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Subject: Re: [xacml] Summary of what I think I said on the call about thehierarchical profile
Hi Rich,
What you describe is what I meant with "Way B".
In way A, one first defines the parent-child relations on the resources
themselves, independent of anything with names. After that is done, one
can give name to the nodes. There is no risks of inconsistencies.
In contrast, in Way B, you initially have a set of hierarchies. Once you
start adding a resource to multiple, independently defined hierarchies,
you can get inconsistencies. Two nodes may have reverse ancestor
relationships in two different hierarchies. This is inherent in this
model, and has nothing to do with how we formulate the profile.
Anyway, all this is outside the scope of the profile itself if we define
the profile in terms of the ancestor attributes. This corresponds to
that for the purposes of the profile, there is an already defined,
_single_ "hierarchy" (if we allow a general interpretation of the term),
which is "flattened" to ancestor attributes. The profile does not have
to deal with inconsistencies in multiple hierarchies.
The single "hierarchy" which the profile works on, may be a the result
from a preprocessing step where a number of independent hierarchies have
been merged. But not, it's not always possible to merge hierarchies
consistently.
Best regards,
Erik
Rich.Levinson wrote:
> 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.
>
> Thanks,
> Rich
>
>
>
>
> Erik Rissanen wrote:
>> 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,
>> Erik
>>
>>
>> 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
>>> “urn:oasis::names:tc:xacml:2.0:resource:resource-ancestor”.
>>>
>>> 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
>>> resource.
>>> 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.
>>>
>>> Thanks,
>>> Rich
>>>
>>>
>>> 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.
>>>>
>>>>
>>>>
>>>> Hal
>>>>
>>>>
>>>>
>>>> ________________________________
>>>>
>>>> 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.
>>>>
>>>> Thanks,
>>>> Rich
>>>>
>>>>
>>>> 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.
>>>>
>>>> Hal
>>>>
>>>>
>>>> -----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,
>>>> Erik
>>>> 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
>>>> hierarchy.
>>>>
>>>> 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
>>>>
>>>> interest.
>>>>
>>>> 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.
>>>>
>>>> Hal
>>>>
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>>>>
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>>>>
>>>>
>>
>>
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