Internet Draft Igor Bryskin (Movaz Networks)
Category: Informational Dimitri Papadimitriou (Alcatel)
Expiration Date: January 2007 Lou Berger (LabN Consulting, LLC)
July 2006
Policy-Enabled Path Computation Framework
draft-ietf-pce-policy-enabled-path-comp-00.txt
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Abstract
The PCE architecture [PCE-ARCH] introduces the concept of policy in
the context of path computation. This document provides additional
details on policy within the PCE Architecture and also provides
context for the support of PCE Policy. This document introduces the
use of the Policy Core Information Model (PCIM) as a framework for
supporting path computation policy. This document also provides
representative scenarios for the support of PCE Policy.
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Contents
1 Terminology ............................................... 3
2 Introduction .............................................. 3
3 Background ................................................ 4
3.1 Motivations ............................................... 4
3.2 Representative Policy Scenarios ........................... 6
3.2.1 Scenario: Policy Configured Paths ......................... 6
3.2.2 Scenario: Provider Selection Policy ....................... 7
3.2.3 Scenario: Policy Based Constraints ........................ 8
4 Requirements .............................................. 10
5 Path Computation Policy Information Model (PCPIM) ......... 11
6 Policy-Enabled Path Computation Framework Components ...... 13
7 Policy Component Configurations ........................... 15
7.1 PCC-PCE Configurations .................................... 15
7.2 Policy Repositories ....................................... 17
7.3 Cooperating PCE Configurations ............................ 18
7.4 Policy Configuration Management ........................... 20
8 Inter-Component Communication ............................. 20
8.1 Policy Communication ..................................... 20
8.2 PCE Discovery Policy Considerations ....................... 22
9 Path Computation Sequence of Events ....................... 22
9.1 Policy-enabled PCC, Policy-enabled PCE .................... 23
9.2 Policy-ignorant PCC, Policy-enabled PCE ................... 24
10 Introduction of New Constraints ........................... 25
11 Security Considerations ................................... 26
12 Acknowledgements .......................................... 27
13 IANA Considerations ....................................... 27
14 References ................................................ 27
14.1 Normative References ...................................... 27
14.2 Informative References .................................... 28
15 Authors' Addresses ........................................ 28
16 Full Copyright Statement .................................. 29
17 Intellectual Property ..................................... 29
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Conventions used in this document
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].
1. Terminology
The reader is assumed to be familiar with the following terms:
CSPF: Constraint-based Shortest Path First, see [RFC3630].
LSP: Label Switched Path, see [RFC3031].
LSR: Label Switching Router, see [RFC3031].
PCC: Path Computation Client, see [PCE-ARCH].
PCE: Path Computation Element, see [PCE-ARCH].
TE LSP: Traffic Engineering MPLS Label Switched Path, see
[RFC3209] and [RFC3473].
CIM: Common Information Model, see [DMTF].
PCIM: Policy Core Information Model, see [RFC3060].
PC: Path Computation.
PCCIM: Path Computation Core Information Model.
QPIM: QoS Policy Information Model, see [RFC3644].
PBM: Policy-based Management, see [RFC3198].
PEP: Policy Enforcement Points, see [RFC2753].
PDP: Policy Decision Points, see [RFC2753].
COPS: Common Open Policy Service, see [RFC2748].
COPS-PR: COPS Usage for Policy Provisioning, see [RFC3084].
2. Introduction
The PCE architecture is introduced in [PCE-ARCH]. This document
describes the impact of policy on the PCE architecture and provides
additional details on, and context for, policy within the PCE
Architecture.
Policy-based Management (PBM) enables network administrators to
operate in a high-level manner through rule-based policies; the
latter are translated automatically into individual device
configuration directives, aimed at controlling a network as a whole.
Two IETF Working Groups have considered policy networking in the
past: The Resource Allocation Protocol working group and the Policy
Framework working group.
A framework for policy-based admission control [RFC2753] was defined
and a protocol for use between Policy Enforcement Points (PEP) and
Policy Decision Points (PDP) was specified: Common Open Policy
Service (COPS) [RFC2748]. This document uses the terms PEP and PDP to
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refer to the functions defined in the COPS context. This document
makes no assumptions nor requires that the actual COPS protocol be
used.
The IETF has also produced a general framework for representing,
managing, sharing, and reusing policies in a vendor independent,
interoperable, and scalable manner. It has also defined an extensible
information model for representing policies, called the Policy Core
Information Model (PCIM) [RFC3060], and an extension to this model to
address the need for QoS management, called the QoS Policy
Information Model (QPIM) [RFC3644]. However, additional mechanisms
are needed in order to specify policies related to the path
computation logic as well as its control.
This document introduces PCIM as a core component in a framework for
providing policy-enabled path computation. This document also covers
the other aspects of the framework. It also discusses related
background to provide a context for this work. Specific PCIM
definition to support path computation will be discussed in a
separate document.
3. Background
This section provides some general background on the use of policy
within the PCE architecture. It presents some rational behind the use
of policy, as well as representative policy usage scenarios. This
information is intended to provide context for the presented policy
framework. This section does not attempt to present an exhaustive
list of rational or scenarios.
3.1. Motivations
The PCE architecture is introduced in [PCE-ARCH]. It includes policy
as an integral part of the PCE architecture. This section presents
some of the rational for this inclusion.
Network operators require a certain level of flexibility to shape the
TE path computation process, so that the process can be aligned with
their business and operational needs. Many aspects of the path
computation may be governed by policies. For example, a PCC may use
policies configured by the operator to decide which optimizations,
constraints, diversities and their relaxation strategies to request
while computing path(s) for a particular service. Depending on SLAs,
TE and cost/performance ratio goals, path computation requests may be
built differently for different services. Service A, for instance,
may require two SRLG-disjoint paths for building end-to-end recovery
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scheme, while for service B link-disjoint paths may be sufficient.
Service A may need paths with minimal end-to-end delay, while service
B may be looking for shortest (minimal-cost) paths. Different
constraint relaxation strategies may be applied while computing paths
for service A and for service B, and so forth.
Likewise, a PCE may apply policies to decide which algorithm or
algorithms to use while performing path computations requested from a
particular PCC or for a particular domain, see [PCECP-IA-REQ];
whether to seek the cooperation of other PCEs to satisfy a particular
request or to handle a request on its own (possibly responding with
non explicit paths); or how the request should be modified before
being sent to other member(s) of a group of cooperating PCEs, etc.
Additional motivations for supporting policy within the PCE
architecture can be described as follows. Historically, a path
computation entity was an intrinsic part of an LSR's control plane
and always co-located with the LSR's signaling and routing
subsystems. This approach allowed for unlimited flexibility in
providing various path computation enhancements, such as: adding new
types of constraints, diversities and their relaxation strategies,
adopting new objective functions and optimization criterions, etc.
All that had to be done to support an enhancement was to upgrade the
control plane software of a particular LSR (and no other LSRs or any
other network elements).
With the introduction of the PCE architecture, the introduction of
new PCE capabilities becomes more complicated: it isn't enough for a
PCE to upgrade its own software. In order to take advantage of a
PCE's new capabilities, new advertising and signaling objects may
need to be standardized, all PCCs may need to be upgraded with new
software, and new interoperability problems may need to be resolved,
etc.
Within the context of the PCE architecture, it is therefore highly
desirable to find a way to introduce new path computation
capabilities without requiring modifying either the
discovery/communication protocols or PCC software. One way to achieve
this objective is to consider path selection constraints, their
relaxations and objective functions, as path computation request
specific policies. Furthermore that such policies may, on one hand,
be configured and managed by an operator as any other policies or, on
the other hand, may be interpreted in real time by PCCs and PCEs.
There are a number of advantages and useful by-products of such an
approach:
- New path computation capabilities may be introduced without
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changing PCE-PCC communication and discovery protocols or PCC
software. Only the PCE module providing the path computation
capabilities (referred in this document as path computation
engine) needs to be updated.
- Existing constraints, objective functions and their relaxations
may be aggregated and otherwise associated together, thus,
producing new more complex ones that do not require change of
code even on PCEs supporting them.
- Different elements such as conditions, actions, variables,
etc. may be re-used by multiple constraints, diversities, and
optimizations.
- PCCs and PCEs need to handle other (that is, not request
specific) policies. Path computation related policies of all
types can be placed within the same policy repositories, can be
managed by the same policy management tools and can be
interpreted using the same mechanisms. Also policies need to be
supported by PCCs and PCEs independently from peculiarities of a
specific PCC-PCE communication protocol. Thus, introducing a new
(request specific) type of policies describing constraints and
other elements of a path computation request will be a natural
and relatively inexpensive addition to the policy-enabled path
computation architecture.
3.2. Representative Policy Scenarios
This section provides example scenarios of how policy may be applied
using the PCE policy framework within the PCE architecture context.
Actual networks may use one of the scenarios discussed, some
combination of the presented scenarios or other scenarios (not
discussed). This section should not be viewed as limiting other
applications of policy within the PCE architecture.
3.2.1. Scenario: Policy Configured Paths
A very simple usage scenario for PCE policy would be to use PCE to
centrally administer configured paths. Configured paths are composed
of strict and loose hops in the form of EROs (see [RFC3209]), and are
used by one or more LSPs. Typically such paths are configured at the
LSP ingress. In the context of policy-enabled path computation, an
alternate approach is possible.
In particular, service specific policies can be installed that will
provide configured path(s) for a specific service request. The
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request may be identified based on service parameters such as end-
points, requested QoS or even a token that identifies the end-user.
The configured path(s) would then be used as input to PCE computation
process, which would return explicit routes by expanding of all
specified loose hops.
The described policies may be applied at either PCC or PCE, see
Figure 5. In the PCC case, the configured path would be processed at
the PCC and then passed to the PCE along with the PCE request,
probably in the form of (inclusion) constraints. When applied at the
PCE, the configured path would be used locally. Both cases require
some method to configure and manage policies. In the PCC case, the
real benefit would come when there is an automated policy
distribution mechanism.
Policy-configured paths may also be used in multiple PCE environments
(see Figures 7 and 8). For example, consider the case when there is
limited TE visibility and independent PCEs are used to determine
path(s) within each area of the TE visibility. In such a case, it may
not be possible (or desirable) to configure entire explicit path(s)
on a single PCE. However, it is possible to configure explicit
path(s) for each area of the TE visibility and each responsible PCE.
One by one, the PCEs would then map an incoming signaling request to
appropriate configured path(s). Note that to make such scenario work
it would likely be necessary to start and finish the configured paths
on TE domain boundary nodes. Clearly, consistent PC Policy
Repositories are also critical in this example.
3.2.2. Scenario: Provider Selection Policy
A potentially more interesting usage scenario is applying PC policies
in multi-domain multi-provider networks. There are numerous
interesting policy applications in such networks. A rudimentary
example is simple access control, that is, deciding which clients are
permitted to request inter-domain path computation.
A more complicated example is applying policy to determine which
domain or provider network will be used to support a particular PCE
request. Consider the topology presented in Figure 1. In this example
there are multiple transit networks available to provide a path from
a source domain to a destination domain. Furthermore, each transit
network may have one or more options for reaching a particular
domain. Each domain will need to select which of the multiple
available paths will be used to satisfy a particular PCE request.
Clearly, TE reachability, availability and optimality are the basic
criterions for path selection, however, policies can provide an
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important added consideration in the decision process. For example,
transit network A may be more expensive and provide lower delay or
loss than transit network C. Likewise, a transit network may wish to
treat PCE requests from its own customers differently than requests
from other providers. In both cases, computation based on traffic
engineering databases will result in multiple transit networks that
provide reachability, and policies can be used to govern which PCE
requests get better service.
+----------------+
|Transit Domain A| +----------------+
+----------------+ |Transit Domain D|
+------+ +----------------+ +----------------+ +------+
|Source| |Transit Domain B| +----------------+ |Target|
|Domain| +----------------+ |Transit Domain E| |Domain|
+------+ +----------------+ +----------------+ +------+
|Transit Domain C| +----------------+
+----------------+ |Transit Domain F|
+----------------+
Figure 1: Multi-domain network with multiple transit options
There are multiple options for differentiating which PCE requests use
a particular transit domain and get a particular (better or worse)
level of service. For example, a PCE in the source domain may use
user and request specific policies to determine the level of service
to provide. A PCE in the source domain may also use domain specific
policies to choose which transit domains are acceptable. A PCE in a
transit domain may use request specific policies to determine if a
request is from a direct customer or another provider, and then use
domain specific policies to identify how the request should be
processed.
3.2.3. Scenario: Policy Based Constraints
Another usage scenario is the use of policy to provide new
constraints in a PCE request. Consider an LSR with a policy enabled
PCC, as shown in Figure 3, which receives a service request via
signaling, including over a NNI or UNI reference point, or receives a
configuration request over a management interface to establish a
service. In either case the path(s) needed to support the service are
not explicitly specified in the message/request, and hence path
computation is needed.
In this case, the PCC may apply user or service specific policies to
decide how the path selection process should be constrained, that is,
which constraints, diversities, optimizations and relaxations should
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be applied in order for the service LSP(s) to have a likelihood to be
successfully established and provide necessary QoS and resilience
against network failures. When deciding on the set of constraints the
PCC uses as an input all information it knows about the user and
service, such as the contents of the received message, port ID over
which message was received, associated VPN ID, signaling/reference
point type, request time, etc. Once the constraints and other
parameters of the required path computation are determined, the PCC
generates a path computation request which includes the request-
specific policies that describe the determined set of constraints,
optimizations, and other parameters that indicate how the request is
to be considered in the path computation process.
The PCC may also apply server specific policies for each of the known
(i.e., discovered or configured) PCEs in order to select which PCE to
use. The PCC may also use server specific policies to form the
request to match the PCE's capabilities so that the request will not
be rejected and has a higher likelihood of being satisfied in an
efficient way. An example of a request modification as the result of
a server specific policy is removing a constraint not supported by
the PCE. Once the policy processing is completed at the PCC, and the
path computation request resulting from the original service request
is updated by the policy processing, the request is sent to the PCE.
The PCE that receives the request validates and otherwise processes
the request, applying the policies found in the request as well as
any policies that are available at the PCE, e.g., client and domain
specific polices. As a result of the policy processing, the PCE may
decide to reject the request. It also may decide to respond with one
or several pre-computed paths if user or client specific polices
instruct the PCE to do so. If the PCE decides to satisfy the request
by performing a path computation, it determines if it needs the
cooperation of other PCEs and defines parameters for path
computations to be performed locally and remotely. After that, the
PCE instructs a co-located path computation engine to perform the
local path computation(s) and, if necessary, sends path computation
requests to one or more other PCEs. It then waits for the responses
from the local path computation engine and, when used, the remote
PCE. It then combines the resulting paths and sends the result back
to the requesting PCC. The response may indicate policies describing
the resulting paths, their characteristics (summary cost, expected
end-to-end delay, etc.) as well as additional information related to
the request, e.g., which of constraints were honored, which were
dismissed and which were relaxed and in what way.
The PCC processes the response and instructs the LSR to encode the
received path(s) into the outgoing signaling message(s).
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4. Requirements
The following requirements must be addressed by mechanisms and
protocols that enable policy-based control over path computation
requests and decisions:
- (G)MPLS path computation-specific
The mechanisms must meet the policy-based control requirements
specific to the problem of path computation using RSVP-TE as the
signaling protocol on MPLS and GMPLS LSRs.
- Support for non (G)MPLS PCCs
The mechanisms must be sufficiently generic to support
non-(G)MPLS (LSR) clients such as an NMS, or network planner,
etc.
- Support for many policies
The mechanisms must include support for many policies and policy
configurations. In general, the determination and configuration
of viable policies are the responsibility of the service
provider.
- Provision for monitoring and accounting information
The mechanisms must include support for monitoring policy state,
and provide access information. In particular, mechanisms must
provide usage and access information that may be used for
accounting purposes.
- Fault tolerance and recovery
The mechanisms must include provisions for fault tolerance and
recovery from failure cases such as failure of PCC/PCE PDPs,
disruption in communication that separate a PCC/PCE PDP from its
associated PCC/PCE PEPs.
- Support for policy-ignorant nodes
The mechanisms should not be mandatory for every node in a
network. Policy based path computation control may be enforced at
a subset of nodes, for example, on boundary nodes within an
administrative domain. These policy-capable nodes will function
as trusted nodes from the point of view of the policy-ignorant
nodes in that administrative domain. Alternatively, policy may be
applied solely on PCEs with all PCCs being policy-ignorant nodes.
- Scalability
One of the important requirements for the mechanisms is
scalability. The mechanisms must scale at least to the same
extent that RSVP-TE signaling scales in terms of accommodating
multiple LSPs and network nodes in the path of an LSP. There are
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several sensitive areas in terms of scalability of policy based
path computation control. First, not every policy aware node in
an infrastructure should be expected to contact a remote
PDP. This would cause potentially long delays in verifying
requests. Additionally, the policy control architecture must
scale at least as well as RSVP-TE protocol based on factors such
as the size of RSVP-TE messages, the time required for the
network to service an RSVP-TE request, local processing time
required per node, and local memory consumed per node. These
scaling considerations are of particular importance during
re-routing of a set of LSPs.
- Security and denial of service considerations
The policy control architecture, protocols and mechanisms must be
secure as far as the following aspects are concerned:
o First, the mechanisms proposed must minimize theft and denial
of service threats.
o Second, it must be ensured that the entities (such as PEPs
and PDPs) involved in policy control can verify each other's
identity and establish necessary trust before communicating.
- Inter-AS and inter-area requirements
There are several inter-AS policy related requirements discussed
in [RFC4216] and [INTERAS-PCEP], and inter-area policy related
requirements discussed in [PCECP-IA-REQ]. These requirements
must be addressed by policy-enabled PCE mechanisms and protocols.
It should be noted that this document only outlines the communication
elements and mechanisms needed to allow a wide variety of possible
policies to be applied for path computation control. It does not
include any discussion of any specific policy behavior. Nor does it
define or require use of specific policies.
5. Path Computation Policy Information Model (PCPIM)
The Policy Core Information Model (PCIM) introduced in [RFC3060] and
expanded in [RFC3460] presents the object-oriented information model
for representing general policy information.
This model defines two hierarchies of object classes:
- structural classes representing policy information and control of
policies
- association classes that indicate how instances of the structural
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classes are related to each other.
These classes can be mapped to various concrete implementations, for
example, to a directory that uses LDAPv3 as its access protocol.
Figure 2 shows an abstract from the class inheritance hierarchy for
PCIM.
ManagedElement (abstract)
|
+--Policy (abstract)
| |
| +---PolicySet (abstract)
| | |
| | +---PolicyGroup
| | |
| | +---PolicyRule
| |
| +---PolicyCondition (abstract)
| | |
| | +---PolicyTimePeriodCondition
| | |
| | +---VendorPolicyCondition
| | |
| | +---SimplePolicyCondition
| | |
| | +---CompoundPolicyCondition
| | |
| | +---CompoundFilterCondition
| |
| +---PolicyAction (abstract)
| | |
| | +---VendorPolicyAction
| | |
| | +---SimplePolicyAction
| | |
| | +---CompoundPolicyAction
| |
| +---PolicyVariable (abstract)
| | |
| | +---PolicyExplicitVariable
| | |
| | +---PolicyImplicitVariable
| | |
| | +---(subtree of more specific classes)
| |
| +---PolicyValue (abstract)
| |
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| +---(subtree of more specific classes)
|
Figure 2: PCIM Class Inheritance
The policy classes and associations defined in PCIM are sufficiently
generic to allow them to represent policies related to anything.
Policy models for application-specific areas such as the Path
Computation Service may extend the PCIM 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 defined to provide a standard extension
mechanism for vendor-specific extensions to the Policy Core
Information Model.
Policy Quality of Service Information Model [RFC3644] further extends
the PCIM to represent QoS policy information for large-scale policy
domains. New classes introduced in this document describing QoS and
RSVP related variables, conditions and actions can be used as a
foundation for the PCPIM.
Detailed description of the PCPIM will be provided in a separate
documents.
6. Policy-Enabled Path Computation Framework Components
The following components are defined as part of the framework to
support policy-enabled path computation:
- PC Policy Repository
A database from which PC policies are available in the form of
instances of PCPIM classes. PC Policies are configured and
managed by PC Policy Management Tools.
- PCE Policy Decision Point (PCE-PDP)
A logical entity capable of retrieving relevant path computation
policies from one or more Policy Repositories and delivering the
information to associated PCE-PEP(s);
- PCE Policy Enforcement Point (PCE-PEP)
A logical entity capable of issuing device specific Path
Computation Engine configuration requests for the purpose of
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enforcing the policies
- PCC Policy Decision Point (PCC-PDP)
A logical entity capable of retrieving relevant path computation
policies from one or more Policy Repositories and delivering the
information to associated PCC-PEP(s;
- PCC Policy Enforcement Point (PCC-PEP)
A logical entity capable of issuing device specific Path
Computation Service User configuration requests for the purpose
of enforcing the policies.
From the policy perspective a PCC is logically decomposed into two
parts: PCC-PDP and PCC-PEP. When present, a PCC-PEP is co-located
with a Path Computation Service User entity that requires remote path
computation (for example, the GMPLS control plane of an LSR). The
PCC-PEP and PCC-PDP may be physically co-located (as per [RFC2748])
or separated. In the later case they talk to each other via such
protocols as COPS and/or COPS-PR [RFC3084].
Likewise, a PCE is logically decomposed into two parts: PCE-PEP and
PCE-PDP. When present, PCE-PEP is co-located with a Path Computation
Engine entity that actually provides the Path Computation Service
(that is, runs path computation algorithms). PCE-PEP and PCE-PDP may
be physically co-located or separated. In the later case they
communicate using such protocols as COPS and/or COPS-PR.
PCC-PDP/PCE-PDP may be co-located with, or separated from, an
associated PC Policy Repository. In the latter case, the PDPs use
some access protocol (for example, LDAPv3 or SNMP). The task of PDPs
is to retrieve policies from the repository(ies) and convey them to
respective PEPs either in unsolicited way or upon the PEPs requests.
A PCC-PEP may receive policy information not only from PCC-PDPs(s)
but also from PCE-PEP(s) via PCC-PCE communication and/or PCE
discovery protocols. Likewise, a PCE-PEP may receive policy
information not only from PCE-PDPs(s) but also from PCC-PEP(s) via
PCC-PCE communication protocol.
Any given policy can be interpreted (that is, translated into a
sequence of concrete device specific configuration requests) either
on a PDP or on the associated PEP or partly on the PDP and partly on
the PEP.
Generally speaking, the task of the PCC-PEP is to select the PCE and
build path computation requests applying service specific policies
provided by the PCC-PDP. The task of the PCE-PEP is to control path
computations by applying request-specific policies found in the
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requests as well as client-specific and domain-specific policies
supplied by the PCE-PDP.
7. Policy Component Configurations
7.1. PCC-PCE Configurations
The PCE policy architecture supports policy being applied at a PCC
and at a PCE. While the architecture supports policy being applied at
both, there is no requirement for policy to always be applied at
both, or even at either. The use of policy in a network, on PCCs and
on PCEs, is a specific network design choice. Some networks may
choose to apply policy only at PCCs (Figure 3), some at PCEs (Figure
4), and others at both PCCs and PCEs (Figure 5). Regardless of where
policy is applied it must be applied in a consistent fashion in order
to achieve the intended results.
........................
. .
. PC Policy Management .
. .
........................
.
.
--------- Policy ----------------------
| PCC-PDP |<--------- | PC Policy Repository |
--------- ----------------------
^
| e.g., COPS
v
--------- ---------
| PCC-PEP |<------------------------------------------->| PCE |
--------- PCC-PCE Communication Protocol ---------
Figure 3: Policies are applied on PCC only
Along with supporting flexibility in where policy may be applied, the
PCE architecture is also flexible in terms of where specific types of
policies may be applied. Also the PCE architecture allows for the
application of only a subset of policy types. [PCE-ARCH] defines
several PC policy types. Each of these may be applied at either a PCC
or a PCE or both. Clearly when policy is only applied at PCCs or at
PCEs, all PC policy types used in the network must be applied at
those locations.
Bryskin, Papadimitriou & Berger [Page 15]
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........................
. .
. PC Policy Management .
. .
........................
.
.
---------------------- Policy ---------
| PC Policy Repository | -------->| PCE-PDP |
---------------------- ---------
^
e.g., COPS |
v
--------- ---------
| PCC |<------------------------------------------->| PCE-PEP |
--------- PCC-PCE Communication Protocol ---------
Figure 4: Policies are applied on PCE only
In the case where policy is only applied at a PCE, it is expected
that the PCC will pass to the PCE all information about the service
that it can gather in the path computation request (most likely in
the form of PCPIM policy variables). The PCE is expected to
understand this information and apply appropriate policies while
defining the actual parameters of the path computation to be
performed. Note that in this scenario the PCC cannot apply server-
specific or any other policies, and PCE selection is static.
When applying policy at both PCC and PCE, it is necessary to select
which types of policies are applied at each. In such configurations,
it is likely that the application of policy types will be distributed
across PCC and PCE rather than applying all of them at both. For
example, user specific and server specific policies may be applied at
a PCC, request and client specific policies may be applied at a PCE,
while domain specific policies may be applied at both the PCC and
PCE.
In the case when policy is only applied at a PCC, the PCC must apply
all the types of required policies, for example user, service, server
and domain-specific policies. The PCC uses the policies to construct
a path computation request that appropriately represents the applied
policies. The request will necessarily be limited to the set of
"basic" (that is, non-policy capable) constraints explicitly defined
by the PCC-PCE communication protocol in use.
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7.2. Policy Repositories
Within the policy-enabled path computation framework policy
repositories may be used in a single or multiple PC policy repository
configuration:
o) Single PC Policy Repository
In this configuration there is a single PC Policy Repository shared
between PCCs and PCEs.
........................
. .
. PC Policy Management .
. .
........................
.
.
--------- Policy a ---------------------- Policy b ---------
| PCC-PDP |<--------- | PC Policy Repository | -------->| PCE-PDP |
--------- ---------------------- ---------
^ ^
| e.g., COPS e.g., COPS |
v v
--------- ---------
| PCC-PEP |<------------------------------------------->| PCE-PEP |
--------- PCC-PCE Communication Protocol ---------
Figure 5: Single PCC/PCE Policy Repository
o) Multiple PC Policy Repositories
The repositories in this case may be fully or partially synchronized
by some discovery/ synchronization management protocol or may be
completely independent. Note that the situation when PC Policy
Repository A exactly matches PC Policy Repository B, results in the
single PC Policy Repository configuration case.
Bryskin, Papadimitriou & Berger [Page 17]
Internet Draft draft-ietf-pce-policy-enabled-path-comp-00.txt July 2006
-------------- --------------
| PC Policy | | PC Policy |
---| Repository A | | Repository B |---
| -------------- -------------- |
| |
| Policy a Policy b |
| |
v v
--------- ---------
| PCC-PDP | | PCE-PDP |
--------- ---------
^ ^
| e.g., COPS e.g., COPS |
v v
--------- ---------
| PCC-PEP |<------------------------------------------->| PCE-PEP |
--------- PCC-PCE Communication Protocol ---------
Figure 6: Multiple PCE/PCC Policy Repositories
7.3. Cooperating PCE Configurations
The previous section shows the relationship between PCCs and PCEs. A
parallel relationship exists between cooperating PCEs, and, in fact,
this relationship can be viewed as the same as the relationship
between PCCs and PCEs. The one notable difference is that there will
be cases where having a shared PC Policy Repository will not be
desirable, for example, when the PCEs are managed by different
entities. Note that in this case it still remains necessary for the
policies to be consistent across the domains in order to identify
usable paths. The other notable difference is that a PCE, while
processing a path computation request, may need to apply requestor-
specific (that is, client-specific) policies in order to modify the
request before sending it to other cooperating PCE(s). This
relationship is particularly important as the PCE Architecture allows
for configuration where all PCCs are not policy-enabled.
The following are example configurations. These examples do not
represent an exhaustive list and other configurations are expected.
o) Single Policy Repository
In this configuration there is a single PC Policy repository shared
between PCEs. This configuration is likely to be useful within a
single administrative domain where multiple PCEs are provided for
redundancy or load distribution purposes.
Bryskin, Papadimitriou & Berger [Page 18]
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........................
. .
. PC Policy Management .
. .
........................
.
.
--------- Policy a ---------------------- Policy b ---------
| PCE-PDP |<--------- | PC Policy Repository | -------->| PCE-PDP |
--------- ---------------------- ---------
^ ^
| e.g., COPS e.g., COPS |
v v
--------- ---------
| PCE-PEP |<------------------------------------------->| PCE-PEP |
--------- PCE-PCE Communication Protocol ---------
Figure 7: Single PCC Policy Repository
o) Multiple Policy Repositories
The repositories in this case may be fully or partially synchronized
by some discovery/synchronization management protocol(s) or may be
completely independent. In the multi-domain case it is expected that
the repositories will be distinct, providing, however. consistent
policies.
-------------- --------------
| PC Policy | | PC Policy |
---| Repository A | | Repository B |---
| -------------- -------------- |
| |
| Policy a Policy b |
| |
v v
--------- ---------
| PCE-PDP | | PCE-PDP |
--------- ---------
^ ^
| e.g., COPS e.g., COPS |
v v
--------- ---------
| PCE-PEP |<------------------------------------------->| PCE-PEP |
--------- PCC-PCE Communication Protocol ---------
Figure 8: Multiple PCC Policy Repositories
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7.4. Policy Configuration Management
The management of path computation policy information used by PCCs
and PCEs is largely out of scope of the described framework. The
framework assumes that such information is installed, removed and
otherwise managed using typical policy management techniques. Policy
Repositories may be populated and managed via static configuration,
standard and proprietary policy management tools, or even dynamically
via policy management/discovery protocols and applications.
8. Inter-Component Communication
8.1. Policy Communication
Flexibility in the application of policy types is imperative from the
architecture perspective. However, this commodity implies added
complexity on the part of the PCE related communication protocols.
One added complexity is that PCE communication protocols must carry
certain information to support various policy types that may be
applied. For example, in the case where policy is only applied at a
PCE, a PCC-PCE request must carry sufficient information for the PCE
to apply service or user specific policies. This does imply that a
PCC must have sufficient understanding of what policies can be
applied at the PCE. Such information may be obtained via local
configuration, static coding or even via a PCE discovery mechanism.
The PCC must also have sufficient understanding to properly encode
the required information for each policy type.
Another added complexity is that PCE communication protocols must
also be able to carry information that may result from a policy
decision. For example, user or service specific policy applied at a
PCC may result in policy related information that must be carried
along with the request for use by a PCE. This complexity is
particularly important as it may be used to introduce new path
computation parameters (e.g. constraints, objection functions, etc.)
without modification of the core PCC and PCE. This communication will
likely simply require the PCE communication protocols to support
opaque policy related information elements.
A final added complexity is that PCE communication protocols must
also be able to support updated or unsolicited responses from a PCE.
For example, changes in PCE policy may force a change to a previously
provided path. Such updated or unsolicited responses may contain
information that the PCC must act on, and may contain policy
information that must be provided to a PCC.
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PCC-PEP and PCE-PEP or a pair of PCE-PEPs communicate via a request-
response type PCC-PCE Communication Protocol. This document makes no
assumptions as to what exact protocol is used to support this
communication. This document does assume that the semantics of a
path computation request are sufficiently abstract and general, and
support both PCE-PCC and PCE-PCE communication.
A path computation request at minimum should include:
o One or several source addresses;
o One or several destination addresses;
o Computation type (P2P, P2MP, MP2P, etc.);
o Number of required paths;
o Zero or more policy descriptors in the following format:
<policy name>,
<policy variable1 name>, <param11>, <param12>,...,<param1N>
<policy variable2 name>, <param21>, <param12>,...,<param2N>
...
<policy variableM name>, <paramM1>, <paramM2>,...,<paramMN>
A successful path computation response, at minimum, should include
the list of computed paths and may include policies (in the form of
policy descriptors as in path computation request, see above) for use
in evaluating and otherwise applying the computed paths.
PCC-PCE Communication Protocol provides transport for policy
information and should not understand nor make any assumptions about
the semantics of policies specified in path computation requests and
responses.
Note: This document explicitly allows for (but does not require) the
PCC to decide that all necessary constraints, objective functions,
etc. pertinent to the computation of paths for the service in
question are to be determined by the PCE performing the computation.
In this case the PCC will use a set of policies (more precisely,
PCPIM policy variables) describing the service specific information.
These policies may be placed within the path computation request and
delivered to the PCE via a PCC-PCE communication protocol. The PCE
(more precisely, PCE-PEP) is expected to understand this information
and use it to determine the constraints and optimization functions
applying local policies (that is, policies locally configured or
provided by the associated PCE-PDP(s)).
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8.2. PCE Discovery Policy Considerations
Dynamic PCE discovery allows for PCCs and PCEs to automatically
discover a set of PCEs (including information required for the PCE
selection). It also allows for PCCs and PCEs to dynamically detect
new PCEs or any modification of PCEs information. Policy can be
applied in two ways in this context:
1. Restricting the scope of information distribution for the
mandatory set of information (in particular the PCE presence
and location).
2. Restricting the type and nature of the optional information
distributed by the discovery protocol. The latter is also
subject to policy since the PCE architecture allows for
distributing this information using either PCE discovery
protocol(s) or PCC-PCE communication protocol(s). One important
policy decision in this context is the nature of the information
to be distributed, especially, when this information is not
strictly speaking a "discovery" information, rather, the PCE
state changes. Client-specific and domain-specific policies may
be applied when deciding whether this information should be
distributed and to which clients of the path computation service
(that is, which PCCs and/or PCEs)
Another place where policy applies is at the administrative
boundaries. In multi-domain networks multiple PCEs will communicate
with each other and across administrative boundaries. In such cases,
domain-specific polices would be applied to 1) filter the information
exchanged between peering PCEs during the discovery process (to the
bare minimum in most cases if at all allowed by the security policy)
and 2) limit the content of information being passed in path
computation request and responses.
9. Path Computation Sequence of Events
This section presents representative scenarios; other scenarios are
also possible.
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9.1. Policy-enabled PCC, Policy-enabled PCE
When a GMPLS LSR receives a Setup (RSVP Path) message from an
upstream LSR, the LSR may decide to use a remote Path Computation
Entity. The following sequence of events occurs in this case:
- A PCC-PEP co-located with the LSR applies the service specific
policies to select a PCE for the service path computation as
well as to build the path computation request (that is, to
select a list of policies, their variables, conditions and
actions expressing constraints, diversities, objective functions
and relaxation strategies appropriate for the service path
computation). The policies may be:
a) Statically configured on the PCC-PEP;
b) Communicated to the PCC-PEP by a remote or local PCC-PDP
via protocol such as COPS either pro-actively (most of the
cases) or upon an explicit request by the PCC-PEP in case when
some specifics of the new service have not been covered yet by
the policies so far known to the PCC-PEP)
The input for the decision process on the PCC-PEP is the
information found in the signaling message as well as any other
service specific information such as port ID over which the message
was received, associated VPN ID, the reference point type (UNI, E-
NNI, etc.) and so forth. After the path computation request is
built it is sent directly to the PCE-PEP using the PCC-PCE
Communication Protocol.
- PCE-PEP validates and otherwise processes the request applying
the policies found in the request as well as client and domain
specific polices. The latter, again, may be either statically
configured on the PCE-PEP or provided by the associated local or
remote PCE-PDP via a protocol such as COPS. The outcome of the
decision process is the following information:
a) Whether the request should be satisfied, rejected or
dismissed.
b) The sets of sources and destinations for which paths should
be locally computed.
c) The set of constraints, diversities, optimization functions
and relaxations to be considered in each of locally performed
path computation.
d) The address of the next-in-chain PCE.
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e) The path computation request to be sent to the
next-in-chain PCE.
The PCE-PEP instructs a co-located path computation engine to
perform the local path computation(s) and, if necessary, sends the
path computation request to the next-in-chain PCE using a PCC-PCE
Communication Protocol. Then it waits for the responses from the
local path computation engine and the remote PCE, combines the
resulting paths and sends them back to the PCC-PEP using the PCC-
PCE Communication Protocol. The response contains the resulting
paths as well as policies describing some additional information
(for example, which of constraints were honored, which were
dismissed and which were relaxed and in what way)
- PCC-PEP instructs the signaling sub-system of the GMPLS LSR to
encode the received path(s) into the outgoing Setup message(s).
9.2. Policy-ignorant PCC, Policy-enabled PCE
This case parallels the previous example, but the user and service
specific policies should be applied at the PCE as the PCC is policy
ignorant. Again, when a GMPLS LSR has received a Setup (RSVP Path)
message from an upstream LSR, the LSR may decide to use a non co-
located Path Computation Entity. The following sequence of events
occurs in this case:
- The PCC constructs a PCE request using information found in the
signaling/provisioning message as well as any other service
specific information such as port ID over which the message was
received, associated VPN ID, the reference point type (UNI, E-
NNI, etc.) and so forth. This information is encoded in the
request in the form of policy variables. After the request is
built it is sent directly to the PCE-PEP using a PCC-PCE
Communication Protocol.
- PCE-PEP validates and otherwise processes the request
interpreting the policy variables found in the request and
applying user, service- and also client- and domain- specific
polices to build the actual path computation request. The
policies, again, may be either statically configured on the
PCE-PEP or provided by the associated local or remote PCE-PDP via
a protocol such as COPS. The outcome of the decision process is
the following information:
a) Whether the request should be satisfied, rejected or
dismissed.
Bryskin, Papadimitriou & Berger [Page 24]
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b) The sets of sources and destinations for which paths should
be locally computed.
c) The set of constraints, diversities, optimization functions
and relaxations to be considered in each of locally performed
path computation.
d) The address of the next-in-chain PCE.
e) The path computation request to be sent to the next-in-
chain PCE.
The PCE-PEP instructs a co-located path computation engine to
perform the local path computation(s) and, if necessary, sends the
path computation request to the next-in-chain PCE using the PCC-PCE
Communication Protocol. Then it waits for the responses from the
local path computation engine and the remote PCE, combines the
resulting paths and sends them back to the PCC-PEP using the PCC-
PCE Communication Protocol. The response contains the resulting
paths as well as policies describing some additional information
(for example, which of constraints were honored, which were
dismissed and which were relaxed and in what way)
- PCC-PEP instructs the signaling sub-system of the GMPLS LSR to
encode the received path(s) into the outgoing Setup message(s).
10. Introduction of New Constraints
An important aspect of the policy-enable path computation framework
discussed above is the ability to introduce new constraints with
minimal impact. In particular, only those components and mechanisms
that will use a new constraint need to change in order to support a
new constraints. Importantly, those components and mechanisms that
will not use a new constraint must not require change for a new
constraint to be utilized. For example, the PCE communication
protocols must not require any changes to support a new constraint.
Likewise, PCC and PCEs that will not process a new constraint must
not require any modification.
Consider the case where a PCE has been upgraded with software
supporting optical physical impairment constraint, such as
Polarization Mode Dispersion (PMD), that previously was not supported
in the domain. In this case, one or more new policies will be
installed in the PC Policy Repository (associated with the PCE)
defining the constraint (rules that determine application criteria,
set of policy variables, conditions, actions, etc.) and its
relaxation strategy(ies). The new policies will be also propagated
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into other PC Policy Repositories within the domain via discovery and
synchronization protocols or via local configuration. PCE-PDPs and
PCC-PDPs will then retrieve the corresponding policies from the
repository(ies). From then on PCC-PDPs will instruct associated PCC-
PEPs to add the new policy information into path computation requests
for services with certain parameters (for example, for services
provisioned in the OCh layer).
It is important to note that policy-enabled path computation model
naturally solves the PCE capability discovery issues. Suppose a PCE
working in a single PC Policy Repository configuration starts to
support a new constraint. Once a corresponding policy installed in
the repository, it automatically becomes available for all repository
users, that is, PCCs. In the multi-repository case some policy
synchronization must be provided, however, this problem is one of the
management plane which is solved already.
11. Security Considerations
This document adds to the policy security considerations mentioned in
[PCE-ARCH]. In particular it is now necessary to consider the
security of policy information maintained in PC Policy Repositories
and policy related transactions. The most notable issues, some of
which are also listed in [PCE-ARCH], are:
- Unauthorized access to the PC Policy Repositories;
- Interception of policy information when it is retrieved from
the repositories and/or transported from PDPs to PEPs;
- Interception of policy related information in path computation
requests and responses;
o Impersonation of user and client identities;
o Falsification of policy information and/or PCE capabilities;
o Denial of service attacks on policy related communication
mechanisms.
As with [PCE-ARCH], it is expected that PCE solutions will address
these issues in detail.
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12. Acknowledgements
We would like to thank Bela Berde for fruitful discussions on PBM
and Policy-driven path computation.
13. IANA Considerations
None.
14. References
14.1. Normative References
[PCE-ARCH] Farrel, A., Vasseur, JP., Ash, J., "Path Computation
Element (PCE) Architecture", April 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," BCP 14, RFC 2119, March 1997.
[RFC2753] R. Yavatkar, D. Pendarakis, R. Guerin, A Framework for
Policy Based Admission Control, RFC 2753, January 2000.
[RFC2748] D. Durham, et al., The COPS (Common Open Policy Service)
protocol, RFC 2748, IETF, January 2000.
[RFC3060] B. Moore, et al., Policy Information Model Version1
Specification, RFC 3060, February 2001.
[RFC3209] Awduche, D., et al., "Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC3460] Moore, B. Ed., "Policy Core Information Model (PCIM)
Extensions", RFC 3460, January 2003.
[RFC3473] Berger, L., et al., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
3473, January 2003.
[RFC3644] Y. Snir, et al., Policy Quality of Service (QoS)
Information Model, RFC 3644, November 2003.
[RFC4216] Zhang, R., Vasseur, J-P., Eds., "MPLS Inter-Autonomous
System (AS) Traffic Engineering (TE) Requirements",
RFC4216, November 2005.
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[INTERAS-PCEP] Bitar, N., Zhang, R., Kumaki, K., Eds., "Inter-AS
Requirements for the Path Computation Element
Communication Protocol (PCECP)", February 2006.
[PCECP-IA-REQ] Le Roux, J-L., Ed., "PCE Communication Protocol
(PCECP) Specific Requirements for Inter-Area
(G)MPLS Traffic Engineering", February 2006.
14.2. Informative References
[DMTF] Common Information Model (CIM) Schema, version 2.x.
Distributed Management Task Force, Inc. The components
of the CIM v2.x schema are available via links on the
following DMTF web page:
http://www.dmtf.org/standards/standard_cim.php.
[RFC3031] Rosen, E,. Viswanathan, V. Callon, R., "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3084] Chan, K., Seligson, J., Durham, D., Gai, S., McCloghrie,
K., Herzog, S., Reichmeyer, F., Yavatkar, R. and A.
Smith, "COPS Usage for Policy Provisioning (COPS-PR)",
RFC 3084, February 2001.
[RFC3198] Westerinen, A., et al., "Terminology for Policy-Based
Management", RFC 3198, November 2001.
[RFC3630] Katz, D., Kompella, K., Yeung., D., "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630, September
2003.
15. Authors' Addresses
Igor Bryskin
Movaz Networks, Inc.
7926 Jones Branch Drive
Suite 615
McLean, VA 22102
Email: ibryskin@movaz.com
Dimitri Papadimitriou (Alcatel)
Fr. Wellesplein 1,
B-2018 Antwerpen, Belgium
Phone: +32 3 240-8491
Email: dimitri.papadimitriou@alcatel.be
Bryskin, Papadimitriou & Berger [Page 28]
Internet Draft draft-ietf-pce-policy-enabled-path-comp-00.txt July 2006
Lou Berger
LabN Consulting, LLC
Phone: +1 301 468 9228
Email: lberger@labn.net
16. Full Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
17. Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
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attempt made to obtain a general license or permission for the use of
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The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
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