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Subject: Re: [xacml] Summary of what I think I said on the call about thehierarchical profile
In my previous email I tried to show how the forest can cleanly handle the use case.
I am not sure why you say:
I showed how multiple hierarchies across a common set of resources could be represented by an equal number of columns in a table, where each column represented one hierarchy. Optionally each column could have an adjunct column with the names that are used for the members within each hierarchy, but, functionally, that's a "bellandwhistle".
I also showed that the DAG could be made from this by considering the columns to altogether be considered as a 2 dimensional table, and that the DAG could be formed by "compressing" the data in the rows to the left. This would save some space, but also destroy the columns.
If I am not mistaken, there is no actual requirement to create the DAG, except for possibly some performance gains. However, if the cost of performance is such that the operations no longer can be made to work, then this does not appear to be reasonable choice to make.
Finally, none of these discussions have really changed anything as far as I can see in terms of the basis on which I put together the v5-02 spec. The descriptive text in the glossary is optional. It is only there to be helpful. Section 3.2.2 and 220.127.116.11 describe the forest and URI alternatives respectively for those use cases where creating a DAG does not work or, in general, for people who prefer to keep their hierarchies intact when creating a model around which to develop policies.
Erik Rissanen wrote:
49B87AB7.firstname.lastname@example.org" type="cite">Hi Rich, See my previous email for an explanation why it will not be possible to define the profile in terms of multiple hierarchies. We must have a single consistent hierarchy to work on. The 2.0 profile talks about multiple parents, so it works on a DAG or forest in addition to a plain tree. Anyway, I've completely lost track of the various proposals for how to proceed with the profile text. :-) Maybe it's time for a new working draft after today's call? It would be so much easier to talk about a specific document, rather than lots of "imaginary" proposals from several people. :-) Regards, Erik Rich.Levinson wrote:Hi Erik, Previous email was simply to respond to what I believe was the inaccurate premise about on which you started your email. That aside, let me respond to the content of your email. You say: "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 assumption you are making here appears to be that the resources are simply part of a predefined hierarchy. *There is a big problem with this assumption, when you consider it in terms of the algorithms defined in section 3.2.* * These algorithms (3.2) presume that the requested node can have multiple parents and those parents also can have multiple parents etc. * If we are also to take as a given, that this "hierarchy" has already been "flattened" in terms of ancestor attributes, then that assumes we have already erased any original hierarchical relationships. That is the problem: the prescription in section 3.2 essentially tells customers that they must turn their multiple hierarchically arranged forest of resources into a DAG in order to use this profile. My proposal allows you to do exactly that, and if that's what you want to do, use section 3.2.1 of my proposed revision which is exactly what is in the existing profile. However, if as I believe is perfectly reasonable, that customers do not want to do a preprocessing step that effectively destroys their existing hierarchies in order to create a DAG with all of its problems, then I am offering section 3.2.2, which saves them the trouble of having to do this DAG pre-processing, and allows them to work with their pre-existing forest as is, instead. And, finally, once again, if they have their resources hierarchically named, as prescribed in section 2.2, they can use the forest model from section 3.2.2 without the need to create the ancestors, because they already have them in the URIs. The issue here seems to be the ancestor model for forest. The existing algorithms basically suggest that the customer cut down their original forest and replace it with a DAG. Section 3.2.2 says that instead of replacing the forest with a DAG, determine the ancestors by following the original hierarchies directly, which of course you can't do if you have already replaced your forest with a DAG. Or if you didn't replace your forest and just created a DAG in front of it, section 3.2.2 says disregard this DAG and process the forest directly. One final note: you made the following point twice in your email, so I want to clarify that I do not believe what you are claiming is, in general, a problem: * "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." * "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" These "inconsistencies" you are referring to, only exist if you want to create a DAG. If you just work on the forest, they are not inconsistencies and do not create any problems. It is only when you map the forest into the highly constrained DAG that you find that by forcing a DAG on the forest, you effectively force rules on the forest that only need to be there if you want to create a DAG. As has been discussed, not everyone believes that transforming a set of hierarchies into a DAG is a good idea. It may be for some applications, like creating dictionaries of resources, but that is not an access control application. It is an inventory-type application. For access control there is no requirement to create a DAG, and little incentive to do so, if it will force rules on assignment of relations for resources that are not needed and prevent creation of organizational resource relations that are considered desirable. On the one hand, technically, this issue boils down to: * How to define the ancestors (DAG or forest) On the other hand, operationally, the issue boils down to: * Do customers want to represent their resources in the form of a DAG, which will effectively make it impossible for them to manage multiple hierarchies independently. For the first technical point, my proposal offers both options. For the second operational point, my proposal offers 3.2.2, 18.104.22.168 if they find the DAG too restrictive. All this is done with no change in functionality, but simply be explaining the problem being solved by the profile more clearly, which makes obvious the alternate set of algorithm in section 3.2.2. Thanks, Rich Rich.Levinson wrote:Hi Erik, I think that you are misinterpreting what I said in the previous email. Let me try to be more explicit: 1. Start with an unnamed collection of resources. Let's say there are N individual identical resources. 2. On a laptop, using a program, create an array of N variables, for example int parent = new parent[N]; 3. Now I can start creating my hierarchy by assigning a value to these parent ints which somehow identifies another of the resources in the collection. I believe at this point that I must pick some method to identify the elements of the collection, otherwise there is no meaning to this value. 4. For the sake of discussion, let me choose to use the index of the element in the array as the way to identify the parent. Therefore, I can pick any element in the array and assign it a value in the range 0->N-1. For example, I can say: parent[i] = j; 5. Using this technique I can create a hierarchy of these resources. This is basically an implementation of the example I gave in this email I was asked to prepare to help the TC understand the issue by example: http://lists.oasis-open.org/archives/xacml/200902/msg00058.html The bottom line is that in order to assign parent child relationships, you need to have something to hold the assignment artifact. There will be one artifact holder for each resource. When you assign an artifact to a holder, this artifact must somehow identify another resource in the collection in order for it to have any meaning in terms of this operation of assigning a parent child relationship. So, in the example I gave in the email, each of the 2 customers assigned parent child relations to the empty boxes on the piece of paper they were each handed. When customer 1 handed the paper to the collection manager, the manager did the equivalent of the above. The fact that I had id-numbers painted on the box was just to help people understand the problem. The example works equally well if you paint the numbers on the boxes later after the customers have handed in their papers and the manager entered their data into 2 arrays. The manager would then create a 3rd array and assign these painted numbers then. This is totally consistent with what you are calling "Way A": "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." So, I do not understand, what you are trying to indicate in your email, since it appears to premised on an assumption you are making that appears to be inconsistent with what I have been saying in these emails. Since I believe I have been totally consistent in my representation of the problem, we should be able to unambiguously resolve this "Way A", "Way B" discussion, and possibly come up with some terminology on which we agree so that we may continue this analysis without having to continually re-explain what we are talking about. Thanks, Rich Erik Rissanen wrote: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:email@example.com] Sent: Tuesday, March 10, 2009 8:56 AM To: firstname.lastname@example.org Cc: Erik Rissanen; email@example.com 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:firstname.lastname@example.org] Sent: Monday, March 09, 2009 7:43 AM To: email@example.com Cc: firstname.lastname@example.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 --------------------------------------------------------------------- To unsubscribe from this mail list, you must leave the OASIS TC that generates this mail. 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