[e2e] Multiple TCP-friendly Sessions and Cong. Control in user-mode?

Joe Touch touch at ISI.EDU
Fri Apr 12 15:31:41 PDT 2002

David P. Reed wrote:
> At 01:21 PM 4/12/2002 -0700, Joe Touch wrote:
> My position is that the paper communicates only one point directly 
> (which has been called 'weak' in other places):
>>         an E2E service cannot be created exclusively by the
>>         composition of HBH services
>> 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.
> 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:
>     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:
>     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.)
>     We call this line of reasoning against low-level function
>     implementation the "end-to-end argument."
> Some observations, compared with your point above:
> - we never said Hop-by-Hop, because the argument applies even if the 
> communications system has no "graph structure".

Agreed - there are two ways to do non-E2E:
	1) HBH (staging at intermediate points, whether inside the net
	or even talking to multiple endpoints in succession may qualify)

	2) from a non-endpoint
	this is harder to define; the only definition of endpoint I
	have been able to determine is "that which _I_ can kick" (or
	that which _I_ care about). everything else is 'middle'.
	which means an endpoint might be an application, a host,
	a router, a link interface, or just about anything, depending
	on who is talking :-)

> - we never said this applied only to the functions contained in the 
> "transport layer".

Agreed - nor do I, FWIW. Though talking about E2E about a link is a 
little odd, but certainly a good example of #2 above.

> - 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.

I'd say one follows from the other, i.e., it is the lack of sufficient 
information which prevents composition.


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