🚀 go-pugleaf

In article <1990Oct5.002410.3924@cbnewsi.att.com>
hrs1@cbnewsi.att.com (herman.r.silbiger) writes:

>SDIF provides an ASN.1 envelope for an SGML encoded document,

In article <5166@hemuli.tik.vtt.fi>
Markku.Savela@tel.vtt.fi (Markku Savela) writes:

>   I have always thought that SDIF was a presentation of ODA
> structure in with SGML, a DTD matching ODA structures (e.g.
> nothing to do with ASN.1).

at which point

enag@ifi.uio.no (Erik Naggum) writes:

> You're probably thinking of ODL (Office Document Language), which
> makes ODA an SGML application, then uses SDIF to encode it.
> (As far as I got it.  Correction solicited.)

Erik and Herman are both right, and indeed, Markku later re-posted
explaining he had found the explanation. Nonetheless, I'm taking the
liberty of posting a slightly more detailed description of some of
the finer points. (This may be construed as the correction that Erik
solicited.)

SDIF can and does provide an ASN.1 envelope for an SGML encoded document,
and is generally assumed to do so, although strictly speaking,
one could use other methods for the interchange.


ISO 9069 (SDIF) defines the SGML Document Interchange Format as:
"A data structure that enables a main document and its related documents,
each of which may be stored in several entities, to be combined into a
single data stream for interchange in a manner that will permit the
recipient to reconstitute the separate entities."

That's it. Nothing about ODL or ODA or even ASN.1.

An ODA document, encoded in ODL, may, **like any other SGML document**
be enveloped in an SDIF data stream. ODL contents, like SGML documents
from any other application, may be "packed" in SDIF.


ISO 9069 formally defines SDIF in the abstract, and uses
ASN.1 as the grammar for the abstract definition. As Charles 
Goldfarb says in The SGML Handbook (available within a
few weeks, I understand), "A specific encoding
for SDIF (as, indeed, for any structure defined in ASN.1) can be
derived automatically from the abstract syntax by applying the
ASN.1 basic encoding rules, defined in ISO 8825. It is expected
that SDIF data streams in Open Systems Interconnection (OSI)
environments will be encoded in this way. However, the standard
allows any encoding accepted by the medium of interchange."

Goldfarb then goes on to explain how SGML documents in SDIF can
be interchanged with the FTAM (ISO 8571), JTM (ISO 8832) and
MHS or X.400 protocols, and using the MOTIS functions defined in
ISO 8505 and 8883.


The most valuable feature of this interchange is that it can include
the logical and layout structures within ODA and still take advantage
of SGML's entity structure: This allows users critical flexibility:
SGML documents may exist in any number of files on a system, separated
as to content notation (graphic formats, for example) or
as chapters in a book, or whatever. SDIF
"packs" them together and provides for full "unpacking" to recreate
that entity structure.

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