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Framework for Abstraction and Control of Traffic Engineered Networks

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8453.
Authors Daniele Ceccarelli , Young Lee
Last updated 2018-05-24 (Latest revision 2018-05-11)
Replaces draft-ceccarelli-teas-actn-framework
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Vishnu Pavan Beeram
Shepherd write-up Show Last changed 2018-01-26
IESG IESG state Became RFC 8453 (Informational)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Deborah Brungard
Send notices to Vishnu Beeram <>
IANA IANA review state IANA OK - No Actions Needed
TEAS Working Group                              Daniele Ceccarelli (Ed)
Internet Draft                                                 Ericsson
Intended status: Informational                           Young Lee (Ed)
Expires: November 11, 2018                                       Huawei

                                                          May 11, 2018

  Framework for Abstraction and Control of Traffic Engineered Networks



   Traffic Engineered 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
   Traffic Engineered Networks (ACTN) to support virtual network
   services and connectivity services.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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."

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   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on November 11, 2018.

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
   ( 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...................................10
         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..................................17
      5.1. Abstraction Factors......................................17
      5.2. Abstraction Types........................................18
         5.2.1. Native/White Topology...............................18
         5.2.2. Black Topology......................................19
         5.2.3. Grey Topology.......................................20
      5.3. Methods of Building Grey Topologies......................21

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         5.3.1. Automatic Generation of Abstract Topology by
         5.3.2. On-demand Generation of Supplementary Topology via Path
         Compute Request/Reply......................................21
      5.4. Hierarchical Topology Abstraction Example................22
      5.5. VN Recursion with Network Layers.........................24
   6. Access Points and Virtual Network Access Points...............25
      6.1. Dual-Homing Scenario.....................................27
   7. Advanced ACTN Application: Multi-Destination Service..........28
      7.1. Pre-Planned End Point Migration..........................29
      7.2. On the Fly End-Point Migration...........................30
   8. Manageability Considerations..................................30
      8.1. Policy...................................................31
      8.2. Policy Applied to the Customer Network Controller........32
      8.3. Policy Applied to the Multi-Domain Service Coordinator...32
      8.4. Policy Applied to the Provisioning Network Controller....32
   9. Security Considerations.......................................33
      9.1. CNC-MDSC Interface (CMI).................................34
      9.2. MDSC-PNC Interface (MPI).................................34
   10. IANA Considerations..........................................34
   11. References...................................................35
      11.1. Informative References..................................35
   12. Contributors.................................................36
   Authors' Addresses...............................................37
   APPENDIX A - Example of MDSC and PNC Functions Integrated in A
   Service/Network Orchestrator.....................................37

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.  Traffic Engineered (TE)
   networks have a variety of mechanisms to facilitate separation of
   data plane and control plane 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 networks, Multiprotocol Label Switching
   (MPLS) Transport Profile (MPLS-TP) networks [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

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   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 represented to customers and that are built
   from abstractions of the underlying TE networks so that, for
   example, a link in 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 Traffic Engineered
   Networks" (ACTN).

2. Overview

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

     . 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 remains responsive
        to the customer's services and business needs.

     . 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

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

   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

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   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.

   The ACTN framework described in this document facilitates:

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

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

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

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

     . 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 [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.  Network elements will often be grouped into
        domains based on technology types, vendor profiles, and
        geographic proximity.

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     . 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.

     . 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

     . 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.

     . A Virtual Network (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

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        of a VN as follows:

        a) 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.

        b) 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 end points, border nodes, and internal nodes as well
          as 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 a operator

     . 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 VNS. Note that
        the term "TE Link Termination Point" (LTP) defined in [TE-Topo]
        describes the end points 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

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   connectivity service becomes blurred. There are three types of VNS
   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.

   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 off-line pre-configuration
             or through dynamic requests specifying attributes to a
             Service Level Agreement (SLA) to satisfy the customer's

          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:

     - Customers
     - Service Providers
     - Network Operators

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

                           |       Customer       |
                      VNS       ||    |   /\     VNS
                     Request    ||    |   ||    Reply
                                \/    |   ||

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                           |  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.

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, governments, and utility
   companies.  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 virtual network service.
   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, and 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.

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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, this 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 to 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, service provider is also a
   network operator when it owns network infrastructure on which
   service is provided.

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 3-tier reference model and
   allows for hierarchy and recursion.  The main functionalities within
   an ACTN system are:

     . 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

     . 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

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        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.

     . 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 OSS/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.

     . Virtual service coordination: This function translates customer
        service-related information into virtual network service
        operations in order to seamlessly operate virtual networks
        while meeting a customer's service requirements.  In the
        context of ACTN, service/virtual service coordination includes
        a number of service orchestration functions such as multi-
        destination load balancing, guarantees of service quality,
        bandwidth and throughput.  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:

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

   Figure 2 also shows the following interfaces:

     . CMI - CNC-MDSC Interface
     . MPI - MDSC-PNC Interface
     . 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.

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 end-points associated
   with the VNS (expressed as APs), the service policy, and other QoS
   information related to the service.

   As the Customer Network Controller directly interfaces to the
   applications, it understands multiple application requirements and

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   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.  The two functions of the MDSC,
   namely, multi-domain coordination and virtualization/abstraction are
   referred to as network-related functions while 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 PNC to allow for network resource
   partitioning/sharing among different customers not necessarily
   connected to the same MDSC (e.g., different service providers) but
   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 [Centralized] or any

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   other means to dynamically control a set of nodes and that is
   implementing an NBI compliant with ACTN specification.

   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.  In addition, some
   networks may operate a control plane and as such it is not practical
   for the customer to directly interface with network elements.
   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

     . 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

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        configuration) where it is necessary to specify technology-
        specific details.

     . 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 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.

     . 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

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.

                                       |   CNC  |
                                            |          +-----+
                                            | CMI      | CNC |
                                      +----------+     +-----+

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                               -------|  MDSC-H  |----    |
                              |       +----------+    |   | CMI
                          MPI |                   MPI |   |
                              |                       |   |
                         +---------+               +---------+
                         |  MDSC-L |               |  MDSC-L |
                         +---------+               +---------+
                       MPI |     |                   |     |
                           |     |                   |     |
                        -----   -----             -----   -----
                       | PNC | | PNC |           | PNC | | PNC |
                        -----   -----             -----   -----

                         Figure 4: MDSC Hierarchy

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
   Yet Another Next Generation (YANG) service model architecture
   described in [Service-YANG].  Figure 5 depicts this and shows how
   the ACTN interfaces may map to YANG models.

                                |           Customer |
                                |   +-----+          |
                                |   | CNC |          |
                                |   +-----+          |
                                         CMI |  Customer Service Model
                        |                          Service      |
                ********|***********************   Orchestrator |
                * MDSC  |  +-----------------+ *                |
                *       |  | Service-related | *                |
                *       |  |    Functions    | *                |
                *       |  +-----------------+ *                |
                *       +----------------------*----------------+
                *                              *  |  Service Delivery Model
                *                              *  |
                *       +----------------------*----------------+
                *       |                      *   Network      |
                *       |  +-----------------+ *   Orchestrator |

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                *       |  | 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
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 to the creation
   of a VN (and particularly a Type 2 VN) which is not abstraction but
   construction of virtual resources.

5.1. Abstraction Factors

   As discussed in [RFC7926], abstraction is tied with 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, policy of the
   networks may determine the abstraction type as described in Section

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

   - Abstraction depends on the nature of the underlying domain
     networks.  For instance, packet networks may be abstracted with

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     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.

   - 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 PNC may not have the ability to
     abstract native topology while other PNCs may have the ability to
     use sophisticated algorithms.

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

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

   - 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.

   - 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

     . Native/White Topology (Section 5.2.1)
     . Black Topology (Section 5.2.2)
     . 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

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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 inter-connected 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.
                   : PNC Domain                        :
                   :  +--+     +--+     +--+     +--+  :
                ------+  +-----+  +-----+  +-----+  +------
                   :  ++-+     ++-+     +-++     +-++  :
                   :   |        |         |        |   :
                   :   |        |         |        |   :
                   :   |        |         |        |   :
                   :   |        |         |        |   :
                   :  ++-+     ++-+     +-++     +-++  :
                ------+  +-----+  +-----+  +-----+  +------
                   :  +--+     +--+     +--+     +--+  :

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

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

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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 domains border nodes
   and an abstraction of the connectivity between those border nodes.
   This abstraction may contain either physical or abstract

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

                  : PNC Domain                        :
                  :  +--+     +--+     +--+     +--+  :
               ------+  +-----+  +-----+  +-----+  +------
                  :  ++-+     ++-+     +-++     +-++  :
                  :   |        |         |        |   :
                  :   |        |         |        |   :
                  :   |        |         |        |   :
                  :   |        |         |        |   :
                  :  ++-+     ++-+     +-++     +-++  :
               ------+  +-----+  +-----+  +-----+  +------
                  :  +--+     +--+     +--+     +--+  :

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

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                           :   +--+    +--+   :

         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

     . Automatic generation of abstract topology by configuration
        (Section 5.3.1)
     . On-demand generation of supplementary topology via path
        computation request/reply (Section 5.3.2)

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

5.3.2. On-demand Generation of Supplementary Topology via Path Compute

   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

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   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

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

                     Figure 8: A Multi-Domain Example
   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.

                            | CNC |  CNC wants to create a VN
                            +-----+  between CE A and CE B
                   |         MDSC-H        |
                         /           \
                        /             \
                +---------+         +---------+
                | MDSC-L1 |         | MDSC-L2 |
                +---------+         +---------+

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                  /    \               /    \
                 /      \             /      \
              +----+  +----+       +----+  +----+
    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
                (   )    (   )                   (   )    (   )
                 (_)      (_)                     (_)      (_)

                             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

        o  is a node
       --- is a link
       === 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

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   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 MSDC-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.

   To support this, Operator 1 has a CNC that communicates to 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.

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

                                    -----    CNC wants to create a VN
      Customer                     | CNC |   between CE A and CE B
      Operator 1         ---------------------------
                        |           MDSC            |
                          :           :           :
                          :           :           :
                        -----   -------------   -----
                       | PNC | |     PNC     | | PNC |
                        -----   -------------   -----
                          :     :     :     :     :

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      Higher              v     v     :     v     v
      Layer      CE A o---P-----Q===========R-----S---o CE B
      Network                   |     :     |
                                |     :     |
                                |   -----   |
                                |  | CNC |  |
                                |   -----   |
                                |     :     |
                                |     :     |
      Operator 2                |  ------   |
                                | | MSDC |  |
                                |  ------   |
                                |     :     |
                                |  -------  |
                                | |  PNC  | |
                                |  -------  |
                                 \ :  :  : /
      Lower                       \v  v  v/
      Layer                        X--Y--Z

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

                Figure 10: VN recursion with Network Layers

6. Access Points and Virtual Network Access Points

   In order to map identification of connections between the customer's
   sites and the TE networks and to scope the connectivity requested in
   the VNS, the CNC and the MDSC refer to the connections using the
   Access Point (AP) construct as shown in Figure 11.

                               (             )
                              -               -
               +---+ X       (                 )      Z +---+
               |CE1|---+----(                   )---+---|CE2|
               +---+   |     (                 )    |   +---+
                        AP1   -               -    AP2
                               (             )

                      Figure 11: Customer View of APs

   Let's take as an example a scenario shown in Figure 11.  CE1 is
   connected to the network via a 10 Gbps link and CE2 via a 40 Gbps

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   link.  Before the creation of any VN between AP1 and AP2 the
   customer view can be summarized as shown in Table 1.

                         |End Point | Access Link Bandwidth  |
                   |AP id| CE,port  | MaxResBw | AvailableBw |
                   | AP1 |CE1,portX |   10Gbps |    10Gbps   |
                   | AP2 |CE2,portZ |   40Gbps |    40Gbps   |

                      Table 1: AP - Customer View

   On the other hand, what the provider sees is shown in Figure 12.

                          -------            -------
                         (       )          (       )
                        -         -        -         -
                    W  (+---+       )     (       +---+)  Y
                 -+---( |PE1| Dom.X  )---(  Dom.Y |PE2| )---+-
                  |    (+---+       )     (       +---+)    |
                  AP1   -         -        -         -     AP2
                         (       )          (       )
                          -------            -------

                    Figure 12: Provider view of the AP

   Which results in a summarization as shown in Table 2.

                         |End Point | Access Link Bandwidth  |
                   |AP id| PE,port  | MaxResBw | AvailableBw |
                   | AP1 |PE1,portW |   10Gbps |    10Gbps   |
                   | AP2 |PE2,portY |   40Gbps |    40Gbps   |

                        Table 2: AP - Operator View

   A Virtual Network Access Point (VNAP) needs to be defined as binding
   between an AP and a VN. It is used to allow for different VNs to
   start from the same AP. It also allows for traffic engineering on

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   the access and/or inter-domain links (e.g., keeping track of
   bandwidth allocation).  A different VNAP is created on an AP for
   each VN.

   In this simple scenario we suppose we want to create two virtual
   networks.  The first with VN identifier 9 between AP1 and AP2 with
   bandwidth of 1 Gbps, while the second with VN identifier 5, again
   between AP1 and AP2 and with bandwidth 2 Gbps.

   The operator view would evolve as shown in Table 3.

                           |End Point |  Access Link/VNAP Bw   |
                 |AP/VNAPid| PE,port  | MaxResBw | AvailableBw |
                 |AP1      |PE1,portW | 10 Gbps  |   7 Gbps    |
                 | -VNAP1.9|          |  1 Gbps  |     N.A.    |
                 | -VNAP1.5|          |  2 Gbps  |     N.A     |
                 |AP2      |PE2,portY | 4 0Gbps  |   37 Gbps   |
                 | -VNAP2.9|          |  1 Gbps  |     N.A.    |
                 | -VNAP2.5|          |  2 Gbps  |     N.A     |
        Table 3: AP and VNAP - Operator View after VNS Creation

6.1. Dual-Homing Scenario

   Often there is a dual homing relationship between a CE and a pair of
   PEs.  This case needs to be supported by the definition of VN, APs,
   and VNAPs.  Suppose CE1 connected to two different PEs in the
   operator domain via AP1 and AP2 and that the customer needs 5 Gbps
   of bandwidth between CE1 and CE2.  This is shown in Figure 12.

                              AP1    (            )    AP3
                             -------(PE1)      (PE3)-------
                          W /      (                )      \ X
                      +---+/      (                  )      \+---+
                      |CE1|      (                    )      |CE2|
                      +---+\      (                  )      /+---+
                          Y \      (                )      / Z
                             -------(PE2)      (PE4)-------
                              AP2    (____________)

                      Figure 12: Dual-Homing Scenario

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   In this case, the customer will request for a VN between AP1, AP2,
   and AP3 specifying a dual homing relationship between AP1 and AP2.
   As a consequence no traffic will flow between AP1 and AP2.  The dual
   homing relationship would then be mapped against the VNAPs (since
   other independent VNs might have AP1 and AP2 as end points).

   The customer view would be shown in Table 4.

                      |End Point |  Access Link/VNAP Bw   |
            |AP/VNAPid| CE,port  | MaxResBw | AvailableBw |Dual Homing|
            |AP1      |CE1,portW | 10 Gbps  |   5 Gbps    |           |
            | -VNAP1.9|          |  5 Gbps  |     N.A.    | VNAP2.9   |
            |AP2      |CE1,portY | 40 Gbps  |  35 Gbps    |           |
            | -VNAP2.9|          |  5 Gbps  |     N.A.    | VNAP1.9   |
            |AP3      |CE2,portX | 50 Gbps  |  45 Gbps    |           |
            | -VNAP3.9|          |  5 Gbps  |     N.A.    |   NONE    |

          Table 4: Dual-Homing - Customer View after VN Creation

7. Advanced ACTN Application: Multi-Destination Service

   A further advanced application of ACTN is in the case of Data Center
   selection, where the customer requires the Data Center selection to
   be based on the network status; this is referred to as Multi-
   Destination in [ACTN-REQ].  In terms of ACTN, a CNC could request a
   VNS between a set of source APs and destination APs and leave it up
   to the network (MDSC) to decide which source and destination access
   points to be used to set up the VNS. The candidate list of source
   and destination APs is decided by a CNC (or an entity outside of
   ACTN) based on certain factors which are outside the scope of ACTN.

   Based on the AP selection as determined and returned by the network
   (MDSC), the CNC (or an entity outside of ACTN) should further take
   care of any subsequent actions such as orchestration or service
   setup requirements.  These further actions are outside the scope of

   Consider a case as shown in Figure 14, where three data centers are
   available, but the customer requires the data center selection to be
   based on the network status and the connectivity service setup
   between the AP1 (CE1) and one of the destination APs (AP2 (DC-A),

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   AP3 (DC-B), and AP4 (DC-C)).  The MDSC (in coordination with PNCs)
   would select the best destination AP based on the constraints,
   optimization criteria, policies, etc., and setup the connectivity
   service (virtual network).

                          -------            -------
                         (       )          (       )
                        -         -        -         -
          +---+        (           )      (           )        +----+
          |CE1|---+---(  Domain X   )----(  Domain Y   )---+---|DC-A|
          +---+   |    (           )      (           )    |   +----+
                   AP1  -         -        -         -    AP2
                         (       )          (       )
                          ---+---            ---+---
                             |                  |
                         AP3-+              AP4-+
                             |                  |
                          +----+              +----+
                          |DC-B|              |DC-C|
                          +----+              +----+

          Figure 14: End-Point Selection Based on Network Status

7.1. Pre-Planned End Point Migration

   Furthermore, in case of Data Center selection, customer could
   request for a backup DC to be selected, such that in case of
   failure, another DC site could provide hot stand-by protection.  As
   shown in Figure 15 DC-C is selected as a backup for DC-A.  Thus, the
   VN should be setup by the MDSC to include primary connectivity
   between AP1 (CE1) and AP2 (DC-A) as well as protection connectivity
   between AP1 (CE1) and AP4 (DC-C).

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                    -------            -------
                   (       )          (       )
                  -         -        -         -
   +---+         (           )      (           )        +----+
   |CE1|---+----(  Domain X   )----(  Domain Y   )---+---|DC-A|
   +---+   |     (           )      (           )    |   +----+
           AP1    -         -        -         -    AP2    |
                   (       )          (       )            |
                    ---+---            ---+---             |
                       |                  |                |
                   AP3-+              AP4-+         HOT STANDBY
                       |                  |                |
                    +----+             +----+              |
                    |DC-D|             |DC-C|<-------------
                    +----+             +----+

                Figure 15: Pre-planned End-Point Migration

7.2. On the Fly End-Point Migration

   Compared to pre-planned end point migration, on the fly end point
   selection is dynamic in that the migration is not pre-planned but
   decided based on network condition.  Under this scenario, the MDSC
   would monitor the network (based on the VN Service-level Agreement
   (SLA) and notify the CNC in case where some other destination AP
   would be a better choice based on the network parameters.  The CNC
   should instruct the MDSC when it is suitable to update the VN with
   the new AP if it is required.

8. Manageability Considerations

   The objective of ACTN is to manage traffic engineered resources, and
   provide a set of mechanisms to allow customers to request virtual
   connectivity across server network resources.  ACTN supports
   multiple customers each with its own view of and control of a
   virtual network built on the server network, the network operator
   will need to partition (or "slice") their network resources, and
   manage the resources accordingly.

   The ACTN platform will, itself, need to support the request,
   response, and reservations of client and network layer connectivity.
   It will also need to provide performance monitoring and control of
   traffic engineered resources.  The management requirements may be
   categorized as follows:

     . Management of external ACTN protocols

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     . Management of internal ACTN interfaces/protocols
     . Management and monitoring of ACTN components
     . Configuration of policy to be applied across the ACTN system

   The ACTN framework and interfaces are defined to enable traffic
   engineering for virtual network services and connectivity services.
   Network operators may have other Operations, Administration, and
   Maintenance (OAM) tasks for service fulfillment, optimization, and
   assurance beyond traffic engineering. The realization of OAM beyond
   abstraction and control of traffic engineered networks is not
   considered in this document.

8.1. Policy

   Policy is an important aspect of ACTN control and management.
   Policies are used via the components and interfaces, during
   deployment of the service, to ensure that the service is compliant
   with agreed policy factors and variations (often described in SLAs),
   these include, but are not limited to: connectivity, bandwidth,
   geographical transit, technology selection, security, resilience,
   and economic cost.

   Depending on the deployment of the ACTN architecture, some policies
   may have local or global significance.  That is, certain policies
   may be ACTN component specific in scope, while others may have
   broader scope and interact with multiple ACTN components.  Two
   examples are provided below:

     . A local policy might limit the number, type, size, and
       scheduling of virtual network services a customer may request
       via its CNC.  This type of policy would be implemented locally
       on the MDSC.

     . A global policy might constrain certain customer types (or
       specific customer applications) to only use certain MDSCs, and
       be restricted to physical network types managed by the PNCs.  A
       global policy agent would govern these types of policies.

   The objective of this section is to discuss the applicability of
   ACTN policy: requirements, components, interfaces, and examples.
   This section provides an analysis and does not mandate a specific
   method for enforcing policy, or the type of policy agent that would
   be responsible for propagating policies across the ACTN components.
   It does highlight examples of how policy may be applied in the

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   context of ACTN, but it is expected further discussion in an
   applicability or solution specific document, will be required.

8.2. Policy Applied to the Customer Network Controller

   A virtual network service for a customer application will be
   requested by the CNC.  The request will reflect the application
   requirements and specific service needs, including bandwidth,
   traffic type and survivability.  Furthermore, application access and
   type of virtual network service requested by the CNC, will be need
   adhere to specific access control policies.

8.3. Policy Applied to the Multi-Domain Service Coordinator

   A key objective of the MDSC is to support the customer's expression
   of the application connectivity request via its CNC as set of
   desired business needs, therefore policy will play an important

   Once authorized, the virtual network service will be instantiated
   via the CNC-MDSC Interface (CMI), it will reflect the customer
   application and connectivity requirements, and specific service
   transport needs.  The CNC and the MDSC components will have agreed
   connectivity end-points, use of these end-points should be defined
   as a policy expression when setting up or augmenting virtual network
   services.  Ensuring that permissible end-points are defined for CNCs
   and applications will require the MDSC to maintain a registry of
   permissible connection points for CNCs and application types.

   Conflicts may occur when virtual network service optimization
   criteria are in competition.  For example, to meet objectives for
   service reachability a request may require an interconnection point
   between multiple physical networks; however, this might break a
   confidentially policy requirement of specific type of end-to-end
   service.  Thus an MDSC may have to balance a number of the
   constraints on a service request and between different requested
   services.  It may also have to balance requested services with
   operational norms for the underlying physical networks.  This
   balancing may be resolved using configured policy and using hard and
   soft policy constraints.

8.4. Policy Applied to the Provisioning Network Controller

   The PNC is responsible for configuring the network elements,
   monitoring physical network resources, and exposing connectivity
   (direct or abstracted) to the MDSC.  It is therefore expected that

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   policy will dictate what connectivity information will be exported
   between the PNC, via the MDSC-PNC Interface (MPI), and MDSC.

   Policy interactions may arise when a PNC determines that it cannot
   compute a requested path from the MDSC, or notices that (per a
   locally configured policy) the network is low on resources (for
   example, the capacity on key links become exhausted).  In either
   case, the PNC will be required to notify the MDSC, which may (again
   per policy) act to construct a virtual network service across
   another physical network topology.

   Furthermore, additional forms of policy-based resource management
   will be required to provide virtual network service performance,
   security and resilience guarantees.  This will likely be implemented
   via a local policy agent and additional protocol methods.

9. Security Considerations

   The ACTN framework described in this document defines key components
   and interfaces for managed traffic engineered networks.  Securing
   the request and control of resources, confidentially of the
   information, and availability of function, should all be critical
   security considerations when deploying and operating ACTN platforms.

   Several distributed ACTN functional components are required, and
   implementations should consider encrypting data that flows between
   components, especially when they are implemented at remote nodes,
   regardless these data flows are on external or internal network

   The ACTN security discussion is further split into two specific
   categories described in the following sub-sections:

     . Interface between the Customer Network Controller and Multi-
       Domain Service Coordinator (MDSC), CNC-MDSC Interface (CMI)

     . Interface between the Multi-Domain Service Coordinator and
       Provisioning Network Controller (PNC), MDSC-PNC Interface (MPI)

   From a security and reliability perspective, ACTN may encounter many
   risks such as malicious attack and rogue elements attempting to
   connect to various ACTN components.  Furthermore, some ACTN
   components represent a single point of failure and threat vector,
   and must also manage policy conflicts, and eavesdropping of
   communication between different ACTN components.

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   The conclusion is that all protocols used to realize the ACTN
   framework should have rich security features, and customer,
   application and network data should be stored in encrypted data
   stores.  Additional security risks may still exist.  Therefore,
   discussion and applicability of specific security functions and
   protocols will be better described in documents that are use case
   and environment specific.

9.1. CNC-MDSC Interface (CMI)

   Data stored by the MDSC will reveal details of the virtual network
   services, and which CNC and customer/application is consuming the
   resource.  The data stored must therefore be considered as a
   candidate for encryption.

   CNC Access rights to an MDSC must be managed.  The MDSC must
   allocate resources properly, and methods to prevent policy
   conflicts, resource wastage, and denial of service attacks on the
   MDSC by rogue CNCs, should also be considered.

   The CMI will likely be an external protocol interface.  Suitable
   authentication and authorization of each CNC connecting to the MDSC
   will be required, especially, as these are likely to be implemented
   by different organizations and on separate functional nodes.  Use of
   the AAA-based mechanisms would also provide role-based authorization
   methods, so that only authorized CNC's may access the different
   functions of the MDSC.

9.2. MDSC-PNC Interface (MPI)

   Where the MDSC must interact with multiple (distributed) PNCs, a
   PKI-based mechanism is suggested, such as building a TLS or HTTPS
   connection between the MDSC and PNCs, to ensure trust between the
   physical network layer control components and the MDSC.

   Which MDSC the PNC exports topology information to, and the level of
   detail (full or abstracted), should also be authenticated, and
   specific access restrictions and topology views should be
   configurable and/or policy-based.

10. IANA Considerations

   This document has no actions for IANA.

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11. References

11.1. Informative References

   [RFC2702] Awduche, D., et. al., "Requirements for Traffic
             Engineering Over MPLS", RFC 2702, September 1999.

   [RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
             Computation Element (PCE)-Based Architecture", IETF RFC
             4655, August 2006.

   [RFC5654] Niven-Jenkins, B. (Ed.), D. Brungard (Ed.), and M. Betts
             (Ed.), "Requirements of an MPLS Transport Profile", RFC
             5654, September 2009.

   [RFC7149] Boucadair, M. and Jacquenet, C., "Software-Defined
             Networking: A Perspective from within a Service Provider
             Environment", RFC 7149, March 2014.

   [RFC7926] A. Farrel (Ed.), "Problem Statement and Architecture for
             Information Exchange between Interconnected Traffic-
             Engineered Networks", RFC 7926, July 2016.

   [RFC3945] Manning, E., et al., "Generalized Multi-Protocol Label
             Switching (GMPLS) Architecture2, RFC 3945, October 2004.

   [ONF-ARCH] Open Networking Foundation, "SDN architecture", Issue
             1.1, ONF TR-521, June 2016.

   [Centralized] Farrel, A., et al., "An Architecture for Use of PCE
             and PCEP in a Network with Central Control", draft-ietf-
             teas-pce-central-control, work in progress.

   [Service-YANG] Lee, Y., Dhody, D., and Ceccarelli, C., "Traffic
             Engineering and Service Mapping Yang Model", draft-lee-
             teas-te-service-mapping-yang, work in progress.

   [ACTN-YANG] Lee, Y., et al., "A Yang Data Model for ACTN VN
             Operation", draft-lee-teas-actn-vn-yang, work in progress.

   [ACTN-REQ] Lee, Y., et al., "Requirements for Abstraction and
             Control of TE Networks", draft-ietf-teas-actn-
             requirements, work in progress.

   [TE-Topo]  X. Liu et al., "YANG Data Model for TE Topologies", draft-
             ietf-teas-yang-te-topo, work in progress.

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12. Contributors

   Adrian Farrel
   Old Dog Consulting

   Italo Busi

   Khuzema Pithewan

   Michael Scharf

   Luyuan Fang

   Diego Lopez
   Telefonica I+D
   Don Ramon de la Cruz, 82
   28006 Madrid, Spain

   Sergio Belotti
   Alcatel Lucent
   Via Trento, 30
   Vimercate, Italy

   Daniel King
   Lancaster University

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

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   Gert Grammel
   Juniper Networks

Authors' Addresses

   Daniele Ceccarelli
   Stockholm, Sweden

   Young Lee
   Huawei Technologies
   5340 Legacy Drive
   Plano, TX 75023, USA
   Phone: (469)277-5838

APPENDIX A - Example of MDSC and PNC Functions Integrated in A
Service/Network Orchestrator

   This section provides an example of a possible deployment scenario,
   in which Service/Network Orchestrator can include a number of
   functionalities, among which, in the example below, PNC
   functionalities for domain 2 and MDSC functionalities to coordinate
   the PNC1 functionalities (hosted in a separate domain controller)
   and PNC2 functionalities (co-hosted in the network orchestrator).

               |    +-----+                    |
               |    | CNC |                    |
               |    +-----+                    |
   Service/Network     | CMI
   Orchestrator        |
               |    +------+   MPI   +------+   |
               |    | MDSC |---------| PNC2 |   |
               |    +------+         +------+   |
                       | MPI              |
   Domain Controller   |                  |
               +-------|-----+            |
               |   +-----+   |            | SBI

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               |   |PNC1 |   |            |
               |   +-----+   |            |
               +-------|-----+            |
                       v SBI              v
                    -------            -------
                   (       )          (       )
                  -         -        -         -
                 (           )      (           )
                (  Domain 1   )----(  Domain 2   )
                 (           )      (           )
                  -         -        -         -
                   (       )          (       )
                    -------            -------

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