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Re: T is for Transfer

From: Mark Baker <distobj@acm.org>
Date: Thu, 28 Mar 2002 15:32:27 -0500 (EST)
Message-Id: <200203282032.PAA23516@markbaker.ca>
To: jhui@digisle.net (Joseph Hui)
Cc: PMAppleton@bemis.com (Appleton Pete M), highland.m.mountain@intel.com, xml-dist-app@w3.org
Hey Joseph, good to see you over here on dist-app. 8-)

> This is interesting.
> 
> Mark's right that HTTP, unlike TCP or UDP, is not a transport
> protocol in an OSI or DARPA networking stack.
> 
> However, Highland's claim, as far as I can tell from the quoted text
> in Mark's message, is only that HTTP can be seen as a transport.
> I have no problem with referring to HTTP -- which itself assumes an
> underlying reliable transport protocol, say TCP, for transferring
> HTTP messages reliably -- as a transport, where HTTP message headers
> and bodies are used to carry application-level payloads.

Well, you may not be aware of the history here, but there are two
views of this, and SOAP is trying to straddle the fence to please both.
One view is that SOAP can use HTTP as a transport protocol, and the
other is that SOAP can use HTTP as an application protocol.

BTW, what do you mean by "application-level payloads"?  Does HTTP not
already carry these?  It is an application protocol, in the OSI sense
of the term.

> HTTP is not a transport protocol by design; but it doesn't stop people
> from seeing it as one creatively, often for its "port-80 firewall
> friendliness."  I'd be quite comfortable to design an application
> protocol that specifies HTTP as its transport where in-band data
> (e.g. HTML content) are carried in HTTP bodies, and out-of-band
> data ( e.g. application-specific content signals) are embedded
> in HTTP extended headers.

I think the main reason that people want to use HTTP as a transport
protocol is that they don't understand how they can achieve their
goals by using it as an application protocol.  i.e. they don't
understand what the HTTP methods, particular POST, mean.

Let me explain ...

If I wrote up a purchase order on a piece of paper, and walked up to
a stranger on the street and handed it to them, they could either
say "ok, I'll take care of it", or "what the heck is this"?  This
is what POST does.  Of course, "I'll take care of it" can mean many
things; if the person shreds paper, then that will yield a different
result than if the person works at the purchasing department of Walmart.
That's why it always helps to know what type of person they are.  On
the Web, I might find that out through several means such as;

<a href="http://strange-person.org" rel="http://walmart.com/purchaser">

or

HEAD http://strange-person.org
response;
Resource-Type: http://walmart.com/purchaser

or

<http://strange-person.org> rdf:type <http://walmart.com/purchaser>

etc..

MB
-- 
Mark Baker, Chief Science Officer, Planetfred, Inc.
Ottawa, Ontario, CANADA.      mbaker@planetfred.com
http://www.markbaker.ca   http://www.planetfred.com
Received on Thursday, 28 March 2002 15:31:06 GMT

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