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At 01:21 PM 4/12/2002 -0700, Joe Touch wrote:<br>
My position is that the paper communicates only one point directly (which
has been called 'weak' in other places):<br><br>
<blockquote type=cite class=cite cite><x-tab> </x-tab>an
E2E service cannot be created exclusively by the<br>
<x-tab> </x-tab>composition
of HBH services<br><br>
There may have been other agendas at work when writing the document, but
this is the only one that I feel is established to readers of the
document. I don't feel that the document sufficiently asserts that there
are reasons for preferring E2E over HBH, only that HBH cannot be composed
to create E2E.</blockquote><br>
I don't think we miscommunicated our point, nor were there hidden agendas
we didn't state. Here's a direct quote from the opening of our
paper, which defines what an end-to-end argument is:<br>
<dl>
<dd>In a system that includes communications, one usually draws a modular
boundary around the communication subsystem and defines a firm interface
between it and the rest of the system. When doing so, it becomes apparent
that there is a list of functions each of which might be implemented in
any of several ways: by the communication subsystem, by its client, as a
joint venture, or perhaps redundantly, each doing its own version. In
reasoning about this choice, the requirements of the application provide
the basis for a class of arguments, which go as follows:<br><br>
<dd>The function in question can completely and correctly be implemented
only with the knowledge and help of the application standing at the end
points of the communication system. Therefore, providing that questioned
function as a feature of the communication system itself is not possible.
(Sometimes an incomplete version of the function provided by the
communication system may be useful as a performance
enhancement.)<br><br>
<dd>We call this line of reasoning against low-level function
implementation the "end-to-end argument." <br><br>
</dl>Some observations, compared with your point above:<br><br>
- we never said Hop-by-Hop, because the argument applies even if the
communications system has no "graph structure". And for
that matter, we never limited the number of endpoints to 2, though for
simplicity we used an example that only had two endpoints, nor do the
end-points have to be symmetrical in function. So the collection of
web browsers and web servers around the Internet implement the WWW in a
way that closely follows the "end-to-end" argument where the
communications system consists of TCP, IP, DNS, as well as the links and
routers involved. That is, HTTP and HTML are "outside"
the communications system.<br><br>
- we never said this applied only to the functions contained in the
"transport layer". In fact at the end of the paper we
talked about other "communications systems" that can be thought
of this way (operating system interprocess communication, file systems,
...).<br><br>
- the inability to compose HBH solutions to form an E2E solution is only
one way in which one satisfies the predicate of an end-to-end argument
(i.e. that one cannot implement the function completely within the
communications system). A more fundamental reason would be that the
communications system doesn't have and can't have enough information to
perform the function correctly... and that has nothing to do with
hop-by-hop composition.<br><br>
Feel free to consult the text of the paper at:
<a href="http://www.reed.com/Papers/EndtoEnd.html" eudora="autourl">http://www.reed.com/Papers/EndtoEnd.html</a><br><br>
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