I agree with Marty.
When the attribute is missing,
we can discuss what the default
value should be. I proposed "perMessage"
as the default in that case.
Chris F. doesn't like that proposal,
from what I could understand.
But I have no idea how
to decide between yes (always)
versus no (never) as a default
for requesting acknowledgements,
for example. To me it
makes sense to say that absent
an agreement upon
an explicit fixed value, make it
perMessage.