The problem presented by the commenter is analagous to:
Just because two separate cities have a street named MainStreet does
not mean these are the same street. These are two distinct normative
identifiers for two distinct resources.
A forest is a DAG, but a DAG is not necessarily a forest. When you
start using DAGs that are not forests or hierarchies, because the
member trees are not disjoint, then you are no longer talking about a
Possibly we can consider splitting the Hierarchical Profile into two
However, the current situation, where the profile named "Hierarchical"
deals with structures that are explicitly non-hierarchical, and
actually anti-hierarchical, in that they destroy the hierarchical
properties is not an acceptable state of affairs.
- One called the Hierarchical Resource Profile, which deals with
hierarchies and forests
- One called the DAG Resource Profile, which deals with more
general structures that are not necessarily hierarchies or forests, but
may incidentally be in special cases.
Labeling the profile "Hierarchical" and then representing it with
anti-hierarchical algorithms is a misrepresentation of its content and
a Severe Error, because it leads people to expect one thing and then
delivers something which is exactly NOT what it is labeled to be, and
has potential extreme severe unintended consequences when used as such.
My recommendation is to correct the content so that the specification
is no longer mislabeled, which is done by simple modifications to the
algorithms in section 3.2, which simply changes the node collection
algorithm to only collect the one and only parent node of the
requested node for each normative identity of the requested node
and then recursively for ancestors. (i.e. a node can have multiple
parents but they must be in disjoint trees)
Erik Rissanen wrote:
I don't think I understand what you mean.
The commenter's resources are not disjoint. They are a DAG. Therefore
he wants to use the "ancestor scheme" without URIs.
The commenter states the problem he is trying to solve as:
* "I have a rule that says "permit if resource has ancestor
It is only after he follows the erroneous directions in section 3.2
that he runs into trouble, whereas if he used the suggestion in my
previous email the problem would already be solved and we would have no
I have done further research into this and I think that I can now say
unambiguously that section the bulleted algorithms in section 3.2 are
simply wrong, because they are based on an erroneous premise.
The erroneous premise is that it is assumed that a forest is the same
as a DAG. In fact, they are not the same. In particular, a forest is
generally defined as a *disjoint set of trees*. The important word in
this phrase is "disjoint". There are many reasons why this problem must
be modeled as a disjoint set, several of which have been covered in
The algorithms in section 3.2 are specifically not disjoint, which is
why they are in error. This error is simply corrected by a minor
modification to ensure disjointness.
At this point I would like to raise this to a formal issue, which I
would categorize as a recommendation to correct a SEVERE ERROR in the
Hierarchical Profile Specification.
We have already seen evidence as reported by this commenter that the
errors in this specification are leading users to do wrong and
irrelevant work, and we now have specific formal definitions of the
terms that express exactly what the error is and how it can be
Erik Rissanen wrote:
My understanding is that the commenter does not want to use URIs at
all. Instead he wants to do it like this:
URI matching wouldn't help him.
With regard to your 2nd point on the comments/users list, it appears to
me that because the Hierarchical spec has the problems that I have
described in earlier emails, that the user has posed his question in
such a way that instead of advising the commenter on what appears to me
to be a simple obvious way to solve his problem, you instead have been
drawn into considering the user's suggestion that we change the specs
from the current explicit description in section 2.2 to one that
appears to me would significantly undermine the intent of the current
Now that we have hammered away at the hierarchical profile for several
emails, and if people have been reading the details, we can now take
another look at the question the commenter raised, which I had
previously said we should revisit in the context of my proposal (item 3
The commenter's question is here:
Your reply to him, where you said you would bring the issue to the TC,
This example shows exactly why I believe it is NECESSARY to add the
clarifying text that I have been talking about to section 3.2.
It appears to me that this user has been clearly misled by the spec to
attempt to do what seems to me to be a wasteful and pointless exercise
of parsing URIs and then trying to make sense of the collection of
artifacts that he has produced. It appears to me that this is totally
unnecessary, and I would suggest that it has been a waste of his time
for being led down this path by the specifications, which is the point
I have repeatedly been making that the specs are insufficient in
section 3.2 in this regard .
I am concerned that instead of addressing this type of problem with the
capabilities currently in the spec, that we are being drawn down a
path, which will effectively dismantle the perfectly good functionality
that is in the spec by removing the URI capability as it currently
I think this user should be told that since his nodes are currently
identified by URIs that are in the form called for in section 2.2 that
he does not need to worry about collecting ancestors etc. and all he
needs to do is apply the XACML 2.0 anyURI-regexp-match as described in
section A.3.13 of the core spec and his problem is solved.
As described in section 4.3 of the example doc in the archive
this function is simply specified as:
This is the kind of guidance I am recommending that we add to section
3.2 of the Hierarchical Profile.
As I understand your email, an alternative suggestion to address this
user situation is to recommend that the user parse his URIs into
components, and that we change the spec to alter the currently defined
URI capability (to no longer be a URI capability, with all the inherent
capabilities of a URI) in order that the customer be able to do this?
Let me further point out that I don't think there is any need to change
the spec, because it already says in section 2 that governs 2.2 that:
"The following sections describe RECOMMENDED representations for
nodes in hierarchical resources. Alternative representations of
nodes in a given resource are permitted so long as all Policy
Administration Points and all Policy Enforcement Points that deal
with that resource have contracted to use the alternative
Therefore, if for some reason we advise against using what appears to
me to be an obvious standard capability described above with
anyURI-regexp-match, and would like users instead to parse their URIs
into components so that they can supply the components as a collection
of ancestors, I believe we are still able to do this because the
statement above appears to explicitly allows for nonURI
representations, in which case we would not have to do anything to
However, to be perfectly honest, I still see no advantage to applying
the bulleted capability procedure of section 3.2 to nodes that are
already identified by URIs.
Comments welcome. I would really like to understand if there is a
legitimate reason for not using the URIs as is and replacing them by
collections of components.
Erik Rissanen wrote:
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:
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
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.
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:
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.
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
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
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.
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"
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.
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
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
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.
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.
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.
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
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
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.
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