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PCE Working Group                                               D. Dhody
Internet-Draft                                                    Y. Lee
Intended status: Informational                       Huawei Technologies
Expires: June 8, 2019                                      D. Ceccarelli
                                                                Ericsson
                                                        December 5, 2018


  Applicability of Path Computation Element (PCE) for Abstraction and
                     Control of TE Networks (ACTN)
                  draft-ietf-pce-applicability-actn-08

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 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 (PCE) is 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 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 June 8, 2019.




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Copyright Notice

   Copyright (c) 2018 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
     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
       1.1.3.  Relationship to PCE Based Central Control . . . . . .   4
     1.2.  Abstraction and Control of TE Networks (ACTN) . . . . . .   5
     1.3.  PCE and ACTN  . . . . . . . . . . . . . . . . . . . . . .   7
   2.  Architectural Considerations  . . . . . . . . . . . . . . . .   7
     2.1.  Multi Domain Coordination via Hierarchy . . . . . . . . .   7
     2.2.  Abstraction . . . . . . . . . . . . . . . . . . . . . . .   8
     2.3.  Customer Mapping  . . . . . . . . . . . . . . . . . . . .   9
     2.4.  Virtual Service Coordination  . . . . . . . . . . . . . .  10
   3.  Interface Considerations  . . . . . . . . . . . . . . . . . .  10
   4.  Realizing ACTN with PCE (and PCEP)  . . . . . . . . . . . . .  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

1.1.  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.




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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 [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
   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.

1.1.1.  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].




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1.1.2.  PCE in Multi-domain and Multi-layer Deployments

   Computing paths across large multi-domain 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 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, 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 is a large set of interconnected
   domains.

   [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.ietf-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 Virtual
   Network Topology (VNT) [RFC5212], called the VNT Manager (VNTM).

1.1.3.  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, and introduces the implications for
   the protocol.  The section 2.1.3 of [RFC8283] describe a hierarchy of



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   PCE-based controller as per the Hierarchy of PCE framework defined in
   [RFC6805].

1.2.  Abstraction and Control of TE Networks (ACTN)

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

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






































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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)


                         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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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.

   Further, ACTN, Stateful H-PCE, and PCECC are based on the same basic
   hierarchy framework and thus compatible with each other.

2.  Architectural Considerations

   ACTN [RFC8453] 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

   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 funcitonl components.  Operator may
   choose to use the PCEP for multi domain coordination via stateful
   H-PCE, but alternatively use RESTCONF [RFC8040] or BGP-LS [RFC7752]
   to get access to the topology and support 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 [RFC8453], 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




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   detach from the underlying network technology and express customer
   concerns by business needs.

   [RFC6805] and [I-D.ietf-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.ietf-pce-stateful-hpce] is well suited for multi-domain
   coordination function.  This includes domain sequence selection; End-
   to-End (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.  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 the 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 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



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   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.
   [RFC8453] 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] and its optical network
   extension [I-D.lee-pce-pcep-ls-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.

2.3.  Customer Mapping

   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.

   [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
   [I-D.ietf-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 Label
   Switching Router (LSR).  The customer request is received by the MDSC



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   (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 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 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
   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 [RFC8453], 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 two 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.





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   o  The MDSC-PNC Interface (MPI) is an interface between a Multi
      Domain 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 multi-domain environments, the MDSC needs to establish
      multiple MPIs, one for each PNC, as there are multiple PNCs
      responsible for its domain control.

   In case of hierarchy of MDSC, 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 MSDCs.

   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 multi-level 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.
   The Section 4 describes how PCE and PCEP could help realize ACTN on
   the MPI.

4.  Realizing 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
   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 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
      [RFC6805] or it can have full topology information of all domains.
      The latter is not scalable and thus an abstracted topology of each




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      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
      (OF) 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 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,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.ietf-pce-stateful-hpce] and
         [I-D.leedhody-pce-vn-association]).  A suitable stitching
         mechanism would be used to stitch these per domain LSPs.  One
         such mechanism is described in
         [I-D.lee-pce-lsp-stitching-hpce], 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 PCE ([I-D.ietf-pce-stateful-hpce]).  The




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         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.

      *  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).  As described in [I-D.ietf-pce-stateful-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 are reported to the MDSC (via PCRpt message) as per
      [I-D.ietf-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.






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5.  IANA Considerations

   This document makes no requests for IANA action.

6.  Security Considerations

   The ACTN framework described in [RFC8453] 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, this interface needs to be secured.
   That can be achieved using the procdecures described in [RFC8253].
   Each PCEP extension listed in this document, presents its individual
   security considerations, which continue to apply.

7.  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.

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>.




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8.2.  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,
              <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>.

   [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>.







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   [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>.

   [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>.

   [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>.

   [I-D.ietf-pce-stateful-hpce]
              Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., King, D.,
              and O. Dios, "Hierarchical Stateful Path Computation
              Element (PCE).", draft-ietf-pce-stateful-hpce-06 (work in
              progress), October 2018.

   [I-D.ietf-teas-actn-requirements]
              Lee, Y., Ceccarelli, D., Miyasaka, T., Shin, J., and K.
              Lee, "Requirements for Abstraction and Control of TE
              Networks", draft-ietf-teas-actn-requirements-09 (work in
              progress), March 2018.

   [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-11 (work in progress), June 2018.

   [I-D.lee-pce-pcep-ls-optical]
              Lee, Y., Zheng, H., Ceccarelli, D., weiw@bupt.edu.cn, w.,
              Park, P., and B. Yoon, "PCEP Extension for Distribution of
              Link-State and TE information for Optical Networks",
              draft-lee-pce-pcep-ls-optical-05 (work in progress),
              September 2018.

   [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-06 (work in progress), October 2018.




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   [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-04 (work in
              progress), October 2018.

   [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-03 (work in progress),
              June 2018.

   [I-D.lee-pce-lsp-stitching-hpce]
              Lee, Y., Dhody, D., and D. Ceccarelli, "PCEP Extensions
              for Stitching LSPs in Hierarchical Stateful PCE Model",
              draft-lee-pce-lsp-stitching-hpce-01 (work in progress),
              December 2017.

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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