path: root/doc/rfc/rfc8637.txt

Internet Engineering Task Force (IETF)                          D. Dhody
Request for Comments: 8637                           Huawei Technologies
Category: Informational                                           Y. Lee
ISSN: 2070-1721                                   Futurewei Technologies
                                                           D. Ceccarelli
                                                                Ericsson
                                                               July 2019


          Applicability of the Path Computation Element (PCE)
          to the Abstraction and Control of TE Networks (ACTN)

Abstract

   Abstraction and Control of TE Networks (ACTN) refers to the set of
   virtual network (VN) operations needed to orchestrate, control, and
   manage large-scale multidomain 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 (PCE) is a component, application, or
   network node that is capable of computing a network path or route
   based on a network graph and applying computational constraints.  The
   PCE serves requests from Path Computation Clients (PCCs) that
   communicate with it over a local API or using the Path Computation
   Element Communication Protocol (PCEP).

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

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8637.





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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Background Information  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Path Computation Element (PCE)  . . . . . . . . . . . . .   3
       2.1.1.  Role of PCE in SDN  . . . . . . . . . . . . . . . . .   4
       2.1.2.  PCE in Multidomain and Multilayer Deployments . . . .   4
       2.1.3.  Relationship to PCE-Based Central Control . . . . . .   5
     2.2.  Abstraction and Control of TE Networks (ACTN) . . . . . .   5
   3.  Architectural Considerations  . . . . . . . . . . . . . . . .   7
     3.1.  Multidomain Coordination via Hierarchy  . . . . . . . . .   7
     3.2.  Abstraction . . . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  Customer Mapping  . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Virtual Service Coordination  . . . . . . . . . . . . . .  10
   4.  Interface Considerations  . . . . . . . . . . . . . . . . . .  10
   5.  Realizing ACTN with PCE (and PCEP)  . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Additional Information . . . . . . . . . . . . . . .  21
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Abstraction and Control of TE Networks (ACTN) [RFC8453] refers to the
   set of virtual network (VN) operations needed to orchestrate,
   control, and manage large-scale multidomain 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.



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   The Path Computation Element (PCE) [RFC4655] is a component,
   application, or network node that is capable of computing a network
   path or route based on a network graph and applying computational
   constraints.  The PCE serves requests from Path Computation Clients
   (PCCs) that communicate with it over a local API or using the Path
   Computation Element Communication Protocol (PCEP).

   This document examines the PCE and ACTN architecture and describes
   how 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.

   Further, ACTN, stateful Hierarchical PCE (H-PCE) [PCE-HPCE], and PCE
   as a central controller (PCECC) [RFC8283] are based on the same basic
   hierarchy framework and are thus compatible with each other.

2.  Background Information

2.1.  Path Computation Element (PCE)

   The Path Computation Element Communication Protocol (PCEP) [RFC5440]
   provides mechanisms for Path Computation Clients (PCCs) to request a
   Path Computation Element (PCE) [RFC4655] to perform path
   computations.

   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 [RFC8231] 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 (LSP-DB).

   [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.

   [RFC8231] 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




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   constrained paths while considering individual LSPs and their
   interactions.  [RFC8281] describes the setup, maintenance, and
   teardown of PCE-initiated LSPs under the stateful PCE model.

   [RFC8231] 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.

2.1.1.  Role of PCE in SDN

   Software-Defined Networking (SDN) [RFC7149] 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].

2.1.2.  PCE in Multidomain and Multilayer Deployments

   Computing paths across large multidomain environments requires
   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 multidomain 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 interdomain-constrained MPLS and
   GMPLS TE networks.  However, per-domain technique assumes 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.  BRPC can work best with a known domain sequence, and it will
   also work nicely with a small set of interconnected domains.
   However, it doesn't work well for a large set of interconnected
   domains.






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   [RFC6805] describes a Hierarchical PCE (H-PCE) architecture that can
   be used for computing end-to-end paths for interdomain 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 multidomain
   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 intradomain path based on its domain
   topology information.

   [PCE-HPCE] states the considerations for stateful PCEs in
   Hierarchical PCE architecture.  In particular, the behavior changes
   and adds 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 interlayer (G)MPLS TE.  It provides suggestions for
   the deployment of PCE in support of multilayer networks.  It also
   describes the relationship between PCE and a functional component in
   charge of the control and management of the Virtual Network Topology
   (VNT) [RFC5212] called the VNT Manager (VNTM).

2.1.3.  Relationship to PCE-Based Central Control

   [RFC8283] introduces the architecture for PCE as a central controller
   (PCECC); it further examines the motivations and applicability for
   PCEP as a southbound interface (SBI) and introduces the implications
   for the protocol.  Section 2.1.3 of [RFC8283] describes a hierarchy
   of PCE-based controllers as per the PCE Hierarchy Framework defined
   in [RFC6805].

2.2.  Abstraction and Control of TE Networks (ACTN)

   [RFC8453] describes the high-level ACTN requirements and the
   architecture model for ACTN, including the entities Customer Network
   Controller (CNC), Multidomain Service Coordinator (MDSC), and
   Provisioning Network Controller (PNC) and their interfaces.

   The ACTN reference architecture is shown in Figure 1, which is
   reproduced here from [RFC8453] for convenience.  [RFC8453] remains
   the definitive reference for the ACTN architecture.  As depicted in
   Figure 1, the ACTN architecture identifies a three-tier hierarchy.








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              +---------+           +---------+           +---------+
              |   CNC   |           |   CNC   |           |   CNC   |
              +---------+           +---------+           +---------+
                        \                |                /
                         \               |               /
   Boundary  =============\==============|==============/============
   Between                 \             |             /
   Customer &               -------      | CMI  -------
   Network Operator                \     |     /
                                 +---------------+
                                 |     MDSC      |
                                 +---------------+
                                   /     |     \
                       ------------      | MPI  -------------
                      /                  |                   \
                 +-------+          +-------+             +-------+
                 |  PNC  |          |  PNC  |             |  PNC  |
                 +-------+          +-------+             +-------+
                     | SBI            /   |                /   \
                     |               /    | SBI           /     \
                 ---------        -----   |              /       \
                (         )      (     )  |             /         \
                - Control -     ( Phys. ) |            /        -----
               (  Plane    )     ( Net )  |           /        (     )
              (  Physical   )     -----   |          /        ( Phys. )
               (  Network  )            -----      -----       ( Net )
                -         -            (     )    (     )       -----
                (         )           ( Phys. )  ( Phys. )
                 ---------             ( Net )    ( Net )
                                        -----      -----

   CMI - (CNC-MDSC Interface)
   MPI - (MDSC-PNC Interface)
   SBI - (Southbound Interface)

                         Figure 1: ACTN Hierarchy

   There are two interfaces with respect to the MDSC: one north of the
   MDSC (the CNC-MDSC Interface (CMI)), and one south (the MDSC-PNC
   Interface (MPI)).  A hierarchy of MDSCs is possible with a recursive
   MPI interface.

   [RFC8454] provides an information model for ACTN interfaces.








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3.  Architectural Considerations

   The ACTN architecture [RFC8453] is based on the hierarchy and
   recursiveness of controllers.  It defines three types of controllers
   (depending on the functionalities they implement).  The main
   functionalities are:

   o  Multidomain coordination

   o  Abstraction

   o  Customer mapping/translation

   o  Virtual service coordination

   Section 3 of [RFC8453] describes these functions.

   It should be noted that this document lists all possible ways in
   which PCE could be used for each of the above functions, but all
   functions are not required to be implemented via PCE.  Similarly,
   this document presents the ways in which PCEP could be used as the
   communications medium between functional components.  Operators may
   choose to use the PCEP for multidomain coordination via stateful
   H-PCE but alternatively use Network Configuration Protocol (NETCONF)
   [RFC6241], RESTCONF [RFC8040], or BGP - Link State (BGP-LS) [RFC7752]
   to get access to the topology and support abstraction function.

3.1.  Multidomain Coordination via Hierarchy

   With the definition of domain being everything that is under the
   control of the single logical controller, as per [RFC8453], it is
   needed both 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 [PCE-HPCE] describe a hierarchy of PCEs with the Parent
   PCE coordinating multidomain path computation function between Child
   PCEs.  It is easy to see how these principles align, and thus how the
   stateful H-PCE architecture can be used to realize ACTN.






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   The per-domain stitched LSP in the Hierarchical stateful PCE
   architecture, described in Section 3.3.1 of [PCE-HPCE], is well
   suited for multidomain coordination function.  This includes domain
   sequence selection, End-to-End (E2E) path computation, and
   controller-initiated (PCE-initiated) path setup and reporting.  This
   is also applicable to multilayer coordination in case of IP+optical
   networks.

   [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 are thus useful for ACTN as well.

3.2.  Abstraction

   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 includes abstract topology created as per the
   customer service connectivity requests and represented as a VN 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 [PCE-INTER-AREA] suggest that BGP-LS could be used as a
   "northbound" TE advertisement from the Child PCE to the Parent PCE.

   [PCEP-LS] proposes another approach 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 the
   PNC (Child PCE) to the MDSC (Parent PCE) should support abstraction.
   [RFC8453] describes a few alternative approaches of abstraction.  The
   resulting abstracted topology can be encoded using the PCEP-LS
   mechanisms [PCEP-LS] and its optical network extension



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   [PCEP-OPTICAL].  PCEP-LS is an attractive option when the operator
   would wish to have a single control-plane protocol (PCEP) to achieve
   ACTN functions.

   [RFC8453] discusses two ways to build abstract topology from an MDSC
   standpoint with interaction with PNCs.  The primary method is called
   automatic generation of abstract topology by configuration.  With
   this method, automatic generation is based on the abstraction/
   summarization of the whole domain by the PNC and its advertisement on
   the MPI.  The secondary method is called on-demand generation of
   supplementary topology via Path Compute Request/Reply.  This method
   may be needed to obtain further complementary information such as
   potential connectivity from Child PCEs in order to facilitate an end-
   to-end path provisioning.  PCEP is well suited to support both
   methods.

3.3.  Customer Mapping

   In ACTN, there is a need to map customer virtual network (VN)
   requirements into a network provisioning request to the PNC.  That
   is, the customer requests/commands are mapped by the MDSC into
   network provisioning requests that can be sent to the PNC.
   Specifically, the MDSC 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 the network configuration
   process is made possible.

   [RFC8281] 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 [PCE-HPCE], for
   interdomain LSP in Hierarchical PCE architecture, the initiation
   operations can be carried out at the Parent PCE.  In this 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 Label Switching Router (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 a 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
   [ASSOC-POLICY] can be used.





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3.4.  Virtual Service Coordination

   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 customers' service requirements.

   [PCEP-VN] 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 ensure optimization and SLA attainment for the VN rather than for
   a single LSP.  Similarly, during reoptimization, advanced path
   computation algorithms and optimization techniques can be considered
   for all the LSPs belonging to a VN/customer and optimize them all
   together.

4.  Interface Considerations

   As per [RFC8453], to allow virtualization and multidomain
   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 two ACTN interfaces are as follows:

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






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   In the case of a hierarchy of MDSCs, the MPI is applied recursively.
   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 MDSCs.

   PCEP is especially suitable on the MPI as it meets the requirement
   and the functions as set out in the ACTN framework [RFC8453].  Its
   recursive nature is well suited via the multilevel hierarchy of PCE.
   PCEP can also be applied to the CMI as the CNC can be a path
   computation client while the MDSC can be a path computation server.
   Section 5 describes how PCE and PCEP could help realize ACTN on the
   MPI.

5.  Realizing ACTN with PCE (and PCEP)

   As per the example in Figure 2, there are 4 domains, each with their
   own PNC and MDSC on top.  The PNC and MDSC need PCE as an important
   function.  The PNC (or Child PCE) already uses PCEP to communicate to
   the network device.  It can utilize the PCEP as the MPI to
   communicate between controllers too.































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                             ******
                   ..........*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 the path in its domain.  As described in
      Section 3.2, 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 [PCEP-LS].  At the end of
      this step, the MDSC (or Parent PCE) has the abstracted topology
      from each of its PNCs (or Child PCE).  This could be as simple as
      a domain topology map as described in [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 interdomain links is the most common case.



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      *  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: When an 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 3.4 and [PCEP-VN], 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 (OF), is already defined in
      PCEP in terms of a single path.

      *  Path Computation: As per the example in 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 3.1) using the PCReq and PCRep
         messages to get the end-to-end path with the help of the Child
         PCEs (PNC).  Either way, the resultant E2E paths may be broken
         into per-domain paths.

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

      *  A-C: (A-B13,B13-B31,B31-B34,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 [PCE-HPCE] and [PCEP-VN]).
         A suitable stitching mechanism would be used to stitch these
         per-domain LSPs.  One such mechanism is described in
         [PCE-INTERDOMAIN], where PCEP is extended to support stitching
         in stateful H-PCE context.

      *  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 PCEs ([PCE-HPCE]).  The status of the
         end-to-end LSP (A-B and A-C) is made up when all the per-domain
         LSPs are reported up by the PNCs.

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



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      *  State Synchronization: The state needs to be synchronized
         between the Parent PCE and Child PCE.  The mechanism described
         in [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 an existing per-intradomain path (via PCUpd message) or
      the creation (or deletion) of a per-domain path (via PCInitiate
      message).  As described in [PCE-HPCE], 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 is reported to the MDSC (via PCRpt message) as per [PCE-HPCE].
      This may result in changes in the E2E path or VN status.  This may
      also trigger a reoptimization leading to a new per-domain path, an
      update to an existing path, or the deletion of the path.

   o  VN Protection: The VN protection/restoration requirements need to
      be 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 a PNC generates an abstract topology towards the MDSC, the
      PCInitiate/PCUpd messages from the MDSC to a PNC will contain a
      path with abstract nodes and links.  A PNC would need to take that
      as an input for path computation to get a path with physical nodes
      and links.  Similarly, a PNC would convert the path received from
      the device (with physical nodes and links) into an abstract path
      (based on the abstract topology generated before with abstract
      nodes and links) and report it to the MDSC.

6.  IANA Considerations

   This document has no IANA actions.










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7.  Security Considerations

   Various security considerations for PCEP are described in [RFC5440]
   and [RFC8253].  Security considerations as stated in Sections 10.1,
   10.6, and 10.7 of [RFC5440] continue to apply on PCEP when used as an
   ACTN interface.  Further, this document lists various extensions of
   PCEP that are applicable; each of them specify various security
   considerations that continue to apply here.

   The ACTN framework described in [RFC8453] defines key components and
   interfaces for managed traffic-engineered networks.  It also lists
   various security considerations such as request and control of
   resources, confidentially of the information, and availability of
   function, which should be taken into consideration.

   As per [RFC8453], securing the request and control of resources,
   confidentiality of the information, and availability of function
   should all be critical security considerations when deploying and
   operating ACTN platforms.  From a security and reliability
   perspective, ACTN may encounter many risks such as malicious attack
   and rogue elements attempting to connect to various ACTN components
   (with PCE being one of them).  Furthermore, some ACTN components
   represent a single point of failure and threat vector, and must also
   manage policy conflicts and eavesdropping of communication between
   different ACTN components.  [RFC8453] further states that all
   protocols used to realize the ACTN framework should have rich
   security features, and customer, application, and network data should
   be stored in encrypted data stores.  When PCEP is used as an ACTN
   interface, the security of PCEP provided by Transport Layer Security
   (TLS) [RFC8253], as per the recommendations and best current
   practices in [RFC7525] (unless explicitly set aside in [RFC8253]), is
   used.

   As per [RFC8453], regarding the MPI, a PKI-based mechanism is
   suggested, such as building a TLS or HTTPS connection between the
   MDSC and PNCs, to ensure trust between the physical network layer
   control components and the MDSC.  Which MDSC the PNC exports topology
   information to, and the level of detail (full or abstracted), should
   also be authenticated, and specific access restrictions and topology
   views should be configurable and/or policy based.  When PCEP is used
   in MPI, the security functions, as per [RFC8253], are used to fulfill
   these requirements.

   As per [RFC8453], regarding the CMI, suitable authentication and
   authorization of each CNC connecting to the MDSC will be required.
   If PCEP is used in CMI, the security functions, as per [RFC8253], can
   be used to support peer authentication, message encryption, and
   integrity checks.



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8.  References

8.1.  Normative References

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

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

   [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,
              <https://www.rfc-editor.org/info/rfc6805>.

   [RFC8453]  Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
              Abstraction and Control of TE Networks (ACTN)", RFC 8453,
              DOI 10.17487/RFC8453, August 2018,
              <https://www.rfc-editor.org/info/rfc8453>.

8.2.  Informative References

   [ASSOC-POLICY]
              Litkowski, S., Sivabalan, S., Tantsura, J., Hardwick, J.,
              and M. Negi, "Path Computation Element communication
              Protocol extension for associating Policies and LSPs",
              Work in Progress, draft-ietf-pce-association-policy-05,
              February 2019.

   [EXP]      Casellas, R., Vilalta, R., Martinez, R., Munoz, R., Zheng,
              H., and Y. Lee, "Experimental Validation of the ACTN
              architecture for flexi-grid optical networks using Active
              Stateful Hierarchical PCEs", 19th International Conference
              on Transparent Optical Networks (ICTON),
              DOI 10.5281/zenodo.832904, July 2017,
              <https://zenodo.org/record/832904>.

   [PCE-HPCE]
              Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., and D. King,
              "Hierarchical Stateful Path Computation Element (PCE).",
              Work in Progress, draft-ietf-pce-stateful-hpce-10, June
              2019.




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   [PCE-INTER-AREA]
              King, D. and H. Zheng, "Applicability of the Path
              Computation Element to Interarea and Inter-AS MPLS and
              GMPLS Traffic Engineering", Work in Progress,
              draft-ietf-pce-inter-area-as-applicability-07, December
              2018.

   [PCE-INTERDOMAIN]
              Dugeon, O., Meuric, J., Lee, Y., and D. Ceccarelli, "PCEP
              Extension for Stateful Interdomain Tunnels", Work in
              Progress, draft-dugeon-pce-stateful-interdomain-02, March
              2019.

   [PCE-STATE-SYNC]
              Litkowski, S., Sivabalan, S., Li, C., and H. Zheng, "Inter
              Stateful Path Computation Element (PCE) Communication
              Procedures.", Work in Progress,
              draft-litkowski-pce-state-sync-05, March 2019.

   [PCEP-LS]  Dhody, D., Lee, Y., and D. Ceccarelli, "PCEP Extension for
              Distribution of Link-State and TE Information.", Work in
              Progress, draft-dhodylee-pce-pcep-ls-13, February 2019.

   [PCEP-OPTICAL]
              Lee, Y., Zheng, H., Ceccarelli, D., Wang, W., Park, P.,
              and B. Yoon, "PCEP Extension for Distribution of Link-
              State and TE information for Optical Networks", Work in
              Progress, draft-lee-pce-pcep-ls-optical-07, March 2019.

   [PCEP-VN]  Lee, Y., Zhang, X., and D. Ceccarelli, "Path Computation
              Element communication Protocol (PCEP) extensions for
              Establishing Relationships between sets of LSPs and
              Virtual Networks", Work in Progress,
              draft-leedhody-pce-vn-association-08, June 2019.

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

   [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,
              <https://www.rfc-editor.org/info/rfc4203>.







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   [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,
              <https://www.rfc-editor.org/info/rfc5152>.

   [RFC5212]  Shiomoto, K., Papadimitriou, D., Le Roux, JL., Vigoureux,
              M., and D. Brungard, "Requirements for GMPLS-Based Multi-
              Region and Multi-Layer Networks (MRN/MLN)", RFC 5212,
              DOI 10.17487/RFC5212, July 2008,
              <https://www.rfc-editor.org/info/rfc5212>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://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,
              <https://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,
              <https://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, <https://www.rfc-editor.org/info/rfc5623>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC7149]  Boucadair, M. and C. Jacquenet, "Software-Defined
              Networking: A Perspective from within a Service Provider
              Environment", RFC 7149, DOI 10.17487/RFC7149, March 2014,
              <https://www.rfc-editor.org/info/rfc7149>.

   [RFC7399]  Farrel, A. and D. King, "Unanswered Questions in the Path
              Computation Element Architecture", RFC 7399,
              DOI 10.17487/RFC7399, October 2014,
              <https://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,
              <https://www.rfc-editor.org/info/rfc7491>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [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,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

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

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

   [RFC8253]  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
              "PCEPS: Usage of TLS to Provide a Secure Transport for the
              Path Computation Element Communication Protocol (PCEP)",
              RFC 8253, DOI 10.17487/RFC8253, October 2017,
              <https://www.rfc-editor.org/info/rfc8253>.

   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.








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   [RFC8283]  Farrel, A., Ed., Zhao, Q., Ed., Li, Z., and C. Zhou, "An
              Architecture for Use of PCE and the PCE Communication
              Protocol (PCEP) in a Network with Central Control",
              RFC 8283, DOI 10.17487/RFC8283, December 2017,
              <https://www.rfc-editor.org/info/rfc8283>.

   [RFC8454]  Lee, Y., Belotti, S., Dhody, D., Ceccarelli, D., and B.
              Yoon, "Information Model for Abstraction and Control of TE
              Networks (ACTN)", RFC 8454, DOI 10.17487/RFC8454,
              September 2018, <https://www.rfc-editor.org/info/rfc8454>.









































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Appendix A.  Additional Information

   In the paper [EXP], the application of the ACTN architecture is
   presented to demonstrate the control of a multidomain flexi-grid
   optical network by proposing, adopting, and extending:

   o  the Hierarchical active stateful PCE architectures and protocols

   o  the PCEP protocol to support efficient and incremental link-state
      topological reporting, known as PCEP-LS

   o  the per-link partitioning of the optical spectrum based on
      variable-sized allocated frequency slots enabling network sharing
      and virtualization

   o  the use of a model-based interface to dynamically request the
      instantiation of virtual networks for specific clients/tenants.

   The design and implementation of the test bed are reported in order
   to validate the approach.

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.

   Thanks to Adrian Farrel and Daniel King for their substantial
   reviews.

   Thanks to Yingzhen Qu for RTGDIR review.

   Thanks to Rifaat Shekh-Yusef for SECDIR review.

   Thanks to Meral Shirazipour for GENART review.

   Thanks to Roman Danyliw and Barry Leiba for IESG review comments.

   Thanks to Deborah Brungard for being the responsible AD.












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Authors' Addresses

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

   Email: dhruv.ietf@gmail.com


   Young Lee
   Futurewei Technologies
   5340 Legacy Drive, Suite 173
   Plano, TX  75024
   United States of America

   Email: younglee.tx@gmail.com


   Daniele Ceccarelli
   Ericsson
   Torshamnsgatan,48
   Stockholm
   Sweden

   Email: daniele.ceccarelli@ericsson.com
























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