Internet Draft                                          Paul Hoffman
draft-ietf-idn-race-02.txt                                IMC & VPNC
October 16, 2000
Expires in six months                         

        RACE: Row-based ASCII Compatible Encoding for IDN

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

This document describes a transformation method for representing
non-ASCII characters in host name parts in a fashion that is completely
compatible with the current DNS. It is a potential candidate for an
ASCII-Compatible Encoding (ACE) for internationalized host names, as
described in the comparison document from the IETF IDN Working Group.
This method is based on the observation that many internationalized
host name parts will have all their characters in one row of the ISO
10646 repertoire.


1. Introduction

There is a strong world-wide desire to use characters other than plain
ASCII in host names. Host names have become the equivalent of business
or product names for many services on the Internet, so there is a need
to make them usable by people whose native scripts are not representable
by ASCII. The requirements for internationalizing host names are
described in the IDN WG's requirements document, [IDNReq].

The IDN WG's comparison document [IDNComp] describes three potential
main architectures for IDN: arch-1 (just send binary), arch-2 (send
binary or ACE), and arch-3 (just send ACE). RACE is an ACE, called
Row-based ACE or RACE, that can be used with protocols that match arch-2
or arch-3. RACE specifies an ACE format as specified in ace-1 in
[IDNComp]. Further, it specifies an identifying mechanism for ace-2 in
[IDNComp], namely ace-2.1.1 (add hopefully-unique legal tag to the
beginning of the name part).

Author's note: although earlier drafts of this document supported the
ideas in arch-3, I no longer support that idea and instead only support
arch-2. Of course, someone else might right an IDN proposal that matches
arch-3 and use RACE as the protocol.

In formal terms, RACE describes a character encoding scheme of the ISO
10646 [ISO10646] coded character set and the rules for using that scheme
in the DNS. As such, it could also be called a "charset" as defined in
[IDNReq].

The RACE protocol has the following features:

- There is exactly one way to convert internationalized host parts to
and from RACE parts. Host name part uniqueness is preserved.

- Host parts that have no international characters are not changed.

- Names using RACE can include more internationalized characters than
with other ACE protocols that have been suggested to date. Names in the
Han, Yi, Hangul syllables, or Ethiopic scripts can have up to 17
characters, and names in most other scripts can have up to 35
characters. Further, a name that consist of characters from one
non-Latin script but also contains some Latin characters such as digits
or hyphens can have close to 33 characters.

It is important to note that the following sections contain many
normative statements with "MUST" and "MUST NOT". Any implementation that
does not follow these statements exactly is likely to cause damage to
the Internet by creating non-unique representations of host names.

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
[RFC2119].

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 names. For example, the letter "a"
may be represented as either "U+0061" or "LATIN SMALL LETTER A".

RACE converts strings with internationalized characters into
strings of US-ASCII that are acceptable as host name parts in current
DNS host naming usage. The former are called "pre-converted" and the
latter are called "post-converted".

1.2 IDN summary

Using the terminology in [IDNComp], RACE specifies an ACE format as
specified in ace-1. Further, it specifies an identifying mechanism for
ace-2, namely ace-2.1.1 (add hopefully-unique legal tag to the beginning
of the name part).

RACE has the following length characteristics. In this list, "row" means
a row from ISO 10646.

- If the characters in the input all come from the same row, up to 35
characters per name part are allowed.

- If the characters in the input come from two or more rows, neither of
which is row 0, up to 17 characters per name part are allowed.

- If the characters in the input come from two rows, one of which is row
0, between 17 and 33 characters per name part are allowed.


2. Host Part Transformation

According to [STD13], host parts must be case-insensitive, start and
end with a letter or digit, and contain only letters, digits, and the
hyphen character ("-"). This, of course, excludes any internationalized
characters, as well as many other characters in the ASCII character
repertoire. Further, domain name parts must be 63 octets or shorter in
length.

2.1 Name tagging

All post-converted name parts that contain internationalized characters
begin with the string "bq--". (Of course, because host name parts are
case-insensitive, this might also be represented as "Bq--" or "bQ--" or
"BQ--".) The string "bq--" was chosen because it is extremely unlikely
to exist in host parts before this specification was produced. As a
historical note, in late August 2000, none of the second-level host name
parts in any of the .com, .edu, .net, and .org top-level domains began
with "bq--"; there are many tens of thousands of other strings of three
characters followed by a hyphen that have this property and could be
used instead. The string "bq--" will change to other strings with the
same properties in future versions of this draft.

Note that a zone administrator might still choose to use "bq--" at the
beginning of a host name part even if that part does not contain
internationalized characters. Zone administrators SHOULD NOT create host
part names that begin with "bq--" unless those names are post-converted
names. Creating host part names that begin with "bq--" but that are not
post-converted names may cause two distinct problems. Some display
systems, after converting the post-converted name part back to an
internationalized name part, might display the name parts in a
possibly-confusing fashion to users. More seriously, some resolvers,
after converting the post-converted name part back to an
internationalized name part, might reject the host name if it contains
illegal characters.

2.2 Converting an internationalized name to an ACE name part

To convert a string of internationalized characters into an ACE name
part, the following steps MUST be preformed in the exact order of the
subsections given here.

If a name part consists exclusively of characters that conform to the
host name requirements in [STD13], the name MUST NOT be converted to
RACE. That is, a name part that can be represented without RACE MUST NOT
be encoded using RACE. This absolute requirement prevents there from
being two different encodings for a single DNS host name.

If any checking for prohibited name parts (such as ones that are
prohibited characters, case-folding, or canonicalization) is to be done,
it MUST be done before doing the conversion to an ACE name part.

The input name string consists of characters from the ISO 10646
character set in big-endian UTF-16 encoding. This is the pre-converted
string.

Characters outside the first plane of characters (that is, outside the
first 0xFFFF characters) MUST be represented using surrogates, as
described in the UTF-16 description in ISO 10646.

2.2.1 Compress the pre-converted string

The entire pre-converted string MUST be compressed using the compression
algorithm specified in section 2.4. The result of this step is the
compressed string.

2.2.2 Check the length of the compressed string

The compressed string MUST be 36 octets or shorter. If the compressed
string is 37 octets or longer, the conversion MUST stop with an error.

2.2.3 Encode the compressed string with Base32

The compressed string MUST be converted using the Base32 encoding
described in section 2.5. The result of this step is the encoded string.

2.2.4 Prepend "bq--" to the encoded string and finish

Prepend the characters "bq--" to the encoded string. This is the host
name part that can be used in DNS resolution.

2.3 Converting a host name part to an internationalized name

The input string for conversion is a valid host name part. Note that if
any checking for prohibited name parts (such as ones that are already
legal DNS name parts), prohibited characters, case-folding, or
canonicalization is to be done, it MUST be done after doing the
conversion from an ACE name part. (Previous versions of this draft
specified these steps.)

2.3.1 Strip the "bq--"

The input string MUST begin with the characters "bq--". If it does not,
the conversion MUST stop with an error. Otherwise, remove the characters
"bq--" from the input string. The result of this step is the stripped
string.

2.3.2 Decode the stripped string with Base32

The entire stripped string MUST be checked to see if it is valid Base32
output. The entire stripped string MUST be changed to all lower-case
letters and digits. If any resulting characters are not in Table 1, the
conversion MUST stop with an error; the input string is the
post-converted string. Otherwise, the entire resulting string MUST be
converted to a binary format using the Base32 decoding described in
section 2.5. The result of this step is the decoded string.

2.3.3 Decompress the decoded string

The entire decoded string MUST be converted to ISO 10646 characters
using the decompression algorithm described in section 2.4. The result
of this is the internationalized string.

2.4 Compression algorithm

The basic method for compression is to reduce a full string that
consists of characters all from a single row of the ISO 10646
repertoire, or all from a single row plus from row 0, to as few octets
as possible. Any full string that has characters that come from two
rows, neither of which are row 0, or three or more rows, has all the
octets of the input string in the output string.

If the string comes from only one row, compression is to one octet per
character in the string. If the string comes from only one row other
than row 0, but also has characters only from row 0, compression is to
one octet for the characters from the non-0 row and two octets for the
characters from row 0. Otherwise, there is no compression and the output
is a string that has two octets per input character.

The compressed string always has a one-octet header. If the string comes
from only one row, the header octet is the upper octet of the
characters. If the string comes from only one row other than row 0, but
also has characters only from row 0, the header octet is the upper octet
of the characters from the non-0 row. Otherwise, the header octet is
0xD8, which is the upper octet of a surrogate pair. Design note: It is
impossible to have a legal stream of UTF-16 characters that has all the
upper octets being 0xD8 because a character whose upper octet is 0xD8
must be followed by one whose upper octet is in the range 0xDC through
0xDF.

Although the two-octet mode limits the number of characters in a RACE
name part to 17, this is still generally enough for almost all names in
almost scripts. Also, this limit is close to the limits set by other
encoding proposals.

Note that the compression and decompression rules MUST be followed
exactly. This requirement prevents a single host name part from having
two encodings. Thus, for any input to the algorithm, there is only one
possible output. An implementation cannot chose to use one-octet mode or
two-octet mode using anything other than the logic given in this
section.

2.4.1 Compressing a string

The input string is in UTF-16 encoding with no byte order mark.

Design note: No checking is done on the input to this algorithm. It is
assumed that all checking for valid ISO 10646 characters has already
been done by a previous step in the conversion process.

Design note: In step 5, 0xFF was chosen as the escape character because
it appears in the fewest number of scripts in ISO 10646, and therefore
the "escaped escape" will be needed the least. 0x99 was chosen as the
second octet for the "escaped escape" because the character U+0099 has
no value, and is not even used as a control character in the C1 controls
or in ISO 6429.

1) Read each pair of octets in the input stream, comparing the upper
octet of each. If all of the upper octets (called U1) are the same, go
to step 4.

2) Read each pair of octets in the input stream, comparing the upper
octet of each. If all of the upper octets are either 0 or one single
other value (called U1), go to step 4.

3) Output 0xD8, followed by the entire input stream. Finish.

4) Output U1.

5) If you are at the end of the input string, finish. Otherwise, read
the next octet, called U2, and the octet after that, called N1.

6) If U2 is equal to U1, and N1 is not equal to 0xFF, output N1, and go
to step 5.

7) If U2 is equal to U1, and N1 is equal to 0xFF, output 0xFF followed
by 0x99, and go to step 5.

8) Output 0xFF followed by N1. Go to step 5.

2.4.2 Decompressing a string

1) Read the first octet of the input string. Call the value of the first
octet U1. If U1 is 0xD8, go to step 7.

2) If you are at the end of the input string, finish. Otherwise, read
the next octet in the input string, called N1. If N1 is 0xFF, go to step
4.

3) Output U1 followed by N1. Go to step 2.

4) If you are at the end of the input string, stop with an error.

5) Read the next octet of the input string, called N1. If N1 is 0x99,
output U1 followed by 0xFF, and go to step 2.

6) Output 0x00 followed by N1. Go to step 2.

7) Read the rest of the input stream and put it in the output stream.
Finish.

2.4.3 Compression examples

For the input string of <U+012E><U+0110><U+014A>, all characters are in
the same row, 0x01. Thus, the output is 0x012E104A.

For the input string of <U+012E><U+00D0><U+014A>, the characters are all
in row 0x01 or row 0x00. Thus, the output is 0x012EFFD04A.

For the input string of <U+1290><U+12FF><U+120C>, the characters are all
in row 0x12. Thus, the output is 0x1290FF990C.

For the input string of <U+012E><U+00D0><U+24C3>, the characters are
from two rows other than 0x00. Thus, the output is 0xD8012E00D024C3.

2.5 Base32

In order to encode non-ASCII characters in DNS-compatible host name parts,
they must be converted into legal characters. This is done with Base32
encoding, described here.

Table 1 shows the mapping between input bits and output characters in
Base32. Design note: the digits used in Base32 are "2" through "7"
instead of "0" through "6" in order to avoid digits "0" and "1". This
helps reduce errors for users who are entering a Base32 stream and may
misinterpret a "0" for an "O" or a "1" for an "l".

                    Table 1: Base32 conversion
             bits   char  hex         bits   char  hex
             00000   a    0x61        10000   q    0x71
             00001   b    0x62        10001   r    0x72
             00010   c    0x63        10010   s    0x73
             00011   d    0x64        10011   t    0x74
             00100   e    0x65        10100   u    0x75
             00101   f    0x66        10101   v    0x76
             00110   g    0x67        10110   w    0x77
             00111   h    0x68        10111   x    0x78
             01000   i    0x69        11000   y    0x79
             01001   j    0x6a        11001   z    0x7a
             01010   k    0x6b        11010   2    0x32
             01011   l    0x6c        11011   3    0x33
             01100   m    0x6d        11100   4    0x34
             01101   n    0x6e        11101   5    0x35
             01110   o    0x6f        11110   6    0x36
             01111   p    0x70        11111   7    0x37

2.5.1 Encoding octets as Base32

The input is a stream of octets. However, the octets are then treated
as a stream of bits.

Design note: The assumption that the input is a stream of octets
(instead of a stream of bits) was made so that no padding was needed.
If you are reusing this algorithm for a stream of bits, you must add a
padding mechanism in order to differentiate different lengths of input.

1) Set the read pointer to the beginning of the input bit stream.

2) Look at the five bits after the read pointer. If there are not five
bits, go to step 5.

3) Look up the value of the set of five bits in the bits column of
Table 1, and output the character from the char column (whose hex value
is in the hex column).

4) Move the read pointer five bits forward. If the read pointer is at
the end of the input bit stream (that is, there are no more bits in the
input), stop. Otherwise, go to step 2.

5) Pad the bits seen until there are five bits.

6) Look up the value of the set of five bits in the bits column of
Table 1, and output the character from the char column (whose hex value
is in the hex column).

2.5.2 Decoding Base32 as octets

The input is octets in network byte order. The input octets MUST be
values from the second column in Table 1.

1) Set the read pointer to the beginning of the input octet stream.

2) Look up the character value of the octet in the char column (or hex
value in hex column) of Table 1, and output the five bits from the bits
column.

3) Move the read pointer one octet forward. If the read pointer is at
the end of the input octet stream (that is, there are no more octets in
the input), stop. Otherwise, go to step 2.

2.5.3 Base32 example

Assume you want to encode the value 0x3a270f93. The bit string is:

3   a    2   7    0   f    9   3
00111010 00100111 00001111 10010011

Broken into chunks of five bits, this is:

00111 01000 10011 10000 11111 00100 11

Padding is added to make the last chunk five bits:

00111 01000 10011 10000 11111 00100 11000

The output of encoding is:

00111 01000 10011 10000 11111 00100 11000
  h     i     t     q     7     e     y
or "hitq7ey".


3. 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. Thus, RACE makes no changes to the DNS
itself.

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.

RACE is designed so that every internationalized host name part
can be represented as one and only one DNS-compatible string. If there
is any way to follow the steps in this document and get two or more
different results, it is a severe and fatal error in the protocol.


4. References

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

[IDNReq] James Seng, "Requirements of Internationalized Domain Names",
draft-ietf-idn-requirement.

[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 UPDATED AFTER DETERMINING ACCEPTABLE WORDING ]]]

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

[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
<http://www.unicode.org/unicode/standard/versions/Unicode3.0.html>.


A. Acknowledgements

Mark Davis contributed many ideas to the initial draft of this document.
Graham Klyne and Martin Duerst offered technical comments on the
algorithms used. GIM Gyeongseog and Pongtorn Jentaweepornkul helped fix
technical errors in early drafts.

Base32 is quite obviously inspired by the tried-and-true Base64
Content-Transfer-Encoding from MIME.



B. Changes from Versions -01 to -02 of this Draft

Removed section 1.3 (open issues) because no one said anything
in support of either proposal.

Added the prohibition on encoding a string that is already in
STD13 format in section 2.2.


C. IANA Considerations

There are no IANA considerations in this document.


D. Author Contact Information

Paul Hoffman
Internet Mail Consortium and VPN Consortium
127 Segre Place
Santa Cruz, CA  95060 USA
paul.hoffman@imc.org and paul.hoffman@vpnc.org
