HTML Working Group                                           D. Connolly
INTERNET-DRAFT                                               MIT/W3C
draft-ietf-html-charset-harmful-00.txt                       May 2, 1995
Expires November, 1995



Character Set Considered Harmful



Status of this Document



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Abstract



The term character set is often used to describe a ditigal
representation of text. ASCII is perhaps the most widely deployed
representation of text, and in the interest of interoperability,
information systems on the Internet traditionally rely on it
exclusively.

The Multipurpose Internet Mail Extensions (MIME) introduces Internet
Media Types, including text representations besides ASCII. The Hypertext
Markup Language (HTML) used in the World-Wide Web is a proposed Internet
Media Type. But HTML is also an application of Standard Generalized
Markup Language (SGML).




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In the MIME and SGML specifications, the discussion of characters
representation is notoriously complex, and apparently subtly
inconsistent or incompatible. This document presents a collection of
terms intended to reconcile the two specifications and serve as a basis
for rigorous discussion of characters and their digital representations.

Introduction



The term character set is often used to describe a ditigal
representation of text. The specification of such a representation
typically involves identifying a sufficiently expressive collection of
characters, and giving each of them a number.

In conventional mathematics terminology then, a "character set" is not
just a set of characters, but a function whose domain is a set of
integers, and whose range is a set of characters.

Some standards documents, including the SGML standard, make little or no
use of such conventional mathematical terms as function, domain and
range. Perhaps the authors of those documents intend the documents to be
comprehensible without a prior understanding of mathematics. But the
specification of notions such as the conformance of an SGML document or
SGML system are much more complex than the basics of logic and
mathematics.

In his text on Calculus [Spivak] , Michael Spivak writes:
     
     
     
     Every aspect of this book was influenced by the desire to
     present calculus not merely as a prelude to but as the first
     real encounter with mathematics. Since the foundation of
     analysis provided the arena in which modern modes of
     mathematical thinking developed, calculus ought to be the
     place in which to expect, rather than avoid, the strengthening
     of insight with logic. In addition to developing the students'
     intuition about the beautiful concepts of analysis, it is
     surely equally important to persuade them that precision and
     rigor are neither deterrents to intuition, nor ends in
     themselves, but the natural medium in which to formulate and
     think about mathematical questions.



This document is not intended as the first real encounter with
mathematics. But neither will we make any effort to avoid or apologize
for mathematical terminology. The reader is referred to the large body
of literature on logic and set theory, including a history of writings
on math and logic[SET] and Douglas Hofstadter's fascinating book [GEB] .


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Coded Character Sets



Using "character set" rather than something such as character table or
even character sequence to denote the functions that maps integers to
characters is unfortunate, but it is water under the bridge, and a lot
of it by now. Rather than attempting to divert all that water at this
point, we introduce the primitive notion of character and use it to
define the term coded character set from [ISO10646] and other standards:

character
     An atom of information
coded character set
     A function whose domain is a subset of the integers, and whose
     range is a set of characters.


Note that by the term character, we do not mean a glyph, a name, a
phoneme, nor a bit combination. A character is simply an atomic unit of
communication. It is typically a symbol whose various representations
are understood to mean the same thing by a community of people.

It might seem more intuitive to map from characters to integers, rather
than the way it is defined here. But in practice there are some coded
character sets that assign two different numbers to the same character
[Lee] , and so the inverse is not a function in the general case.

There are two other terms used in standards such as [ISO10646] that we
define in relation to the first two:

code position
     An integer. A coded character set and a code position from its
     domain determine a character.
character repertoire
     A set of characters; that is, the range of a coded character set.


Character Encoding Schemes



The only practical means for exchanging information on the Internet is
to represent it as a sequence of octets (bytes).

One way to transmit a sequence of characters is to agree on a coded
character set and transmit the character numbers of each of the
characters.





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But in practice, characters are encoded using a variety of optimizations
of this brute-force approach: code switching techniques, escape
sequences, etc. The encoding of a sequence of characters is not, in
general, the result of encoding each character independently and then
concatenating them. But it is sufficiently general to note that
sequences of characters are encoded as a sequence of bytes. So we
define:

octetan element of the set {0, 1, 2, ..., 255}
character encoding scheme
     a function whose domain is the set of sequences of octets, and
     whose range is the set of sequences of characters over some
     character repertoire.


Representation of SGML Text Entities



An SGML document is made up of entities: a text entity called the
document entity, and possibly some other text entities and data
entities.

A text entity is a sequence of characters. The representation of a text
entity is not specified by the SGML standard. For the purpose of
MIME-based interchange of SGML text entities, we define the following:

text entity
     a sequence of characters
message entity
     a pair (T, OS) where T is an Internet Media Type and OS is a
     sequence of octets.


Note that each text/* media type has an associated charset parameter,
which designates a character encoding scheme. The character encoding
scheme maps the body -- a sequence of octets -- to a text entity -- a
sequence of characters. Hence any message entity of type text/* is
equivalent to a text entity.

Numeric Character References



Numeric character references are a great source of confusion. The key
insights are that:
   * Every SGML document has exactly one document character set, which
     is a coded character set
   * Numeric character references give code positions in the document
     character set



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Example: ISO2022 Encoding with ISO10646 Coded Character Set


Consider the following message entity:
Date: Saturday, 29-Apr-95 03:53:33 GMT
MIME-version: 1.0
Content-Type: text/html; charset=iso-2022-jp

<TITLE>...</TITLE>
<BODY>
Here is some normal text.
Here is a 10646 numeric character reference &#2432;.
Here is some ISO-2022-JP text: ...
</BODY>



To interpret the message entity, we notice that the Content-Type is
text/html , so this represents a text entity. The charset parameter
iso-2022-jp , along with the octet sequence of the body, determines a
sequence of characters. The octets denoted above by '...' represent
characters, as per iso-2022-jp .

To parse the resulting text entity as per SGML, the sender and receiver
must agree on an SGML declaration, since none is present in the document
entity. For this example, we assume that SGML declaration specifies
ISO10646 as the document character set. So the numeric character
reference &#2432; is resolved with respect to ISO10646.

It may seem contradictory that the ISO-2022-JP character encoding scheme
is defined in terms of a collection of coded character sets, none of
which is ISO10646. But there is no contradiction. Each character encoded
by ISO-2022-JP is in the repertoire of one of those coded character
sets, each of which is a subset of the repertoire of ISO10646.

So while ISO-2022-JP is not sufficient for every ISO10646 document, it
is the case that ISO10646 is a sufficient document character set for any
entity encoded with ISO-2022-JP .

Example: Reducing the Repertoire of an Entity


Suppose we have an SGML document D whose document character set is the
coded character set ISO10646. We find the document entity DE in the form
of sequence of octets OS in a disk file, encoded using the Unicode-UCS-2
character encoding scheme.
        Unicode-UCS-2(OS) = DE






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We can reduce the character repertoire necessary to represent the
document entity by replacing characters outside the ISO-646-IRV
character repertoire with numeric character references:
        DE' = reduce(DE, ISO10646, ISO-646-IRV)

where

  reduce : SEQ(char) X Coded Character Set X Character Repertoire -> 
SEQ(char)

and

  reduce(c . rest, CCS, R) = if c in R, c . reduce(rest, CCS, R)
                                        else &#N; . reduce(rest, CCS, R)
                                        where CCS(N) = c


The resulting entity, DE' can then be endoded using US-ASCII
        US-ASCII(OS') = DE' = reduce(DE, ISO10646, ISO-646-IRV)


Hence, we can represent the document D as a message entity whose content
type is "text/plain; charset=US-ASCII" and whose body is OS'.

Conclusion



It is critical to keep separate the notion of a simple table of
characters and their numbers, i.e. a coded character set, separate from
the various algorithms to encoded sequences of characters, i.e.
character encoding schemes. This separation allows a representation of a
text entity which is consistent with both the MIME and SGML
specifications.

Acknowledgements



The idea for the title of this document actually came from John Klensin.
The notion of character encoding scheme was inspired by the MIME
specification by Ned Freed. James Clark, Ed Levinson, and several other
members of the MIMESGML working group collaborated in discussions
leading up to this draft. Liam Quin from SoftQuad and Gavin Nicol from
EBT have provided guidance on these issues in the past. Erik Naggum has
provided invaluable aid in understanding the SGML standard.

References





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[MIME]
     N. Borenstein and N. Freed. "MIME (Multipurpose Internet Mail
     Extensions) Part One: Mechanisms for Specifying and Describing the
     Format of Internet Message Bodies." RFC 1521, Bellcore, Innosoft,
     September 1993.
[ASCII]
     US-ASCII. Coded Character Set - 7-Bit American Standard Code for
     Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.
[ISO-8859]
     ISO 8859. International Standard -- Information Processing -- 8-bit
     Single-Byte Coded Graphic Character Sets -- Part 1: Latin Alphabet
     No. 1, ISO 8859-1:1987. Part 2: Latin alphabet No. 2, ISO 8859-2,
     1987. Part 3: Latin alphabet No. 3, ISO 8859-3, 1988. Part 4: Latin
     alphabet No. 4, ISO 8859-4, 1988. Part 5: Latin/Cyrillic alphabet,
     ISO 8859-5, 1988. Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987.
     Part 7: Latin/Greek alphabet, ISO 8859-7, 1987. Part 8:
     Latin/Hebrew alphabet, ISO 8859-8, 1988. Part 9: Latin alphabet No.
     5, ISO 8859-9, 1990.
[SGML]
     ISO 8879. Information Processing -- Text and Office Systems --
     Standard Generalized Markup Language (SGML), 1986.
[Nicol]
     The Multilingual World Wide Web , Gavin T. Nicol, Electronic Book
     Technologies, Japan gtn@ebt.com
[Lee]Private communication with Liam Quin, from SoftQuad.
[Spivak]
     Spivak, Michael. Calculus. 2nd Ed. 1967 ISBN 0-914098-77-2
[GEB]Hofstadter, Douglas R. G&ouml;del, Escher, Bach: An Eternal Golden
     Braid, 1979 ISBN 0-394-75682-7
[SET]"Investigations in the foundations of set theory I", in Jean van
     Heijenoort (ed.) _From Frege to Godel: A Source Book in
     Mathematical Logic, 1879-1931_ (Harvard U.P., 1967)





















Connolly                                                       [Page 7]