It is possible that to a large extent this flexibility can be accommodated by
saying that whether an RAO is required may depend on the MRM, but that all
current ones do require it. Then a more detailed discussion of the policies and
guidelines can go in the extensibility document. My personal preference would be
that for anything which requires a message to be picked out of the forwarding
path, the RAO is the right thing to use (at least as a default), even if
logically other types of filtering would also be possible.