OASIS Mailing List ArchivesView the OASIS mailing list archive below
or browse/search using MarkMail.


Help: OASIS Mailing Lists Help | MarkMail Help

xacml message

[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]

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 

You could also have a hierarchy where http://example.com/A is not the 
parent of http://example.com/A/B


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

[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]