draft-ietf-idn-requirements-05.txt

bind-3.2.

IETF IDN Working Group               Editors Zita Wenzel, James Seng
Internet Draft                       draft-ietf-idn-requirements-05.txt
24 April 2001                        Expires 24 October 2001

             Requirements of Internationalized Domain Names

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-
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"work in progress."

The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Intended Scope  

The intended scope of this document is to explore requirements for the
internationalization of domain names on the Internet. It is not
intended to document user requirements. It is recommended that
solutions not necessarily be within the DNS itself, but could be a layer
interjected between the application and the DNS. Proposals SHOULD
fulfill most, if not all, of the requirements. This document MAY be
updated based on clinical trials.

Abstract

This document describes the requirement for encoding international
characters into DNS names and records. This document is guidance for
developing protocols for internationalized domain names.

1. Introduction

At present, the encoding of Internet domain names is restricted to a
subset of 7-bit ASCII (ISO/IEC 646). HTML, XML, IMAP, FTP, and many
other text based items on the Internet have already been at least
partially internationalized. It is important for domain names to be
similarly internationalized or for an equivalent solution to be found.
This document assumes that the most effective solution involves putting
non-ASCII names inside some parts of the overall DNS system.

This document is being discussed on the "idn" mailing list. To join the
list, send a message to <[e-mail]> with the words
"subscribe idn" in the body of the message. Archives of the mailing
list can also be found at ftp://ops.ietf.org/pub/lists/idn*.

1.1 Definitions and Conventions

A language is a way that humans interact. In computerised form, a text
in a written language can be expressed as a string of characters.
The same set of characters can often be used for many written languages,
and many written languages can be expressed using different scripts.
The same characters are often shown with somewhat different glyphs
(shapes) for display of a text depending on the font used, the
automatic shaping applied, or the automatic formation of ligatures. In
addition, the same characters can be shown with somewhat different
glyphs (shapes) for display of a text depending on the language being
used, even within the same font or trough automatic font change.

A character is a member of a set of elements used for organization,
control, or representation of textual data.

A graphic character is a character, other than a control function,
that has a visual representation normally handwritten, printed, or
displayed.

Characters mentioned in this document are identified by their position
in the Unicode [UNICODE] character set.  This character set is also
known as the UCS [ISO10646]. The notation U+12AB, for example, indicates
the character at position 12AB (hexadecimal) in the Unicode character
set.  Note that the use of this notation is not an indication of a
requirement to use Unicode.

Examples quoted in this document should be considered as a method to
further explain the meanings and principles adopted by the document. It
is not a requirement for the protocol to satisfy the examples.

Unicode Technical Report 17 [UTR17] defines a character encoding
model in several levels (much of the text below is quoted from
Unicode Technical Report 17 [UTR17]):

1. A abstract character repertoire (ACR) is defined as the set of
   abstract characters to be encoded, normally a familiar alphabet
   or symbol set. The word abstract just means that these objects
   are defined by convention (such as the 26 letters of the English
   alphabet, uppercase and lowercase forms). Examples: the ASCII
   repertoire, the Latin-15 repertoire, the JIS X 0208 repertoire,
   the UCS repertiore (of a particular version).

2. A coded character set (CCS) is defined to be a mapping from a
   set of abstract characters to the set of non-negative integers.
   This range of integers need not be contiguous. An abstract
   character is defined to be in a coded character set if the coded
   character set maps from it to an integer. That integer is said
   to be the code point for the abstract character. That abstract
   character is then an encoded character. Examples: ASCII, Latin-15,
   JIS X 0208, the UCS.

3. A character encoding form (CEF) is a mapping from the set of integers
   used in a CCS to the set of sequences of code units. A code unit
   is an integer occupying a specified binary width in a computer
   architecture, such as a septet, an octet, or a 16-bit unit. The
   encoding form enables character representation as actual data in
   a computer. The sequences of code units do not necessarily have the
   same length. Examples: ASCII, Latin-15, Shift-JIS, UTF-16, UTF-8.

4. A character encoding scheme (CES) is a mapping of code units into
   serialized octet sequences. Character encoding schemes are relevant
   to the issue of cross-platform persistent data involving code units
   wider than a byte, where byte-swapping may be required to put data
   into the byte polarity canonical for a particular platform.

   The CES may involve two or more CCS's, and may include code units
   (e.g. single shifts, SI/SO, or escape sequences) that are not part
   of the CCS per se, but which are defined by the character encoding
   architecture and which may require an external registry of particular
   values (as for the ISO 2022 escape sequences). In such a case, the
   CES is called a compound CES. (A CES that only involves a single
   CCS is called a simple CES.)

   Examples: ASCII, Latin-15, Shift-JIS, UTF-16BE, UTF-16LE, UTF-8.

5. The mapping from an abstract character repertoire (ACR) to a
   serialised sequence of octets is called a Character Map (CM). A simple
   character map thus implicitly includes a CCS, a CEF, and a CES,
   mapping from abstract characters to code units to octets. A compound
   character map includes a compound CES, and thus includes more than one
   CCS and CEF. In that case, the abstract character repertoire for the
   character map is the union of the repertoires covered by the coded
   character sets involved.

   Character Maps are the things that in the IAB architecture get IANA
   charset identifiers. A sequence of encoded characters must be
   unambiguously mapped onto a sequence of octets by the charset. The
   charset must be specified in all instances, as in Internet
   protocols, where textual content is treated as a ordered sequence
   of octets, and where the textual content must be reconstructible
   from that sequence of octets.  Charset names are registered by the
   IANA according to procedures documented in [RFC2278]. In many cases,
   the same name is used for both a character map and for a character
   encoding scheme, such as UTF-16BE. Typically this is done for simple
   character maps when such usage is clear from context.

6. A transfer encoding syntax (TES) is a reversible transform of encoded
   data which may (or may not) include textual data represented in
   one or more character encoding schemes.  Examples: 8bit,
   Quoted-Printable, BASE64, UTF-7 (defunct), (UTF-5, and RACE).

1.2 Description of the Domain Name System

The Domain Name System is defined by [RFC1034] and [RFC1035], with
clarifications, extensions and modifications given in [RFC1123],
[RFC1996], [RFC2181], and others. Of special importance here is the
security extensions described in [RFC2535] and companions.

Over the years, many different words have been used to describe the
components of resource naming on the Internet (e.g., URI, URN); to make
certain that the set of terms used in this document are well-defined and
non-ambiguous, the definitions are given here.

A master server for a zone holds the main copy of that zone. This copy
is sometimes stored in a zone file. A slave server for a zone holds a
complete copy of the records for that zone. Slave servers MAY be either
authorized by the zone owner (secondary servers) or unauthorized
(so-called "stealth secondaries"). Master and authorized slave servers
are listed in the NS records for the zone, and are termed
"authoritative" servers. In many contexts, outside this document the
term "primary" is used interchangeably with "master" and "secondary" is
used interchangeably with "slave".

A caching server holds temporary copies of DNS records; it uses records
to answer queries about domain names. Further explanation of these terms
can be found in [RFC1034] and [RFC1996].

DNS names can be represented in multiple forms, with different
properties for internationalization. The most important ones are:

- Domain name: The binary representation of a name used internally in
  the DNS protocol. This consists of a series of components of 1-63
  octets, with an overall length limited to 255 octets (including the
  length fields).

- Master file format domain name: This is a representation of the name
  as a sequence of characters in some character sets; the common
  convention (derived from [RFC1035] section 5.1) is to represent the
  octets of the name as ASCII characters where the octet is in the set
  corresponding to the ASCII values for [a-zA-Z0-9-], using an escape
  mechanism (\x or \NNN) where not, and separating the components of the
  name by the dot character (".").

The form specified for most protocols using the DNS is a limited form of
the master file format domain name. This limited form is defined in
[RFC1034] Section 3.5 and [RFC1123]. In most implementations of
applications today, domain names in the Internet have been limited to
the much more restricted forms used, e.g., in email.  Those names are
limited to the upper- and lower-case letters a-z (interpreted in a
case-independent fashion), the digits, and the hyphen-minus, all in
ASCII.

1.3 Definition of "hostname" and "Internationalized Domain Name"

In the DNS protocols, a name is referred to as a sequence of octets.
However, when discussing requirements for internationalized domain
names, what we are looking for are ways to represent characters that
are meaningful for humans.

In this document, this is referred to as a "hostname". While this term
has been used for many different purposes over the years, it is used
here in the sense of sequence of characters (not octets) representing a
domain name conforming to the limited hostname syntax [RFC952].

This document attempts to define the requirements for an
"Internationalized Domain Name" (IDN). This is defined as a sequence of
characters that can be used in the context of functions where a hostname
is used today, but contains one or more characters that are outside the
set of characters specified as legal characters for host names
[RFC1123].

1.4 A multilayer model of the DNS function

The DNS can be seen as a multilayer function:

- The bottom layer is where the packets are passed across the Internet
  in a DNS query and a DNS response. At this level, what matters is
  the format and meaning of bits and octets in a DNS packet.

- Above that is the "DNS service", created by an infrastructure of DNS
  servers, NS records that point to those DNS servers, that is
  pointed to by the root servers (listed in the "root cache file" on
  each DNS server, often called "named.cache". It is at this level
  that the statement "the DNS has a single root" [RFC2826] makes
  sense, but still, what are being transferred are octets, not
  characters.

- Interfacing to the user is a service layer, often called "the resolver
  library", and often embedded in the operating system or system
  libraries of the client machines. It is at the top of this layer that
  the API calls commonly known as "gethostbyname" and "gethostbyaddress"
  reside.  These calls are modified to support IPv6 [RFC2553]. A
  conceptually similar layer exists in authoritative DNS servers,
  comprising the parts that generate "meaningful" strings in DNS files.
  Due to the popularity of the "master file" format, this layer often
  exists only in the administrative routines of the service maintainers.

- The user of this layer (resolver library) is the application programs
  that use the DNS, such as mailers, mail servers, Web clients, Web
  servers, Web caches, IRC clients, FTP clients, distributed file
  systems, distributed databases, and almost all other applications on
  TCP/IP.

Graphically, one can illustrate it like this:

+---------------+                            +---------------------+
| Application   |                            | (Base data)         |
+---------------+                            +---------------------+
      |  Application service interface                 |
      |  For ex. GethostbyXXXX interface               | (no standard)
+---------------+                            +---------------------+
| Resolver      |                            | Auth DNS server     |
+---------------+                            +---------------------+
      |     <-----   DNS service interface   ----->    |
+------------------------------------------------------------------+
|  DNS service                                                     |
|  +-----------------------+         +--------------------+        |
|  | Forwarding DNS server |         | Caching DNS server |        |
|  +-----------------------+         +--------------------+        |
|                                                                  |
|                 +-------------------------+                      |
|                 | Parent-zone DNS servers |                      |
|                 +-------------------------+                      |
|                                                                  |
|                 +-------------------------+                      |
|                 | Root DNS servers        |                      |
|                 +-------------------------+                      |
|                                                                  |
+------------------------------------------------------------------+

1.5 Service model of the DNS

The Domain Name Service is used for multiple purposes, each of which is
characterized by what it puts into the system (the query) and what it
expects as a result (the reply).

The most used ones in the current DNS are:

- Hostname-to-address service (A, AAAA, A6): Enter a hostname, and get
  back an IPv4 or IPv6 address.

- Hostname-to-Mail server service (MX): As above, but the expected
  return value is a hostname and a priority for SMTP servers.

- Address-to-hostname service (PTR): Enter an IPv4 or IPv6 address (in
  in-addr.arpa or ip6.int form respectively) and get back a hostname.

- Domain delegation service (NS). Enter a domain name and get back
  nameserver records (designated hosts who provides authoritive
  nameservice) for the domain.

New services are being defined, either as entirely new services (IPv6 to
hostname mapping using binary labels) or as embellishments to other
services (DNSSEC returning information about whether a given DNS service
is performed securely or not).

These services exist, conceptually, at the Application/Resolver
interface, NOT at the DNS-service interface. This document attempts to
set requirements for an equivalent of the "used services" given above,
where "hostname" is replaced by "Internationalized Domain Name". This
doesn't preclude the fact that IDN should work with any kind of DNS
queries.  IDN is a new service. Since existing protocols like SMTP or
HTTP use the old service, it is a matter of great concern how the new
and old services work together, and how other protocols can take
advantage of the new service.