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RFC 8453

Informational
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Framework for Abstraction and Control of TE Networks (ACTN)

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Internet Engineering Task Force (IETF)                D. Ceccarelli, Ed.
Request for Comments: 8453                                      Ericsson
Category: Informational                                      Y. Lee, Ed.
ISSN: 2070-1721                                                   Huawei
                                                             August 2018


      Framework for Abstraction and Control of TE Networks (ACTN)

Abstract

   Traffic Engineered (TE) networks have a variety of mechanisms to
   facilitate the separation of the data plane and control plane.  They
   also have a range of management and provisioning protocols to
   configure and activate network resources.  These mechanisms represent
   key technologies for enabling flexible and dynamic networking.  The
   term "Traffic Engineered network" refers to a network that uses any
   connection-oriented technology under the control of a distributed or
   centralized control plane to support dynamic provisioning of end-to-
   end connectivity.

   Abstraction of network resources is a technique that can be applied
   to a single network domain or across multiple domains to create a
   single virtualized network that is under the control of a network
   operator or the customer of the operator that actually owns the
   network resources.

   This document provides a framework for Abstraction and Control of TE
   Networks (ACTN) to support virtual network services and connectivity
   services.

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/rfc8453.

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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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  VNS Model of ACTN . . . . . . . . . . . . . . . . . . . .   7
       2.2.1.  Customers . . . . . . . . . . . . . . . . . . . . . .   9
       2.2.2.  Service Providers . . . . . . . . . . . . . . . . . .   9
       2.2.3.  Network Operators . . . . . . . . . . . . . . . . . .  10
   3.  ACTN Base Architecture  . . . . . . . . . . . . . . . . . . .  10
     3.1.  Customer Network Controller . . . . . . . . . . . . . . .  12
     3.2.  Multi-Domain Service Coordinator  . . . . . . . . . . . .  13
     3.3.  Provisioning Network Controller . . . . . . . . . . . . .  13
     3.4.  ACTN Interfaces . . . . . . . . . . . . . . . . . . . . .  14
   4.  Advanced ACTN Architectures . . . . . . . . . . . . . . . . .  15
     4.1.  MDSC Hierarchy  . . . . . . . . . . . . . . . . . . . . .  15
     4.2.  Functional Split of MDSC Functions in Orchestrators . . .  16
   5.  Topology Abstraction Methods  . . . . . . . . . . . . . . . .  18
     5.1.  Abstraction Factors . . . . . . . . . . . . . . . . . . .  18
     5.2.  Abstraction Types . . . . . . . . . . . . . . . . . . . .  19
       5.2.1.  Native/White Topology . . . . . . . . . . . . . . . .  19
       5.2.2.  Black Topology  . . . . . . . . . . . . . . . . . . .  19
       5.2.3.  Grey Topology . . . . . . . . . . . . . . . . . . . .  20
     5.3.  Methods of Building Grey Topologies . . . . . . . . . . .  21
       5.3.1.  Automatic Generation of Abstract Topology by
               Configuration . . . . . . . . . . . . . . . . . . . .  22
       5.3.2.  On-Demand Generation of Supplementary Topology via
               Path Compute Request/Reply  . . . . . . . . . . . . .  22
     5.4.  Hierarchical Topology Abstraction Example . . . . . . . .  23
     5.5.  VN Recursion with Network Layers  . . . . . . . . . . . .  25
   6.  Access Points and Virtual Network Access Points . . . . . . .  28
     6.1.  Dual-Homing Scenario  . . . . . . . . . . . . . . . . . .  30

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   7.  Advanced ACTN Application: Multi-Destination Service . . . .   31
     7.1.  Preplanned Endpoint Migration . . . . . . . . . . . . . .  32
     7.2.  On-the-Fly Endpoint Migration . . . . . . . . . . . . . .  33
   8.  Manageability Considerations  . . . . . . . . . . . . . . . .  33
     8.1.  Policy  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     8.2.  Policy Applied to the Customer Network Controller . . . .  34
     8.3.  Policy Applied to the Multi-Domain Service Coordinator  .  35
     8.4.  Policy Applied to the Provisioning Network Controller . .  35
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  36
     9.1.  CNC-MDSC Interface (CMI)  . . . . . . . . . . . . . . . .  37
     9.2.  MDSC-PNC Interface (MPI)  . . . . . . . . . . . . . . . .  37
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
   11. Informative References  . . . . . . . . . . . . . . . . . . .  38
   Appendix A.  Example of MDSC and PNC Functions Integrated in a
                Service/Network Orchestrator . . . . . . . . . . . .  40
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

1.  Introduction

   The term "Traffic Engineered network" refers to a network that uses
   any connection-oriented technology under the control of a distributed
   or centralized control plane to support dynamic provisioning of end-
   to-end connectivity.  TE networks have a variety of mechanisms to
   facilitate the separation of data planes and control planes including
   distributed signaling for path setup and protection, centralized path
   computation for planning and traffic engineering, and a range of
   management and provisioning protocols to configure and activate
   network resources.  These mechanisms represent key technologies for
   enabling flexible and dynamic networking.  Some examples of networks
   that are in scope of this definition are optical, MPLS Transport
   Profile (MPLS-TP) [RFC5654], and MPLS-TE networks [RFC2702].

   One of the main drivers for Software-Defined Networking (SDN)
   [RFC7149] is a decoupling of the network control plane from the data
   plane.  This separation has been achieved for TE networks with the
   development of MPLS/GMPLS [RFC3945] and the Path Computation Element
   (PCE) [RFC4655].  One of the advantages of SDN is its logically
   centralized control regime that allows a global view of the
   underlying networks.  Centralized control in SDN helps improve
   network resource utilization compared with distributed network
   control.  For TE-based networks, a PCE may serve as a logically
   centralized path computation function.

   This document describes a set of management and control functions
   used to operate one or more TE networks to construct virtual networks
   that can be presented to customers and that are built from
   abstractions of the underlying TE networks.  For example, a link in

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   the customer's network is constructed from a path or collection of
   paths in the underlying networks.  We call this set of functions
   "Abstraction and Control of TE Networks" or "ACTN".

2.  Overview

   Three key aspects that need to be solved by SDN are:

   o  Separation of service requests from service delivery so that the
      configuration and operation of a network is transparent from the
      point of view of the customer but it remains responsive to the
      customer's services and business needs.

   o  Network abstraction: As described in [RFC7926], abstraction is the
      process of applying policy to a set of information about a TE
      network to produce selective information that represents the
      potential ability to connect across the network.  The process of
      abstraction presents the connectivity graph in a way that is
      independent of the underlying network technologies, capabilities,
      and topology so that the graph can be used to plan and deliver
      network services in a uniform way

   o  Coordination of resources across multiple independent networks and
      multiple technology layers to provide end-to-end services
      regardless of whether or not the networks use SDN.

   As networks evolve, the need to provide support for distinct
   services, separated service orchestration, and resource abstraction
   have emerged as key requirements for operators.  In order to support
   multiple customers each with its own view of and control of the
   server network, a network operator needs to partition (or "slice") or
   manage sharing of the network resources.  Network slices can be
   assigned to each customer for guaranteed usage, which is a step
   further than shared use of common network resources.

   Furthermore, each network represented to a customer can be built from
   virtualization of the underlying networks so that, for example, a
   link in the customer's network is constructed from a path or
   collection of paths in the underlying network.

   ACTN can facilitate virtual network operation via the creation of a
   single virtualized network or a seamless service.  This supports
   operators in viewing and controlling different domains (at any
   dimension: applied technology, administrative zones, or vendor-
   specific technology islands) and presenting virtualized networks to
   their customers.

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   The ACTN framework described in this document facilitates:

   o  Abstraction of the underlying network resources to higher-layer
      applications and customers [RFC7926].

   o  Virtualization of particular underlying resources, whose selection
      criterion is the allocation of those resources to a particular
      customer, application, or service [ONF-ARCH].

   o  TE Network slicing of infrastructure to meet specific customers'
      service requirements.

   o  Creation of an abstract environment allowing operators to view and
      control multi-domain networks as a single abstract network.

   o  The presentation to customers of networks as a virtual network via
      open and programmable interfaces.

2.1.  Terminology

   The following terms are used in this document.  Some of them are
   newly defined, some others reference existing definitions:

   Domain:  A domain as defined by [RFC4655] is "any collection of
      network elements within a common sphere of address management or
      path computation responsibility".  Specifically, within this
      document we mean a part of an operator's network that is under
      common management (i.e., under shared operational management using
      the same instances of a tool and the same policies).  Network
      elements will often be grouped into domains based on technology
      types, vendor profiles, and geographic proximity.

   Abstraction:  This process is defined in [RFC7926].

   TE Network Slicing:  In the context of ACTN, a TE network slice is a
      collection of resources that is used to establish a logically
      dedicated virtual network over one or more TE networks.  TE
      network slicing allows a network operator to provide dedicated
      virtual networks for applications/customers over a common network
      infrastructure.  The logically dedicated resources are a part of
      the larger common network infrastructures that are shared among
      various TE network slice instances, which are the end-to-end
      realization of TE network slicing, consisting of the combination
      of physically or logically dedicated resources.

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   Node:  A node is a vertex on the graph representation of a TE
      topology.  In a physical network topology, a node corresponds to a
      physical network element (NE) such as a router.  In an abstract
      network topology, a node (sometimes called an "abstract node") is
      a representation as a single vertex of one or more physical NEs
      and their connecting physical connections.  The concept of a node
      represents the ability to connect from any access to the node (a
      link end) to any other access to that node, although "limited
      cross-connect capabilities" may also be defined to restrict this
      functionality.  Network abstraction may be applied recursively, so
      a node in one topology may be created by applying abstraction to
      the nodes in the underlying topology.

   Link:  A link is an edge on the graph representation of a TE
      topology.  Two nodes connected by a link are said to be "adjacent"
      in the TE topology.  In a physical network topology, a link
      corresponds to a physical connection.  In an abstract network
      topology, a link (sometimes called an "abstract link") is a
      representation of the potential to connect a pair of points with
      certain TE parameters (see [RFC7926] for details).  Network
      abstraction may be applied recursively, so a link in one topology
      may be created by applying abstraction to the links in the
      underlying topology.

   Abstract Topology:  The topology of abstract nodes and abstract links
      presented through the process of abstraction by a lower-layer
      network for use by a higher-layer network.

   Virtual Network (VN):  A VN is a network provided by a service
      provider to a customer for the customer to use in any way it wants
      as though it was a physical network.  There are two views of a VN
      as follows:

      o  The VN can be abstracted as a set of edge-to-edge links (a Type
         1 VN).  Each link is referred as a "VN member" and is formed as
         an end-to-end tunnel across the underlying networks.  Such
         tunnels may be constructed by recursive slicing or abstraction
         of paths in the underlying networks and can encompass edge
         points of the customer's network, access links, intra-domain
         paths, and inter-domain links.

      o  The VN can also be abstracted as a topology of virtual nodes
         and virtual links (a Type 2 VN).  The operator needs to map the
         VN to actual resource assignment, which is known as "virtual
         network embedding".  The nodes in this case include physical
         endpoints, border nodes, and internal nodes as well as

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         abstracted nodes.  Similarly, the links include physical access
         links, inter-domain links, and intra-domain links as well as
         abstract links.

      Clearly, a Type 1 VN is a special case of a Type 2 VN.

   Access link:  A link between a customer node and an operator node.

   Inter-domain link:  A link between domains under distinct management
      administration.

   Access Point (AP):  An AP is a logical identifier shared between the
      customer and the operator used to identify an access link.  The AP
      is used by the customer when requesting a Virtual Network Service
      (VNS).  Note that the term "TE Link Termination Point" defined in
      [TE-TOPO] describes the endpoints of links, while an AP is a
      common identifier for the link itself.

   VN Access Point (VNAP):  A VNAP is the binding between an AP and a
      given VN.

   Server Network:  As defined in [RFC7926], a server network is a
      network that provides connectivity for another network (the Client
      Network) in a client-server relationship.

2.2.  VNS Model of ACTN

   A Virtual Network Service (VNS) is the service agreement between a
   customer and operator to provide a VN.  When a VN is a simple
   connectivity between two points, the difference between VNS and
   connectivity service becomes blurred.  There are three types of VNSs
   defined in this document.

   o  Type 1 VNS refers to a VNS in which the customer is allowed to
      create and operate a Type 1 VN.

   o  Type 2a and 2b VNS refer to VNSs in which the customer is allowed
      to create and operates a Type 2 VN.  With a Type 2a VNS, the VN is
      statically created at service configuration time, and the customer
      is not allowed to change the topology (e.g., by adding or deleting
      abstract nodes and links).  A Type 2b VNS is the same as a Type 2a
      VNS except that the customer is allowed to make dynamic changes to
      the initial topology created at service configuration time.

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   VN Operations are functions that a customer can exercise on a VN
   depending on the agreement between the customer and the operator.

   o  VN Creation allows a customer to request the instantiation of a
      VN.  This could be through offline preconfiguration or through
      dynamic requests specifying attributes to a Service Level
      Agreement (SLA) to satisfy the customer's objectives.

   o  Dynamic Operations allow a customer to modify or delete the VN.
      The customer can further act upon the virtual network to
      create/modify/delete virtual links and nodes.  These changes will
      result in subsequent tunnel management in the operator's networks.

   There are three key entities in the ACTN VNS model:

   o  Customers
   o  Service Providers
   o  Network Operators

   These entities are related in a three tier model as shown in
   Figure 1.

                           +----------------------+
                           |       Customer       |
                           +----------------------+
                                      |
                       VNS       ||   |   /\     VNS
                      Request    ||   |   ||    Reply
                                 \/   |   ||
                           +----------------------+
                           |  Service Provider    |
                           +----------------------+
                           /          |           \
                          /           |            \
                         /            |             \
                        /             |              \
    +------------------+   +------------------+   +------------------+
    |Network Operator 1|   |Network Operator 2|   |Network Operator 3|
    +------------------+   +------------------+   +------------------+

                      Figure 1: The Three-Tier Model

   The commercial roles of these entities are described in the following
   sections.

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2.2.1.  Customers

   Basic customers include fixed residential users, mobile users, and
   small enterprises.  Each requires a small amount of resources and is
   characterized by steady requests (relatively time invariant).  Basic
   customers do not modify their services themselves: if a service
   change is needed, it is performed by the provider as a proxy.

   Advanced customers include enterprises and governments.  Such
   customers ask for both point-to point and multipoint connectivity
   with high resource demands varying significantly in time.  This is
   one of the reasons why a bundled service offering is not enough, and
   it is desirable to provide each advanced customer with a customized
   VNS.  Advanced customers may also have the ability to modify their
   service parameters within the scope of their virtualized
   environments.  The primary focus of ACTN is Advanced Customers.

   As customers are geographically spread over multiple network operator
   domains, they have to interface to multiple operators and may have to
   support multiple virtual network services with different underlying
   objectives set by the network operators.  To enable these customers
   to support flexible and dynamic applications, they need to control
   their allocated virtual network resources in a dynamic fashion; that
   means that they need a view of the topology that spans all of the
   network operators.  Customers of a given service provider can, in
   turn, offer a service to other customers in a recursive way.

2.2.2.  Service Providers

   In the scope of ACTN, service providers deliver VNSs to their
   customers.  Service providers may or may not own physical network
   resources (i.e., may or may not be network operators as described in
   Section 2.2.3).  When a service provider is the same as the network
   operator, the case is similar to existing VPN models applied to a
   single operator (although it may be hard to use this approach when
   the customer spans multiple independent network operator domains).

   When network operators supply only infrastructure, while distinct
   service providers interface with the customers, the service providers
   are themselves customers of the network infrastructure operators.
   One service provider may need to keep multiple independent network
   operators because its end users span geographically across multiple
   network operator domains.  In some cases, a service provider is also
   a network operator when it owns network infrastructure on which
   service is provided.

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2.2.3.  Network Operators

   Network operators are the infrastructure operators that provision the
   network resources and provide network resources to their customers.
   The layered model described in this architecture separates the
   concerns of network operators and customers, with service providers
   acting as aggregators of customer requests.

3.  ACTN Base Architecture

   This section provides a high-level model of ACTN, showing the
   interfaces and the flow of control between components.

   The ACTN architecture is based on a three-tier reference model and
   allows for hierarchy and recursion.  The main functionalities within
   an ACTN system are:

   o  Multi-domain coordination: This function oversees the specific
      aspects of different domains and builds a single abstracted end-
      to-end network topology in order to coordinate end-to-end path
      computation and path/service provisioning.  Domain sequence path
      calculation/determination is also a part of this function.

   o  Abstraction: This function provides an abstracted view of the
      underlying network resources for use by the customer -- a customer
      may be the client or a higher-level controller entity.  This
      function includes network path computation based on customer-
      service-connectivity request constraints, path computation based
      on the global network-wide abstracted topology, and the creation
      of an abstracted view of network resources allocated to each
      customer.  These operations depend on customer-specific network
      objective functions and customer traffic profiles.

   o  Customer mapping/translation: This function is to map customer
      requests/commands into network provisioning requests that can be
      sent from the Multi-Domain Service Coordinator (MDSC) to the
      Provisioning Network Controller (PNC) according to business
      policies provisioned statically or dynamically at the Operations
      Support System (OSS) / Network Management System (NMS).
      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.

   o  Virtual service coordination: This function translates information
      that is customer service related into virtual network service
      operations in order to seamlessly operate virtual networks while
      meeting a customer's service requirements.  In the context of

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      ACTN, service/virtual service coordination includes a number of
      service orchestration functions such as multi-destination load-
      balancing and guarantees of service quality.  It also includes
      notifications for service fault and performance degradation and so
      forth.

   The base ACTN architecture defines three controller types and the
   corresponding interfaces between these controllers.  The following
   types of controller are shown in Figure 2:

   o  CNC - Customer Network Controller
   o  MDSC - Multi-Domain Service Coordinator
   o  PNC - Provisioning Network Controller

   Figure 2 also shows the following interfaces

   o  CMI - CNC-MDSC Interface
   o  MPI - MDSC-PNC Interface
   o  SBI - Southbound Interface

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

                     Figure 2: ACTN Base Architecture

   Note that this is a functional architecture: an implementation and
   deployment might collocate one or more of the functional components.
   Figure 2 shows a case where the service provider is also a network
   operator.

3.1.  Customer Network Controller

   A Customer Network Controller (CNC) is responsible for communicating
   a customer's VNS requirements to the network operator over the CNC-
   MDSC Interface (CMI).  It has knowledge of the endpoints associated
   with the VNS (expressed as APs), the service policy, and other QoS
   information related to the service.

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   As the CNC directly interfaces with the applications, it understands
   multiple application requirements and their service needs.  The
   capability of a CNC beyond its CMI role is outside the scope of ACTN
   and may be implemented in different ways.  For example, the CNC may,
   in fact, be a controller or part of a controller in the customer's
   domain, or the CNC functionality could also be implemented as part of
   a service provider's portal.

3.2.  Multi-Domain Service Coordinator

   A Multi-Domain Service Coordinator (MDSC) is a functional block that
   implements all of the ACTN functions listed in Section 3 and
   described further in Section 4.2.  Two functions of the MDSC, namely,
   multi-domain coordination and virtualization/abstraction are referred
   to as network-related functions; whereas the other two functions,
   namely, customer mapping/translation and virtual service
   coordination, are referred to as service-related functions.  The MDSC
   sits at the center of the ACTN model between the CNC that issues
   connectivity requests and the Provisioning Network Controllers (PNCs)
   that manage the network resources.  The key point of the MDSC (and of
   the whole ACTN framework) is detaching the network and service
   control from underlying technology to help the customer express the
   network as desired by business needs.  The MDSC envelopes the
   instantiation of the right technology and network control to meet
   business criteria.  In essence, it controls and manages the
   primitives to achieve functionalities as desired by the CNC.

   In order to allow for multi-domain coordination, a 1:N relationship
   must be allowed between MDSCs and PNCs.

   In addition to that, it could also be possible to have an M:1
   relationship between MDSCs and PNCs to allow for network-resource
   partitioning/sharing among different customers that are not
   necessarily connected to the same MDSC (e.g., different service
   providers) but that are all using the resources of a common network
   infrastructure operator.

3.3.  Provisioning Network Controller

   The Provisioning Network Controller (PNC) oversees configuring the
   network elements, monitoring the topology (physical or virtual) of
   the network, and collecting information about the topology (either
   raw or abstracted).

   The PNC functions can be implemented as part of an SDN domain
   controller, a Network Management System (NMS), an Element Management
   System (EMS), an active PCE-based controller [RFC8283], or any other
   means to dynamically control a set of nodes that implements a

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   northbound interface from the standpoint of the nodes (which is out
   of the scope of this document).  A PNC domain includes all the
   resources under the control of a single PNC.  It can be composed of
   different routing domains and administrative domains, and the
   resources may come from different layers.  The interconnection
   between PNC domains is illustrated in Figure 3.

                     _______                        _______
                   _(       )_                    _(       )_
                 _(           )_                _(           )_
                (               )     Border   (               )
               (     PNC     ------   Link   ------     PNC     )
              (   Domain X  |Border|========|Border|  Domain Y   )
              (             | Node |        | Node |             )
               (             ------          ------             )
                (_             _)              (_             _)
                  (_         _)                  (_         _)
                    (_______)                      (_______)

                       Figure 3: PNC Domain Borders

3.4.  ACTN Interfaces

   Direct customer control of transport network elements and virtualized
   services is not a viable proposition for network operators due to
   security and policy concerns.  Therefore, the network has to provide
   open, programmable interfaces, through which customer applications
   can create, replace, and modify virtual network resources and
   services in an interactive, flexible, and dynamic fashion.

   Three interfaces exist in the ACTN architecture as shown in Figure 2.

   o  CMI: The CNC-MDSC Interface (CMI) is an interface between a CNC
      and an MDSC.  The CMI is a business boundary between customer and
      network operator.  It is used to request a VNS for an application.
      All service-related information is conveyed over this interface
      (such as the VNS type, topology, bandwidth, and service
      constraints).  Most of the information over this interface is
      agnostic of the technology used by network operators, but there
      are some cases (e.g., access link configuration) where it is
      necessary to specify technology-specific details.

   o  MPI: The MDSC-PNC Interface (MPI) is an interface between an MDSC
      and a PNC.  It communicates requests for new connectivity or for
      bandwidth changes in the physical network.  In multi-domain
      environments, the MDSC needs to communicate with multiple PNCs,

Top      ToC       Page 15 
      each responsible for control of a domain.  The MPI presents an
      abstracted topology to the MDSC hiding technology-specific aspects
      of the network and hiding topology according to policy.

   o  SBI: The Southbound Interface (SBI) is out of scope of ACTN.  Many
      different SBIs have been defined for different environments,
      technologies, standards organizations, and vendors.  It is shown
      in Figure 3 for reference reason only.

4.  Advanced ACTN Architectures

   This section describes advanced configurations of the ACTN
   architecture.

4.1.  MDSC Hierarchy

   A hierarchy of MDSCs can be foreseen for many reasons, among which
   are scalability, administrative choices, or putting together
   different layers and technologies in the network.  In the case where
   there is a hierarchy of MDSCs, we introduce the terms "higher-level
   MDSC" (MDSC-H) and "lower-level MDSC" (MDSC-L).  The interface
   between them is a recursion of the MPI.  An implementation of an
   MDSC-H makes provisioning requests as normal using the MPI, but an
   MDSC-L must be able to receive requests as normal at the CMI and also
   at the MPI.  The hierarchy of MDSCs can be seen in Figure 4.

   Another implementation choice could foresee the usage of an MDSC-L
   for all the PNCs related to a given technology (e.g., Internet
   Protocol (IP) / Multiprotocol Label Switching (MPLS)) and a different
   MDSC-L for the PNCs related to another technology (e.g., Optical
   Transport Network (OTN) / Wavelength Division Multiplexing (WDM)) and
   an MDSC-H to coordinate them.

Top      ToC       Page 16 
                                  +--------+
                                  |   CNC  |
                                  +--------+
                                       |          +-----+
                                       | CMI      | CNC |
                                 +----------+     +-----+
                          -------|  MDSC-H  |----    |
                         |       +----------+    |   | CMI
                     MPI |                   MPI |   |
                         |                       |   |
                    +---------+               +---------+
                    |  MDSC-L |               |  MDSC-L |
                    +---------+               +---------+
                  MPI |     |                   |     |
                      |     |                   |     |
                   -----   -----             -----   -----
                  | PNC | | PNC |           | PNC | | PNC |
                   -----   -----             -----   -----

                         Figure 4: MDSC Hierarchy

   The hierarchy of MDSC can be recursive, where an MDSC-H is, in turn,
   an MDSC-L to a higher-level MDSC-H.

4.2.  Functional Split of MDSC Functions in Orchestrators

   An implementation choice could separate the MDSC functions into two
   groups: one group for service-related functions and the other for
   network-related functions.  This enables the implementation of a
   service orchestrator that provides the service-related functions of
   the MDSC and a network orchestrator that provides the network-related
   functions of the MDSC.  This split is consistent with the YANG
   service model architecture described in [RFC8309].  Figure 5 depicts
   this and shows how the ACTN interfaces may map to YANG data models.

Top      ToC       Page 17 
                                +--------------------+
                                |           Customer |
                                |   +-----+          |
                                |   | CNC |          |
                                |   +-----+          |
                                +--------------------+
                                         CMI |  Customer Service Model
                                             |
                        +---------------------------------------+
                        |                          Service      |
                ********|***********************   Orchestrator |
                * MDSC  |  +-----------------+ *                |
                *       |  | Service-related | *                |
                *       |  |    Functions    | *                |
                *       |  +-----------------+ *                |
                *       +----------------------*----------------+
                *                              *  |  Service Delivery
                *                              *  |  Model
                *       +----------------------*----------------+
                *       |                      *   Network      |
                *       |  +-----------------+ *   Orchestrator |
                *       |  | Network-related | *                |
                *       |  |    Functions    | *                |
                *       |  +-----------------+ *                |
                ********|***********************                |
                        +---------------------------------------+
                                         MPI |  Network Configuration
                                             |  Model
                               +------------------------+
                               |            Domain      |
                               |  +------+  Controller  |
                               |  | PNC  |              |
                               |  +------+              |
                               +------------------------+
                                         SBI |  Device Configuration
                                             |  Model
                                         +--------+
                                         | Device |
                                         +--------+

   Figure 5: ACTN Architecture in the Context of the YANG Service Models

Top      ToC       Page 18 
5.  Topology Abstraction Methods

   Topology abstraction is described in [RFC7926].  This section
   discusses topology abstraction factors, types, and their context in
   the ACTN architecture.

   Abstraction in ACTN is performed by the PNC when presenting available
   topology to the MDSC, or by an MDSC-L when presenting topology to an
   MDSC-H.  This function is different from the creation of a VN (and
   particularly a Type 2 VN) that is not abstraction but construction of
   virtual resources.

5.1.  Abstraction Factors

   As discussed in [RFC7926], abstraction is tied with the policy of the
   networks.  For instance, per an operational policy, the PNC would not
   provide any technology-specific details (e.g., optical parameters for
   Wavelength Switched Optical Network (WSON) in the abstract topology
   it provides to the MDSC.  Similarly, the policy of the networks may
   determine the abstraction type as described in Section 5.2.

   There are many factors that may impact the choice of abstraction:

   o  Abstraction depends on the nature of the underlying domain
      networks.  For instance, packet networks may be abstracted with
      fine granularity while abstraction of optical networks depends on
      the switching units (such as wavelengths) and the end-to-end
      continuity and cross-connect limitations within the network.

   o  Abstraction also depends on the capability of the PNCs.  As
      abstraction requires hiding details of the underlying network
      resources, the PNC's capability to run algorithms impacts the
      feasibility of abstraction.  Some PNCs may not have the ability to
      abstract native topology while other PNCs may have the ability to
      use sophisticated algorithms.

   o  Abstraction is a tool that can improve scalability.  Where the
      native network resource information is of a large size, there is a
      specific scaling benefit to abstraction.

   o  The proper abstraction level may depend on the frequency of
      topology updates and vice versa.

   o  The nature of the MDSC's support for technology-specific
      parameters impacts the degree/level of abstraction.  If the MDSC
      is not capable of handling such parameters, then a higher level of
      abstraction is needed.

Top      ToC       Page 19 
   o  In some cases, the PNC is required to hide key internal
      topological data from the MDSC.  Such confidentiality can be
      achieved through abstraction.

5.2.  Abstraction Types

   This section defines the following three types of topology
   abstraction:

   o  Native/White Topology (Section 5.2.1)
   o  Black Topology (Section 5.2.2)
   o  Grey Topology (Section 5.2.3)

5.2.1.  Native/White Topology

   This is a case where the PNC provides the actual network topology to
   the MDSC without any hiding or filtering of information, i.e., no
   abstraction is performed.  In this case, the MDSC has the full
   knowledge of the underlying network topology and can operate on it
   directly.

5.2.2.  Black Topology

   A black topology replaces a full network with a minimal
   representation of the edge-to-edge topology without disclosing any
   node internal connectivity information.  The entire domain network
   may be abstracted as a single abstract node with the network's
   access/egress links appearing as the ports to the abstract node and
   the implication that any port can be "cross-connected" to any other.
   Figure 6 depicts a native topology with the corresponding black
   topology with one virtual node and inter-domain links.  In this case,
   the MDSC has to make a provisioning request to the PNCs to establish
   the port-to-port connection.  If there is a large number of
   interconnected domains, this abstraction method may impose a heavy
   coordination load at the MDSC level in order to find an optimal end-
   to-end path since the abstraction hides so much information that it
   is not possible to determine whether an end-to-end path is feasible
   without asking each PNC to set up each path fragment.  For this
   reason, the MPI might need to be enhanced to allow the PNCs to be
   queried for the practicality and characteristics of paths across the
   abstract node.

Top      ToC       Page 20 
                   .....................................
                   : PNC Domain                        :
                   :  +--+     +--+     +--+     +--+  :
                ------+  +-----+  +-----+  +-----+  +------
                   :  ++-+     ++-+     +-++     +-++  :
                   :   |        |         |        |   :
                   :   |        |         |        |   :
                   :   |        |         |        |   :
                   :   |        |         |        |   :
                   :  ++-+     ++-+     +-++     +-++  :
                ------+  +-----+  +-----+  +-----+  +------
                   :  +--+     +--+     +--+     +--+  :
                   :....................................

                                +----------+
                             ---+          +---
                                | Abstract |
                                |   Node   |
                             ---+          +---
                                +----------+

               Figure 6: Native Topology with Corresponding
               Black Topology Expressed as an Abstract Node

5.2.3.  Grey Topology

   A grey topology represents a compromise between black and white
   topologies from a granularity point of view.  In this case, the PNC
   exposes an abstract topology containing all PNC domain border nodes
   and an abstraction of the connectivity between those border nodes.
   This abstraction may contain either physical or abstract nodes/links.

   Two types of grey topology are identified:

   o  In a type A grey topology, border nodes are connected by a full
      mesh of TE links (see Figure 7).

   o  In a type B grey topology, border nodes are connected over a more-
      detailed network comprising internal abstract nodes and abstracted
      links.  This mode of abstraction supplies the MDSC with more
      information about the internals of the PNC domain and allows it to
      make more informed choices about how to route connectivity over
      the underlying network.

Top      ToC       Page 21 
                  .....................................
                  : PNC Domain                        :
                  :  +--+     +--+     +--+     +--+  :
               ------+  +-----+  +-----+  +-----+  +------
                  :  ++-+     ++-+     +-++     +-++  :
                  :   |        |         |        |   :
                  :   |        |         |        |   :
                  :   |        |         |        |   :
                  :   |        |         |        |   :
                  :  ++-+     ++-+     +-++     +-++  :
               ------+  +-----+  +-----+  +-----+  +------
                  :  +--+     +--+     +--+     +--+  :
                  :....................................

                           ....................
                           : Abstract Network :
                           :                  :
                           :   +--+    +--+   :
                        -------+  +----+  +-------
                           :   ++-+    +-++   :
                           :    |  \  /  |    :
                           :    |   \/   |    :
                           :    |   /\   |    :

                           :    |  /  \  |    :
                           :   ++-+    +-++   :
                        -------+  +----+  +-------
                           :   +--+    +--+   :
                           :..................:

        Figure 7: Native Topology with Corresponding Grey Topology

5.3.  Methods of Building Grey Topologies

   This section discusses two different methods of building a grey
   topology:

   o  Automatic generation of abstract topology by configuration
      (Section 5.3.1)

   o  On-demand generation of supplementary topology via path
      computation request/reply (Section 5.3.2)

Top      ToC       Page 22 
5.3.1.  Automatic Generation of Abstract Topology by Configuration

   Automatic generation is based on the abstraction/summarization of the
   whole domain by the PNC and its advertisement on the MPI.  The level
   of abstraction can be decided based on PNC configuration parameters
   (e.g., "provide the potential connectivity between any PE and any
   ASBR in an MPLS-TE network").

   Note that the configuration parameters for this abstract topology can
   include available bandwidth, latency, or any combination of defined
   parameters.  How to generate such information is beyond the scope of
   this document.

   This abstract topology may need to be periodically or incrementally
   updated when there is a change in the underlying network or the use
   of the network resources that make connectivity more or less
   available.

5.3.2.  On-Demand Generation of Supplementary Topology via Path Compute
        Request/Reply

   While abstract topology is generated and updated automatically by
   configuration as explained in Section 5.3.1, additional supplementary
   topology may be obtained by the MDSC via a path compute request/reply
   mechanism.

   The abstract topology advertisements from PNCs give the MDSC the
   border node/link information for each domain.  Under this scenario,
   when the MDSC needs to create a new VN, the MDSC can issue path
   computation requests to PNCs with constraints matching the VN request
   as described in [ACTN-YANG].  An example is provided in Figure 8,
   where the MDSC is creating a P2P VN between AP1 and AP2.  The MDSC
   could use two different inter-domain links to get from domain X to
   domain Y, but in order to choose the best end-to-end path, it needs
   to know what domain X and Y can offer in terms of connectivity and
   constraints between the PE nodes and the border nodes.

                        -------                 --------
                       (       )               (        )
                      -      BrdrX.1------- BrdrY.1      -
                     (+---+       )          (       +---+)
               -+---( |PE1| Dom.X  )        (  Dom.Y |PE2| )---+-
                |    (+---+       )          (       +---+)    |
               AP1    -      BrdrX.2------- BrdrY.2      -    AP2
                       (       )               (        )
                        -------                 --------

                     Figure 8: A Multi-Domain Example

Top      ToC       Page 23 
   The MDSC issues a path computation request to PNC.X asking for
   potential connectivity between PE1 and border node BrdrX.1 and
   between PE1 and BrdrX.2 with related objective functions and TE
   metric constraints.  A similar request for connectivity from the
   border nodes in domain Y to PE2 will be issued to PNC.Y.  The MDSC
   merges the results to compute the optimal end-to-end path including
   the inter-domain links.  The MDSC can use the result of this
   computation to request the PNCs to provision the underlying networks,
   and the MDSC can then use the end-to-end path as a virtual link in
   the VN it delivers to the customer.

5.4.  Hierarchical Topology Abstraction Example

   This section illustrates how topology abstraction operates in
   different levels of a hierarchy of MDSCs as shown in Figure 9.

Top      ToC       Page 24 
                             +-----+
                             | CNC |  CNC wants to create a VN
                             +-----+  between CE A and CE B
                                |
                                |
                    +-----------------------+
                    |         MDSC-H        |
                    +-----------------------+
                          /           \
                         /             \
                 +---------+         +---------+
                 | MDSC-L1 |         | MDSC-L2 |
                 +---------+         +---------+
                   /    \               /    \
                  /      \             /      \
               +----+  +----+       +----+  +----+
     CE A o----|PNC1|  |PNC2|       |PNC3|  |PNC4|----o CE B
               +----+  +----+       +----+  +----+

                   Virtual Network Delivered to CNC

                     CE A o==============o CE B

                   Topology operated on by MDSC-H

                  CE A o----o==o==o===o----o CE B

     Topology operated on by MDSC-L1     Topology operated on by MDSC-L2
                  _        _                       _        _
                 ( )      ( )                     ( )      ( )
                (   )    (   )                   (   )    (   )
       CE A o--(o---o)==(o---o)==Dom.3   Dom.2==(o---o)==(o---o)--o CE B
                (   )    (   )                   (   )    (   )
                 (_)      (_)                     (_)      (_)

Top      ToC       Page 25 
                              Actual Topology
                ___          ___          ___          ___
               (   )        (   )        (   )        (   )
              (  o  )      (  o  )      ( o--o)      (  o  )
             (  / \  )    (   |\  )    (  |  | )    (  / \  )
   CE A o---(o-o---o-o)==(o-o-o-o-o)==(o--o--o-o)==(o-o-o-o-o)---o CE B
             (  \ /  )    ( | |/  )    (  |  | )    (  \ /  )
              (  o  )      (o-o  )      ( o--o)      (  o  )
               (___)        (___)        (___)        (___)

              Domain 1     Domain 2     Domain 3     Domain 4

   Where
        o   is a node
        --- is a link
        === is a border link

        Figure 9: Illustration of Hierarchical Topology Abstraction

   In the example depicted in Figure 9, there are four domains under
   control of PNCs: PNC1, PNC2, PNC3, and PNC4.  MDSC-L1 controls PNC1
   and PNC2, while MDSC-L2 controls PNC3 and PNC4.  Each of the PNCs
   provides a grey topology abstraction that presents only border nodes
   and links across and outside the domain.  The abstract topology
   MDSC-L1 that operates is a combination of the two topologies from
   PNC1 and PNC2.  Likewise, the abstract topology that MDSC-L2 operates
   is shown in Figure 9.  Both MDSC-L1 and MDSC-L2 provide a black
   topology abstraction to MDSC-H in which each PNC domain is presented
   as a single virtual node.  MDSC-H combines these two topologies to
   create the abstraction topology on which it operates.  MDSC-H sees
   the whole four domain networks as four virtual nodes connected via
   virtual links.

5.5.  VN Recursion with Network Layers

   In some cases, the VN supplied to a customer may be built using
   resources from different technology layers operated by different
   operators.  For example, one operator may run a packet TE network and
   use optical connectivity provided by another operator.

   As shown in Figure 10, a customer asks for end-to-end connectivity
   between CE A and CE B, a virtual network.  The customer's CNC makes a
   request to Operator 1's MDSC.  The MDSC works out which network
   resources need to be configured and sends instructions to the
   appropriate PNCs.  However, the link between Q and R is a virtual
   link supplied by Operator 2: Operator 1 is a customer of Operator 2.

Top      ToC       Page 26 
   To support this, Operator 1 has a CNC that communicates with Operator
   2's MDSC.  Note that Operator 1's CNC in Figure 10 is a functional
   component that does not dictate implementation: it may be embedded in
   a PNC.

Top      ToC       Page 27 
      Virtual     CE A o===============================o CE B
      Network

                                    -----    CNC wants to create a VN
      Customer                     | CNC |   between CE A and CE B
                                    -----
                                      :
               ***********************************************
                                      :
      Operator 1         ---------------------------
                        |           MDSC            |
                         ---------------------------
                          :           :           :
                          :           :           :
                        -----   -------------   -----
                       | PNC | |     PNC     | | PNC |
                        -----   -------------   -----
                          :     :     :     :     :
      Higher              v     v     :     v     v
      Layer      CE A o---P-----Q===========R-----S---o CE B
      Network                   |     :     |
                                |     :     |
                                |   -----   |
                                |  | CNC |  |
                                |   -----   |
                                |     :     |
               ***********************************************
                                |     :     |
      Operator 2                |  ------   |
                                | | MDSC |  |
                                |  ------   |
                                |     :     |
                                |  -------  |
                                | |  PNC  | |
                                |  -------  |
                                 \ :  :  : /
      Lower                       \v  v  v/
      Layer                        X--Y--Z
      Network

      Where

      --- is a link
      === is a virtual link

                Figure 10: VN Recursion with Network Layers


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