Network Working Group                                      Adrian Farrel
IETF Internet Draft                                   Old Dog Consulting
Proposed Status: Informational                     Jean-Philippe Vasseur
Expires: January 2006                                Cisco Systems, Inc.
                                                               Jerry Ash

                                                               July 2005


              Path Computation Element (PCE) Architecture

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   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 highly 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

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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/Global Optimization ....... 6
       4.2. Partial Visibility ....................................... 6
       4.3. Absence of the TED or Use of Non-TE-Enabled IGP .......... 7
       4.4. Node Outside the Routing Domain .......................... 7
       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 ..................................... 8
   5. Overview of the PCE-Based Architecture ......................... 9
       5.1. Composite PCE Node ....................................... 9
       5.2. External PCE ............................................ 10
       5.3. Multiple PCE Path Computation ........................... 11
       5.4. Multiple PCE Path Computation with Inter-PCE Communication
                       .............................................. 12
       5.5. Areas for Standardization ............................... 13
   6. PCE Architectural Considerations .............................. 13
       6.1. Centralized Computation Model ........................... 14
       6.2. Distributed Computation Model ........................... 14
       6.3. Synchronization ......................................... 14
       6.4. PCE Discovery and Load Balancing ........................ 15
       6.5. Detecting PCE Liveness .................................. 16
       6.6. PCC-PCE & PCE-PCE Communication ......................... 16
       6.7. PCE TED Synchronization ................................. 18
       6.8. Stateful Versus Stateless PCEs .......................... 19
       6.9. Monitoring .............................................. 21
       6.10. Policy and Confidentiality  ............................ 21
       6.11. Unsolicited Interactions ............................... 21
   7. Evaluation Metrics ............................................ 22
   8. Manageability Considerations .................................. 23
       8.1 Information and Data Models .............................. 23
       8.2 Liveness Detection and Monitoring ........................ 24
       8.3 Verifying Correct Operation .............................. 24
       8.4 Requirements on Other Protocols and Functional Components  25
       8.5 Impact on Network Operation .............................. 25
   9. Security Considerations ....................................... 26
   10. IANA Considerations .......................................... 26
   11. Acknowledgements ............................................. 27
   12. Intellectual Property Considerations ......................... 27
   13. Normative References ......................................... 27
   14. Informational References ..................................... 27
   15. Authors' Addresses ........................................... 28
   16. Full Copyright Statement ..................................... 29

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1. Introduction

   Constraint-based path computation is a fundamental building block for
   traffic engineering in MPLS and GMPLS networks. Path computation in
   large, multi-domain networks is highly 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

   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, PCC-PCE and PCE-PCE
   communication, TED synchronization, stateful and stateless PCEs,
   monitoring, policy and confidentiality, and evaluation metrics.

2. Terminology

   CSPF: Constraint-based Shortest Path First.

   LER: Label Edge Router.

   LSDB: Link State Database.

   LSP: Label Switched Path.

   LSR: Label Switching Router.

   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 IGP
   extensions or potentially by other means.

   TE LSP: Traffic Engineering MPLS Label Switched Path.

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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 (ASs), multiple ASs
   within a service provider network, or multiple ASs across multiple
   service provider networks. However, domains of path computation
   responsibility may also exist as sub-domains of areas or ASs.

   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 correlation of
         topology and routing information between domains.

      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.

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

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      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 of 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 which 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 PCE path" (that
      is, the full explicit path from start to destination, made of a
      list of strict hops) or a "strict/loose PCE 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
      a PCE-based path computation model in some IGP areas, whereas the
      set of traversed ASs 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.

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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.

4.1. CPU-intensive Path Computation/Global Optimization

   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:

   - Global optimization in 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)

   - Computation of minimal cost Point to Multipoint trees (Steiner

   In these situations, it may not be possible or desirable for a router
   to perform path computation because of the constraints on its CPU, in
   which case the path computation may be off-loaded to some other

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 an 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 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, nor even
   that a viable path will be discovered except, possibly, through
   repeated trial and error using crankback or other signaling

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   This problem of inter-domain path computation may most probably be
   addressed through distributed computation with cooperation among PCEs
   within each of the domains, or perhaps by using a central
   "all-seeing" PCE that has access to the complete set of topology
   information. In this latter 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

   Note that the issues described here can be further highlighted in the
   context of LSP reoptimization, or the establishment of multiple
   diverse 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) both from
   a memory perspective and because it may require non-negligible CPU
   activity to maintain. 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.

   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 each router so that each router can perform path computation, or
   held centrally (on a distinct node that supports PCE) for centralized
   path 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 [RFC2547] 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 router, and that does not rely on static
   loose hops configuration in which case optimal shortest paths could
   not be achieved. A distinct PCE-based solution can help here. Note
   that the PCE in this case may, itself, provide a path that includes
   loose hops.

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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
   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 NMS or OSS.

   This scenario is important for ASON-capable networks, and may also be
   used for interworking between GMPLS-capable and GMPLS-incapable

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
   protection independent elements, while avoiding any extensions to 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 TDM network may provide connectivity for client-layer
   networks such as IP, MPLS or Layer 2 [MRN].

   The server-layer network is unlikely to provide the same connectivity
   paradigm as the client networks so that 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.
   Further, 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

   The different client and server layer networks may be considered as
   distinct path computation regions within a PCE domain, and 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.

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   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.

5. Overview of the PCE-Based Architecture

   This section is 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

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
   which is requested from a PCE. The PCE operates on the TED in order
   to respond with the requested path.

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             |   ---------   | 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.

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
   as input to the computation and returns a response.

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           |  -----   |
           | | TED |<-+------------>
           |  -----   |  TED synchronization
           |    |     |  mechanism (for example, routing protocol)
           |    |     |
           |    v     |
           |  -----   |
           | | PCE |  |
           |  -----   |
                | Request/
                | Response
    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 composit PCE node), but those functions are purely
   orthogonal to the operation of the function in the instance being
   considered here.

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.

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

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            ----------           ----------
           |          |         |          |
           |   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

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 this resulted in the adjacent node needing 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

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

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

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   Multiple PCE path computation with inter-PCE communication involves
   coordination between distributed PCEs such that the result of the
   computation performed by one PCE depends on information supplied by
   other PCEs. This model does not provide a distributed computaiton
   algorithm, but 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.

5.5 Areas for Standardization

   The following areas require standardization within the PCE

   - communication between PCCs and PCEs, and between cooperating PCEs

   - requirements for extensions to existing routing and/or signaling
     protocols in support of PCE discovery and signaling of inter-domain

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

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

   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 strictly relates to the path
   computation aspects of such TE LSPs.

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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 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

   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. 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 which impacts on this choice. It
   will often be the case that 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 LSP.

6.3. Synchronization

   It is often the case that 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, the PCE will 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.

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

   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 which 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 supplies a set of alternate paths for use by the PCC
   should the establishment of the 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

   The PCE architecture requires that the PCC/PCE know the location of
   one or more PCEs that it can use for the computation of a path. Such
   knowledge may come through a discovery mechanism that simply relies
   on local configuration, or can imply dynamic PCE discovery along with
   various static (for example, Boolean capability) or dynamically
   computed variables (for example, computing resources). 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 later case, it is a requirement
   for the proxy to 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, in order for instance to
   efficiently share the computation load across multiple PCEs, is local
   to the PCC and out of the scope of this document.

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   A PCE SHOULD advertise its capabilities, such as:

   - set of constraints that it can account for (diversity, SRLGs,
     Optical impairments, wavelength continuity, etc.)
   - number of switching capability layers (and which ones)
   - 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 that dynamically learns about
   PCEs available on the network to decide which of them to use.
   Alternatively, a PCC might ask a PCE to perform a particular type
   of service and receive a response that says that the PCE is unable to
   perform the service, but specifying the things that the PCE can do.
   Note that the parameters mentioned above are not meant to be
   exhaustive and are listed for the sake of illustration.

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

   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. Further, 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 & 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.

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   The path computation request may include a significant set of
   requirements including

   - the source and destination of the path
   - the bandwidth and other QoS parameters desired
   - resources, resource affinities and shared risk link groups (SRLGs)
     to use/avoid
   - the number of disjoint paths required and if near-disjoint paths
     are acceptable
   - the level of robustness of the path resources
   - and so on.

   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 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
   to be more likely to achieve a positive result 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 to be
   more likely to achieve a positive result. 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.
   This may require coordination through the communication between the

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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 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, and is achieved through
   configuration not through protocol communications.

   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
      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 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 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. 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 LSPs provisioned within the network. This information
      can include LSP routes, reserved bandwidth, and measured traffic
      volume passing through the LSP.

      Such LSP information can enhance LSP reoptimization to provide
      "full network" reoptimization, and can allow traffic fluctuations
      to be taken into account. Detailed LSP information may also
      facilitate reconfiguration of the Virtual Network Topology (VNT)
      [MRN], in which lower layer 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. Further, 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.

   It must also be highlighted 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 for any PCE to
   handle the complete TED related to the set of traversed domains by
   the LSP path 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).

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   For example, if there is only one PCE in the domain, all LSP
   computation is done by this PCE, which can then track all the
   existing LSPs and stay synchronized. However, this model could
   require substantial control plane resources. If there are multiple
   PCEs in the network, LSP computation and information is 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 of the LSPs the PCE has computed,
   if it can also be known that the LSPs were actually set up, 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 LSPs are set up using
   distributed computation performed among several PCEs, the problem of
   synchronization becomes larger and more complex.

   There is benefit in knowing which LSPs exist, and their routing, to
   support such applications as placing a high priority LSP in a crowded
   network such that it preempts as few other LSPs as possible. Note
   that preempting based on the minimum number of links might not result
   in the smallest number of LSPs being disrupted. Another application
   concerns the construction and maintenance of a Virtual Network
   Topology [MRN]. It is also helpful to understand which other 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

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   It should be observed 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

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

6.10. Policy and Confidentiality

   As stated in [INTER-AS], the case of inter-provider TE LSP path
   computation requires the ability to compute a path while preserving
   confidentiality across multiple Service Providers cores. Thus any PCE
   architecture solution must support the ability to return partial
   paths by means of loose hops (for example, where each loose hops
   would for instance identify a boundary LSR). Confidentiality and
   security of PCC-PCE and PCE-PCE messages must also be ensured.

   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.

6.11. 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.

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   Further, 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),
   and in this case the PCE-PCC communication could support an
   "unsolicited" path computation message to supply this new path to the
   PCC. It should be noted, 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. 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

7. 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) have an inherent risk of lower quality
     paths and might create path loops unless careful policy is applied.

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

   - Load sharing: The ability to allow multiple PCEs to spread the path
     computation load by allowing multiple PCEs to each 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.

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   - Path computation time: The time to compute individual paths,
     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.

   - Speed with which TED synchronization is achieved.

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

   Note that other metrics may also be considered. Such metrics should
   be used when evaluating a particular PCE-based architecture. It must
   also be highlighted that 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

8. 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.

8.1 Information and Data Models

   It is expected that the operations of PCEs and PCCs will be modeled
   and controlled through appropriate MIB modules. These will be
   relatively simple constructs since the relationships between PCEs and
   PCCs are quite simple. 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 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

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   PCE, including issues such as malformed messages, unauthorized
   messages and messages discarded owing to 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
   (formerly known as traps) when key thresholds are crossed or when
   important events occur. Great care must be exercised to ensure that
   the network is not flooded with SNMP notifications. Thus it might be
   inappropriate to issue a notification every time that a PCE receives
   a request to compute a path. In any case, full control must be
   provided through the MIB modules to allow notifications to be

8.2 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

   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

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

8.3 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 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 8.1.

   Verifying computed paths is more complex. The information to perform
   this function can, however, be made available to the operator through
   MIB tables provided 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.

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8.4 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 solutions 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 LSP signaling

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

     As well as 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 path
     used for an LSP was supplied in a signaling message, 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.

8.5 Impact on Network Operation

   This architecture may have two impacts on the operation of a network.
   It increases 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.

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

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   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. 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. There is unlikely to be any impact on intra-domain
   security, 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

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

   It should be observed that the use of a non-local PCE (that is, not
   co-resident with the PCC) does introduce additional security issues.
   Most notable amongst these are:

   - Interception of PCE requests or responses

   - Impersonation of PCE

   - Falsification of TE information

   - 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.

10. IANA Considerations

   This informational document makes no requests for IANA action.

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11. Acknowledgements

   The authors would like to extend their warmest thanks to (in
   alphabetical order) Arthi Ayyangar, Zafar Ali, Mohamed Boucadair,
   Igor Bryskin, Dean Cheng, Vivek Dubey, Kireeti Kompella,
   Jean-Louis Le Roux, Stephen Morris, Eiji Oki, Dimitri Papadimitriou,
   Richard Rabbat, Takao Shimizu, and Raymond Zhang for their review and

12. Intellectual Property Considerations

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights. Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard. Please address the information to the IETF at

13. Normative References

   [RFC3667]    Bradner, S., "IETF Rights in Contributions", BCP 78,
                RFC 3667, February 2004.

   [RFC3668]    Bradner, S., "Intellectual Property Rights in IETF
                Technology", BCP 79, RFC 3668, February 2004.

14. Informational References

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

   [RFC2547]    Rosen, E. and  Rekhter, Y. "BGP/MPLS VPNs", RFC2547,
                March 1999.

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   [RFC3209]    Awduche, D., et. all, "Extensions to RSVP for LSP
                Tunnels", RFC 3209, December 2001.

   [RFC3630]    Katz et al., "Traffic Engineering (TE) Extensions to
                OSPF Version 2", RFC3630, September 2003.

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

   [RFC3748]    Smit, H. and Li, T., "Intermediate System to
                Intermediate System (IS-IS) - Extensions for Traffic
                Engineering (TE)", RFC3784, June 2004.

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

   [INTER-AS]   Zhang, R., Vasseur, JP., et. al., "MPLS Inter-AS Traffic
                Engineering requirements",
                draft-ietf-tewg-interas-mpls-te-req, work in progress.

   [MRN]        Shiomoto, K., et. al., "Requirements for GMPLS-based
                multi-region and multi-layer networks",
                draft-shiomoto-ccamp-gmpls-mrn-reqs, work in progress.

15. Authors' Addresses

   Adrian Farrel
   Old Dog Consulting

   Jean-Philippe Vasseur
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough , MA - 01719

   Jerry Ash
   Room MT D5-2A01
   200 Laurel Avenue
   Middletown, NJ 07748, USA
   Phone: +1-(732)-420-4578
   Fax:   +1-(732)-368-8659

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16. Full Copyright Statement

   Copyright (C) The Internet Society (2005). This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an

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