Internet Draft Dan Oscarsson
draft-ietf-idn-sace-00.txt Telia ProSoft
Expires: 27 February 2001 27 August 2000
Simple ASCII Compatible Encoding (SACE)
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 way to encode non-ASCII characters in host
names in a way that is completely compatible with the current ASCII
only host names that are used in DNS. It can be used both with DNS to
support software only handling ASCII host names and as a way to
downgrade from 8-bit text to ASCII in protocols.
1. Introduction
This document defines an ASCII Compatible Encoding (ACE) of names
that can be used when communicating with DNS. It is needed during a
transition period when non-ASCII names are introduced in DNS to avoid
breaking programs expecting ASCII only.
The Simple ASCII Compatible Encoding (SACE) defined here can be
compared to [RACE]. The main differences are:
- RACE encodes by first compressing and the encoding the resulting
bit stream into ASCII. SACE encodes each character directly in one
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pass.
- SACE recognises that at lot of latin based names are mostly
composed of ASCII characters and gives a higher compression for
those. In the 63 byte limit of DNS RACE will allow 36 characters
for ISO 8859-1 and less if characters from the additional Latin
characters are needed. SACE will allow around 40 characters if
about 10 % of a Latin name is non-ASCII (in the UCS [ISO10646]
range 0-0x217). SACE is closer to the compression that UTF-8 have
than RACE.
- Most ASCII characters will not be encoded so Latin based names
composed of mostly ASCII characters will be somewhat readable.
1.1 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Simple ASCII Compatible Encoding
The encoding encodes values using the available characters allowed in
a ASCII host name (a-z0-9 and hyphen).
Values are encoded as follows:
Character - value mapping
value character value character
0 a 18 s
1 b 19 t
2 c 20 u
3 d 21 v
4 e 22 w
5 f 23 x
6 g 24 y
7 h 25 z
8 i 26 1
9 j 27 2
10 k 28 3
11 l 29 4
12 m 30 7
13 n 31 9
14 o 32 0
15 p 33 8
16 q 34 5
17 r 35 6
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In the following description the following syntax will be used:
B => one value in the range 0-35 mapped to a character as above
X => one value in the range 0-31 mapped to a character as above
Each UCS character is identified as follows:
latin => a character in the range 0-0x217
10bit => a character in the range 0x218-0x2FFF
base36 => all other characters
During encoding/decoding a string a current mode is used. In each
mode characters are encoded like this:
latin => as themselves, 00 for 0, 88 for 8 or as 10 bit value
encoded as 0XX (two 5 bit values)
10bit => as 15 bits represented by its current prefix of 5 bits
followed by 10 bits encoded as XX
(the value is the 15 bits of prefix and
10 bits concatenated)
base36 => as a base 36 value represented by its current base 36
prefix followed by three base 36 digits encoded as BBB
(the value is prefix*36*36*36*36+B*36*36+B*36+B)
Before encoding the character value must first be
reduced:
if >= 0xd800 reduce by 8192 (private/surrogate start)
then reduce by 0x2FFF.
After decoding the character value need to be restored
as
add 0x2FFF
followed by adding 8192 if >= 0xd800
2.1 Decoding a string
During decode you start with:
Mode: latin
10bit prefix: 0
base36 prefix: 0
Then the characters in an encoded string are interpreted as follows
depending on current mode:
When in latin mode:
00 => the character 0
0XX => XX represents 10 bits which decodes to one character
88 => the character 8
85 => switch to 10bit mode with same prefix as last time
8X5 => switch 10 10bit mode setting X as current 10bit prefix
87 => switch to base36 mode with same prefix as last time
8X7 => switch to base36 mode setting X as current base36 prefix
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other => the characters represent itself
When in 10bit mode
- => the character -
0 => switch to latin mode
X5 => switch 10 10bit mode using X as current prefix
7 => switch to base36 mode with same prefix as last time
X7 => switch to base36 mode using X as current prefix
XX => current 10bit prefix plus XX gives the character
When in base36 mode
-- => the character -
-0 => switch to latin mode
-5 => switch to 10bit mode with same prefix as last time
-X5 => switch 10 10bit mode setting X as current prefix
-X7 => switch to base36 mode setting X as current prefix
XXX => current base36 prefix plus XXX as base 36 values gives
character
2.2 Encoding a string
To encode a string you start with the data as UCS characters and:
Mode: latin
10bit prefix: 0
base36 prefix: 0
Then for each UCS character, the mode and/or prefix is switched if
needed and then the character is encoded as defined above.
3. References
[RFC2119] Scott Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", March 1997, RFC 2119.
[RFC2279] F. Yergeau, "UTF-8, a transformation format of ISO 10646",
RFC 2279, January 1998.
[ISO10646] ISO/IEC 10646-1:2000. International Standard --
Information technology -- Universal Multiple-Octet Coded
Character Set (UCS)
[Unicode] 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
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[IDNREQ] James Seng, "Requirements of Internationalized Domain
Names", draft-ietf-idn-requirement.
[RACE] Paul Hoffman, "RACE: Row-based ASCII Compatible Encoding
for IDN", draft-ietf-idn-race.
4. Acknowledgements
Paul Hoffman for many good ideas.
Author's Address
Dan Oscarsson
Telia ProSoft AB
Box 85
201 20 Malmo
Sweden
E-mail: Dan.Oscarsson@trab.se
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