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Subject: Re: [xacml] Possible Issue: XACML 3.0 WD 9 - 2 questions on Obligation

Hi again, Erik and TC,

I think we can now consider this possible issue as being "almost resolved".
  • The only remaining suggestion I have is that we also add optional Issuer to the AttributeAssignment element.
The following is an explanation in case anyone is interested:

As indicated a couple emails back, I had a certain expectation that an Obligation was the functional equivalent of the Attributes element and was effectively equivalent to a general collection of Attributes.

However, I also had the expectation that the AttributeValue subelement of Attribute was relevant in some way to that functionality, and was concerned for the lack of an example to show how this functionality was expressed.

So, first, wrt to the example, the thing that made this different from XACML 2.0 was that the example in Ch 4 was no longer complete because Obligation had been replaced policy-side by ObligationExpression. However, in 2.0 the policy-side and response-side were equal, so the 2.0 policy served as an example and does show AttributeAssignment w attributes: AttributeId and DataType, and a child text value. Noticing this, I was then satisfied that AttributeValue never originally was child to AttributeAssignment, so at least we were not losing anything from 2.0 in this regard, which I was wondering about.

Now wrt to the functional equivalence of Obligation to a collection of attributes, as indicated in prev emails, I was thinking that AttributeValue first was meaningful, then possibly extraneous, then meaningful again. Now, I think I have effectively reached the conclusion that it is "functionally" extraneous, but may serve useful purposes in other respects, but maybe not. In any case, I am not recommending changing it for any non-functional purpose.

Effectively there are 4 xml attributes that totally define the metadata associated with the text content (the 5th item in list) of an Attribute. They are:
  • Attributes@Category
  • Attributes/Attribute@AttributeId
  • Attributes/Attribute@Issuer
  • Attributes/Attribute/AttributeValue@DataType
  • Attributes/Attribute/AttributeValue/text()
For an Obligation, we can list the equivalent information (as of WD10):
  • Obligation/AttributeAssignment@Category
  • Obligation/AttributeAssignment@AttributeId
  • Obligation/AttributeAssignment@DataType
  • Obligation/AttributeAssignment/text()
The purpose of my recommendation should be obvious, now, which is to fill in the missing piece of potentially useful metadata, which is:
  • Obligation/AttributeAssignment@Issuer
One final note is that Obligation does have one additional attribute that Attributes does not have a functional equivalent, which is:
  • Obligation@ObligationId
which effectively means that Obligation, in current form, w @Issuer added, is effectively one dimension greater than already extremely general Attributes element. i.e. while we can have multiple Attributes elements they are only distinguishable by Category, whereas Obligation can distinguish by Category by only including elements within a single Category, but then multiple collections of this type can then be distinguished by ObligationId.

It probably turns out that, for the multi-resource profile that xml:id fills this gap as well.


Rich.Levinson wrote:
49C2EA27.5090107@oracle.com" type="cite">Hi Erik and TC,

I have found the answer on the multiple DataTypes. The AttributeDesignator has a required DataType attribute, which restricts the bag returned to be of a single DataType.

This was also in XACML 2.0. The change in 3.0 effectively consolidates the need in 2.0 to specify separate Attribute elements for each DataType, by moving the DataType down to the AttributeValue, which enables one Attribute element to contain all AttributeValues with same AttributeId.

Please ignore question 2, however, I would still like clarification on question 1.


Rich.Levinson wrote:
Hi again, Erik,

Thinking about this some more, I realized that I jumped the gun calling the AttributeValue possibly functionally extraneous. It has an explicit function which is to enable multi-valued Attribute elements.

However, this suddenly puts additional spotlight on the move of DataType in 2.0 from the Attribute element, to the AttributeValue element in 3.0.

   * This now means that in 3.0 an attribute with a specific
     AttributeId can have multiple values, each with a different
     DataType! Was this intended? I took a quick run thru 3.0 and could
     not find any explicit indicators that this was functionality that
     was being utilized in any specific manner.
   * Does this impact the "bag functions" in section A.3.12? All the
     examples there appear to assume that all the elements in the bag
     have the same DataType. That was a good assumption in 2.0, because
     DataType was defined at the parent element, but apparently not in
     3.0, because each child element can now have its own DataType.

In any event, if my interpretation of AttributeAssignment in prev email is correct, that would mean that each AttributeAssignment can only be single-valued, because it does not "contain" an AttributeValue.

The net effect of this is to make Obligation now the functional equivalent of a single multi-value multi-data-type element comparable to our regular definition of Attribute. However, unlike an Attribute, now each AttributeValue equivalent (AttributeAssignment) has its own AttributeId.

The net effect of all this I am finding really confusing and would appreciate some guidance to bring it under a more reasonable conceptual framework. If my interpretation of your original response is correct, it appears that Obligations, in effect, are another conceptualization of how to define Attributes.

I think it would be much easier to have AttributeAssignment be equivalent to Attribute and be able to contain multiple AttributeValues in the familiar way.

So, I guess there are two potential issues against core here:

  1. Are we making Obligations unnecessarily complex by not allowing
     them to contain multiple AttributeValues within an
     AttributeAssignment, and forcing this functionality back up, which
     remakes a single Obligation into the functional equivalent of a
     single Attribute (both multi-valued, but constructed quite
  2. Does putting DataType in the AttributeValue element force new
     functionality to be required now that a single attribute can not
     only contain multiple values, but each value may be of its own


Rich.Levinson wrote:
Hi Erik,

Thanks for the feedback and attempted clarification on AttributeValueType :).  I admit I am still a little confused, let me try to explain. I think the problem is that a definitive example is needed. Here is my perspective: A typical attribute in the RequestContext has the following form:

<Attributes Category="abc">
 <Attribute AttributeId="def" Issuer="ghi" IncludeInResult="jkl">
   <AttributeValue DataType="mno"
     >content data</AttributeValue>

(Couple points worth noting are that DataType has moved from being an attribute of Attribute in XACML 2.0 to being an attribute of AttributeValue in XACML 3.0., also that IncludeInResult is new in 3.0, and Category is expanded in 3.0 to be general, from the SubjectCategory special case from 2.0)

Presumably, all the attributes in the response sent by IncludeInResult appear in this same form in the Response.

The first additional case we addressed was MissingAttributeDetail, section 5.56, which appears to come back in the following form:

 <MissingAttributeDetail Category="abc" AttributeId="def" DataType="mno">
     >content data</AttributeValue>

This has essentially the same form as the other attributes, except:

   * Category is pushed down so it now appears alongside AttributeId etc.
   * DataType is above <AttributeValue> as it was in XACML 2.0

I am ok w those differences, since an AttributeValue may not be returned, in general, because, after all, the point is that it is "missing" and the PDP may tell the PEP what dataType is needed, it probably is not going to say what value to provide. However, and empty AttributeValue w DataType attribute might be an improvement.

Also, I am satisfied with pushing down Category into the MissingAttributeDetail, because the StatusDetail can hold multiple of these elements and there is not reason why they would all have the same Category, which is unlike the case on Input, and to some degree IncludeInResult as well.

With that context in mind, what I am "expecting" to see for AttributeAssignment is something along the following lines:

<Obligation ObligationId="123">
 <AttributeAssignment Category="abc" AttributeId="def" Issuer="ghi">
   <AttributeValue DataType="mno"
     >do this and that</AttributeValue>

I have not found any examples to confirm the above "expectation", so I have been relying on interpreting the xml, which is the reason for my original question.

From the meeting minutes, I see the TC agreed w your suggestion to include Category as optional, which seems fine to me.

So, all that remains in my mind is straightening out what is actually returned. It sounds from your description that

   * "AttributeAssignment has the same content as AttributeValueType,
     except that the XML attribute AttributeId is required"

I apologize for being  picky, but  with the absence of  examples,  I still find this sentence difficult to parse in a way that makes sense. I interpret "content" as "child", so in this case if I take the first part of the sentence literally, AttributeAssignment "replaces" AttributeValue, so there is no "AttributeValue" and it just has the text content, no contained elements.

Now the second part of the sentence says that "except that the XML AttributeId is required", which I interpret that AttributeId is attribute of AttributeAssignment. Finally, w decision for optional Category, we would have:

<Obligation ObligationId="123">
 <AttributeAssignment Category="abc" AttributeId="def" DataType="mno"
     >do this and that</AttributeAssignment>

Is this correct?

If so, then I guess my comment is why the need to squeeze out the AttributeValue element? Also, if it can be squeezed out why is it there in the first place?

Bottom line: I think the reason I am still confused it that there appears to be some non-intuitive behavior, which appears somewhat arbitrary, possibly because AttributeValue may be functionally extraneous, which doesn't bother me, and even if it is, my recommendation is that we use it consistently and not arbitrarily squeeze it out.

Also, might want to consider "Issuer" in the AttributeAssignment as well in case, policy designers want to distinguish where Obligations are coming from, at a finer granularity than just "the PDP".


Erik Rissanen wrote:
Hi Rich,

I presume you are referring to the following schema fragment:

        <xs:element name="AttributeAssignment" type="xacml:AttributeAssignmentType"/>
        <xs:complexType name="AttributeAssignmentType" mixed="true">
                <xs:complexContent mixed="true">
                        <xs:extension base="xacml:AttributeValueType">
                                <xs:attribute name="AttributeId" type="xs:anyURI" use="required"/>

This means that an AttributeAssignment has the same content as AttributeValueType, except that the XML attribute AttributeId is required. Like this:

<AttributeAssignment AttributeId="urn:....:foo" DataType="urn:...:bar">
  some value here

We want to declare the XML attribute, although xs:anyAttribute is already allowed, since without the declaration the AttributeId is not _required_.

I haven't thought about the category for attributes in obligations. I have thought them as parameters of the obligation, not parts of the request. But I see the point. What about making the Category an optional XML attribute? That way it won't "pollute" those obligations which just contain obligation parameters, but it will still be possible to return parts of the request.

Best regards,

Rich.Levinson wrote:
While reviewing where we have ended up with the handling of Obligation elements in 3.0, I have 2 questions which I am unable to resolve based on my reading of the text (question 2 contains a possible issue of functionality, question 1 might just be clarification either by response to this email or by issue  for more explanatory info in the text):

   1. AttributeId in Obligation in Response: In section 5.41,
      AttributeAssignmentExpression, it says:
          * "It SHALL contain an AttributeId and an expression which
            SHALL by evaluated into the corresponding attribute value."
          * Presumably, this means that these two items will be what
            the PDP puts into the Obligation element that is put into
            the Response. This interpretation is also in agreement, I
            believe, with the description of this element in section 5.39:
                o "The expressions SHALL be evaluated by the PDP to
                  constant <AttributeValue> elements, which shall be
                  the attribute assignments in the <Obligation>
                  returned to the PEP. "
          * Presumably the two items above (AttributeValue,
            AttributeId) are then put by the PDP into the
            AttributeAssignment element (section 5.36) which is child
            to the Obligation (section 5.34)
          * Here is my basic question on section 5.36, which may be
            simply that I do not understand the mechanics of the
            extension element in the schema: it appears on lines
            2543-2546 that AttributeId might be defined here as an
            attribute of AttributeValue:
                o "

                        <xs:extension base="xacml:AttributeValueType">

                           <xs:attribute name="AttributeId"



                o So, that's the 1st part of the question. Is this the
                  same AttributeId identified in section 5.41, and
                  does it show up in the output Obligation as an
                  attribute of the AttributeAssignment element or of
                  the AttributeValue element? (It appears based on the
                  above that it might be the latter, if not please
                o If it is the an attribute of AttributeValue, the 2nd
                  part of the question is does this not kind of
                  violate section 5.31 AttributeValue, because this
                  AttributeId would presumably now be part of the
   2. (2nd question) Should we include the "Category" in the
      Obligation (probably not because that would apply to all
      AttributeAssignments) or preferably in the AttributeAssignment
      (assuming the AttributeId is already there from question 1)?
          * The reason for asking is that it does not seem
            unreasonable that in many cases the AttributeId assigned
            to the Obligation/AttributeAssignment will be the same
            AttributeId used to pull an attribute out of the Request.
            Granted, it doesn't have to be, but let's assume that is
            what some people might want to do.
          * Assuming people want to do this, we now run into the same
            ambiguity that led to the addition of Category to
            MissingAttributeDetail (section 5.56), namely that if the
            PEP needs to know how to correlate the returned attributes
            with the input request, then both AttributeId and Category
            are needed, in general.


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