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Versions: 00 01 02 draft-ietf-pce-applicability-actn

PCE Working Group                                               D. Dhody
Internet-Draft                                                    Y. Lee
Intended status: Informational                       Huawei Technologies
Expires: September 14, 2017                                D. Ceccarelli
                                                                Ericsson
                                                          March 13, 2017


  Applicability of Path Computation Element (PCE) for Abstraction and
                     Control of TE Networks (ACTN)
                 draft-dhody-pce-applicability-actn-02

Abstract

   Abstraction and Control of TE Networks (ACTN) refers to the set of
   virtual network operations needed to orchestrate, control and manage
   large-scale multi-domain TE networks so as to facilitate network
   programmability, automation, efficient resource sharing, and end-to-
   end virtual service aware connectivity and network function
   virtualization services.

   The Path Computation Element Communication Protocol (PCEP) provides
   mechanisms for Path Computation Elements (PCEs) to perform path
   computations in response to Path Computation Clients (PCCs) requests.

   This document examines the applicability of PCE to the ACTN
   framework.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 14, 2017.







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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Path Computation Element (PCE)  . . . . . . . . . . . . .   2
       1.1.1.  Role of PCE in SDN  . . . . . . . . . . . . . . . . .   3
       1.1.2.  PCE in multi-domain and multi-layer deployments . . .   4
     1.2.  Abstraction and Control of TE Networks (ACTN) . . . . . .   4
     1.3.  PCE and ACTN  . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Architectural Considerations  . . . . . . . . . . . . . . . .   6
     2.1.  Multi domain coordination via Hierarchy . . . . . . . . .   6
     2.2.  Virtualization/Abstraction function . . . . . . . . . . .   7
     2.3.  Customer mapping function . . . . . . . . . . . . . . . .   8
     2.4.  Virtual Network Operations  . . . . . . . . . . . . . . .   8
   3.  Interface Considerations  . . . . . . . . . . . . . . . . . .   9
   4.  Realizining ACTN with PCE (and PCEP)  . . . . . . . . . . . .   9
   5.  Relationship to PCE based central control . . . . . . . . . .  12
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

1.1.  Path Computation Element (PCE)

   The Path Computation Element communication Protocol (PCEP) [RFC5440]
   provides mechanisms for Path Computation Elements (PCEs) [RFC4655] to
   perform path computations in response to Path Computation Clients
   (PCCs) requests.




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   The ability to compute shortest constrained TE LSPs in Multiprotocol
   Label Switching (MPLS) and Generalized MPLS (GMPLS) networks across
   multiple domains has been identified as a key motivation for PCE
   development.

   A stateful PCE is capable of considering, for the purposes of path
   computation, not only the network state in terms of links and nodes
   (referred to as the Traffic Engineering Database or TED) but also the
   status of active services (previously computed paths, and currently
   reserved resources, stored in the Label Switched Paths Database
   (LSPDB).

   [RFC8051] describes general considerations for a stateful PCE
   deployment and examines its applicability and benefits, as well as
   its challenges and limitations through a number of use cases.

   [I-D.ietf-pce-stateful-pce] describes a set of extensions to PCEP to
   provide stateful control.  A stateful PCE has access to not only the
   information carried by the network's Interior Gateway Protocol (IGP),
   but also the set of active paths and their reserved resources for its
   computations.  The additional state allows the PCE to compute
   constrained paths while considering individual LSPs and their
   interactions.  [I-D.ietf-pce-pce-initiated-lsp] describes the setup,
   maintenance and teardown of PCE-initiated LSPs under the stateful PCE
   model.

   [I-D.ietf-pce-stateful-pce] also describes the active stateful PCE.
   The active PCE functionality allows a PCE to reroute an existing LSP
   or make changes to the attributes of an existing LSP, or a PCC to
   delegate control of specific LSPs to a new PCE.

1.1.1.  Role of PCE in SDN

   Software-Defined Networking (SDN) refers to a separation between the
   control elements and the forwarding components so that software
   running in a centralized system called a controller, can act to
   program the devices in the network to behave in specific ways.  A
   required element in an SDN architecture is a component that plans how
   the network resources will be used and how the devices will be
   programmed.  It is possible to view this component as performing
   specific computations to place flows within the network given
   knowledge of the availability of network resources, how other
   forwarding devices are programmed, and the way that other flows are
   routed.  It is concluded in [RFC7399], that this is the same function
   that a PCE might offer in a network operated using a dynamic control
   plane.  This is the function and purpose of a PCE, and the way that a
   PCE integrates into a wider network control system including SDN is
   presented in Application-Based Network Operation (ABNO) [RFC7491].



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1.1.2.  PCE in multi-domain and multi-layer deployments

   Computing paths across large multi-domain environments require
   special computational components and cooperation between entities in
   different domains capable of complex path computation.  The PCE
   provides an architecture and a set of functional components to
   address this problem space.  A PCE may be used to compute end-to-end
   paths across multi-domain environments using a per-domain path
   computation technique [RFC5152].  The Backward recursive PCE based
   path computation (BRPC) mechanism [RFC5441] defines a PCE-based path
   computation procedure to compute inter-domain constrained MPLS and
   GMPLS TE networks.  However, both per-domain and BRPC techniques
   assume that the sequence of domains to be crossed from source to
   destination is known, either fixed by the network operator or
   obtained by other means.

   [RFC6805] describes a Hierarchical PCE (H-PCE) architecture which can
   be used for computing end-to-end paths for inter-domain MPLS Traffic
   Engineering (TE) and GMPLS Label Switched Paths (LSPs) when the
   domain sequence is not known.  Within the Hierarchical PCE (H-PCE)
   architecture, the Parent PCE (P-PCE) is used to compute a multi-
   domain path based on the domain connectivity information.  A Child
   PCE (C-PCE) may be responsible for a single domain or multiple
   domains, it is used to compute the intra-domain path based on its
   domain topology information.

   [I-D.dhodylee-pce-stateful-hpce] state the considerations for
   stateful PCE(s) in hierarchical PCE architecture.  In particular, the
   behavior changes and additions to the existing stateful PCE
   mechanisms (including PCE- initiated LSP setup and active PCE usage)
   in the context of networks using the H-PCE architecture.

   [RFC5623] describes a framework for applying the PCE-based
   architecture to inter-layer to (G)MPLS TE.  It provides suggestions
   for the deployment of PCE in support of multi-layer networks.  It
   also describes the relationship between PCE and a functional
   component in charge of the control and management of the VNT, called
   the Virtual Network Topology Manager (VNTM).

1.2.  Abstraction and Control of TE Networks (ACTN)

   [I-D.ietf-teas-actn-requirements] describes the high-level ACTN
   requirements.  [I-D.ietf-teas-actn-framework] describes the
   architecture model for ACTN including the entities (Customer Network
   Controller(CNC), Mult-domain Service Coordinator(MDSC), and Physical
   Network Controller(PNC)) and their interfaces.





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   The ACTN reference architecture identified a three-tier control
   hierarchy as depicted in Figure 1:



      +-------+                 +-------+                 +-------+
      | CNC-A |                 | CNC-B |                 | CNC-C |
      +-------+                 +-------+                 +-------+
            \                       |                        /
             ----------             |-CMI I/F     -----------
                        \           |            /
                         +-----------------------+
                         |         MDSC          |
                         +-----------------------+
                         /          |            \
               ----------           |-MMI I/F     -----------
              /                     |                        \
      +----------+              +----------+             +--------+
      |   MDSC   |              |   MDSC   |             |  MDSC  |
      +----------+              +----------+             +--------+
           |                    /     |-MPI I/F             /    \
           |                   /      |                    /      \
        +-----+           +-----+  +-----+            +-----+  +-----+
        | PNC |           | PNC |  | PNC |            | PNC |  | PNC |
        +-----+           +-----+  +-----+            +-----+  +-----+

   CMI - (CNC-MDSC Interface)
   MMI - (MDSC-MDSC Interface)
   MPI - (MDSC-PNC Interface)


                         Figure 1: ACTN Hierarchy

   The two interfaces with respect to the MDSC, one north of the MDSC
   Interface) and MPI (MDSC-PNC Interface), respectively.  MMI (MDSC-
   MDSC interface) is used to support recursion.

   [I-D.ietf-teas-actn-info-model] provides an information model for
   ACTN interfaces.

1.3.  PCE and ACTN

   This document examines the PCE and ACTN architecture and describes
   how the PCE architecture is applicable to ACTN.  It also lists the
   PCEP extensions that are needed to use PCEP as an ACTN interface.
   This document also identifies any gaps in PCEP, that exist at the
   time of publication of this document.




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2.  Architectural Considerations

   ACTN [I-D.ietf-teas-actn-framework] architecture is based on
   hierarchy and recursiveness of controllers.  It defines three types
   of controllers (depending on the functionalities they implement).
   The main functionalities are -

   o  Multi domain coordination function

   o  Virtualization/Abstraction function

   o  Customer mapping/translation function

   o  Virtual service coordination function

   Section 3 of [I-D.ietf-teas-actn-framework] describes these
   functions.

   It should be noted that, in this document we list all possible ways
   in which PCEP could be used for each of the above functions, but all
   functions are not required to be implemented via PCEP.  Operator may
   choose to use the PCEP for multi domain coordination via stateful
   H-PCE but use RestConf or BGP-LS to get the topology and support
   virtualization/abstraction function.

2.1.  Multi domain coordination via Hierarchy

   With the definition of domain being "everything that is under the
   control of the single logical controller", as per
   [I-D.ietf-teas-actn-framework], it is needed to have a control entity
   that oversees the specific aspects of the different domains and to
   build a single abstracted end-to-end network topology in order to
   coordinate end-to-end path computation and path/service provisioning.

   The MDSC in ACTN framework realizes this function by coordinating the
   per-domain PNCs in a hierarchy of controllers.  It also needs to
   detach from the underlying network technology and express customer
   concerns by business needs.

   [RFC6805] and [I-D.dhodylee-pce-stateful-hpce] describes a hierarchy
   of PCE with Parent PCE coordinating multi-domain path computation
   function between Child PCE(s).  It is easy to see how these
   principles align, and thus how stateful H-PCE architecture can be
   used to realize ACTN.

   The Per domain stitched LSP in the Hierarchical stateful PCE
   architecture, described in Section 3.3.1 of
   [I-D.dhodylee-pce-stateful-hpce] is well suited for multi-domain



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   coordination function.  This includes domain sequence selection; E2E
   path computation; Controller (PCE) initiated path setup and
   reporting.  This is also applicable to multi-layer coordination in
   case of IP+optical networks.

   [I-D.litkowski-pce-state-sync]" describes the procedures to allow a
   stateful communication between PCEs for various use-cases.  The
   procedures and extensions are also applicable to Child and Parent PCE
   communication and thus useful for ACTN as well.

2.2.  Virtualization/Abstraction function

   To realize ACTN, an abstracted view of the underlying network
   resources needs to be built.  This includes global network-wide
   abstracted topology based on the underlying network resources of each
   domain.  This also include abstract topology created as per the
   customer service connectivity requests and represented as a network
   slice allocated to each customer.

   In order to compute and provide optimal paths, PCEs require an
   accurate and timely Traffic Engineering Database (TED).
   Traditionally this TED has been obtained from a link state (LS)
   routing protocol supporting traffic engineering extensions.  PCE may
   construct its TED by participating in the IGP ([RFC3630]  and
   [RFC5305]  for MPLS-TE; [RFC4203]  and [RFC5307] for GMPLS).  An
   alternative is offered by BGP-LS [RFC7752].

   In case of H-PCE [RFC6805], the parent PCE needs to build the domain
   topology map of the child domains and their interconnectivity.
   [RFC6805] and [I-D.ietf-pce-inter-area-as-applicability] suggest that
   BGP-LS could be used as a "northbound" TE advertisement from the
   child PCE to the parent PCE.

   [I-D.dhodylee-pce-pcep-ls] proposes another approaches for learning
   and maintaining the Link-State and TE information as an alternative
   to IGPs and BGP flooding, using PCEP itself.  The child PCE can use
   this mechanism to transport Link-State and TE information from child
   PCE to a Parent PCE using PCEP.

   In ACTN, there is a need to control the level of abstraction based on
   the deployment scenario and business relationship between the
   controllers.  The mechanism used to disseminate information from PNC
   (child PCE) to MDSC (parent PCE) should support abstraction.
   [I-D.lee-teas-actn-abstraction] describes a few alternative
   approaches of abstraction.  The resulting abstracted topology can be
   encoded using the PCEP-LS mechanisms [I-D.dhodylee-pce-pcep-ls].
   PCEP-LS is an attractive option when the operator would wish to have
   a single control plane protocol (PCEP) to achieve ACTN functions.



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2.3.  Customer mapping function

   In ACTN, there is a need to map customer virtual network (VN)
   requirements into network provisioning request to the PNC.  That is,
   the customer requests/commands are mapped into network provisioning
   requests that can be sent to the PNC.  Specifically, it provides
   mapping and translation of a customer's service request into a set of
   parameters that are specific to a network type and technology such
   that network configuration process is made possible.

   [I-D.ietf-pce-pce-initiated-lsp] describes the setup, maintenance and
   teardown of PCE-initiated LSPs under the stateful PCE model, without
   the need for local configuration on the PCC, thus allowing for a
   dynamic network that is centrally controlled and deployed.  To
   instantiate or delete an LSP, the PCE sends the Path Computation LSP
   Initiate Request (PCInitiate) message to the PCC.  As described in
   [I-D.dhodylee-pce-stateful-hpce], for inter-domain LSP in
   Hierarchical PCE architecture, the initiation operations can be
   carried out at the parent PCE.  In which case after parent PCE
   finishes the E2E path computation, it can send the PCInitiate message
   to the child PCE, the child PCE further propagates the initiate
   request to the LSR.  The customer request is received by the MDSC
   (parent PCE) and based on the business logic, global abstracted
   topology, network conditions and local policy, the MDSC (parent PCE)
   translates this into per domain LSP initiation request that a PNC
   (child PCE) can understand and act on.  This can be done via the
   PCInitiate message.

   PCEP extensions for associating opaque policy between PCEP peer
   [I-D.ietf-pce-association-policy] can be used.

2.4.  Virtual Network Operations

   Virtual service coordination function in ACTN incorporates customer
   service-related information into the virtual network service
   operations in order to seamlessly operate virtual networks while
   meeting customer's service requirements.

   [I-D.leedhody-pce-vn-association] describes the need for associating
   a set of LSPs with a VN "construct" to facilitate VN operations in
   PCE architecture.  This association allows the PCEs to identify which
   LSPs belong to a certain VN.

   This association based on VN is useful for various optimizations at
   the VN level which can be applied to all the LSPs that are part of
   the VN slice.  During path computation, the impact of a path for an
   LSP is compared against the paths of other LSPs in the VN.  This is
   to make sure that the overall optimization and SLA of the VN rather



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   than of a single LSP.  Similarly, during re-optimization, advanced
   path computation algorithm and optimization technique can be
   considered for all the LSPs belonging to a VN/customer and optimize
   them all together.

3.  Interface Considerations

   As per [I-D.ietf-teas-actn-framework], to allow virtualization and
   multi domain coordination, the network has to provide open,
   programmable interfaces, in which customer applications can create,
   replace and modify virtual network resources and services in an
   interactive, flexible and dynamic fashion while having no impact on
   other customers.  The 3 ACTN interfaces are -

   o  The CNC-MDSC Interface (CMI) is an interface between a Customer
      Network Controller and a Multi Domain Service Coordinator.  It
      requests the creation of the network resources, topology or
      services for the applications.  The MDSC may also report potential
      network topology availability if queried for current capability
      from the Customer Network Controller.

   o  The MDSC-PNC Interface (MPI) is an interface between a Multi
      Domain Service Coordinator and a Physical Network Controller.  It
      communicates the creation request, if required, of new
      connectivity of bandwidth changes in the physical network, via the
      PNC.  In multi-domain environments, the MDSC needs to establish
      multiple MPIs, one for each PNC, as there are multiple PNCs
      responsible for its domain control.

   o  The MDSC-MDSC Interface (MMI) is a special case of the MPI and
      behaves similarly to an MPI to support general functions performed
      by the MDSCs such as abstraction function and provisioning
      function.  From an abstraction point of view, the top level MDSC
      which interfaces the CNC operates on a higher level of abstraction
      (i.e., less granular level) than the lower level MSDCs.  As such,
      the MMI carries more abstract TE information than the MPI.

   PCEP is especially suitable on the MPI and MMI as it meets the
   requirement and the functions as set out in the ACTN framework
   [I-D.ietf-teas-actn-framework].  Its recursive nature is well suited
   via the multi-level hierarchy of PCE.  The Section 4 describe how PCE
   and PCEP could help realize ACTN.

4.  Realizining ACTN with PCE (and PCEP)

   As per the example in the Figure 2, there are 4 domains, each with
   its own PNC and a MDSC at top.  The PNC and MDSC need PCE as a
   important function.  The PNC (or child PCE) already uses PCEP to



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   communicate to the network device.  It can utilize the PCEP as the
   MPI to communicate between controllers too.

                             ******
                   ..........*MDSC*..............................
                .            ****** ..                   MPI    .
             .                .        .                        .
          .                   .          .                      .
        .                    .             .                    .
       .                    .                .                  .
      .                    .                  .                 .
     .                    .                    .                .
     v                    v                    v                .
   ******               ******               ******             .
   *PNC1*               *PNC2*               *PNC4*             .
   ******               ******               ******             .
   +---------------+    +---------------+    +---------------+  .
   |A              |----|               |----|              C|  .
   |               |    |               |    |               |  .
   |DOMAIN 1       |----|DOMAIN 2       |----|DOMAIN 4       |  .
   +------------B13+    +---------------+    +B43------------+  .
                   \                         /                  .
                    \   ******              /                   .
                     \  *PNC3*<............/.....................
                      \ ******            /
                       \+---------------+/
                        B31           B34
                        |               |
                        |DOMAIN 3      B|
                        +---------------+


   MDSC -> Parent PCE
   PNC  -> Child  PCE
   MPI  -> PCEP

                          Figure 2: ACTN with PCE

   o  Building Domain Topology at MDSC: PNC (or child PCE) needs to have
      the TED to compute path in its domain.  As described in
      Section 2.2, it can learn the topology via IGP or BGP-LS.  PCEP-LS
      is also a proposed mechanism to carry link state and traffic
      engineering information within PCEP.  A mechanism to carry
      abstracted topology while hiding technology specific information
      between PNC and MDSC is described in [I-D.dhodylee-pce-pcep-ls].
      At the end of this step the MDSC (or parent PCE) has the
      abstracted topology from each of its PNC (or child PCE).  This
      could be as simple as a domain topology map as described in



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      [RFC6805] or it can have full topology information of all domains.
      The latter is not scalable and thus an abstracted topology of each
      domain interconnected by inter-domain links is the most common
      case.

      *  Topology Change: When the PNC learns of any topology change,
         the PNC needs to decide if the change needs to be notified to
         the MDSC.  This is dependent on the level of abstraction
         between the MDSC and the PNC.

   o  VN Instantiate: MDSC is requested to instantiate a VN, the minimal
      information that is required would be a VN identifier and a set of
      end points.  Various path computation, setup constraints and
      objective functions may also be provided.  In PCE terms, a VN
      Instantiate can be considered as a set of paths belonging to the
      same VN.  As described in Section 2.4 and
      [I-D.leedhody-pce-vn-association] the VN association can help in
      identifying the set of paths that belong to a VN.  The rest of the
      information like the endpoints, constraints and objective function
      is already defined in PCEP in terms of a single path.

      *  Path Computation: As per the example in the Figure 2, the VN
         instantiate requires two end to end paths between (A in Domain
         1 to B in Domain 3) and (A in Domain 1 to C in Domain 4).  The
         MDSC (or parent PCE) triggers the end to end path computation
         for these two paths.  MDSC can do path computation based on the
         abstracted domain topology that it already has or it may use
         the H-PCE procedures (Section 2.1) using the PCReq and PCRep
         messages to get the end to end path with the help of PNC.
         Either way, the resulted E2E paths may be broken into per-
         domain paths.

      *  A-B: (A-B13,B13-B31,B31-B)

      *  A-C: (A-B13,B13-B31,B34-B43,B43-C)

      *  Per Domain Path Instantiation: Based on the above path
         computation, MDSC can issue the path instantiation request to
         each PNC via PCInitiate message (see
         [I-D.dhodylee-pce-stateful-hpce] and
         [I-D.leedhody-pce-vn-association]).  A suitable stitching
         mechanism would be use to stitch these per domain LSPs.

      *  Per Domain Path Report: Each PNC should report the status of
         the per-domain LSP to the MDSC via PCRpt message, as per the
         Hierarchy of stateful PCE ([I-D.dhodylee-pce-stateful-hpce]).
         The status of the end to end LSP (A-B and A-C) is made up when
         all the per domain LSP are reported up by the PNCs.



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      *  Delegation: It is suggested that the per domain LSPs are
         delegated to respective PNC, so that they can control the path
         and attributes based on each domain network conditions.

      *  State Synchronization: The state needs to be synchronized
         between the parent PCE and child PCE.  The mechanism described
         in [I-D.litkowski-pce-state-sync] can be used.

   o  VN Modify: MDSC is requested to modify a VN, for example the
      bandwidth for VN is increased.  This may trigger path computation
      at MDSC as described in the previous step and can trigger an
      update to existing per-intra-domain path (via PCUpd message) or
      creation (or deletion) of a per-domain path (via PCInitiate
      message).  This should be done in make-before-break fashion.

   o  VN Delete: MDSC is requested to delete a VN, in this case, based
      on the E2E paths and the resulting per-domain paths need to be
      removed (via PCInitiate message).

   o  VN Update (based on network changes): Any change in the per-domain
      LSP are reported to the MDSC (via PCRpt message) as per
      [I-D.dhodylee-pce-stateful-hpce].  This may result in changes in
      the E2E path or VN status.  This may also trigger a re-
      optimization leading to a new per-domain path, update to existing
      path, or deletion of the path.

   o  VN Protection: The VN protection/restoration requirements, need to
      applied to each E2E path as well as each per domain path.  The
      MDSC needs to play a crucial role in coordinating the right
      protection/restoration policy across each PNC.  The existing
      protection/restoration mechanism of PCEP can be applied on each
      path.

   o  In case PNC generates an abstract topology to the MDSC, the
      PCInitiate/PCUpd messages from the MDSC to a PNC will contain a
      path with abstract nodes and links.  PNC would need to take that
      as an input for path computation to get a path with physical nodes
      and links.  Similarly PNC would convert the path received from the
      device (with physical nodes and links) into abstract path (based
      on the abstract topology generated before with abstract nodes and
      links) and reported to the MDSC.

5.  Relationship to PCE based central control

   [I-D.ietf-teas-pce-central-control] introduces the architecture for
   PCE as a central controller (PCECC), it further examines the
   motivations and applicability for PCEP as a southbound interface, and
   introduces the implications for the protocol.  The section 2.1.3 of



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   [I-D.ietf-teas-pce-central-control] describe an hierarchy of PCE-
   based controller as per the Hierarchy of PCE framework defined in
   [RFC6805].  Both ACTN and PCECC is based on the same basic framework
   and thus compatible with each other.

6.  IANA Considerations

   This is an informational document and thus does not have any IANA
   allocations to be made.

7.  Security Considerations

   The ACTN framework described in [I-D.ietf-teas-actn-framework]
   defines key components and interfaces for managed traffic engineered
   networks.  It also list various security considerations such as
   request and control of resources, confidentially of the information,
   and availability of function which should be taken into
   consideration.

   When PCEP is used on the MPI/MMI, this interface needs to be secured,
   use of [I-D.ietf-pce-pceps] is RECOMENDED.  Each PCEP extension
   listed in this document, presents its individual security
   considerations, which continue to apply.

8.  Acknowledgments

   The authors would like to thank Jonathan Hardwick for the inspiration
   behind this document.  Further thanks to Avantika for her comments
   with suggested text.

9.  References

9.1.  Normative References

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <http://www.rfc-editor.org/info/rfc5440>.

9.2.  Informative References

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <http://www.rfc-editor.org/info/rfc3630>.






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   [RFC4203]  Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
              Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
              <http://www.rfc-editor.org/info/rfc4203>.

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <http://www.rfc-editor.org/info/rfc4655>.

   [RFC5152]  Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
              Per-Domain Path Computation Method for Establishing Inter-
              Domain Traffic Engineering (TE) Label Switched Paths
              (LSPs)", RFC 5152, DOI 10.17487/RFC5152, February 2008,
              <http://www.rfc-editor.org/info/rfc5152>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <http://www.rfc-editor.org/info/rfc5305>.

   [RFC5307]  Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
              <http://www.rfc-editor.org/info/rfc5307>.

   [RFC5441]  Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
              "A Backward-Recursive PCE-Based Computation (BRPC)
              Procedure to Compute Shortest Constrained Inter-Domain
              Traffic Engineering Label Switched Paths", RFC 5441,
              DOI 10.17487/RFC5441, April 2009,
              <http://www.rfc-editor.org/info/rfc5441>.

   [RFC5623]  Oki, E., Takeda, T., Le Roux, JL., and A. Farrel,
              "Framework for PCE-Based Inter-Layer MPLS and GMPLS
              Traffic Engineering", RFC 5623, DOI 10.17487/RFC5623,
              September 2009, <http://www.rfc-editor.org/info/rfc5623>.

   [RFC6805]  King, D., Ed. and A. Farrel, Ed., "The Application of the
              Path Computation Element Architecture to the Determination
              of a Sequence of Domains in MPLS and GMPLS", RFC 6805,
              DOI 10.17487/RFC6805, November 2012,
              <http://www.rfc-editor.org/info/rfc6805>.

   [RFC7399]  Farrel, A. and D. King, "Unanswered Questions in the Path
              Computation Element Architecture", RFC 7399,
              DOI 10.17487/RFC7399, October 2014,
              <http://www.rfc-editor.org/info/rfc7399>.




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   [RFC7491]  King, D. and A. Farrel, "A PCE-Based Architecture for
              Application-Based Network Operations", RFC 7491,
              DOI 10.17487/RFC7491, March 2015,
              <http://www.rfc-editor.org/info/rfc7491>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <http://www.rfc-editor.org/info/rfc7752>.

   [RFC8051]  Zhang, X., Ed. and I. Minei, Ed., "Applicability of a
              Stateful Path Computation Element (PCE)", RFC 8051,
              DOI 10.17487/RFC8051, January 2017,
              <http://www.rfc-editor.org/info/rfc8051>.

   [I-D.ietf-pce-stateful-pce]
              Crabbe, E., Minei, I., Medved, J., and R. Varga, "PCEP
              Extensions for Stateful PCE", draft-ietf-pce-stateful-
              pce-18 (work in progress), December 2016.

   [I-D.ietf-pce-pce-initiated-lsp]
              Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "PCEP
              Extensions for PCE-initiated LSP Setup in a Stateful PCE
              Model", draft-ietf-pce-pce-initiated-lsp-09 (work in
              progress), March 2017.

   [I-D.dhodylee-pce-stateful-hpce]
              Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., King, D.,
              and O. Dios, "Hierarchical Stateful Path Computation
              Element (PCE).", draft-dhodylee-pce-stateful-hpce-03 (work
              in progress), March 2017.

   [I-D.ietf-teas-pce-central-control]
              Farrel, A., Zhao, Q., Li, Z., and C. Zhou, "An
              Architecture for Use of PCE and PCEP in a Network with
              Central Control", draft-ietf-teas-pce-central-control-01
              (work in progress), December 2016.

   [I-D.ietf-teas-actn-requirements]
              Lee, Y., Dhody, D., Belotti, S., Pithewan, K., and D.
              Ceccarelli, "Requirements for Abstraction and Control of
              TE Networks", draft-ietf-teas-actn-requirements-04 (work
              in progress), January 2017.







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   [I-D.ietf-teas-actn-framework]
              Ceccarelli, D. and Y. Lee, "Framework for Abstraction and
              Control of Traffic Engineered Networks", draft-ietf-teas-
              actn-framework-04 (work in progress), February 2017.

   [I-D.ietf-teas-actn-info-model]
              Lee, Y., Belotti, S., Dhody, D., Ceccarelli, D., and B.
              Yoon, "Information Model for Abstraction and Control of TE
              Networks (ACTN)", draft-ietf-teas-actn-info-model-00 (work
              in progress), February 2017.

   [I-D.ietf-pce-inter-area-as-applicability]
              King, D., Meuric, J., Dugeon, O., Zhao, Q., Dhody, D., and
              O. Dios, "Applicability of the Path Computation Element to
              Inter-Area and Inter-AS MPLS and GMPLS Traffic
              Engineering", draft-ietf-pce-inter-area-as-
              applicability-06 (work in progress), July 2016.

   [I-D.dhodylee-pce-pcep-ls]
              Dhody, D., Lee, Y., and D. Ceccarelli, "PCEP Extension for
              Distribution of Link-State and TE Information.", draft-
              dhodylee-pce-pcep-ls-07 (work in progress), March 2017.

   [I-D.leedhody-pce-vn-association]
              Lee, Y., Dhody, D., Zhang, X., and D. Ceccarelli, "PCEP
              Extensions for Establishing Relationships Between Sets of
              LSPs and Virtual Networks", draft-leedhody-pce-vn-
              association-02 (work in progress), March 2017.

   [I-D.litkowski-pce-state-sync]
              Litkowski, S., Sivabalan, S., and D. Dhody, "Inter
              Stateful Path Computation Element communication
              procedures", draft-litkowski-pce-state-sync-01 (work in
              progress), February 2017.

   [I-D.ietf-pce-association-policy]
              Dhody, D., Sivabalan, S., Litkowski, S., Tantsura, J., and
              J. Hardwick, "Path Computation Element communication
              Protocol extension for associating Policies and LSPs",
              draft-ietf-pce-association-policy-00 (work in progress),
              December 2016.

   [I-D.ietf-pce-pceps]
              Lopez, D., Dios, O., Wu, W., and D. Dhody, "Secure
              Transport for PCEP", draft-ietf-pce-pceps-11 (work in
              progress), January 2017.





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   [I-D.lee-teas-actn-abstraction]
              Lee, Y., Dhody, D., Ceccarelli, D., and O. Dios,
              "Abstraction and Control of TE Networks (ACTN) Abstraction
              Methods", draft-lee-teas-actn-abstraction-00 (work in
              progress), October 2016.

Authors' Addresses

   Dhruv Dhody
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore, Karnataka  560066
   India

   EMail: dhruv.ietf@gmail.com


   Young Lee
   Huawei Technologies
   5340 Legacy Drive, Building 3
   Plano, TX  75023
   USA

   EMail: leeyoung@huawei.com


   Daniele Ceccarelli
   Ericsson
   Torshamnsgatan,48
   Stockholm
   Sweden

   EMail: daniele.ceccarelli@ericsson.com


















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