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Re: FW: LC Comments: Web Method Feature

From: Mark Baker <distobj@acm.org>
Date: Thu, 4 Jul 2002 12:10:04 -0400
To: "Williams, Stuart" <skw@hplb.hpl.hp.com>
Cc: "'xml-dist-app@w3.org'" <xml-dist-app@w3.org>
Message-ID: <20020704121004.N10550@www.markbaker.ca>

Hey again,

FWIW, I think we're doing a fine job at getting to the point here,
unlike some of our previous "blossoming" exchanges. 8-)

On Thu, Jul 04, 2002 at 09:07:54AM +0100, Williams, Stuart wrote:
> > The access and manipulation operations *are* messages.  I don't
> > understand the distinction you're making.
> 
> I guess I'm making a distinction between mean and ends. Messages are the
> means to transfer representations of resource state.  But the ends is the
> access and manipulation of resource state, not the exchange of messages.

Right, I understood that.  But a message needs an intent in order for it
to be a message, even if that intent is just "here, take this" (like
HTTP POST or SMTP DATA).  Otherwise it's more like a "packet".

I'm not sure if that adds value to this conversation, but I thought I'd
toss it out. 8-)

> > "The difference between an application-level
> > protocol and a transport-level protocol is that an application-level
> > includes application semantics, by standard agreement, within
> > the messages that the protocol transfers.  That is why HTTP is called
> > a Transfer protocol.  It is impossible to conform to an
> > application-level protocol without also conforming faithfully to
> > its message semantics."
> >  -- http://lists.w3.org/Archives/Public/www-tag/2002Apr/0303
> 
> Firstly I agree with Roy, but I would also generalise this as:
> 
> "The difference between an N layer protocol and a M layer protocol is that a
> N layer protocol includes layer N semantics, by standard agreement, within
> the messages that the protocol transfers.... It is impossible to conform to
> an N layer protocol without conforming faithfully to its message semantics."
> 
> With which I also agree.

Good point.  I guess Roy's use of the word "conform" there wasn't as
precise as it should have been.

Let's look at this another way.  Typically, a layer has two uses; it
can be extended, or it can be layered on top of.  For example, TCP has
been extended many times by RFCs such as 2414, 1323, 1072, etc..  But of
course, TCP has also been layered on top of many times; HTTP, for
example.

What's special about the application layer, is that it is the "top dog".
You can't layer on top of it and still call yourself a valid use of
that protocol.  You can only extend it.

MB
-- 
Mark Baker, CTO, Idokorro Mobile (formerly Planetfred)
Ottawa, Ontario, CANADA.               distobj@acm.org
http://www.markbaker.ca        http://www.idokorro.com
Received on Thursday, 4 July 2002 12:12:18 GMT

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