Internet Draft Paul Hoffman
draft-ietf-idn-idna-00.txt IMC & VPNC
September 12, 2000 Patrik Faltstrom
Expires in six months Cisco
Internationalizing Host Names In Applications (IDNA)
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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Abstract
The current DNS infrastructure does not provide a way to use
internationalized host names (IDN). This document describes a mechanism
that requires no changes to any DNS server or resolver that will allow
internationalized host names to be used by end users with changes only
to applications. It allows flexibility for user input and display, and
assures that host names that have non-ASCII characters are not sent to
DNS servers or resolvers.
1. Introduction
In the discussion of IDN solutions, a great deal of discussion has
focused on transition issues and how IDN will work in a world where not
all of the components have been updated. Earlier proposed solutions
require that user applications, resolvers, and DNS servers to be updated
in order for a user to use an internationalized host name. Instead of
this requirement for widespread updating of all components, the current
proposal is that only user applications be updated; no changes are
needed to the DNS protocol or any DNS servers or the resolvers on user's
computers.
1.1 Design philosophy
To date, the proposals for IDN protocols have required that DNS servers
be updated to handle internationalized host names. Because of this, the
person who wanted to use an internationalized host name had to be sure
that their request went to a DNS server that was updated for IDN.
Further, that server could only send queries to other servers that had
been updated for IDN because the queries contain new protocol elements
to differentiate IDN name parts from current host parts. In addition,
these proposals require that resolvers must be updated to use the new
protocols, and in most cases the applications would need to be updated
as well.
Updating all (or even a significant percentage) of the DNS servers in
the world will be difficult, to say the least. Because of this, we have
designed a protocol that requires no updating of any name servers. IDNA
still requires the updating of applications, but once a
user has updated these, she or he could immediately start using
internationalized host names. The cost of implementing IDN would thus be
much lower, and the speed of implementation will be much higher.
1.2 Terminology
The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", and
"MAY" in this document are to be interpreted as described in RFC 2119
[RFC2119].
1.3 IDN summary
Using the terminology in [IDNCOMP], this protocol specifies an IDN
architecture of arch-3 (just send ACE). The format is ace-1.2 (RACE),
and the method for distinguishing ACE name parts from current name parts
is ace-2.1.1 (add hopefully-unique legal tag). Because there is no
changes needed to the DNS, the transition strategy is trans-1 (always do
current plus new architecture).
2. Structural Overview
In IDNA, users' applications are updated to perform the processing
needed to input internationalized host names from users, display
internationalized host names that are returned from the DNS to users,
and process the inputs and outputs from the DNS.
2.1 Interfaces between DNS components in IDNA
The interfaces in IDNA can be represented pictorially as:
+------+
| User |
+------+
^
| Input and display: local interface methods
| (pen, keyboard, glowing phosphorus, ...)
+--------------- v -------------------------------+
| +-------------+ |
| | Application | | End system
| +-------------+
| ^
| | API call and return: nameprepped RACE
| v
| +----------+
| | Resolver |
| +----------+ |
+----------------^--------------------------------+
| DNS query and response: nameprepped RACE
v
+-------------+
| DNS servers |
+-------------+
2.1.1 Users and applications
Applications can accept host names using any character set or sets
desired by the application developer, and can display host names in any
charset. That is, this protocol does not affect the interface between
users and applications.
An IDNA-aware application can accept and display internationalized host
names in two formats: the internationalized character set(s) supported
by the application, and in RACE [RACE] ASCII-compatible encoding.
Applications MAY allow RACE input and output, but are not encouraged to
do so except as an interface for advanced users, possibly for debugging.
RACE encoding is opaque and ugly, and should thus only be exposed to
users who absolutely need it. The optional use, especially during a
transition period, of RACE encodings in the user interface is described
in section 3. Since RACE can be rendered either as the encoded ASCII
glyphs or the proper decoded character glyphs, the rendering engine for
an application SHOULD have an option for the user to select the
preferred display; if it does, rendering the RACE SHOULD NOT be the
default.
2.1.2 Applications and resolvers
Applications communicate with resolver libraries through a programming
interface (API). Typically, the IETF does not standardize APIs, although
it has for IPv6. This protocol does not specify a specific API, but
instead specifies only the input and output formats of the host names to
the resolver library.
Before converting the name parts into RACE, the application MUST prepare
each name part as specified in [NAMEPREP]. The application MUST use RACE
ASCII-compatible encoding for the name parts that are sent to the
resolver, and will always get name parts encoded in RACE from the
resolver.
IDNA-aware applications MUST be able to work with both
non-internationalized host name parts (those that conform to RFC 1035
[STD13]) and internationalized host name parts. An IDNA-aware
application that is resolving a non-internationalized host name parts
MUST NOT do any preparation or conversion to RACE on any
non-internationalized name part.
2.1.3 Resolvers and DNS servers
An operating system might have a set of libraries for converting host
names to nameprepped RACE. The input to such a library might be in one
or more charsets that are used in applications (UTF-8 and UTF-16 are
likely candidates for almost any operating system, and script-specific
charsets are likely for localized operating systems). The output would
be either the unchanged name part (if the input already conforms to [STD
13]), or the nameprepped, RACE-encoded name part. Such a library would
help keep applications smaller.
DNS servers MUST use the RACE format for internationalized host name
parts.
If a signalling system which makes negotiation possible between old and
new DNS clients and servers is standardized in the future, the encoding
of the query in the DNS protocol itself can be changed from RACE to
something else, such as UTF-8. The question whether or not this should
be used is, however, a separate problem and is not discussed in this
memo.
2.1.4 Avoiding exposing users to the raw ACE encoding
All applications that might show the user a host name that was received
from a gethostbyaddr or other such lookup SHOULD update as soon as
possible in order to prevent users from seeing the ACE. However, this is
not considered a big problem because so few applications show this type
of resolution to users.
3. Name Server Considerations
It is imperative that there be only one encoding for a particular host
name. RACE is an encoding for host name parts that use characters
outside those allowed for host names [STD 13]. Thus, a primary master
name server MUST NOT contain an RACE-encoded name that decodes to a host
name that is allowed in [STD 13].
Name servers MUST NOT have any records with host names that contain
internationalized name parts unless those name parts have be prepared
according to [NAMEPREP]. If names that are not legal in [NAMEPREP] are
passed to an application, it will result in an error being passed to the
application with no error being reported to the name server. Further, no
application will ever ask for a name that is not legal in [NAMEPREP]
because requests always go through [NAMEPREP] before getting to the DNS.
The host name data in zone files (as specified by section 5 of RFC 1035)
MUST be both nameprepped and RACE encoded.
4. Root Server Considerations
Because there are no changes to the DNS protocols, adopting this
protocol has no effect on the root servers.
5. Security Considerations
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
Internet.
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], it includes the
security considerations from that document as well.
6. References
[IDNCOMP] Paul Hoffman, "Comparison of Internationalized Domain Name
Proposals", draft-ietf-idn-compare.
[NAMEPREP] Paul Hoffman & Marc Blanchet, "Preparation of
Internationalized Host Names", draft-ietf-idn-nameprep.
[RACE] RACE: Row-based ASCII Compatible Encoding for IDN,
draft-ietf-idn-race.
[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.
[STD13] Paul Mockapetris, "Domain names - implementation and
specification", November 1987, STD 13 (RFC 1035).
A. Authors' Addresses
Paul Hoffman
Internet Mail Consortium and VPN Consortium
127 Segre Place
Santa Cruz, CA 95060 USA
phoffman@imc.org
Patrik Faltstrom
Cisco Systems
170 West Tasman Drive SJ-13/2
San Jose, CA 95134 USA
paf@cisco.com