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   RE: Another Question

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  • From: Bob Kline <bkline@rksystems.com>
  • To: "John F. Schlesinger" <johns@syscore.com>
  • Date: Tue, 20 Jun 2000 10:25:27 -0400 (EDT)

On Mon, 19 Jun 2000, John F. Schlesinger wrote:

> Mike Kay wrote:
> "Think OSI 7-layer model. XML is layer 6, adxml is layer 7."
> I agree with Mike (I'm enjoying your XSL book by the way) that XML
> is a layer 6 (presentation) kind of thing. So are BNF and ASN-1.
> That raises the question, why is XML better than ASN-1 or BNF?

Let's not say "better"; how about "preferred for some applications"?

1. Easier to debug.
2. More accessible to more common tools.
3. Easier to get back on track with garbled transmission.

> Here's one thing I miss from ASN-1 - BER (Basic Encoding Rules). BER
> lets ASN-1 be represented "on the wire" in a deterministic and
> changeable way (I can't stream XML unless I change the
> representation).
> XML's heritage is documents not data and so the idea of BER is new
> to those from the SGML community. At the moment, all we get is the
> XML processing directive with its encoding attribute.
> I would like to be able to represent the XML message "on the wire"
> as a Generalised Data Stream (that is, with a code point
> representing the tag and a length to the end of the tag). This would
> be a much better way of streaming a message and would enable the
> first part of a message to appear on a browser or in a program
> before the last part has been processed. I know SAX can stream
> events, but for SAX to work the whole tag must be parsed, whereas
> with GDS whole tags can be skipped.
> Any thoughts?

BER doesn't always give you the length of an element.  Sometimes it
does, and other times you have to watch for the EOC bytes (and pray they
didn't get stepped on by the transmission).

Bob Kline

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