RFC3060 - Policy Core Information Model -- Version 1 Specification

Network Working Group B. Moore
Request for Comments: 3060 IBM
Category: Standards Track E. Ellesson
LongBoard, Inc.
J. Strassner
A. Westerinen
Cisco Systems
February 2001
Policy Core Information Model -- Version 1 Specification
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document presents the object-oriented information model for
representing policy information developed jointly in the IETF Policy
Framework WG and as extensions to the Common Information Model (CIM)
activity in the Distributed Management Task Force (DMTF). This model
defines two hierarchies of object classes: strUCtural classes
representing policy information and control of policies, and
association classes that indicate how instances of the structural
classes are related to each other. Subsequent documents will define
mappings of this information model to various concrete
implementations, for example, to a Directory that uses LDAPv3 as its
Access protocol.
Table of Contents
1. Introduction.................................................... 4
2. Modeling Policies............................................... 5
2.1. Policy Scope............................................... 8
2.2. Declarative versus Procedural Model........................ 8
3. Overview of the Policy Core Information Model.................. 10
4. Inheritance Hierarchies for the Policy Core Information Model.. 13
4.1. Implications of CIM Inheritance........................... 15
5. Details of the Model........................................... 15
5.1. Reusable versus Rule-Specific Conditions and Actions...... 15
5.2. Roles..................................................... 17
5.2.1. Roles and Role Combinations............................. 17
5.2.2. The PolicyRoles Property................................ 21
5.3. Local Time and UTC Time in PolicyTimePeriodConditions..... 21
5.4. CIM Data Types............................................ 23
5.5. Comparison between CIM and LDAP Class Specifications...... 24
6. Class Definitions.............................................. 25
6.1. The Abstract Class "Policy"............................... 25
6.1.1. The Property "CommonName (CN)".......................... 26
6.1.2. The Multi-valued Property "PolicyKeyWords".............. 26
6.1.3. The Property "Caption" (Inherited from ManagedElement).. 27
6.1.4. The Property "Description" (Inherited from
ManagedElement)......................................... 27
6.2. The Class "PolicyGroup"................................... 27
6.3. The Class "PolicyRule".................................... 29
6.3.1. The Property "Enabled".................................. 31
6.3.2. The Property "ConditionListType"........................ 31
6.3.3. The Property "RuleUsage"................................ 31
6.3.4. The Property "Priority"................................. 32
6.3.5. The Property "Mandatory"................................ 32
6.3.6. The Property "SequencedActions"......................... 33
6.3.7. The Multi-valued Property "PolicyRoles"................. 33
6.4. The Abstract Class "PolicyCondition"...................... 34
6.5. The Class "PolicyTimePeriodCondition"..................... 36
6.5.1. The Property "TimePeriod"............................... 38
6.5.2. The Property "MonthOfYearMask".......................... 39
6.5.3. The Property "DayOfMonthMask"........................... 39
6.5.4. The Property "DayOfWeekMask"............................ 40
6.5.5. The Property "TimeOfDayMask"............................ 41
6.5.6. The Property "LocalOrUtcTime"........................... 42
6.6. The Class "VendorPolicyCondition"......................... 42
6.6.1. The Multi-valued Property "Constraint".................. 43
6.6.2. The Property "ConstraintEncoding"....................... 43
6.7. The Abstract Class "PolicyAction"......................... 44
6.8. The Class "VendorPolicyAction"............................ 45
6.8.1. The Multi-valued Property "ActionData".................. 45
6.8.2. The Property "ActionEncoding"........................... 46
6.9. The Class "PolicyRepository".............................. 46
7. Association and Aggregation Definitions........................ 46
7.1. Associations.............................................. 47
7.2. Aggregations.............................................. 47
7.3. The Abstract Aggregation "PolicyComponent................. 47
7.4. The Aggregation "PolicyGroupInPolicyGroup"................ 47
7.4.1. The Reference "GroupComponent".......................... 48
7.4.2. The Reference "PartComponent"........................... 48
7.5. The Aggregation "PolicyRuleInPolicyGroup"................. 48
7.5.1. The Reference "GroupComponent".......................... 49
7.5.2. The Reference "PartComponent"........................... 49
7.6. The Aggregation "PolicyConditionInPolicyRule"............. 49
7.6.1. The Reference "GroupComponent".......................... 50
7.6.2. The Reference "PartComponent"........................... 50
7.6.3. The Property "GroupNumber".............................. 50
7.6.4. The Property "ConditionNegated"......................... 51
7.7. The Aggregation "PolicyRuleValidityPeriod"................ 51
7.7.1. The Reference "GroupComponent".......................... 52
7.7.2. The Reference "PartComponent"........................... 52
7.8. The Aggregation "PolicyActionInPolicyRule"................ 52
7.8.1. The Reference "GroupComponent".......................... 53
7.8.2. The Reference "PartComponent"........................... 53
7.8.3. The Property "ActionOrder".............................. 53
7.9. The Abstract Association "PolicyInSystem"................. 54
7.10. The Weak Association "PolicyGroupInSystem"............... 55
7.10.1. The Reference "Antecedent"............................. 55
7.10.2. The Reference "Dependent".............................. 55
7.11. The Weak Association "PolicyRuleInSystem"................ 56
7.11.1. The Reference "Antecedent"............................. 56
7.11.2. The Reference "Dependent".............................. 56
7.12. The Association "PolicyConditionInPolicyRepository"...... 56
7.12.1. The Reference "Antecedent"............................. 57
7.12.2. The Reference "Dependent".............................. 57
7.13. The Association "PolicyActionInPolicyRepository"......... 57
7.13.1. The Reference "Antecedent"............................. 58
7.13.2. The Reference "Dependent".............................. 58
7.14. The Aggregation "PolicyRepositoryInPolicyRepository"..... 58
7.14.1. The Reference "GroupComponent"......................... 58
7.14.2. The Reference "PartComponent".......................... 59
8. Intellectual Property.......................................... 59
9. Acknowledgements............................................... 59
10. Security Considerations....................................... 60
11. References.................................................... 62
12. Authors" Addresses............................................ 64
13. Appendix A: Class Identification in a Native CIM
Implementation................................................ 65
13.1. Naming Instances of PolicyGroup and PolicyRule........... 65
13.1.1. PolicyGroup"s CIM Keys................................. 65
13.1.2. PolicyRule"s CIM Keys.................................. 66
13.2. Naming Instances of PolicyCondition and Its Subclasses... 67
13.2.1. PolicyCondition"s CIM Keys............................. 69
13.3. Naming Instances of PolicyAction and Its Subclasses...... 71
13.4. Naming Instances of PolicyRepository..................... 72
13.5. Role of the CreationClassName Property in Naming......... 73
13.6. Object References........................................ 73
14. Appendix B: The Core Policy MOF.............................. 75
15. Full Copyright Statement..................................... 100
1. Introduction
This document presents the object-oriented information model for
representing policy information currently under joint development in
the IETF Policy Framework WG and as extensions to the Common
Information Model (CIM) activity in the Distributed Management Task
Force (DMTF). This model defines two hierarchies of object classes:
structural classes representing policy information and control of
policies, and association classes that indicate how instances of the
structural classes are related to each other. Subsequent documents
will define mappings of this information model to various concrete
implementations, for example, to a directory that uses LDAPv3 as its
access protocol. The components of the CIM schema are available via
the following URL: http://www.dmtf.org/spec/cims.Html [1].
The policy classes and associations defined in this model are
sufficiently generic to allow them to represent policies related to
anything. However, it is eXPected that their initial application in
the IETF will be for representing policies related to QoS (DiffServ
and IntServ) and to IPSec. Policy models for application-specific
areas such as these may extend the Core Model in several ways. The
preferred way is to use the PolicyGroup, PolicyRule, and
PolicyTimePeriodCondition classes directly, as a foundation for
representing and communicating policy information. Then, specific
subclasses derived from PolicyCondition and PolicyAction can capture
application-specific definitions of conditions and actions of
policies.
Two subclasses, VendorPolicyCondition and VendorPolicyAction, are
also included in this document, to provide a standard extension
mechanism for vendor-specific extensions to the Policy Core
Information Model.
This document fits into the overall framework for representing,
deploying, and managing policies being developed by the Policy
Framework Working Group. It traces its origins to work that was
originally done for the Directory-enabled Networks (DEN)
specification, reference [5]. Work on the DEN specification by the
DEN Ad-Hoc Working Group itself has been completed. Further work to
standardize the models contained in it will be the responsibility of
selected working groups of the CIM effort in the Distributed
Management Task Force (DMTF). DMTF standardization of the core
policy model is the responsibility of the SLA Policy working group in
the DMTF.
This document is organized in the following manner:
o Section 2 provides a general overview of policies and how they are
modeled.
o Section 3 presents a high-level overview of the classes and
associations comprising the Policy Core Information Model.
o The remainder of the document presents the detailed specifications
for each of the classes and associations.
o Appendix A overviews naming for native CIM implementations. Other
mappings, such as LDAPv3, will have their own naming mechanisms.
o Appendix B reproduces the DMTF"s Core Policy MOF specification.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119, reference
[3].
2. Modeling Policies
The classes comprising the Policy Core Information Model are intended
to serve as an extensible class hierarchy (through specialization)
for defining policy objects that enable application developers,
network administrators, and policy administrators to represent
policies of different types.
One way to think of a policy-controlled network is to first model the
network as a state machine and then use policy to control which state
a policy-controlled device should be in or is allowed to be in at any
given time. Given this approach, policy is applied using a set of
policy rules. Each policy rule consists of a set of conditions and a
set of actions. Policy rules may be aggregated into policy groups.
These groups may be nested, to represent a hierarchy of policies.
The set of conditions associated with a policy rule specifies when
the policy rule is applicable. The set of conditions can be
expressed as either an ORed set of ANDed sets of condition statements
or an ANDed set of ORed sets of statements. Individual condition
statements can also be negated. These combinations are termed,
respectively, Disjunctive Normal Form (DNF) and Conjunctive Normal
Form (CNF) for the conditions.
If the set of conditions associated with a policy rule evaluates to
TRUE, then a set of actions that either maintain the current state of
the object or transition the object to a new state may be executed.
For the set of actions associated with a policy rule, it is possible
to specify an order of execution, as well as an indication of whether
the order is required or merely recommended. It is also possible to
indicate that the order in which the actions are executed does not
matter.
Policy rules themselves can be prioritized. One common reason for
doing this is to express an overall policy that has a general case
with a few specific exceptions.
For example, a general QoS policy rule might specify that traffic
originating from members of the engineering group is to get Bronze
Service. A second policy rule might express an exception: traffic
originating from John, a specific member of the engineering group, is
to get Gold Service. Since traffic originating from John satisfies
the conditions of both policy rules, and since the actions associated
with the two rules are incompatible, a priority needs to be
established. By giving the second rule (the exception) a higher
priority than the first rule (the general case), a policy
administrator can get the desired effect: traffic originating from
John gets Gold Service, and traffic originating from all the other
members of the engineering group gets Bronze Service.
Policies can either be used in a stand-alone fashion or aggregated
into policy groups to perform more elaborate functions. Stand-alone
policies are called policy rules. Policy groups are aggregations of
policy rules, or aggregations of policy groups, but not both. Policy
groups can model intricate interactions between objects that have
complex interdependencies. Examples of this include a sophisticated
user logon policy that sets up application access, security, and
reconfigures network connections based on a combination of user
identity, network location, logon method and time of day. A policy
group represents a unit of reusability and manageability in that its
management is handled by an identifiable group of administrators and
its policy rules would be consistently applied
Stand-alone policies are those that can be expressed in a simple
statement. They can be represented effectively in schemata or MIBs.
Examples of this are VLAN assignments, simple YES/NO QoS requests,
and IP address allocations. A specific design goal of this model is
to support both stand-alone and aggregated policies.
Policy groups and rules can be classified by their purpose and
intent. This classification is useful in querying or grouping policy
rules. It indicates whether the policy is used to motivate when or
how an action occurs, or to characterize services (that can then be
used, for example, to bind clients to network services). Describing
each of these concepts in more detail,
o Motivational Policies are solely targeted at whether or how a
policy"s goal is accomplished. Configuration and Usage Policies
are specific kinds of Motivational Policies. Another example is
the scheduling of file backup based on disk write activity from
8am to 3pm, M-F.
o Configuration Policies define the default (or generic) setup of a
managed entity (for example, a network service). Examples of
Configuration Policies are the setup of a network forwarding
service or a network-hosted print queue.
o Installation Policies define what can and cannot be put on a
system or component, as well as the configuration of the
mechanisms that perform the install. Installation policies
typically represent specific administrative permissions, and can
also represent dependencies between different components (e.g., to
complete the installation of component A, components B and C must
be previously successfully installed or uninstalled).
o Error and Event Policies. For example, if a device fails between
8am and 9pm, call the system administrator, otherwise call the
Help Desk.
o Usage Policies control the selection and configuration of entities
based on specific "usage" data. Configuration Policies can be
modified or simply re-applied by Usage Policies. Examples of
Usage Policies include upgrading network forwarding services after
a user is verified to be a member of a "gold" service group, or
reconfiguring a printer to be able to handle the next job in its
queue.
o Security Policies deal with verifying that the client is actually
who the client purports to be, permitting or denying access to
resources, selecting and applying appropriate authentication
mechanisms, and performing accounting and auditing of resources.
o Service Policies characterize network and other services (not use
them). For example, all wide-area backbone interfaces shall use a
specific type of queuing.
Service policies describe services available in the network.
Usage policies describe the particular binding of a client of the
network to services available in the network.
These categories are represented in the Policy Core Information Model
by special values defined for the PolicyKeywords property of the
abstract class Policy.
2.1. Policy Scope
Policies represent business goals and objectives. A translation must
be made between these goals and objectives and their realization in
the network. An example of this could be a Service Level Agreement
(SLA), and its objectives and metrics (Service Level Objectives, or
SLOs), that are used to specify services that the network will
provide for a given client. The SLA will usually be written in
high-level business terminology. SLOs address more specific metrics
in support of the SLA. These high-level descriptions of network
services and metrics must be translated into lower-level, but also
vendor-and device-independent specifications. The Policy Core
Information Model classes are intended to serve as the foundation for
these lower-level, vendor- and device-independent specifications.
It is envisioned that the definition of the Policy Core Informational
Model in this document is generic in nature and is applicable to
Quality of Service (QoS), to non-QoS networking applications (e.g.,
DHCP and IPSec), and to non-networking applications (e.g., backup
policies, auditing access, etc.).
2.2. Declarative versus Procedural Model
The design of the Policy Core Information Model is influenced by a
declarative, not procedural, approach. More formally, a declarative
language is used to describe relational and functional languages.
Declarative languages describe relationships between variables in
terms of functions or inference rules, to which the interpreter or
compiler can apply a fixed algorithm in order to produce a result.
An imperative (or procedural) language specifies an explicit sequence
of steps to follow in order to produce a result.
It is important to note that this information model does not rule out
the use of procedural languages. Rather, it recognizes that both
declarative as well as procedural languages can be used to implement
policy. This information model is better viewed as being declarative
because the sequence of steps for doing the processing of declarative
statements tends to be left to the implementer. However, we have
provided the option of expressing the desired order of action
execution in this policy information model, and for expressing
whether the order is mandatory or not. In addition, rather than
trying to define algorithms or sets of instructions or steps that
must be followed by a policy rule, we instead define a set of modular
building blocks and relationships that can be used in a declarative
or procedural fashion to define policies.
Compare this to a strictly procedural model. Taking such an approach
would require that we specify the condition testing sequence, and the
action execution sequence, in the policy repository itself. This
would, indeed, constrain the implementer. This is why the policy
model is characterized as a declarative one. That is, the
information model defines a set of attributes, and a set of entities
that contain these attributes. However, it does NOT define either
the algorithm to produce a result using the attributes or an explicit
sequence of steps to produce a result.
There are several design considerations and trade-offs to make in
this respect.
1. On the one hand, we would like a policy definition language to be
reasonably human-friendly for ease of definitions and diagnostics.
On the other hand, given the diversity of devices (in terms of
their processing capabilities) which could act as policy decision
points, we would like to keep the language somewhat machine-
friendly. That is, it should be relatively simple to automate the
parsing and processing of the language in network elements. The
approach taken is to provide a set of classes and attributes that
can be combined in either a declarative or procedural approach to
express policies that manage network elements and services. The
key point is to avoid trying to standardize rules or sets of steps
to be followed in defining a policy. These must be left up to an
implementation. Interoperability is achieved by standardizing the
building blocks that are used to represent policy data and
information.
2. An important decision to make is the semantic style of the
representation of the information.
The declarative approach that we are describing falls short of
being a "true" declarative model. Such a model would also specify
the algorithms used to combine the information and policy rules to
achieve particular behavior. We avoid specifying algorithms for
the same reason that we avoid specifying sets of steps to be
followed in a policy rule. However, the design of the information
model more closely follows that of a declarative language, and may
be easier to understand if such a conceptual model is used. This
leads to our third point, acknowledging a lack of "completeness"
and instead relying on presenting information that the policy
processing entity will work with.
3. It is important to control the complexity of the specification,
trading off richness of expression of data in the core information
model for ease of implementation and use. It is important to
acknowledge the collective lack of experience in the field
regarding policies to control and manage network services and
hence avoid the temptation of aiming for "completeness". We
should instead strive to facilitate definition of a set of common
policies that customers require today (e.g., VPN and QoS) and
allow migration paths towards supporting complex policies as
customer needs and our understanding of these policies evolve with
experience. Specifically, in the context of the declarative style
language discussed above, it is important to avoid having full
blown predicate calculus as the language, as it would render many
important problems such as consistency checking and policy
decision point algorithms intractable. It is useful to consider a
reasonably constrained language from these perspectives.
The Policy Core Information Model strikes a balance between
complexity and lack of power by using the well understood logical
concepts of Disjunctive Normal Form and Conjunctive Normal Form for
combining simple policy conditions into more complex ones.
3. Overview of the Policy Core Information Model
The following diagram provides an overview of the five central
classes comprising the Policy Core Information Model, their
associations to each other, and their associations to other classes
in the overall CIM model. Note that the abstract class Policy and
the two extension classes VendorPolicyCondition and
VendorPolicyAction are not shown.
NOTE: For cardinalities, "*" is an abbreviation for "0..n".
+-----------+
System
..... +--^-----^--+ .....
. . 1. 1. . .
*.(a).* .(b) .(c) *.(d).*
+--v---v---------+ . . +-v---v------------+
PolicyGroup <........ . PolicyRepository
w * .
+------^---------+ . +-----^---------^--+
*. . 0..1 . 0..1 .
.(e) . .(f) .(g)
*. . . .
+------v------+ w * . . .
<................. . .
PolicyRule . .
. .
. .
<........................ . .
* (h) . . .
. . .
. . .
. . .
. . .
. . .
. . .
.* .* .
+---------v-------v--+ .
PolicyCondition .
*+--------------------+ .
(i) ^ .
<.............. I .
* . I .
.* ^ .
+----v----------------------+ .
PolicyTimePeriodCondition .
+---------------------------+ .
(j) .
<......................... .
* . .
.* .
+----------v---------+* .
PolicyAction <.......
+-------------+ +--------------------+
Figure 1. Overview of the Core Policy Classes and Relationships
In this figure the boxes represent the classes, and the dotted arrows
represent the associations. The following associations appear:
(a) PolicyGroupInPolicyGroup
(b) PolicyGroupInSystem
(c) PolicyRuleInSystem
(d) PolicyRepositoryInPolicyRepository
(e) PolicyRuleInPolicyGroup
(f) PolicyConditionInPolicyRepository
(g) PolicyActionInPolicyRepository
(h) PolicyConditionInPolicyRule
(i) PolicyRuleValidityPeriod
(j) PolicyActionInPolicyRule
An association always connects two classes. The "two" classes may,
however, be the same class, as is the case with the
PolicyGroupInPolicyGroup association, which represents the recursive
containment of PolicyGroups in other PolicyGroups. The
PolicyRepositoryInPolicyRepository association is recursive in the
same way.
An association includes cardinalities for each of the related
classes. These cardinalities indicate how many instances of each
class may be related to an instance of the other class. For example,
the PolicyRuleInPolicyGroup association has the cardinality range "*"
(that is, "0..n") for both the PolicyGroup and PolicyRule classes.
These ranges are interpreted as follows:
o The "*" written next to PolicyGroup indicates that a PolicyRule
may be related to no PolicyGroups, to one PolicyGroup, or to more
than one PolicyGroup via the PolicyRuleInPolicyGroup association.
In other words, a PolicyRule may be contained in no PolicyGroups,
in one PolicyGroups, or in more than one PolicyGroup.
o The "*" written next to PolicyRule indicates that a PolicyGroup
may be related to no PolicyRules, to one PolicyRule, or to more
than one PolicyRule via the PolicyRuleInPolicyGroup association.
In other words, a PolicyGroup may contain no PolicyRules, one
PolicyRule, or more than one PolicyRule.
The "w" written next to the PolicyGroupInSystem and
PolicyRuleInSystem indicates that these are what CIM terms
"aggregations with weak references", or more briefly, "weak
aggregations". A weak aggregation is simply an indication of a
naming scope. Thus these two aggregations indicate that an instance
of a PolicyGroup or PolicyRule is named within the scope of a System
object. A weak aggregation implicitly has the cardinality 1..1 at
the end opposite the "w".
The associations shown in Figure 1 are discussed in more detail in
Section 7.
4. Inheritance Hierarchies for the Policy Core Information Model
The following diagram illustrates the inheritance hierarchy for the
core policy classes:
ManagedElement (abstract)

+--Policy (abstract)

+---PolicyGroup

+---PolicyRule

+---PolicyCondition (abstract)

+---PolicyTimePeriodCondition

+---VendorPolicyCondition

+---PolicyAction (abstract)

+---VendorPolicyAction

+--ManagedSystemElement (abstract)

+--LogicalElement (abstract)

+--System (abstract)

+--AdminDomain (abstract)

+---PolicyRepository
Figure 2. Inheritance Hierarchy for the Core Policy Classes
ManagedElement, ManagedSystemElement, LogicalElement, System, and
AdminDomain are defined in the CIM schema [1]. These classes are not
discussed in detail in this document.
In CIM, associations are also modeled as classes. For the Policy
Core Information Model, the inheritance hierarchy for the
associations is as follows:
[unrooted]

+---PolicyComponent (abstract)

+---PolicyGroupInPolicyGroup

+---PolicyRuleInPolicyGroup

+---PolicyConditionInPolicyRule

+---PolicyRuleValidityPeriod

+---PolicyActionInPolicyRule

+---Dependency (abstract)

+---PolicyInSystem (abstract)

+---PolicyGroupInSystem

+---PolicyRuleInSystem

+---PolicyConditionInPolicyRepository

+---PolicyActionInPolicyRepository

+---Component (abstract)

+---SystemComponent

+---PolicyRepositoryInPolicyRepository
Figure 3. Inheritance Hierarchy for the Core Policy Associations
The Dependency, Component, and SystemComponent associations are
defined in the CIM schema [1], and are not discussed further in this
document.
4.1. Implications of CIM Inheritance
From the CIM schema, both properties and associations are inherited
to the Policy classes. For example, the class ManagedElement is
referenced in the associations Dependency, Statistics and
MemberOfCollection. And, the Dependency association is in turn
referenced in the DependencyContext association. At this very
abstract and high level in the inheritance hierarchy, the number of
these associations is very small and their semantics are quite
general.
Many of these inherited associations convey additional semantics that
are not needed in understanding the Policy Core Information Model.
In fact, they are defined as OPTIONAL in the CIM Schema - since their
cardinality is "0..n" on all references. The PCIM document
specifically discusses what is necessary to support and instantiate.
For example, through subclassing of the Dependency association, the
exact Dependency semantics in PCIM are described.
So, one may wonder what to do with these other inherited
associations. The answer is "ignore them unless you need them". You
would need them to describe additional information and semantics for
policy data. For example, it may be necessary to capture statistical
data for a PolicyRule (either for the rule in a repository or for
when it is executing in a policy system). Some examples of
statistical data for a rule are the number of times it was
downloaded, the number of times its conditions were evaluated, and
the number of times its actions were executed. (These types of data
would be described in a subclass of CIM_StatisticalInformation.) In
these cases, the Statistics association inherited from ManagedElement
to PolicyRule may be used to describe the tie between an instance of
a PolicyRule and the set of statistics for it.
5. Details of the Model
The following subsections discuss several specific issues related to
the Policy Core Information Model.
5.1. Reusable versus Rule-Specific Conditions and Actions
Policy conditions and policy actions can be partitioned into two
groups: ones associated with a single policy rule, and ones that are
reusable, in the sense that they may be associated with more than one
policy rule. Conditions and actions in the first group are termed
"rule-specific" conditions and actions; those in the second group are
characterized as "reusable".
It is important to understand that the difference between a rule-
specific condition or action and a reusable one is based on the
intent of the policy administrator for the condition or action,
rather than on the current associations in which the condition or
action participates. Thus a reusable condition or action (that is,
one that a policy administrator has created to be reusable) may at
some point in time be associated with exactly one policy rule,
without thereby becoming rule-specific.
There is no inherent difference between a rule-specific condition or
action and a reusable one. There are, however, differences in how
they are treated in a policy repository. For example, it"s natural
to make the access permissions for a rule-specific condition or
action identical to those for the rule itself. It"s also natural for
a rule-specific condition or action to be removed from the policy
repository at the same time the rule is. With reusable conditions
and actions, on the other hand, access permissions and existence
criteria must be expressible without reference to a policy rule.
The preceding paragraph does not contain an exhaustive list of the
ways in which reusable and rule-specific conditions should be treated
differently. Its purpose is merely to justify making a semantic
distinction between rule-specific and reusable, and then reflecting
this distinction in the policy model itself.
An issue is highlighted by reusable and rule-specific policy
conditions and policy actions: the lack of a programmatic capability
for expressing complex constraints involving multiple associations.
Taking PolicyCondition as an example, there are two aggregations to
look at. PolicyConditionInPolicyRule has the cardinality * at both
ends, and PolicyConditionInPolicyRepository has the cardinality * at
the PolicyCondition end, and [0..1] at the PolicyRepository end.
Globally, these cardinalities are correct. However, there"s more to
the story, which only becomes clear if we examine the cardinalities
separately for the two cases of a rule-specific PolicyCondition and a
reusable one.
For a rule-specific PolicyCondition, the cardinality of
PolicyConditionInPolicyRule at the PolicyRule end is [1..1], rather
than [0..n] (recall that * is an abbreviation for [0..n]), since the
condition is unique to one policy rule. And the cardinality of
PolicyConditionInPolicyRepository at the PolicyRepository end is
[0..0], since the condition is not in the "re-usable" repository.
This is OK, since these are both subsets of the specified
cardinalities.
For a reusable PolicyCondition, however, the cardinality of
PolicyConditionInPolicyRepository at the PolicyRepository end is
[1..1], since the condition must be in the repository. And, the
cardinality of PolicyConditionInPolicyRule at the PolicyRule end is
[0..n]. This last point is important: a reusable PolicyCondition
may be associated with 0, 1, or more than 1 PolicyRules, via exactly
the same association PolicyConditionInPolicyRule that binds a rule-
specific condition to its PolicyRule.
Currently the only way to document constraints of this type is
textually. More formal methods for documenting complex constraints
are needed.
5.2. Roles
5.2.1. Roles and Role Combinations
The concept of role is central to the design of the entire Policy
Framework. The idea behind roles is a simple one. Rather than
configuring, and then later having to update the configuration of,
hundreds or thousands (or more) of resources in a network, a policy
administrator assigns each resource to one or more roles, and then
specifies the policies for each of these roles. The Policy Framework
is then responsible for configuring each of the resources associated
with a role in such a way that it behaves according to the policies
specified for that role. When network behavior must be changed, the
policy administrator can perform a single update to the policy for a
role, and the Policy Framework will ensure that the necessary
configuration updates are performed on all the resources playing that
role.
A more formal definition of a role is as follows:
A role is a type of attribute that is used to select one or more
policies for a set of entities and/or components from among a much
larger set of available policies.
Roles can be combined together. Here is a formal definition of a
"role- combination":
A role-combination is a set of attributes that are used to select
one or more policies for a set of entities and/or components from
among a much larger set of available policies. As the examples
below illustrate, the selection process for a role combination
chooses policies associated with the combination itself, policies
associated with each of its sub-combinations, and policies
associated with each of the individual roles in the role-
combination.
It is important to note that a role is more than an attribute. A
role defines a particular function of an entity or component that can
be used to identify particular behavior associated with that entity
or component. This difference is critical, and is most easily
understood by thinking of a role as a selector. When used in this
manner, one role (or role-combination) selects a different set of
policies than a different role (or role-combination) does.
Roles and role-combinations are especially useful in selecting which
policies are applicable to a particular set of entities or components
when the policy repository can store thousands or hundreds of
thousands of policies. This use emphasizes the ability of the role
(or role- combination) to select the small subset of policies that
are applicable from a huge set of policies that are available.
An example will illustrate how role-combinations actually work.
Suppose an installation has three roles defined for interfaces:
"Ethernet", "Campus", and "WAN". In the Policy Repository, some
policy rules could be associated with the role "Ethernet"; these
rules would apply to all Ethernet interfaces, regardless of whether
they were on the campus side or the WAN side. Other rules could be
associated with the role-combination "Campus"+"Ethernet"; these rules
would apply to the campus-side Ethernet interfaces, but not to those
on the WAN side. Finally, a third set of rules could be associated
with the role-combination "Ethernet"+"WAN"; these rules would apply
to the WAN-side Ethernet interfaces, but not to those on the campus
side. (The roles in a role-combination appear in alphabetical order
in these examples, because that is how they appear in the information
model.)
If we have a specific interface A that"s associated with the role-
combination "Ethernet"+"WAN", we see that it should have three
categories of policy rules applied to it: those for the "Ethernet"
role, those for the "WAN" role, and those for the role-combination
"Ethernet"+"WAN". Going one step further, if interface B is
associated with the role- combination "branch-
Office"+"Ethernet"+"WAN", then B should have seven categories of
policy rules applied to it - those associated with the following
role-combinations:
o "branch-office"
o "Ethernet"
o "WAN"
o "branch-office"+"Ethernet"
o "branch-office"+"WAN"
o "Ethernet"+"WAN"
o "branch-office"+"Ethernet"+"WAN".
In order to get all of the right policy rules for a resource like
interface B, a PDP must expand the single role-combination it
receives for B into this list of seven role-combinations, and then
retrieve from the Policy Repository the corresponding seven sets of
policy rules. Of course this example is unusually complicated: the
normal case will involve expanding a two-role combination into three
values identifying three sets of policy rules.
Role-combinations also help to simplify somewhat the problem of
identifying conflicts between policy rules. With role-combinations,
it is possible for a policy administrator to specify one set of
policy rules for campus-side Ethernet interfaces, and a second set of
policy rules for WAN-side Ethernet interfaces, without having to
worry about conflicts between the two sets of rules. The policy
administrator simply "turns off" conflict detection for these two
sets of rules, by telling the policy management system that the roles
"Campus" and "WAN" are incompatible with each other. This indicates
that the role combination will never occur, and therefore conflicts
will never occur. In some cases the technology itself might identify
incompatible roles: "Ethernet" and "FrameRelay", for example. But
for less precise terms like "Campus" and "WAN", the policy
administrator must say whether they identify incompatible roles.
When the policy administrator does this, there are three effects:
1. If an interface has assigned to it a role-combination involving
both "Campus" and "WAN", then the policy management system can
flag it as an error.
2. If a policy rule is associated with a role-combination involving
both "Campus" and "WAN", then the policy management system can
flag it as an error.
3. If the policy management system sees two policy rules, where one
is tied to the role "Campus" (or to a role-combination that
includes the role "Campus") and the other is tied to the role
"WAN" (or to a role- combination that includes the role "WAN"),
then the system does not need to look for conflicts between the
two policy rules: because of the incompatible roles, the two
rules cannot possibly conflict.
+-------------------+
Policy Repository
+-------------------+
V
V retrieval of policy
V
+---------+
PDP/PEP
+---------+
v
v application of policy
v
+----------------+
Network Entity
+----------------+
Figure 4. Retrieval and Application of a Policy
Figure 4, which is introduced only as an example of how the Policy
Framework might be implemented by a collection of network
components, illustrates how roles operate within the Policy
Framework. Because the distinction between them is not important
to this discussion, the PDP and the PEP are combined in one box.
The points illustrated here apply equally well, though, to an
environment where the PDP and the PEP are implemented separately.
A role represents a functional characteristic or capability of a
resource to which policies are applied. Examples of roles include
Backbone interface, Frame Relay interface, BGP-capable router, web
server, firewall, etc. The multiple roles assigned to a single
resource are combined to form that resource"s role combination.
Role combinations are represented in the PCIM by values of the
PolicyRoles property in the PolicyRule class. A PDP uses policy
roles as follows to identify the policies it needs to be aware of:
1. The PDP learns in some way the list of roles that its PEPs
play. This information might be configured at the PDP, the
PEPs might supply it to the PDP, or the PDP might retrieve it
from a repository.
2. Using repository-specific means, the PDP determines where to
look for policy rules that might apply to it.
3. Using the roles and role-combinations it received from its PEPs
as indicated in the examples above, the PDP is able to locate
and retrieve the policy rules that are relevant to it.
5.2.2. The PolicyRoles Property
As indicated earlier, PolicyRoles is a property associated with a
policy rule. It is an array holding "role combinations" for the
policy rule, and correlates with the roles defined for a network
resource. Using the PolicyRoles property, it is possible to mark a
policy rule as applying, for example, to a Frame Relay interface or
to a backbone ATM interface. The PolicyRoles property take strings
of the form:
<RoleName>[&&<RoleName>]*
Each value of this property represents a role combination, including
the special case of a "combination" containing only one role. As the
format indicates, the role names in a role combination are ANDed
together to form a single selector. The multiple values of the
PolicyRoles property are logically ORed, to make it possible for a
policy rule to have multiple selectors.
The individual role names in a role combination must appear in
alphabetical order (according to the collating sequence for UCS-2
characters), to make the string matches work correctly. The role
names used in an environment are specified by the policy
administrator.
5.3. Local Time and UTC Time in PolicyTimePeriodConditions
An instance of PolicyTimePeriodCondition has up to five properties
that represent times: TimePeriod, MonthOfYearMask, DayOfMonthMask,
DayOfWeekMask, and TimeOfDayMask. All of the time-related properties
in an instance of PolicyTimePeriodCondition represent one of two
types of times: local time at the place where a policy rule is
applied, or UTC time. The property LocalOrUtcTime indicates which
time representation applies to an instance of
PolicyTimePeriodCondition.
Since the PCIM provides only for local time and UTC time, a Policy
Management Tool that provides for other time representations (for
example, a fixed time at a particular location) will need to map from
these other representations to either local time or UTC time. An
example will illustrate the nature of this mapping.
Suppose a policy rule is tied to the hours of operation for a Help
Desk: 0800 to 2000 Monday through Friday [US] Eastern Time. In
order to express these times in PolicyTimePeriodCondition, a
management tool must convert them to UTC times. (They are not local
times, because they refer to a single time interval worldwide, not to
intervals tied to the local clocks at the locations where the
PolicyRule is being applied.) As reference [10] points out, mapping
from [US] Eastern Time to UTC time is not simply a matter of applying
an offset: the offset between [US] Eastern Time and UTC time
switches between -0500 and -0400 depending on whether Daylight
Savings Time is in effect in the US.
Suppose the policy administrator"s goal is to have a policy rule be
valid from 0800 until 1200 [US] Eastern Time on every Monday, within
the overall time period from the beginning of 2000 until the end of
2001. The Policy Management Tool could either be configured with the
definition of what [US] Eastern Time means, or it could be configured
with knowledge of where to go to get this information. Reference
[10] contains further discussion of time zone definitions and where
they might reside.
Armed with knowledge about [US] Eastern Time, the Policy Management
Tool would create however many instances of PolicyTimePeriodCondition
it needed to represent the desired intervals. Note that while there
is an increased number of PolicyTimePeriodCondition instances, there
is still just one PolicyRule, which is tied to all the
PolicyTimePeriodCondition instances via the aggregation
PolicyRuleValidityPeriod. Here are the first two of these instances:
1. TimePeriod: 20000101T050000/20000402T070000
DayOfWeekMask: { Monday }
TimeOfDayMask: T130000/T170000
LocalOrUtcTime: UTC
2. TimePeriod: 20000402T070000/20001029T070000
DayOfWeekMask: { Monday }
TimeOfDayMask: T120000/T160000
LocalOrUtcTime: UTC
There would be three more similar instances, for winter 2000-2001,
summer 2001, and winter 2001 up through December 31.
Had the example been chosen differently, there could have been even
more instances of PolicyTimePeriodCondition. If, for example, the
time interval had been from 0800 - 2200 [US] Eastern Time on Mondays,
instance 1 above would have split into two instances: one with a UTC
time interval of T130000/T240000 on Mondays, and another with a UTC
time interval of T000000/T030000 on Tuesdays. So the end result
would have been ten instances of PolicyTimePeriodCondition, not five.
By restricting PolicyTimePeriodCondition to local time and UTC time,
the PCIM places the difficult and expensive task of mapping from
"human" time representations to machine-friendly ones in the Policy
Management Tool. Another approach would have been to place in
PolicyTimePeriodCondition a means of representing a named time zone,
such as [US] Eastern Time. This, however, would have passed the
difficult mapping responsibility down to the PDPs and PEPs. It is
better to have a mapping such as the one described above done once in
a Policy Management Tool, rather than having it done over and over in
each of the PDPs (and possibly PEPs) that need to apply a PolicyRule.
5.4. CIM Data Types
Since PCIM extends the CIM Schema, a correspondence between data
types used in both CIM and PCIM is needed. The following CIM data
types are used in the class definitions that follow in Sections 6 and
7:
o uint8 unsigned 8-bit integer
o uint16 unsigned 16-bit integer
o boolean Boolean
o string UCS-2 string.
Strings in CIM are stored as UCS-2 characters, where each character
is encoded in two octets. Thus string values may need to be
converted when moving between a CIM environment and one that uses a
different string encoding. For example, in an LDAP-accessible
directory, attributes of type DirectoryString are stored in UTF-8
format. RFC2279 [7] explains how to convert between these two
formats.
When it is applied to a CIM string, a MaxLen value refers to the
maximum number of characters in the string, rather than to the
maximum number of octets.
In addition to the CIM data types listed above, the association
classes in Section 7 use the following type:
o <classname> ref strongly typed reference.
There is one obvious omission from this list of CIM data types:
octet strings. This is because CIM treats octet strings as a derived
data type. There are two forms of octet strings in CIM - an ordered
uint8 array for single-valued strings, and a string array for multi-
valued properties. Both are described by adding an "OctetString"
qualifier (meta-data) to the property. This qualifier functions
exactly like an SMIv2 (SNMP) Textual Convention, refining the syntax
and semantics of the existing CIM data type.
The first four numeric elements of both of the "OctetString"
representations are a length field. (The reason that the "numeric"
adjective is added to the previous sentence is that the string
property also includes "0" and "x", as its first characters.) In
both cases, these 4 numeric elements (octets) are included in
calculating the length. For example, a single-valued octet string
property having the value X"7C" would be represented by the uint8
array, X"00 00 00 05 7C".
The strings representing the individual values of a multi-valued
property qualified with the "OctetString" qualifier are constructed
similarly:
1. Take a value to be encoded as an octet string (we"ll use X"7C" as
above), and prepend to it a four-octet length. The result is the
same, X"00 00 00 05 7C".
2. Convert this to a character string by introducing "0" and "x" at
the front, and removing all white space. Thus we have the 12-
character string "0x000000057C". This string is the value of one
of the array elements in the CIM string array. Since CIM uses the
UCS-2 character set, it will require 24 octets to encode this 12-
character string.
Mappings of the PCIM to particular data models are not required to
follow this CIM technique of representing multi-valued octet strings
as length- prefixed character strings. In an LDAP mapping, for
example, it would be much more natural to simply use the Octet String
syntax, and omit the prepended length octets.
5.5. Comparison between CIM and LDAP Class Specifications
There are a number of differences between CIM and LDAP class
specifications. The ones that are relevant to the abbreviated class
specifications in this document are listed below. These items are
included here to help introduce the IETF community, which is already
familiar with LDAP, to CIM modeling, and by extension, to information
modeling in general.
o Instead of LDAP"s three class types (abstract, auxiliary,
structural), CIM has only two: abstract and instantiable. The
type of a CIM class is indicated by the Boolean qualifier
ABSTRACT.
o CIM uses the term "property" for what LDAP terms an "attribute".
o CIM uses the array notation "[ ]" to indicate that a property is
multi-valued. CIM defines three types of arrays: bags (contents
are unordered, duplicates allowed), ordered bags (contents are
ordered but duplicates are allowed) and indexed arrays (contents
are ordered and no duplicates are allowed).
o CIM classes and properties are identified by name, not by OID.
o CIM classes use a different naming scheme for native
implementations, than LDAP. The CIM naming scheme is documented
in Appendix A since it is not critical to understanding the
information model, and only applies when communicating with a
native CIM implementation.
o In LDAP, attribute definitions are global, and the same attribute
may appear in multiple classes. In CIM, a property is defined
within the scope of a single class definition. The property may
be inherited into subclasses of the class in which it is defined,
but otherwise it cannot appear in other classes. One side effect
of this difference is that CIM property names tend to be much
shorter than LDAP attribute names, since they are implicitly
scoped by the name of the class in which they are defined.
There is also a notational convention that this document follows, to
improve readability. In CIM, all class and property names are
prefixed with the characters "CIM_". These prefixes have been
omitted throughout this document, with one exception regarding
naming, documented in Appendix A.
For the complete definition of the CIM specification language, see
reference [2].
6. Class Definitions
The following sections contain the definitions of the PCIM classes.
6.1. The Abstract Class "Policy"
The abstract class Policy collects several properties that may be
included in instances of any of the Core Policy classes (or their
subclasses). For convenience, the two properties that Policy
inherits from ManagedElement in the CIM schema are shown here as
well.
The class definition is as follows:
NAME Policy
DESCRIPTION An abstract class with four properties for
describing a policy-related instance.
DERIVED FROM ManagedElement
ABSTRACT TRUE
PROPERTIES CommonName (CN)
PolicyKeywords[ ]
// Caption (inherited)
// Description (inherited)
6.1.1. The Property "CommonName (CN)"
The CN, or CommonName, property corresponds to the X.500 attribute
commonName (cn). In X.500 this property specifies one or more user-
friendly names (typically only one name) by which an object is
commonly known, names that conform to the naming conventions of the
country or culture with which the object is associated. In the CIM
model, however, the CommonName property is single-valued.
NAME CN
DESCRIPTION A user-friendly name of a policy-related object.
SYNTAX string
6.1.2. The Multi-valued Property "PolicyKeywords"
This property provides a set of one or more keywords that a policy
administrator may use to assist in characterizing or categorizing a
policy object. Keywords are of one of two types:
o Keywords defined in this document, or in documents that define
subclasses of the classes defined in this document. These
keywords provide a vendor-independent, installation-independent
way of characterizing policy objects.
o Installation-dependent keywords for characterizing policy objects.
Examples include "Engineering", "Billing", and "Review in December
2000".
This document defines the following keywords: "UNKNOWN",
"CONFIGURATION", "USAGE", "SECURITY", "SERVICE", "MOTIVATIONAL",
"INSTALLATION", and "EVENT". These concepts were defined earlier in
Section 2.
One additional keyword is defined: "POLICY". The role of this
keyword is to identify policy-related instances that would not
otherwise be identifiable as being related to policy. It may be
needed in some repository implementations.
Documents that define subclasses of the Policy Core Information Model
classes SHOULD define additional keywords to characterize instances
of these subclasses. By convention, keywords defined in conjunction
with class definitions are in uppercase. Installation-defined
keywords can be in any case.
The property definition is as follows:
NAME PolicyKeywords
DESCRIPTION A set of keywords for characterizing /categorizing
policy objects.
SYNTAX string
6.1.3. The Property "Caption" (Inherited from ManagedElement)
This property provides a one-line description of a policy-related
object.
NAME Caption
DESCRIPTION A one-line description of this policy-related object.
SYNTAX string
6.1.4. The Property "Description" (Inherited from ManagedElement)
This property provides a longer description than that provided by the
caption property.
NAME Description
DESCRIPTION A long description of this policy-related object.
SYNTAX string
6.2. The Class "PolicyGroup"
This class is a generalized aggregation container. It enables either
PolicyRules or PolicyGroups to be aggregated in a single container.
Loops, including the degenerate case of a PolicyGroup that contains
itself, are not allowed when PolicyGroups contain other PolicyGroups.
PolicyGroups and their nesting capabilities are shown in Figure 5
below. Note that a PolicyGroup can nest other PolicyGroups, and
there is no restriction on the depth of the nesting in sibling
PolicyGroups.
+---------------------------------------------------+
PolicyGroup

+--------------------+ +-----------------+
PolicyGroup A PolicyGroup X

+----------------+ ooo
PolicyGroup A1
+----------------+
+--------------------+ +-----------------+
+---------------------------------------------------+
Figure 5. Overview of the PolicyGroup class
As a simple example, think of the highest level PolicyGroup shown in
Figure 5 above as a logon policy for US employees of a company. This
PolicyGroup may be called USEmployeeLogonPolicy, and may aggregate
several PolicyGroups that provide specialized rules per location.
Hence, PolicyGroup A in Figure 5 above may define logon rules for
employees on the West Coast, while another PolicyGroup might define
logon rules for the Midwest (e.g., PolicyGroup X), and so forth.
Note also that the depth of each PolicyGroup does not need to be the
same. Thus, the WestCoast PolicyGroup might have several additional
layers of PolicyGroups defined for any of several reasons (different
locales, number of subnets, etc..). The PolicyRules are therefore
contained at n levels from the USEmployeeLogonPolicyGroup. Compare
this to the Midwest PolicyGroup (PolicyGroup X), which might directly
contain PolicyRules.
The class definition for PolicyGroup is as follows:
NAME PolicyGroup
DESCRIPTION A container for either a set of related
PolicyRules or a set of related PolicyGroups.
DERIVED FROM Policy
ABSTRACT FALSE
PROPERTIES NONE
No properties are defined for this class since it inherits all its
properties from Policy. The class exists to aggregate PolicyRules or
other PolicyGroups. It is directly instantiable. In an
implementation, various key/identification properties MUST be
defined. The keys for a native CIM implementation are defined in
Appendix A, Section 13.1.1. Keys for an LDAP implementation will be
defined in the LDAP mapping of this information model [11].
6.3. The Class "PolicyRule"
This class represents the "If Condition then Action" semantics
associated with a policy. A PolicyRule condition, in the most
general sense, is represented as either an ORed set of ANDed
conditions (Disjunctive Normal Form, or DNF) or an ANDed set of ORed
conditions (Conjunctive Normal Form, or CNF). Individual conditions
may either be negated (NOT C) or unnegated (C). The actions
specified by a PolicyRule are to be performed if and only if