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Subject: Re: [xacml] Issue: Hierarchical profile appears ambiguous and inconsistent
Hi Rich, The ancestor scheme is more general since the hierarchy is completely independent from naming. You could have a hierarchy where http://example.com/A is the parent of urn:oasis:xacml:D You could also have a hierarchy where http://example.com/A is not the parent of http://example.com/A/B Regards, Eri,k Rich.Levinson wrote: > Hi Erik, > > A few points I'd like you to consider with respect to your first point: > > 1. I am not sure why you have chosen to refer the URI scheme as the > "limited URI scheme", because as I showed in previous emails, it > appears that the URI scheme is, in fact, more general than what > you refer to as the "more general approach advocated by Daniel". > The reasons for this are quite simple. First, the URI scheme is > functionally equivalent and more efficient as can be easily seen > as follows: > * in the ancestor scheme, as I understand it, we might have > a node, "a", which is a normative identity for a > particular node, and it might have 2 ancestors, a parent, > "b", and ancestor "c". > Presumably, if a request to access "a" comes in, the CH > will have to gather the ancestors. This means the CH must > have some means of finding out that "b" and "c" are the > ancestors of "a". Since this information is not included > in the node name of "a", the CH must look elsewhere. > * in the URI scheme, I would name my "a" node as "/c/b/a" > using the same strings as above. In the URI scheme, a > request for "a" would come in as "/c/b/a". In this case, > the CH is done, it doesn't have to go looking for > ancestors for 2 reasons: 1. because it doesn't need them, > 2. even if it did need them, they are already there. > > 2. The reason why the URI scheme is more general is a bit subtle, > but still should be straight-forward to understand. The subtlety > is that when presented from an inverted perspective, it might at > first appear that the ancestor scheme is more general, but a few > quick points should explain why that is not the case: > * Sticking with the same use case as above as a starting > point, let us consider the case where "b" has 2 parent > nodes "c" and "d". I don't believe there are many people > who would argue that this structure is still a hierarchy, > however a rational case could be made that it is actually > 2 hierarchies, one headed by "c" and one headed by "d". In > this case "b" would be a member of 2 hierarchies. > * At this point the differences in the two methods becomes > clear. Let us first look at the ancestor case. Because a > relationship between "d" and "b" has been established, > does that mean there is now a relation between "d" and > "a". In the ancestor case, the answer is "yes". > * If we now look at the same use case with the URI method > and ask the question whether there is now a relation > between "d" and "a" the answer is "no". > * One says yes, the other says no. Does this mean they are > simply "equal" in capability, but different in "result"? > Similar to a xacml Rule that can be designed for Permit or > Deny? > * If that was the end of the story, then one could say they > are equal in capability, but it is not the end of the story. > * In the URI case, we may now establish a 2nd relation > /d/b/a, which now will produce the answer yes, when asked > the question whether there is now a relation between "d" > and "a". > * Therefore the URI scheme can produce both a yes and a no > to this question, while the ancestor scheme can produce > only a yes. Therefore the URI scheme has greater > capabilities and is thus, more general than the ancestor > scheme. > * QED > > Again, this is not a change to the profile, this is simply exposing a > capability of using the URIs in the profile that is not present when > using the ancestors. > > There is a 2nd issue that I will also comment on shortly in a separate > email, so that we may reference these use cases in a discrete manner. > > Thanks, > Rich > > > Erik Rissanen wrote: >> All, >> >> I've been in a rush today, so I haven't followed every detail in the >> discussion, but basically, here is how it appears to me: >> >> * The profile, as it stands today, does specify the limited URI >> scheme which Rich describes. It says in section 2.2 that: >> >> --8<-- >> The <pathname> portion of the URI SHALL be of the form >> >> <root name> [ “/” <node name> ]* >> >> The sequence of <root name> and <node name> values SHALL correspond >> to the individual hierarchical component names of ancestors of the >> represented node along the path from a <root> node to the represented >> node. >> --8<-- >> >> So it in fact says that the identifiers must consists of paths with >> the names of the ancestors. >> >> * If I understand Daniel correctly, he says that each node should be >> allowed to have a name which is entirely independent of the other >> nodes in the hierarchy. Relations between the nodes are maintained in >> a manner not specified by XACML and are expressed in XACML Requests >> and policies in the form of the attributes resource-parent, >> resource-ancestor, etc. I think that the more general approach >> advocated by Daniel would be the correct way to go, so I agree with >> him (and Seth I believe. :-)) >> >> * I also think as suggested on the XACML comments/users list that the >> data type of the node identifier should not be limited to URIs only. >> >> But I would prefer to leave major changes to the hierarchical profile >> out of the first batch of CD documents. >> >> Best regards, >> Erik >> >> >> Rich.Levinson wrote: >>> Hi Daniel and TC, >>> >>> Hopefully, those who have followed the details of these emails >>> recognize that each step in the sequence has advanced the discussion >>> in a consistent manner and as a result we have done a fairly >>> thorough job of mapping out the problem space that is under >>> discussion. In any event I believe my comments in this email >>> continue to advance the discussion in a worthwhile manner, and I >>> think will describe the complete problem space as well as give a >>> clear description of the options available, all of which offer full >>> functionality. >>> >>> In the current phase, if I am not mistaken, it is a straight-forward >>> matter to apply definitions to the distinct categories of problems >>> and simply observe that we have two sets of tools which are equally >>> effective at solving each category of problem, where >>> >>> * one set of tools (let's call it the "ancestor method") is most >>> effective when one is dealing with resources where it is not >>> possible or desirable to apply URIs as normative identifiers >>> * a second set of tools (let's call it the "URI method") which is >>> available when one is dealing with resources where URIs can be >>> applied as normative identifiers, and the designers want to take >>> advantage of the powerful features inherent in URI objects, esp >>> when applied to hierarchical problems. >>> >>> Let me address Daniel's points below, then try to summarize the >>> present state of the discussion: >>> >>> Daniel Engovatov wrote: >>>> >>>> On Feb 18, 2009, at 3:05 PM, Rich.Levinson wrote: >>>> >>>>> Daniel, Seth, Erik, and TC, >>>>> >>>>> If we stick to the generally accepted definition that an object in >>>>> a hierarchy can have at most one parent, then a URI solves the >>>>> problem without having to look beyond the URI itself >>>> >>>> It is NOT a generally accepted definition and we did not stick to >>>> it on purpose. >>>> >>> Whether it is generally accepted or not is probably not important >>> here, however, it is consistent with the structure of XML documents, >>> such that when we are talking about a "single hierarchy" of nonXML >>> resources that if we assert that this implies a structural >>> relationship equivalent to the structural relationship of the nodes >>> of a well-formed XML document, which is that each element can have >>> at most one parent, and the top element or node has zero parents. >>> >>> This gives us a crisp unambiguous definition of the term "hierarchy" >>> which can be applied both to the XML and nonXML resources, and it >>> totally avoids trying to determine whether it is an "accepted" >>> definition or not, since that property is no longer relevant. >>> >>> The point of this definition is to give us a conceptual framework >>> within which to evaluate the two primary use cases of the DAG, which >>> as will be explained are also clear and unambiguous well-defined use >>> cases. >>>> >>>>> However, if we allow the hierarchies to break down and lose their >>>>> inherent hierarchical properties, then more complicated >>>>> approaches, such as going outside the initial request context to >>>>> get more nodes, although still solvable w URIs as demo'd below, >>>>> are needed. >>>>> >>>> >>>> It is NOT "broken down" >>> True, in an absolute sense nothing is "broken", however, what has >>> happened is that we have allowed one class of DAG representation to >>> be impacted in such a way that we have allowed it to become the >>> second class of DAG representation because we did not clearly define >>> the distinction and what was to be allowed and not allowed. This has >>> nothing to with whether the ancestor or URI method is used. It has >>> only to do with the relationships that are allowed to be represented >>> when two resources are "connected" by virtue of their hierarchical >>> relationship being established. >>> >>> Specifically, one has a choice of: >>> >>> 1. only allowing the relationship that is being established to be >>> active. For example if my boss is assigned to be subordinate to >>> a task force leader when a cross functional team is being set >>> up, this case would say that has no impact on my relationship >>> with the task force leader unless I am a member of the task >>> force. i.e. the task force leader has control over my boss's >>> resources to whatever degree is implied by the task force >>> situation, however the task force leader has zero direct control >>> over my resources as a result of this assignment. In this model, >>> that direct control could be simply be established by either >>> assigning me directly to the task force leader, or assigning me >>> a second subordinate relationship to my boss in the context of >>> the task force relationship. >>> This is a clearly defined process, where there is no ambiguity >>> about relationships between the resources. If you want the >>> relationship, you explicitly establish it, if not, you don't. >>> >>> 2. the other choice is the exact opposite, namely allowing >>> incidental relationships to be established simply because they >>> connect to a node with direct relationships. To take an extreme >>> light-spirited example, for the purpose of showing how >>> "extraneous" relations are introduced, if the company CEO was a >>> member of a company bowling team, where the captain of the >>> bowling team happened to be a junior software engineer who just >>> joined the company, then everyone in the company would suddenly >>> have this junior engineer as their ancestor. Possibly this would >>> be disallowed by acyclic graph rules, but a similar situation >>> would occur if the VP of engineering was on a bowling team >>> captained by the junior sales trainee, who would now be ancestor >>> to everyone in engineering organization. >>> >>> Both methods are acceptable for assigning relationships, but one or >>> the other may be more effective for one or another type of >>> organization. Personally, I think most enterprise security >>> departments would favor the first approach, because it appears to >>> offer more direct control and less chance of unintended consequences >>> resulting from the assignment of a direct relationship. >>> >>> However, either choice can be used with either the "ancestor method" >>> or the "URI method". Which choice is made is a function of the node >>> collecting algorithm that is used for policy evaluation. i.e. when >>> you collect the parent nodes of the requested node, >>> >>> * choice 1 above means only collect those nodes to which the >>> parent has a direct relationship with the requested node, >>> * and choice 2 means collect all the nodes of choice 1 plus all >>> other nodes where the parent has a hierarchical relationship >>> that does not directly involve the requested node. >>> >>> These are the two primary use cases of the DAG, which were mentioned >>> above. Which use case is chosen depends only on the node collection >>> algorithm and not how the nodes are represented. i.e. parents and >>> ancestors exist whether or not they are incorporated for handy >>> access within a URI or not. >>> When the URI can be used, the URI collection within the requested >>> node itself contains all the nodes that will be collected with >>> method 1 and there is no need to access any additional information. >>> >>>> >>>> >>>>> Again, I am not trying to add or change any of the existing >>>>> functionality, >>>> >>>> You are proposing an addition that is a subset of the more general >>>> approach. >>> Hopefully, the description above satisfactorily demonstrates that >>> the URIs are simply a concrete mechanism to implement the general >>> solution. It is also a mechanism that, if used effectively, appears >>> to be much more efficient since all nodes that need to be collected >>> in method 1 actually are already contained in the URI collection of >>> the requested node. >>> Therefore it is a concrete representation of the general approach, >>> however it is a concrete representation that capitalizes on the fact >>> that the object used to represent the node (the URI) has an >>> equivalent structure to the spatial relationships of the nodes in >>> the DAG that need to be collected in method 1, and so those nodes do >>> not need to collected at all since they are already present. >>> The same structural relationship exists in method 2, however, method >>> 2 fans out so far so fast that collection outside the requested node >>> will be required to fulfill the needs of method 2. >>> It is functionally equivalent to the general approach, however, it >>> has the advantage that a single URI contains the normative identity >>> of all the required nodes for method 1 and some for method 2. >>>> >>>> I understand that you favor a different approach to this problem. >>>> It may be worth our while to create a separate profile for such an >>>> approach, but I do not see any reason to muddy the existing one. >>>> >>> It should be clear from the above discussion that showing how URIs >>> address the same problem is not a "different approach". It is the >>> same approach, except the work required to collect the nodes is a >>> lot less, and can be eliminated almost completely depending on what >>> node collection strategy is chosen, method 1 or method 2. >>> >>> Finally, it should be clear that the bulleted algorithms in section >>> 3.2 of the spec represent a nonURI approach using a method 2 >>> collection algorithm. >>> >>> Now that the problem is clearly defined, I expect it will take much >>> fewer words than have been exchanged in these emails to explain the >>> available options in section 3.2, which are: >>> >>> 1. method 1 node collection, URI method (all nodes required are in >>> requested node) >>> 2. method 1 node collection, ancestor method: (requested node has >>> pointers to parents, but need to recursively navigate to parent >>> to advance up the hierarchy, but does not navigate thru nodes of >>> which the requested node is not a hierarchy member) >>> 3. method 2 node collection, URI method (subset of nodes required >>> are in requested node, the rest must be obtained by recursively >>> navigating based on parent hierarchy nodes of which requested >>> node is not a member) >>> 4. method 2 node collection, ancestor method (this is the algorithm >>> currently in section 3.2 bullets and need to recursively >>> navigate thru all parent nodes regardless of whether requested >>> node is a member of the hierarchy or not.) >>> >>> These 2 choices of node collection are implicit in the DAG problem >>> definition and are not currently explained in the document and I >>> believe need to be. i.e. a DAG is the result of a set of hierarchies >>> (as defined above) being layed across a set of resources. i.e. it is >>> the result of a set of explicit relations being applied between >>> pairs of nodes. The "choice" is whether to retain the "history" of >>> why those relations were applied (i.e. the direct relations) or not. >>> If you don't then additional, indirect, extraneous relations >>> automatically appear and there is no way distinguish between them >>> and the direct relations, at least in the "general" or "ancestor" >>> case. In the URI case, the direct and indirect relations are always >>> present and may be used or not as a matter of choice. >>> >>> The choice of ancestor or URI method for node identification is >>> simply whether URI "can" be used and whether URI is "desired" to be >>> used. Functionally, URI will produce the same results. >>> >>> Thanks, >>> Rich >>> >>>> >>>> Daniel; >> >> >> >> --------------------------------------------------------------------- >> To unsubscribe from this mail list, you must leave the OASIS TC that >> generates this mail. Follow this link to all your TCs in OASIS at: >> https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php
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