Motivation for sub-service choreography?

[Joe Lapp <jlapp@webMethods.com>]

I just read the ebXML TR&P spec dated 5/26/2000.  I'm curious about the
motivation for the "sub-service" and "Multiple Round Trip Document
Exchange" approach.  This seems to contrast with an approach that layers
business process protocol strictly above the delivery protocol.  The spec
proposes multi-document exchanges would be alternatives to the "one-way"
and "simple" document exchanges instead of layering above these latter two
exchanges.  Wouldn't this approach require that business process designers
have smarts about reliable messaging?

Maybe you can help me by shooting holes in the alternative that I have in
mind.  TR&P would define protocols for synchronous and asychronous
messaging with reliability and error recovery in mind.  The result is an
abstraction layer that, from the outside, provides for the reliable
delivery of messages, for the provision of responses, and for the reporting
of failures.  Neither acknowledgements nor errors are directly exposed.  If
ackknowledgements are not received or errors cannot be recovered, only then
are failures reported above this abstraction layer.  Business processes are
then designed as a higher level protocol that is concerned only with the
choreography of business level messages.

Also, to clarify my question, I'm aware that in choreographing a sequence
of messages, you need to give the action of the sequence a name ("service")
as well as the action of each message ("sub-service").  Given these
definitions there will always be a "sub-service choreography."  My question
regards the particular approach taken.

Please accept my apologies for being so ignorant about TR&P.  I haven't
been following along.  Thanks!

- Joe