Network Working Group P. Faltstrom
Request for Comments: 3490 Cisco
Category: Standards Track P. Hoffman
 IMC & VPNC
 A. Costello
 UC Berkeley
 March 2003
 Internationalizing Domain Names in Applications (IDNA)
Status of this Memo
 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements. Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
 Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
 Until now, there has been no standard method for domain names to use
 characters outside the ASCII repertoire. This document defines
 internationalized domain names (IDNs) and a mechanism called
 Internationalizing Domain Names in Applications (IDNA) for handling
 them in a standard fashion. IDNs use characters drawn from a large
 repertoire (Unicode), but IDNA allows the non-ASCII characters to be
 represented using only the ASCII characters already allowed in so-
 called host names today. This backward-compatible representation is
 required in existing protocols like DNS, so that IDNs can be
 introduced with no changes to the existing infrastructure. IDNA is
 only meant for processing domain names, not free text.
Table of Contents
 1. Introduction.................................................. 2
 1.1 Problem Statement......................................... 3
 1.2 Limitations of IDNA....................................... 3
 1.3 Brief overview for application developers................. 4
 2. Terminology................................................... 5
 3. Requirements and applicability................................ 7
 3.1 Requirements.............................................. 7
 3.2 Applicability............................................. 8
 3.2.1. DNS resource records................................ 8
 3.2.2. Non-domain-name data types stored in domain names... 9
 4. Conversion operations......................................... 9
 4.1 ToASCII................................................... 10
 4.2 ToUnicode................................................. 11
 5. ACE prefix.................................................... 12
 6. Implications for typical applications using DNS............... 13
 6.1 Entry and display in applications......................... 14
 6.2 Applications and resolver libraries....................... 15
 6.3 DNS servers............................................... 15
 6.4 Avoiding exposing users to the raw ACE encoding........... 16
 6.5 DNSSEC authentication of IDN domain names................ 16
 7. Name server considerations.................................... 17
 8. Root server considerations.................................... 17
 9. References.................................................... 18
 9.1 Normative References...................................... 18
 9.2 Informative References.................................... 18
 10. Security Considerations...................................... 19
 11. IANA Considerations.......................................... 20
 12. Authors' Addresses........................................... 21
 13. Full Copyright Statement..................................... 22
1. Introduction
 IDNA works by allowing applications to use certain ASCII name labels
 (beginning with a special prefix) to represent non-ASCII name labels.
 Lower-layer protocols need not be aware of this; therefore IDNA does
 not depend on changes to any infrastructure. In particular, IDNA
 does not depend on any changes to DNS servers, resolvers, or protocol
 elements, because the ASCII name service provided by the existing DNS
 is entirely sufficient for IDNA.
 This document does not require any applications to conform to IDNA,
 but applications can elect to use IDNA in order to support IDN while
 maintaining interoperability with existing infrastructure. If an
 application wants to use non-ASCII characters in domain names, IDNA
 is the only currently-defined option. Adding IDNA support to an
 existing application entails changes to the application only, and
 leaves room for flexibility in the user interface.
 A great deal of the discussion of IDN solutions has focused on
 transition issues and how IDN will work in a world where not all of
 the components have been updated. Proposals that were not chosen by
 the IDN Working Group would depend on user applications, resolvers,
 and DNS servers being updated in order for a user to use an
 internationalized domain name. Rather than rely on widespread
 updating of all components, IDNA depends on updates to user
 applications only; no changes are needed to the DNS protocol or any
 DNS servers or the resolvers on user's computers.
1.1 Problem Statement
 The IDNA specification solves the problem of extending the repertoire
 of characters that can be used in domain names to include the Unicode
 repertoire (with some restrictions).
 IDNA does not extend the service offered by DNS to the applications.
 Instead, the applications (and, by implication, the users) continue
 to see an exact-match lookup service. Either there is a single
 exactly-matching name or there is no match. This model has served
 the existing applications well, but it requires, with or without
 internationalized domain names, that users know the exact spelling of
 the domain names that the users type into applications such as web
 browsers and mail user agents. The introduction of the larger
 repertoire of characters potentially makes the set of misspellings
 larger, especially given that in some cases the same appearance, for
 example on a business card, might visually match several Unicode code
 points or several sequences of code points.
 IDNA allows the graceful introduction of IDNs not only by avoiding
 upgrades to existing infrastructure (such as DNS servers and mail
 transport agents), but also by allowing some rudimentary use of IDNs
 in applications by using the ASCII representation of the non-ASCII
 name labels. While such names are very user-unfriendly to read and
 type, and hence are not suitable for user input, they allow (for
 instance) replying to email and clicking on URLs even though the
 domain name displayed is incomprehensible to the user. In order to
 allow user-friendly input and output of the IDNs, the applications
 need to be modified to conform to this specification.
 IDNA uses the Unicode character repertoire, which avoids the
 significant delays that would be inherent in waiting for a different
 and specific character set be defined for IDN purposes by some other
 standards developing organization.
1.2 Limitations of IDNA
 The IDNA protocol does not solve all linguistic issues with users
 inputting names in different scripts. Many important language-based
 and script-based mappings are not covered in IDNA and need to be
 handled outside the protocol. For example, names that are entered in
 a mix of traditional and simplified Chinese characters will not be
 mapped to a single canonical name. Another example is Scandinavian
 names that are entered with U+00F6 (LATIN SMALL LETTER O WITH
 DIAERESIS) will not be mapped to U+00F8 (LATIN SMALL LETTER O WITH
 STROKE).
 An example of an important issue that is not considered in detail in
 IDNA is how to provide a high probability that a user who is entering
 a domain name based on visual information (such as from a business
 card or billboard) or aural information (such as from a telephone or
 radio) would correctly enter the IDN. Similar issues exist for ASCII
 domain names, for example the possible visual confusion between the
 letter 'O' and the digit zero, but the introduction of the larger
 repertoire of characters creates more opportunities of similar
 looking and similar sounding names. Note that this is a complex
 issue relating to languages, input methods on computers, and so on.
 Furthermore, the kind of matching and searching necessary for a high
 probability of success would not fit the role of the DNS and its
 exact matching function.
1.3 Brief overview for application developers
 Applications can use IDNA to support internationalized domain names
 anywhere that ASCII domain names are already supported, including DNS
 master files and resolver interfaces. (Applications can also define
 protocols and interfaces that support IDNs directly using non-ASCII
 representations. IDNA does not prescribe any particular
 representation for new protocols, but it still defines which names
 are valid and how they are compared.)
 The IDNA protocol is contained completely within applications. It is
 not a client-server or peer-to-peer protocol: everything is done
 inside the application itself. When used with a DNS resolver
 library, IDNA is inserted as a "shim" between the application and the
 resolver library. When used for writing names into a DNS zone, IDNA
 is used just before the name is committed to the zone.
 There are two operations described in section 4 of this document:
 - The ToASCII operation is used before sending an IDN to something
 that expects ASCII names (such as a resolver) or writing an IDN
 into a place that expects ASCII names (such as a DNS master file).
 - The ToUnicode operation is used when displaying names to users,
 for example names obtained from a DNS zone.
 It is important to note that the ToASCII operation can fail. If it
 fails when processing a domain name, that domain name cannot be used
 as an internationalized domain name and the application has to have
 some method of dealing with this failure.
 IDNA requires that implementations process input strings with
 Nameprep [NAMEPREP], which is a profile of Stringprep [STRINGPREP],
 and then with Punycode [PUNYCODE]. Implementations of IDNA MUST
 fully implement Nameprep and Punycode; neither Nameprep nor Punycode
 are optional.
2. Terminology
 The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
 and "MAY" in this document are to be interpreted as described in BCP
 14, RFC 2119 [RFC2119].
 A code point is an integer value associated with a character in a
 coded character set.
 Unicode [UNICODE] is a coded character set containing tens of
 thousands of characters. A single Unicode code point is denoted by
 "U+" followed by four to six hexadecimal digits, while a range of
 Unicode code points is denoted by two hexadecimal numbers separated
 by "..", with no prefixes.
 ASCII means US-ASCII [USASCII], a coded character set containing 128
 characters associated with code points in the range 0..7F. Unicode
 is an extension of ASCII: it includes all the ASCII characters and
 associates them with the same code points.
 The term "LDH code points" is defined in this document to mean the
 code points associated with ASCII letters, digits, and the hyphen-
 minus; that is, U+002D, 30..39, 41..5A, and 61..7A. "LDH" is an
 abbreviation for "letters, digits, hyphen".
 [STD13] talks about "domain names" and "host names", but many people
 use the terms interchangeably. Further, because [STD13] was not
 terribly clear, many people who are sure they know the exact
 definitions of each of these terms disagree on the definitions. In
 this document the term "domain name" is used in general. This
 document explicitly cites [STD3] whenever referring to the host name
 syntax restrictions defined therein.
 A label is an individual part of a domain name. Labels are usually
 shown separated by dots; for example, the domain name
 "www.example.com" is composed of three labels: "www", "example", and
 "com". (The zero-length root label described in [STD13], which can
 be explicit as in "www.example.com." or implicit as in
 "www.example.com", is not considered a label in this specification.)
 IDNA extends the set of usable characters in labels that are text.
 For the rest of this document, the term "label" is shorthand for
 "text label", and "every label" means "every text label".
 An "internationalized label" is a label to which the ToASCII
 operation (see section 4) can be applied without failing (with the
 UseSTD3ASCIIRules flag unset). This implies that every ASCII label
 that satisfies the [STD13] length restriction is an internationalized
 label. Therefore the term "internationalized label" is a
 generalization, embracing both old ASCII labels and new non-ASCII
 labels. Although most Unicode characters can appear in
 internationalized labels, ToASCII will fail for some input strings,
 and such strings are not valid internationalized labels.
 An "internationalized domain name" (IDN) is a domain name in which
 every label is an internationalized label. This implies that every
 ASCII domain name is an IDN (which implies that it is possible for a
 name to be an IDN without it containing any non-ASCII characters).
 This document does not attempt to define an "internationalized host
 name". Just as has been the case with ASCII names, some DNS zone
 administrators may impose restrictions, beyond those imposed by DNS
 or IDNA, on the characters or strings that may be registered as
 labels in their zones. Such restrictions have no impact on the
 syntax or semantics of DNS protocol messages; a query for a name that
 matches no records will yield the same response regardless of the
 reason why it is not in the zone. Clients issuing queries or
 interpreting responses cannot be assumed to have any knowledge of
 zone-specific restrictions or conventions.
 In IDNA, equivalence of labels is defined in terms of the ToASCII
 operation, which constructs an ASCII form for a given label, whether
 or not the label was already an ASCII label. Labels are defined to
 be equivalent if and only if their ASCII forms produced by ToASCII
 match using a case-insensitive ASCII comparison. ASCII labels
 already have a notion of equivalence: upper case and lower case are
 considered equivalent. The IDNA notion of equivalence is an
 extension of that older notion. Equivalent labels in IDNA are
 treated as alternate forms of the same label, just as "foo" and "Foo"
 are treated as alternate forms of the same label.
 To allow internationalized labels to be handled by existing
 applications, IDNA uses an "ACE label" (ACE stands for ASCII
 Compatible Encoding). An ACE label is an internationalized label
 that can be rendered in ASCII and is equivalent to an
 internationalized label that cannot be rendered in ASCII. Given any
 internationalized label that cannot be rendered in ASCII, the ToASCII
 operation will convert it to an equivalent ACE label (whereas an
 ASCII label will be left unaltered by ToASCII). ACE labels are
 unsuitable for display to users. The ToUnicode operation will
 convert any label to an equivalent non-ACE label. In fact, an ACE
 label is formally defined to be any label that the ToUnicode
 operation would alter (whereas non-ACE labels are left unaltered by
 ToUnicode). Every ACE label begins with the ACE prefix specified in
 section 5. The ToASCII and ToUnicode operations are specified in
 section 4.
 The "ACE prefix" is defined in this document to be a string of ASCII
 characters that appears at the beginning of every ACE label. It is
 specified in section 5.
 A "domain name slot" is defined in this document to be a protocol
 element or a function argument or a return value (and so on)
 explicitly designated for carrying a domain name. Examples of domain
 name slots include: the QNAME field of a DNS query; the name argument
 of the gethostbyname() library function; the part of an email address
 following the at-sign (@) in the From: field of an email message
 header; and the host portion of the URI in the src attribute of an
 HTML <IMG> tag. General text that just happens to contain a domain
 name is not a domain name slot; for example, a domain name appearing
 in the plain text body of an email message is not occupying a domain
 name slot.
 An "IDN-aware domain name slot" is defined in this document to be a
 domain name slot explicitly designated for carrying an
 internationalized domain name as defined in this document. The
 designation may be static (for example, in the specification of the
 protocol or interface) or dynamic (for example, as a result of
 negotiation in an interactive session).
 An "IDN-unaware domain name slot" is defined in this document to be
 any domain name slot that is not an IDN-aware domain name slot.
 Obviously, this includes any domain name slot whose specification
 predates IDNA.
3. Requirements and applicability
3.1 Requirements
 IDNA conformance means adherence to the following four requirements:
 1) Whenever dots are used as label separators, the following
 characters MUST be recognized as dots: U+002E (full stop), U+3002
 (ideographic full stop), U+FF0E (fullwidth full stop), U+FF61
 (halfwidth ideographic full stop).
 2) Whenever a domain name is put into an IDN-unaware domain name slot
 (see section 2), it MUST contain only ASCII characters. Given an
 internationalized domain name (IDN), an equivalent domain name
 satisfying this requirement can be obtained by applying the
 ToASCII operation (see section 4) to each label and, if dots are
 used as label separators, changing all the label separators to
 U+002E.
 3) ACE labels obtained from domain name slots SHOULD be hidden from
 users when it is known that the environment can handle the non-ACE
 form, except when the ACE form is explicitly requested. When it
 is not known whether or not the environment can handle the non-ACE
 form, the application MAY use the non-ACE form (which might fail,
 such as by not being displayed properly), or it MAY use the ACE
 form (which will look unintelligle to the user). Given an
 internationalized domain name, an equivalent domain name
 containing no ACE labels can be obtained by applying the ToUnicode
 operation (see section 4) to each label. When requirements 2 and
 3 both apply, requirement 2 takes precedence.
 4) Whenever two labels are compared, they MUST be considered to match
 if and only if they are equivalent, that is, their ASCII forms
 (obtained by applying ToASCII) match using a case-insensitive
 ASCII comparison. Whenever two names are compared, they MUST be
 considered to match if and only if their corresponding labels
 match, regardless of whether the names use the same forms of label
 separators.
3.2 Applicability
 IDNA is applicable to all domain names in all domain name slots
 except where it is explicitly excluded.
 This implies that IDNA is applicable to many protocols that predate
 IDNA. Note that IDNs occupying domain name slots in those protocols
 MUST be in ASCII form (see section 3.1, requirement 2).
3.2.1. DNS resource records
 IDNA does not apply to domain names in the NAME and RDATA fields of
 DNS resource records whose CLASS is not IN. This exclusion applies
 to every non-IN class, present and future, except where future
 standards override this exclusion by explicitly inviting the use of
 IDNA.
 There are currently no other exclusions on the applicability of IDNA
 to DNS resource records; it depends entirely on the CLASS, and not on
 the TYPE. This will remain true, even as new types are defined,
 unless there is a compelling reason for a new type to complicate
 matters by imposing type-specific rules.
3.2.2. Non-domain-name data types stored in domain names
 Although IDNA enables the representation of non-ASCII characters in
 domain names, that does not imply that IDNA enables the
 representation of non-ASCII characters in other data types that are
 stored in domain names. For example, an email address local part is
 sometimes stored in a domain label (hostmaster@example.com would be
 represented as hostmaster.example.com in the RDATA field of an SOA
 record). IDNA does not update the existing email standards, which
 allow only ASCII characters in local parts. Therefore, unless the
 email standards are revised to invite the use of IDNA for local
 parts, a domain label that holds the local part of an email address
 SHOULD NOT begin with the ACE prefix, and even if it does, it is to
 be interpreted literally as a local part that happens to begin with
 the ACE prefix.
4. Conversion operations
 An application converts a domain name put into an IDN-unaware slot or
 displayed to a user. This section specifies the steps to perform in
 the conversion, and the ToASCII and ToUnicode operations.
 The input to ToASCII or ToUnicode is a single label that is a
 sequence of Unicode code points (remember that all ASCII code points
 are also Unicode code points). If a domain name is represented using
 a character set other than Unicode or US-ASCII, it will first need to
 be transcoded to Unicode.
 Starting from a whole domain name, the steps that an application
 takes to do the conversions are:
 1) Decide whether the domain name is a "stored string" or a "query
 string" as described in [STRINGPREP]. If this conversion follows
 the "queries" rule from [STRINGPREP], set the flag called
 "AllowUnassigned".
 2) Split the domain name into individual labels as described in
 section 3.1. The labels do not include the separator.
 3) For each label, decide whether or not to enforce the restrictions
 on ASCII characters in host names [STD3]. (Applications already
 faced this choice before the introduction of IDNA, and can
 continue to make the decision the same way they always have; IDNA
 makes no new recommendations regarding this choice.) If the
 restrictions are to be enforced, set the flag called
 "UseSTD3ASCIIRules" for that label.
 4) Process each label with either the ToASCII or the ToUnicode
 operation as appropriate. Typically, you use the ToASCII
 operation if you are about to put the name into an IDN-unaware
 slot, and you use the ToUnicode operation if you are displaying
 the name to a user; section 3.1 gives greater detail on the
 applicable requirements.
 5) If ToASCII was applied in step 4 and dots are used as label
 separators, change all the label separators to U+002E (full stop).
 The following two subsections define the ToASCII and ToUnicode
 operations that are used in step 4.
 This description of the protocol uses specific procedure names, names
 of flags, and so on, in order to facilitate the specification of the
 protocol. These names, as well as the actual steps of the
 procedures, are not required of an implementation. In fact, any
 implementation which has the same external behavior as specified in
 this document conforms to this specification.
4.1 ToASCII
 The ToASCII operation takes a sequence of Unicode code points that
 make up one label and transforms it into a sequence of code points in
 the ASCII range (0..7F). If ToASCII succeeds, the original sequence
 and the resulting sequence are equivalent labels.
 It is important to note that the ToASCII operation can fail. ToASCII
 fails if any step of it fails. If any step of the ToASCII operation
 fails on any label in a domain name, that domain name MUST NOT be
 used as an internationalized domain name. The method for dealing
 with this failure is application-specific.
 The inputs to ToASCII are a sequence of code points, the
 AllowUnassigned flag, and the UseSTD3ASCIIRules flag. The output of
 ToASCII is either a sequence of ASCII code points or a failure
 condition.
 ToASCII never alters a sequence of code points that are all in the
 ASCII range to begin with (although it could fail). Applying the
 ToASCII operation multiple times has exactly the same effect as
 applying it just once.
 ToASCII consists of the following steps:
 1. If the sequence contains any code points outside the ASCII range
 (0..7F) then proceed to step 2, otherwise skip to step 3.
 2. Perform the steps specified in [NAMEPREP] and fail if there is an
 error. The AllowUnassigned flag is used in [NAMEPREP].
 3. If the UseSTD3ASCIIRules flag is set, then perform these checks:
 (a) Verify the absence of non-LDH ASCII code points; that is, the
 absence of 0..2C, 2E..2F, 3A..40, 5B..60, and 7B..7F.
 (b) Verify the absence of leading and trailing hyphen-minus; that
 is, the absence of U+002D at the beginning and end of the
 sequence.
 4. If the sequence contains any code points outside the ASCII range
 (0..7F) then proceed to step 5, otherwise skip to step 8.
 5. Verify that the sequence does NOT begin with the ACE prefix.
 6. Encode the sequence using the encoding algorithm in [PUNYCODE] and
 fail if there is an error.
 7. Prepend the ACE prefix.
 8. Verify that the number of code points is in the range 1 to 63
 inclusive.
4.2 ToUnicode
 The ToUnicode operation takes a sequence of Unicode code points that
 make up one label and returns a sequence of Unicode code points. If
 the input sequence is a label in ACE form, then the result is an
 equivalent internationalized label that is not in ACE form, otherwise
 the original sequence is returned unaltered.
 ToUnicode never fails. If any step fails, then the original input
 sequence is returned immediately in that step.
 The ToUnicode output never contains more code points than its input.
 Note that the number of octets needed to represent a sequence of code
 points depends on the particular character encoding used.
 The inputs to ToUnicode are a sequence of code points, the
 AllowUnassigned flag, and the UseSTD3ASCIIRules flag. The output of
 ToUnicode is always a sequence of Unicode code points.
 1. If all code points in the sequence are in the ASCII range (0..7F)
 then skip to step 3.
 2. Perform the steps specified in [NAMEPREP] and fail if there is an
 error. (If step 3 of ToASCII is also performed here, it will not
 affect the overall behavior of ToUnicode, but it is not
 necessary.) The AllowUnassigned flag is used in [NAMEPREP].
 3. Verify that the sequence begins with the ACE prefix, and save a
 copy of the sequence.
 4. Remove the ACE prefix.
 5. Decode the sequence using the decoding algorithm in [PUNYCODE] and
 fail if there is an error. Save a copy of the result of this
 step.
 6. Apply ToASCII.
 7. Verify that the result of step 6 matches the saved copy from step
 3, using a case-insensitive ASCII comparison.
 8. Return the saved copy from step 5.
5. ACE prefix
 The ACE prefix, used in the conversion operations (section 4), is two
 alphanumeric ASCII characters followed by two hyphen-minuses. It
 cannot be any of the prefixes already used in earlier documents,
 which includes the following: "bl--", "bq--", "dq--", "lq--", "mq--",
 "ra--", "wq--" and "zq--". The ToASCII and ToUnicode operations MUST
 recognize the ACE prefix in a case-insensitive manner.
 The ACE prefix for IDNA is "xn--" or any capitalization thereof.
 This means that an ACE label might be "xn--de-jg4avhby1noc0d", where
 "de-jg4avhby1noc0d" is the part of the ACE label that is generated by
 the encoding steps in [PUNYCODE].
 While all ACE labels begin with the ACE prefix, not all labels
 beginning with the ACE prefix are necessarily ACE labels. Non-ACE
 labels that begin with the ACE prefix will confuse users and SHOULD
 NOT be allowed in DNS zones.
6. Implications for typical applications using DNS
 In IDNA, applications perform the processing needed to input
 internationalized domain names from users, display internationalized
 domain names to users, and process the inputs and outputs from DNS
 and other protocols that carry domain names.
 The components and interfaces between them can be represented
 pictorially as:
 +------+
 | User |
 +------+
 ^
 | Input and display: local interface methods
 | (pen, keyboard, glowing phosphorus, ...)
 +-------------------|-------------------------------+
 | v |
 | +-----------------------------+ |
 | | Application | |
 | | (ToASCII and ToUnicode | |
 | | operations may be | |
 | | called here) | |
 | +-----------------------------+ |
 | ^ ^ | End system
 | | | |
 | Call to resolver: | | Application-specific |
 | ACE | | protocol: |
 | v | ACE unless the |
 | +----------+ | protocol is updated |
 | | Resolver | | to handle other |
 | +----------+ | encodings |
 | ^ | |
 +-----------------|----------|----------------------+
 DNS protocol: | |
 ACE | |
 v v
 +-------------+ +---------------------+
 | DNS servers | | Application servers |
 +-------------+ +---------------------+
 The box labeled "Application" is where the application splits a
 domain name into labels, sets the appropriate flags, and performs the
 ToASCII and ToUnicode operations. This is described in section 4.
6.1 Entry and display in applications
 Applications can accept domain names using any character set or sets
 desired by the application developer, and can display domain names in
 any charset. That is, the IDNA protocol does not affect the
 interface between users and applications.
 An IDNA-aware application can accept and display internationalized
 domain names in two formats: the internationalized character set(s)
 supported by the application, and as an ACE label. ACE labels that
 are displayed or input MUST always include the ACE prefix.
 Applications MAY allow input and display of ACE labels, but are not
 encouraged to do so except as an interface for special purposes,
 possibly for debugging, or to cope with display limitations as
 described in section 6.4.. ACE encoding is opaque and ugly, and
 should thus only be exposed to users who absolutely need it. Because
 name labels encoded as ACE name labels can be rendered either as the
 encoded ASCII characters or the proper decoded characters, the
 application MAY have an option for the user to select the preferred
 method of display; if it does, rendering the ACE SHOULD NOT be the
 default.
 Domain names are often stored and transported in many places. For
 example, they are part of documents such as mail messages and web
 pages. They are transported in many parts of many protocols, such as
 both the control commands and the RFC 2822 body parts of SMTP, and
 the headers and the body content in HTTP. It is important to
 remember that domain names appear both in domain name slots and in
 the content that is passed over protocols.
 In protocols and document formats that define how to handle
 specification or negotiation of charsets, labels can be encoded in
 any charset allowed by the protocol or document format. If a
 protocol or document format only allows one charset, the labels MUST
 be given in that charset.
 In any place where a protocol or document format allows transmission
 of the characters in internationalized labels, internationalized
 labels SHOULD be transmitted using whatever character encoding and
 escape mechanism that the protocol or document format uses at that
 place.
 All protocols that use domain name slots already have the capacity
 for handling domain names in the ASCII charset. Thus, ACE labels
 (internationalized labels that have been processed with the ToASCII
 operation) can inherently be handled by those protocols.
6.2 Applications and resolver libraries
 Applications normally use functions in the operating system when they
 resolve DNS queries. Those functions in the operating system are
 often called "the resolver library", and the applications communicate
 with the resolver libraries through a programming interface (API).
 Because these resolver libraries today expect only domain names in
 ASCII, applications MUST prepare labels that are passed to the
 resolver library using the ToASCII operation. Labels received from
 the resolver library contain only ASCII characters; internationalized
 labels that cannot be represented directly in ASCII use the ACE form.
 ACE labels always include the ACE prefix.
 An operating system might have a set of libraries for performing the
 ToASCII operation. The input to such a library might be in one or
 more charsets that are used in applications (UTF-8 and UTF-16 are
 likely candidates for almost any operating system, and script-
 specific charsets are likely for localized operating systems).
 IDNA-aware applications MUST be able to work with both non-
 internationalized labels (those that conform to [STD13] and [STD3])
 and internationalized labels.
 It is expected that new versions of the resolver libraries in the
 future will be able to accept domain names in other charsets than
 ASCII, and application developers might one day pass not only domain
 names in Unicode, but also in local script to a new API for the
 resolver libraries in the operating system. Thus the ToASCII and
 ToUnicode operations might be performed inside these new versions of
 the resolver libraries.
 Domain names passed to resolvers or put into the question section of
 DNS requests follow the rules for "queries" from [STRINGPREP].
6.3 DNS servers
 Domain names stored in zones follow the rules for "stored strings"
 from [STRINGPREP].
 For internationalized labels that cannot be represented directly in
 ASCII, DNS servers MUST use the ACE form produced by the ToASCII
 operation. All IDNs served by DNS servers MUST contain only ASCII
 characters.
 If a signaling system which makes negotiation possible between old
 and new DNS clients and servers is standardized in the future, the
 encoding of the query in the DNS protocol itself can be changed from
 ACE to something else, such as UTF-8. The question whether or not
 this should be used is, however, a separate problem and is not
 discussed in this memo.
6.4 Avoiding exposing users to the raw ACE encoding
 Any application that might show the user a domain name obtained from
 a domain name slot, such as from gethostbyaddr or part of a mail
 header, will need to be updated if it is to prevent users from seeing
 the ACE.
 If an application decodes an ACE name using ToUnicode but cannot show
 all of the characters in the decoded name, such as if the name
 contains characters that the output system cannot display, the
 application SHOULD show the name in ACE format (which always includes
 the ACE prefix) instead of displaying the name with the replacement
 character (U+FFFD). This is to make it easier for the user to
 transfer the name correctly to other programs. Programs that by
 default show the ACE form when they cannot show all the characters in
 a name label SHOULD also have a mechanism to show the name that is
 produced by the ToUnicode operation with as many characters as
 possible and replacement characters in the positions where characters
 cannot be displayed.
 The ToUnicode operation does not alter labels that are not valid ACE
 labels, even if they begin with the ACE prefix. After ToUnicode has
 been applied, if a label still begins with the ACE prefix, then it is
 not a valid ACE label, and is not equivalent to any of the
 intermediate Unicode strings constructed by ToUnicode.
6.5 DNSSEC authentication of IDN domain names
 DNS Security [RFC2535] is a method for supplying cryptographic
 verification information along with DNS messages. Public Key
 Cryptography is used in conjunction with digital signatures to
 provide a means for a requester of domain information to authenticate
 the source of the data. This ensures that it can be traced back to a
 trusted source, either directly, or via a chain of trust linking the
 source of the information to the top of the DNS hierarchy.
 IDNA specifies that all internationalized domain names served by DNS
 servers that cannot be represented directly in ASCII must use the ACE
 form produced by the ToASCII operation. This operation must be
 performed prior to a zone being signed by the private key for that
 zone. Because of this ordering, it is important to recognize that
 DNSSEC authenticates the ASCII domain name, not the Unicode form or
 the mapping between the Unicode form and the ASCII form. In the
 presence of DNSSEC, this is the name that MUST be signed in the zone
 and MUST be validated against.
 One consequence of this for sites deploying IDNA in the presence of
 DNSSEC is that any special purpose proxies or forwarders used to
 transform user input into IDNs must be earlier in the resolution flow
 than DNSSEC authenticating nameservers for DNSSEC to work.
7. Name server considerations
 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 [STD13] and DNS update messages
 [RFC2136]) are IDN-unaware because they predate IDNA, and therefore
 requirement 2 of section 3.1 of this document provides the needed
 shielding, by ensuring that internationalized domain names entering
 DNS server databases through such channels have already been
 converted to their equivalent ASCII forms.
 It is imperative that there be only one ASCII encoding for a
 particular domain name. Because of the design of the ToASCII and
 ToUnicode operations, there are no ACE labels that decode to ASCII
 labels, and therefore name servers cannot contain multiple ASCII
 encodings of the same domain name.
 [RFC2181] explicitly allows domain labels to contain octets beyond
 the ASCII range (0..7F), and this document does not change that.
 Note, however, that there is no defined interpretation of octets
 80..FF as characters. If labels containing these octets are returned
 to applications, unpredictable behavior could result. The ASCII form
 defined by ToASCII is the only standard representation for
 internationalized labels in the current DNS protocol.
8. Root server considerations
 IDNs are likely to 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 today, so more queries and
 responses may be forced to go to TCP instead of UDP.
9. References
9.1 Normative References
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119, March 1997.
 [STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of
 Internationalized Strings ("stringprep")", RFC 3454,
 December 2002.
 [NAMEPREP] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
 Profile for Internationalized Domain Names (IDN)", RFC
 3491, March 2003.
 [PUNYCODE] Costello, A., "Punycode: A Bootstring encoding of
 Unicode for use with Internationalized Domain Names in
 Applications (IDNA)", RFC 3492, March 2003.
 [STD3] Braden, R., "Requirements for Internet Hosts --
 Communication Layers", STD 3, RFC 1122, and
 "Requirements for Internet Hosts -- Application and
 Support", STD 3, RFC 1123, October 1989.
 [STD13] Mockapetris, P., "Domain names - concepts and
 facilities", STD 13, RFC 1034 and "Domain names -
 implementation and specification", STD 13, RFC 1035,
 November 1987.
9.2 Informative References
 [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
 RFC 2535, March 1999.
 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
 Specification", RFC 2181, July 1997.
 [UAX9] Unicode Standard Annex #9, The Bidirectional Algorithm,
 <http://www.unicode.org/unicode/reports/tr9/>.
 [UNICODE] The Unicode Consortium. The Unicode Standard, Version
 3.2.0 is defined by The Unicode Standard, Version 3.0
 (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5),
 as amended by the Unicode Standard Annex #27: Unicode
 3.1 (http://www.unicode.org/reports/tr27/) and by the
 Unicode Standard Annex #28: Unicode 3.2
 (http://www.unicode.org/reports/tr28/).
 [USASCII] Cerf, V., "ASCII format for Network Interchange", RFC
 20, October 1969.
10. Security Considerations
 Security on the Internet partly relies on the DNS. Thus, any change
 to the characteristics of the DNS can change the security of much of
 the Internet.
 This memo describes an algorithm which encodes characters that are
 not valid according to STD3 and STD13 into octet values that are
 valid. No security issues such as string length increases or new
 allowed values are introduced by the encoding process or the use of
 these encoded values, apart from those introduced by the ACE encoding
 itself.
 Domain names are used by users to identify and connect to Internet
 servers. The security of the Internet is compromised if a user
 entering a single internationalized name is connected to different
 servers based on different interpretations of the internationalized
 domain name.
 When systems use local character sets other than ASCII and Unicode,
 this specification leaves the the problem of transcoding between the
 local character set and Unicode up to the application. If different
 applications (or different versions of one application) implement
 different transcoding rules, they could interpret the same name
 differently and contact different servers. This problem is not
 solved by security protocols like TLS that do not take local
 character sets into account.
 Because this document normatively refers to [NAMEPREP], [PUNYCODE],
 and [STRINGPREP], it includes the security considerations from those
 documents as well.
 If or when this specification is updated to use a more recent Unicode
 normalization table, the new normalization table will need to be
 compared with the old to spot backwards incompatible changes. If
 there are such changes, they will need to be handled somehow, or
 there will be security as well as operational implications. Methods
 to handle the conflicts could include keeping the old normalization,
 or taking care of the conflicting characters by operational means, or
 some other method.
 Implementations MUST NOT use more recent normalization tables than
 the one referenced from this document, even though more recent tables
 may be provided by operating systems. If an application is unsure of
 which version of the normalization tables are in the operating
 system, the application needs to include the normalization tables
 itself. Using normalization tables other than the one referenced
 from this specification could have security and operational
 implications.
 To help prevent confusion between characters that are visually
 similar, it is suggested that implementations provide visual
 indications where a domain name contains multiple scripts. Such
 mechanisms can also be used to show when a name contains a mixture of
 simplified and traditional Chinese characters, or to distinguish zero
 and one from O and l. DNS zone adminstrators may impose restrictions
 (subject to the limitations in section 2) that try to minimize
 homographs.
 Domain names (or portions of them) are sometimes compared against a
 set of privileged or anti-privileged domains. In such situations it
 is especially important that the comparisons be done properly, as
 specified in section 3.1 requirement 4. For labels already in ASCII
 form, the proper comparison reduces to the same case-insensitive
 ASCII comparison that has always been used for ASCII labels.
 The introduction of IDNA means that any existing labels that start
 with the ACE prefix and would be altered by ToUnicode will
 automatically be ACE labels, and will be considered equivalent to
 non-ASCII labels, whether or not that was the intent of the zone
 adminstrator or registrant.
11. IANA Considerations
 IANA has assigned the ACE prefix in consultation with the IESG.
12. Authors' Addresses
 Patrik Faltstrom
 Cisco Systems
 Arstaangsvagen 31 J
 S-117 43 Stockholm Sweden
 EMail: paf@cisco.com
 Paul Hoffman
 Internet Mail Consortium and VPN Consortium
 127 Segre Place
 Santa Cruz, CA 95060 USA
 EMail: phoffman@imc.org
 Adam M. Costello
 University of California, Berkeley
 URL: http://www.nicemice.net/amc/
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