Proposed text on reliable messaging

I've taken my action item to work on the reliable messaging text. I've tried
to improve the text on the following aspects:

1. To clarify the concept of "reliable messaging" under the context of Web
services.
2. To articulate more on the difference between "guarantee of delivery" and
"reliable messaging"
3. To add the "end-to-end argument".
4. To explain why the "life cycle" of a message may be important to improve
reliability.
5. To correct a few typos.

As always, your comments are appreciated.

Hao

2.3.1.13 Reliable messaging
2.3.1.13.1 Definition

Reliable messaging is a feature that may contribute to the overall
reliability and efficiency of Web services. Reliable messaging requires
participating software agents to act on a message after receiving it.

Relationships to other elements

reliable messaging is

a feature

reliable messaging may be realized by
a combination of message acknowledgement and correlation.

a good practise of reliable messaging is
to make the life cycles of messages available to participating software
agents.

2.3.1.13.3 Explanation

Reliable messaging can improve the overall reliability and efficiency of Web
services. Reliable messaging in Web services not only requires the delivery
of messages but also the contact on the receiving agent to act on those
messages as defined in service contracts.  

The goal of reliable messaging is to both reduce the error frequency for
messaging and to provide sufficient information about the life cycle of
messages. Such information enables a participating agent to make a
compensating decision when errors or less than desired results occur.  It is
important to note that a guarantee of the delivery of message  alone does
not improve the overall reliability due to the "end-to-end
argument."[1]  It may, however, improve the performance of messaging.
Requiring an agent to act on a message after receiving it would improve the
overall  reliability of Web services that only involve two software agents.
High level correlation such "two-phase commit" is needed if more than two
agents are involved.

Reliable messaging may be realized with a combination of message receipt
acknowledgement and correlation. In the event that a message has not been
properly received and acted upon, the sender may attempt a resend, or some
other compensating action at the application level. Note that in a
distributed system, it theoretically
not possible to guarantee correct notification of delivery; however, in
practice, simple techniques can greatly increase the overall confidence in
the message delivery.  For example, a good practise is to make messaging
idempotent so a sender can attempt sending a message again without worrying
of causing undesired effects, if it is unsure about the status of the
previous message.

Message correlation may be used by the receivers of messages to ensure that
messages are only acted on once - with duplicate messages being ignored or
treated as errors. Whether two messages are duplicated is defined at the
application level.

[1]http://www.reed.com/Papers/EndtoEnd.html