Arvola,
I read the proposal with much interest. Looks good to me
based on my current understanding. I have a few minor
items you may consider.
1. There was a "signed" attribute in your first proposal which seems
to have disappeared in the second one. Was that a typo?
2. Please consider "perCPA" instead of "fixed" since we already have
"perMessage" (unless "fixed" has other semantics)
Another problem which may touch upon negotiation is,
can we have a message that does not require a CPA?
(don't you need one to bootstrap CPA creation negotiations?)
Should we have some statement that covers this issue?
Regards,
-Suresh