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Subject: Re: [xacml] Is the choice element in Policy correct

Hi Paul,

This issue is to determine what is correct. Once that is determined we
can discuss possible implications and what, if anything, should be
done about the problem, assuming a problem exists.

I think we need to focus on what the schema should contain, not
whether changing the schema would break existing implementations.
Otherwise, there won't be anything we can discuss if it involves
potential changes.


On 1/30/2012 9:06 AM, Tyson, Paul H wrote:

We have attestations of use for the current schema, and any change would break those implementations.


Policy evaluation is specified by section 7 of the spec, and I believe it is robust enough to handle nonsensical instances allowed by this schema oddity.


Unless the current schema allows a policy instance that cannot be evaluated successfully and unambiguously by the procedures in section 7, I suggest we leave it alone.





From: xacml@lists.oasis-open.org [mailto:xacml@lists.oasis-open.org] On Behalf Of rich levinson
Sent: Sunday, 29 January, 2012 21:15
To: Erik Rissanen
Cc: xacml@lists.oasis-open.org
Subject: Re: [xacml] Is the choice element in Policy correct


Hi Erik,

At this point we are simply trying to determine what the schema should be,
in the sense that if it was reflecting the intended behavior, what changes
would be appropriate.

For example it would seem the appropriate changes might be:

  • A Policy must have one or more Rules.
  • A Policy may have zero or more VariableDefinitions
  • A Policy may have zero or one CombinerParameters
  • A Policy may have zero or one RuleCombinerParameters

If these are the actual requirements that are intended, or if some
other specific requirements are more accurate or appropriate, then
the purpose of the issue being raised is to determine what are the
correct requirements.

Once the requirements are established, then we can evaluate what,
if any, impact that might have on the schema, and whether the
TC thinks the schema should be changed.

At present, it appears that the current schema does not reflect
any kind of plausible real requirements, and this represents
a problem for developers using the specification to build products,
because they may be uncertain how to handle the use cases that
are made possible w the current schema.


On 1/27/2012 6:10 PM, Erik Rissanen wrote:

Hi Rich,

I don't think there is anything which needs to be changed here. It's true that the schema is a bit weird in this respect. It's a carry over from 2.0, and does not represent any practical concern.

Empty policies are fine I think, though it does not really make sense to have an empty policy with variables since there would be no rule to use them.

But there is not really any issue in how to interpret any combination which the schema allows and there is no reason for a product to produce meaningless policies.

I would say that we keep it as it is.

Best regards,

On 2012-01-24 07:23, rich levinson wrote:

To TC:

We have been looking at the xsd for Policy and there is a central
"choice" element that does not appear correct, although for
mainstream Policies it probably does not show up.

The choice element is the following:

<xs:choice maxOccurs="unbounded">
    <xs:element ref="xacml:CombinerParameters" minOccurs="0"/>
    <xs:element ref="xacml:RuleCombinerParameters" minOccurs="0"/>
    <xs:element ref="xacml:VariableDefinition"/>
    <xs:element ref="xacml:Rule"/>

This is the construct that allows multiple Rules in a Policy, which, looking at
the Rule element alone, seems ok, as default is minOccurs="1", and it
inherits maxOccurs="unbounded" from the choice element itself.

However, very little else about this element appears to make sense:

  1. Since this is a choice element, w minOccurs="1", one could choose any
    of the other 3 elements and nothing else, and the result would be a
    Policy with zero Rules. Does this make sense?
  2. With zero Rules, even if you used the Policy to define VariableDefinitions,
    they cannot be referenced outside the Policy, and since there are no
    Rules in the Policy, there is nothing that would ever use the
  3. Does it make sense to have multiple instances of either CombinerParameters
    or RuleCombinerParameters? i.e. can't all the parameters be put in one
    element in both cases? If not then why are these elements in the choice
    block that allows unbounded instances?

Please advise as to whether the above interpretation is accurate. If so, we would
like to consider raising this as an issue for action.



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