A Framework for Automating Service and Network Management with YANG
draft-wu-model-driven-management-virtualization-06

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Networking Working Group                                           Q. Wu
Internet-Draft                                                    Huawei
Intended status: Informational                              M. Boucadair
Expires: April 12, 2020                                     C. Jacquenet
                                                                  Orange
                                             L. Miguel Contreras Murillo
                                                              Telifonica
                                                                D. Lopez
                                                          Telefonica I+D
                                                                  C. Xie
                                                           China Telecom
                                                                W. Cheng
                                                                 L. Geng
                                                            China Mobile
                                                                  Y. Lee
                                                               Futurewei
                                                        October 10, 2019

  A Framework for Automating Service and Network Management with YANG
           draft-wu-model-driven-management-virtualization-06

Abstract

   Data models for service and network management provides a
   programmatic approach for representing (virtual) services or networks
   and deriving (1) configuration information that will be communicated
   to network and service components that are used to build and deliver
   the service and (2) state information that will be monitored and
   tracked.  Indeed, data models can be used during various phases of
   the service and network management life cycle, such as service
   instantiation, service provisioning, optimization, monitoring, and
   diagnostic.  Also, data models are instrumental in the automation of
   network management.  They also 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 that takes
   advantage of YANG modeling technologies.  This framework is drawn
   from a network provider perspective irrespective of the origin of a
   data module; it can accommodate even modules that are developed
   outside the IETF.

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

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Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Architectural Concepts & Goals  . . . . . . . . . . . . . . .   5
     3.1.  Data Models: Layering and Representation  . . . . . . . .   5
     3.2.  Automation of Service Delivery Procedures . . . . . . . .   7
     3.3.  Service Fullfillment Automation . . . . . . . . . . . . .   8
     3.4.  YANG Modules Integration  . . . . . . . . . . . . . . . .   8
   4.  Architecture Overview . . . . . . . . . . . . . . . . . . . .   9
     4.1.  Service Lifecycle Management Procedure  . . . . . . . . .  10
       4.1.1.  Service Exposure  . . . . . . . . . . . . . . . . . .  11
       4.1.2.  Service Creation/Modification . . . . . . . . . . . .  11
       4.1.3.  Service Optimization  . . . . . . . . . . . . . . . .  11
       4.1.4.  Service Diagnosis . . . . . . . . . . . . . . . . . .  12
       4.1.5.  Service Decommission  . . . . . . . . . . . . . . . .  12

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     4.2.  Service Fullfillment Management Procedure . . . . . . . .  12
       4.2.1.  Intended Configuration Provision  . . . . . . . . . .  12
       4.2.2.  Configuration Validation  . . . . . . . . . . . . . .  13
       4.2.3.  Operational State Telemetry . . . . . . . . . . . . .  13
       4.2.4.  Fault Diagnostic  . . . . . . . . . . . . . . . . . .  13
     4.3.  Multi-layer/Multi-domain Service Mapping  . . . . . . . .  14
     4.4.  Service Decomposing . . . . . . . . . . . . . . . . . . .  14
   5.  YANG Data Model Integration Examples  . . . . . . . . . . . .  14
     5.1.  L3VPN Service Delivery  . . . . . . . . . . . . . . . . .  14
     5.2.  VN Lifecycle Management Example . . . . . . . . . . . . .  16
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  17
   Appendix A.  Layered YANG Modules Example Overview  . . . . . . .  25
     A.1.  Service Models: Definition and Samples  . . . . . . . . .  25
     A.2.  Network Models: Definitions and Samples . . . . . . . . .  26
     A.3.  Device Models: Definitions and Samples  . . . . . . . . .  28
       A.3.1.  Model Composition . . . . . . . . . . . . . . . . . .  29
       A.3.2.  Device Models: Definitions and Samples  . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   The service management system usually comprises service activation/
   provision and service operation.  Current service delivery
   procedures, from the processing of customer's requirements and order
   to service delivery and operation, typically assume the manipulation
   of data sequentially into multiple OSS/BSS applications that may be
   managed by different departments within the service provider's
   organization (e.g., billing factory, design factory, network
   operation center, etc.).  In addition, many of these applications
   have been developed in-house over the years and operating in a silo
   mode:

   o  The lack of standard data input/output (i.e., data model) also
      raises many challenges in system integration and often results in
      manual configuration tasks.

   o  Secondly, many current service fulfillment might not support real
      time streaming telemetry capability in high frequency and in high
      throughput on the current state of networking and therefore have
      slow response to the network changes.

   Software Defined Networking (SDN) becomes crucial to address these
   challenges.  SDN techniques [RFC7149] are meant to automate the
   overall service delivery procedures and typically rely upon
   (standard) data models that are used to not only reflect service

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   providers'savoir-faire but also to dynamically instantiate and
   enforce a set of (service-inferred) policies that best accommodate
   what has been (contractually) defined (and possibly negotiated) with
   the customer.  [RFC7149] provides a first tentative to rationalize
   that service provider's view on the SDN 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.

   Models are key for each of these technical items.  Service and
   network management automation is an important step to improve the
   agility of network operations and infrastructures.  Models are also
   important to ease integrating multi-vendor solutions.

   YANG module developers have taken both top-down and bottom-up
   approaches to develop modules [RFC8199], and also to establish a
   mapping between network technology and customer requirements on the
   top or abstracting common construct from various network technologies
   on the bottom.  At the time of writing this document (2019), there
   are many data models including configuration and service models that
   have been specified or are being specified by the IETF.  They cover
   many of the networking protocols and techniques.  However, how these
   models work together to configure a device, manage a set of devices
   involved in a service, or even provide a service is something that is
   not currently documented either within the IETF or other SDOs (e.g.,
   MEF).

   This document provides a framework that describes and discusses an
   architecture for service and network management automation that takes
   advantage of YANG modeling technologies and investigates how
   different layer YANG data models interact with each other (e.g.,
   service mapping, model composing) in the context of service delivery
   and fulfillment.

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   This framework is drawn from a network provider perspective
   irrespective of the origin of a data module; it can accommodate even
   modules that are developed outside the IETF.

   The document also identifies a list of use cases to exemplify the
   proposed approach, but it 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.  Terminology

   The following terms are defined in [RFC8309][RFC8199] and are not
   redefined here:

   o  Network Operator

   o  Customer

   o  Service

   o  Data Model

   o  Service Model

   o  Network Element Module

   The document makes use of the following terms:

   Network Resource Model:  is used by a network operator to allocate a
      network resource (e.g., tunnel resource, topology resource) for
      the service or schedule the resource to meet the service
      requirements captured in a Service Model.

   Device Model:  Network Element YANG data module described in
      [RFC8199].

3.  Architectural Concepts & Goals

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 data modules developed by the IETF can be classified into service
   level, network level, and device level modules.  Different service

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   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 modules providing a common model construct for higher
   level network services (e.g., L3VPN), which can be further mapped to
   network technology-specific modules at lower layer (e.g., tunnel,
   routing, QoS, security).  For example, the service level can be used
   to characterise the network service(s) to be ensured between service
   nodes (ingress/egress) such as the communication scope (pipe, hose,
   funnel, ...), the directionality, the traffic performance guarantees
   (one-way delay (OWD), one-way loss, ...), etc.

   Network level modules mostly follow a bottom-up approach and are
   mainly network resource-facing modules 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 traffic-
   engineering Tunnel modules).

   Device (and function) level modules usually follow a bottom-up
   approach and are mostly technology-specific modules used to realize a
   service (e.g., BGP, NAT).

   Each level maintains a view of the supported YANG modules provided by
   low-levels (see for example, Appendix A).

   Figure 1 illustrates the overall layering model.

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   +-----------------------------------------------------------------+
   | +-----------------------+                                       |
   | |    Orchestrator       |               Hierarchy Abstraction   |
   | |+---------------------+|                                       |
   | ||  Service Modeling   ||                 Service Model         |
   | |+---------------------+|               (Customer Oriented)     |
   | |                       |               Scope: "1:1" Pipe model |
   | |                       |                   Bidirectional       |
   | |+---------------------+|             +-+ BW:100M,OWD   +-+     |
   | ||Service Orchestration||             | +---------------+ |     |
   | |+---------------------+|             +-+               +-+     |
   | +-----------------------+          1. Ingress        2. Egress  |
   |                                                                 |
   |                                                                 |
   |                                                                 |
   | +-----------------------+                Network Model          |
   | |   Controller          |               (Operator Oriented)     |
   | |+---------------------+|           +-+    +--+    +---+   +-+  |
   | || Network Modeling    ||           | |    |  |    |   |   | |  |
   | |+---------------------+|           | o----o--o----o---o---o |  |
   | |+---------------------+|           +-+    +--+    +---+   +-+  |
   | ||network Orchestration|            src                    dst  |
   | |+---------------------+|                L3VPN over TE          |
   | |                       |         Instance Name/Access Interface|
   | +-----------------------+         Proto Type/BW/RD,RT,..mapping |
   |                                           for hop               |
   |                                                                 |
   |                                                                 |
   | +-----------------------+                                       |
   | |    Device             |                 Device Model          |
   | |+--------------------+ |                                       |
   | || Device Modeling    | |           Interface add,BGP Peer,     |
   | |+--------------------+ |           Tunnel id,QoS/TE config     |
   | +-----------------------+                                       |
   +-----------------------------------------------------------------+

                   Figure 1: Layering and representation

3.2.  Automation of Service Delivery Procedures

   To dynamically offer and deliver service offerings, Service level
   modules can be used by an operator.  One or more monolithic Service
   modules can be used in the context of a composite service activation
   request (e.g., delivery of a caching infrastructure over a VPN).
   Such modules are used to feed a decision-making intelligence to
   adequately accommodate customer's needs.

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   Also, such modules may be used jointly with services that require
   dynamic invocation.  An example is provided by 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 modules can be derived from service level modules and
   used to provision, monitor, instantiate the service, and provide
   lifecycle management of network resources (e.g., expose network
   resources to customers or operators to provide service fulfillment
   and assurance and allow customers or operators to dynamically adjust
   the network resources based on service requirements as described in
   service level modules and the current network performance information
   described in the telemetry modules).

3.3.  Service Fullfillment Automation

   To operate the service, Device level modules derived from Service
   level modules or Network level modules can be used to provision each
   involved network function/device with the proper configuration
   information, and operate the network based on service requirements as
   described in the Service level module(s).

   In addition, the operational state including configuration that is in
   effect together with statistics should be exposed to upper layers to
   provide better network visibility (and assess to what extent the
   derived low level modules are consistent with the upper level
   inputs).

   Note that it is important to correlate telemetry data with
   configuration data to be used for closed loops at the different
   stages of service delivery, from resource allocation to service
   operation, in particular.

3.4.  YANG Modules Integration

   To support top-down service delivery, YANG modules at different level
   or at the same level need to be integrated together to enable
   function, feature in the network device and get network setup.  For
   example, the service parameters captured in service level modules
   need to be decomposed into a set of (configuration/notification)
   parameters that may be specific to one or more technologies; these
   technology-specific parameters are grouped together to define
   technology-specific device level models or network level models.

   In addition, these technology-specific device level models or network
   level models can be further integrated with each other using schema
   mount mechanism [RFC8528] to provision each involved network
   function/device or each involved administrative domain to support

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   newly added module or features.  A collection of device models
   integrated together can be loaded and validated during implementation
   time.

   Policies provide a higher layer of abstraction.  Policy models can be
   defined at service level, network level, or device level to provide
   policy-based management and telemetry automation,e.g., telemetry data
   can trigger a new policy that captures new network service
   requirements.

   Performance measurement telemetry can be used to provide service
   assurance at service level or at the network level.  Performance
   measurement telemetry model can tie with network level model or
   service level model to monitor network performance or service level
   agreement.

4.  Architecture Overview

   The architectural considerations described in Section 3 lead to the
   architecture described in this section and illustrated in Figure 2.

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                    +------------------+
     Service level  |                  |
    -----------     V                  |
      E2E         E2E                E2E
    Service  -- Service -------->   Service      --->Service   ---+
    Exposure    Creation     ^    Optimization    | Diagnosis     |
               /Modification |                    |               |
                    |        |Diff                |               V
     Multi-layer    |        |         E2E        |        E2E
     Multi-domain   |        |       Service      |       Service
     Service Mapping|        +------ Assurance ---+      Decommission
                    |                     ^
                    |<-----------------+  |
     Network level  |                  |  +----+
   ------------     V                  |       |
                Specific           Specific    |      Specific
                Service  ----+--->  Service ---+--+->  Service --+
                Creation     ^    Optimization |  |  Diagnosis   |
               /Modification |                 |  |              V
                    |        |Diff             |  |       Specific
                    |        |     Specific----+  |       Service
           Service  |        |      Service       |     Decommission
        Decomposing |        +------Assurance -----+
                    |                  ^
                    |                  |    Aggregation
     Device level   |                  +------------+
   ------------     V                               |
   Service      Intent                         Operational
   Fulfillment  Config  ------> Config   ----> State      -->Fault
                Provision       Validate       Telemetry   Diagnostic

            Figure 2: Service and Network Lifecycle Management

4.1.  Service Lifecycle Management Procedure

   Service lifecycle management includes end to end service lifecycle
   management at the service level and specific network lifecycle
   management at the network level.  The end-to-end service lifecycle
   management is multi-domain or multi-layer service management while
   specific service lifecycle management is domain specific or layer
   specific service lifecycle management.

   o  Note: Clarify what is meant by "domain".

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4.1.1.  Service Exposure

   A service in the context of this document (sometimes called a Network
   Service) is some form of connectivity between customer sites and the
   Internet or between customer sites across the network operator's
   network and across the Internet.

   Service exposure is used to capture services offered to customers
   (ordering and order handling).  One typical example is that a
   customer can use a L3SM service model to request L3VPN service by
   providing the abstract technical characterization of the intended
   service between cutsomer sites.

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

4.1.2.  Service Creation/Modification

   A customer is (usually) unaware of the technology that the network
   operator has available to deliver the service, so the customer does
   not make requests specific to the underlying technology but is
   limited to making requests specific to the service that is to be
   delivered.  This service request can be issued using the service
   model.

   The service orchestrator/management system maps such service request
   to its view.  This view can be described as a network model and this
   mapping may include a choice of which networks and technologies to
   use depending on which service features have been requested.

   In addition, a customer may require to change underlying network
   infrastructure to adapt to new customer's needs and service
   requirements.  This service modification can be issued in the same
   service model used by the service request.

4.1.3.  Service Optimization

   Service optimization is a technique that gets the configuration of
   the network updated due to network change, incident mitigation, or
   new service requirements.  One typical example is once the tunnel or
   the VPN is setup, Performance monitoring information or telemetry
   information per tunnel or per VPN can be collected and fed into the
   management system, if the network performance doesn't meet the
   service requirements, the management system can create new VPN
   policies capturing network service requirements and populate them
   into the network.

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   Both network performance information and policies can be modelled
   using YANG.  With Policy-based management, self-configuration and
   self-optimization behavior can be specified and implemented.

4.1.4.  Service Diagnosis

   Operations, Administration, and Maintenance (OAM) are important
   networking functions for service diagnosis that allow operators to:

   o  monitor network communications (i.e., reachability verification
      and Continuity Check)

   o  troubleshoot failures (i.e., fault verification and localization)

   o  monitor service-level agreements and performance (i.e.,
      performance management)

   When the network is down, service diagnosis should be in place to
   pinpoint the problem and provide recommendation (or instructions) for
   the network recovery.

   The service diagnosis information can be modelled as technology-
   independent RPC operations for OAM protocols and technology-
   independent abstraction of key OAM constructs for OAM protocols
   [RFC8531][RFC8533].  These models can provide consistent
   configuration, reporting, and presentation for the OAM mechanisms
   used to manage the network.

4.1.5.  Service Decommission

   Service decommission allow the customer to stop the service and
   remove the service from active status and release the network
   resource that is allocated to the service.  Customer can also use the
   service model to withdraw the subscription to a service.

4.2.  Service Fullfillment Management Procedure

4.2.1.  Intended Configuration Provision

   Intended configuration at the device level is derived from network
   model at the network level or service model at the service level and
   represents the configuration that the system attempts to apply.  Take
   L3SM service model as an example, to deliver a L3VPN service, we need
   to map L3VPN service view defined in Service model into detailed
   intended configuration view defined by specific configuration models
   for network elements, configuration information includes:

   o  VRF definition, including VPN Policy expression

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   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 function

   This specific configuration models can be used to configure PE and CE
   devices within the site, e.g., A BGP policy model can be used to
   establish VPN membership between sites and VPN Service Topology.

4.2.2.  Configuration Validation

   Configuration validation is used to validate intended configuration
   and ensure the configuration take effect.  For example, a customer
   creates an interface "et-0/0/0" but the interface does not physically
   exist at this point, then configuration data appears in the
   <intended> status but does not appear in <operational> datastore.

4.2.3.  Operational State Telemetry

   <operational> datastore holds the complete operational state of the
   device including learned, system, default configuration and system
   state. <operational> datastore can be used as telemetry data source
   and allows the client subscribe to updates of a YANG datastore.

   Based on criteria negotiated as part of a subscription, updates will
   be pushed to targeted recipients using YANG push mechanism [RFC8641].

4.2.4.  Fault Diagnostic

   Technology-dependent nodes and remote procedure call (RPC) commands
   are defined in technology-specific YANG modules which use and extend
   the base model described in Section 4.1.4.

   These RPC commands received in the technology dependent node can be
   used to trigger technology specific OAM message exchange for fault
   verification and fault isolation.

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4.3.  Multi-layer/Multi-domain Service Mapping

   Multi-layer/Multi-domain Service Mapping allow you map end to end
   abstract view of the service segmented at different layer or
   different administrative domain into domain specific view.  One
   example is to map service parameters in L3VPN service model into
   configuration parameters such as RD, RT, and VRF in L3VPN network
   model.  Another example is to map service parameters in L3VPN service
   model into TE tunnel parameter (e.g.,Tunnel ID) in TE model and VN
   parameters (e.g., AP list, VN member) in TEAS VN model [I-D.ietf-
   teas-actn-vn-yang].

4.4.  Service Decomposing

   Service Decomposing allows to decompose service model at the service
   level or network model at the network level into a set of device/
   function models at the device level.  These device models may be tied
   to specific device type or classified into a collection of related
   YANG modules based on service type and feature offered and load at
   the implementation time before configuration is loaded and validated.

5.  YANG Data Model Integration Examples

5.1.  L3VPN Service Delivery

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                 L3SM    |
               Service   |
                Model    |
    +--------------------+----------------------------+
    |              +-----V- -------+                  |
    | Orchestrator |Service Mapping|                  |
    |              +-----+---------+                  |
    |                    |                            |
    +--------------------+----------------------------+
                   L3NM  |
                  Network|
                   Model |
    +--------------------+----------------------------+
    | Controller+--------V-----------+                |
    |           | Service Decomposing|                |
    |           +-++------------++---+                |
    |             ||            ||                    |
    |             ||            ||                    |
    +-------------++----------  ++--------------------+
                  ||            ||
                  ||            ||
                  ||BGP,QoS     ||
                  ||            ||
       +----------+|NI,Intf,IP  |+-----------------+
    +--+--+      +++---+      --+---+           +--+--+
    | CE1 |------| PE1 |      | PE2 |  ---------+ CE2 |
    +-----+      +-----+      +-----+           +-----+

                 Figure 3: L3VPN Service Delivery Example

   In reference to Figure 3, the following steps are performed to
   deliver the L3VPN service within the network management automation
   architecture defined in this document:

   1.  Customer Requests 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

   2.  The Orchestrator extracts the service parameters from the L3SM
       model.  Then, it uses them as input to translate them into an
       orchestrated configuration of network elements (e.g., RD, RT,
       VRF, etc.) that is part of the L3NM network model.

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   3.  The Controller takes orchestrated configuration parameters in the
       L3NM network model and translates them into orchestrated
       configuration of network elements that is part of BGP model, QoS
       model, Network Instance model, IP management model, interface
       model, etc.

5.2.  VN Lifecycle Management Example

                           |
                    VN     |
                   Service |
                   Model   |
    +------------------- --|--------------------------+
    | Orchestrator         |                          |
    |             +--------V--------+   +----------+  |
    |             | Service Mapping | +-+ECA Engine|  |
    |             +-----------------+ | +--------^-+  |
    +----------------------+----------+----------+----+
                   TE      |     ECA  |     Telemetry
                  Tunnel   |    Policy|        Model
                  Model    |          |          |
    +----------------------V----------V----------+----+
    | Controller                                      |
    |                                                 |
    +-------------------------------------------------+

    +-----+      +-----+        +-----+         +-----+
    | CE1 |------| PE1 |        | PE2 |---------+ CE2 |
    +-----+      +-----+        +-----+         +-----+

                                 Figure 4

   In reference to Figure 4, the following steps are performed to
   deliver the VN service within the network management automation
   architecture defined in this document:

   1.  Customer requests to create 'VN' based on Access point,
       association between VN and Access point, VN member defined in the
       VN YANG module.

   2.  The orchestrator creates the single abstract node topology based
       on the information captured in an VN YANG module.

   3.  The Customer exchanges connectivity-matrix on abstract node and
       explicit path using TE topology model with the orchestrator.
       This information can be used to instantiate VN and setup tunnels
       between source and destination endpoints.

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   4.  The telemetry which augments the TEAS VN model and corresponding
       TE Tunnel model can be used to notify all the parameter changes
       and network performance change related to VN topology or Tunnel
       [I-D.ietf-teas-actn-pm-telemetry-autonomics].  This information
       can be further used as input to ECA engine in the orchestrator
       and generate ECA policy model to optimize the network.

6.  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

7.  IANA Considerations

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

8.  Acknowledgements

   Thanks to Joe Clark, Greg Mirsky, and Shunsuke Homma for the review.

9.  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.

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   [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-01 (work in progress), July 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.

   [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-10 (work in progress),
              July 2019.

   [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., King, D., and
              Y. Lee, "YANG data model for Flexi-Grid Optical Networks",
              draft-ietf-ccamp-flexigrid-yang-04 (work in progress),
              July 2019.

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   [I-D.ietf-ccamp-l1csm-yang]
              Lee, Y., Lee, K., Zheng, H., Dhody, D., Dios, O., and D.
              Ceccarelli, "A YANG Data Model for L1 Connectivity Service
              Model (L1CSM)", draft-ietf-ccamp-l1csm-yang-10 (work in
              progress), September 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.

   [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-08 (work in progress), September 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-07
              (work in progress), July 2019.

   [I-D.ietf-ccamp-wson-tunnel-model]
              Lee, Y., Zheng, H., Guo, A., Lopezalvarez, V., King, D.,
              Yoon, B., and R. Vilata, "A Yang Data Model for WSON
              Tunnel", draft-ietf-ccamp-wson-tunnel-model-04 (work in
              progress), September 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-31 (work in progress), July
              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-37 (work in progress), July 2019.

   [I-D.ietf-idr-bgp-model]
              Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP
              YANG Model for Service Provider Networks", draft-ietf-idr-
              bgp-model-07 (work in progress), October 2019.

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   [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-04 (work in progress), September 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.

   [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-11 (work in progress), September 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-07 (work in progress), September 2019.

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   [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-13 (work in progress), July 2019.

   [I-D.ietf-supa-generic-policy-data-model]
              Halpern, J. and J. Strassner, "Generic Policy Data Model
              for Simplified Use of Policy Abstractions (SUPA)", draft-
              ietf-supa-generic-policy-data-model-04 (work in progress),
              June 2017.

   [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-06 (work in progress), July 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., Fioccola, G., WU, Q., Ceccarelli, D.,
              and J. Tantsura, "Traffic Engineering (TE) and Service
              Mapping Yang Model", draft-ietf-teas-te-service-mapping-
              yang-02 (work in progress), September 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-05 (work in progress),
              July 2019.

   [I-D.ietf-teas-yang-path-computation]
              Busi, I. and S. Belotti, "Yang model for requesting Path
              Computation", draft-ietf-teas-yang-path-computation-06
              (work in progress), July 2019.

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   [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-07 (work in progress), July
              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-05 (work in
              progress), July 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.

   [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-22 (work in
              progress), June 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>.

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

   [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>.

   [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>.

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   [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>.

   [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>.

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   [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>.

   [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>.

Appendix A.  Layered YANG Modules Example Overview

A.1.  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  VN model [I-D.ietf-teas-actn-vn-yang]provides a YANG data model
      generally applicable to any mode of Virtual Network (VN)
      operation.

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A.2.  Network Models: Definitions and Samples

   Figure 5 depicts a set of Network models such as topology models or
   tunnel models:

                        |                             |
      Topo YANG modules |   Tunnel YANG modules       |
      ------------------------------------------------|
   +------------+       |                             |
   |Network Top |       | +------+  +-----------+     |
   |   Model    |       | |Other |  | TE Tunnel |     |
   +----+-------+       | |Tunnel|  +------+----+     |
        |   +--------+  | +------+         |          |
        |---+Svc Topo|  |         +--------+-+--------+
        |   +--------+  |    +----+---+  +---+----+ +-+-----+
        |   +--------+  |    |MPLS-TE |  |RSVP-TE | |SR TE  |
        |---+L2 Topo |  |    | Tunnel |  | Tunnel | |Tunnel |
        |   +--------+  |    +--------+  +--------+ +-------+
        |   +--------+  |
        |---+TE Topo |  |
        |   +--------+  |
        |   +--------+  |
        +---+L3 Topo |  |
            +--------+  |

              Figure 5: Sample Resource Facing Network Models

   Topology YANG module Examples:

   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.

   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.

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   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.

   Tunnel YANG module Examples:

   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.

   Other Network 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]

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      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
      [I-D.ietf-supa-generic-policy-data-model]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.

A.3.  Device Models: Definitions and Samples

   Network Element models (Figure 6) are used to describe how a service
   can be implemented by activating and tweaking a set of functions
   (enabled in one or multiple devices, or hosted in cloud
   infrastructures) that are involved in the service delivery.  The
   following figure uses IETF defined models as an example.

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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 6: Network Element Modules Overview

A.3.1.  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).

A.3.1.1.  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.

A.3.2.  Device Models: Definitions and Samples

   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).

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

   Christian
   Orange
   Rennes 35000
   France

   Email: christian.jacquenet@orange.com

   Luis Miguel Contreras Murillo
   Telifonica

   Email: luismiguel.contrerasmurillo@telefonica.com

   Diego R. Lopez
   Telefonica I+D
   Spain

   Email: diego.r.lopez@telefonica.com

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   Chongfeng Xie
   China Telecom
   Beijing
   China

   Email: xiechf.bri@chinatelecom.cn

   Weiqiang Cheng
   China Mobile

   Email: chengweiqiang@chinamobile.com

   Liang Geng
   China Mobile

   Email: gengliang@chinamobile.com

   Young Lee
   Futurewei

   Email: younglee.tx@gmail.com

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