Network Working Group                                         J. Klensin
Internet-Draft                                        September 12, 2008
Obsoletes: 3490 (if approved)
Intended status: Standards Track
Expires: March 16, 2009

    Internationalized Domain Names in Applications (IDNA): Protocol

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on March 16, 2009.


   This document supplies the protocol definition for a revised and
   updated specification for internationalized domain names (IDNs).  The
   rationale for these changes, the relationship to the older
   specification, and important terminology are provided in other
   documents.  This document specifies the protocol mechanism, called
   Internationalizing Domain Names in Applications (IDNA), for
   registering and looking up IDNs in a way that does not require
   changes to the DNS itself.  IDNA is only meant for processing domain
   names, not free text.

Klensin                  Expires March 16, 2009                 [Page 1]

Internet-Draft              IDNA2008 Protocol             September 2008

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Discussion Forum . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Requirements and Applicability . . . . . . . . . . . . . . . .  4
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  4
       3.2.1.  DNS Resource Records . . . . . . . . . . . . . . . . .  5
       3.2.2.  Non-domain-name Data Types Stored in the DNS . . . . .  5
   4.  Registration Protocol  . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Proposed label . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Conversion to Unicode and Normalization  . . . . . . . . .  6
     4.3.  Permitted Character and Label Validation . . . . . . . . .  6
       4.3.1.  Rejection of Characters that are not Permitted . . . .  6
       4.3.2.  Label Validation . . . . . . . . . . . . . . . . . . .  7
       4.3.3.  Registration Validation Summary  . . . . . . . . . . .  7
     4.4.  Registry Restrictions  . . . . . . . . . . . . . . . . . .  8
     4.5.  Punycode Conversion  . . . . . . . . . . . . . . . . . . .  8
     4.6.  Insertion in the Zone  . . . . . . . . . . . . . . . . . .  8
   5.  Domain Name Lookup Protocol  . . . . . . . . . . . . . . . . .  8
     5.1.  Label String Input . . . . . . . . . . . . . . . . . . . .  9
     5.2.  Conversion to Unicode  . . . . . . . . . . . . . . . . . .  9
     5.3.  Character Changes in Preprocessing or the User
           Interface  . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.4.  A-label Input  . . . . . . . . . . . . . . . . . . . . . . 10
     5.5.  Validation and Character List Testing  . . . . . . . . . . 10
     5.6.  Punycode Conversion  . . . . . . . . . . . . . . . . . . . 12
     5.7.  DNS Name Resolution  . . . . . . . . . . . . . . . . . . . 12
   6.  Name Server Considerations . . . . . . . . . . . . . . . . . . 12
     6.1.  Processing Non-ASCII Strings . . . . . . . . . . . . . . . 12
     6.2.  DNSSEC Authentication of IDN Domain Names  . . . . . . . . 12
     6.3.  Root and other DNS Server Considerations . . . . . . . . . 13
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   9.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Changes between Version -00 and -01 of
           draft-ietf-idnabis-protocol  . . . . . . . . . . . . . . . 14
     9.2.  Version -02  . . . . . . . . . . . . . . . . . . . . . . . 14
     9.3.  Version -03  . . . . . . . . . . . . . . . . . . . . . . . 15
     9.4.  Version -04  . . . . . . . . . . . . . . . . . . . . . . . 15
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 15
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     12.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 19

Klensin                  Expires March 16, 2009                 [Page 2]

Internet-Draft              IDNA2008 Protocol             September 2008

1.  Introduction

   This document supplies the protocol definition for a revised and
   updated specification for internationalized domain names.  The
   rationale for these changes and relationship to the older
   specification and some new terminology is provided in other
   documents, notably [IDNA2008-Rationale].

   IDNA works by allowing applications to use certain ASCII string
   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.

   IDNA is applied only to DNS labels.  Standards for combining labels
   into fully-qualified domain names and parsing labels out of those
   names are covered in the base DNS standards [RFC1035].  An
   application may, of course, apply locally-appropriate conventions to
   the presentation forms of domain names as discussed in

   While they share terminology, reference data, and some operations,
   this document describes two separate protocols, one for IDN
   registration (Section 4) and one for IDN lookup (Section 5).

   A good deal of the background material that appeared in RFC 3490 has
   been removed from this update.  That material is either of historical
   interest only or has been covered from a more recent perspective in
   RFC 4690 [RFC4690] and [IDNA2008-Rationale].

   [[anchor2: Note in Draft: This document still needs more specifics
   about how to perform some of the tests in the Registration and Lookup
   protocols described below.  Those details will be supplied in a later
   revision, but the intent should be clear from the existing text.]]

1.1.  Discussion Forum

   [[anchor4: RFC Editor: please remove this section.]]

   This work is being discussed in the IETF IDNABIS WG and on the
   mailing list

2.  Terminology

   General terminology applicable to IDNA, but with meanings familiar to

Klensin                  Expires March 16, 2009                 [Page 3]

Internet-Draft              IDNA2008 Protocol             September 2008

   those who have worked with Unicode or other character set standards
   and the DNS, appears in [IDNA2008-Rationale].  Terminology that is an
   integral, normative, part of the IDNA definition, including the
   definitions of "ACE", appears in that document as well.  Familiarity
   with the terminology materials in that document is assumed for
   reading this one.  The reader of this document is assumed to be
   familiar with DNS-specific terminology as defined in RFC 1034

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

3.  Requirements and Applicability

3.1.  Requirements

   IDNA conformance means adherence to the following requirements:

   1.  Whenever a domain name is put into an IDN-unaware domain name
       slot (see Section 2 and [IDNA2008-Rationale]), it MUST contain
       only ASCII characters (i.e., must be either an A-label or an LDH-
       label), or must be a label associated with a DNS application that
       is not subject to either IDNA or the historical recommendations
       for "hostname"-style names [RFC1034].

   2.  Comparison of labels MUST be done on the A-label form, using an
       ASCII case-insensitive comparison as with all comparisons of DNS

   3.  Labels being registered MUST conform to the requirements of
       Section 4.  Labels being looked up and the lookup process MUST
       conform to the requirements of Section 5.

3.2.  Applicability

   IDNA is applicable to all domain names in all domain name slots
   except where it is explicitly excluded.  It is not applicable to
   domain name slots which do not use the LDH syntax rules.

   This implies that IDNA is applicable to many protocols that predate
   IDNA.  Note that IDNs occupying domain name slots in those older
   protocols MUST be in A-label form until and unless those protocols
   and implementations of them are upgraded.

Klensin                  Expires March 16, 2009                 [Page 4]

Internet-Draft              IDNA2008 Protocol             September 2008

3.2.1.  DNS Resource Records

   IDNA applies only to domain names in the NAME and RDATA fields of DNS
   resource records whose CLASS is IN.

   There are currently no other exclusions on the applicability of IDNA
   to DNS resource records.  Applicability depends entirely on the
   CLASS, and not on the TYPE except as noted below.  This will remain
   true, even as new types are defined, unless there is a compelling
   reason for a new type that requires type-specific rules.  The special
   naming conventions applicable to SRV records are examples of type-
   specific rules that are incompatible with IDNA coding.  Hence the
   first two labels (the ones required to start in "_") on a record with
   TYPE SRV MUST NOT be A-labels or U-labels (while it would be possible
   to write a non-ASCII string with a leading underscore, conversion to
   an A-label would be impossible without loss of information because
   the underscore is not a letter, digit, or hyphen).  Of course, those
   labels may be part of a domain that uses IDN labels at higher levels
   in the tree.

3.2.2.  Non-domain-name Data Types Stored in the DNS

   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, specifically in the RDATA field for types
   that have structured RDATA format.  For example, an email address
   local part is stored in a domain name in the RNAME field as part of
   the RDATA of an SOA record ( would be
   represented as  IDNA specifically does not
   update the existing email standards, which allow only ASCII
   characters in local parts.  Even though work is in progress to define
   internationalization for email addresses [RFC4952], changes to the
   email address part of the SOA RDATA would require action in other
   standards, specifically those that specify the format of the SOA RR.

4.  Registration Protocol

   This section defines the procedure for registering an IDN.  The
   procedure is implementation independent; any sequence of steps that
   produces exactly the same result for all labels is considered a valid

   Note that, while the registration and lookup protocols (Section 5)
   are very similar in most respects, they are different and
   implementers should carefully follow the steps they are implementing.

Klensin                  Expires March 16, 2009                 [Page 5]

Internet-Draft              IDNA2008 Protocol             September 2008

4.1.  Proposed label

   The registrant submits a request for an IDN.  The user typically
   produces the request string by the keyboard entry of a character
   sequence in the local native character set (which might, of course,
   be Unicode).  The registry MAY permit submission of labels in A-label
   form.  If it does so, it SHOULD perform a conversion to a U-label,
   perform the steps and tests described below, and verify that the
   A-label produced by the step in Section 4.5 matches the one provided
   as input.  If, for some reason, it does not, the registration MUST be
   [[anchor9: Editorial: Should the sentences starting with "The
   registry" be moved to 4.3?  I.e., would they be more in sequence

4.2.  Conversion to Unicode and Normalization

   Some system routine, or a localized front-end to the IDNA process,
   ensures that the proposed label is a Unicode string or converts it to
   one as appropriate.  That string MUST be in Unicode Normalization
   Form C (NFC [Unicode-UAX15]).

   As a local implementation choice, the implementation MAY choose to
   map some forbidden characters to permitted characters (for instance
   mapping uppercase characters to lowercase ones), displaying the
   result to the user, and allowing processing to continue.  However, it
   is strongly recommended that, to avoid any possible ambiguity,
   entities responsible for zone files ("registries") accept
   registrations only for A-labels (to be converted to U-labels by the
   registry) or U-labels actually produced from A-labels, not forms
   expected to be converted by some other process.

4.3.  Permitted Character and Label Validation

4.3.1.  Rejection of Characters that are not Permitted

   The Unicode string is checked to verify that no characters that IDNA
   does not permit in input appear in it.  Those characters are
   identified in the "DISALLOWED" and "UNASSIGNED" lists that are
   discussed in [IDNA2008-Rationale].  The normative rules for producing
   that list and the initial version of it are specified in
   [IDNA2008-Tables].  Characters that are either DISALLOWED or
   UNASSIGNED MUST NOT be part of labels being processed for
   registration in the DNS.

Klensin                  Expires March 16, 2009                 [Page 6]

Internet-Draft              IDNA2008 Protocol             September 2008

4.3.2.  Label Validation

   The proposed label (in the form of a Unicode string, i.e., a putative
   U-label) is then examined, performing tests that require examination
   of more than one character.  Rejection of Confusing or Hostile Sequences in U-labels

   The Unicode string MUST NOT contain "--" (two consecutive hyphens) in
   the third and fourth character positions.  Leading Combining Marks

   The first character of the string is examined to verify that it is
   not a combining mark.  If it is a combining mark, the string MUST NOT
   be registered.  Contextual Rules

   Each code point is checked for its identification as characters
   requiring contextual processing for registration (the list of
   characters appears as the combination of CONTEXTJ and CONTEXTO in
   [IDNA2008-Tables]).  If that indication appears, the table of
   contextual rules is checked for a rule for that character.  If no
   rule is found, the proposed label is rejected and MUST NOT be
   installed in a zone file.  If one is found, it is applied (typically
   as a test on the entire label or on adjacent characters).  If the
   application of the rule does not conclude that the character is valid
   in context, the proposed label MUST BE rejected.  (See the IANA
   Considerations: IDNA Context Registry section of
   [IDNA2008-Rationale].)  Labels Containing Characters Written Right to Left

   Additional special tests for right-to-left strings are applied (See
   [IDNA2008-BIDI].  Strings that contain right to left characters that
   do not conform to the rule(s) identified there MUST NOT be inserted
   as labels in zone files.

4.3.3.  Registration Validation Summary

   Strings that have been produced by the steps above, and whose
   contents pass the above tests, are U-labels.

   To summarize, tests are made here for invalid characters, invalid
   combinations of characters, and for labels that are invalid even if
   the characters they contain are valid individually.  For example,
   labels containing invisible ("zero-width") characters may be

Klensin                  Expires March 16, 2009                 [Page 7]

Internet-Draft              IDNA2008 Protocol             September 2008

   permitted in context with characters whose presentation forms are
   significantly changed by the presence or absence of the zero-width
   characters, while other labels in which zero-width characters appear
   may be rejected.
   [[anchor16: Should the example text be removed or moved?  Note that
   I've been strongly encouraged to supply specific examples to reduce
   abstraction and questions about the appropriateness of the text.

4.4.  Registry Restrictions

   Registries at all levels of the DNS, not just the top level, are
   expected to establish policies about the labels that may be
   registered, and for the processes associated with that action.  While
   exact policies are not specified as part of IDNA2008 and it is
   expected that different registries may specify different policies,
   there SHOULD be policies.  Even a trivial policy (e.g., "anything can
   be registered in this zone that can be represented as an A-label -
   U-label pair") has value because it provides notice to users and
   applications implementers that the registry cannot be relied upon to
   provide even minimal user-protection restrictions.  These per-
   registry policies and restrictions are an essential element of the
   IDNA registration protocol even for registries (and corresponding
   zone files) deep in the DNS hierarchy.  As discussed in
   [IDNA2008-Rationale], such restrictions have always existed in the

   The string produced by the above steps is checked and processed as
   appropriate to local registry restrictions.  Application of those
   registry restrictions may result in the rejection of some labels or
   the application of special restrictions to others.

4.5.  Punycode Conversion

   The resulting U-label is converted to an A-label (i.e., the encoding
   of that label according to the Punycode algorithm [RFC3492] with the
   ACE prefix added, i.e., the "xn--..." form).
   [[anchor17: Explain why 3492 failures cannot occur or explain what to
   do if they do.]]

4.6.  Insertion in the Zone

   The A-label is registered in the DNS by insertion into a zone.

5.  Domain Name Lookup Protocol

   Lookup is conceptually different from registration and different

Klensin                  Expires March 16, 2009                 [Page 8]

Internet-Draft              IDNA2008 Protocol             September 2008

   tests are applied on the client.  Although some validity checks are
   necessary to avoid serious problems with the protocol (see
   Section 5.5 ff.), the lookup-side tests are more permissive and rely
   heavily on the assumption that names that are present in the DNS are

5.1.  Label String Input

   The user supplies a string in the local character set, typically by
   typing it or clicking on, or copying and pasting, a resource
   identifier, e.g., a URI [RFC3986] or IRI [RFC3987] from which the
   domain name is extracted.  Or some process not directly involving the
   user may read the string from a file or obtain it in some other way.
   Processing in this step and the next two are local matters, to be
   accomplished prior to actual invocation of IDNA, but at least these
   two steps must be accomplished in some way.

5.2.  Conversion to Unicode

   The string is converted from the local character set into Unicode, if
   it is not already Unicode.  The exact nature of this conversion is
   beyond the scope of this document, but may involve normalization, as
   described in Section 4.2.

5.3.  Character Changes in Preprocessing or the User Interface

   The Unicode string MAY then be processed to prevent confounding of
   user expectations.  For instance, it would be reasonable, at this
   step, to convert all upper case characters to lower case, if this
   makes sense in the user's environment.  The procedures described in
   this section are ordinarily useful only for processing direct user
   input and when needed for backward compatibility with IDNA2003.  In
   general, IDNs appearing in files and those transmitted across the
   network as part of protocols are expected to be in either ASCII form
   (including A-labels) or to contain U-labels, not forms requiring
   mapping or other conversions.

   Other examples of processing for localization might be applied,
   especially to direct user input, at this point.  They include
   interpreting various characters as separating domain name components
   from each other (label separators) because they either look like
   periods or are used to separate sentences, mapping halfwidth or
   fullwidth East Asian characters to the common form permitted in
   labels, or giving special treatment to characters whose presentation
   forms are dependent only on placement in the label.  Such
   localization changes are also outside the scope of this

Klensin                  Expires March 16, 2009                 [Page 9]

Internet-Draft              IDNA2008 Protocol             September 2008

   Recommendations for preprocessing for global contexts (i.e., when
   local considerations do not apply or cannot be used) and for maximum
   interoperability with labels that might have been specified under
   liberal readings of IDNA2003 are given in [IDNA2008-Rationale].  It
   is important to note that the intent of these specifications is that
   labels in application protocols, files, or links are intended to be
   in U-label or A-label form.  Preprocessing MUST NOT map a character
   that is valid in a label (i.e., one that is PROTOCOL-VALID or
   permitted in any context) into another character.  Excessively
   liberal use of preprocessing, especially to strings stored in files,
   poses a threat to consistent and predictable behavior for the user
   even if not to actual interoperability.

   Because these transformations are local, it is important that domain
   names that might be passed between systems (e.g., in IRIs) be
   U-labels or A-labels and not forms that might be accepted locally as
   a consequence of this step.  This step is not standardized as part of
   IDNA, and is not further specified here.

5.4.  A-label Input

   If the input to this procedure appears to be an A-label (i.e., it
   starts in "xn--"), the lookup application MAY attempt to convert it
   to a U-label and apply the tests of Section 5.5 and, of course, the
   conversion of Section 5.6 to that form.  If the A-label is converted
   to a U-label then the processing specified in those two sections MUST
   yield an A-label identical to the original one.  See also
   Section 6.1.

   In general, that conversion and testing should be performed if the
   domain name will later be presented to the user in native character
   form (this requires that the lookup application be IDNA-aware).
   Applications that are not IDNA-aware will obviously omit that
   testing; others may treat the string as opaque to avoid the
   additional processing at the expense of providing less protection and
   information to users.

5.5.  Validation and Character List Testing

   As with the registration procedure, the Unicode string is checked to
   verify that all characters that appear in it are valid for IDNA
   lookup processing input.  As discussed above and in
   [IDNA2008-Rationale], the lookup check is more liberal than the
   registration one.  Putative labels with any of the following
   characteristics MUST BE rejected prior to DNS lookup:

   o  Labels containing code points that are unassigned in the version
      of Unicode being used by the application, i.e., in the

Klensin                  Expires March 16, 2009                [Page 10]

Internet-Draft              IDNA2008 Protocol             September 2008

      "Unassigned" Unicode category or the UNASSIGNED category of

   o  Labels that are not in NFC form.

   o  Labels containing prohibited code points, i.e., those that are
      assigned to the "DISALLOWED" category in the permitted character
      table [IDNA2008-Tables].

   o  Labels containing code points that are shown in the permitted
      character table as requiring a contextual rule and that are
      flagged as requiring exceptional special processing on lookup
      ("CONTEXTJ" in the Tables) MUST conform to the rule, which MUST be

   o  Labels containing other code points that are shown in the
      permitted character table as requiring a contextual rule
      ("CONTEXTO" in the tables), but for which no such rule appears in
      the table of rules.  With the exception in the rule immediately
      above, applications resolving DNS names or carrying out equivalent
      operations are not required to test contextual rules, only to
      verify that a rule exists.

   o  Labels whose first character is a combining mark.>

   In addition, the application SHOULD apply the following test.  The
   test may be omitted in special circumstances, such as when the lookup
   application knows that the conditions are enforced elsewhere, because
   an attempt to look up and resolve such strings will almost certainly
   lead to a DNS lookup failure.  However, applying the test is likely
   to give much better information about the reason for a lookup failure
   -- information that may be usefully passed to the user when that is
   feasible -- then DNS resolution failure information alone.  In any
   event, lookup applications should avoid attempting to resolve labels
   that are invalid under that test.

   o  Verification that the string is compliant with the requirements
      for right to left characters, specified in [IDNA2008-BIDI].

   For all other strings, the lookup application MUST rely on the
   presence or absence of labels in the DNS to determine the validity of
   those labels and the validity of the characters they contain.  If
   they are registered, they are presumed to be valid; if they are not,
   their possible validity is not relevant.  A lookup application that
   declines to process and resolve up a string that conforms to the
   above rules is not in conformance with this protocol.

Klensin                  Expires March 16, 2009                [Page 11]

Internet-Draft              IDNA2008 Protocol             September 2008

5.6.  Punycode Conversion

   The validated string, a U-label, is converted to an A-label using the
   Punycode algorithm with the ACE prefix added.

5.7.  DNS Name Resolution

   The A-label is looked up in the DNS, using normal DNS resolver

6.  Name Server Considerations

6.1.  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]) are IDN-unaware 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 design of the algorithms in Section 4 and Section 5 (a
   domain name containing only ASCII codepoints can not be converted to
   an A-label), there can not be more than one A-label form for any
   given U-label.

   The current update to the definition of the DNS protocol [RFC2181]
   explicitly allows domain labels to contain octets beyond the ASCII
   range (0000..007F), and this document does not change that.  Note,
   however, that there is no defined interpretation of octets 0080..00FF
   as characters.  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 current DNS

6.2.  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

Klensin                  Expires March 16, 2009                [Page 12]

Internet-Draft              IDNA2008 Protocol             September 2008

   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
   A-label form.  Conversion to A-labels 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 a
   domain name containing A-labels or conventional LDH-labels, not
   U-labels.  In the presence of DNSSEC, no form of a zone file or query
   response that contains a U-label may be signed or the signature

   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 lookup flow
   than DNSSEC authenticating nameservers for DNSSEC to work.

6.3.  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 EDNS0 [RFC2671] support may be more important (otherwise, queries
   and responses may be forced to go to TCP instead of UDP).

7.  Security Considerations

   The general security principles and issues for IDNA appear in
   [IDNA2008-Rationale].  The comments below are specific to this pair
   of protocols, but should be read in the context of that material and
   the definitions and specifications, identified there, on which this
   one depends.

   This memo describes procedures for registering and looking up labels
   that are not compatible with the preferred syntax described in the
   base DNS specifications (STD13 [RFC1034] [RFC1035] and Host
   Requirements [RFC1123]) because they contain non-ASCII characters.
   These procedures depend on the use of a special ASCII-compatible
   encoding form that contains only characters permitted in host names
   by those earlier specifications.  The encoding is specified in
   [RFC3492].  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

   Domain names (or portions of them) are sometimes compared against a

Klensin                  Expires March 16, 2009                [Page 13]

Internet-Draft              IDNA2008 Protocol             September 2008

   set domains to be given special treatment if a match occurs, e.g.,
   treated as more privileged than others or blocked in some way.  In
   such situations it is especially important that the comparisons be
   done properly, as specified in requirement 2 of Section 3.1.  For
   labels already in ASCII form (i.e., are LDH-labels or A-labels), 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 would be construed as A-labels, at least until
   they failed one of the relevant tests, whether or not that was the
   intent of the zone administrator or registrant.  There is no evidence
   that this has caused any practical problems since RFC 3490 was
   adopted, but the risk still exists in principle.

8.  IANA Considerations

   IANA actions for this version of IDNA are specified in
   [IDNA2008-Rationale].  The component of IDNA described in this
   document does not require any IANA actions.

9.  Change Log

   [[anchor24: RFC Editor: Please remove this section.]]

9.1.  Changes between Version -00 and -01 of draft-ietf-idnabis-protocol

   o  Corrected discussion of SRV records.

   o  Several small corrections for clarity.

   o  Inserted more "open issue" placeholders.

9.2.  Version -02

   o  Rewrote the "conversion to Unicode" text in Section 5.2 as
      requested on-list.

   o  Added a comment (and reference) about EDNS0 to the "DNS Server
      Conventions" section, which was also retitled.

   o  Made several editorial corrections and improvements in response to
      various comments.

   o  Added several new discussion placeholder anchors and updated some
      older ones.

Klensin                  Expires March 16, 2009                [Page 14]

Internet-Draft              IDNA2008 Protocol             September 2008

9.3.  Version -03

   o  Trimmed change log, removing information about pre-WG drafts.

   o  Incorporated a number of changes suggested by Marcos Sanz in his
      note of 2008.07.17 and added several more placeholder anchors.

   o  Several minor editorial corrections and improvements.

   o  "Editor" designation temporarily removed because the automatic
      posting machinery does not accept it.

9.4.  Version -04

   o  Removed Contextual Rule appendices for transfer to Tables.

   o  Several changes, including removal of discussion anchors, based on
      discussions at IETF 72 (Dublin)

   o  Rewrote the preprocessing material (Section 5.3) somewhat.

10.  Contributors

   While the listed editor held the pen, the original versions of this
   document represent 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.  This document
   draws significantly on the original version of IDNA [RFC3490] both
   conceptually and for specific text.  This second-generation version
   would not have been possible without the work that went into that
   first version and its authors, Patrik Faltstrom, Paul Hoffman, and
   Adam Costello.  While Faltstrom was actively involved in the creation
   of this version, Hoffman and Costello were not and should not be held
   responsible for any errors or omissions.

11.  Acknowledgements

   This revision to IDNA would have been impossible without the
   accumulated experience since RFC 3490 was published and resulting
   comments and complaints of many people in the IETF, ICANN, and other
   communities, too many people to list here.  Nor would it have been
   possible without RFC 3490 itself and the efforts of the Working Group
   that defined it.  Those people whose contributions are acknowledged
   in RFC 3490, [RFC4690], and [IDNA2008-Rationale] were particularly

Klensin                  Expires March 16, 2009                [Page 15]

Internet-Draft              IDNA2008 Protocol             September 2008

   Specific textual changes were incorporated into this document after
   suggestions from Stephane Bortzmeyer, Mark Davis, Paul Hoffman,
   Marcos Sanz and others.

12.  References

12.1.  Normative References

              Alvestrand, H. and C. Karp, "An updated IDNA criterion for
              right-to-left scripts", July 2008, <https://

              Klensin, J., Ed., "Internationalizing Domain Names for
              Applications (IDNA): Issues, Explanation, and Rationale",
              July 2008, <

              Faltstrom, P., "The Unicode Codepoints and IDNA",
              July 2008, <

              A version of this document is available in HTML format at

   [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.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

              The Unicode Consortium, "Unicode Character Database:
              PropertyValueAliases", March 2008, <http://

Klensin                  Expires March 16, 2009                [Page 16]

Internet-Draft              IDNA2008 Protocol             September 2008


              The Unicode Consortium, "Unicode Technical Standard #18:
              Unicode Regular Expressions", May 2005,

              The Unicode Consortium, "Unicode Standard Annex #24:
              Unicode Script Property", February 2008,

              The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms", 2006,

12.2.  Informative References

   [ASCII]    American National Standards Institute (formerly United
              States of America Standards Institute), "USA Code for
              Information Interchange", ANSI X3.4-1968, 1968.

              ANSI X3.4-1968 has been replaced by newer versions with
              slight modifications, but the 1968 version remains
              definitive for the Internet.

   [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.

   [RFC2535]  Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
              RFC 2671, August 1999.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

Klensin                  Expires March 16, 2009                [Page 17]

Internet-Draft              IDNA2008 Protocol             September 2008

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC4690]  Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
              Recommendations for Internationalized Domain Names
              (IDNs)", RFC 4690, September 2006.

   [RFC4952]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 4952, July 2007.

   [Unicode]  The Unicode Consortium, "The Unicode Standard, Version
              5.0", 2007.

              Boston, MA, USA: Addison-Wesley.  ISBN 0-321-48091-0

Author's Address

   John C Klensin
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA  02140

   Phone: +1 617 245 1457

Klensin                  Expires March 16, 2009                [Page 18]

Internet-Draft              IDNA2008 Protocol             September 2008

Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at

Klensin                  Expires March 16, 2009                [Page 19]