





Network Working Group                                          A. Farrel
Request for Comments: 4655                            Old Dog Consulting
Category: Informational                                    J.-P. Vasseur
                                                     Cisco Systems, Inc.
                                                                  J. Ash
                                                                    AT&T
                                                             August 2006


          A Path Computation Element (PCE)-Based Architecture


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) The Internet Society (2006).

Abstract

   Constraint-based path computation is a fundamental building block for
   traffic engineering systems such as Multiprotocol Label Switching
   (MPLS) and Generalized Multiprotocol Label Switching (GMPLS)
   networks.  Path computation in large, multi-domain, multi-region, or
   multi-layer networks is complex and may require special computational
   components and cooperation between the different network domains.

   This document specifies the architecture for a Path Computation
   Element (PCE)-based model to address this problem space.  This
   document does not attempt to provide a detailed description of all
   the architectural components, but rather it describes a set of
   building blocks for the PCE architecture from which solutions may be
   constructed.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. Definitions .....................................................4
   4. Motivation for a PCE-Based Architecture .........................6
      4.1. CPU-Intensive Path Computation .............................6
      4.2. Partial Visibility .........................................7
      4.3. Absence of the TED or Use of Non-TE-Enabled IGP ............7
      4.4. Node Outside the Routing Domain ............................8



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      4.5. Network Element Lacks Control Plane or Routing Capability ..8
      4.6. Backup Path Computation for Bandwidth Protection ...........8
      4.7. Multi-layer Networks .......................................9
      4.8. Path Selection Policy ......................................9
      4.9. Non-Motivations ...........................................10
           4.9.1. The Whole Internet .................................10
           4.9.2. Guaranteed TE LSP Establishment ....................10
   5. Overview of the PCE-Based Architecture .........................11
      5.1. Composite PCE Node ........................................11
      5.2. External PCE ..............................................12
      5.3. Multiple PCE Path Computation .............................13
      5.4. Multiple PCE Path Computation with Inter-PCE
           Communication .............................................14
      5.5. Management-Based PCE Usage ................................15
      5.6. Areas for Standardization .................................16
   6. PCE Architectural Considerations ...............................16
      6.1. Centralized Computation Model .............................16
      6.2. Distributed Computation Model .............................17
      6.3. Synchronization ...........................................17
      6.4. PCE Discovery and Load Balancing ..........................18
      6.5. Detecting PCE Liveness ....................................20
      6.6. PCC-PCE and PCE-PCE Communication .........................20
      6.7. PCE TED Synchronization ...................................22
      6.8. Stateful versus Stateless PCEs ............................23
      6.9. Monitoring ................................................25
      6.10. Confidentiality ..........................................25
      6.11. Policy ...................................................26
           6.11.1. PCE Policy Architecture ...........................26
           6.11.2. Policy Realization ................................28
           6.11.3. Type of Policies ..................................28
           6.11.4. Relationship to Signaling .........................29
      6.12. Unsolicited Interactions .................................30
      6.13. Relationship with Crankback ..............................30
   7. The View from the Path Computation Client ......................31
   8. Evaluation Metrics .............................................32
   9. Manageability Considerations ...................................33
      9.1. Control of Function and Policy ............................33
      9.2. Information and Data Models ...............................34
      9.3. Liveness Detection and Monitoring .........................34
      9.4. Verifying Correct Operation ...............................35
      9.5. Requirements on Other Protocols and Functional
           Components ................................................35
      9.6. Impact on Network Operation ...............................36
      9.7. Other Considerations ......................................36
   10. Security Considerations .......................................37
   11. Acknowledgements ..............................................37
   12. Informative References ........................................38




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1.  Introduction

   Constraint-based path computation is a fundamental building block for
   traffic engineering in MPLS [RFC3209] and GMPLS [RFC3473] networks.
   [RFC2702] describes requirements for traffic engineering in MPLS
   networks, while [RFC4105] and [RFC4216] describe traffic engineering
   requirements in inter-area and inter-AS environments, respectively.

   Path computation in large, multi-domain networks is complex and may
   require special computational components and cooperation between the
   elements in different domains.  This document specifies the
   architecture for a Path Computation Element (PCE)-based model to
   address this problem space.

   This document describes a set of building blocks for the PCE
   architecture from which solutions may be constructed.  For example,
   it discusses PCE-based implementations including composite, external,
   and multiple PCE path computation.  Furthermore, it discusses
   architectural considerations including centralized computation,
   distributed computation, synchronization, PCE discovery and load
   balancing, detection of PCE liveness, communication between Path
   Computation Clients (PCCs) and the PCE (PCC-PCE communication) and
   PCE-PCE communication, Traffic Engineering Database (TED)
   synchronization, stateful and stateless PCEs, monitoring, policy and
   confidentiality, and evaluation metrics.

   The model of the Internet is to distribute network functionality
   (e.g., routing) within the network.  PCE functionality is not
   intended to contradict this model and can be used to match the model
   exactly, for example, when the PCE functionality coexists with each
   Label Switching Router (LSR) in the network.  PCE is also able to
   augment functionality in the network where the Internet model cannot
   supply adequate solutions, for example, where traffic engineering
   information is not exchanged between network domains.

2.  Terminology

   CSPF: Constraint-based Shortest Path First.

   LER: Label Edge Router.

   LSDB: Link State Database.

   LSP: Label Switched Path.

   LSR: Label Switching Router.





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   PCC: Path Computation Client.  Any client application requesting a
   path computation to be performed by the Path Computation Element.

   PCE: Path Computation Element.  An entity (component, application, or
   network node) that is capable of computing a network path or route
   based on a network graph and applying computational constraints (see
   further description in Section 3).

   TED: Traffic Engineering Database, which contains the topology and
   resource information of the domain.  The TED may be fed by Interior
   Gateway Protocol (IGP) extensions or potentially by other means.

   TE LSP: Traffic Engineering MPLS Label Switched Path.

3.  Definitions

   A Path Computation Element (PCE) is an entity that is capable of
   computing a network path or route based on a network graph, and of
   applying computational constraints during the computation.  The PCE
   entity is an application that can be located within a network node or
   component, on an out-of-network server, etc.  For example, a PCE
   would be able to compute the path of a TE LSP by operating on the TED
   and considering bandwidth and other constraints applicable to the TE
   LSP service request.

   A domain is any collection of network elements within a common sphere
   of address management or path computation responsibility.  Examples
   of domains include IGP areas, Autonomous Systems (ASes), and multiple
   ASes within a Service Provider network.  Domains of path computation
   responsibility may also exist as sub-domains of areas or ASes.

   In order to fully characterize a PCE and clarify these definitions,
   the following important considerations must also be examined:

   1) Path computation is applicable in intra-domain, inter-domain, and
      inter-layer contexts.

      a. Inter-domain path computation may involve the association of
         topology, routing, and policy information from multiple domains
         from which relationships may be deduced in order to help in
         performing path computation.

      b. Inter-layer path computation refers to the use of PCE where
         multiple layers are involved and when the objective is to
         perform path computation at one or multiple layers while taking
         into account topology and resource information at these layers.





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      Overlapping domains are not within the scope of this document.  In
      the inter-domain case, the domains may belong to a single or to
      multiple Service Providers.

   2) a. In "single PCE path computation", a single PCE is used to
         compute a given path in a domain.  There may be multiple PCEs
         in a domain, but only one PCE per domain is involved in any
         single path computation.

      b. In "multiple PCE path computation", multiple PCEs are used to
         compute a given path in a domain.

   3) a. "Centralized computation model" refers to a model whereby all
         paths in a domain are computed by a single, centralized PCE.

      b. Conversely, "distributed computation model" refers to the
         computation of paths in a domain being shared among multiple
         PCEs.

      Paths that span multiple domains may be computed using the
      distributed model with one or more PCEs responsible for each
      domain, or the centralized model by defining a domain that
      encompasses all the other domains.

      From these definitions, a centralized computation model inherently
      uses single PCE path computation.  However, a distributed
      computation model could use either single PCE path computation or
      multiple PCE path computations.  There would be no such thing as a
      centralized model that uses multiple PCEs.

   4) The PCE may or may not be located at the head-end of the path.
      For example, a conventional intra-domain solution is to have path
      computation performed by the head-end LSR of an MPLS TE LSP; in
      this case, the head-end LSR contains a PCE.  But solutions also
      exist where other nodes on the path must contribute to the path
      computation (for example, loose hops), making them PCEs in their
      own right.  At the same time, the path computation may be made by
      some other PCE physically distinct from the computed path.

   5) The path computed by the PCE may be an "explicit path" (that is,
      the full explicit path from start to destination, made of a list
      of strict hops) or a "strict/loose path" (that is, a mix of strict
      and loose hops comprising at least one loose hop representing the
      destination), where a hop may be an abstract node such as an AS.

   6) A PCE-based path computation model does not mean to be exclusive
      and can be used in conjunction with other path computation models.
      For instance, the path of an inter-AS TE LSP may be computed using



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      a PCE-based path computation model in some ASes, whereas the set
      of traversed ASes may be specified by other means (not determined
      by a PCE).  Furthermore, different path computation models may be
      used for different TE LSPs.

   7) This document does not make any assumptions about the nature or
      implementation of a PCE.  A PCE could be implemented on a router,
      an LSR, a dedicated network server, etc.  Moreover, the PCE
      function is orthogonal to the forwarding capability of the node on
      which it is implemented.

4.  Motivation for a PCE-Based Architecture

   Several motivations for a PCE-based architecture (described in
   Section 5) are listed below.  This list is not meant to be exhaustive
   and is provided for the sake of illustration.

   It should be highlighted that the aim of this section is to provide
   some application examples for which a PCE-based path may be suitable:
   this also clearly states that such a model does not aim to replace
   existing path computation models but would apply to specific existing
   or future situations.

   As can be seen from these examples, PCE does not replace the existing
   Internet model where intelligence is distributed within the network.
   Instead, it builds on this model and makes use of distributed centers
   of information or computational ability.  PCE should not, therefore,
   necessarily be seen as a centralized, "all-seeing oracle in the sky",
   but as the cooperative operation of distributed functionality used to
   address specific challenges such as the computation of a shortest
   inter-domain constrained path.

4.1.  CPU-Intensive Path Computation

   There are many situations where the computation of a path may be
   highly CPU-intensive; examples of CPU-intensive path computations
   include the resolution of problems such as:

   - Placing a set of TE LSPs within a domain so as to optimize an
     objective function (for example, minimization of the maximum link
     utilization)

   - Multi-criteria path computation (for example, delay and link
     utilization, inclusion of switching capabilities, adaptation
     features, encoding types and optical constraints within a GMPLS
     optical network)





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   - Computation of minimal cost Point to Multipoint trees (Steiner
     trees)

   In these situations, it may not be possible or desirable for some
   routers to perform path computation because of the constraints on
   their CPUs, in which case the path computations may be off-loaded to
   some other PCE(s) that may, themselves, be routers or may be
   dedicated PCE servers.

4.2.  Partial Visibility

   There are several scenarios where the node responsible for path
   computation has limited visibility of the network topology to the
   destination.  This limitation may occur, for instance, when an
   ingress router attempts to establish a TE LSP to a destination that
   lies in a separate domain, since TE information is not exchanged
   across the domain boundaries.  In such cases, it is possible to use
   loose routes to establish the TE LSP, relying on routers at the
   domain borders to establish the next piece of the path.  However, it
   is not possible to guarantee that the optimal (shortest) path will be
   used, or even that a viable path will be discovered except, possibly,
   through repeated trial and error using crankback or other signaling
   extensions.

   This problem of inter-domain path computation may most probably be
   addressed through distributed computation with cooperation among PCEs
   within each of the domains, and potentially using crankback between
   the domains to dynamically resolve provisioning issues.
   Alternatively, a central "all-seeing" PCE that has access to the
   complete set of topology information may be used, but in this case
   there are challenges of scalability (both the size of the TED and the
   responsiveness of a single PCE handling requests for many domains)
   and of preservation of confidentiality when the domains belong to
   different Service Providers.

   Note that the issues described here can be further highlighted in the
   context of TE LSP reoptimization, or the establishment of multiple
   diverse TE LSPs for protection or load sharing.

4.3.  Absence of the TED or Use of Non-TE-Enabled IGP

   The traffic engineering database (TED) may be a large drain on the
   resources of a network node (such as an edge router or LER).
   Maintaining the TED may require a lot of memory and may require non-
   negligible CPU activity.  The use of a distinct PCE may be
   appropriate in such circumstances, and a separate node can be used to
   establish and maintain the TED, and to make it available for path
   computation.



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   The IGPs run within some networks are not sufficient to build a full
   TED.  For example, a network may run OSPF/IS-IS without the
   OSPF-TE/ISIS-TE extensions, or some routers in the network may not
   support the TE extensions.  In these cases, in order to successfully
   compute paths through the network, the TED must be constructed or
   supplemented through configuration action and updated as network
   resources are reserved or released.  Such a TED could be distributed
   to the routers that need to perform path computation or held
   centrally (on a distinct node that supports PCE) for centralized
   computation.

4.4.  Node Outside the Routing Domain

   An LER might not be part of the routing domain for administrative
   reasons (for example, a customer-edge (CE) router connected to the
   provider-edge (PE) router in the context of MPLS VPN [RFC4364] and
   for which it is desired to provide a CE to CE TE LSP path).

   This scenario suggests a solution that does not involve doing
   computation on the ingress (TE LSP head-end, CE) router, and that
   does not rely on the configuration of static loose hops.  In this
   case, optimal shortest paths cannot be guaranteed.  A solution that a
   distinct PCE can help here.  Note that the PCE in this case may,
   itself, provide a path that includes loose hops.

4.5.  Network Element Lacks Control Plane or Routing Capability

   It is common in legacy optical networks for the network elements not
   to have a control plane or routing capability.  Such network elements
   only have a data plane and a management plane, and all cross-
   connections are made from the management plane.  It is desirable in
   this case to run the path computation on the PCE, and to send the
   cross-connection commands to each node on the computed path.  That
   is, the PCC would be an element of the management plane, perhaps
   residing in the Network Management System (NMS) or Operations Support
   System (OSS).

   This scenario is important for Automatically Switched Optical Network
   (ASON)-capable networks and may also be used for interworking between
   GMPLS-capable and GMPLS-incapable networks.

4.6.  Backup Path Computation for Bandwidth Protection

   A PCE can be used to compute backup paths in the context of fast
   reroute protection of TE LSPs.  In this model, all backup TE LSPs
   protecting a given facility are computed in a coordinated manner by a
   PCE.  This allows complete bandwidth sharing between backup tunnels
   protecting independent elements, while avoiding any extensions to TE



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   LSP signaling.  Both centralized and distributed computation models
   are applicable.  In the distributed case each LSR can be a PCE to
   compute the paths of backup tunnels to protect against the failure of
   adjacent network links or nodes.

4.7.  Multi-layer Networks

   A server-layer network of one switching capability may support
   multiple networks of another (more granular) switching capability.
   For example, a Time-Division Multiplexing (TDM) network may provide
   connectivity for client-layer networks such as IP, MPLS, or Layer 2
   [MLN].

   The server-layer network is unlikely to provide the same connectivity
   paradigm as the client networks, so bandwidth granularity in the
   server-layer network may be much coarser than in the client-layer
   network.  Similarly, there is likely to be a management separation
   between the two networks providing independent address spaces.
   Furthermore, where multiple client-layer networks make use of the
   same server-layer network, those client-layer networks may have
   independent policies, control parameters, address spaces, and routing
   preferences.

   The different client- and server-layer networks may be considered
   distinct path computation regions within a PCE domain, so the PCE
   architecture is useful to allow path computation from one client-
   layer network region, across the server-layer network, to another
   client-layer network region.

   In this case, the PCEs are responsible for resolving address space
   issues, handling differences in policy and control parameters, and
   coordinating resources between the networks.  Note that, because of
   the differences in bandwidth granularity, connectivity across the
   server-layer network may be provided through virtual TE links or
   Forwarding Adjacencies: the PCE may offer a point of control
   responsible for the decision to provision new TE links or Forwarding
   Adjacencies across the server-layer network.

4.8.  Path Selection Policy

   A PCE may have a local policy that impacts path computation and
   selection in response to a path computation request.  Such policy may
   act on information provided by the requesting PCC.  The result of
   applying such policy includes, for example, rejection of the path
   computation request, or provision of a path that does not meet all of
   the requested constraints.  Further, the policy may support





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   administratively configured paths, or selection among transit
   providers.  Inclusion of policy within PCE may simplify the
   application of policy within the path computation/selection process.

   Similarly, a PCC may apply local policy to the selection of a PCE to
   compute a specific path, and to the constraints that are requested.

   In a PCE context, the policy may be sensitive to the type of path
   that is being computed.  For example, a different set of policies may
   be applied for an intra-area or single-layer path than would be
   provided for an inter-area or multi-layer path.

   Note that synchronization of policy between PCEs or between PCCs and
   PCEs may be necessary.  Such issues are outside the scope of the PCE
   architecture, but within scope for the PCE policy framework and
   application which is described in a separate document.

4.9.  Non-Motivations

4.9.1.  The Whole Internet

   PCE is not considered to be a solution that is applicable to the
   entire Internet.  That is, the applicability of PCE is limited to a
   set of domains with known relationships.  The scale of this
   limitation is similar to the peering relationships between Service
   Providers.

4.9.2.  Guaranteed TE LSP Establishment

   When two or more paths for TE LSPs are computed on the same set of TE
   link state information, it is possible that the resultant paths will
   compete for limited resources within the network.  This may result in
   success for only the first TE LSP to be signaled, or it might even
   mean that no TE LSP can be established.

   Batch processing of computation requests, back-off times, computation
   of alternate paths, and crankback can help to mitigate this sort of
   problem, and PCE may also improve the chances of successful TE LSP
   setup.  However, a single, centralized PCE is not viewed as a
   solution that can guarantee TE LSP establishment since the potential
   for network failures or contention for resources still exists where
   the centralized TED cannot fully reflect current (i.e., real-time)
   network state.








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5.  Overview of the PCE-Based Architecture

   This section gives an overview of the architecture of the PCE model.
   It needs to be read in conjunction with the details provided in the
   next section to provide a full view of the flexibility of the model.

5.1.  Composite PCE Node

   Figure 1 below shows the components of a typical composite PCE node
   (that is, a router that also implements the PCE functionality) that
   utilizes path computation.  The routing protocol is used to exchange
   TE information from which the TED is constructed.  Service requests
   to provision TE LSPs are received by the node and converted into
   signaling requests, but this conversion may require path computation
   that is requested from a PCE.  The PCE operates on the TED subject to
   local policy in order to respond with the requested path.

                 ---------------
                |   ---------   | Routing   ----------
                |  |         |  | Protocol |          |
                |  |   TED   |<-+----------+->        |
                |  |         |  |          |          |
                |   ---------   |          |          |
                |      |        |          |          |
                |      | Input  |          |          |
                |      v        |          |          |
                |   ---------   |          |          |
                |  |         |  |          | Adjacent |
                |  |   PCE   |  |          |   Node   |
                |  |         |  |          |          |
                |   ---------   |          |          |
                |      ^        |          |          |
                |      |Request |          |          |
                |      |Response|          |          |
                |      v        |          |          |
                |   ---------   |          |          |
       Service  |  |         |  | Signaling|          |
        Request |  |Signaling|  | Protocol |          |
          ------+->| Engine  |<-+----------+->        |
                |  |         |  |          |          |
                |   ---------   |           ----------
                 ---------------

                    Figure 1.  Composite PCE Node

   Note that the routing adjacency between the composite PCE node and
   any other router may be performed by means of direct connectivity or
   any tunneling mechanism.



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5.2.  External PCE

   Figure 2 shows a PCE that is external to the requesting network
   element.  A service request is received by the head-end node, and
   before it can initiate signaling to establish the service, it makes a
   path computation request to the external PCE.  The PCE uses the TED
   subject to local policy as input to the computation and returns a
   response.

               ----------
              |  -----   |
              | | TED |<-+----------->
              |  -----   |  TED synchronization
              |    |     |  mechanism (for example, routing protocol)
              |    |     |
              |    v     |
              |  -----   |
              | | PCE |  |
              |  -----   |
               ----------
                   ^
                   | Request/
                   | Response
                   v
      Service  ----------  Signaling   ----------
      Request | Head-End | Protocol   | Adjacent |
         ---->|  Node    |<---------->|   Node   |
               ----------              ----------

                    Figure 2.  External PCE Node

   Note that in this case, the node that supports the PCE function may
   also be an LSR or router performing forwarding in its own right
   (i.e., it may be a composite PCE node), but those functions are
   purely orthogonal to the operation of the function in the instance
   being considered here.















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5.3.  Multiple PCE Path Computation

   Figure 3 illustrates how multiple PCE path computations may be
   performed along the path of a signaled service.  As in the previous
   example, the head-end PCC makes a request to an external PCE, but the
   path that is returned is such that the next network element finds it
   necessary to perform further computation.  This may be the case when
   the path returned is a partial path that does not reach the intended
   destination or when the computed path is loose.  The downstream
   network element consults another PCE to establish the next hop(s) in
   the path.  In this case, all policy decisions are made independently
   at each PCE based on information passed from the PCC.

   Note that either or both PCEs in this case could be composite PCE
   nodes, as in Section 5.1.

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

                 Figure 3.  Multiple PCE Path Computation

















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5.4.  Multiple PCE Path Computation with Inter-PCE Communication

   The PCE in Section 5.3 was not able to supply a full path for the
   requested service, and as a result the adjacent node needs to make
   its own computation request.  As illustrated in Figure 4, the same
   problem may be solved by introducing inter-PCE communication, and
   cooperation between PCEs so that the PCE consulted by the head-end
   network node makes a request of another PCE to help with the
   computation.

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

   Figure 4.  Multiple PCE Path Computation with Inter-PCE Communication

   Multiple PCE path computation with inter-PCE communication involves
   coordination between distinct PCEs such that the result of the
   computation performed by one PCE depends on path fragment information
   supplied by other PCEs.  This model does not provide a distributed
   computation algorithm, but it allows distinct PCEs to be responsible
   for computation of parts (segments) of the path.

   PCE-PCE communication is discussed further in Section 6.6.

   Note that a PCC might not see the difference between centralized
   computation and multiple PCE path computation with inter-PCE
   communication.  That is, the PCC network node or component that
   requests the computation makes a single request and receives a full
   or partial path in response, but the response is actually achieved
   through the coordinated, cooperative efforts of more than one PCE.







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   In this model, all policy decisions may be made independently at each
   PCE based on computation information passed from the previous PCE.
   Alternatively, there may be explicit communication of policy
   information between PCEs.

5.5.  Management-Based PCE Usage

   It must be observed that the PCC is not necessarily an LSR.  For
   example, in Figure 5 the NMS supplies the head-end LSR with a fully
   computed explicit path for the TE LSP that it is to establish through
   signaling.  The NMS uses a management plane mechanism to send this
   request and encodes the data using a representation such as the TE
   MIB module [RFC3812].

   The NMS constructs the explicit path that it supplies to the head-end
   LSR using information provided by the operator.  It consults the PCE,
   which returns a path for the NMS to use.

   Although Figure 5 shows the PCE as remote from the NMS, it could, of
   course, be collocated with the NMS.

                                 -----------
                                |   -----   |
            Service             |  | TED |<-+----------->
            Request             |   -----   |  TED synchronization
               |                |     |     |  mechanism (for example,
               v                |     |     |  routing protocol)
         ------------- Request/ |     v     |
        |             | Response|   -----   |
        |     NMS     |<--------+> | PCE |  |
        |             |         |   -----   |
         -------------           -----------
       Service |
       Request |
               v
          ----------  Signaling   ----------
         | Head-End | Protocol   | Adjacent |
         |  Node    |<---------->|   Node   |
          ----------              ----------

                 Figure 5.  Management-Based PCE Usage










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5.6.  Areas for Standardization

   The following areas require standardization within the PCE
   architecture.

   - communication between PCCs and PCEs, and between cooperating PCEs,
     including the communication of policy-related information

   - requirements for extending existing routing and signaling protocols
     in support of PCE discovery and signaling of inter-domain paths

   - definition of metrics to evaluate path quality, scalability,
     responsiveness, robustness, and policy support of path computation
     models.

   - MIB modules related to communication protocols, routing and
     signaling extensions, metrics, and PCE monitoring information

6.  PCE Architectural Considerations

   This section provides a list of the PCE architectural components.
   Specific realizations and implementation details (state machines or
   algorithms, etc.) of PCE-based solutions are out of the scope of this
   document.

   Note also that PCE-based path computation does not affect in any way
   the use of the computed paths.  For example, the use of PCE does not
   change the way in which Traffic Engineering LSPs are signaled,
   maintained, and torn down, but it strictly relates to the path
   computation aspects of such TE LSPs.

   This section presents an architectural view of PCE.  That is, it
   describes the components that exist and how they interact.  Note that
   the architectural model, and in particular the functional model, may
   be perceived differently by different components of the PCE system.
   For example, the PCC will not be aware of whether a PCE consults
   other PCEs.  The PCC view of the PCE architecture is discussed in
   Section 7.

6.1.  Centralized Computation Model

   A "centralized computation model" considers that all path
   computations for a given domain will be performed by a single,
   centralized PCE.  This may be a dedicated server (for example, an
   external PCE node), or a designated router (for example, a composite
   PCE node) in the network.  In this model, all PCCs in the domain
   would send their path computation requests to the central PCE.  While




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   a domain in this context might be an IGP area or AS, it might also be
   a sub-group of network nodes that is defined by its dependence on the
   PCE.

   This model has a single point of failure: the PCE.  In order to avoid
   this issue, the centralized computation model may designate a backup
   PCE that can take over the computation responsibility in a controlled
   manner in the event of a failure of the primary PCE.  Any policies
   present on the primary PCE should also be present on the backup,
   although the primary policies may themselves be subject to policy
   governing how they are implemented on the backup.  Note that at any
   moment in time there is only one active PCE in any domain.

6.2.  Distributed Computation Model

   A "distributed computation model" refers to a domain or network that
   may include multiple PCEs, and where computation of paths is shared
   among the PCEs.  A given path may in turn be computed by a single PCE
   ("single PCE path computation") or multiple PCEs ("multiple PCE path
   computation").  A PCC may be linked to a particular PCE or may be
   able to choose freely among several PCEs; the method of choice
   between PCEs is out of scope of this document, but see Section 6.4
   for a discussion of PCE discovery that affects this choice.
   Implementation of policy should be consistent across the set of
   available PCEs.

   Often, the computation of an individual path is performed entirely by
   a single PCE.  For example, this is usually the case in MPLS TE
   within a single IGP area where the ingress LSR/composite PCE node is
   responsible for computing the path or for contacting an external PCE.
   Conversely, multiple PCE path computation implies that more than one
   PCE is involved in the computation of a single path.  An example of
   this is where loose hop expansion is performed by transit
   LSRs/composite PCE nodes on an MPLS TE LSP.  Another example is the
   use of multiple cooperating PCEs to compute the path of a single TE
   LSP across multiple domains.

6.3.  Synchronization

   Often, multiple paths need to be computed to support a single service
   (for example, for protection or load sharing).  A PCC that determines
   that it requires more than one path to be computed may send a series
   of individual requests to the PCE.  In this case of non-synchronized
   path computation requests, the PCE may make multiple individual path
   computations to generate the paths, and the PCC may send its
   individual requests to different PCEs.





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   Alternatively, the PCC may send a single request to a PCE asking for
   a set of paths to be computed, but specifying that non-synchronized
   path computation is acceptable.  The PCE may compute each path in
   turn exactly as it would have done had the PCC made multiple
   requests, and the PCE may devolve some computations to other PCEs if
   it chooses.  On the other hand, the PCE is not prohibited from
   performing all computations together in a synchronized manner as
   described below.

   The PCC may also issue a single request to the PCE asking for all the
   paths to be computed in a synchronized manner.  The PCE will then
   perform simultaneous computation of the set of requested paths.  Such
   synchronized computation can often provide better results.

   The involvement of more than one PCE in the computation of a series
   of paths is by its nature non-synchronized.  However, a set of
   cooperating PCEs may be synchronized under the control of a single
   PCE.  For example, a PCC may send a request to a PCE that invokes
   domain-specific computations by other PCEs before supplying a result
   to the PCC.

   It is desirable to add a parameter to the PCC-PCE protocol to request
   that the PCE supply a set of alternate paths for use by the PCC,
   should the establishment of the TE LSP using the principal path fail
   to complete.  While alternate paths may not always be successful if
   the first path fails, including alternate paths in a PCE response
   could have less overhead than having the PCC make separate requests
   for subsequent path computations as the need arises.  This technique
   is used in some existing CSPF implementations.

6.4.  PCE Discovery and Load Balancing

   In order that a PCC can communicate efficiently with a PCE, it must
   know the location of the PCE.  That is, it is an architectural
   decision made here that PCC requests be targeted to a specific PCE,
   and not broadcast to the network for any PCE to respond.  This
   decision means that only the selected PCE will operate on any single
   request, and it saves network resources during request propagation
   and processing resources at the PCEs that are not required to
   respond.

   The knowledge of the location of a PCE may be achieved through local
   configuration at the PCC or may rely on a protocol-based discovery
   mechanism that may be governed by policy.

   Where more than one PCE is known to a PCC, the PCC must have
   sufficient information to select an appropriate PCE for its purposes,
   under the control of policy.  Such a selection procedure allows for



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   load sharing between PCEs and supports PCEs with different
   computation capabilities including different visibility scopes.
   Thus, the information available to the PCC must include details of
   the PCE capabilities, which may be fixed or may vary dynamically in
   time.

   The PCC may learn PCE capabilities through static configuration, or
   it may discover the information dynamically.  Note that even when the
   location of the PCE is configured at the PCC, the PCC may still
   discover the PCE capabilities dynamically.  Dynamic PCE capabilities
   cannot be configured and can only be discovered.

   Proxy PCE advertisement whereby the existence of a PCE is advertised
   via a proxy PCE is a viable alternative, should the PCE be incapable
   of such advertisement itself.  In this case, it is a requirement that
   the proxy adequately advertise the PCE status and capability in a
   timely and synchronized fashion.

   In the event that multiple PCEs are available to serve a particular
   path computation request, the PCC must select a PCE to satisfy the
   request.  The details of such a selection (for instance, to
   efficiently share the computation load across multiple PCEs or to
   request secondary computations after partial or failed computations)
   are local to the PCC, may be based on policy, and are out of the
   scope of this document.

   PCE capabilities that may be advertised or configured could include
   (and are not be limited to):

   - a set of constraints that it can account for (diversity, shared
     risk link groups (SRLGs), optical impairments, wavelength
     continuity, etc.)

   - computational capacity (for example, the number of computations it
     can perform per second)

   - the number of switching capability layers (and which ones)

   - the number of path selection criteria (and which ones)

   - whether it is a stateless PCE or it can send updates about better
     paths that might be available in the future

   - whether it can compute P2MP trees (and which types)

   - whether it can ensure resource sharing between backup tunnels

   This information would help a PCC to decide which PCE to use.



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   Requirements for PCE advertisement will be documented separately.
   Note that there is no restriction within the architecture about how
   location and capabilities are advertised, and the two elements should
   be considered functionally distinct.

   A PCC might also ask a PCE to perform a particular type of service
   without knowledge of the PCE's capabilities and receive a response
   that says that the PCE is unable to perform the service.  The
   response could specify the capabilities of the PCE and might also
   suggest another PCE that has the requested capabilities.

6.5.  Detecting PCE Liveness

   The ability to detect a PCE's liveness is a mandatory piece of the
   overall architecture and could be achieved by several means.  If some
   form of regular advertisement (such as through IGP extensions) is
   used for PCE discovery, it is expected that the PCE liveness will be
   determined by means of status advertisement (for example, IGP
   LSA/LSPs).

   The inability of a PCE to service a request (perhaps due to excessive
   load) may be reported to the PCC through a failure message, but the
   failure of a PCE or the communications mechanism while processing a
   request cannot be reported in this way.  Furthermore, in the case of
   excessive load, the PCE may not have sufficient resources to send a
   failure message.  Thus, the PCC should employ other mechanisms, such
   as protocol timers, to determine the liveness of the PCE.  This is
   particularly important in the case of inter-domain path computation
   where the PCE liveness may not be detected by means of the IGP that
   runs in the PCC's domain.

6.6.  PCC-PCE and PCE-PCE Communication

   Once the PCC has selected a PCE, and provided that the PCE is not
   local to the PCC, a request/response protocol is required for the PCC
   to communicate the path computation requests to the PCE and for the
   PCE to return the path computation response.  Discussion of the
   security requirements and implications for this protocol is provided
   in Section 10 of this document.

   The path computation request may include a significant set of
   requirements, including the following:

   - the source and destination of the path

   - the bandwidth and other Quality of Service (QoS) parameters desired





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   - resources, resource affinities, and shared risk link groups (SRLGs)
     to use/avoid

   - the number of disjoint paths required and whether near-disjoint
     paths are acceptable

   - the levels of resiliency, reliability, and robustness of the path
     resources

   - policy-related information

   The level of robustness of the path resources covers a qualitative
   assessment of the vulnerability of the resources that may be used.
   For example, one might grade resources based on empirical evidence
   (mean time between failures), on known risks (there is major building
   work going on near this conduit), or on prejudice (vendor X's
   software is always crashing).  A PCC could request that only robust
   resources be used, or it could allow any resource.

   In case of a positive response from the PCE, one or more paths would
   be returned to the requesting node.  In the event of a failure to
   compute the desired path(s), an error is returned together with as
   much information as possible about the reasons for the failure(s),
   and potentially with advice about which constraints might be relaxed
   so that a positive result is more likely in a future request.

   Note that the resultant path(s) may be made up of a set of strict or
   loose hops, or any combination of strict and loose hops.  Moreover, a
   hop may have the form of a non-explicit abstract node.

   A request/response protocol is also required for a PCE to communicate
   path computation requests to another PCE and for the PCE to return
   the path computation response.  The path computation request may
   include a significant set of requirements including those defined
   above.  In case of a positive response from the PCE, one or more
   paths would be returned to the requesting PCE.  In the event of a
   failure to compute the desired path(s), an error is returned together
   with as much information as possible about the reasons for the
   failure, and potentially advice about which constraints might be
   relaxed so that a positive result is more likely.  Note that the
   resultant path(s) may be made up of a set of strict or loose hops, or
   any combination of strict and loose hops.  Moreover, a hop may have
   the form of a non-explicit abstract node.

   An important feature of PCEs that are cooperating to compute a path
   is that they apply compatible or identical computation algorithms and
   coordinated policies.  This may require coordination through the
   communication between the PCEs.



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   Note that when multiple PCEs cooperate to compute a path, it is
   important that they have a coordinated view of the meaning of
   constraints such as costs, resource affinities, and class of service.
   This is particularly significant where the PCEs are responsible for
   different domains.  It is assumed that this is a matter of policy
   between domains and between PCEs.

   No assumption is made in this architecture about whether the PCC-PCE
   and PCE-PCE communication protocols are identical.

6.7.  PCE TED Synchronization

   As previously described, the PCE operates on a TED.  Information on
   network status to build the TED may be provided in the domain by
   various means:

   1) Participation in IGP distribution of TE information.  The standard
      method of distribution of TE information within an IGP area is
      through the use of extensions to the IGP [RFC3630, RFC3748].  This
      mechanism allows participating nodes to build a TED, and this is
      the standard technique, for example, within a single area MPLS or
      GMPLS network.  A node that hosts the PCE function may collect TE
      information in this way by maintaining at least one routing
      adjacency with a router in the domain.  The PCE node may be
      adjacent or non-adjacent (via some tunneling techniques) to the
      router.  Such a technique provides a mechanism for ensuring that
      the TED is efficiently synchronized with the network state and is
      the normal case, for example, when the PCE is co-resident with the
      LSRs in an MPLS or GMPLS network.

   2) Out-of-band TED synchronization.  It may not be convenient or
      possible for a PCE to participate in the IGPs of one or more
      domains (for example, when there are very many domains, when IGP
      participation is not desired, or when some domains are not running
      TE-aware IGPs).  In this case, some mechanism may need to be
      defined to allow the PCE node to retrieve the TED from each
      domain.  Such a mechanism could be incremental (like the IGP in
      the previous case), or it could involve a bulk transfer of the
      complete TED.  The latter might significantly limit the capability
      to ensure TED synchronization, which might result in an increase
      in the failure rate of computed paths, or the computation of sub-
      optimal paths.  Consideration should also be given to the impact
      of the TED distribution on the network and on the network node
      within the domain that is asked to distribute the database.  This
      is particularly relevant in the case of frequent network state
      changes.





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   3) Information in the TED can include information obtained from
      sources other than the IGP.  For example, information about link
      usage policies can be configured by the operator.  Path
      computation can also act on a far wider set of information that
      includes data about the TE LSPs provisioned within the network.
      This information can include TE LSP routes, reserved bandwidth,
      and measured traffic volume passing through the TE LSP.

      Such TE LSP information can enhance TE LSP (re)optimization to
      provide "full network" (re)optimization and can allow traffic
      fluctuations to be taken into account.  Detailed TE LSP
      information may also facilitate reconfiguration of the Virtual
      Network Topology (VNT) [MLN], in which lower-layer TE LSPs, such
      as optical paths, provide TE links for use by the higher layer,
      since this reconfiguration is also a "full network" problem.

   Note that synchronization techniques may apply to both intra- and
   inter-domain TEDs.  Furthermore, the techniques can be mixed for use
   in different domains.  The degree of synchronization between the PCE
   and the network is subject to implementation and/or policy.  However,
   better synchronization generally leads to paths that are more likely
   to succeed.

   Note also that the PCE may have access to only a partial TED: for
   instance, in the case of inter-domain path computation where each
   such domain may be managed by different entities.  In such cases,
   each PCE may have access to a partial TED, and cooperative techniques
   between PCEs may be used to achieve end-to-end path computation
   without any requirement that any PCE handle the complete TED related
   to the set of traversed domains by the TE LSP in question.

6.8.  Stateful versus Stateless PCEs

   A PCE can be either stateful or stateless.  In the former case, there
   is a strict synchronization between the PCE and not only the network
   states (in term of topology and resource information), but also the
   set of computed paths and reserved resources in use in the network.
   In other words, the PCE utilizes information from the TED as well as
   information about existing paths (for example, TE LSPs) in the
   network when processing new requests.  Note that although this allows
   for optimal path computation and increased path computation success,
   stateful PCEs require reliable state synchronization mechanisms, with
   potentially significant control plane overhead and the maintenance of
   a large amount of data/states (for example, full mesh of TE LSPs).

   For example, if there is only one PCE in the domain, all TE LSP
   computation is done by this PCE, which can then track all the
   existing TE LSPs and stay synchronized (each TE LSP state change must



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   be tracked by the PCE).  However, this model could require
   substantial control plane resources.  If there are multiple PCEs in
   the network, TE LSP computation and information are distributed among
   PCEs and so the resources required to perform the computations are
   also distributed.  However, synchronization issues discussed in
   Section 6.7 also come into play.

   The maintenance of a stateful database can be non-trivial.  However,
   in a single centralized PCE environment, a stateful PCE is almost a
   simple matter of remembering all the TE LSPs the PCE has computed,
   that the TE LSPs were actually set up (if this can be known), and
   when they were torn down.  Out-of-band TED synchronization can also
   be complex, with multiple PCE setup in a distributed PCE computation
   model, and could be prone to race conditions, scalability concerns,
   etc.  Even if the PCE has detailed information on all paths,
   priorities, and layers, taking such information into account for path
   computation could be highly complex.  PCEs might synchronize state by
   communicating with each other, but when TE LSPs are set up using
   distributed computation performed among several PCEs, the problems of
   synchronization and race condition avoidance become larger and more
   complex.

   There is benefit in knowing which TE LSPs exist, and their routing,
   to support such applications as placing a high-priority TE LSP in a
   crowded network such that it preempts as few other TE LSPs as
   possible (also known as the "minimal perturbation" problem).  Note
   that preempting based on the minimum number of links might not result
   in the smallest number of TE LSPs being disrupted.  Another
   application concerns the construction and maintenance of a Virtual
   Network Topology [MLN].  It is also helpful to understand which other
   TE LSPs exist in the network in order to decide how to manage the
   forward adjacencies that exist or need to be set up.  The cost-
   benefit of stateful PCE computation would be helpful to determine if
   the benefit in path computation is sufficient to offset the
   additional drain on the network and computational resources.

   Conversely, stateless PCEs do not have to remember any computed path
   and each set of request(s) is processed independently of each other.
   For example, stateless PCEs may compute paths based on current TED
   information, which could be out of sync with actual network state
   given other recent PCE-computed paths changes.  Note that a PCC may
   include a set of previously computed paths in its request, in order
   to take them into account, for instance, to avoid double bandwidth
   accounting or to try to minimize changes (minimum perturbation
   problem).






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   Note that the stateless PCE does operate on information about network
   state.  The TED contains link state and bandwidth availability
   information as distributed by the IGPs or collected through some
   other means.  This information could be further enhanced to provide
   increased granularity and more detail to cover, for example, the
   current bandwidth usage on certain links according to resource
   affinities or forwarding equivalence classes.  Such information is,
   however, not PCE state information and so a model that uses it is
   still described as stateless in the PCE context.

   A limited form of statefulness might be applied within an otherwise
   stateless PCE.  The PCE may retain some context from paths it has
   recently computed so that it avoids suggesting the use of the same
   resources for other TE LSPs.

6.9.  Monitoring

   PCE monitoring is undoubtedly of the utmost importance in any PCE
   architecture.  This must include the collection of variables related
   to the PCE status and operation.  For example, it will be necessary
   to understand the way in which the TED is being kept synchronized,
   the rate of arrival of new requests and the computation times, the
   range of PCCs that are using the PCE, and the operation of any PCC-
   PCE protocol.

6.10.  Confidentiality

   As stated in [RFC4216], the case of inter-provider TE LSP computation
   requires the ability to compute a path while preserving
   confidentiality across multiple Service Providers cores.  That is,
   one Service Provider must not be required to divulge any information
   about its resources or topology in order to support inter-provider TE
   LSP path computation.  Thus, any PCE architecture solution must
   support the ability to return partial paths by means of loose hops
   (for example, where each loose hop would, for instance, identify a
   boundary LSR).

   This requirement is not a security issue, but relates to Service
   Provider policy.  Confidentiality, integrity, and authentication of
   PCC-PCE and PCE-PCE messages must also be ensured and are described
   in Section 10.

   The ability to compute a path at the request of the head-end PCC, but
   to supply the path in segments to the domain boundary PCCs, may also
   be desirable.






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6.11.  Policy

   Policy impacts multiple aspects of the PCE architecture.  There are
   two applications of policy for consideration:

   - application of policy within an architectural entity (PCC or PCE)

   - application of policy to PCE-related communications

   As directly applicable to TE LSPs, policy forms part of the signaling
   mechanism for the establishment of the TE LSPs and is not described
   here.

   It is envisioned that policy will be largely applied as a local
   matter within each PCC and PCE.  However, this document needs to
   define policy models that can be supported within the PCE
   architecture and by PCE-related communication.

   Some example policies include:

   - selection of a PCE by a PCC

   - rejection of a request by the PCE based on the identity of the
     requesting PCC

   - selection by the PCE of a path or application of additional
     constraints to a computation based on the PCC, the computation
     target, the time of day, etc.

6.11.1.  PCE Policy Architecture

   Two examples of the use of policy components within the PCE
   architecture are illustrated in Figures 6 and 7.  Policy components
   could equally be applied to the other PCE configurations shown in
   Section 5.  In each configuration, policy may be consulted before a
   response is provided by a PCE and may also be consulted by the
   PCC/PCE that receives the response.

   A PCE may have a local policy that impacts the paths selected to
   satisfy a particular PCE request.  A policy may be applied based on
   any information provided from a PCC.

   In Figure 6, the policy component is shown providing input to the PCE
   component.  This policy component may consult an external policy
   database, but this is outside the scope of this document.






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              ------------------------------
             |                  ---------   | Routing   ----------
             |                 |         |  | Protocol |          |
             |                 |   TED   |<-+----------+->        |
             |                 |         |  |          |          |
             |                  ---------   |          |          |
             |                     |        |          |          |
             |                     | Input  |          |          |
             |                     v        |          |          |
             |   ---------      ---------   |          |          |
             |  | Policy  |    |         |  |          | Adjacent |
             |  |Component|--->|   PCE   |  |          |   Node   |
             |  |         |    |         |  |          |          |
             |   ---------      ---------   |          |          |
             |                     ^        |          |          |
             |                     |Request |          |          |
             |                     |Response|          |          |
             |                     v        |          |          |
             |                  ---------   |          |          |
    Service  |                 |         |  | Signaling|          |
     Request |                 |Signaling|  | Protocol |          |
       ------+---------------->| Engine  |<-+----------+->        |
             |                 |         |  |          |          |
             |                  ---------   |           ----------
              ------------------------------

            Figure 6.  Policy Component in the Composite PCE Node

   Note that policy information may be conveyed on the internal
   interfaces, and on the external protocol interfaces.

   Figure 7 displays the case of a distinct PCE function through the
   example of the multiple PCE with inter-PCE communication example
   (compare with Figure 4).  Each PCE takes input from local policy as
   part of the router computation/determination process.  The local
   policy components may consult external policy components or
   databases, but that is out of the scope of this document.

   Note that policy information may be conveyed on the external protocol
   interfaces, including the inter-PCE interface.











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      ------------------                             ------------------
     |                  | 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 7.  Policy Components in Multiple PCEs

6.11.2.  Policy Realization

   There are multiple options for how policy information is coordinated.

   - Policy decisions may be made by PCCs before consulting PCEs.  This
     type of decision includes selection of PCE, application of
     constraints, and interpretation of service requests.

   - Policy decisions may be made independently at a PCE, or at each
     cooperating PCE.  That is, the PCE(s) may make policy decisions
     independent of other policy decisions made at PCCs or other PCEs.

   - There may also be explicit communication of policy information
     between PCC and PCE, or between PCEs to achieve some level of
     coordination of policy between entities.  The type of information
     conveyed to support policy has important implications on what
     policies may be applied at each PCE, and the requirements for the
     exchange of policy information inform the choice or implementation
     of communication protocols including PCC-PCE, PCE-PCE, and
     discovery protocols.

6.11.3.  Type of Policies

   Within the context of PCE, we identify several types of policies:

   o User-specific policies operate on information that is specific to
     the user of a service or the service itself, that is, the service
     for which the path is being computed, not the computation service.
     Examples of such information includes the contents of objects of a



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     signaling or provisioning message, the port ID over which the
     message was received, a VPN ID, a reference point type, or the
     identity of the user initiating the request.  User-specific
     policies could be applied by a PCC while building a path
     computation request, or by a PCE while processing the request
     provided that sufficient information is supplied by the PCC to the
     PCE.

   o Request-specific policies operate on information that is specific
     to a path computation request and is carried in the request.
     Examples of such information include constraints, diversities,
     constraint and diversity relaxation strategies, and optimization
     functions.  Request-specific policies directly affect the path
     selection process because they specify which links, nodes, path
     segments, and/or paths are not acceptable or, on the contrary, may
     be desirable in the resulting paths.

   o Domain-specific policies operate on the identify of the domain in
     which the requesting PCC exists, and upon the identities of the
     domains through which the resulting paths are routed.  These
     policies have the same effect as user-specific policies, with the
     difference that they can be applied to a group of users rather than
     an individual user.  One example of domain-specific policy is a
     restriction on what information a PCE publishes within a given
     domain.  In such a case, PCEs in some domains may advertise just
     their presence, while others may advertise details regarding their
     capabilities, client authentication process, and computation
     resource availability.

6.11.4.  Relationship to Signaling

   When a path for an inter-domain TE LSP is being computed, it is not
   required to consider signaling plane policy.  However, failure to do
   so may result in the TE LSP failing to be established, or being
   assigned fewer resources than intended resulting in a substandard
   service.  Thus, where a PCE invoked by a head-end LSR has visibility
   into other domains, it should be capable of applying policy
   considerations to the computation and should be aware of the inter-
   domain policy agreements.  Where path computation is the result of
   cooperation between PCEs, each of which is responsible for a
   particular domain, the policy issues should, where possible, be
   resolved at the time of computation so that the TE LSP is more likely
   to be signaled successfully.  In this context, policy violation
   during inter-domain TE LSP computation may lead to path computation
   interruption, about which the requester should be notified along with
   the cause.





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6.12.  Unsolicited Interactions

   It may be that the PCC-PCE communications (see Section 6.6) can be
   usefully extended beyond a simple request/response interaction.  For
   example, the PCE and PCC could exchange capabilities using this
   protocol.  Additionally, the protocol could be used to collect and
   report information in support of a stateful PCE.

   Furthermore, it may be the case that a PCE is able to update a path
   that it computed earlier (perhaps in reaction to a change in the
   network or a change in policy), and in this case the PCE-PCC
   communication could support an "unsolicited" path computation message
   to supply this new path to the PCC.  Note, however, that this
   function would require that the PCE retained a record of previous
   computations and had a clear trigger for performing recomputations.
   The PCC would also need to be able to identify the new path with the
   old path and determine whether it should act on the new path.
   Further, the PCC should be able to report the outcome of such path
   changes to the requesting PCE.  Note that the PCE-PCC interaction is
   not a management interaction and the PCC is not obliged to utilize
   any additional path supplied by the PCE.

   These functions fit easily within the architecture described here but
   are left for further discussion within separate requirements
   documents.

6.13.  Relationship with Crankback

   Crankback routing is a mechanism whereby a failure to establish a
   path or a failure of an existing path may be corrected by a new path
   computation and fresh signaling.  Crankback routing relies on the
   distribution of crankback information along with the failure
   notification so that the new computation can be performed avoiding
   the failure or blockage point.

   In the context of PCE, crankback information may be passed back to
   the head-end where the process of computation and signaling can be
   repeated using the failed resource as an exclusion in the computation
   process.  But crankback may be used to attempt to correct the problem
   at intermediate points along the path.  Such crankback recomputation
   nodes are most likely to be domain boundaries where the PCC had
   already invoked a PCE.  Thus, a failure within a domain is reported
   to the ingress domain boundary, which will attempt to compute an
   alternate path across the domain.  Failing this, the problem may be
   reported to the previous domain and communicated to the ingress
   boundary for that domain, which may attempt to select a more





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   successful path either by choosing a different entry point into the
   next domain, or by selecting a route through a different set of
   domains.

7.  The View from the Path Computation Client

   The view of the PCE architecture, and particularly the functional
   model, is subtly different from the PCC's perspective.  This is
   partly because the PCC has limited knowledge of the way in which the
   PCEs cooperate to answer its requests, but depends more on the fact
   that the PCC is concerned with different questions.

   The PCC is interested in the following:

   - Selecting a PCE that is able to promptly provide a computed path
     that meets the supplied constraints.

   - How many computation requests will the PCC have to send? Will the
     desired path be computed by the first PCE contacted (possibly in
     cooperation with other PCEs), or will the PCC have to consult other
     PCEs to fill in gaps in the path?

   - How many other path computations will need to be issued from within
     the network in order to establish the TE LSP?

   This last question might be considered out of scope for the head-end
   LSR, but an important constraint that the PCC may wish to apply is
   that the path should be computed in its entirety and supplied without
   loose hops or non-simple abstract nodes.

   Thus, with its limited perspective, the PCC will see Multiple PCE
   Path Computation (Section 5.3) as important and will distinguish two
   subcases.  The first is as shown in Figure 3 with subsequent
   computation requests made by other PCCs along the path of the TE LSP.
   In the second, multiple computation requests are issued by the head-
   end LSR.  On the other hand, the PCC will not be aware of Multiple
   PCE Path Computation with Inter-PCE Communication (Section 5.4),
   which it will perceive as no different from the simple External PCE
   Node case (Section 5.2).

   The PCC, therefore, will be acutely aware that a Centralized PCE
   Model (Section 6.1) might still require Multiple PCE Path
   Computations with the head-end or subsequent PCCs required to issue
   further requests to the central PCE.  Conversely, the PCC may be
   protected from the Distributed PCE Model (Section 6.2) because the
   first PCE it consults uses inter-PCE communication to achieve a
   complete computation result so that no further computation requests
   are required.



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   These distinctions can be completely classified by determining
   whether the computation response includes all necessary paths, and
   whether those paths are fully explicit (that is, containing only
   strict hops between simple abstract nodes).

8.  Evaluation Metrics

   Evaluation metrics that may be used to evaluate the efficiency and
   applicability of any PCE-based solution are listed below.  Note that
   these metrics are not being used to determine paths, but are used to
   evaluate potential solutions to the PCE architecture.

   - Optimality: The ability to maximize network utilization and
     minimize cost, considering QoS objectives, multiple regions, and
     network layers.  Note that models that require the sequential
     involvement of multiple PCEs (for example, the multiple PCE model
     described in Section 5.3) might create path loops unless careful
     policy is applied.

   - Scalability: The implications of routing, TE LSP signaling, and PCE
     communication overhead, such as the number of messages and the size
     of messages (including LSAs, crankback information, queries,
     distribution mechanisms, etc.).

   - Load sharing: The ability to allow multiple PCEs to spread the path
     computation load by allowing multiple PCEs each to take
     responsibility for a subset of the total path computation requests.

   - Multi-path computation: The ability to compute multiple and
     potentially diverse paths to satisfy load-sharing of traffic and
     protection/restoration needs including end-to-end diversity and
     protection within individual domains.

   - Reoptimization: The ability to perform TE LSP path reoptimization.
     This also includes the ability to perform inter-layer correlation
     when considering the reoptimization at any specific layer.

   - Path computation time: The time to compute individual paths and
     multiple diverse paths and to satisfy bulk path computation
     requests.  (Note that such a metric can only be applied to problems
     that are not NP-complete.)

   - Network stability: The ability to minimize any perturbation on
     existing TE state resulting from the computation and establishment
     of new TE paths.

   - Ability to maintain accurate synchronization between TED and
     network topology and resource states.



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   - Speed with which TED synchronization is achieved.

   - Impact of the synchronization process on the data flows in the
     network.

   - Ability to deal with situations where paths satisfying a required
     set of constraints cannot be found by the PCE.

   - Policy: Application of policy to the PCC-PCE and PCE-PCE
     communications as well as to the computation of paths that respect
     inter-domain TE LSP establishment policies.

   Note that other metrics may also be considered.  Such metrics should
   be used when evaluating a particular PCE-based architecture.  The
   potential tradeoffs of the optimization of such metrics should be
   evaluated (for instance, increasing the path optimality is likely to
   have consequences on the computation time).

9.  Manageability Considerations

   The PCE architecture introduces several elements that are subject to
   manageability.  The PCE itself must be managed, as must its
   communications with PCCs and other PCEs.  The mechanism by which PCEs
   and PCCs discover each other are also subject to manageability.

   Many of the issues of manageability are already covered in other
   sections of this document.

9.1.  Control of Function and Policy

   It must be possible to enable and disable the PCE function at a PCE,
   and this will lead to the PCE accepting, rejecting, or simply not
   receiving requests from PCCs.  Graceful shutdown of the PCE function
   should also be considered so that in controlled circumstances (such
   as software upgrade) a PCE does not just 'disappear' but warns its
   PCCs and gracefully handles any queued computation requests (perhaps
   by completing them, forwarding them to another PCE, or rejecting
   them).

   Similarly it must be possible to control the application of policy at
   the PCE through configuration.  This control may include the
   restriction of certain functions or algorithms, the configuration of
   access rights and priorities for PCCs, and the relationships with
   other PCEs both inside and outside the domain.

   The policy configuration interface is yet to be determined.  The
   interface may be purely a local matter, or it may be supported via a
   standardized interface (such as a MIB module).



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9.2.  Information and Data Models

   It is expected that the operations of PCEs and PCCs will be modeled
   and controlled through appropriate MIB modules.  The tables in the
   new MIB modules will need to reflect the relationships between
   entities and to control and report on configurable options.

   Statistics gathering will form an important part of the operation of
   PCEs.  The operator must be able to determine the historical
   interactions of a PCC with its PCEs, the performance that it has
   seen, and the success rate of its requests.  Similarly, it is
   important for an operator to be able to inspect a PCE and determine
   its load and whether an individual PCC is responsible for a
   disproportionate amount of the load.  It will also be important to be
   able to record and inspect statistics about the communications
   between the PCC and PCE, including issues such as malformed messages,
   unauthorized messages, and messages discarded because of congestion.
   In this respect, there is clearly an overlap between manageability
   and security.

   Statistics for the PCE architecture can be made available through
   appropriate tables in the new MIB modules.

   The new MIB modules should also be used to provide notifications when
   key thresholds are crossed or when important events occur.  Great
   care must be exercised to ensure that the network is not flooded with
   Simple Network Management Protocol (SNMP) notifications.  Thus, it
   might be inappropriate to issue a notification every time a PCE
   receives a request to compute a path.  In any case, full control must
   be provided to allow notifications to be disabled using, for example,
   the mechanisms defined in the SNMP-NOTIFICATION-MIB module in
   [RFC3413].

9.3.  Liveness Detection and Monitoring

   Section 6.5 discusses the importance of a PCC being able to detect
   the liveness of a PCE.  PCE-PCC communications techniques must enable
   a PCC to determine the liveness of a PCE both before it sends a
   request and in the period between sending a request and receiving a
   response.

   It is less important for a PCE to know about the liveness of PCCs,
   and within the simple request/response model, this is only helpful

   - to gain a predictive view of the likely loading of a PCE in the
     future, or

   - to allow a PCE to abandon processing of a received request.



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9.4.  Verifying Correct Operation

   Correct operation for the PCE architecture can be classified as
   determining the correct point-to-point connectivity between PCCs and
   PCEs, and as assessing the validity of the computed paths.  The
   former is a security issue that may be enhanced by authentication and
   monitored through event logging and records as described in Section
   9.1.  It may also be a routing issue to ensure that PCC-PCE
   connectivity is possible.

   Verifying computed paths is more complex.  The information to perform
   this function can, however, be made available to the operator through
   MIB tables, provided that full records are kept of the constraints
   passed on the request, the path computed and provided on the
   response, and any additional information supplied by the PCE such as
   the constraint relaxation policies applied.

9.5.  Requirements on Other Protocols and Functional Components

   At the architectural stage, it is impossible to make definitive
   statements about the impact on other protocols and functional
   components since the solution's work has not been completed.
   However, it is possible to make some observations.

   - Dependence on underlying transport protocols

     PCE-PCC communications may choose to utilize underlying protocols
     to provide transport mechanisms.  In this case, some of the
     manageability considerations described in the previous sections may
     be devolved to those protocols.

   - Re-use of existing protocols for discovery

     Without prejudicing the requirements and solutions work for PCE
     discovery (see Section 6.4), it is possible that use will be made
     of existing protocols to facilitate this function.  In this case
     some of the manageability considerations described in the previous
     sections may be devolved to those protocols.

   - Impact on LSRs and TE LSP signaling

     The primary example of a PCC identified in this architecture is an
     MPLS or a GMPLS LSR.  Consideration must therefore be given to the
     manageability of the LSRs and the additional manageability
     constraints applicable to the TE LSP signaling protocols.






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     In addition to allowing the PCC management described in the
     previous sections, an LSR must be configurable to determine whether
     it will use a remote PCE at all, the options being to use hop-by-
     hop routing or to supply the PCE function itself.  It is likely to
     be important to be able to distinguish within an LSR whether the
     route used for a TE LSP was supplied in a signaling message from
     another LSR, by an operator, or by a PCE, and, in the case where it
     was supplied in a signaling message, whether it was enhanced or
     expanded by a PCE.

   - Reuse of existing policy models and mechanisms

     As policy support mechanisms can be quite extensive, it is
     worthwhile to explore to what extent this prior work can be
     leveraged and applied to PCE.  This desire to leverage prior work
     should not be interpreted as a requirement to use any particular
     solution or protocol.

9.6.  Impact on Network Operation

   This architecture may have two impacts on the operation of a network.
   It increases TE LSP setup times while requests are sent to and
   processed by a remote PCE, and it may cause congestion within the
   network if a significant number of computation requests are issued in
   a small period of time.  These issues are most severe in busy
   networks and after network failures, although the effect may be
   mitigated if the protection paths are precomputed or if the path
   computation load is distributed among a set of PCEs.

   Issues of potential congestion during recovery from failures may be
   mitigated through the use of pre-established protection schemes such
   as fast reroute.

   It is important that network congestion be managed proactively
   because it may be impossible to manage it reactively once the network
   is congested.  It should be possible for an operator to rate limit
   the requests that a PCC sends to a PCE, and a PCE should be able to
   report impending congestion (according to a configured threshold)
   both to the operator and to its PCCs.

9.7.  Other Considerations

   No other management considerations have been identified.








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10.  Security Considerations

   The impact of the use of a PCE-based architecture must be considered
   in the light of the impact that it has on the security of the
   existing routing and signaling protocols and techniques in use within
   the network.  The impact may be less likely to be an issue in the
   case of intra-domain use of PCE, but an increase in inter-domain
   information flows and the facilitation of inter-domain path
   establishment may increase the vulnerability to security attacks.

   Of particular relevance are the implications for confidentiality
   inherent in a PCE-based architecture for multi-domain networks.  It
   is not necessarily the case that a multi-domain PCE solution will
   compromise security, but solutions MUST examine their effects in this
   area.

   Applicability statements for particular combinations of signaling,
   routing and path computation techniques are expected to contain
   detailed security sections.

   Note that the use of a non-local PCE (that is, one not co-resident
   with the PCC) does introduce additional security issues.  Most
   notable among these are:

   - interception of PCE requests or responses;

   - impersonation of PCE or PCC;

   - falsification of TE information, policy information, or PCE
     capabilities; and

   - denial-of-service attacks on PCE or PCE communication mechanisms.

   It is expected that PCE solutions will address these issues in detail
   using authentication and security techniques.

11.  Acknowledgements

   The authors would like to extend their warmest thanks to (in
   alphabetical order) Arthi Ayyangar, Zafar Ali, Lou Berger, Mohamed
   Boucadair, Igor Bryskin, Dean Cheng, Vivek Dubey, Kireeti Kompella,
   Jean-Louis Le Roux, Stephen Morris, Eiji Oki, Dimitri Papadimitriou,
   Richard Rabbat, Payam Torab, Takao Shimizu, and Raymond Zhang for
   their review and suggestions.  Lou Berger provided valuable and
   detailed contributions to the discussion of policy in this document.

   Thanks also to Pekka Savola, Russ Housley and Dave Kessens for review
   and constructive discussions during the final stages of publication.



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12.  Informative References

   [RFC2702]  Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
              McManus, "Requirements for Traffic Engineering Over MPLS",
              RFC 2702, September 1999.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.

   [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.

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

   [RFC3413]  Levi, D., Meyer, P., and B. Stewart, "Simple Network
              Management Protocol (SNMP) Applications", STD 62, RFC
              3413, December 2002.

   [RFC3473]  Berger, L., "Generalized Multi-Protocol Label Switching
              (GMPLS) Signaling Resource ReserVation Protocol-Traffic
              Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.

   [RFC3748]  Smit, H. and T. Li, "Intermediate System to Intermediate
              System (IS-IS) Extensions for Traffic Engineering (TE)",
              RFC 3784, June 2004.

   [RFC3812]  Srinivasan, C., Viswanathan, A., and T. Nadeau,
              "Multiprotocol Label Switching (MPLS) Traffic Engineering
              (TE) Management Information Base (MIB)", RFC 3812, June
              2004.

   [RFC4105]  Le Roux, J.-L., Vasseur, J.-P., and J. Boyle,
              "Requirements for Inter-Area MPLS Traffic Engineering",
              RFC 4105, June 2005.

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

   [MLN]      Shiomoto, K., Papdimitriou, D., Le Roux, J.-L., Vigoureux,
              M., and D. Brungard, "Requirements for GMPLS-based multi-
              region and multi-layer networks (MRN/MLN)", Work in
              Progress, June 2006.





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Authors' Addresses

   Adrian Farrel
   Old Dog Consulting

   EMail: adrian@olddog.co.uk


   Jean-Philippe Vasseur
   1414 Massachussetts Avenue
   Boxborough, MA 01719
   USA

   EMail: jpv@cisco.com


   Jerry Ash
   AT&T
   Room MT D5-2A01
   200 Laurel Avenue
   Middletown, NJ 07748,
   USA

   Phone: (732)-420-4578
   Fax:   (732)-368-8659
   EMail: gash@att.com

























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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
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