Internet Draft Mark Davis
draft-ietf-idn-lace-00.txt IBM
November 6, 2000 Paul Hoffman
Expires May 6, 2001 IMC & VPNC
LACE: Length-based ASCII Compatible Encoding for IDN
Status of this memo
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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 a few substrings from a small number of rows of the
ISO 10646 repertoire. Run-length encoding for these types of
host names will be fairly compact, and is fairly easy to describe.
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). LACE is an ACE, called
Row-based ACE or LACE, that can be used with protocols that match arch-2
or arch-3. LACE 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).
In formal terms, LACE describes a character encoding scheme of the
ISO/IEC 10646 [ISO10646] coded character set (whose assignment of
characters is synchronized with Unicode [Unicode3]) and the rules for
using that scheme in the DNS. As such, it could also be called a
"charset" as defined in [IDNReq].
The LACE protocol has the following features:
- There is exactly one way to convert internationalized host parts to
and from LACE parts. Host name part uniqueness is preserved.
- Host parts that have no international characters are not changed.
- Names using LACE can include more internationalized characters than
with other ACE protocols that have been suggested to date. LACE-encoded
names are variable length, depending on the number of transitions
between rows in the ISO 10646 repertoire that appear in the name part.
Name parts that cannot be compressed using run-length encoding can have
up to 17 characters, and names that can be compressed can have up to 35
characters. Further, a name that has just a few row transitions
typically can have over 30 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".
LACE 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], LACE 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).
LACE has the following length characteristics. In this list, "row" means
a row from ISO 10646.
- LACE-encoded names are variable length, depending on the number of
transitions between rows that appear in the name part.
- Name parts that cannot be compressed using run-length encoding can
have up to 17 characters.
- Names that can be compressed can have up to 35 characters.
-A name that has just a few row transitions typically can have over 30
characters.
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
LACE. That is, a name part that can be represented without LACE MUST NOT
be encoded using LACE. 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
(those with codepoints above U+FFFF) 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 prohibited characters,
case-folding, or canonicalization is to be done, it MUST be done after
doing the conversion from an ACE name part.
If a decoded name part consists exclusively of characters that conform
to the host name requirements in [STD13], the conversion from LACE MUST
fail. Because a name part that can be represented without LACE MUST NOT
be encoded using LACE, the decoding process MUST check for name parts
that consists exclusively of characters that conform to the host name
requirements in [STD13] and, if such a name part is found, MUST
beconsidered an error (and possibly a security violation).
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 substring that consists
of characters all from a single row of the ISO 10646 repertoire to a
count octet followed by the row header followed by the lower octets of
the characters. If this ends up being longer than the input, the string
is not compressed, but instead has a unique one-octet header attached.
Although the uncompressed mode limits the number of characters in a LACE
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 big-endian 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/IEC 10646 characters has already
been done by a previous step in the conversion process.
1) If the length of the input is not even, or is less than 2, stop with
an error.
2) Set the input pointer, called IP, to the first octet of the input
string.
3) Set the variable called HIGH to the octet at IP.
4) Determine the number of pairs at or after IP that have HIGH as the
first octet; call this COUNT.
5) Put into an output buffer the single octet for COUNT followed by the
single octet for HIGH, followed by all those low octets. Move IP to the
end of those pairs; that is, set IP to IP+(2*(COUNT+1)).
6) If IP is not at the end of the input string, go to step 3.
7) If the length of the output buffer is less than or equal to the
length of the input buffer (in octets, not in characters), output the
buffer. Otherwise, output the octet 0xFF followed by the input buffer.
Note that there can only be one possible representation for a name part,
so that outputting the wrong name part is a serious security error.
Decompression schemes MUST accept only the valid form and MUST NOT
accept invalid forms.
2.4.2 Decompressing a string
1. Set the input pointer, called IP, to the first octet of the input
string. If there is no first octet, stop with an error.
2. If the octet at IP is 0xFF, go to step 10.
3. Get the octet at IP, call it COUNT. Set IP to IP+1. If IP is now at
the end of the input string, stop with an error.
4. Get the octet at IP, call it HIGH. Set IP to IP+1. If IP is now at
the end of the input string, stop with an error.
5. Get the octet at IP, call it LOW. Set IP to IP+1.
6. Output HIGH, then LOW, to the output buffer.
7. Decrement COUNT. If COUNT is greater than 0, go to step 5.
8. If IP is not at the end of the input buffer, go to step 3.
9. Compare the length of the input string with the length of the output
buffer. If the length of the output buffer is longer than the length of
the input buffer, stop with an error because the wrong compression form
was used. Otherwise, send out the output buffer and stop.
10. Set IP to IP+1. Copy the rest of the input buffer to the output
buffer. Compress the output buffer into a separate comparison buffer
following the steps for compression above. If the length of the
comparison buffer is less than or equal to the length of the output
buffer, stop with an error because the wrong compression form was used.
Otherwise, send out the output buffer and stop.
2.4.3 Compression examples
The five input characters <U+30E6 U+30CB U+30B3 U+30FC U+30C9> are
represented in big-endian UTF-16 as the ten octets <30 E6 30 CB 30 B3 30
FC 30 C9>. All the code units are in the same row (03). The output
buffer has seven octets <05 30 E6 CB B3 FC C9>, which is shorter than
the input string. Thus the output is <05 30 E6 CB B3 FC C9>.
The four input characters <U+012E U+0110 U+014A U+00C5> are represented
in big-endian UTF-16 as the eight octets <01 2E 01 10 01 4A 00 C5>. The
output buffer has eight octets <03 01 2E 10 4A 01 00 C5>, which is the
same length as the input string. Thus, the output is <03 01 2E 10 4A 01
00 C5>.
The three input characters <U+012E U+00D0 U+014A> are represented in
big-endian UTF-16 as the six octets <01 2E 00 D0 01 4A>. The output
buffer is nine octets <01 01 2E 01 00 D0 01 01 4A>, which is longer than
the input buffer. Thus, the output is <FF 01 2E 00 D0 01 4A>.
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, 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, LACE 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.
LACE 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
Base32 is quite obviously inspired by the tried-and-true Base64
Content-Transfer-Encoding from MIME.
B. IANA Considerations
There are no IANA considerations in this document.
C. Author Contact Information
Mark Davis
IBM
10275 N. De Anza Blvd
Cupertino, CA 95014
mark.davis@us.ibm.com and mark.davis@macchiato.com
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