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Versions: 00 01 02

Internet Draft                                   Marc Blanchet
draft-ietf-idn-idne-02.txt                            Viagenie
March 19, 2001                                   Paul  Hoffman
Expires in six months                               IMC & VPNC

            Internationalized domain names using EDNS (IDNE)

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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-Drafts.

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

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The current DNS infrastructure does not provide a way to use
internationalized domain names (IDN). This document describes an
extension mechanism based on EDNS which enables the use of IDN without
causing harm to the current DNS. IDNE enables IDN host names with a as
many characters as current ASCII-only host names. It fully supports
UTF-8 and conforms to the IDN requirements.

1. Introduction

Various proposals for IDN have tried to integrate IDN into the current
limited ASCII DNS. However, the compatibility issues make too many
constraints on the architecture. Many of these proposals require
modifications to the applications or to the DNS protocol or to the
servers. This proposal take a different approach: it uses the
standardized extension mechanism for DNS (EDNS) and uses UTF-8 as the
mandatory charset. It causes no harm to the current DNS because it uses
the EDNS extension mechanism. The major drawback of this proposal is
that all protocols, applications and DNS servers will have to be
upgraded to support this proposal.

1.1 Terminology

The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", and
"MAY" in this document are to be interpreted as described in RFC 2119

Hexadecimal values are shown preceded with an "0x". For example,
"0xa1b5" indicates two octets, 0xa1 followed by 0xb5. Binary values are
shown preceded with an "0b". For example, a nine-bit value might be
shown as "0b101101111".

Examples in this document use the notation from the Unicode Standard
[UNICODE3] as well as the ISO 10646 [ISO10646] names. For example, the
letter "a" may be represented as either "U+0061" or "LATIN SMALL LETTER
A". In the lists of prohibited characters, the "U+" is left off to make
the lists easier to read.

1.2 IDN summary

Using the terminology in [IDNCOMP],  this protocol specifies an IDN
architecture of arch-2 (send binary or ACE). The binary format is
bin-1.1 (UTF-8), and the method for distinguishing binary from current
names is bin-2.4 (mark binary with EDNS0). The transition period is not

2. Functional Description

DNS query and responses containing IDNE labels have the following

- The string in the label MUST be pre-processed as described in
[NAMEPREP] before the query or response is prepared.

- The characters in the label MUST be encoded using UTF-8 [RFC2279].

- The entire label MUST be encoded EDNS [RFC2671].

- The version of the IDN protocol MUST be identified.

3. Encoding

An IDNE label uses the EDNS extended label type prefix (0b01), as
described in [RFC2671]. (A normal label type always begin with 0b00). A
new extended label type for IDNE is used to identify an IDNE label. This
document uses 0b000010 as the extended label type; however, the label
type will be assigned by IANA and it may not be 0b000010.

          0                   1                   2
bits  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2     . . .
         |0 1|    ELT    |     Size      |        IDN label ...        |

ELT: The six-bit extended label type to be assigned by the IANA for an
IDN label. In this document, the value 0b000010 is used, although that
might be changed by IANA.

Size: Size (in octets) of the IDN label following. This MUST NOT
be zero.

IDN label: Label, encoded in UTF-8 [RFC2279]. Note that this label might
contain all ASCII characters, and thus can be used for host name labels
that are legal in [STD13].

IDNE labels can be mixed with STD13 labels in a domain name.

The compression scheme in section 4.1.4 of [STD13] is supported as is.
Pointers can refer to either IDN labels or non-IDN labels.

3.1 Examples

3.1.1 Basic example

The following example shows the label me.com where the "e" in "me" is
replaced by a <LATIN CAPITAL LETTER E WITH ACUTE>, which is U+00C9. The
decomposition and downcasing specified in [NAMEPREP] changes the second
character to <LATIN SMALL LETTER E WITH ACUTE>, U+00E9. This string is
then transformed using UTF-8 [RFC2279] to 0x6DC3A9.

Ignoring the other fields of the message, the domain name portion of the
datagram could look like:

       20 | 0  1  0  0  0  0  1  0| 0  0  0  0  0  0  1  1|
       22 |         0x6D (m)      |       0xC3 (e'(1))    |
       24 |         0xA9 (e'(2))  |       3               |
       26 |         0x63 (c)      |       0x6F (o)        |
       28 |         0x6D (m)      |       0x00            |

Octet 20 means EDNS extended label type (0b01) using the IDN label
        type (0b000010)
Octet 21 means size of label is 3 octets following
Octet 22-24 are the "m*" label encoded in UTF-8
Octet 25-28 are "com" encoded as a STD13 label
Octet 29 is the root domain

3.1.2 Example with compression

Using the previous labels, one datagram might contain "www.m*.com" and
"m*.com" (where the "*" is <LATIN CAPITAL LETTER E WITH ACUTE>).

Ignoring the other fields of the message, the domain name portions of
the datagram could look like:

       20 | 0  1  0  0  0  0  1  0| 0  0  0  0  0  0  1  1|
       22 |         0x6D (m)      |       0xC3 (e'(1))    |
       24 |         0xA9 (e'(2))  |       3               |
       26 |         0x63 (c)      |       0x6F (o)        |
       28 |         0x6D (m)      |       0x00            |
      .    .    .
       40 |           3           |       0x77 (w)        |
       42 |       0x77 (w)        |       0x77 (w)        |
       44 | 1  1|                20                       |

The domain name "m*.com" is shown at offset 20. The domain name
"www.m*.com" is shown at offset 40; this definition uses a pointer to
concatenate a label for www to the previously defined "m*.com".

4. Label Size

In IDNE, the maximum length of a label is 255 octets, and the maximum
size for a domain name is 1023 octets. The reason for using these values
is so that IDNE labels can have the same number of characters as the
ASCII-based labels in [STD13]. Because character encoding in UTF-8 is
variable length, the maximum octet length for characters expected in the
foreseeable future (that is, 4 octets for a single character) was used.
Note that this extension allows some IDNE labels to be longer than 63
characters and some IDNE names to be longer than 255 octets.

Software creating DNS queries or responses using IDNE MUST verify that,
after IDN preparation and transformation to UTF8, that no labels are
longer than 255 octets and that no names are longer than 1023 octets. If
there is a user interface associated with the process creating the query
or response, that interface SHOULD give the user an error message.

Software MUST NOT transmit DNS queries or responses which contain labels
that are longer than 255 octets or names that are longer than 1023
octets. Servers MUST NOT accept DNS queries or responses which contain
labels that are longer than 255 octets or names that are longer than
1023 octets, and MUST send the NOTIMPL RCODE error message if such
queries or responses are received.

5. UDP Packet Size

IDNE-capable senders and receivers MUST support UDP packet sizes of 1220
octets, not including IP and UDP headers (note that the minimum MTU for
IPv6 is 1280 [RFC2460]). A sender MUST announce its capability in the
OPT pseudo-RR described in section 4.3 of [RFC2671] by having the CLASS
sender's UDP payload size be greater than or equal to 1220.

6. Canonalization, Prohibited Characters, and Case Folding

The string in the label MUST be pre-processed as described in [NAMEPREP]
before the query or response is prepared. A query or response MUST NOT
contain a label that does not conform to [NAMEPREP].

7. Versions of IDNE

The IDN protocol version number MUST be included in the OPT RR RDATA of
EDNS (described in Section 4.4 of [RFC2671]). An OPTION-CODE will be
assigned by IANA for storing the IDNE protocol version number; this
document uses 0x0001 for the OPTION-CODE. The value (that
is, the OPTION-DATA) is the version number coded in 8 bits.

All requesters MUST send this information as part of the OPT RR included
in the EDNS packet.

7.1 This version of IDNE

This document describes version 1 of IDNE. This version is a combination
of the protocol in this document and the rules as described in
[NAMEPREP]. Note that [NAMEPREP] describes a single version of the list
of canonicalization, case folding, and prohibited characters, and that
this document is linked to that single version of [NAMEPREP].

The identifiers for this specification are:
OPTION-CODE =   0x0001  (IDNE protocol version)
OPTION-LENGTH = 0x0001  (1 octet following)
OPTION-DATA =   0x01  (IDNE protocol version 1)

7.2 Creating new versions of IDNE

A new version of IDNE is created by a standards-track RFC that

- a normative reference to [NAMEPREP] or a successor document to

- an IDNE version number that is 1 greater than the highest IDNE version
number at the time the RFC is published

If there are any changes to the encoding or interpretation of the
protocol, they must also be specified in the same standards-track RFC.

7.3 Prohibited characters and versions of IDNE

If a server receives a request containing an illegal or unknown
character (as described in the version number in the request), it MUST
send a NOTIMPL RCODE to the client. For example, if a server that
understands both version 1 and version 2 receives a request that is
marked as version 1, but contains a label that includes a character that
is prohibited in version 1 but allowed in version 2, that server must
still send a NOTIMPL RCODE to the client.

8. API Specifications

The current API for TCP/IP uses gethostbyname and gethostbyaddr for IPv4
and getnodeipbyname and getnodeipbyaddr (specified in [RFC 2671]) for
both IPv4 and IPv6. These function calls returns hostent structs, where
the h_name field contains a pointer to a char. In this context,
receiving a UTF-8 string mean that the application should know that
UTF-8 uses more than one octet per char.

A new flag "IDN" (to appear in netdb.h) is defined to be passed in the
flags argument of getnodeipbynode and getnodeipbyaddr. This flag tells
the resolver to request an IDNE-encoded name. No new return code is
defined since the returned codes in RFC 2671 are meaningful in the IDNE

If one has not yet converted his code to IPv6 and still wants to enable
IDNs with this API, one can do a macro of the getnodeipby* functions
mapped to the IPv4 gethostby* ones, including the "IDN" flag, and then
process differently based on the presence of the flag.

9. Transition and Deployment

Deployment of this proposal means updating clients and servers, as well
as applications and protocols, and therefore a transition strategy is
proposed. Because many DNS servers do not yet handle IDNE and may take
years or decades to do so, an ASCII-compatible encoding (ACE) format for
IDN names is also needed as a transition to an all-IDNE DNS. Note that
IDNE and an ACE are not related, and do not interact in the DNS. If the
IETF chooses to have an ACE mechanism in use at the same time as IDNE,
it would be wise to choose an ACE that allows as many characters as
possible in the name parts and full names.

IDNE allows names with as many characters as current names. This means
that it is possible to create names in IDNE that are longer than those
that can be created in the ACE protocols that have been described so
far. Although not prohibited, it is unwise to create a name that can be
legally represented in IDNE but not in the ACE, or a name that can be
legally represented in the ACE but not in IDNE.

The IETF should periodically evaluate the benefits and problems
associated with having three different formats for names (STD13, IDNE,
and ACE). If at some point it is decided that the problems outweigh the
benefits, the IETF can state a time when one or more of the services
should not be used on the Internet.

10. Root Server Considerations

Because this specification uses EDNS, root servers should be prepared to
receive EDNS requests. This specification handles IDN top-level domains
in exactly the same fashion as it does every other domain.
Considerations about IDN top-level domains are outside of this work, but
the first IDN top-level domains would require all root servers to be
ready for IDNE requests.

11. IANA Considerations

[[ TBD. This section will have two parts. The first will request an EDNS
option code. The second will specify how IDNE version numbers are
allocated (namely, standards-track RFC only). ]]

12. Security Considerations

Because IDNE uses EDNS, it inherits the same security considerations as

Much of the security of the Internet relies on the DNS. Thus, any change
to the characteristics of the DNS can change the security of much of the

Host names are used by users to connect to Internet servers. The
security of the Internet would be compromised if a user entering a
single internationalized name could be connected to different servers
based on different interpretations of the internationalized host name.

Because this document normatively refers to [NAMEPREP] and [RFC2671],
it includes the security considerations from those documents as well.

13. References

[IDNCOMP] Paul Hoffman, "Comparison of Internationalized Domain Name
Proposals", draft-ietf-idn-compare.

[ISO10646] ISO/IEC 10646-1:1993. International Standard -- Information
technology -- Universal Multiple-Octet Coded Character Set (UCS) -- Part
1: Architecture and Basic Multilingual Plane.  Five amendments and a
technical corrigendum have been published up to now. UTF-16 is described
in Annex Q, published as Amendment 1. 17 other amendments are currently
at various stages of standardization. [[[ THIS REFERENCE NEEDS TO BE

[NAMEPREP] Paul Hoffman & Marc Blanchet, "Preparation of
Internationalized Host Names", draft-ietf-idn-nameprep.

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

[RFC2279] Francois Yergeau, "UTF-8, a transformation format of ISO
10646", January 1998, RFC 2279.

[RFC2460] Steve Deering & Bob Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", December 1998, RFC 2460.

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

[STD13] Paul Mockapetris, "Domain names - implementation and
specification", November 1987, STD 13 (RFC 1035).

[UNICODE3] The Unicode Consortium, "The Unicode Standard -- Version
3.0", ISBN 0-201-61633-5. Described at

A. Acknowledgements

This document is the result of the thinking of many people. The following
people made significant comments on the early drafts:

Andre Cormier
Andrew Draper
Bill Sommerfeld
Francois Yergeau

B. Changes from -01 to -02


C. Authors' Addresses

Marc Blanchet
2875 boul. Laurier, bureau 300
Sainte-Foy, QC  G1V 2M2 Canada

Paul Hoffman
Internet Mail Consortium and VPN Consortium
127 Segre Place
Santa Cruz, CA  95060 USA

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