A Framework for Automating Service and Network Management with YANG
draft-wu-model-driven-management-virtualization-04

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Networking Working Group                                           Q. Wu
Internet-Draft                                                    Huawei
Intended status: Informational                              M. Boucadair
Expires: December 21, 2019                                        Orange
                                                                  Y. Lee
                                                               Futurewei
                                                           June 19, 2019

  A Framework for Automating Service and Network Management with YANG
           draft-wu-model-driven-management-virtualization-04

Abstract

   Model-driven service and network management provides a programmatic
   and standard-based approach for representing (virtual) services or
   networks and configuration to the network device that are used to
   build and deliver the service.  Models can be used at various phases
   of service and network management life cycle such as service
   instantiation, service provisionning, optimization, monitoring, and
   diagnostic.  Also, models can be designed to automate network
   management and provide closed-loop control for the sake of adaptive
   and deterministic service creation, delivery, and maintenance.

   This document provides a framework that describes and discusses an
   architecture for service and network management automation with YANG
   modeling technologies.  An applicability of YANG data models to
   automation of virtualized network service is also investigated.

   The document aims to exemplify an approach to illustrate the journey
   from technology-agnostic services to technology-specific actions.

Status of This Memo

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   This Internet-Draft will expire on December 21, 2019.

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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   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.  IETF YANG Modules: An Overview  . . . . . . . . . . . . . . .   4
     2.1.  Network Service and Resource Models . . . . . . . . . . .   5
       2.1.1.  Network Service Models: Definition and Samples  . . .   6
       2.1.2.  Network Resource Models . . . . . . . . . . . . . . .   7
     2.2.  Network Element Models  . . . . . . . . . . . . . . . . .  10
       2.2.1.  Model Composition . . . . . . . . . . . . . . . . . .  11
       2.2.2.  Protocol/Function Configuration Models  . . . . . . .  12
   3.  Architectural Concepts  . . . . . . . . . . . . . . . . . . .  15
     3.1.  Data Models: Layering and Representation  . . . . . . . .  15
     3.2.  Service Activation, Provision, and Invocation Automation   15
     3.3.  Service Enforcement Automation  . . . . . . . . . . . . .  16
     3.4.  Modules Decomposition and Composition . . . . . . . . . .  16
   4.  Architecture Overview . . . . . . . . . . . . . . . . . . . .  17
     4.1.  End-to-End Service Delivery and Service Assurance
           Procedure . . . . . . . . . . . . . . . . . . . . . . . .  17
       4.1.1.  Resource Collection and Abstraction (a) . . . . . . .  17
       4.1.2.  Service Exposure & Abstraction (b)  . . . . . . . . .  18
       4.1.3.  IP Service Mapping (c)  . . . . . . . . . . . . . . .  19
       4.1.4.  IP Service Composition (d)  . . . . . . . . . . . . .  20
       4.1.5.  IP Service Provision (e)  . . . . . . . . . . . . . .  20
       4.1.6.  Performance Measurement and Alarm Telemetry (f) . . .  20
       4.1.7.  IP Service to TE Mapping (g)  . . . . . . . . . . . .  20
       4.1.8.  Path Management (h) . . . . . . . . . . . . . . . . .  21
       4.1.9.  TE Resource Exposure (i)  . . . . . . . . . . . . . .  21
   5.  Sample Service Coordination via YANG Moodules . . . . . . . .  22
     5.1.  L3VPN Service Delivery via Coordinated YANG Modules . . .  22
     5.2.  5G Transport Service Delivery via Coordinated YANG
           Modules . . . . . . . . . . . . . . . . . . . . . . . . .  22
   6.  Modules Usage in Automated Virtualized Network Environment:
       Sample Examples . . . . . . . . . . . . . . . . . . . . . . .  24

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     6.1.  Network-initiated Resource Creation . . . . . . . . . . .  24
     6.2.  Customer-initiated Dynamic Resource Creation  . . . . . .  26
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  29
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29
   11. Informative References  . . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

1.  Introduction

   The service management system usually comprises service activation/
   provision and service enforcement.  Traditional service delivery work
   flow process, from customer order to the actual service provision,
   typically involves input data sequentially into multiple OSS/BSS
   applications managed by different departments.  Many of these
   applications are custom built over the years and operating in a silo
   mode.  The lack of standard data input/output also causes many
   challenges in system integration and results in manual data entry.
   Secondly, traditional service fulfillment lack a programmatic and
   standards-based way of writing configurations to any network device
   and has slow response to the network changes and doesn't provide real
   time monitoring capability in high frequency and in high throughput
   on the current state of networking.  Therefore, model-driven network
   management becomes crucial to address these challenges.

   For years, the IETF has been driving the industry transition from an
   overloaded Software Defined Networking (SDN) buzzword to focus on
   specific areas such as modeling-driven network management.  [RFC7149]
   provides a first tentative to rationalize that space by identifying
   concrete technical domains that need to be considered and for which
   solutions can be provided:

   o  Techniques for the dynamic discovery of topology, devices, and
      capabilities, along with relevant information and data models that
      are meant to precisely document such topology, devices, and their
      capabilities.

   o  Techniques for exposing network services [RFC8309] and their
      characteristics.

   o  Techniques used by service-requirement-derived dynamic resource
      allocation and policy enforcement schemes, so that networks can be
      programmed accordingly.

   o  Dynamic feedback mechanisms that are meant to assess how
      efficiently a given policy (or a set thereof) is enforced from a
      service fulfillment and assurance perspective.

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   Models are key for each of these technical items.  Automation is an
   important step to improve the agility of network operations and
   infrastructure.

   In the later development, as described in [RFC8199], YANG module
   developers have taken both top-down and bottom-up approaches to
   develop modules and establish mapping between network technology and
   customer requirements on the top or abstracting common construct from
   various network technologies.  At the time of writing this document
   (2019), we see the large number of data models including
   configuration models and service models developed or under
   development in IETF covering much of networking protocols and
   techniques.  In addition, how these models work together to fully
   configure a device, manage a set of devices involved in a service, or
   even provide a service aren't developed yet in IETF.

   This document takes both bottom up approach and top down approach to
   provide an architectural framework for network management automation,
   with a focus on network virtualization environment.

   This document also describes specific YANG modules needed to realize
   connectivity services and investigates how top down built model
   (e.g., customer-facing data models) interact with bottom up built
   model (network resource-facing data models) in the context of service
   delivery and assurance.

   The document identifies a comprehensive list of modules to exemplify
   the proposed approach, but the document does not claim to be
   exhaustive.

   It is not the intent of this document to provide an inventory of
   tools and mechanisms used in specific network and service management
   domains; such inventory can be found in documents such as [RFC7276].

2.  IETF YANG Modules: An Overview

   Figure 1 provides an overview of various macro-functional blocks to
   which belong the various IETF-defined modules.

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   <<Network Service and Resource Models>>
+-----------------------------------------------------------------------+
| << Network Service Models>>                                           |
| +----------------+ +----------------+ +-------------+ +-------------+ |
| |      L3SM      | |     L2SM       | | TEAS VN     | |   L1CSM     | |
| |  Service Model | |  Service Model | |Service Model| |Service Model| |
| +----------------+ +----------------+ +-------------+ +-------------+ |
|-------------------------------------------------------------------    |
| << Network Resource Models >>                                         |
|      +------------+  +-------+  +----------------+   +------------+   |
|      |Network Topo|  | Tunnel|  |Path Computation|   |FM/PM/Alarm |   |
|      |   Models   |  | Models|  | API Models     |   | OAM  Models|   |
|      +------------+  +-------+  +----------------+   +------------+   |
+-----------------------------------------------------------------------+
 --------------------------------------------------------------------
 <Network Element Models>>
+-----------------------------------------------------------------------+
|  <<Composition Models>>                                               |
|      +-------------+ +---------------+ +----------------+             |
|      |Device Model | |Logical Network| |Network Instance|             |
|      |             | |Element Model  | |   Model        |             |
|      +-------------+ +---------------+ +----------------+             |
|---------------------------------------------------------------------- |
| << Component Models>>                                                 |
|+---------++---------++---------++----------++---------++---------+    |
||         ||         ||         ||Common    ||         || OAM:    |    |
|| Routing ||Transport|| Policy  ||(interface||Multicast||         |    |
||(e.g.,BGP||(e.g.,   ||(e.g, ACL||multicast || (IGMP   ||FM,PM,   |    |
|| OSPF)   || MPLS)   ||  QoS)   || IP, ... )|| MLD,...)||Alarm    | ...|
|+---------++---------++---------++----------++---------++---------+    |
+-----------------------------------------------------------------------+

                Figure 1: An overview of IETF YANG Modules

2.1.  Network Service and Resource Models

   Service and Network Resource modules define what the
   "service"/"resource" is.  These modules can be classified into two
   categories:

   1.  Network Service Models (Section 2.1.1)

   2.  Network Resource Models (Section 2.1.2)

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2.1.1.  Network Service Models: Definition and Samples

   As described in [RFC8309], the service is some form of connectivity
   between customer sites and the Internet and/or between customer sites
   across the network operator's network and across the Internet.  More
   concretely, an IP connectivity service can be defined as the IP
   transfer capability characterized by a (Source Nets, Destination
   Nets, Guarantees, Scope) tuple where "Source Nets" is a group of
   unicast IP addresses, "Destination Nets" is a group of IP unicast
   and/or multicast addresses, and "Guarantees" reflects the guarantees
   (expressed in terms of Quality Of Service (QoS), performance, and
   availability, for example) to properly forward traffic to the said
   "Destination" [RFC7297].

   For example,

   o  L3SM model [RFC8299] defines the L3VPN service ordered by a
      customer from a network operator.

   o  L2SM model [RFC8466] defines the L2VPN service ordered by a
      customer from a network operator.

   o  L1CSM model [I-D.ietf-ccamp-l1csm-yang] defines a YANG module for
      Layer 1 Connectivity Service Model (L1CSM).

   o  TEAS VN model [I-D.ietf-teas-actn-vn-yang] defines a YANG module
      for the Abstraction and Control of Traffic Engineered (TE)
      networks (ACTN) Virtual Network Service (VNS) operation.  Unlike
      L3SM model, ACTN model can also be used as operator-facing model,
      e.g., establish interconnections between L3VPN sites across
      multiple ASes.

   o  [I-D.ietf-teas-te-service-mapping-yang] defines a YANG module to
      map service model (e.g., L3SM) and Traffic Engineering model
      (e.g., TE Tunnel or the ACTN model).  This model is applicable to
      the operation's need for a control and management of VPN services
      with TE tunnel support and principally used to allow monitoring
      and diagnostic of the management systems to assess how the service
      requests are mapped onto underlying network resources and TE
      models.

   o  Composed VPN model [I-D.evenwu-opsawg-yang-composed-vpn] defines a
      YANG module that can be used by a network operator to configure a
      VPN service in multiple administrative domain environment
      consisting of L2VPN or L3VPN or a mixture of the two.  This model
      provides an abstracted view of VPN service configuration
      components at different layer.

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2.1.2.  Network Resource Models

   Figure 2 depicts a set of Network resource YANG modules such as
   topology models or tunnel models:

                       |                             |
     Topo YANG modules |   Tunnel YANG modules       |Resource NM Tool
     ------------------------------------------------|-- ------------
  +------------+       |                             |
  |Network Top |       | +------+  +-----------+     |       +-------+
  |   Model    |       | |Other |  | TE Tunnel |     |       | LIME  |
  +----+-------+       | |Tunnel|  +------+----+     |       | Model |
       |   +--------+  | +------+         |          |       |/PM/FM |
       |---+Svc Topo|  |         +--------+-+--------+       |Model  |
       |   +--------+  |    +----+---+  +---+----+ +-+-----+ +-------+
       |   +--------+  |    |MPLS-TE |  |RSVP-TE | |SR TE  | +--------+
       |---+L2 Topo |  |    | Tunnel |  | Tunnel | |Tunnel | |  Alarm |
       |   +--------+  |    +--------+  +--------+ +-------+ |  Model |
       |   +--------+  |                                     +--------+
       |---+TE Topo |  |                                   +-----------+
       |   +--------+  |                                   |Path       |
       |   +--------+  |                                   |Computation|
       +---+L3 Topo |                                      |API Model  |
           +----|---+                                      +-----------+
      +---------+---------+
      |         |         |
  +---|---+  +--|---+ +---|-+
  |SR Topo|  |SR TE | |L3 TE|
  | Model |  | Topo | |Topo |
  +-------+  +------+ +-----+

              Figure 2: Sample Resource Facing Network Models

   Topology YANG modules:

   o  Network Topology Models: [RFC8345] defines a base model for
      network topology and inventories.  Network topology data include
      link resource, node resource, and terminate-point resources.

   o  TE Topology Models: [I.D-ietf-teas-yang-te-topo] defines a data
      model for representing and manipulating TE topologies.

      This module is extended from network topology model defined in
      [RFC8345] with TE topologies specifics.  This model contains
      technology agnostic TE Topology building blocks that can be
      augmented and used by other technology-specific TE Topology
      models.

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   o  L3 Topology Models

      [RFC8346] defines a data model for representing and manipulating
      L3 Topologies.  This model is extended from the network topology
      model defined in [RFC8345] with L3 topologies specifics.

   o  L2 Topology Models

      [I.D-ietf-i2rs-yang-l2-topology] defines a data model for
      representing and manipulating L2 Topologies.  This model is
      extended from the network topology model defined in [RFC8345] with
      L2 topologies specifics.

   o  L3 TE Topology Models

      When traffic engineering is enabled on a layer 3 network topology,
      there will be a corresponding TE topology.  [I.D-ietf-teas-yang-
      l3-te-topo] defines data models for layer 3 traffic engineering
      topologies.  Two data models are defined, one is layer 3 TE
      topology model, the other is packet switching TE topology model.
      Layer 3 TE topology model is extended from Layer 3 topology model.
      Packet switching TE topology model is extended from TE topology
      model.

   o  SR TE Topology Models

      [I-D.ietf-teas-yang-sr-te-topo] defines a YANG module for Segment
      Routing (SR) topology and Segment Routing (SR) traffic engineering
      (TE) topology.  Two models are defined, one is SR topology model,
      the other is SR TE topology model, SR topology model is extended
      from L3 Topology model.  SR TE topology model is extended from
      both SR Topology model and L3 TE topology model.

   o  SF Aware TE Topology YANG module

      [I-D.  ietf-teas-sf-aware-topo-model] defines a YANG module for TE
      network topologies that are network service and function aware.

   o  Optical Transport Topology Models:

      *  OTN Transport Topology Model: [I-D.ietf-ccamp-otn-topo-yang]
         defines a YANG module to describe the topologies of an Optical
         Transport Network (OTN).

      *  WSON Transport Topology Model: [I-D.ietf-ccamp-wson-yang]
         defines a YANG module for the routing and wavelength assignment
         (RWA) Traffic Engineering (TE) topology in wavelength switched
         optical networks (WSONs).

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      *  Flex-Grid Transport Topology Model: [I-D.ietf-ccamp-flexigrid-
         yang] defines a YANG module for flexi-grid objects in the
         dynamic optical network, including the nodes, transponders and
         links between them, as well as how such links interconnect
         nodes and transponders.

   Tunnel YANG modules:

   o  Tunnel identities [I-D.ietf-softwire-iftunnel] to ease
      manipulating extensions to specific tunnels.

   o  TE Tunnel Model

      [I.D-ietf-teas-yang-te] defines a YANG module for the
      configuration and management of TE interfaces, tunnels and LSPs.

   o  SR TE Tunnel Model

      [I.D-ietf-teas-yang-te] augments the TE generic and MPLS-TE
      model(s) and defines a YANG module for Segment Routing (SR) TE
      specific data.

   o  MPLS TE Model

      [I.D-ietf-teas-yang-te] augments the TE generic and MPLS-TE
      model(s) and defines a YANG module for MPLS TE configurations,
      state, RPC and notifications.

   o  RSVP-TE MPLS Model

      [I.D-ietf-teas-yang-rsvp-te] augments the RSVP-TE generic module
      with parameters to configure and manage signaling of MPLS RSVP-TE
      LSPs.

   o  Optical Transport Tunnel Models:

      *  Flexigrid Media Channel Tunnel Models: [I-D.ccamp-flexigrid-
         media-channel-yang] defines a YANG module for the flexi-grid
         media-channel.  This YANG module defines the whole path from a
         source transponder or node to the destination through a number
         of intermediate nodes in the flexi-grid network.

      *  WSON Tunnel Model: [I-D.ccamp-wson-tunnel-model] defines a YANG
         module for WSON tunnel model.

      *  OTN Tunnel Model: [I-D.  ietf-ccamp-otn-tunnel-model]defines a
         YANG module for OTN tunnel Model.

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   Resource NM Tool Models:

   o  Path Computation API Model

      [I.D-ietf-teas-path-computation] YANG module for a stateless RPC
      which complements the stateful solution defined in [I.D-ietf-teas-
      yang-te].

   o  OAM Models (including Fault Management (FM) and Performance
      Monitoring)

      [RFC8532] defines a base YANG module for the management of OAM
      protocols that use Connectionless Communications.  [RFC8533]
      defines a retrieval method YANG module for connectionless OAM
      protocols.  [RFC8531] defines a base YANG module for connection
      oriented OAM protocols.  These three models are intended to
      provide consistent reporting, configuration and representation for
      connection-less OAM and Connection oriented OAM separately.

      Alarm monitoring is a fundamental part of monitoring the network.
      Raw alarms from devices do not always tell the status of the
      network services or necessarily point to the root cause.  [I.D-
      ietf-ccamp-alarm-module]defines a YANG module for alarm
      management.

   o  Generic Policy Model

      The Simplified Use of Policy Abstractions (SUPA) policy-based
      management framework [RFC8328] defines base YANG modules to encode
      policy.  These models point to device-, technology-, and service-
      specific YANG modules developed elsewhere.  Policy rules within an
      operator's environment can be used to express high-level, possibly
      network-wide, policies to a network management function (within a
      controller, an orchestrator, or a network element).  The network
      management function can then control the configuration and/or
      monitoring of network elements and services.  This document
      describes the SUPA basic framework, its elements, and interfaces.

2.2.  Network Element Models

   Network Element models (Figure 3) are used to describe how a service
   can be implemented by activating and tweaking a set of functions
   (enabled in one or multiple devices) that are involved in the service
   delivery.

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                                          +----------------+
                                        --|Device Model    |
                                        | +----------------+
                                        | +------------------+
                     +---------------+  | |Logical Network   |
                     |               |  --|  Element Mode    |
                     | Architecture  |  | +------------------+
                     |               |  | +----------------------+
                     +-------+-------+  --|Network Instance Mode |
                             |          | +----------------------+
                             |          | +-------------------+
                             |          --|Routing Type Model |
                             |            +-------------------+
     +-------+----------+----+------+------------+-----------+-------+
     |       |          |           |            |           |       |
   +-+-+ +---+---+   +--+------+  +-+-+    +-----+---+   +---+-+     |
   |ACL| |Routing|   |Transport|  |OAM|    |Multicast|   |  PM |  Others
   +---+ |-------+   +---------+  +---+    +---------+   +-----+
         | +-------+  +----------+ +-------+   +-----+    +-----+
         --|Core   |  |MPLS Basic| |BFD    |   |IGMP |    |TWAMP|
         | |Routing|  +----------+ +-------+   |/MLD |    +-----+
         | +-------+  |MPLS LDP  | |LSP Ping   +-----+    |OWAMP|
         --|BGP    |  +----------+ +-------+   |PIM  |    +-----+
         | +-------+  |MPLS Static |MPLS-TP|   +-----+    |LMAP |
         --|ISIS   |  +----------+ +-------+   |MVPN |    +-----+
         | +-------+                           +-----+
         --|OSPF   |
         | +-------+
         --|RIP    |
         | +-------+
         --|VRRP   |
         | +-------+
         --|SR/SRv6|
         | +-------+
         --|ISIS-SR|
         | +-------+
         --|OSPF-SR|
           +-------+

                     Figure 3: Network Element Modules

2.2.1.  Model Composition

   o  Device Model

      [I.D-ietf-rtgwg-device-model] presents an approach for organizing
      YANG modules in a comprehensive logical structure that may be used
      to configure and operate network devices.  The structure is itself

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      represented as an example YANG module, with all of the related
      component models logically organized in a way that is
      operationally intuitive, but this model is not expected to be
      implemented.

   o  Logical Network Element Model

      [RFC8530] defines a logical network element module which can be
      used to manage the logical resource partitioning that may be
      present on a network device.  Examples of common industry terms
      for logical resource partitioning are Logical Systems or Logical
      Routers.

   o  Network Instance Model

      [RFC8529] defines a network instance module.  This module can be
      used to manage the virtual resource partitioning that may be
      present on a network device.  Examples of common industry terms
      for virtual resource partitioning are Virtual Routing and
      Forwarding (VRF) instances and Virtual Switch Instances (VSIs).

2.2.1.1.  Schema Mount

   Modularity and extensibility were among the leading design principles
   of the YANG data modeling language.  As a result, the same YANG
   module can be combined with various sets of other modules and thus
   form a data model that is tailored to meet the requirements of a
   specific use case.  [RFC8528] defines a mechanism, denoted schema
   mount, that allows for mounting one data model consisting of any
   number of YANG modules at a specified location of another (parent)
   schema.

   That capability does not cover design time.

2.2.2.  Protocol/Function Configuration Models

   BGP:       [I-D.ietf-idr-bgp-yang-model] defines a YANG module for
              configuring and managing BGP, including protocol, policy,
              and operational aspects based on data center, carrier and
              content provider operational requirements.

   MPLS:      [I-D.ietf-mpls-base-yang] defines a base model for MPLS
              which serves as a base framework for configuring and
              managing an MPLS switching subsystem.  It is expected that
              other MPLS technology YANG modules (e.g.  MPLS LSP Static,
              LDP or RSVP-TE models) will augment the MPLS base YANG
              module.

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   QoS:       [I-D.asechoud-netmod-diffserv-model] describes a YANG
              module of Differentiated Services for configuration and
              operations.

   ACL:       Access Control List (ACL) is one of the basic elements
              used to configure device forwarding behavior.  It is used
              in many networking technologies such as Policy Based
              Routing, Firewalls, etc.  [RFC8519] describes a data model
              of Access Control List (ACL) basic building blocks.

   NAT:       For the sake of network automation and the need for
              programming Network Address Translation (NAT) function in
              particular, a data model for configuring and managing the
              NAT is essential.  [RFC8512] defines a YANG module for the
              NAT function covering a variety of NAT flavors such as
              Network Address Translation from IPv4 to IPv4 (NAT44),
              Network Address and Protocol Translation from IPv6 Clients
              to IPv4 Servers (NAT64), customer-side translator (CLAT),
              Stateless IP/ICMP Translation (SIIT), Explicit Address
              Mappings (EAM) for SIIT, IPv6-to-IPv6 Network Prefix
              Translation (NPTv6), and Destination NAT.  [RFC8513]
              specifies a YANG module for the DS-Lite AFTR.

   Stateless Address Sharing:  [I-D.ietf-softwire-yang] specifies a YANG
              module for A+P address sharing, including Lightweight
              4over6, Mapping of Address and Port with Encapsulation
              (MAP-E), and Mapping of Address and Port using Translation
              (MAP-T) softwire mechanisms.

   Multicast: [I-D.ietf-pim-yang] defines a YANG module that can be used
              to configure and manage Protocol Independent Multicast
              (PIM) devices.  [I-D.ietf-pim-igmp-mld-yang] defines a
              YANG module that can be used to configure and manage
              Internet Group Management Protocol (IGMP) and Multicast
              Listener Discovery (MLD) devices.  [I-D.ietf-pim-igmp-mld-
              snooping-yang] defines a YANG module that can be used to
              configure and manage Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              devices.

   EVPN:      [I-D.ietf-bess-evpn-yang] defines a YANG module for
              Ethernet VPN services.  The model is agnostic of the
              underlay.  It apply to MPLS as well as to VxLAN
              encapsulation.  The model is also agnostic of the services
              including E-LAN, E-LINE and E-TREE services.  This
              document mainly focuses on EVPN and Ethernet-Segment
              instance framework.

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   L3VPN:     [I-D.ietf-bess-l3vpn-yang] defines a YANG module that can
              be used to configure and manage BGP L3VPNs [RFC4364].  It
              contains VRF specific parameters as well as BGP specific
              parameters applicable for L3VPNs.

   L2VPN:     [I-D.ietf-bess-l2vpn-yang] defines a YANG module for MPLS
              based Layer 2 VPN services (L2VPN) [RFC4664] and includes
              switching between the local attachment circuits.  The
              L2VPN model covers point-to-point VPWS and Multipoint VPLS
              services.  These services use signaling of Pseudowires
              across MPLS networks using LDP [RFC8077][RFC4762] or BGP
              [RFC4761].

   Routing Policy:  [I-D.ietf-rtgwg-policy-model] defines a YANG module
              for configuring and managing routing policies in a vendor-
              neutral way and based on actual operational practice.  The
              model provides a generic policy framework which can be
              augmented with protocol-specific policy configuration.

   BFD:       [I-D.ietf-bfd-yang]defines a YANG module that can be used
              to configure and manage Bidirectional Forwarding Detection
              (BFD) [RFC5880].  BFD is a network protocol which is used
              for liveness detection of arbitrary paths between systems.

   SR/SRv6:   [I-D.ietf-spring-sr-yang] a YANG module for segment
              routing configuration and operation.  [I-D.raza-spring-
              srv6-yang] defines a YANG module for Segment Routing IPv6
              (SRv6) base.  The model serves as a base framework for
              configuring and managing an SRv6 subsystem and expected to
              be augmented by other SRv6 technology models accordingly.

   Core Routing:  [RFC8349] defines the core routing data model, which
              is intended as a basis for future data model development
              covering more-sophisticated routing systems.  It is
              expected that other Routing technology YANG modules (e.g.,
              VRRP, RIP, ISIS, OSPF models) will augment the Core
              Routing base YANG module.

   PM:

              [I.D-ietf-ippm-twamp-yang] defines a data model for client
              and server implementations of the Two-Way Active
              Measurement Protocol (TWAMP).

              [I.D-ietf-ippm-stamp-yang] defines the data model for
              implementations of Session-Sender and Session-Reflector
              for Simple Two-way Active Measurement Protocol (STAMP)
              mode using YANG.

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              [RFC8194] defines a data model for Large-Scale Measurement
              Platforms (LMAPs).

3.  Architectural Concepts

3.1.  Data Models: Layering and Representation

   As described in [RFC8199], layering of modules allows for better
   reusability of lower-layer modules by higher-level modules while
   limiting duplication of features across layers.

   The IETF has developed a number of service level, network level and
   device level modules.  Different service level modules may rely on
   the same set of network level or device level modules.  Service level
   modules usually follow top down approach and are mostly customer-
   facing models providing a common model construct for higher level
   network services, which can be further mapped to network technology-
   specific models at lower layer.

   Network level modules mostly follow bottom up approach and are mainly
   network resource-facing model and describe various aspects of a
   network infrastructure, including devices and their subsystems, and
   relevant protocols operating at the link and network layers across
   multiple devices (e.g., Network topology and TE Tunnel modules).

   Device level modules usually follow bottom up approach and are mostly
   technology-specific modules used to realize a service.

3.2.  Service Activation, Provision, and Invocation Automation

   To provide more adaptive (a.k.a., agile) service offerings, Service
   level modules can be used by an operator to structure how it
   communicates with the customer.  One or more monolithic Service
   modules can be used in teh context of a composite service activation
   requets (e.g., deliver of a caching infrastructure over a VPN).  Such
   modules are used to feed a decision-making intelligence to rapidly
   accommodate customer' needs.

   Also, such modules may be used jointly with services that require
   dynamic service invocation.  A typical example is the service modules
   defined by the DOTS WG to dynamically trigger requests to handle DDoS
   attacks [I-D.ietf-dots-signal-channel][I-D.ietf-dots-data-channel].

   Network level module can be translated from service level module and
   used to provision, monitor, instantiate the service and provide life
   cycle management of network resource,e.g., expose network resource to
   the customer or operators to provide service assurance on network

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   service and allow customer or operator to re-optimize the network
   based on service requirements described in the service level model.

3.3.  Service Enforcement Automation

   To provide network management automation, Device level modules
   translated from Service level modules or Network level modules can be
   used to provision each involved network function/device and operate
   the network based on service requirements described in the Service
   level module(s).

   In addition, the operational state including configuration that is in
   effect and status together with statistics should be exposed to upper
   layers to provide better network visibility (and assess to what
   extent the translated low level modules are honoring the upper level
   inputs).  Note that it is important is to stitch telemetry data with
   configuration data to provide closed loop life cycle management on
   the network as a system (including device-centric views).

3.4.  Modules Decomposition and Composition

   To support top-down service delivery, the service parameters captured
   in service level module(s) need to be decomposed into a set of
   configuration parameters specific to one or more technologies; these
   technology-specific parameters will be grouped together per
   technology to define technology-specific device level model or
   network level model.

   In addition, these technology-specific device level models can be
   further assembled together to provision each involved network
   function/device or each involved administrative domain to improve
   provision efficiency.

   For example, IETF rtgwg and netmod working groups have already been
   tasked to define model composition mechanism (i.e., Schema Mount
   mechanism) and relevant grouping base models such as network instance
   model, logical network element model.  The model composition
   mechanism can be used to assembler different model together while
   grouping based models can be used to setup and administrate both
   virtualized system and physical system.

   IETF also developed YANG catalog tool to manage metadata around IETF-
   defined modules; it allows both YANG developers and operators to
   discover appropriate YANG modules that may be used to automate
   services operations.  This YANG catalog tools can be used to select
   appropriate models for grouping purposes or even to identify gaps.

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4.  Architecture Overview

   The architectural considerations and conclusions described in the
   previous section lead to the architecture described in this section
   and illustrated in Figure 4.

   The interfaces and interactions shown in the figure and labeled (a)
   through (j) are further described in Section 4.1.

       +-----------------+                              ----------------
       |Service Requester|                              Service Level|
       +-----------------+                                           |
 +-------------|--------------------------------------------------+  |
 |    +--------V---------+                +------------+          |  |
 |    | Service Exposure |----------------- IP Service |          |  |
 |    +-------(b)--------+                |  Mapping   |          |  |
 |             |                          +--(c)-|-----+          |  |
 |             |                                 |      ----------------
 | |---------->|<----------------+               |      Network Level|
 | |  +--------V---------+       |               |                |  |
 | |  | IP Service to TE         |      +------->|<-----------+   |  |
 | |  |    Mapping       |       |      |        |            |   |  |
 | |  +-------(f)--------+       |      | +------|-----+      |   |  |
 | |           |           +-----|-----+| | IP Service |  +---+--+|  |
 | |  +--------V---------+ |TE Resource|| | Composition|  |Alarm/||  |
 | |  |     TE Path      | | Exposure  || +--(d)-|-----+  |  PM  ||  |
 | |  |   Management       +----(h)----+|        |        +-(g) -+|  |
 | |  +-------(e)--------+       |      | +------|------+         |  |
 | |           |                 |      | | IP Service  |     |   |  |
 | |           +-----------------+      | | Provision   +-----|   |  |
 | |                                    | +-(e)--|------+         |  |
 | |                        +-----------++                        |  |
 | |                        | Resource   |                        |  |
 | |                        | Collection |                        |  |
 | |------------------------+&Abstraction|                        |  |
 |                          +----(a)-----+              ----------------
 +----------------------------------------------------------------+

       Figure 4: Service and Network Management Automation with YANG

4.1.  End-to-End Service Delivery and Service Assurance Procedure

4.1.1.  Resource Collection and Abstraction (a)

   Network Resource such as links, nodes, or terminate-point resources
   can be collected from the network and aggregated or abstracted to the
   management system.  Periodic fetching of data is not an adequate
   solution for applications requiring frequent or prompt updates of

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   network resource.  Applying polling-based solutions to retrieve
   network resource also imposes a load on networks, devices, and
   applications.  These limitations can be addressed by including
   generic object subscription mechanisms within network elements.

   These resources can be modelled using network topology model, L3
   topology model, L2 topology model, TE topology model, L3 TE topology
   model, SR TE topology models at different layers.

   In some cases, there may have multiple overlay topologies built on
   top of the same underlay topology, and the underlay topology can be
   also built from one or more lower layer underlay topology.

   In some cases, there may have multiple overlay topologies built on
   top of the same underlay topology, and the underlay topology can be
   also built from one or more lower layer underlay topology.  The
   network resources and management objects in these multi-layer
   topologies are not recommended to be exposed to customers who (will)
   order the service from the management system, instead it will be
   exposed to the management system for IP service mapping and TE path
   Management.

   The abstract view is likely to be technology-agnostic.

4.1.2.  Service Exposure & Abstraction (b)

   Service exposure & abstraction is used to capture services offered to
   customers.

   Service abstraction can be used by a customer to request a service
   (ordering and order handling).  One typical example is that a
   customer can use L3SM service model to request L3VPN service by
   providing the abstract technical characterization of the intended
   service.  Such L3VPN service describes various aspects of network
   infrastructure, including devices and their subsystems, and relevant
   protocols operating at the link and network layers across multiple
   device.  The L3SM service model can be used to interact with the
   network infrastructure, e.g., configure sites, decide QoS parameters
   to be applied to end to end connectivity between VPN sites, select
   PEs, CEs, etc.

   Service catalogs can be created to expose the various services and
   the information needed to invoke/order a given service.

   YANG modules can be grouped into various service bundles; each
   service bundle is corresponding to a set of YANG modules that have
   been released or published.  Then, a mapping can be established

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   between service abstraction at higher layer and service bundle or a
   set of YANG modules at lower layer.

4.1.3.  IP Service Mapping (c)

   Service abstraction starts with high-level abstractions exposing the
   business capabilities or capturing customer requirements.  Then, it
   needs to maps them to resource abstraction and specific network
   technologies.

   Therefore, the interaction between service abstraction in the overlay
   and network resource abstraction in the underlay is required.  For
   example, in the L3SM service model, we describe VPN service topology
   including sites relationship, e.g., hub and spoke and any to any,
   single homed, dual-homed, multi-homed relation between PEs and CEs,
   but we don't know how this service topology can be mapped into
   underlying network topology.  For detailed interaction, please refer
   to Section 4.1.8

   In addition, there is a need to decide on a mapping between service
   abstraction and underlying specific network technologies.  Take L3SM
   service model as an example, to realize L3VPN service, we need to map
   L3SM service view defined in Service model into detailed
   configuration view defined by specific configuration models for
   network elements, these configuration models include:

   o  VRF definition, including VPN Policy expression

   o  Physical Interface

   o  IP layer (IPv4, IPv6).

   o  QoS features such as classification, profiles, etc.

   o  Routing protocols: support of configuration of all protocols
      listed in the document, as well as routing policies associated
      with those protocols.

   o  Multicast Support

   o  NAT or address sharing

   o  Security functions

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4.1.4.  IP Service Composition (d)

   These detailed configuration models are further assembled together
   into service bundle described inFigure 3 using, e.g., device model,
   logical network element model or network instance model defined in
   [I.D-ietf-rtgwg-device-model] [RFC8530] [RFC8529] and provide the
   association between an interface and its associated LNE and NI and
   populate them into appropriate devices(e.g., PE and CE).

4.1.5.  IP Service Provision (e)

   IP Service Provision is used to provision network infrastructure
   using various configuration models, e.g., use network element models
   such as BGP, ACL, QoS, Interface model, Network instance models to
   configure PE and CE device within the site.  BGP Policy model is used
   to establish VPN membership between sites and VPN Service Topology.
   Traditionally, "push" service element configuration model one by one
   to the network device and provide association between an interface
   and each service element configuration model is not efficient.

   To automate configuration of the service elements, we first assemble
   all related network elements models into logical network element
   model defined in [RFC8530] and then establish association with an
   interface and a set of network element configurations.

   In addition, IP Service Provision can be used to setup tunnels
   between sites and setup tunnels between PE and CE within the site
   when tunnels related configuration parameters can be generated from
   service abstraction.  However when tunnels related configuration
   parameters can not be generated from service abstraction, IP Service
   to TE Mapping procedure is required.

4.1.6.  Performance Measurement and Alarm Telemetry (f)

   Once the tunnel is setup, PM and Warning information per tunnel or
   per link based on network topology can be collected and report to the
   management system.  This information can be used to optimize the
   network or provide troubleshooting support.

4.1.7.  IP Service to TE Mapping (g)

   Take L3VPN service model as an example, the management system will
   use L3SM service model to determine where to connect each site-
   network-access of a particular site to the provider network (e.g.,
   PE, aggregation switch).  L3SM Service model proposes parameters and
   constraints that can influence the meshing of the site-network-
   access.

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   Nodes used to connect a site may be captured in relevant clauses of a
   service exposure model (e.g., Customer Nodes Map [RFC7297]).

   When Site location is determined, PE and CE device location will be
   selected.  Then we can replace parameters and constraints that can
   influence the meshing of the site-network-access with specified PE
   and CE device information associated with site-network-access and
   generate resource facing VN Overlay Resource model.  One example of
   resource facing VN Overlay Resource model is TEAS VN Service Model
   [I-D.ietf-teas-actn-vn-yang].

   This VN Overlay Resource model can be used to calculate node and link
   resource to Meet service requirements based on Network Topology
   models collected at step (a).

4.1.8.  Path Management (h)

   Path Management includes Path computation and Path setup.  For
   example, we can translate L3SM service model into resource facing VN
   Model, with selected PE and CE in each site, we can calculate point
   to point or multipoint end to end path between sites based on VN
   Overlay Resource Model.

   After identifying node and link resources required to meet service
   requirements, the mapping between overlay topology and underlay
   topology can be established, e.g., establish an association between
   VPN service topology defined in customer facing model and underlying
   network topology defined in the TE topology model (e.g., one overlay
   node is supported by multiple underlay nodes, one overlay link is
   supported by multiple underlay nodes) and generate end to end VN
   topology.

4.1.9.  TE Resource Exposure (i)

   When tunnels related configuration parameters can not be generated
   from service abstraction, IP Service to TE Mapping procedure can be
   used to generate TE Resource Exposure view, this TE resource Exposure
   view can be modeled as resource facing VN model which is translated
   and instantiated from L3SM model and manage TE resource based on path
   management information and PM and alarm telemetry information.

   Operators may use this dedicated TE resource Exposure view to
   dynamically capture the overall network status and topology to:

   o  Perform all the requested recovery operations upon detecting
      network failures affecting the network service.

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   o  Adjust resource distribution and update to end to end Service
      topology models

   o  Provide resource scheduling to better guarantee services for
      customers and to improve the efficiency of network resource usage.

5.  Sample Service Coordination via YANG Moodules

5.1.  L3VPN Service Delivery via Coordinated YANG Modules

   Take L3VPN service as an example, IETF has already developed L3VPN
   service model [RFC8299] which can be used to describe L3VPN service.
   To enforce L3VPN service and program the network, a set of network
   element models are needed, e.g., BGP model, Network Instance model,
   ACL model, Multicast Model, QoS model, or NAT model.

   These network element models can be grouped into different release
   bundles or feature bundle using Schema Mount technology to meet
   different tailored requirements and realize L3VPN service.

   To support the creation of logical network elements on a network
   device and enable automation of virtualized network, Logical Network
   Element (LNE) model can be used to manage its own set of modules such
   as ACL, QoS, or Network Instance modules.

5.2.  5G Transport Service Delivery via Coordinated YANG Modules

   The overview of network slice structure as defined in the 3GPP 5GS is
   shown in Figure 5.  The terms are described in specific 3GPP
   documents (e.g., [TS.23.501-3GPP] and [TS.28.530-3GPP]).

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   <==================          E2E-NSI         =======================>
                :                 :                  :           :  :
                :                 :                  :           :  :
   <======  RAN-NSSI  ======><=TRN-NSSI=><====== CN-NSSI  ======>VL[APL]
       :        :        :        :         :       :        :   :  :
       :        :        :        :         :       :        :   :  :
   RW[NFs ]<=TRN-NSSI=>[NFs ]<=TRN-NSSI=>[NFs ]<=TRN-NSSI=>[NFs ]VL[APL]

    . . . . . . . . . . . . ..          . . . . . . . . . . . . ..
    .,----.   ,----.   ,----..  ,----.  .,----.   ,----.   ,----..
 UE--|RAN |---| TN |---|RAN |---| TN |---|CN  |---| TN |---|CN  |--[APL]
    .|NFs |   `----'   |NFs |.  `----'  .|NFs |   `----'   |NFs |.
    .`----'            `----'.          .`----'            `----'.
    . . . . . . . . . . . . ..          . . . . . . . . . . . . ..

   RW         RAN                MBH               CN               DN

 *Legends
  UE: User Equipment
  RAN: Radio Access Network
  CN: Core Network
  DN: Data Network
  TN: Transport Network
  MBH: Mobile Backhaul
  RW: Radio Wave
  NF: Network Function
  APL: Application Server
  NSI: Network Slice Instance
  NSSI: Network Slice Subnet Instance

             Figure 5: Overview of Structure of NS in 3GPP 5GS

   To support 5G service (e.g., 5G MBB service), L3VPN service model
   [RFC8299] and TEAS VN model [I-D. ietf-teas-actn-vn-yang] can be both
   provided to describe 5G MBB Transport Service or connectivity
   service.  L3VPN service model is used to describe end-to-end
   connectivity service while TEAS VN model is used to describe TE
   connectivity service between VPN sites or between RAN NFs and Core
   network NFs.

   VN in TEAS VN model and support point-to-point or multipoint-to-
   multipoint connectivity service and can be seen as one example of
   network slice.

   TE Service mapping model can be used to map L3VPN service requests
   onto underlying network resource and TE models to get TE network
   setup.

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   For IP VPN service provision, L3VPN service model is translated into
   a set of network element configuration parameters, these
   configuration parameters will be bound to different network element
   models and group them together to form feature bundle or service
   bundle to get L3VPN network setup.

6.  Modules Usage in Automated Virtualized Network Environment: Sample
    Examples

6.1.  Network-initiated Resource Creation

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                                     |(2)
                                     |
                                     V
                         +-------------------+
                         | Management System | (3)(4)(5)
                         +-------------------+

           +--------------------------------------------------------+
          /     _[CE2]                      _[CE3]                 /
         /    _/  :   \_                  _/  :   \_              /
        /   _/     :    \_              _/     :    \_           /
       /  _/        :     \_          _/        :     \_        /
      /  /           :      \        /           :      \      /
     /[CE1]_________________[PE1] [PE2]_________________[CE4] /
    +---------:--------------:------------:--------------:---+
                                                             "Service"
   --------------------------------------------------------------------
           +---------------------+    +---------------------+"Resource"
          /   [Y5]...           /    / [Z5]______[Z3]      /
         /    /  \  :          /    /  : \_       / :     /
        /    /    \  :        /    /   :   \_    /  :    /
       /    /      \  :      /    /   :      \  /   :   /
      /   [Y4]____[Y1] :    /    /   :       [Z2]   :  /
     +------:-------:---:--+    +---:---------:-----:-+       ^
     vNet1  :        :   :         :          :     :  vNet2  |
            :         :   :       :           :     :         |(1)
            :  +-------:---:-----:------------:-----:-----+   |
            : /       [X1]__:___:___________[X2]   :     /    |
            :/         / \_  : :       _____/ /   :     /     |
            :         /    \_ :  _____/      /   :     /
           /:        /       \: /           /   :     /
          / :       /        [X5]          /   :     /
         /   :     /       __/ \__        /   :     /
        /     :   /    ___/       \__    /   :     /
       /       : / ___/              \  /   :     /
      /        [X4]__________________[X3]..:     /
     +------------------------------------------+
                          L3 Topology

   The following steps are performed to deliver the service within the
   network management automation architecture proposed in this document:

   o  Pre-provision multiple virtualized networks on top of the same
      basic network infrastructure based on pre-configured service
      requirements and establish resource pool for each virtualized
      network and expose to the customer with several service templates
      through web portal.

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   o  Selects and uses one which fulfills most its requirement among the
      service templates.

   o  Create resource facing VN Network based on selected service
      template, and calculate the node resource, link resource
      corresponding to connectivity between sites.

   o  Setup tunnels between sites and map them into the selected
      virtualized network topology and establish resource facing VN
      topology based on TEAS VN model [I-D.ietf-teas-actn-vn-yang] and
      TE tunnel based on TE Tunnel model.

      The resource facing VN model and corresponding TE Tunnel model can
      be further used to notify all the parameter changes and event
      related to VN topology or Tunnel.  These information can be
      further used to adjust network resource distributed in the
      network.

   The network initiated resource creation is similar to ready made
   Network Slice creation pattern discussed in Section 5.1 of [I-
   D.homma-slice-provision-models].

6.2.  Customer-initiated Dynamic Resource Creation

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                                  |(2)
                                  |
                                  V
                      +-------------------+
                      | Management System | (3)(4)(5)
                      +-------------------+

        +--------------------------------------------------------+
       /     _[CE2]                      _[CE3]                 /
      /    _/  :   \_                  _/  :   \_              /
     /   _/     :    \_              _/     :    \_           /
    /  _/        :     \_          _/        :     \_        /
   /  /           :      \        /           :      \      /
  /[CE1]_________________[PE1] [PE2]_________________[CE4] /
 +---------:--------------:------------:--------------:---+
                                                          "Service"
--------------------------------------------------------------------
                                                        "Resource"                             ^
         :                                                 |
         :         :   :                                   |(1)
         :  +-------:---:-----:------------:-----:-----+   |
         : /       [X1]__:___  __________[X2]         /    |
         :/         / \_  :         _____/ /         /     |
         :         /    \_ :  _____/      /         /
        /:        /       \: /           /         /
       / :       /        [X5]          /         /
      /   :     /       __/ \__        /         /
     /     :   /    ___/       \__    /         /
    /       : / ___/              \  /         /
   /        [X4]__________________[X3].       /
  +------------------------------------------+
                       L3 Topology

   The following steps are performed to deliver the service within the
   network management automation architecture proposed in this document:

   o  Establish resources pool for the basic common network
      infrastructure.

   o  Request to create two sites based on L3SM Service model with each
      having one network access connectivity:

         Site A: Network-Access A, Bandwidth=20M, for class "foo",
         guaranteed-bw-percent = 10, One-Way-Delay=70 msec

         Site B: Network-Access B, Bandwidth=30M, for class "foo1",
         guaranteed-bw-percent = 15, One-Way-Delay=60 msec

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   o  Create a new service topology based on Service Type and service
      requirements (e.g., Slice Service Type, Slice location, Number of
      Slices, QoS requirements corresponding to network connectivity
      within a Slice) defined in L3SM service model.

   o  Translate L3SM service model into resource facing TEAS VN Model
      [I-D.ietf-teas-actn-vn-yang], and calculate the node resource,
      link resource corresponding to connectivity between sites or
      connectivity between PE and CE within Site in the service topology
      based on generated resource facing TEAS VN model.

   o  Setup tunnels between sites and tunnel between PE and CE within
      Site and map them into basic network infrastructure and establish
      resource facing VN topology based on TEAS VN model and TE tunnel
      based on TE Tunnel model.  The resource facing TEAS VN model and
      corresponding TE Tunnel model can be used to notify all the
      parameter changes and event related to VN topology or Tunnel.
      These information can be further used to adjust network resource
      distributed within the network.

   The customer initiated resource creation is similar to customer made
   Network Slice creation pattern discussed in Section 5.2 of [I-
   D.homma-slice-provision-models].

7.  Security Considerations

   Security considerations specific to each of the technologies and
   protocols listed in the document are discussed in the specification
   documents of each of these techniques.

   (Potential) security considerations specific to this document are
   listed below:

   o  Create forwarding loops by mis-configuring the underlying network.

   o  Leak sensitive information: special care should be considered when
      translating between the various layers introduced in the document.

   o  ...tbc

8.  IANA Considerations

   There are no IANA requests or assignments included in this document.

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9.  Contributors

      Shunsuke Homma
      Japan

      Email: s.homma0718+ietf@gmail.com

10.  Acknowledgements

   Thanks to Joe Clarck and Greg Mirsky for the review.

11.  Informative References

   [I-D.arkko-arch-virtualization]
              Arkko, J., Tantsura, J., Halpern, J., and B. Varga,
              "Considerations on Network Virtualization and Slicing",
              draft-arkko-arch-virtualization-01 (work in progress),
              March 2018.

   [I-D.asechoud-netmod-diffserv-model]
              Choudhary, A., Shah, S., Jethanandani, M., Liu, B., and N.
              Strahle, "YANG Model for Diffserv", draft-asechoud-netmod-
              diffserv-model-03 (work in progress), June 2015.

   [I-D.clacla-netmod-model-catalog]
              Clarke, J. and B. Claise, "YANG module for
              yangcatalog.org", draft-clacla-netmod-model-catalog-03
              (work in progress), April 2018.

   [I-D.evenwu-opsawg-yang-composed-vpn]
              Even, R., Bo, W., Wu, Q., and Y. Cheng, "YANG Data Model
              for Composed VPN Service Delivery", draft-evenwu-opsawg-
              yang-composed-vpn-03 (work in progress), March 2019.

   [I-D.homma-slice-provision-models]
              Homma, S., Nishihara, H., Miyasaka, T., Galis, A., OV, V.,
              Lopez, D., Contreras, L., Ordonez-Lucena, J., Martinez-
              Julia, P., Qiang, L., Rokui, R., Ciavaglia, L., and X.
              Foy, "Network Slice Provision Models", draft-homma-slice-
              provision-models-00 (work in progress), February 2019.

   [I-D.ietf-bess-evpn-yang]
              Brissette, P., Shah, H., Hussain, I., Tiruveedhula, K.,
              and J. Rabadan, "Yang Data Model for EVPN", draft-ietf-
              bess-evpn-yang-07 (work in progress), March 2019.

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   [I-D.ietf-bess-l2vpn-yang]
              Shah, H., Brissette, P., Chen, I., Hussain, I., Wen, B.,
              and K. Tiruveedhula, "YANG Data Model for MPLS-based
              L2VPN", draft-ietf-bess-l2vpn-yang-09 (work in progress),
              October 2018.

   [I-D.ietf-bess-l3vpn-yang]
              Jain, D., Patel, K., Brissette, P., Li, Z., Zhuang, S.,
              Liu, X., Haas, J., Esale, S., and B. Wen, "Yang Data Model
              for BGP/MPLS L3 VPNs", draft-ietf-bess-l3vpn-yang-04 (work
              in progress), October 2018.

   [I-D.ietf-bfd-yang]
              Rahman, R., Zheng, L., Jethanandani, M., Networks, J., and
              G. Mirsky, "YANG Data Model for Bidirectional Forwarding
              Detection (BFD)", draft-ietf-bfd-yang-17 (work in
              progress), August 2018.

   [I-D.ietf-ccamp-alarm-module]
              Vallin, S. and M. Bjorklund, "YANG Alarm Module", draft-
              ietf-ccamp-alarm-module-09 (work in progress), April 2019.

   [I-D.ietf-ccamp-flexigrid-media-channel-yang]
              Madrid, U., Perdices, D., Lopezalvarez, V., Dios, O.,
              King, D., Lee, Y., and G. Galimberti, "YANG data model for
              Flexi-Grid media-channels", draft-ietf-ccamp-flexigrid-
              media-channel-yang-02 (work in progress), March 2019.

   [I-D.ietf-ccamp-flexigrid-yang]
              Madrid, U., Perdices, D., Lopezalvarez, V., Dios, O.,
              King, D., Lee, Y., and G. Galimberti, "YANG data model for
              Flexi-Grid Optical Networks", draft-ietf-ccamp-flexigrid-
              yang-03 (work in progress), March 2019.

   [I-D.ietf-ccamp-l1csm-yang]
              Fioccola, G., Lee, K., Lee, Y., Dhody, D., and D.
              Ceccarelli, "A YANG Data Model for L1 Connectivity Service
              Model (L1CSM)", draft-ietf-ccamp-l1csm-yang-09 (work in
              progress), March 2019.

   [I-D.ietf-ccamp-mw-yang]
              Ahlberg, J., Ye, M., Li, X., Spreafico, D., and M.
              Vaupotic, "A YANG Data Model for Microwave Radio Link",
              draft-ietf-ccamp-mw-yang-13 (work in progress), November
              2018.

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   [I-D.ietf-ccamp-otn-topo-yang]
              Zheng, H., Guo, A., Busi, I., Sharma, A., Liu, X.,
              Belotti, S., Xu, Y., Wang, L., and O. Dios, "A YANG Data
              Model for Optical Transport Network Topology", draft-ietf-
              ccamp-otn-topo-yang-06 (work in progress), February 2019.

   [I-D.ietf-ccamp-otn-tunnel-model]
              Zheng, H., Guo, A., Busi, I., Sharma, A., Rao, R.,
              Belotti, S., Lopezalvarez, V., Li, Y., and Y. Xu, "OTN
              Tunnel YANG Model", draft-ietf-ccamp-otn-tunnel-model-06
              (work in progress), February 2019.

   [I-D.ietf-ccamp-wson-tunnel-model]
              Lee, Y., Dhody, D., Guo, A., Lopezalvarez, V., King, D.,
              Yoon, B., and R. Vilata, "A Yang Data Model for WSON
              Tunnel", draft-ietf-ccamp-wson-tunnel-model-03 (work in
              progress), March 2019.

   [I-D.ietf-dots-data-channel]
              Boucadair, M. and R. K, "Distributed Denial-of-Service
              Open Threat Signaling (DOTS) Data Channel Specification",
              draft-ietf-dots-data-channel-29 (work in progress), May
              2019.

   [I-D.ietf-dots-signal-channel]
              K, R., Boucadair, M., Patil, P., Mortensen, A., and N.
              Teague, "Distributed Denial-of-Service Open Threat
              Signaling (DOTS) Signal Channel Specification", draft-
              ietf-dots-signal-channel-34 (work in progress), May 2019.

   [I-D.ietf-idr-bgp-model]
              Jethanandani, M., Patel, K., and S. Hares, "BGP YANG Model
              for Service Provider Networks", draft-ietf-idr-bgp-
              model-06 (work in progress), June 2019.

   [I-D.ietf-ippm-stamp-yang]
              Mirsky, G., Xiao, M., and W. Luo, "Simple Two-way Active
              Measurement Protocol (STAMP) Data Model", draft-ietf-ippm-
              stamp-yang-03 (work in progress), March 2019.

   [I-D.ietf-ippm-twamp-yang]
              Civil, R., Morton, A., Rahman, R., Jethanandani, M., and
              K. Pentikousis, "Two-Way Active Measurement Protocol
              (TWAMP) Data Model", draft-ietf-ippm-twamp-yang-13 (work
              in progress), July 2018.

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   [I-D.ietf-mpls-base-yang]
              Saad, T., Raza, K., Gandhi, R., Liu, X., and V. Beeram, "A
              YANG Data Model for MPLS Base", draft-ietf-mpls-base-
              yang-10 (work in progress), February 2019.

   [I-D.ietf-pim-igmp-mld-snooping-yang]
              Zhao, H., Liu, X., Liu, Y., Sivakumar, M., and A. Peter,
              "A Yang Data Model for IGMP and MLD Snooping", draft-ietf-
              pim-igmp-mld-snooping-yang-08 (work in progress), June
              2019.

   [I-D.ietf-pim-igmp-mld-yang]
              Liu, X., Guo, F., Sivakumar, M., McAllister, P., and A.
              Peter, "A YANG Data Model for Internet Group Management
              Protocol (IGMP) and Multicast Listener Discovery (MLD)",
              draft-ietf-pim-igmp-mld-yang-15 (work in progress), June
              2019.

   [I-D.ietf-pim-yang]
              Liu, X., McAllister, P., Peter, A., Sivakumar, M., Liu,
              Y., and f. hu, "A YANG Data Model for Protocol Independent
              Multicast (PIM)", draft-ietf-pim-yang-17 (work in
              progress), May 2018.

   [I-D.ietf-rtgwg-device-model]
              Lindem, A., Berger, L., Bogdanovic, D., and C. Hopps,
              "Network Device YANG Logical Organization", draft-ietf-
              rtgwg-device-model-02 (work in progress), March 2017.

   [I-D.ietf-rtgwg-policy-model]
              Qu, Y., Tantsura, J., Lindem, A., and X. Liu, "A YANG Data
              Model for Routing Policy Management", draft-ietf-rtgwg-
              policy-model-06 (work in progress), March 2019.

   [I-D.ietf-softwire-iftunnel]
              Boucadair, M., Farrer, I., and R. Asati, "Tunnel Interface
              Types YANG Module", draft-ietf-softwire-iftunnel-07 (work
              in progress), June 2019.

   [I-D.ietf-softwire-yang]
              Farrer, I. and M. Boucadair, "YANG Modules for IPv4-in-
              IPv6 Address plus Port (A+P) Softwires", draft-ietf-
              softwire-yang-16 (work in progress), January 2019.

   [I-D.ietf-spring-sr-yang]
              Litkowski, S., Qu, Y., Lindem, A., Sarkar, P., and J.
              Tantsura, "YANG Data Model for Segment Routing", draft-
              ietf-spring-sr-yang-12 (work in progress), February 2019.

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   [I-D.ietf-teas-actn-vn-yang]
              Lee, Y., Dhody, D., Ceccarelli, D., Bryskin, I., and B.
              Yoon, "A Yang Data Model for VN Operation", draft-ietf-
              teas-actn-vn-yang-05 (work in progress), June 2019.

   [I-D.ietf-teas-sf-aware-topo-model]
              Bryskin, I., Liu, X., Lee, Y., Guichard, J., Contreras,
              L., Ceccarelli, D., and J. Tantsura, "SF Aware TE Topology
              YANG Model", draft-ietf-teas-sf-aware-topo-model-03 (work
              in progress), March 2019.

   [I-D.ietf-teas-te-service-mapping-yang]
              Lee, Y., Dhody, D., Ceccarelli, D., Tantsura, J.,
              Fioccola, G., and Q. Wu, "Traffic Engineering and Service
              Mapping Yang Model", draft-ietf-teas-te-service-mapping-
              yang-01 (work in progress), March 2019.

   [I-D.ietf-teas-yang-l3-te-topo]
              Liu, X., Bryskin, I., Beeram, V., Saad, T., Shah, H., and
              O. Dios, "YANG Data Model for Layer 3 TE Topologies",
              draft-ietf-teas-yang-l3-te-topo-04 (work in progress),
              March 2019.

   [I-D.ietf-teas-yang-path-computation]
              Busi, I., Belotti, S., Lopezalvarez, V., Dios, O., Sharma,
              A., Shi, Y., Vilata, R., Sethuraman, K., Scharf, M., and
              D. Ceccarelli, "Yang model for requesting Path
              Computation", draft-ietf-teas-yang-path-computation-05
              (work in progress), March 2019.

   [I-D.ietf-teas-yang-rsvp-te]
              Beeram, V., Saad, T., Gandhi, R., Liu, X., Bryskin, I.,
              and H. Shah, "A YANG Data Model for RSVP-TE Protocol",
              draft-ietf-teas-yang-rsvp-te-06 (work in progress), April
              2019.

   [I-D.ietf-teas-yang-sr-te-topo]
              Liu, X., Bryskin, I., Beeram, V., Saad, T., Shah, H., and
              S. Litkowski, "YANG Data Model for SR and SR TE
              Topologies", draft-ietf-teas-yang-sr-te-topo-04 (work in
              progress), March 2019.

   [I-D.ietf-teas-yang-te]
              Saad, T., Gandhi, R., Liu, X., Beeram, V., and I. Bryskin,
              "A YANG Data Model for Traffic Engineering Tunnels and
              Interfaces", draft-ietf-teas-yang-te-21 (work in
              progress), April 2019.

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   [I-D.ietf-teas-yang-te-topo]
              Liu, X., Bryskin, I., Beeram, V., Saad, T., Shah, H., and
              O. Dios, "YANG Data Model for Traffic Engineering (TE)
              Topologies", draft-ietf-teas-yang-te-topo-21 (work in
              progress), May 2019.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC4664]  Andersson, L., Ed. and E. Rosen, Ed., "Framework for Layer
              2 Virtual Private Networks (L2VPNs)", RFC 4664,
              DOI 10.17487/RFC4664, September 2006,
              <https://www.rfc-editor.org/info/rfc4664>.

   [RFC4761]  Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
              LAN Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
              <https://www.rfc-editor.org/info/rfc4761>.

   [RFC4762]  Lasserre, M., Ed. and V. Kompella, Ed., "Virtual Private
              LAN Service (VPLS) Using Label Distribution Protocol (LDP)
              Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,
              <https://www.rfc-editor.org/info/rfc4762>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

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

   [RFC7276]  Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
              Weingarten, "An Overview of Operations, Administration,
              and Maintenance (OAM) Tools", RFC 7276,
              DOI 10.17487/RFC7276, June 2014,
              <https://www.rfc-editor.org/info/rfc7276>.

   [RFC7297]  Boucadair, M., Jacquenet, C., and N. Wang, "IP
              Connectivity Provisioning Profile (CPP)", RFC 7297,
              DOI 10.17487/RFC7297, July 2014,
              <https://www.rfc-editor.org/info/rfc7297>.

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   [RFC8077]  Martini, L., Ed. and G. Heron, Ed., "Pseudowire Setup and
              Maintenance Using the Label Distribution Protocol (LDP)",
              STD 84, RFC 8077, DOI 10.17487/RFC8077, February 2017,
              <https://www.rfc-editor.org/info/rfc8077>.

   [RFC8194]  Schoenwaelder, J. and V. Bajpai, "A YANG Data Model for
              LMAP Measurement Agents", RFC 8194, DOI 10.17487/RFC8194,
              August 2017, <https://www.rfc-editor.org/info/rfc8194>.

   [RFC8199]  Bogdanovic, D., Claise, B., and C. Moberg, "YANG Module
              Classification", RFC 8199, DOI 10.17487/RFC8199, July
              2017, <https://www.rfc-editor.org/info/rfc8199>.

   [RFC8299]  Wu, Q., Ed., Litkowski, S., Tomotaki, L., and K. Ogaki,
              "YANG Data Model for L3VPN Service Delivery", RFC 8299,
              DOI 10.17487/RFC8299, January 2018,
              <https://www.rfc-editor.org/info/rfc8299>.

   [RFC8309]  Wu, Q., Liu, W., and A. Farrel, "Service Models
              Explained", RFC 8309, DOI 10.17487/RFC8309, January 2018,
              <https://www.rfc-editor.org/info/rfc8309>.

   [RFC8328]  Liu, W., Xie, C., Strassner, J., Karagiannis, G., Klyus,
              M., Bi, J., Cheng, Y., and D. Zhang, "Policy-Based
              Management Framework for the Simplified Use of Policy
              Abstractions (SUPA)", RFC 8328, DOI 10.17487/RFC8328,
              March 2018, <https://www.rfc-editor.org/info/rfc8328>.

   [RFC8345]  Clemm, A., Medved, J., Varga, R., Bahadur, N.,
              Ananthakrishnan, H., and X. Liu, "A YANG Data Model for
              Network Topologies", RFC 8345, DOI 10.17487/RFC8345, March
              2018, <https://www.rfc-editor.org/info/rfc8345>.

   [RFC8346]  Clemm, A., Medved, J., Varga, R., Liu, X.,
              Ananthakrishnan, H., and N. Bahadur, "A YANG Data Model
              for Layer 3 Topologies", RFC 8346, DOI 10.17487/RFC8346,
              March 2018, <https://www.rfc-editor.org/info/rfc8346>.

   [RFC8349]  Lhotka, L., Lindem, A., and Y. Qu, "A YANG Data Model for
              Routing Management (NMDA Version)", RFC 8349,
              DOI 10.17487/RFC8349, March 2018,
              <https://www.rfc-editor.org/info/rfc8349>.

   [RFC8466]  Wen, B., Fioccola, G., Ed., Xie, C., and L. Jalil, "A YANG
              Data Model for Layer 2 Virtual Private Network (L2VPN)
              Service Delivery", RFC 8466, DOI 10.17487/RFC8466, October
              2018, <https://www.rfc-editor.org/info/rfc8466>.

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   [RFC8512]  Boucadair, M., Ed., Sivakumar, S., Jacquenet, C.,
              Vinapamula, S., and Q. Wu, "A YANG Module for Network
              Address Translation (NAT) and Network Prefix Translation
              (NPT)", RFC 8512, DOI 10.17487/RFC8512, January 2019,
              <https://www.rfc-editor.org/info/rfc8512>.

   [RFC8513]  Boucadair, M., Jacquenet, C., and S. Sivakumar, "A YANG
              Data Model for Dual-Stack Lite (DS-Lite)", RFC 8513,
              DOI 10.17487/RFC8513, January 2019,
              <https://www.rfc-editor.org/info/rfc8513>.

   [RFC8519]  Jethanandani, M., Agarwal, S., Huang, L., and D. Blair,
              "YANG Data Model for Network Access Control Lists (ACLs)",
              RFC 8519, DOI 10.17487/RFC8519, March 2019,
              <https://www.rfc-editor.org/info/rfc8519>.

   [RFC8528]  Bjorklund, M. and L. Lhotka, "YANG Schema Mount",
              RFC 8528, DOI 10.17487/RFC8528, March 2019,
              <https://www.rfc-editor.org/info/rfc8528>.

   [RFC8529]  Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
              Liu, "YANG Data Model for Network Instances", RFC 8529,
              DOI 10.17487/RFC8529, March 2019,
              <https://www.rfc-editor.org/info/rfc8529>.

   [RFC8530]  Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
              Liu, "YANG Model for Logical Network Elements", RFC 8530,
              DOI 10.17487/RFC8530, March 2019,
              <https://www.rfc-editor.org/info/rfc8530>.

   [RFC8531]  Kumar, D., Wu, Q., and Z. Wang, "Generic YANG Data Model
              for Connection-Oriented Operations, Administration, and
              Maintenance (OAM) Protocols", RFC 8531,
              DOI 10.17487/RFC8531, April 2019,
              <https://www.rfc-editor.org/info/rfc8531>.

   [RFC8532]  Kumar, D., Wang, Z., Wu, Q., Ed., Rahman, R., and S.
              Raghavan, "Generic YANG Data Model for the Management of
              Operations, Administration, and Maintenance (OAM)
              Protocols That Use Connectionless Communications",
              RFC 8532, DOI 10.17487/RFC8532, April 2019,
              <https://www.rfc-editor.org/info/rfc8532>.

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   [RFC8533]  Kumar, D., Wang, M., Wu, Q., Ed., Rahman, R., and S.
              Raghavan, "A YANG Data Model for Retrieval Methods for the
              Management of Operations, Administration, and Maintenance
              (OAM) Protocols That Use Connectionless Communications",
              RFC 8533, DOI 10.17487/RFC8533, April 2019,
              <https://www.rfc-editor.org/info/rfc8533>.

Authors' Addresses

   Qin Wu
   Huawei
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012
   China

   Email: bill.wu@huawei.com

   Mohamed Boucadair
   Orange
   Rennes 35000
   France

   Email: mohamed.boucadair@orange.com

   Young Lee
   Futurewei

   Email: younglee.tx@gmail.com

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