Re: [xacml] Draft proposal for new work item: properties for newcombining algorithm

I think that Anne raises some excellent points.
These are some of the same concerns that I have, and
I think that her proposal makes a lot of sense:
>  Define ordered versions of permit-overrides and
deny-overrides
>  for XACML 1.1.

In addition to helping with this priority question,
there's another issue that I think this helps
resolve. It's somewhat unclear why or how unordered
combining algorithms are useful. Yes, I understand
the idea that a PDP may be able to re-order
evaluation to make things more effecient, but the
more time I spend thinking about this from the
implementation point of view, the more convinced I
am that this is unlikely. It's much more likely, in
my mind, that the policy writer will have some idea
of how a policy should be ordered, and so there
should be combining algs to support that. It's just
too hard to think about a PDP becoming an optimizing
compiler for any given input and still being
effecient. Because the current permit/deny overrides
 algs are unordered I think they should remain that
way, and if someone can come up with ways to use
this as an optimization, I'll gladly implement it. I
think that having determinisitic algs, however,
helps with priority, helps with obligations, and
will help policy writers think about the policies
they're forming.

seth