Hi Paul,
As I recall the "extensive discussions" on the combining algorithms
last
year had to do w correcting some inconsistencies and unnecessary
gaps
based on the handling of Indeterminates that was in 2.0.
Also this is not a matter of "exclusively" relying on combining
algorithms
vs policy or rule evaluation. The policy and rule evaluation
provides the
inputs to the combining algorithms. Basically, what the table says
is
no Rules means NotApplicable, as explained in previous email.
I agree that there has always been a notion of "biased" evaluation,
where
a "final" answer of either Permit or Deny is required as opposed to
returning Indeterminate or NotApplicable. My understanding is that
this is the purpose of the Deny-unless-permit and Permit-unless-deny
combining algorithms.
As such, in order to allow this special case of zero Rules to exist,
I think
it would be reasonable to say that a Policy with zero Rules is
equivalent
to a Policy with one Rule that always evaluates to NotApplicable.
i.e. we can define default inputs to the combining algorithm in the
absence of any explicit input from the Policy itself, which I think
is a reasonable interpretation of the current contents of Table 5.
Bottom line: I see no reason why Table 5 needs to be changed as
it already appears to cleanly specify the case we have been
discussing,
except that it needs a vehicle for sending the NotApplicable to
the combining algorithm in the absence of any physical Rule.
Thanks,
Rich
On 2/23/2012 4:17 PM, Tyson, Paul H wrote:
Yes, Table 5 assumes the existence of a rule in
the policy. I think our extensive discussion last year of
combining algorithms, which resulted in rewriting the
pseudo-code, reflects our current intent of relying
exclusively on the combining-algorithm. I don’t see any
reason to back away from that position, but whatever we do
Table 5 must be adjusted.
To harmonize in the direction of
combining-algorithm control, we would remove rows 2 and 3 of
the table body, and put “Don’t Care” for all Rule values.
To say that a policy with no rules is
“NotApplicable” we could remove the 1st body row;
change “All rule values are NotApplicable” in the 2nd
body row to “No rules in policy”; and change the existing 3rd
row to “Don’t care” under Rule values.
If we wanted the condition “All rule values not
applicable” to result in “NotApplicable” even when the
algorithm says “permit-unless-deny” or “deny-unless-permit”,
we would say “No rules in policy or all rules NotApplicable”
= NotApplicable.
I prefer the first approach (rely exclusively on
combining algorithm) because it introduces no special cases
and therefore will cause the fewest surprises.
Regards,
--Paul
From:
[mailto:]
On Behalf Of rich levinson
Sent: Thursday, 23 February, 2012 14:58
To: xacml
Subject: [xacml] Comment on issue 8? "choice
element" or "Policy w no Rules"
To TC:
To collect the info from today's discussion, which was ref'd
in the
Feb 9 minutes:
the "latest email" I thought Erik and I had agreement that a
statement
would be made in the "implementor's guide" that a Policy w
no Rules
may be ignored by developers:
http://lists.oasis-open.org/archives/xacml/201202/msg00000.html
"It would seem to me that at a minimum,
we could include
an advisory note to developers that a PDP may ignore
a Policy that contains no Rules, since there is no point
from a XACML functional perspective to provide any logic
to do anything specific with such Policies.
Such an approach would remove any questions from developers,
and could leave the schema unchanged."
During today's discussion, the notion was
introduced that somehow a
combining algorithm could effectively introduce a decision
despite the
fact that there were no Rules in the Policy.
However, I think that interpretation is wrong for the
following reason.
For Policy evaluation, we have to refer to section 7.11
"Policy Evaluation".
According to Table 5 there, the following is normative
behavior:
The policy truth table is shown in Table
5.
Target
Rule values
Policy Value
“Match”
At least one rule value is its
Effect
Specified by the rule-combining
algorithm
“Match”
All rule values are
“NotApplicable”
“NotApplicable”
“Match”
At least one rule value is
“Indeterminate”
Specified by the rule-combining
algorithm
“No-match”
Don‟t care
“NotApplicable”
“Indeterminate”
Don‟t care
“Indeterminate”
Table 5 Policy truth table
I think we
can agree that the Target is a "Match", since, by section
7.7, even
an empty Target matches any request.
Also, I think that rows 1 and 3 that begin with "At least
one rule ..." do
not apply since there are "zero Rules" in the use case we
are discussing.
Since those rows are the only places that cause the
rule-combining
algorithm to be invoked, I think we can assume that even
combining
algorithms, such as "Deny-unless -permit" (section C.6) or
"Permit-unless-deny" (section C.7) will not get invoked.
Therefore, the only thing that is left is row 2, which
states:
"All Rule values are "NotApplicable"". I believe this
statement
is TRUE, because in order to be false there must be at least
one Rule which has a value other than "NotApplicable", which
is FALSE, and therefore the statement is TRUE.
Therefore, a Policy w no Rules must evaluate to
NotApplicable.
QED. :)
Thanks,
Rich
