Comments on MIME/SGML

Daniel W. Connolly (connolly@hal.com)
Fri, 25 Feb 1994 10:59:17 --100

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Content-Type: text/plain; charset="us-ascii"
Content-ID: <10024.761615492.2@ulua>
Content-Description: MIME/SGML as text

Comments on MIME/SGML
PROPOSED CHANGES FOR MIME REPRESENTATION OF SGML

Daniel W. Connolly <connolly@hal.com>

SGML Entites in MIME

I believe that the goals of Mr. Levinson's MIME Content-types for SGML
Documents[1] are essential to the success of the intenet as an Integrated
Open Hypermedia (c.f. HyTime[2]) system.

However, after a careful reading of the SGML standard[3] (specifically
section 6: Entity Structure), I believe that SGML/MIME[4] fails to specify
the most important machine representation of an SGML document; that is, the
SGML document entity (production 2, section 6.2).

In conventional practice, an SGML document "doc" of type "T" is represented
as a file doc.sgml, which looks like:

<!DOCTYPE T SYSTEM "t.dtd" [
<!ENTITY fig1 SYSTEM "foo.ps" postscript>
]>
<T>blah blah blah <figure graphic=fig1> blah blah blah</T>

along with an SGML declaration in the file T.decl. Technically speaking, the
SGML document entity is the concatenation of T.decl and doc.sgml (with
perhaps some system-specific newline->RS/RE conversions). But according to
the standard it's OK to interchange SGML documents with an implied SGML
delcaration, and in practice, the SGML declaration is often compiled into
the processing software.

So for all intents and purposes, the file doc.html is an SGML document
entity. And it seems critical that the file doc.html should correspond to
the body of some MIME body part.

The draft[5] misuses the term "DTD", aka "Document Type Definition" (defn
4.104). An SGML document indeed has three parts: the SGML declaration, the
prologue, and the instance. And the distinction between the term prologue
and the term DTD is not trivial.

First, to be pedantic, a DTD is not generally representable in SGML syntax.
The concept of the DTD includes not only the SGML-representable formal part,
but also the associated application conventions which cannot be represented
in SGML. Also -- a document may have more than one DTD in its prologue.

Second, to be practical, the conventional machine representation of a DTD is
just an SGML text entity in the file t.dtd which looks like:

<!NOTATION postscript PUBLIC "-/Adobe/Postscript">
<!ELEMENT T - - (#PCDATA) >
..

Note that it does not correspond exactly to the prologue of the document, in
that it does not contain the <!DOCTYPE [ ... ]> markup.

For these reasons, I suggest the following modifications to the proposed
MIME representation of SGML documents:

1. We make the following correspondence between the terms of the SGML
standard and the MIME RFC:

SGML notation => MIME content-type

SGML SYSTEM identifier => MIME Content-ID

SGML data entity (= notation, data)

=> MIME body part (=content type, body)

SGML text entity (= sequence of characters)

=> body of MIME text/sgml body part (= seq of chars)

SGML document => a MIME multipart/SGML body part

2. We use text/sgml in stead of application/sgml for the SGML "files", since
they are in general readable on teletype devices.e

3. We change the parameters of the multipart/SGML content type from

sgml-part := "intance" / "declaration"
/ "dtd" / "fosi" / extension-token

sgml-part := "document" / "declaration"
/ "dtd" / "fosi" / extension-token

where "document" is required, declaration is optional, and dtd is acutally
redundant (since it's in the document entity) but useful, since a MIME UA
might want to know what kind of document it is without parsing the document.

References

Network Working Group, Internet Draft: MIME/SGML,
<draft-levinson-sgml-01.txt>, E. Levinson, Accurate Information Systems,
Inc., January 17, 1993

ISO 8879:1986, Information Processing: Text and Office Systems:
Standard Generalized Markup Language (SGML)

ISO/IEC 10744 Information technology -- Hypermedia/Time-based Structuring
Language (HyTime)

Editorial Note

I used HTML because it works to a certain extent, not because I think it's
exactly how I think internet IOH should work. My comments on HTML are still
under development. See my notebook on the design on an HTML successor[6].

Production Note

This document is brought to you by the following tools:

Lucid emacs

html-mode by Marc Andressen

SGMLs

NCSA Mosaic

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Content-Type: multipart/x-sgml; boundary="----- =_aaaaaaaaaa1";
document="10024.761615492.5@ulua";
dtd="10024.761615492.6@ulua";
declaration="10024.761615492.7@ulua"
Content-ID: <10024.761615492.3@ulua>

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Content-ID: <10024.761615492.4@ulua>
Content-Description: comments on MIME/SGML as html
Content-Transfer-Encoding: quoted-printable

<HEAD>
<TITLE>Comments on MIME/SGML</TITLE>
</HEAD>
<BODY>
<H1>Proposed Changes for MIME representation of SGML
</H1>

<ADDRESS>Daniel W. Connolly <connolly@hal.com></ADDRESS>

<H2>SGML Entites in MIME</H2>

I believe that the goals of Mr. Levinson's <CITE><A HREF=3D"#r1">MIME
Content-types for SGML Documents</A></CITE> are essential to the
success of the intenet as an Integrated Open Hypermedia (c.f. <A
HREF=3D"#HyTime">HyTime</A>) system. <P>

However, after a careful reading of <A HREF=3D"#SGML">the SGML
standard</A> (specifically section 6: Entity Structure), I believe
that <A HREF=3D"#r1">SGML/MIME</A> fails to specify the most
important machine representation of an SGML document; that is, the
SGML document entity (production 2, section 6.2). <P>

In conventional practice, an SGML document "doc" of type "T" is
represented as a file doc.sgml, which looks like:

So for all intents and purposes, the file doc.html is an SGML document
entity. And it seems critical that the file doc.html should correspond
to the body of some MIME body part. <P>

<A HREF=3D"#r1">The draft</A> misuses the term "DTD", aka "Document Type
Definition" (defn 4.104). An SGML document indeed has three parts: the
SGML declaration, the <EM>prologue</EM>, and the instance. And the
distinction between the term prologue and the term DTD is not trivial. <P>

First, to be pedantic, a DTD is <EM>not</EM> generally representable
in SGML syntax. The concept of the DTD includes not only the
SGML-representable formal part, but also the associated application
conventions which cannot be represented in SGML. Also -- a document
may have more than one DTD in its prologue. <P>

Second, to be practical, the conventional machine representation of a
DTD is just an SGML text entity in the file t.dtd which looks like: <P>

Note that it does <EM>not</EM> correspond exactly to the prologue of
the document, in that it does not contain the <CODE><!DOCTYPE [ ...
]></CODE> markup. <P>

For these reasons, I suggest the following modifications to the
proposed MIME representation of SGML documents: <P>

1. We make the following correspondence between the terms of the SGML
standard and the MIME RFC:

<UL>
<LI>SGML notation =3D> MIME content-type
<LI> SGML SYSTEM identifier =3D> MIME Content-ID
<LI> SGML data entity (=3D notation, data)
=3D> MIME body part (=3Dcontent type, body)
<LI> SGML text entity (=3D sequence of characters)
=3D> body of MIME text/sgml body part (=3D seq of chars)
<LI> SGML document =3D> a MIME multipart/SGML body part
</UL>

2. We use text/sgml in stead of application/sgml for the SGML "files",
since they are in general readable on teletype devices.e <P>

3. We change the parameters of the multipart/SGML content type from

<PRE>
sgml-part :=3D "intance" / "declaration"
/ "dtd" / "fosi" / extension-token

</PRE>

<PRE>
sgml-part :=3D "document" / "declaration"
/ "dtd" / "fosi" / extension-token

</PRE>

where "document" is required, declaration is optional, and dtd is
acutally redundant (since it's in the document entity) but useful,
since a MIME UA might want to know what kind of document it is without
parsing the document.

<H2>References</H2>

<OL>
<LI> =

<A NAME=3D"r1">
Network Working Group,
Internet Draft: MIME/SGML,
<draft-levinson-sgml-01.txt>,
E. Levinson,
Accurate Information Systems, Inc.,
January 17, 1993
</A>

<LI> =

<A NAME=3D"SGML">ISO 8879:1986, Information Processing: Text and Office Sy=
stems:
Standard Generalized Markup Language (SGML)
</A>

<LI><A NAME=3D"HyTime">ISO/IEC 10744 Information technology -- Hypermedia/=
Time-based
Structuring Language (HyTime)
</A>
</OL>

<H2>Editorial Note</H2>

I used HTML because it works to a certain extent, not because I think
it's exactly how I think internet IOH should work. My comments on HTML
are still under development. See <A
HREF=3D"http://www-external.hal.com/~connolly/html-design.html">my
notebook on the design on an HTML successor</A>.

<H2>Production Note</H2>

This document is brought to you by the following tools:

<UL>
<LI> Lucid emacs
<LI> html-mode by Marc Andressen
<LI> SGMLs
<LI> NCSA Mosaic
</UL>

</BODY>

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Content-Type: text/x-sgml; charset="us-ascii"
Content-ID: <10024.761615492.5@ulua>
Content-Description: SGML document wrapper

<!DOCTYPE HTML SYSTEM "10024.761615492.6@ulua"
-- PUBLIC "-//IETF/DRAFT/ietf-iiir-html-01" @@ -- [

<!ENTITY web-node SYSTEM "10024.761615492.4@ulua">
]>
&web-node;

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Content-Type: text/x-sgml; charset="us-ascii"
Content-ID: <10024.761615492.6@ulua>
Content-Description: HTML dtd
Content-Transfer-Encoding: quoted-printable

<!-- Regarding clause 6.1, SGML Document:

[1] SGML document =3D SGML document entity,
(SGML subdocument entity |
SGML text entity | non-SGML data entity)*

The role of SGML document entity is filled by this DTD,
followed by the conventional HTML data stream.
-->

<!ENTITY % heading "H1|H2|H3|H4|H5|H6" >
<!ENTITY % list " UL | OL | DIR | MENU ">
<!ENTITY % literal " XMP | LISTING ">

<!ENTITY % headelement
" TITLE | NEXTID |ISINDEX" >

<!ENTITY % oldstyle "%headelement | %bodyelement | #PCDATA">

<!ENTITY % URL "CDATA"
-- The term URL means a CDATA attribute
whose value is a Uniform Resource Locator,
as defined. (A URN may also be usable here when defined.)
-->

<!ENTITY % linkattributes
"NAME NMTOKEN #IMPLIED
HREF %URL; #IMPLIED
REL CDATA #IMPLIED -- forward relationship type --
REV CDATA #IMPLIED -- reversed relationship type
to referent data:

PARENT CHILD, SIBLING, NEXT, TOP,
DEFINITION, UPDATE, ORIGINAL etc. --

URN CDATA #IMPLIED -- universal resource number --

TITLE CDATA #IMPLIED -- advisory only --

METHODS NAMES #IMPLIED -- supported public methods of the object:
TEXTSEARCH, GET, HEAD, ... --

<!ELEMENT HTML O O (( HEAD | BODY | %oldstyle )*, PLAINTEXT?)>

<!ELEMENT HEAD - - ( TITLE? & ISINDEX? & NEXTID? & LINK*
& BASE?)>

<!ELEMENT TITLE - - RCDATA
-- The TITLE element is not considered part of the flow of text.
It should be displayed, for example as the page header or
window title.
-->

<!ELEMENT ISINDEX - O EMPTY
-- WWW clients should offer the option to perform a search on
documents containing ISINDEX.
-->

<!ELEMENT NEXTID - O EMPTY>
<!ATTLIST NEXTID N NAME #REQUIRED
-- The number should be a name suitable for use
for the ID of a new element. When used, the value
has its numeric part incremented. EG Z67 becomes Z68
-->
<!ELEMENT LINK - O EMPTY>
<!ATTLIST LINK
%linkattributes>
=

<!ELEMENT BASE - O EMPTY -- Reference context for URLS -->
<!ATTLIST BASE

HREF %URL; #IMPLIED

>
<!ENTITY % inline "EM | TT | STRONG | B | I | U |
CODE | SAMP | KBD | KEY | VAR | DFN | CITE "
>

<!ELEMENT (%inline;) - - (#PCDATA)>

<!ENTITY % text "#PCDATA | IMG | %inline;">

<!ENTITY % htext "A | %text" -- Plus links, no structure -->

<!ELEMENT BODY - - (%bodyelement|%htext;)*>

<!ELEMENT A - - (%text)>
<!ATTLIST A
%linkattributes;
>

<!ELEMENT IMG - O EMPTY -- Embedded image -->
<!ATTLIST IMG
SRC %URL; #IMPLIED -- URL of document to embed --
>

<!ELEMENT P - O EMPTY -- separates paragraphs -->
<!ELEMENT HR - O EMPTY -- horizontal rule -->

<!ELEMENT ( %heading ) - - (%htext;)+>

<!ELEMENT DL - - (DT | DD | %stext;)*>

<!ELEMENT DT - O EMPTY>
<!ELEMENT DD - O EMPTY>

<!ELEMENT LI - O EMPTY>

<!ELEMENT BLOCKQUOTE - - (%htext;|P)+
-- for quoting some other source -->

<!ELEMENT ADDRESS - - (%htext;|P)+>

<!ELEMENT PRE - - (#PCDATA|%inline|A|P)+>
<!ATTLIST PRE
WIDTH NUMBER #implied
>

<!ENTITY AElig "Æ" -- capital AE diphthong (ligature) -->
<!ENTITY Aacute "Á" -- capital A, acute accent -->
<!ENTITY Acirc "Â" -- capital A, circumflex accent -->
<!ENTITY Agrave "À" -- capital A, grave accent -->
<!ENTITY Aring "Å" -- capital A, ring -->
<!ENTITY Atilde "Ã" -- capital A, tilde -->
<!ENTITY Auml "Ä" -- capital A, dieresis or umlaut mark -->
<!ENTITY Ccedil "Ç" -- capital C, cedilla -->
<!ENTITY ETH "Ð" -- capital Eth, Icelandic -->
<!ENTITY Eacute "É" -- capital E, acute accent -->
<!ENTITY Ecirc "Ê" -- capital E, circumflex accent -->
<!ENTITY Egrave "È" -- capital E, grave accent -->
<!ENTITY Euml "Ë" -- capital E, dieresis or umlaut mark -->
<!ENTITY Iacute "Í" -- capital I, acute accent -->
<!ENTITY Icirc "Î" -- capital I, circumflex accent -->
<!ENTITY Igrave "Ì" -- capital I, grave accent -->
<!ENTITY Iuml "Ï" -- capital I, dieresis or umlaut mark -->
<!ENTITY Ntilde "Ñ" -- capital N, tilde -->
<!ENTITY Oacute "Ó" -- capital O, acute accent -->
<!ENTITY Ocirc "Ô" -- capital O, circumflex accent -->
<!ENTITY Ograve "Ò" -- capital O, grave accent -->
<!ENTITY Oslash "Ø" -- capital O, slash -->
<!ENTITY Otilde "Õ" -- capital O, tilde -->
<!ENTITY Ouml "Ö" -- capital O, dieresis or umlaut mark -->
<!ENTITY THORN "Þ" -- capital THORN, Icelandic -->
<!ENTITY Uacute "Ú" -- capital U, acute accent -->
<!ENTITY Ucirc "Û" -- capital U, circumflex accent -->
<!ENTITY Ugrave "Ù" -- capital U, grave accent -->
<!ENTITY Uuml "Ü" -- capital U, dieresis or umlaut mark -->
<!ENTITY Yacute "Ý" -- capital Y, acute accent -->
<!ENTITY aacute "á" -- small a, acute accent -->
<!ENTITY acirc "â" -- small a, circumflex accent -->
<!ENTITY aelig "æ" -- small ae diphthong (ligature) -->
<!ENTITY agrave "à" -- small a, grave accent -->
<!ENTITY amp "&" -- ampersand -->
<!ENTITY aring "å" -- small a, ring -->
<!ENTITY atilde "ã" -- small a, tilde -->
<!ENTITY auml "ä" -- small a, dieresis or umlaut mark -->
<!ENTITY ccedil "ç" -- small c, cedilla -->
<!ENTITY eacute "é" -- small e, acute accent -->
<!ENTITY ecirc "ê" -- small e, circumflex accent -->
<!ENTITY egrave "è" -- small e, grave accent -->
<!ENTITY eth "ð" -- small eth, Icelandic -->
<!ENTITY euml "ë" -- small e, dieresis or umlaut mark -->
<!ENTITY gt ">" -- greater than -->
<!ENTITY iacute "í" -- small i, acute accent -->
<!ENTITY icirc "î" -- small i, circumflex accent -->
<!ENTITY igrave "ì" -- small i, grave accent -->
<!ENTITY iuml "ï" -- small i, dieresis or umlaut mark -->
<!ENTITY lt "<" -- less than -->
<!ENTITY nbsp " " -- should be NON_BREAKING space -->
<!ENTITY ntilde "ñ" -- small n, tilde -->
<!ENTITY oacute "ó" -- small o, acute accent -->
<!ENTITY ocirc "ô" -- small o, circumflex accent -->
<!ENTITY ograve "ò" -- small o, grave accent -->
<!ENTITY oslash "ø" -- small o, slash -->
<!ENTITY otilde "õ" -- small o, tilde -->
<!ENTITY ouml "ö" -- small o, dieresis or umlaut mark -->
<!ENTITY szlig "ß" -- small sharp s, German (sz ligature) -->
<!ENTITY thorn "þ" -- small thorn, Icelandic -->
<!ENTITY uacute "ú" -- small u, acute accent -->
<!ENTITY ucirc "û" -- small u, circumflex accent -->
<!ENTITY ugrave "ù" -- small u, grave accent -->
<!ENTITY uuml "ü" -- small u, dieresis or umlaut mark -->
<!ENTITY yacute "ý" -- small y, acute accent -->
<!ENTITY yuml "ÿ" -- small y, dieresis or umlaut mark -->

<!ELEMENT (%literal) - - CDATA>

<!ELEMENT PLAINTEXT - O EMPTY>

------- =_aaaaaaaaaa1
Content-Type: text/x-sgml; charset="us-ascii"
Content-ID: <10024.761615492.7@ulua>
Content-Description: HTML SGML declaration

<!SGML "ISO 8879:1986"

-- Document Type Definition for the HyperText Markup Language as used by the World Wide Web application (HTML DTD).

NOTE: This is a definition of HTML with respect to SGML, and assumes an understanding of SGML terms.

If you find bugs in this DTD or find it does not compile under some circumstances please mail www-bug@info.cern.ch

CHARSET BASESET "ISO 646:1983//CHARSET International Reference Version (IRV)//ESC 2/5 4/0" DESCSET 0 9 UNUSED 9 2 9 11 2 UNUSED 13 1 13 14 18 UNUSED 32 95 32 127 1 UNUSED BASESET "ISO Registration Number 100//CHARSET ECMA-94 Right Part of Latin Alphabet Nr. 1//ESC 2/13 4/1" DESCSET 128 32 UNUSED 160 95 32 255 1 UNUSED

CAPACITY SGMLREF TOTALCAP 150000 GRPCAP 150000

SCOPE DOCUMENT SYNTAX SHUNCHAR CONTROLS 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 127 255 BASESET "ISO 646:1983//CHARSET International Reference Version (IRV)//ESC 2/5 4/0" DESCSET 0 128 0 FUNCTION RE 13 RS 10 SPACE 32 TAB SEPCHAR 9 NAMING LCNMSTRT "" UCNMSTRT "" LCNMCHAR ".-" UCNMCHAR ".-" NAMECASE GENERAL YES ENTITY NO DELIM GENERAL SGMLREF SHORTREF SGMLREF NAMES SGMLREF QUANTITY SGMLREF NAMELEN 34 TAGLVL 100 LITLEN 1024 GRPGTCNT 150 GRPCNT 64

FEATURES MINIMIZE DATATAG NO OMITTAG NO RANK NO SHORTTAG NO LINK SIMPLE NO IMPLICIT NO EXPLICIT NO OTHER CONCUR NO SUBDOC NO FORMAL YES APPINFO NONE >

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