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Subject: Re: [xacml] Combining algorithms

Hi Erik,

Nice job putting together the combining algorithm sections. I agree with most everything you supplied although I have not yet looked at all the explicit details of each algorithm, but for the sake of discussion will assume that they are correct and just focus on the details of the extended algorithms.

First, the things I agree on are:
  • Agree that we don't need to define biased algorithms. Might want to comment on that and indicate that recommendation is to build any bias directly into the policies, if needed.
  • Might also want to mention that PEP bias is possible, but that it can be removed by defining appropriate policies, especially at top level. For example, if you want a Deny for all cases of errors and indeterminate, a top level PolicySet w permit-overrides can contain one policyset for all other policies which collectively can return Permit, Deny, Indeterminate, NotApplicable, plus a second policyset that has an automatic Deny Rule if the first PolicySet did not return a Permit.
  • Agree on change deny-overrides policy combining to unbiased as you have included.
  • Agree on change permit-overrides policy combining to unbiased, as well, also, it is symmetric w deny-overrides.
On the extended algorithms, I think we have a disagreement as to the potential value, and therefore I will try to point out the value that I see there as the basis of why I think they should be included.

I'd like to initially point out that what we are dealing with here is a pre-existing condition in the current set of combining algorithms that neither Erik nor I were a party to creating in the first place. Erik's proposal is to "fix" some aspects of the existing algorithms, and I agree that those fixes are improvements to the current state. My proposal has been to revisit whether, in fact, that separate policy and rule combining algorithms actually add value or subtract value, when compared to a single combining algorithm that applies to both policy(set)s and rules.

First, I agree that the extended algorithms Erik has written appear to be functionally equivalent to what I was recommending in the earlier discussion on this topic:
(Note: I recommend changing the Indeterminate(DP) defn to be: "an Indeterminate from a policy which could have evaluated to a Deny or Permit." i.e. remove the "or rule" because a rule can either be a "D" or a "P" but not both. This has no algorithmic impact but might be conceptually useful when examining the algorithms, i.e. to know DP applies to policies only. (In fact this brings out a key concept, namely that policies, while inherently can be DP, depending on the contained rules and their evaluations, can effectively be constrained to only "D" or "P", which carries the character of the collection of applicable rules that it contains.)

In particular, the 3 flavors of Indeterminate preserve the results as to whether there was a possible override or not, which is lost in the current "base" permit/deny-overrides algorithms that simply return Indeterminate in the case where there is a potentialDeny or potentialPermit flagged and do not distinguish the cases where those potentials are not present.

In other words, it is obvious by inspection of the base permit/deny-overrides algorithms, that Indeterminate in both algorithms can be returned in 2 distinct cases, which causes you to lose track of whether there was a potential deny in the deny-overrides case or a potential permit in the permit-overrides case.

In fact, one could take the point of view that we are effectively introducing the complexity of having two classes of combining algorithm (policy and rule) in order to remove information that we are presupposing will be of no value to any customer. i.e. the reason that the policy-combining algorithms are simpler than the rule-combining is that potentially useful decision making information is removed by the rule combining algorithms after using only a portion of the information that they have gone to the trouble to collect.

The net result is that there is a non-uniformity of containers for Rules that adds conceptual complexity to the strategy of building PolicySets with no obvious added value, but with subtracted decision-making information.

For example, consider a deny-overrides Policy with 2 Permit Rules, RP1 and RP2, and 1 Deny Rule, RD3. If the Deny Rule, RD3,  is NotApplicable and one Permit Rule, RP2 is Indeterminate then if the remaining Rule, RP1, evaluates to Permit, then the Policy evaluates to Permit.

Now split the Rules into 2 Policies, one with a Permit, RP2, and Deny, RD3, and the other with one Permit RP1, and put the Policies in a deny-overrides PolicySet. If the Deny Rule, RD3, again is NotApplicable and the companion Permit Rule, RP2, is again Indeterminate then that Policy will return Indeterminate. If the remaining Rule, RP1, in the other Policy, again evaluates to Permit, then that Policy will evaluate to Permit. Now the PolicySet, which contains the two Policies which together contain the same three rules as the case above, and all the Rules evaluate the same as above, then now this combined PolicySet evaluates to Indeterminate, even though there was a valid Permit and no applicable Deny.

Interesting, if you switch RP2 to be NotApplicable, and RD3 to be Indeterminate, then both cases evaluate to Indeterminate.

The point is that by introducing a distinction between rule and policy combining we end up with a non-intuitive structure where sometimes behavior is consistent and sometimes not depending on the subtleties of the whether the container is a container of Rules directly or a container of containers of Rules.

Now my proposal is not to take away any of the above-mentioned behavior from those who choose to use it, but to offer an alternative behavior which is container-consistent, which will enable one to focus on managing collections of Rules without worrying about whether the structure of the containers might contain unexpected side-effects such as described above.

The extended algorithms provide this container uniformity, while at the same time not introducing new complexity, but simply by avoiding removing potentially useful decision making information. With the combined algorithms, for example, a top level deny-overrides PolicySet will always know when it receives an Indeterminate, whether that Indeterminate was a potential deny or not.

I believe the combination of retaining information that is lost from the existing algorithms plus the conceptual improvement of having uniform container algorithms is sufficient justification to include these extended algorithms as valuable options that one can choose. No one will have to use them under any circumstances, but I believe that there are organizations that would value the conceptual simplification that the extended algorithms offer from the perspective of getting consistent behavior from the container structures.


Erik Rissanen wrote:
49182883.5010100@axiomatics.com" type="cite">All,

We still have the combining algorithms issue to consider. I have written some text to consider and tried to organize the decisions we need to make.

First, do we want to fix the current deny/permit policy algorithms at all? I think we should since it is not good that the basic algorithms are biased. It can lead to strange effects in policies, like a policy can return Deny although there is no rule with Effect="Deny" in it.

Assuming that we do want to fix the basic combining algorithms so that they are not biased,  then there are two orthogonal decisions for us to make:

1. Do we want to define biased algorithms as well, or do we rely on the PEP bias alone?

2. Do we want to make use of an extended Indeterminate to allow more fine grained treatment of errors in the combining algorithms?

See the attached documents for what the different algorithms look like.

comb-algs.doc contains combining algorithms which makes the basic algorithms unbiased and introduces separate biased algorithms. The word diff is against the current 3.0 working draft 7.

comb-algs-extended.doc shows algorithms which make use of an extended indeterminate. The diff is against the unbiased algorithms in comb-algs.doc. I have not "ported" the other algorithms to the extended Indeterminate yet or written biased variants. Also note that under the extended indeterminate the rule and policy combining algorithms become the same, so I joined up their descriptions.

My preference is that

- The basic combining algorithms are made unbiased. (I feel strongly about this, the rest I care less about.)

- We do not introduce biased alternatives. (I am happy with the PEP bias.)

- We do not introduce an extended indeterminate. (I think it complicates matters for fairly little value.)

Best regards,

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