[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-klensin-idnabis-protocol) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 5891

Network Working Group                                    J. Klensin, Ed.
Internet-Draft                                              May 21, 2008
Obsoletes: 3490 (if approved)
Intended status: Standards Track
Expires: November 22, 2008

    Internationalizing 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 November 22, 2008.


   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 November 22, 2008               [Page 1]

Internet-Draft              IDNA2008 Protocol                   May 2008

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Discussion Forum . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Requirements and Applicability . . . . . . . . . . . . . . . .  5
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  5
       3.2.1.  DNS Resource Records . . . . . . . . . . . . . . . . .  6
       3.2.2.  Non-domain-name Data Types Stored in the DNS . . . . .  6
   4.  Registration Protocol  . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Proposed label . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Conversion to Unicode and Normalization  . . . . . . . . .  7
     4.3.  Permitted Character and Label Validation . . . . . . . . .  7
       4.3.1.  Rejection of Characters that are not Permitted . . . .  7
       4.3.2.  Label Validation . . . . . . . . . . . . . . . . . . .  7
       4.3.3.  Registration Validation Summary  . . . . . . . . . . .  8
     4.4.  Registry Restrictions  . . . . . . . . . . . . . . . . . .  8
     4.5.  Punycode Conversion  . . . . . . . . . . . . . . . . . . .  9
     4.6.  Insertion in the Zone  . . . . . . . . . . . . . . . . . .  9
   5.  Domain Name Resolution (Lookup) Protocol . . . . . . . . . . .  9
     5.1.  Label String Input . . . . . . . . . . . . . . . . . . . .  9
     5.2.  Conversion to Unicode  . . . . . . . . . . . . . . . . . .  9
     5.3.  Character Changes in Preprocessing or the User
           Interface  . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  A-label Input  . . . . . . . . . . . . . . . . . . . . . . 10
     5.5.  Validation and Character List Testing  . . . . . . . . . . 11
     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  . . . . . . . . 13
     6.3.  Root Server Considerations . . . . . . . . . . . . . . . . 13
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   9.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.1.  Version -00 of draft-klensin-idnabis-protocol  . . . . . . 14
     9.2.  Versions -01 and -02 of draft-klensin-idnabis-protocol . . 14
     9.3.  Version -03 of draft-klensin-idnabis-protocol  . . . . . . 14
     9.4.  Version -04 of draft-klensin-idnabis-protocol  . . . . . . 15
     9.5.  Version -00 of draft-ietf-idnabis-protocol . . . . . . . . 15
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 15
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     12.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  The Contextual Rules Registry . . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22

Klensin                 Expires November 22, 2008               [Page 2]

Internet-Draft              IDNA2008 Protocol                   May 2008

   Intellectual Property and Copyright Statements . . . . . . . . . . 23

Klensin                 Expires November 22, 2008               [Page 3]

Internet-Draft              IDNA2008 Protocol                   May 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 idna-update@alvestrand.no

2.  Terminology

   General terminology applicable to IDNA, but with meanings familiar to

Klensin                 Expires November 22, 2008               [Page 4]

Internet-Draft              IDNA2008 Protocol                   May 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 November 22, 2008               [Page 5]

Internet-Draft              IDNA2008 Protocol                   May 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
   label on a record with TYPE SRV MUST NOT be an A-label or U-label
   (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 and because the underscore is not a
   letter, digit, or hyphen.

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 (hostmaster@example.com would be
   represented as hostmaster.example.com).  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

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.  The registry MAY permit
   submission of labels in A-label form.  If it does so, it SHOULD

Klensin                 Expires November 22, 2008               [Page 6]

Internet-Draft              IDNA2008 Protocol                   May 2008

   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 rejected.

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.  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 examined to prohibit characters that IDNA does
   not permit in input.  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.

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

Klensin                 Expires November 22, 2008               [Page 7]

Internet-Draft              IDNA2008 Protocol                   May 2008

   be registered.  Contextual Rules

   Each code point is checked for its identification as characters
   requiring contextual processign 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]
   and Appendix A of this document.)  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
   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
   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.

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.  These per-registry policies and
   restrictions are an essential element of the IDNA registration
   protocol even for registries (and corresponding zone files) deep in

Klensin                 Expires November 22, 2008               [Page 8]

Internet-Draft              IDNA2008 Protocol                   May 2008

   the DNS hierarchy.  As discussed in [IDNA2008-Rationale], such
   restrictions have always existed in the DNS.

   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
   prefix included, i.e., the "xn--..." form).

4.6.  Insertion in the Zone

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

5.  Domain Name Resolution (Lookup) Protocol

   Resolution is conceptually different from registration and different
   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 resolution-side tests are more permissive and
   rely heavily on the assumption that names that are present in the DNS
   are valid.  Among other things, this distinction, applied carefully,
   facilitates expansion of the permitted character lists to include new
   scripts and accommodate new versions of Unicode without introducing
   ambiguity into domain name processing.

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 local character set, character coding conventions, and, as
   necessary, display and presentation conventions, are converted to
   Unicode (without surrogates), paralleling the process described above
   in Section 4.2.

Klensin                 Expires November 22, 2008               [Page 9]

Internet-Draft              IDNA2008 Protocol                   May 2008

5.3.  Character Changes in Preprocessing or the User Interface

   The Unicode string MAY then be processed, in a way specific to the
   local environment, to make the result of the IDNA processing match
   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.

   Other examples of processing for localization might be applied, if
   appropriate, at this point.  They include interpreting the KANA
   MIDDLE DOT as separating domain name components from each other,
   mapping different "width" forms of the same character into the one
   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 even further outside the scope
   of this specification than the ones mentioned above.

   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].

   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.

Klensin                 Expires November 22, 2008              [Page 10]

Internet-Draft              IDNA2008 Protocol                   May 2008

5.5.  Validation and Character List Testing

   In parallel with the registration procedure, the Unicode string is
   checked to verify that all characters that appear in it are valid for
   IDNA resolution input.  As discussed above and in
   [IDNA2008-Rationale], the resolution 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
      "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. [[anchor17: Note
      in Draft: this definition may need to be further tightened.]]

   In addition, the application SHOULD apply the following test.  The
   test may be omitted in special circumstances, such as when the
   resolver application knows that the conditions are enforced
   elsewhere, because an attempt to 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 alone.

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

Klensin                 Expires November 22, 2008              [Page 11]

Internet-Draft              IDNA2008 Protocol                   May 2008

   For all other strings, the resolver 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 resolver that declines to look
   up a string that conforms to the above rules is not in conformance
   with this protocol.

5.6.  Punycode Conversion

   The validated string, a U-label, is converted to an A-label using the
   punycode algorithm.

5.7.  DNS Name Resolution

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

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 each

   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

Klensin                 Expires November 22, 2008              [Page 12]

Internet-Draft              IDNA2008 Protocol                   May 2008

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
   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 resolution flow
   than DNSSEC authenticating nameservers for DNSSEC to work.

6.3.  Root 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 today, so more
   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 valid according to 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-compatable encoded form that contains only characters
   permitted in host names by those earlier specifications.  The

Klensin                 Expires November 22, 2008              [Page 13]

Internet-Draft              IDNA2008 Protocol                   May 2008

   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 itself.

   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 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

9.  Change Log

   [[anchor23: RFC Editor: Please remove this section.]]

9.1.  Version -00 of draft-klensin-idnabis-protocol

   Version -00 of this draft was produced in November 2007 by moving
   text from draft-klensin-idnabis-issues and by copy considerable text
   from RFC 3490.  The result was then extensively edited.

9.2.  Versions -01 and -02 of draft-klensin-idnabis-protocol

   These versions reflected a number of editorial changes, some of them
   significant, and alignment of terminology with

9.3.  Version -03 of draft-klensin-idnabis-protocol

   o  Abstract rewritten to bring its length within RFC Editor

Klensin                 Expires November 22, 2008              [Page 14]

Internet-Draft              IDNA2008 Protocol                   May 2008

   o  Corrections and revisions in response to extensive comments by
      Mark Davis and others.

   o  Small modifications to several operations, including moving the
      Normalization steps to a different place in the sequence.

   o  Many editorial changes.

9.4.  Version -04 of draft-klensin-idnabis-protocol

   o  Revised terminology and removed the MAYBE category as a
      consequence of design discussions on 30 January 2003 and followup
      conversations.  Also restructured the various operations to treat
      CONTEXTUAL RULE REQUIRED as a validation step (paralleling bidi),
      rather than a category.  Those changes required changes elsewhere
      in the document for consistency.

   o  Changed the requirements for normalization, making this a
      requirement on the calling application rather than an action of
      this protocol.  This is consistent with the general "mappings
      belong somewhere else" principle.

   o  Updated references.

   o  More editorial work, some independent of the changes, described
      immediately above.

9.5.  Version -00 of draft-ietf-idnabis-protocol

   o  Clarified actions to be taken if an A-label is supplied as input.

   o  Moved the contextual rules appendix into this document from
      draft-klensin-idnabis-issues and made an initial attempt at
      defining the actual rules.  Synchronized the list of characters in
      that appendix with tables-01.

   o  Added an explicit discussion of A-label input.

   o  Inserted a test for double-hyphen here.

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

Klensin                 Expires November 22, 2008              [Page 15]

Internet-Draft              IDNA2008 Protocol                   May 2008

   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

12.  References

12.1.  Normative References

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

              Klensin, J., Ed., "Internationalizing Domain Names for
              Applications (IDNA): Issues, Explanation, and Rationale",
              February 2008, <http://www.ietf.org/internet-drafts/

              Faltstrom, P., "The Unicode Codepoints and IDN",
              February 2008, <http://stupid.domain.name/idnabis/

              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.

Klensin                 Expires November 22, 2008              [Page 16]

Internet-Draft              IDNA2008 Protocol                   May 2008

   [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://

              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

Klensin                 Expires November 22, 2008              [Page 17]

Internet-Draft              IDNA2008 Protocol                   May 2008

              Specification", RFC 2181, July 1997.

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

   [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

Appendix A.  The Contextual Rules Registry

   [[anchor33: Note in Draft: the WG needs to figure out whether this
   table stays as part of this document, is moved to a separate one, or
   is incorporated into "tables".  Regardless of where they are placed,
   the WG will still need to review the specific content of the rules.
   In this version of the document, the table remains something of a
   illustrative placeholder, not a final specification.]]

   As discussed in the IANA Considerations section of
   [IDNA2008-Rationale], a registry of rules that define the contexts in
   which particular PROTOCOL-VALID characters, characters associated
   with a requirement for Contextual Information, are permitted.  These
   rules are expressed as tests on the label in which the characters
   appear (all, or any part of, the label may be tested).

   For each character specified as requiring a contextual rule, a rule
   MAY be established with the following data elements:

Klensin                 Expires November 22, 2008              [Page 18]

Internet-Draft              IDNA2008 Protocol                   May 2008

   1.  The code point associated with the character.

   2.  The name of the character.

   3.  An indication as to whether the code point requires the rule be
       processed at lookup time (this indication is equivalent to the
       difference between "CONTEXTJ" and "CONTEXTO" in the tables
       document [IDNA2008-Tables]).

   4.  A prose description of the contextual rule.

   5.  A description of the contextual rule using Unicode Regular
       Expression notation [Unicode-RegEx].  Only a Level 1
       implementation is needed for the expressions below, which also
       make reference to the Unicode Script definition [Unicode-Scripts]
       and the Unicode Property Value Aliases list
       [Unicode-PropertyValueAliases].  Note that in these regular
       expressions, the label is taken to be an entire line, i.e., "^"
       refers to the beginning of the label and "$" refers to the end of
       the label.

       These regular expressions are used as tests.  The contextual
       requirement is met if there is a match for the regular expression
       and not met if there is no match.

       [[anchor34: Patrik and I (JcK) would like to find a way to state
       these rules that does not require the reader and implementer to
       understand what we believe to be a fairly exotic element of the
       Unicode specification.  Suggestions welcome.]]

   6.  An optional comment preceded by "#"

   Should there be any conflict between the two statements of a rule,
   the regular expression form MUST be considered normative until the
   registry can be corrected.

   The rules for the characters listed in the Tables document as
   exception cases or Join_Controls and for which rules are being
   defined at this time appear below.

   [[anchor35: Note in draft: This table is not complete and the rule
   entries below are temporarily only examples.]]

      Must not appear at the beginning or end of a label;
      Regular expression:
      [^^]\u002D|\u00SD[^$] ;
      # Note that a prohibition on having two hyphens as the third and

Klensin                 Expires November 22, 2008              [Page 19]

Internet-Draft              IDNA2008 Protocol                   May 2008

      fourth characters of anything but a valid A-label appears in the

      Between two characters from the same script only.  The script must
      be one in which the use of this character causes significant
      visual transformation of one or both of the adjacent characters;
      Regular expression:
      Tamil)] ;
      [[anchor36: That script list is _not_ complete and, in particular,
      more Indic scripts certainly need to be listed.  It also does not
      correctly express the "same script" restriction mentioned in the
      prose, since it only tests adjacent characters.  Whether this
      character is required for Arabic script, and with what
      restrictions if it is, is under discussion in the WG and in other
      forums.  It is clear that a Unicode derived property for script
      groups that would permit testing, e.g., "Indic Script", would be
      very helpful here.]]

      Between two characters from the same script only.  The script must
      be one in which the use of this character causes significant
      visual transformation of one or both of the adjacent characters;
      Regular expression:
      \u200D[\p(Script:Deva)\p(Script:Tamil)]+ ;
      [[anchor37: That script list is _not_ complete and, in particular,
      more Indic scripts certainly need to be listed.  It also does not
      correctly express the "same script" restriction mentioned in the
      prose, since it only tests adjacent characters.  Whether this
      character is required for Arabic script, and with what
      restrictions if it is, is under discussion in the WG and in other

   00B7; MIDDLE DOT; F;
      Between two 'l' (U+006C) characters only, used to permit the
      Catalan character ela geminada to be expressed;
      Regular expression:
      \u006C\u00B7\u006c ;

      Greek script only.  Might be further restricted to specific
      following characters;
      Regular expression:
      \0375\(Script:Greek) ;

Klensin                 Expires November 22, 2008              [Page 20]

Internet-Draft              IDNA2008 Protocol                   May 2008

      # Permitted only in contexts in which GREEK LOWER NUMERAL SIGN,
      U+0375, is permitted.  GREEK NUMERAL SIGN, U+0374, and the Lower
      Numeral Sign (U+0375) are indicators for numeric use of letters in
      older Greek writing systems.  U+02B9 is relevant because
      normalization maps U+0374 into it.;
      Regular expression:
      \(Script:Greek)\02B9\(Script:Greek) ;
      [[anchor38: The test is that the adjacent characters be in the
      Greek script.  It is not clear whether this is sufficient.  The
      requirement for a preceding Greek letter may not be necessary.
      More input needed.]]

      Cyrillic script only.  Might be further restricted to permit only
      a preceding list of characters.
      Regular expression:
      \p(Script:Cyrillic)\u0483 ;

      The script of the preceding character and the subsequent
      character, if any, MUST be Hebrew;
      Regular expression:
      \p(Script:Hebrew)\u05F3\p(Script:Hebrew)? ;

      The script of the preceding character and the subsequent
      character, if any, MUST be Hebrew;
      Regular expression:
      \p(Script:Hebrew)\u05F3\p(Script:Hebrew)? ;

      MUST NOT be at the beginning of the label, and the previous
      character MUST be in Han Script;
      Regular expression:
      \p(Script:Hani)\u30FB ;

      MUST NOT be at the beginning of the label, and the previous
      character MUST be in Han Script;
      Regular expression:
      \p(Script:Hani)\u303B ;

      Adjacent characters MUST be Katakana;
      Regular expression:
      \p(Script:Kana)\u30FB\p(Script:Kana) ;

Klensin                 Expires November 22, 2008              [Page 21]

Internet-Draft              IDNA2008 Protocol                   May 2008

   While the information above is to be used to initialize the registry,
   IANA should treat the table format in this Appendix simply as an
   initial, tentative, suggestion.  Subject to review and comment from
   the IESG and any Expert Reviewers, IANA is responsible for, and
   should develop, a format for that registry, or a copy of it
   maintained in parallel, that is convenient for retrieval and machine
   processing and publish the location of that version.

Author's Address

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

   Phone: +1 617 245 1457
   Email: john+ietf@jck.com

Klensin                 Expires November 22, 2008              [Page 22]

Internet-Draft              IDNA2008 Protocol                   May 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 November 22, 2008              [Page 23]

Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/