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INFORMATIONAL

Network Working Group                                         I. Bryskin
Request for Comments: 5394                                  Adva Optical
Category: Informational                                 D. Papadimitriou
                                                                 Alcatel
                                                               L. Berger
                                                         LabN Consulting
                                                                  J. Ash
                                                                    AT&T
                                                           December 2008


               Policy-Enabled Path Computation Framework

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (c) 2008 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (http://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   The Path Computation Element (PCE) architecture 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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Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................3
   2. Background ......................................................4
      2.1. Motivation .................................................4
      2.2. Policy Attributes ..........................................6
      2.3. Representative Policy Scenarios ............................7
           2.3.1. Scenario: Policy Configured Paths ...................7
           2.3.2. Scenario: Provider Selection Policy ................10
           2.3.3. Scenario: Policy Based Constraints .................12
           2.3.4. Scenario: Advanced Load Balancing (ALB) Example ....14
   3. Requirements ...................................................16
   4. Path Computation Policy Information Model (PCPIM) ..............18
   5. Policy-Enabled Path Computation Framework Components ...........20
   6. Policy Component Configurations ................................21
      6.1. PCC-PCE Configurations ....................................21
      6.2. Policy Repositories .......................................24
      6.3. Cooperating PCE Configurations ............................25
      6.4. Policy Configuration Management ...........................27
   7. Inter-Component Communication ..................................27
      7.1. Policy Communication ......................................27
      7.2. PCE Discovery Policy Considerations .......................29
   8. Path Computation Sequence of Events ............................29
      8.1. Policy-Enabled PCC, Policy-Enabled PCE ....................29
      8.2. Policy-Ignorant PCC, Policy-Enabled PCE ...................31
   9. Introduction of New Constraints ................................32
   10. Security Considerations .......................................33
   11. Acknowledgments ...............................................33
   12. References ....................................................34
      12.1. Normative References .....................................34
      12.2. Informative References ...................................34

1.  Introduction

   The Path Computation Element (PCE) Architecture is introduced in
   [RFC4655].  This document describes the impact of policy-based
   decision making when incorporated into the PCE architecture and
   provides additional details on, and context for, applying policy
   within the PCE architecture.

   Policy-based Management (PBM), see [RFC3198], is a network management
   approach that enables a network to automatically perform actions in
   response to network events or conditions based on pre-established
   rules, also denoted as policies, from a network administrator.  PBM
   enables network administrators to operate in a high-level manner
   through rule-based strategy (policies can be defined as a set of
   rules and actions); the latter are translated automatically (i.e.,



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   dynamically, without human interference) 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 (RAP) 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 refer to the functions defined in the COPS context.  This document
   makes no assumptions nor does it require that the actual COPS
   protocol be used.  Any suitable policy exchange protocol (for
   example, Simple Object Access Protocol (SOAP) [W3CSOAP]) may be
   substituted.

   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 Quality
   of Service (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.

   In Section 2, this document presents policy-related background and
   scenarios to provide a context for this work.  Section 3 provides
   requirements that must be addressed by mechanisms and protocols that
   enable policy-based control over path computation requests and
   decisions.  Section 4 introduces PCIM as a core component in a
   framework for providing policy-enabled path computation.  Section 5
   introduces a set of components that may be used to support policy-
   enabled path computation.  Sections 6, 7, and 8 provide details on
   possible component configurations, communication, and events.
   Section 10 discusses the ability to introduce new constraints with
   minimal impact.  It should be noted that this document, in Section 4,
   only introduces PCIM; specific PCIM definitions to support path
   computation will be discussed in a separate document.

1.1.  Terminology

   The reader is assumed to be familiar with the following terms:

   BEEP:    Blocks Extensible Exchange Protocol, see [RFC3080].
   CIM:     Common Information Model, see [DMTF].
   COPS:    Common Open Policy Service, see [RFC2748].
   CSPF:    Constraint-based Shortest Path First, see [RFC3630].



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   LSP:     Label Switched Path, see [RFC3031].
   LSR:     Label Switching Router, see [RFC3031].
   PBM:     Policy-Based Management, see [RFC3198].
   PC:      Path Computation.
   PCC:     Path Computation Client, see [RFC4655].
   PCCIM:   Path Computation Core Information Model.
   PCE:     Path Computation Element, see [RFC4655].
   PCEP:    Path Computation Element Communication Protocol,
            see [PCEP].
   PCIM:    Policy Core Information Model, see [RFC3060].
   PDP:     Policy Decision Point, see [RFC2753].
   PEP:     Policy Enforcement Point, see [RFC2753].
   QPIM:    QoS Policy Information Model, see [RFC3644].
   SLA:     Service Level Agreement.
   SOAP:    Simple Object Access Protocol, see [W3CSOAP].
   TE:      Traffic Engineering, see [RFC3209] and [RFC3473].
   TED:     Traffic Engineering Database, see [RFC3209] and [RFC3473].
   TE LSP:  Traffic Engineering MPLS Label Switched Path, see
            [RFC3209] and [RFC3473].
   WDM:     Wavelength Division Multiplexing

2.  Background

   This section provides some general background on the use of policies
   within the PCE architecture.  It presents the rationale behind the
   use of policies in the TE path computation process, as well as
   representative policies usage scenarios.  This information is
   intended to provide context for the presented PCE policy framework.
   This section does not attempt to present an exhaustive list of
   rationales or scenarios.

2.1.  Motivation

   The PCE architecture as introduced in [RFC4655] includes policy as an
   integral part of the PCE architecture.  This section presents some of
   the rationale 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 optimization
   criteria, 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 issued differently for different services.  A given
   Service A, for instance, may require two Shared Risk Link Group
   (SRLG)-disjoint paths for building end-to-end recovery scheme, while



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   for a 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.  So, based on distinct
   service requirements, distinct or similar policies may be adopted
   when issuing/handling path computation requests.

   Likewise, a PCE may apply policies to decide which algorithm(s) to
   use while performing path computations requested from a particular
   PCC or for a particular domain, see [RFC4927]; 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 motivation for supporting policies 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 criteria, 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 the 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, such policies may be
   configured and managed by a network operator as any other policies
   and may be interpreted in real time by PCCs and PCEs.






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   There are a number of advantages and useful by-products of such an
   approach:

   - New path computation capabilities may be introduced without
     changing PCE-PCC communication and discovery protocols or PCC
     software.  Only the PCE module providing the path computation
     capabilities (referred to in this document as a path computation
     engine) needs to be updated.

   - Existing constraints, objective functions and their relaxations may
     be aggregated and otherwise associated, thus producing new, more
     complex objective functions that do not require a change of code
     even on the PCEs supporting the functions.

   - Different elements such as conditions, actions, variables, etc.,
     may be reused 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, managed by the same
     policy management tools, and interpreted using the same mechanisms.
     Also, policies need to be supported by PCCs and PCEs independent of
     the peculiarities of a specific PCC-PCE communication protocol, see
     [PCEP].  Thus, introducing a new (request-specific) type of policy
     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.

2.2.  Policy Attributes

   This section provides a summary listing of the policy attributes that
   may be included in the policy exchanges described in the scenarios
   that follow.  This list is provided for guidance and is not intended
   to be exclusive.  Implementation of this framework might include
   additional policy attributes not listed here.

      Identities

      - LSP head-end
      - LSP destination
      - PCC
      - PCE








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      LSP identifiers

      - LSP head-end
      - LSP destination
      - Tunnel identifier
      - Extended tunnel identifier
      - LSP ID
      - Tunnel name

      Requested LSP qualities

      - bandwidth
      - traffic parameters
      - LSP attributes
      - explicit path inclusions
      - explicit path exclusions
      - link protection level
      - setup priority
      - holding priority
      - preexisting LSP route

      Requested path computation behavior

      - objective function
      - other LSPs to be considered

      Additional policy information

      - Transparent policy information as received in Resource
        Reservation Protocol (RSVP)-TE

2.3.  Representative Policy Scenarios

   This section provides example scenarios of how policies may be
   applied using the PCE policy framework within the PCE architecture
   context.  Actual networks may deploy 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 policies within the PCE architecture.

2.3.1.  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 Explicit Route Objects
   (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.



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   In particular, service-specific policies can be installed that will
   provide configured path(s) for a specific service request.  The
   request may be identified based on service parameters such as
   endpoints, requested QoS, or even a token that identifies the
   initiator of a service request.  The configured path(s) would then be
   used as input to the path computation process, which would return
   explicit routes by expanding of all specified loose hops.

   Example of policy:
    if(service_destination matches 10.132.12.0/24)
       Use path: 10.125.13.1 => 10.125.15.1 => 10.132.12.1.
    else
       Compute path dynamically.

          ----------------------
         |              -----   |
         |             | TED |<-+------------>
         |              -----   |  TED synchronization
         |                |     |  mechanism (e.g., routing protocol)
         |                |     |
         |                v     |
         |  ------      -----   |  Inter-PCE Request/Response
         | |Policy|<-->| PCE |<.+...........>  (when present)
         |  ------      -----   |
          ----------------------
                          ^
                          | Request/
                          | Response
                          v
           Service  -------------  Signaling
           Request |[PCC][Policy]| Protocol
           <------>|    Node     |<------->
      or Signaling  -------------
         Protocol

                     Figure 1: Policy Enabled PCC and PCE

   Path computation policies may be applied at either a PCC or a PCE,
   see Figure 1.  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.






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       ------------------       -------------------
      |                  |     |                   |
      |        PCE       |     |        PCE        |
      |                  |     |                   |
      |  ------   -----  |     |   -----   ------  |
      | |Policy| | TED | |     |  | TED | |Policy| |
      |  ------   -----  |     |   -----   ------  |
       ------------------       -------------------
               ^                       ^
               | Request/              | Request/
               | Response              | Response
               v                       v
   Service --------  Signaling  ------------  Signaling  ------------
   Request|Head-End| Protocol  |Intermediate| Protocol  |Intermediate|
     ---->|  Node  |<--------->|    Node    |<--------->|    Node    |
           --------             ------------             ------------

                  Figure 2: Multiple PCE Path Computation

    ------------------                              ------------------
   |                  | Inter-PCE Request/Response |                  |
   |       PCE        |<-------------------------->|       PCE        |
   |                  |                            |                  |
   |  ------   -----  |                            |  ------   -----  |
   | |Policy| | TED | |                            | |Policy| | TED | |
   |  ------   -----  |                            |  ------   -----  |
    ------------------                              ------------------
               ^
               | Request/
               | Response
               v
   Service ----------  Signaling   ----------  Signaling   ----------
   Request| Head-End | Protocol   | Adjacent | Protocol   | Adjacent |
     ---->|  Node    |<---------->|   Node   |<---------->|   Node   |
           ----------              ----------              ----------

   Figure 3: Multiple PCE Path Computation with Inter-PCE Communication

   Policy-configured paths may also be used in environments with
   multiple (more than one) cooperating PCEs (see Figures 2 and 3).  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 a scenario work, it would



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   likely be necessary to start and finish the configured paths on TE
   domain boundary nodes.  Clearly, consistent PCE Policy Repositories
   are also critical in this example.

2.3.2.  Scenario: Provider Selection Policy

   A potentially more interesting scenario is applying PC policies in
   multi-provider topologies.  There are numerous interesting policy
   applications in such topologies.  A rudimentary example is simple
   access control, that is, deciding which PCCs are permitted to request
   inter-domain path computation.

   A more complicated example is applying policy to determine which
   domain or network provider will be used to support a particular PCE
   request.  Consider the topology presented in Figure 4.  In this
   example, there are multiple transit domains available to provide a
   path from a source domain to a destination domain.  Furthermore, each
   transit domain 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.

   In today's typical path computation process, TE reachability,
   availability, and metric are the basic criteria for path selection.
   However, policies can provide an important added consideration in the
   decision process.  For example, transit domain A may be more
   expensive and provide lower delay or loss than transit domain B.
   Likewise, a transit domain 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 domains that provide reachability, and
   policies can be used to govern which PCE requests get better service.




















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                              +-------+
                   +----------+Transit+----------+
               +---+---+      | Domain|      +---+---+
               |Transit|      |   C   |      |Transit|
      +--------+ Domain|      +---+---+      | Domain+--------+
      |        |   A   +--+       |       +--+   F   |        |
   +--+---+    +---+---+  |       |       |  +---+---+     +--+---+
   |Source|        |      |   +---+---+   |      |         |Target|
   |Domain|        |      +---+Transit+---+      |         |Domain|
   +--+---+        |      +---+ Domain|---+      |         +--+---+
      |        +---+---+  |   |   D   |   |  +---+---+        |
      |        |Transit|  |   +---+---+   |  |Transit|        |
      +--------+ Domain+--+       |       +--+ Domain+--------+
               |   B   |          |          |   G   |
               +---+---+      +---+---+      +---+---+
                   |          |Transit|          |
                   +----------+ Domain+----------+
                              |   E   |
                              +-------+

       Figure 4: 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.

   Example of policy:
    if(path computation request issued by a PCC within Source Domain)
       Route the path through Transit Domain A.
    else
       Route the path through Transit Domain B.













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2.3.3.  Scenario: Policy Based Constraints

   Another usage scenario is the use of policy to provide constraints in
   a PCE request.  Consider an LSR with a policy enabled PCC, as shown
   in Figure 1, which receives a service request via signaling,
   including over a Network-Network Interface (NNI) or User Network
   Interface (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, optimization criterion, and
   constraint relaxation strategies should 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 that 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.

   Example of policy:
    if(LSP belongs to a WDM layer network)
       Compute the path with wavelength continuity constraint with the
       maximum Optical Signal Noise Ratio (OSNR) at the path end
       optimization.
    else if(LSP belongs to a connection oriented Ethernet layer network)
       Compute the path with minimum end-to-end delay.
    else
       Compute the shortest path.

   The PCC may also apply server-specific policies in order to select
   which PCE to use from the set of known (i.e., discovered or
   configured) PCEs.  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




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   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.

   Example of policy:
    if(LSP belongs to a WDM layer network)
       Identify a PCE supporting wavelength continuity and optical
       impairment constraints;
       Send a request to such PCE, requesting path computation with the
       following constraints:
          a) wavelength continuity;
          b) maximum Polarization Mode Dispersion (PMD) at the path end.
       if(the path computation fails) remove the maximum PMD constraint
          and try the computation again.

   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 policies.  As a result of the policy processing, the PCE may
   decide to reject the request.

   Example of policy:
    Authenticate the PCC requesting the path computation using the
    PCC ID found in the path computation request;
    Reject the request if the authentication fails.

   The PCE also may decide to respond with one or several pre-computed
   paths if user- or client-specific policies 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
   constraints were honored, which were dismissed, and which were
   relaxed and in what way.









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   Example of policy:
    if(the path destination belongs to domain A)
       Instruct local path computation engine to perform the path
       computation;
    else
       Identify the PCE supporting the destination domain;
       Send path computation request to such PCE;
       Wait for and process the response.
    Send the path computation response to the requesting PCC.

   The PCC processes the response and instructs the LSR to encode the
   received path(s) into the outgoing signaling message(s).

2.3.4.  Scenario: Advanced Load Balancing (ALB) Example

   Figure 5 illustrates a problem that stems from the coupling between
   BGP and IGP in the BGP decision process.  If a significant portion of
   the traffic destined for the data center (or customer network) enters
   a PCE-enabled network from AS 1 and all IGP links' weights are the
   same, then both PE3 and PE4 will prefer to reach the data center
   using the routes advertised by PE2.  PE5 will use the router-IDs of
   PE1 and PE2 to break the tie and might therefore also select to use
   the path through PE2 (if the router ID of PE2 is smaller than that of
   PE1).  Either way, the net result is that the link between PE2 and CE
   will carry most of the traffic while the link between PE1 and the
   Customer Edge (CE) will be mostly idle.

























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                           ..............................
                           .          AS 1              .
                           .                            .
                           .   +---+   +---+   +----+   .
                           ....|PE8|...|PE9|...|PE10|....
                               +---+   +---+   +----+
                                 |       |       |
                               +---+   +---+   +---+
                         ......|PE3|...|PE4|...|PE5|......
                         .     +---+   +---+   +---+     .
    ..............     +---+     \      /    ___/      +---+
    .            .    _|PE2|_____+--+__/    /         _|PE6|
    .           +--+ / +---+     |P1|_____+--+_______/ +---+
    . Customer  |CE|=    .       +--+     |P2|           .
    . Network   +--+ \_+---+        \     +--+           .
    .            .     |PE1|________+--+___/|     x===x  .  PCE used
    ..............     +---+        |P3|    |     |PCE|  .  by all
                         .          +--+    |     x===x  .  AS0 nodes
                         .    AS 0         +---+         .
                         ..................|PE7|..........
                                           +---+

                     Figure 5: Advanced Load Balancing

   This is a common problem for providers and customers alike.  Analysis
   of Netflow records, see [IRSCP], for a large ISP network on a typical
   day has shown that for 71.8% of multi-homed customers, there is a
   complete imbalance, where the most loaded link carries all the
   traffic and the least loaded link carries none.

   PCE policies can address this problem by basing the routing decision
   at the ingress routers on the offered load towards the multi-homed
   customer.  For example, in Figure 5, PCE policies could be configured
   such that traffic load is monitored (e.g., based on Netflow data) at
   ingress routers PE3 to PE7 towards the data center prefixes served by
   egress routers PE1 and PE2.  Using this offered load information, the
   path computations returned by PCE, based on the enabled PCE policies,
   can direct traffic to the appropriate egress router, on a per-ingress
   router basis.  For example, the PCE path computation might direct
   traffic from both PE4 and PE5 to egress PE1, thus overriding the
   default IGP based selection.  Alternatively, traffic from each
   ingress router to each egress link could be split 50-50.

   This scenario is a good example of how a policy-governed PCE can
   account for some information that was not or cannot be advertised as
   TE link/node attributes, and, therefore, cannot be subject for
   explicit path computation constraints.  More generally, such
   information can be pretty much anything.  For example, traffic demand



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   forecasts, flow monitoring feedback, any administrative policies,
   etc.  Further examples are described in [IRSCP] of how PCE policies
   might address certain network routing problems, such as selective
   distributed denial-of-service (DDoS) blackholing, planned
   maintenance, and VPN gateway selection.

   Example of policy:
    for(all traffic flows destined to Customer Network)
       if(flow ingresses on PE3, PE4, or PE5)
          Route the flow over PE1.
       else
          Route the flow over PE2.

3.  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 a Network Management System (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.






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   - 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 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 [RFC5376], and inter-area policy-related requirements
     discussed in [RFC4927].  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.




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4.  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
     classes are related to each other.

   These classes can be mapped to various concrete implementations, for
   example, to a directory that uses Lightweight Directory Access
   Protocol version 3 (LDAPv3) as its access protocol.

   Figure 6 shows an abstract from the class inheritance hierarchy for
   PCIM.































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   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)
      |      |
      |      +---(subtree of more specific classes)

                     Figure 6: PCIM Class Inheritance

   The policy classes and associations defined in PCIM are sufficiently
   generic to allow them to represent policies related to anything.





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   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.

   The 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
   document.

5.  Policy-Enabled Path Computation Framework Components

   The following components are defined as part of the framework to
   support policy-enabled path computation:

   - PCE Policy Repository
     A database from which PCE policies are available in the form of
     instances of PCPIM classes.  PCE Policies are configured and
     managed by PCE 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
     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.




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   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 latter case, they talk to each other via such
   protocols as SOAP [W3CSOAP] or BEEP [RFC3080].

   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 SOAP and/or BEEP.

   PCC-PDP/PCE-PDP may be co-located with, or separated from, an
   associated PCE 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 (or repositories) and
   convey them to respective PEPs either in an unsolicited way or upon
   the PEP's requests.

   A PCC-PEP may receive policy information not only from PCC-PDP(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-PDP(s) but also from PCC-PEP(s), via the PCC-PCE
   communication protocol [PCEP].

   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
   requests as well as client-specific and domain-specific policies
   supplied by the PCE-PDP.

6.  Policy Component Configurations

6.1.  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,



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   and on PCEs, is a specific network design choice.  Some networks may
   choose to apply policy only at PCCs (Figure 7), some at PCEs (Figure
   8), and others at both PCCs and PCEs (Figure 9).  Regardless of where
   policy is applied, it must be applied in a consistent fashion in
   order to achieve the intended results.

                         .........................
                         .                       .
                         . PCE Policy Management .
                         .                       .
                         .........................
                                     .
                                     .
    ---------  Policy     -----------------------
   | PCC-PDP |<--------- | PCE Policy Repository |
    ---------             -----------------------
        ^
        | e.g., SOAP
        v
    ---------                     PCEP                      ---------
   | PCC-PEP |<------------------------------------------->|   PCE   |
    ---------         PCC-PCE Communication Protocol        ---------

                  Figure 7: Policies 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.  [RFC4655] 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 PCE policy types used in the network must be applied
   at those locations.


















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                         .........................
                         .                       .
                         . PCE Policy Management .
                         .                       .
                         .........................
                                     .
                                     .
                          -----------------------  Policy    ---------
                         | PCE Policy Repository | -------->| PCE-PDP |
                          -----------------------            ---------
                                                                ^
                                                     e.g., SOAP |
                                                                v
    ---------                     PCEP                      ---------
   |   PCC   |<------------------------------------------->| PCE-PEP |
    ---------         PCC-PCE Communication Protocol        ---------

                    Figure 8: Policies Applied on 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 the 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 the 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.







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6.2.  Policy Repositories

   Within the policy-enabled path computation framework policy
   repositories may be used in a single or multiple PCE policy
   repository configuration:

   o) Single PCE Policy Repository

   In this configuration, there is a single PCE Policy Repository shared
   between PCCs and PCEs.

                         .........................
                         .                       .
                         . PCE Policy Management .
                         .                       .
                         .........................
                                     .
                                     .
    ---------  Policy a   -----------------------  Policy b  ---------
   | PCC-PDP |<--------- | PCE Policy Repository | -------->| PCE-PDP |
    ---------             -----------------------            ---------
        ^                                                       ^
        | e.g., SOAP                                 e.g., SOAP |
        v                                                       v
    ---------                     PCEP                      ---------
   | PCC-PEP |<------------------------------------------->| PCE-PEP |
    ---------         PCC-PCE Communication Protocol        ---------

                Figure 9: Single PCC/PCE Policy Repository

   o) Multiple PCE 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 PCE Policy
   Repository A exactly matches PC Policy Repository B, results in the
   single PCE Policy Repository configuration case.














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             --------------                   --------------
            |  PCE Policy  |                 |  PCE Policy  |
         ---| Repository A |                 | Repository B |---
        |    --------------                   --------------    |
        |                                                       |
        | Policy a                                     Policy b |
        |                                                       |
        v                                                       v
    ---------                                               ---------
   | PCC-PDP |                                             | PCE-PDP |
    ---------                                               ---------
        ^                                                       ^
        | e.g., SOAP                                 e.g., SOAP |
        v                                                       v
    ---------                     PCEP                      ---------
   | PCC-PEP |<------------------------------------------->| PCE-PEP |
    ---------         PCC-PCE Communication Protocol        ---------

              Figure 10: Multiple PCE/PCC Policy Repositories

6.3.  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 PCE 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 requester-
   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 PCE 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.





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                         .........................
                         .                       .
                         . PCE Policy Management .
                         .                       .
                         .........................
                                     .
                                     .
    ---------  Policy a   -----------------------  Policy b  ---------
   | PCE-PDP |<--------- | PCE Policy Repository | -------->| PCE-PDP |
    ---------             -----------------------            ---------
        ^                                                       ^
        | e.g., SOAP                                 e.g., SOAP |
        v                                                       v
    ---------                                               ---------
   | PCE-PEP |<------------------------------------------->| PCE-PEP |
    ---------         PCE-PCE Communication Protocol        ---------

                  Figure 11: 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.

             --------------                   --------------
            |  PCE Policy  |                 |  PCE Policy  |
         ---| Repository A |                 | Repository B |---
        |    --------------                   --------------    |
        |                                                       |
        | Policy a                                     Policy b |
        |                                                       |
        v                                                       v
    ---------                                               ---------
   | PCE-PDP |                                             | PCE-PDP |
    ---------                                               ---------
        ^                                                       ^
        | e.g., SOAP                                 e.g., SOAP |
        v                                                       v
    ---------                     PCEP                      ---------
   | PCE-PEP |<------------------------------------------->| PCE-PEP |
    ---------         PCC-PCE Communication Protocol        ---------

                Figure 12: Multiple PCC Policy Repositories





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6.4.  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.

7.  Inter-Component Communication

7.1.  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, i.e., [PCEP].  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.

   From a policy perspective, a path computation request should include
   at a minimum:

   o One or more source addresses;
   o One or more destination addresses;
   o Computation type (P2P (point to point), P2MP (point to multipoint),
     MP2P (multipoint to point), 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 such as
   [PCEP].  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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7.2.  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 status.  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
      "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 policies 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.

8.  Path Computation Sequence of Events

   This section presents a non-exhaustive list of representative
   scenarios.

8.1.  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



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     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 SOAP either proactively (most of the cases) or
        upon an explicit request by the PCC-PEP in cases 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, e.g., [PCEP].

   - PCE-PEP validates and otherwise processes the request applying the
     policies found in the request- as well as client- and domain-
     specific policies.  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 SOAP.  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.

     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-



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     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 subsystem of the GMPLS LSR to
     encode the received path(s) into the outgoing Setup message(s).

8.2.  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-, client-, and domain-specific policies 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 SOAP.  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.

     e) The path computation request to be sent to the next-in-chain
        PCE.




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     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).

9.  Introduction of New Constraints

   An important aspect of the policy-enabled 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 be updated in order to support
   the new constraint.  Importantly, those components and mechanisms
   that will not use the new constraint must not require any change in
   order for the new constraint to be utilized.  For example, the PCE
   communication protocols must not require any changes to support new
   constraints.  Likewise, PCC and PCEs that will not process new
   constraints 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 PCE 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 (or strategies).  The new policies will be also
   propagated into other PCE 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 (or repositories).  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 optical channel (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 PCE Policy Repository configuration starts to
   support a new constraint.  Once a corresponding policy installed in



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   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.

10.  Security Considerations

   This document adds to the policy security considerations mentioned in
   [RFC4655].  In particular, it is now necessary to consider the
   security issues related to policy information maintained in PCE
   Policy Repositories and policy-related transactions.  The most
   notable issues, some of which are also listed in [RFC4655], are:

   - Unauthorized access to the PCE 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 [RFC4655], it is expected that PCE solutions will address the
   PCE aspects of these issues in detail.

11.  Acknowledgments

   Adrian Farrel contributed significantly to this document.  We would
   like to thank Bela Berde for fruitful discussions on PBM and policy-
   driven path computation.  We would also like to thank Kobus Van der
   Merwe for providing insights and examples regarding PCE policy
   applications.













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12.  References

12.1.  Normative References

   [RFC2753]  Yavatkar, R., Pendarakis, D., and R. Guerin, "A Framework
              for Policy-based Admission Control", RFC 2753, January
              2000.

   [RFC3060]  Moore, B., Ellesson, E., Strassner, J., and A. Westerinen,
              "Policy Core Information Model -- Version 1
              Specification", RFC 3060, February 2001.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: 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., Ed., "Generalized Multi-Protocol Label
              Switching (GMPLS) Signaling Resource ReserVation
              Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
              3473, January 2003.

   [RFC3644]  Snir, Y., Ramberg, Y., Strassner, J., Cohen, R., and B.
              Moore, "Policy Quality of Service (QoS) Information
              Model", RFC 3644, November 2003.

   [RFC4216]  Zhang, R., Ed., and J.-P. Vasseur, Ed., "MPLS Inter-
              Autonomous System (AS) Traffic Engineering (TE)
              Requirements", RFC 4216, November 2005.

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              August 2006.

   [RFC4927]  Le Roux, J.-L., Ed., "Path Computation Element
              Communication Protocol (PCECP) Specific Requirements for
              Inter-Area MPLS and GMPLS Traffic Engineering", RFC 4927,
              June 2007.

12.2.  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.



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   [IRSCP]    Van der Merwe, J., et al., "Dynamic Connectivity
              Management with an Intelligent Route Service Control
              Point," ACM SIGCOMM Workshop on Internet Network
              Management (INM), Pisa, Italy, September 11, 2006.

   [PCEP]     Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", Work in
              Progress, November 2008.

   [RFC2748]  Durham, D., Ed., Boyle, J., Cohen, R., Herzog, S., Rajan,
              R., and A. Sastry, "The COPS (Common Open Policy Service)
              Protocol", RFC 2748, January 2000.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC3080]  Rose, M., "The Blocks Extensible Exchange Protocol Core",
              RFC 3080, March 2001.

   [RFC3198]  Westerinen, A., Schnizlein, J., Strassner, J., Scherling,
              M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry,
              J., and S. Waldbusser, "Terminology for Policy-Based
              Management", RFC 3198, November 2001.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630, September
              2003.

   [RFC5376]  Bitar, N., Zhang, R., and K. Kumaki, "Inter-AS
              Requirements for the Path Computation Element
              Communication Protocol (PCECP)", RFC 5376, November 2008.

   [W3CSOAP]  Hadley, M., Mendelsohn, N., Moreau, J., Nielsen, H., and
              Gudgin, M., "SOAP Version 1.2 Part 1: Messaging
              Framework", W3C REC REC-soap12-part1-20030624, June 2003.
















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Authors' Addresses

   Igor Bryskin
   ADVA Optical
   7926 Jones Branch Drive
   Suite 615
   McLean, VA 22102
   EMail: ibryskin@advaoptical.com

   Dimitri Papadimitriou
   Alcatel
   Fr. Wellesplein 1,
   B-2018 Antwerpen, Belgium
   Phone: +32 3 240-8491
   EMail: dimitri.papadimitriou@alcatel.be

   Lou Berger
   LabN Consulting, LLC
   Phone: +1 301 468 9228
   EMail: lberger@labn.net

   Jerry Ash
   AT&T
   EMail: gash5107@yahoo.com



























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