard, written by hand onto
some sort of digitizer, spoken into a microphone and interpreted by a
speech-to-text engine, etc.). The process of taking any particular
user input and mapping it into a Unicode code point may be a simple
one: if a user strikes the "A" key on a US English keyboard, without
any modifiers such as the "Shift" key held down, in order to draw a
Latin small letter A ("a"), many (perhaps most) modern operating
system input methods will produce to the calling application the code
point U+0061, encoded in a single octet.
Sometimes the process is somewhat more complicated: a user might
strike a particular set of keys to represent a combining macron
followed by striking the "A" key in order to draw a Latin small
letter A with a macron above it. Depending on the operating system,
the input method chosen by the user, and even the parameters with
which the application communicates with the input method, the result
might be the code point U+0101 (encoded as two octets in UTF-8 or
UTF-16, four octets in UTF-32, etc.), the code point U+0061 followed
by the code point U+0304 (again, encoded in three or more octets,
depending upon the encoding used) or even the code point U+FF41
followed by the code point U+0304 (and encoded in some form). These
examples leave aside the issue of operating systems and input methods
that do not use Unicode code points for their character set.
In every case, applications (with the help of the operating systems
on which they run and the input methods used) need to perform a
mapping from user input into Unicode code points.
IDNA2003 used a model whereby input was taken from the user, mapped
(via whatever input method mechanisms were used) to a set of Unicode
code points, and then further mapped to a set of Unicode code points
using the Nameprep profile [RFC3491]. In this procedure, there are
two separate mapping steps: first, a mapping done by the input method
(which might be controlled by the operating system, the application,
or some combination) and then a second mapping performed by the
Nameprep portion of the IDNA protocol. The mapping done in Nameprep
includes a particular mapping table to re-map some characters to
other characters, a particular normalization, and a set of prohibited
Note that the result of the two-step mapping process means that the
mapping chosen by the operating system or application in the first
step might differ significantly from the mapping supplied by the
Nameprep profile in the second step. This has advantages and
disadvantages. Of course, the second mapping regularizes what gets
looked up in the DNS, making for better interoperability between
implementations that use the Nameprep mapping. However, the
application or operating system may choose mappings in their input
methods, which when passed through the second (Nameprep) mapping
result in characters that are "surprising" to the end user.
The other important feature of IDNA2003 is that, with very few
exceptions, it assumes that any set of Unicode code points provided
to the Nameprep mapping can be mapped into a string of Unicode code
points that are "sensible", even if that means mapping some code
points to nothing (that is, removing the code points from the
string). This allowed maximum flexibility in input strings.
The present version of IDNA (IDNA2008) differs significantly in
approach from the original version. First and foremost, it does not
provide explicit mapping instructions. Instead, it assumes that the
application (perhaps via an operating system input method) will do
whatever mapping it requires to convert input into Unicode code
points. This has the advantage of giving flexibility to the
application to choose a mapping that is suitable for its user given
specific user requirements, and avoids the two-step mapping of the
original protocol. Instead of a mapping, IDNA2008 provides a set of
categories that can be used to specify the valid code points allowed
in a domain name.
In principle, an application ought to take user input of a domain
name and convert it to the set of Unicode code points that represent
the domain name the user intends. As a practical matter, of course,
determining user intent is a tricky business, so an application needs
to choose a reasonable mapping from user input. That may differ
based on the particular circumstances of a user, depending on locale,
language, type of input method, etc. It is up to the application to
make a reasonable choice.
7. Migration from IDNA2003 and Unicode Version Synchronization
7.1. Design Criteria
As mentioned above and in the IAB review and recommendations for IDNs
[RFC4690], two key goals of the IDNA2008 design are:
o to enable applications to be agnostic about whether they are being
run in environments supporting any Unicode version from 3.2
o to permit incrementally adding new characters, character groups,
scripts, and other character collections as they are incorporated
into Unicode, doing so without disruption and, in the long term,
without "heavy" processes (an IETF consensus process is required
by the IDNA2008 specifications and is expected to be required and
used until significant experience accumulates with IDNA operations
and new versions of Unicode).
7.1.1. Summary and Discussion of IDNA Validity Criteria
The general criteria for a label to be considered valid under IDNA
are (the actual rules are rigorously defined in the Protocol
[RFC5891] and Tables [RFC5892] documents):
o The characters are "letters", marks needed to form letters,
numerals, or other code points used to write words in some
language. Symbols, drawing characters, and various notational
characters are intended to be permanently excluded. There is no
evidence that they are important enough to Internet operations or
internationalization to justify expansion of domain names beyond
the general principle of "letters, digits, and hyphen".
(Additional discussion and rationale for the symbol decision
appears in Section 7.6.)
o Other than in very exceptional cases, e.g., where they are needed
to write substantially any word of a given language, punctuation
characters are excluded. The fact that a word exists is not proof
that it should be usable in a DNS label, and DNS labels are not
expected to be usable for multiple-word phrases (although they are
certainly not prohibited if the conventions and orthography of a
particular language cause that to be possible).
o Characters that are unassigned (have no character assignment at
all) in the version of Unicode being used by the registry or
application are not permitted, even on lookup. The issues
involved in this decision are discussed in Section 7.7.
o Any character that is mapped to another character by a current
version of NFKC is prohibited as input to IDNA (for either
registration or lookup). With a few exceptions, this principle
excludes any character mapped to another by Nameprep [RFC3491].
The principles above drive the design of rules that are specified
exactly in the Tables document. Those rules identify the characters
that are valid under IDNA. The rules themselves are normative, and
the tables are derived from them, rather than vice versa.
7.1.2. Labels in Registration
Any label registered in a DNS zone must be validated -- i.e., the
criteria for that label must be met -- in order for applications to
work as intended. This principle is not new. For example, since the
DNS was first deployed, zone administrators have been expected to
verify that names meet "hostname" requirements [RFC0952] where those
requirements are imposed by the expected applications. Other
applications contexts, such as the later addition of special service
location formats [RFC2782] imposed new requirements on zone
administrators. For zones that will contain IDNs, support for
Unicode version-independence requires restrictions on all strings
placed in the zone. In particular, for such zones (the exact rules
appear in Section 4 of the Protocol document [RFC5891]):
o Any label that appears to be an A-label, i.e., any label that
starts in "xn--", must be valid under IDNA, i.e., they must be
valid A-labels, as discussed in Section 2 above.
o The Unicode tables (i.e., tables of code points, character
classes, and properties) and IDNA tables (i.e., tables of
contextual rules such as those that appear in the Tables
document), must be consistent on the systems performing or
validating labels to be registered. Note that this does not
require that tables reflect the latest version of Unicode, only
that all tables used on a given system are consistent with each
Under this model, registry tables will need to be updated (both the
Unicode-associated tables and the tables of permitted IDN characters)
to enable a new script or other set of new characters. The registry
will not be affected by newer versions of Unicode, or newly
authorized characters, until and unless it wishes to support them.
The zone administrator is responsible for verifying validity for IDNA
as well as its local policies -- a more extensive set of checks than
are required for looking up the labels. Systems looking up or
resolving DNS labels, especially IDN DNS labels, must be able to
assume that applicable registration rules were followed for names
entered into the DNS.
7.1.3. Labels in Lookup
Any application processing a label through IDNA so it can be looked
up in a DNS zone is required to (the exact rules appear in Section 5
of the Protocol document [RFC5891]):
o Maintain IDNA and Unicode tables that are consistent with regard
to versions, i.e., unless the application actually executes the
classification rules in the Tables document [RFC5892], its IDNA
tables must be derived from the version of Unicode that is
supported more generally on the system. As with registration, the
tables need not reflect the latest version of Unicode, but they
must be consistent.
o Validate the characters in labels to be looked up only to the
extent of determining that the U-label does not contain
"DISALLOWED" code points or code points that are unassigned in its
version of Unicode.
o Validate the label itself for conformance with a small number of
whole-label rules. In particular, it must verify that:
* there are no leading combining marks,
* the Bidi conditions are met if right-to-left characters appear,
* any required contextual rules are available, and
* any contextual rules that are associated with joiner characters
(and CONTEXTJ characters more generally) are tested.
o Do not reject labels based on other contextual rules about
characters, including mixed-script label prohibitions. Such rules
may be used to influence presentation decisions in the user
interface, but not to avoid looking up domain names.
To further clarify the rules about handling characters that require
contextual rules, note that one can have a context-required character
(i.e., one that requires a rule), but no rule. In that case, the
character is treated the same way DISALLOWED characters are treated,
until and unless a rule is supplied. That state is more or less
equivalent to "the idea of permitting this character is accepted in
principle, but it won't be permitted in practice until consensus is
reached on a safe way to use it".
The ability to add a rule more or less exempts these characters from
the prohibition against reclassifying characters from DISALLOWED to
And, obviously, "no rule" is different from "have a rule, but the
test either succeeds or fails".
Lookup applications that follow these rules, rather than having their
own criteria for rejecting lookup attempts, are not sensitive to
version incompatibilities with the particular zone registry
associated with the domain name except for labels containing
characters recently added to Unicode.
An application or client that processes names according to this
protocol and then resolves them in the DNS will be able to locate any
name that is registered, as long as those registrations are valid
under IDNA and its version of the IDNA tables is sufficiently up to
date to interpret all of the characters in the label. Messages to
users should distinguish between "label contains an unallocated code
point" and other types of lookup failures. A failure on the basis of
an old version of Unicode may lead the user to a desire to upgrade to
a newer version, but will have no other ill effects (this is
consistent with behavior in the transition to the DNS when some hosts
could not yet handle some forms of names or record types).
7.2. Changes in Character Interpretations
As a consequence of the elimination of mapping, the current version
of IDNA changes the interpretation of a few characters relative to
its predecessors. This subsection outlines the issues and discusses
possible transition strategies.
7.2.1. Character Changes: Eszett and Final Sigma
In those scripts that make case distinctions, there are a few
characters for which an obvious and unique uppercase character has
not historically been available to match a lowercase one, or vice
versa. For those characters, the mappings used in constructing the
Stringprep tables for IDNA2003, performed using the Unicode
toCaseFold operation (see Section 5.18 of the Unicode Standard
[Unicode52]), generate different characters or sets of characters.
Those operations are not reversible and lose even more information
than traditional uppercase or lowercase transformations, but are more
useful than those transformations for comparison purposes. Two
notable characters of this type are the German character Eszett
(Sharp S, U+00DF) and the Greek Final Form Sigma (U+03C2). The
former is case folded to the ASCII string "ss", the latter to a
medial (lowercase) Sigma (U+03C3).
7.2.2. Character Changes: Zero Width Joiner and Zero Width Non-Joiner
IDNA2003 mapped both ZERO WIDTH JOINER (ZWJ, U+200D) and ZERO WIDTH
NON-JOINER (ZWNJ, U+200C) to nothing, effectively dropping these
characters from any label in which they appeared and treating strings
containing them as identical to strings that did not. As discussed
in Section 3.1.2 above, those characters are essential for writing
many reasonable mnemonics for certain scripts. However, treating
them as valid in IDNA2008, even with contextual restrictions, raises
approximately the same problem as exists with Eszett and Final Sigma:
strings that were valid under IDNA2003 have different interpretations
as labels, and different A-labels, than the same strings under this
7.2.3. Character Changes and the Need for Transition
The decision to eliminate mandatory and standardized mappings,
including case folding, from the IDNA2008 protocol in order to make
A-labels and U-labels idempotent made these characters problematic.
If they were to be disallowed, important words and mnemonics could
not be written in orthographically reasonable ways. If they were to
be permitted as distinct characters, there would be no information
loss and registries would have more flexibility, but IDNA2003 and
IDNA2008 lookups might result in different A-labels.
With the understanding that there would be incompatibility either way
but a judgment that the incompatibility was not significant enough to
justify a prefix change, the Working Group concluded that Eszett and
Final Form Sigma should be treated as distinct and Protocol-Valid
Since these characters are interpreted in different ways under the
older and newer versions of IDNA, transition strategies and policies
will be necessary. Some actions can reasonably be taken by
applications' client programs (those that perform lookup operations
or cause them to be performed), but because of the diversity of
situations and uses of the DNS, much of the responsibility will need
to fall on registries.
Registries, especially those maintaining zones for third parties,
must decide how to introduce a new service in a way that does not
create confusion or significantly weaken or invalidate existing
identifiers. This is not a new problem; registries were faced with
similar issues when IDNs were introduced (potentially, and especially
for Latin-based scripts, in conflict with existing labels that had
been rendered in ASCII characters by applying more or less
standardized conventions) and when other new forms of strings have
been permitted as labels.
7.2.4. Transition Strategies
There are several approaches to the introduction of new characters or
changes in interpretation of existing characters from their mapped
forms in the earlier version of IDNA. The transition issue is
complicated because the forms of these labels after the
ToUnicode(ToASCII()) translation in IDNA2003 not only remain valid
but do not provide strong indications of what the registrant
intended: a string containing "ss" could have simply been intended to
be that string or could have been intended to contain an Eszett; a
string containing lowercase Sigma could have been intended to contain
Final Sigma (one might make heuristic guesses based on position in a
string, but the long tradition of forming labels by concatenating
words makes such heuristics unreliable), and strings that do not
contain ZWJ or ZWNJ might have been intended to contain them.
Without any preference or claim to completeness, some of these, all
of which have been used by registries in the past for similar
1. Do not permit use of the newly available character at the
registry level. This might cause lookup failures if a domain
name were to be written with the expectation of the IDNA2003
mapping behavior, but would eliminate any possibility of false
2. Hold a "sunrise"-like arrangement in which holders of labels
containing "ss" in the Eszett case, lowercase Sigma in that case,
or that might have contained ZWJ or ZWNJ in context, are given
priority (and perhaps other benefits) for registering the
corresponding string containing Eszett, Final Sigma, or the
appropriate zero-width character respectively.
3. Adopt some sort of "variant" approach in which registrants obtain
labels with both character forms.
4. Adopt a different form of "variant" approach in which
registration of additional strings that would produce the same
A-label if interpreted according to IDNA2003 is either not
permitted at all or permitted only by the registrant who already
has one of the names.
5. Ignore the issue and assume that the marketplace or other
mechanisms will sort things out.
In any event, a registry (at any level of the DNS tree) that chooses
to permit labels to be registered that contains these characters, or
considers doing so, will have to address the relationship with
existing, possibly conflicting, labels in some way, just as
registries that already had a considerable number of labels did when
IDNs were first introduced.
7.3. Elimination of Character Mapping
As discussed at length in Section 6, IDNA2003, via Nameprep (see
Section 7.5), mapped many characters into related ones. Those
mappings no longer exist as requirements in IDNA2008. These
specifications strongly prefer that only A-labels or U-labels be used
in protocol contexts and as much as practical more generally.
IDNA2008 does anticipate situations in which some mapping at the time
of user input into lookup applications is appropriate and desirable.
The issues are discussed in Section 6 and specific recommendations
are made in the Mapping document [IDNA2008-Mapping].
7.4. The Question of Prefix Changes
The conditions that would have required a change in the IDNA ACE
prefix ("xn--", used in IDNA2003) were of great concern to the
community. A prefix change would have clearly been necessary if the
algorithms were modified in a manner that would have created serious
ambiguities during subsequent transition in registrations. This
section summarizes the working group's conclusions about the
conditions under which a change in the prefix would have been
necessary and the implications of such a change.
7.4.1. Conditions Requiring a Prefix Change
An IDN prefix change would have been needed if a given string would
be looked up or otherwise interpreted differently depending on the
version of the protocol or tables being used. This IDNA upgrade
would have required a prefix change if, and only if, one of the
following four conditions were met:
1. The conversion of an A-label to Unicode (i.e., a U-label) would
have yielded one string under IDNA2003 and a different string
2. In a significant number of cases, an input string that was valid
under IDNA2003 and also valid under IDNA2008 would have yielded
two different A-labels with the different versions. This
condition is believed to be essentially equivalent to the one
above except for a very small number of edge cases that were not
found to justify a prefix change (see Section 7.2).
Note that if the input string was valid under one version and not
valid under the other, this condition would not apply. See the
first item in Section 7.4.2, below.
3. A fundamental change was made to the semantics of the string that
would be inserted in the DNS, e.g., if a decision were made to
try to include language or script information in the encoding in
addition to the string itself.
4. A sufficiently large number of characters were added to Unicode
so that the Punycode mechanism for block offsets would no longer
reference the higher-numbered planes and blocks. This condition
is unlikely even in the long term and certain not to arise in the
next several years.
7.4.2. Conditions Not Requiring a Prefix Change
As a result of the principles described above, none of the following
changes required a new prefix:
1. Prohibition of some characters as input to IDNA. Such a
prohibition might make names that were previously registered
inaccessible, but did not change those names.
2. Adjustments in IDNA tables or actions, including normalization
definitions, that affected characters that were already invalid
3. Changes in the style of the IDNA definition that did not alter
the actions performed by IDNA.
7.4.3. Implications of Prefix Changes
While it might have been possible to make a prefix change, the costs
of such a change are considerable. Registries could not have
converted all IDNA2003 ("xn--") registrations to a new form at the
same time and synchronize that change with applications supporting
lookup. Unless all existing registrations were simply to be declared
invalid (and perhaps even then), systems that needed to support both
labels with old prefixes and labels with new ones would be required
to first process a putative label under the IDNA2008 rules and try to
look it up and then, if it were not found, would be required to
process the label under IDNA2003 rules and look it up again. That
process would probably have significantly slowed down all processing
that involved IDNs in the DNS, especially since a fully-qualified
name might contain a mixture of labels that were registered with the
old and new prefixes. That would have made DNS caching very
difficult. In addition, looking up the same input string as two
separate A-labels would have created some potential for confusion and
attacks, since the labels could map to different targets and then
resolve to different entries in the DNS.
Consequently, a prefix change should have been, and was, avoided if
at all possible, even if it means accepting some IDNA2003 decisions
about character distinctions as irreversible and/or giving special
treatment to edge cases.
7.5. Stringprep Changes and Compatibility
The Nameprep specification [RFC3491], a key part of IDNA2003, is a
profile of Stringprep [RFC3454]. While Nameprep is a Stringprep
profile specific to IDNA, Stringprep is used by a number of other
protocols. Were Stringprep to have been modified by IDNA2008, those
changes to improve the handling of IDNs could cause problems for
non-DNS uses, most notably if they affected identification and
authentication protocols. Several elements of IDNA2008 give
interpretations to strings prohibited under IDNA2003 or prohibit
strings that IDNA2003 permitted. Those elements include the new
inclusion information in the Tables document [RFC5892], the reduction
in the number of characters permitted as input for registration or
lookup (Section 3), and even the changes in handling of right-to-left
strings as described in the Bidi document [RFC5893]. IDNA2008 does
not use Nameprep or Stringprep at all, so there are no side-effect
changes to other protocols.
It is particularly important to keep IDNA processing separate from
processing for various security protocols because some of the
constraints that are necessary for smooth and comprehensible use of
IDNs may be unwanted or undesirable in other contexts. For example,
the criteria for good passwords or passphrases are very different
from those for desirable IDNs: passwords should be hard to guess,
while domain names should normally be easily memorable. Similarly,
internationalized Small Computer System Interface (SCSI) identifiers
and other protocol components are likely to have different
requirements than IDNs.
7.6. The Symbol Question
One of the major differences between this specification and the
original version of IDNA is that IDNA2003 permitted non-letter
symbols of various sorts, including punctuation and line-drawing
symbols, in the protocol. They were always discouraged in practice.
In particular, both the "IESG Statement" about IDNA and all versions
of the ICANN Guidelines specify that only language characters be used
in labels. This specification disallows symbols entirely. There are
several reasons for this, which include:
1. As discussed elsewhere, the original IDNA specification assumed
that as many Unicode characters as possible should be permitted,
directly or via mapping to other characters, in IDNs. This
specification operates on an inclusion model, extrapolating from
the original "hostname" rules (LDH, see the Definitions document
[RFC5890]) -- which have served the Internet very well -- to a
Unicode base rather than an ASCII base.
2. Symbol names are more problematic than letters because there may
be no general agreement on whether a particular glyph matches a
symbol; there are no uniform conventions for naming; variations
such as outline, solid, and shaded forms may or may not exist;
and so on. As just one example, consider a "heart" symbol as it
might appear in a logo that might be read as "I love...". While
the user might read such a logo as "I love..." or "I heart...",
considerable knowledge of the coding distinctions made in Unicode
is needed to know that there is more than one "heart" character
(e.g., U+2665, U+2661, and U+2765) and how to describe it. These
issues are of particular importance if strings are expected to be
understood or transcribed by the listener after being read out
3. Design of a screen reader used by blind Internet users who must
listen to renderings of IDN domain names and possibly reproduce
them on the keyboard becomes considerably more complicated when
the names of characters are not obvious and intuitive to anyone
familiar with the language in question.
4. As a simplified example of this, assume one wanted to use a
"heart" or "star" symbol in a label. This is problematic because
those names are ambiguous in the Unicode system of naming (the
actual Unicode names require far more qualification). A user or
would-be registrant has no way to know -- absent careful study of
the code tables -- whether it is ambiguous (e.g., where there are
multiple "heart" characters) or not. Conversely, the user seeing
the hypothetical label doesn't know whether to read it -- try to
transmit it to a colleague by voice -- as "heart", as "love", as
"black heart", or as any of the other examples below.
5. The actual situation is even worse than this. There is no
possible way for a normal, casual, user to tell the difference
between the hearts of U+2665 and U+2765 and the stars of U+2606
and U+2729 without somehow knowing to look for a distinction. We
have a white heart (U+2661) and few black hearts. Consequently,
describing a label as containing a heart is hopelessly ambiguous:
we can only know that it contains one of several characters that
look like hearts or have "heart" in their names. In cities where
"Square" is a popular part of a location name, one might well
want to use a square symbol in a label as well and there are far
more squares of various flavors in Unicode than there are hearts
The consequence of these ambiguities is that symbols are a very poor
basis for reliable communication. Consistent with this conclusion,
the Unicode standard recommends that strings used in identifiers not
contain symbols or punctuation [Unicode-UAX31]. Of course, these
difficulties with symbols do not arise with actual pictographic
languages and scripts which would be treated like any other language
characters; the two should not be confused.
7.7. Migration between Unicode Versions: Unassigned Code Points
In IDNA2003, labels containing unassigned code points are looked up
on the assumption that, if they appear in labels and can be mapped
and then resolved, the relevant standards must have changed and the
registry has properly allocated only assigned values.
In the IDNA2008 protocol, strings containing unassigned code points
must not be either looked up or registered. In summary, the status
of an unassigned character with regard to the DISALLOWED,
PROTOCOL-VALID, and CONTEXTUAL RULE REQUIRED categories cannot be
evaluated until a character is actually assigned and known. There
are several reasons for this, with the most important ones being:
o Tests involving the context of characters (e.g., some characters
being permitted only adjacent to others of specific types) and
integrity tests on complete labels are needed. Unassigned code
points cannot be permitted because one cannot determine whether
particular code points will require contextual rules (and what
those rules should be) before characters are assigned to them and
the properties of those characters fully understood.
o It cannot be known in advance, and with sufficient reliability,
whether a newly assigned code point will be associated with a
character that would be disallowed by the rules in the Tables
document [RFC5892] (such as a compatibility character). In
IDNA2003, since there is no direct dependency on NFKC (many of the
entries in Stringprep's tables are based on NFKC, but IDNA2003
depends only on Stringprep), allocation of a compatibility
character might produce some odd situations, but it would not be a
problem. In IDNA2008, where compatibility characters are
DISALLOWED unless character-specific exceptions are made,
permitting strings containing unassigned characters to be looked
up would violate the principle that characters in DISALLOWED are
not looked up.
o The Unicode Standard specifies that an unassigned code point
normalizes (and, where relevant, case folds) to itself. If the
code point is later assigned to a character, and particularly if
the newly assigned code point has a combining class that
determines its placement relative to other combining characters,
it could normalize to some other code point or sequence.
It is possible to argue that the issues above are not important and
that, as a consequence, it is better to retain the principle of
looking up labels even if they contain unassigned characters because
all of the important scripts and characters have been coded as of
Unicode 5.2 (or even earlier), and hence unassigned code points will
be assigned only to obscure characters or archaic scripts.
Unfortunately, that does not appear to be a safe assumption for at
least two reasons. First, much the same claim of completeness has
been made for earlier versions of Unicode. The reality is that a
script that is obscure to much of the world may still be very
important to those who use it. Cultural and linguistic preservation
principles make it inappropriate to declare the script of no
importance in IDNs. Second, we already have counterexamples, e.g.,
in the relationships associated with new Han characters being added
(whether in the BMP or in Unicode Plane 2).
Independent of the technical transition issues identified above, it
can be observed that any addition of characters to an existing script
to make it easier to use or to better accommodate particular
languages may lead to transition issues. Such additions may change
the preferred form for writing a particular string, changes that may
be reflected, e.g., in keyboard transition modules that would
necessarily be different from those for earlier versions of Unicode
where the newer characters may not exist. This creates an inherent
transition problem because attempts to access labels may use either
the old or the new conventions, requiring registry action whether or
not the older conventions were used in labels. The need to consider
transition mechanisms is inherent to evolution of Unicode to better
accommodate writing systems and is independent of how IDNs are
represented in the DNS or how transitions among versions of those
mechanisms occur. The requirement for transitions of this type is
illustrated by the addition of Malayalam Chillu in Unicode 5.1.0.
7.8. Other Compatibility Issues
The 2003 IDNA model includes several odd artifacts of the context in
which it was developed. Many, if not all, of these are potential
avenues for exploits, especially if the registration process permits
"source" names (names that have not been processed through IDNA and
Nameprep) to be registered. As one example, since the character
Eszett, used in German, is mapped by IDNA2003 into the sequence "ss"
rather than being retained as itself or prohibited, a string
containing that character, but that is otherwise in ASCII, is not
really an IDN (in the U-label sense defined above). After Nameprep
maps out the Eszett, the result is an ASCII string and so it does not
get an xn-- prefix, but the string that can be displayed to a user
appears to be an IDN. IDNA2008 eliminates this artifact. A
character is either permitted as itself or it is prohibited; special
cases that make sense only in a particular linguistic or cultural
context can be dealt with as localization matters where appropriate.
8. Name Server Considerations
8.1. Processing Non-ASCII Strings
Existing DNS servers do not know the IDNA rules for handling
non-ASCII forms of IDNs, and therefore need to be shielded from them.
All existing channels through which names can enter a DNS server
database (for example, master files (as described in RFC 1034) and
DNS update messages [RFC2136]) could not be IDNA-aware because they
predate IDNA. Other sections of this document provide the needed
shielding by ensuring that internationalized domain names entering
DNS server databases through such channels have already been
converted to their equivalent ASCII A-label forms.
Because of the distinction made between the algorithms for
Registration and Lookup in Sections 4 and 5 (respectively) of the
Protocol document [RFC5891] (a domain name containing only ASCII code
points cannot be converted to an A-label), there cannot be more than
one A-label form for any given U-label.
As specified in clarifications to the DNS specification [RFC2181],
the DNS protocol explicitly allows domain labels to contain octets
beyond the ASCII range (0000..007F), and this document does not
change that. However, although the interpretation of octets
0080..00FF is well-defined in the DNS, many application protocols
support only ASCII labels and there is no defined interpretation of
these non-ASCII octets as characters and, in particular, no
interpretation of case-independent matching for them (e.g., see the
clarification on DNS case insensitivity [RFC4343]). If labels
containing these octets are returned to applications, unpredictable
behavior could result. The A-label form, which cannot contain those
characters, is the only standard representation for internationalized
labels in the DNS protocol.
8.2. Root and Other DNS Server Considerations
IDNs in A-label form will generally be somewhat longer than current
domain names, so the bandwidth needed by the root servers is likely
to go up by a small amount. Also, queries and responses for IDNs
will probably be somewhat longer than typical queries historically,
so Extension Mechanisms for DNS (EDNS0) [RFC2671] support may be more
important (otherwise, queries and responses may be forced to go to
TCP instead of UDP).
9. Internationalization Considerations
DNS labels and fully-qualified domain names provide mnemonics that
assist in identifying and referring to resources on the Internet.
IDNs expand the range of those mnemonics to include those based on
languages and character sets other than Western European and Roman-
derived ones. But domain "names" are not, in general, words in any
language. The recommendations of the IETF policy on character sets
and languages (BCP 18 [RFC2277]) are applicable to situations in
which language identification is used to provide language-specific
contexts. The DNS is, by contrast, global and international and
ultimately has nothing to do with languages. Adding languages (or
similar context) to IDNs generally, or to DNS matching in particular,
would imply context-dependent matching in DNS, which would be a very
significant change to the DNS protocol itself. It would also imply
that users would need to identify the language associated with a
particular label in order to look that label up. That knowledge is
generally not available because many labels are not words in any
language and some may be words in more than one.
10. IANA Considerations
This section gives an overview of IANA registries required for IDNA.
The actual definitions of, and specifications for, the first two,
which have been newly created for IDNA2008, appear in the Tables
document [RFC5892]. This document describes the registries, but it
does not specify any IANA actions.
10.1. IDNA Character Registry
The distinction among the major categories "UNASSIGNED",
"DISALLOWED", "PROTOCOL-VALID", and "CONTEXTUAL RULE REQUIRED" is
made by special categories and rules that are integral elements of
the Tables document. While not normative, an IANA registry of
characters and scripts and their categories, updated for each new
version of Unicode and the characters it contains, are convenient for
programming and validation purposes. The details of this registry
are specified in the Tables document.
10.2. IDNA Context Registry
IANA has created and now maintains a list of approved contextual
rules for characters that are defined in the IDNA Character Registry
list as requiring a Contextual Rule (i.e., the types of rules
described in Section 3.1.2). The details for those rules appear in
the Tables document.
10.3. IANA Repository of IDN Practices of TLDs
This registry, historically described as the "IANA Language Character
Set Registry" or "IANA Script Registry" (both somewhat misleading
terms), is maintained by IANA at the request of ICANN. It is used to
provide a central documentation repository of the IDN policies used
by top level domain (TLD) registries who volunteer to contribute to
it and is used in conjunction with ICANN Guidelines for IDN use.
It is not an IETF-managed registry and, while the protocol changes
specified here may call for some revisions to the tables, IDNA2008
has no direct effect on that registry and no IANA action is required
as a result.
11. Security Considerations
11.1. General Security Issues with IDNA
This document is purely explanatory and informational and
consequently introduces no new security issues. It would, of course,
be a poor idea for someone to try to implement from it; such an
attempt would almost certainly lead to interoperability problems and
might lead to security ones. A discussion of security issues with
IDNA, including some relevant history, appears in the Definitions
The editor and contributors would like to express their thanks to
those who contributed significant early (pre-working group) review
comments, sometimes accompanied by text, Paul Hoffman, Simon
Josefsson, and Sam Weiler. In addition, some specific ideas were
incorporated from suggestions, text, or comments about sections that
were unclear supplied by Vint Cerf, Frank Ellerman, Michael Everson,
Asmus Freytag, Erik van der Poel, Michel Suignard, and Ken Whistler.
Thanks are also due to Vint Cerf, Lisa Dusseault, Debbie Garside, and
Jefsey Morfin for conversations that led to considerable improvements
in the content of this document and to several others, including Ben
Campbell, Martin Duerst, Subramanian Moonesamy, Peter Saint-Andre,
and Dan Winship, for catching specific errors and recommending
A meeting was held on 30 January 2008 to attempt to reconcile
differences in perspective and terminology about this set of
specifications between the design team and members of the Unicode
Technical Consortium. The discussions at and subsequent to that
meeting were very helpful in focusing the issues and in refining the
specifications. The active participants at that meeting were (in
alphabetic order, as usual) Harald Alvestrand, Vint Cerf, Tina Dam,
Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary
Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel,
Michel Suignard, and Ken Whistler. We express our thanks to Google
for support of that meeting and to the participants for their
Useful comments and text on the working group versions of the working
draft were received from many participants in the IETF "IDNABIS"
working group and a number of document changes resulted from mailing
list discussions made by that group. Marcos Sanz provided specific
analysis and suggestions that were exceptionally helpful in refining
the text, as did Vint Cerf, Martin Duerst, Andrew Sullivan, and Ken
Whistler. Lisa Dusseault provided extensive editorial suggestions
during the spring of 2009, most of which were incorporated.
While the listed editor held the pen, the core of this document and
the initial working group version represents the joint work and
conclusions of an ad hoc design team consisting of the editor and, in
alphabetic order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and
Cary Karp. Considerable material describing mapping principles has
been incorporated from a draft of the Mapping document
[IDNA2008-Mapping] by Pete Resnick and Paul Hoffman. In addition,
there were many specific contributions and helpful comments from
those listed in the Acknowledgments section and others who have
contributed to the development and use of the IDNA protocols.
14.1. Normative References
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications
(IDNA)", RFC 3490, March 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of
Unicode for Internationalized Domain Names in
Applications (IDNA)", RFC 3492, March 2003.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document
Framework", RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC5892] Faltstrom, P., "The Unicode Code Points and
Internationalized Domain Names for Applications (IDNA)",
RFC 5892, August 2010.
[RFC5893] Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
Internationalized Domain Names for Applications (IDNA)",
RFC 5893, August 2010.
[Unicode52] The Unicode Consortium. The Unicode Standard, Version
5.2.0, defined by: "The Unicode Standard, Version
5.2.0", (Mountain View, CA: The Unicode Consortium,
2009. ISBN 978-1-936213-00-9).
14.2. Informative References
Resnick, P. and P. Hoffman, "Mapping Characters in
Internationalized Domain Names for Applications (IDNA)",
Work in Progress, April 2010.
[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD
Internet host table specification", RFC 952,
[RFC1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, October 1989.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS
UPDATE)", RFC 2136, April 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)",
RFC 2782, February 2000.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC3743] Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
Engineering Team (JET) Guidelines for Internationalized
Domain Names (IDN) Registration and Administration for
Chinese, Japanese, and Korean", RFC 3743, April 2004.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4290] Klensin, J., "Suggested Practices for Registration of
Internationalized Domain Names (IDN)", RFC 4290,
[RFC4343] Eastlake, D., "Domain Name System (DNS) Case
Insensitivity Clarification", RFC 4343, January 2006.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review
and Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
[RFC4713] Lee, X., Mao, W., Chen, E., Hsu, N., and J. Klensin,
"Registration and Administration Recommendations for
Chinese Domain Names", RFC 4713, October 2006.