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Subject: Re: [xacml] Summary of what I think I said on the call about thehierarchical profile

Hi Erik,

In fact, my view has always been "Way A". My conceptualization is that there is a collection of physical resources and that an organization organizes the resources in various ways, and, in particular, as hierarchies. Every time someone adds a resource to a hierarchy, the resource gets a new name and parent attached to it. After this has been done for a while the net result is that the collection of physical resources can be viewed as having one or more hierarchies "draped" over it. For every hierarchy a resource belongs to, it, in general can be assumed to be assigned a name and a parent. Hopefully, we agree that is "Way A" and based on that assumption, it should be fairly straight forward to understand the issue:

Ok, so, based on your formula, what I do is:
  • follow the parent-child relations
  • this is fine at the requested resource, I find all my parents
  • now I go to each parent and collect all of its parents ***
This is where the problem is introduced. As soon as I collect a parent of my parent that is not a member of a hierarchy I am in, we have defined a DAG.

If I stick to parents of parents that are members of hierarchies I am in, then I am in a forest.

The diff in these 2 methods is that the DAG method doesn't bother to look at whether the requested node is a member of the hierarchy that it is following from the requested node's parent.

The problem compounds as you follow each generation.


Erik Rissanen wrote:
49B679AB.1010807@axiomatics.com" type="cite">Rich, Hal and All,

Just a quick comment. I don't think that just because a resource, or an ancestor, has multiple normative names, the hierarchy would become a DAG. We need to differentiate between resources and names of resources.

One way ("Way A") to think of the problem is that the parent-child relations are bound to resources, not the resource names. To collect the ancestors to the request, one has to follow the parent-child relations, and then include all names of all ancestor resources. That is what the profile tries to say. (I think.) And that would be correct in my opinion. And would not necessarily mean that the profile was made to support a DAG.

An alternative way ("Way B") is to think that the parent-child relations are tied to the resource names, not the resources themselves (or that there are no resources independent of names). I get the feeling that at least Rich looks at it like this.

I think way B is problematic since it opens up the possibility that parent-child relations between different names for the same resources conflict with each other. This is what I meant with "consistency" in my post last week. By adopting way A, we avoid the problem.

Best regards,

Rich.Levinson wrote:
/ /I agree with that. However, it is the algorithms that allow and appear to encourage collection of non-member ancestors. Here is the text:

    For each ancestor of the node specified in the “resource-id”
    attribute or attributes, *and for each normative representation of
    that ancestor node*, an <Attribute> element with AttributeId

    The <AttributeValue> of this <Attribute> SHALL be the normative
    identity of the ancestor node.
    The DataType of this <Attribute> SHALL depend on the
    representation chosen for the identity of nodes in this particular
    This <Attribute> MAY specify an Issuer.
    For each “resource-parent” attribute, there SHALL be a
    corresponding “resource-ancestor” attribute.
    If the requested node is part of a forest rather than part of a
    single tree, *or if the ancestor node has more than one normative
    representation*, there SHALL be at least one instance of this
    attribute for each ancestor along each path to the multiple roots
    of which the requested node is a descendant, *and for each
    normative representation of each such ancestor*.

It is the addition of all the "...each normative representation of each such ancestor" which clearly opens up these algorithms to imply a DAG. Without these phrases, it is still not particularly tight, but alone they could be interpreted to imply a forest. With the phrases DAG is inescapable.

The point is that these are ancestor nodes and nothing ties their normative representations to be those that are in the hierarchies of which the requested node is a member.

This IS the problem. It is these specific algorithms and what they say about ancestors that forces you into a DAG. It clearly includes hierarchies of which the requested node is not a member.

Therefore, it sounds like we are in agreement. That there is a problem that needs to be fixed.


Hal Lockhart wrote:
As I have said repeatedly, the only problem with combining the initial hierarchies into a DAG arises if the original hierarchies include hierarchies of which the Resource is NOT A MEMBER.





From: Rich.Levinson [mailto:rich.levinson@oracle.com] Sent: Tuesday, March 10, 2009 8:56 AM
To: hal.lockhart@oracle.com
Cc: Erik Rissanen; xacml@lists.oasis-open.org
Subject: Re: [xacml] Summary of what I think I said on the call about the hierarchical profile


Hi Hal,

The fact is that it is the algorithms in section 3.2 that imply that the hierarchies are combined as a DAG. There is no problem, in general, if the one or more of the original "hierarchies" happens to be a DAG. The problem is that the algorithms force the combination of the originals, DAG or forest.

The recommended changes to the spec that I have proposed is to have a choice of algorithms for combining the hierarchies. That way customers can decide for themselves which is appropriate for their resource collections.


Hal Lockhart wrote:
I think the source of confusion was this. Daniel's point was that the initial representation of each hierarchy could be a DAG, since it is a generalization of a tree. Rich's point was that if you start out with all the hierarchies in whatever form, and you include defined hierarchies which do not include the Resource in question as a member, even though ancestors of the Resource are members of the hierarchy, if you combine all the hierarchies you lose the information about the original hierarchies necessary to be able to distinguish whether the nodes above the Resource are true ancestors or not.
My comments on the call and below on the DAG were based on the premise that we started out with one or more hierarchies merged them into a DAG and then determined the parents and ancestors. Under this premise, the use of a DAG seemed like a intermediate step of no particular interest. I now see that Daniel was trying to say that at the very beginning, any of the distinct hierarchies may be multi-rooted and thus represented as a DAG.
My feeling now is to make minimal changes to the document. I think if we make it clear that the starting point is one or more hierarchy each of which may be singly or multiply rooted, but only hierarchies which contain the resource. I don't object to the individual hierarchies or their union as being described as a DAG, but the ancestors could also be computed by examining each hierarchy in turn.
I have some concerns about the URI part, which I will put in a separate email.
    -----Original Message-----
    From: Erik Rissanen [mailto:erik@axiomatics.com]
    Sent: Monday, March 09, 2009 7:43 AM
    To: hal.lockhart@oracle.com
    Cc: xacml@lists.oasis-open.org
    Subject: Re: [xacml] Summary of what I think I said on the call about the
    hierarchical profile
         Hi Hal and all,
         If I understand you correctly, then what you propose is the exact same
    thing as I proposed, except I used the DAG term because I thought we
    wanted to specify how you would get the list of ancestors from a graph.
    If that is not the case, then we can drop the terms DAG, forest and so on.
         So, basically we just say that you have one or more hierarchies in which
    the resource is part of and for the request context you send in the
    resource itself, and its ancestors.
         The only thing which I am still uncertain about in your email is whether
    you are trying to ban the use of a DAG. Sending a list of ancestors this
    way would work for a DAG, which I think is ok.
         Best regards,
                   Hal Lockhart wrote:
        This is an attempt to summarize what I said on the call today. I have
    changed the order a little and added a few extra comments.
        First, let us agree that the hierarchical profile assumes that some
    party needs an AZ decision about a resource that is part of one or more
    hierarchy. The profile does not define what the hierarchy is, the
    semantics of the relationships among its members or anything like that. It
    does define how to extract a small subset of the information and put it in
    the Request Context.
        Now let us consider the two modes of operation in the draft Rich created.
    He called them DAG and Forest mode. If we look at my msg from Tuesday I
    give a small example of some hierarchies and a case where the two methods
    produce different information in the request context. Note that they will
    never differ in their parents, but the DAG mode can include ancestors
    which are not actually in the same hierarchy as the resource. In the
    example, Z is an ancestor of an ancestor (parent actually).
        Another way to express this is that in the DAG model, the "is an
    ancestor" relationship is transitive. Every ancestor of an ancestor is an
    ancestor. In the forest model, it is only transitive within a given
        It is my opinion that the intent of the 2.0 profile, although it is
    certainly not clear and definitely contains mistakes, was that the
    information put in the request context only include hierarchies of which
    the resource is a member. In my example, the Z-A hierarchy would not even
    be considered. Therefore the issue of transitivity does not arise. In
    effect, we are always using the forest model.
        Therefore I do not believe it is necessary to have the forest and DAG
    modes. I do not see any valid usecases for the transitivity property and I
    do not think it was intended in the 2.0 version of the profile. As an
    example, my father is a navy officer. I am below him in a family hierarchy,
    but that does not make navy admirals my ancestors in any way. If my father
    was the resource, the navy hierarchy would be relevant, but if I am the
    resource, it is not. I think all that is required is to clarify that only
    hierarchies of which the resource is a member will be given any
    consideration in computing parents and ancestors.
        Next I talked about loosening the requirement that resources be named
    using a hierarchical URI. We previously agreed to allow strings. My only
    concern was to allow strings or URIs, not URIs carried in strings. This
    allows URI typed operations to be used when the name actually is a URI.
    Eric proposed that we allow any XACML datatype, and I agree. People who
    want the functionality of parsing a hierarchical URI can use a URI and
    others can use whatever is convenient for them. Of course it is possible
    that the information on ancestors and parents might be inconsistent with
    the structure of the hierarchical URI, but that was true in the 2.0
    profile and there are lots of other legal ways for the request context to
    contain inconsistent information. If you put sand in your car's gas tank,
    it will not run, an XACML PDP is the same. In other words, GIGO.
        Finally I said I generally supported Erik's proposed plan of action with
    one exception. Thinking about the problem independently, I had come to the
    conclusion we should totally eliminate mention of a DAG, before reading
    Erik's email. Here is my reasoning. As I said above, we start out with a
    rich set of information about the various hierarchies, at the end we end
    up with a request context which contains nothing but an unordered list of
    parents and an unordered list of ancestors. A DAG is simply a possible
    intermediate step. It contains more information than the request context,
    but less than the original set of multiple hierarchies. Talking about a
    DAG doesn't seem to me to help in explaining what the context handler must
    do, because it represents neither the starting point nor the ending point,
    just one possible intermediate step.
        What I did not say on the call.
                 During the call I was thinking of the distinct hierarchies as being
    singly rooted as in my simple example. However, after the call I realized
    that the algorithm I mentioned completely eliminates the problem of
    transitivity regardless of whether the initial, distinct hierarchies are
    singly or multiply rooted. Therefore it doesn't matter whether the
    individual hierarchies or their union is represented as a forest, dag,
    polyarchy or database table.
        To be explicit here is what I mean:
        1. Start with all hierarchies in the space of resources of the type of
        2. discard all descendants of the resource.
        3. discard all resource hierarchies (and their members) which do not
    contain the resource.
        Now, however you represent the information, any reasonable algorithm to
    enumerate the parents and ancestors, discarding duplicates will produce
    the same results, ignoring order. The issue of transitivity will not arise,
    thus Rich's concern is satisfied.
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