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Subject: Re: [xacml] New core and multiple resource profile and hierarchical

Hi Erik,

Thanks for the input. I think there are still some fundamental misunderstandings of what exactly the issue is that I have been raising here (maybe not, maybe I am misunderstanding something in which case, I will be happy to close the issue, but not there yet). I will address your comments below after prelim comment:

First, for people who have been observing this seemingly endless sequence of extremely detailed emails, please rest assured that I believe there is a significant issue that needs to be addressed. Here are a couple introductory points to consider:
  • As a result of the natural complexity of the problem of identifying the applicable policies to apply to a requested resource that can be known to policies under more than one "normative hierarchical name" (where the "hierarchical" name either contains the hierarchical info internally, as in the case of URI, or whether the name is an unstructured unique string that uniquely identifies the resource in some external resource collection, whereby the contexthandler before submitting the request is able to identify and collect all the relevant names of "hierarchical" ancestor nodes to the requested resource node), these emails have had to contain enough detail to address some specific somewhat subtle characteristics of the process for collecting the list of ancestor nodes.
  • To further help frame the issue, it should be commonly understood that when a PEP submits a request, it is first handled by a context handler, which is defined in XACML to handle input requests as follows:
    • "converts decision requests in the native request format to the XACML canonical form"
  • Therefore the context handler needs to know, given a requested resource, how to assemble the list of ancestors for that resource to submit with the request as part of its responsibility of converting the request to "XACML canonical form".
  • The current document, before the modifications I made, had effectively defined two methods of ancestor collection:
    • an explicit method: defined by the algorithms in section 3.2
    • an implicit method: implied by the definition in section 2.2 of how to form a URI that could be used with the hierarchical profile, and by the anyURI methods described in section 4.3 intended to be used by policies that apply to resources identified using the guidelines of section 2.2
  • As it turns out, as can be understood in detail by reviewing the chain(s) of emails on this subject in Jan-Feb-Mar 2009,
    • the explicit methods in section 3.2 imply that the resources are collected in a manner that assumes that policies will be written with the "mindset" of protecting a hierarchical resource structured as a DAG.
    • the implicit methods of sections 2.2/4.3 effectively imply that the resources are collected in a manner that assumes that policies will be written with the "mindset" of protecting a hierarchical resource structured as a forest. (For example, if a node has 2 normative URIs, then it is a member of 2 trees in the forest - policies written using anyURI will be applicable if their scope includes either of the 2 normative URIs
  • This is roughly the point I was at when I started raising this issue. The first thing I noticed was that there were effectively 2 different ancestor collection methods at work, which had the following curious properties:
    • the methods in section 3.2 involve collecting ancestors of the resource that are not "direct" ancestors, in the sense that if the resource's parent has a parent that is not directly connected then policies associated with that ancestor apply to the requested resource
      • To use the common "sibling" terminology, a "direct" ancestor would be a bloodline ancestor, such as a grandparent. In "indirect" ancestor would be equivalent to a "step-grandparent", i.e. one not connected by bloodline, but that established a relation with the bloodline parent "after the fact" and now has an "indirect" relation with the "step-grandchild".
    • So the question which "lights all the fires" on this issue is whether a "step-grandparent" should have an equal relation with the requested resource as a bloodline "grandparent".
    • It turns out that with the DAG model there is no way for the context handler to differentiate between "grandparents" and "step-grandparents", so the answer is ALWAYS that these are treated equally.
    • By comparison, it turns out that "by default" (i.e. the contexthandler just collects the set of URIs from the resource with no need to search for ancestors because all the needed URIs are right there) using URIs that only grandparents will be selected. "step-grandparents" could be obtained if desired, but simple out of the box URIs give you only the bloodline ancestors.
  • The reason I am concerned about this issue is that from a security perspective, it makes little sense to me to force commonly understood hierarchies, such as organization charts, geographic breakdowns of organization operations, whether within a building or around the world, to suddenly have policies that are intended only to apply to the resources within these specified domains, suddenly apply to resources outside of these domains.
  • Similarly, resources within these domains will find themselves subject to policies applied to resources outside of these domains.
    • For example, if I am a manager in the United States, and there is a policy that says employees in the United States may treat the 4th of July as a holiday, then anyone outside the United States who has any superior inside the United States will be subject to this policy.
    • Why? Because the resources are treated as a DAG. DAGs do not deal with resources individually, they only deal with subtrees.
With that background, let me address Erik's points:

Erik Rissanen wrote:
49AE6BE2.3060302@axiomatics.com" type="cite">Rich and TC,

I have reviewed the draft you have posted (wd 05-02) as well as the recent discussion on the list, and I think the draft needs some more work.

I agree with you that the old profile had lots of ambiguities and small errors in it, and I think you have done a good job at spotting them and you have improved the text in many places.

However, I also think that the new work you have added in there complicates matters needlessly. There are two reasons for that:

1. You try to write the profile so it works with multiple intersecting hieararchies. I think that is the wrong way to do it. It should be specified for a single hierarchy only. If you need to apply it on multiple hierarchies, the way to do it is to preprocess the hierarchies by merging them into a single hierarchy. This joint hierarchy also needs to meet some consistency criteria, or the whole thing becomes meaningless and inconsistent. See more about that below.
This is an invalid assertion. I leave the profile unchanged, except for distinguishing the DAG and forest/polyarchy distinctions.
The DAG is inherently is multiple "overlapping" hierarchies that can be combined into a "single multiroot hierarchy" (see ref prev email) http://en.wikipedia.org/wiki/Directed_acyclic_graph#Properties
The forest is inherently multiple "intersecting" hierarchies, that can be combined into a "single multiroot disjoint hierarchy" identical in every way to the DAG except that:
  1. it retains the identity of the multiple hierarchies as its disjoint property
  2. it is not restricted from having "cycles" as long as the circuit travels over at least segments from 2 disjoint hierarchies
As should be clear from above description, and all previous emails on this subject, the preprocessing is done by creating a joint hierarchy. The only difference between the DAG and the forest is that with the DAG, potentially useful information about the joint hierarchy is thrown away, with the forest it is kept. All is done before contexthandler submits request based on what contexthandler is capable of doing with the resource collection of which the requested resource is a member.

Unfortunately, this statement appears to characterize the conceptual gap on this issue:
"This joint hierarchy also needs to meet some consistency criteria, or the whole thing becomes meaningless and inconsistent."
All one needs to do is peruse the emails to quickly realize that it is the profile that mixes the DAG/forest concepts, which renders the existing spec "meaningless and inconsistent" and that my effort has been to separate these concepts in order to establish meaning and consistency to the contents of the spec.
49AE6BE2.3060302@axiomatics.com" type="cite">
2. You use the "root" concept. That is actually not required at all. As you have realized, the concept of a root in a DAG becomes messy to define and handle. I think we should just drop any reference to a "root" in the whole profile. All we need are "ancestors", and those are found by following the edges upwards. Not down from the root, so the root doesn't have to be defined. (In fact, in a DAG, there is not necessarily a unique root anyway.)
DAG is a directed graph. If you keep following any parent relationship recursively from the requested node you will come to one of the roots of the DAG. If you chose different parents along the way you will come to a different root, in general. In a DAG, it doesn't matter from the perspective of the requested node, all roots are functionally equivalent in the sense that there is no inherent distinction as to the status of one root vs another.

In the forest model, you can have exactly the same layout as any DAG. And you can traverse the same recursive paths to any of the same roots. The only difference along the way is at each step you know whether you are proceeding along the same hierarchy as in your previous step or whether you are switching to another hierarchy.

In the world of security, these hierarchical paths are commonly known as "lines of authority". Generally, in security applications the notion of "clear lines of authority" is desirable, and the notion of "tangled lines of authority" is detrimental. This is precisely the distinction of forest (clear lines) and DAG (tangled lines).
49AE6BE2.3060302@axiomatics.com" type="cite">
Finally, I think we should write the normative sections so they target a DAG only. Trees and forests follow as special cases. We can add explanatory text to make this clear, but the normative parts become much simpler if we don't define many different types of hierarchies.
Obviously, from my above remarks, if we were to target "only" one of DAG or forest, I would choose forest because
  • forest  represents "clear lines of authority" for security applications
  • URI as recommended in section 2.2 already provides it out of the box
and I would not choose DAG, because
  • DAG represents uncontrolled ambiguous lines of authority
  • DAG is intended for sub-tree, or whole "sub-assembly at a time" type of operations, which is why it is popular in source code control systems. If we think global enterprises can be modeled as special case of source code control system then DAG will be great, otherwise, a forest is needed.
Bottom line: I think when the dust and smoke is removed from this issue, we are left with what kind of model do we want for resources: a highly structured model as in forest, or a more loosely structured, "social network" type of model such as DAG.

Or we could follow the path I have represented in the profile which is that you can choose either, as appropriate, for specific subset of the overall enterprise resources.


49AE6BE2.3060302@axiomatics.com" type="cite">
I propose the following changes:

- First, a small nitpick. ;-) I don't like "Working draft 05-02". I think working drafts should progress 5, 6, 7, 8 and so on. It's confusing that there are several documents, all called working draft 5.

- Remove all the new definitions of "polyarchy", etc. They are not needed. Use only the term "DAG".

- Define a hierarchy as a DAG, where the nodes correspond to resources and the edges correspond to child-parent relationships.

- Define that each node in the hierarchy has one or more "normative representations" (names). A normative representations is defined as an XACML datatype value instance, which is provided in the request (through the context handler or the PEP). A representation which is not provided in the request is not normative, and may not be referenced in policies.

- This is actually already implied, but maybe worth stating: if one merges a number of hierarchies in the pre-processing step, the merged hiearchy must be consistent with respect to node representation/naming and the parent-child relationship. That is, if A and B are two representations of a node and C and D are representations of another node, and there is a relationship between the nodes, the same relationship applies to all representations/names. So all of the following would apply, not just some of them: A-C, A-D, B-C and B-D. I think the complexities in what you have done Rich is much due to that you have tried to cover hierarchies where this is not true. But those hierarchies are not internally consistent, so we cannot make them work.

- The ancestors of a node are defined as simply the transitive closure of the parent-child relationship.

Note that the above points imply that there is a single hierarchy which is a DAG. Also note that I don't make use of the term "root". It's not needed.

- Add in a couple of examples with illustrations showing a simple tree formed DAG and a more complex DAG which is not a tree and show how the ancestors are found.

- Remove section 1.1.1. It's not needed if we specify the algorithms on a sigle DAG only.

- Make a separate section about the URI-only scheme, saying that "in some cases when the resources are represented as URIs, it may be possible to simply do matching on portions of the resource representations". Also make it clear that the PEP and PDP must agree on which scheme is used, or the policies won't work.

- I think that a node may be represented by any XACML data type, like a string for instance, not just URIs. This is what the user posted to the comments list and requested that we change.

- I think there are some problems with section 2.2. In particular, it should not say anything about UTF-8 encoding. That's already defined by the core schema and it's unicode consistency requirements. I think actually that we can drop the whole of section 2.2 and allow any form on the normative representations of nodes. There is no reason to restrict it to a particular form of a URI. (The section on the URI-based schema should of course contain some restrictions for how that scheme is made to work.)

- We should not define a new identifier for a functionality named "...:forest:...".

- There is no need to various subsections on multi-rooted hierarchies or polyarchies. See for instance 3.2.1 and 3.2.2.

In short, I think we need very few changes compared to the original profile. We just need to clarify the terminology and definitions, put in a couple of illustrations, relax the data type restrictions and add a section of a URI matching based scheme as an alternative to the other two schemes.

Best regards,

Rich.Levinson wrote:
Hi Erik,

The hierarchical profile is ready for review as is. There are no more changes planned.

    Note: The two added identifiers in section 2.2,
    "...hierarchical:forest:non-xml-node-id" and 3.2.2,
    "...hierarchical:forest:non-xml-node-req" are for convenience only
    and the spec could be rephrased without them, if necessary.

The profile is located at:

The example that Hal requested, which provides further motivation for the changes, as well as detailed explanatory technical structure, is at:

A fresh example of application of the profile using URIs that came up yesterday on the xacml-users list is at:


Erik Rissanen wrote:

I have posted new drafts of the core and the multiple resource profile. See the change logs and tracked changes for details.

As far as I can tell, we don't have any open issues on the following specs:

- Core
- Multiple resource
- Administration
- Privacy
- Dsig

The hierarchical profile is being discussed currently and there was discussion about improving the RBAC profile.

The proposed work on the RBAC profile seems in very early stages and the issue (policies about management of roles) is a major topic, so I propose that we don't bring this in 3.0.

So, could we agree on a feature freeze on the above mentioned profiles? If so, all of the expect hierarchical are ready for review before going to committee draft.

I also propose that if we don't get resolutions on the issues in the hierarchical profile soon and it would appear that there are major changes required, then we use the old version of that profile. However, my understanding is that Rich has pretty much completed the work on that. I haven't had the time to review it myself yet, but I will do so now.

So, given the above, can we agree on the following?

- everybody reviews the above mentioned profiles
- We correct any mistakes
- I will fix the metadata, references, namespaces, etc,
- Go CD with the above

One final issue: we need to update the acknowledgements section of the core. What goes in there? My name is not in there, and I would like to include it. :-) I presume that we keep all the old names, right? John Tolbert has requested to be added. Anybody else?

Best regards,

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