



Internet Engineering Task Force (IETF)                        Q. Wu, Ed.
Request for Comments: 8969                                        Huawei
Category: Informational                                M. Boucadair, Ed.
ISSN: 2070-1721                                                   Orange
                                                                D. Lopez
                                                          Telefonica I+D
                                                                  C. Xie
                                                           China Telecom
                                                                 L. Geng
                                                            China Mobile
                                                            January 2021


  A Framework for Automating Service and Network Management with YANG

Abstract

   Data models provide a programmatic approach to represent services and
   networks.  Concretely, they can be used to derive configuration
   information for network and service components, and state information
   that will be monitored and tracked.  Data models can be used during
   the service and network management life cycle (e.g., service
   instantiation, service provisioning, service optimization, service
   monitoring, service diagnosing, and service assurance).  Data models
   are also instrumental in the automation of network management, and
   they can provide closed-loop control for adaptive and deterministic
   service creation, delivery, and maintenance.

   This document describes a framework for service and network
   management automation that takes advantage of YANG modeling
   technologies.  This framework is drawn from a network operator
   perspective irrespective of the origin of a data model; thus, it can
   accommodate YANG modules that are developed outside the IETF.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8969.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology and Abbreviations
     2.1.  Terminology
     2.2.  Abbreviations
   3.  Architectural Concepts and Goals
     3.1.  Data Models: Layering and Representation
     3.2.  Automation of Service Delivery Procedures
     3.3.  Service Fulfillment Automation
     3.4.  YANG Module Integration
   4.  Functional Blocks and Interactions
     4.1.  Service Life-Cycle Management Procedure
       4.1.1.  Service Exposure
       4.1.2.  Service Creation/Modification
       4.1.3.  Service Assurance
       4.1.4.  Service Optimization
       4.1.5.  Service Diagnosis
       4.1.6.  Service Decommission
     4.2.  Service Fulfillment Management Procedure
       4.2.1.  Intended Configuration Provision
       4.2.2.  Configuration Validation
       4.2.3.  Performance Monitoring
       4.2.4.  Fault Diagnostic
     4.3.  Multi-layer/Multi-domain Service Mapping
     4.4.  Service Decomposition
   5.  YANG Data Model Integration Examples
     5.1.  L2VPN/L3VPN Service Delivery
     5.2.  VN Life-Cycle Management
     5.3.  Event-Based Telemetry in the Device Self Management
   6.  Security Considerations
     6.1.  Service Level
     6.2.  Network Level
     6.3.  Device Level
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  Layered YANG Module Examples Overview
     A.1.  Service Models: Definition and Samples
     A.2.  Schema Mount
     A.3.  Network Models: Samples
     A.4.  Device Models: Samples
       A.4.1.  Model Composition
       A.4.2.  Device Management
       A.4.3.  Interface Management
       A.4.4.  Some Device Model Examples
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   Service management systems usually comprise service activation/
   provision and service operation.  Current service delivery
   procedures, from the processing of customer requirements and orders
   to service delivery and operation, typically assume the manipulation
   of data sequentially into multiple Operations Support System (OSS) or
   Business Support System (BSS) applications that may be managed by
   different departments within the service provider's organization
   (e.g., billing factory, design factory, network operation center).
   Many of these applications have been developed in house over the
   years and operate in a silo mode.  As a result:

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

   *  Service fulfillment systems might have a limited visibility on the
      network state and may therefore have a slow response to network
      changes.

   Software-Defined Networking (SDN) becomes crucial to address these
   challenges.  SDN techniques are meant to automate the overall service
   delivery procedures and typically rely upon standard data models.
   These models are used not only to reflect service providers' savoir
   faire, but also to dynamically instantiate and enforce a set of
   service-inferred policies that best accommodate what has been defined
   and possibly negotiated with the customer.  [RFC7149] provides a
   first tentative attempt 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.  These
   include:

   *  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.

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

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

   *  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 the four technical items above.  Service
   and network management automation is an important step to improve the
   agility of network operations.  Models are also important to ease
   integrating multi-vendor solutions.

   YANG module [RFC7950] developers have taken both top-down and bottom-
   up approaches to develop modules [RFC8199] and to establish a mapping
   between a network technology and customer requirements at the top or
   abstracting common constructs from various network technologies at
   the bottom.  At the time of writing this document (2020), there are
   many YANG 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 function, manage a set of
   devices involved in a service, or provide a service is something that
   is not currently documented either within the IETF or other Standards
   Development Organizations (SDOs).

   Many of the YANG modules listed in this document are used to exchange
   data between NETCONF/RESTCONF clients and servers [RFC6241][RFC8040].
   Nevertheless, YANG is a transport-independent data modeling language.
   It can thus be used independently of NETCONF/RESTCONF.  For example,
   YANG can be used to define abstract data structures [RFC8791] that
   can be manipulated by other protocols (e.g., [DOTS-DDOS]).

   This document describes an architectural framework for service and
   network management automation (Section 3) that takes advantage of
   YANG modeling technologies and investigates how YANG data models at
   different layers interact with each other (e.g., Service Mapping,
   model composition) in the context of service delivery and fulfillment
   (Section 4).  Concretely, the following benefits can be provided:

   *  Vendor-agnostic interfaces managing a service and the underlying
      network are allowed.

   *  Movement from deployment schemes where vendor-specific network
      managers are required to a scheme where the entities that are
      responsible for orchestrating and controlling services and network
      resources provided by multi-vendor devices are unified is allowed.

   *  Data inheritance and reusability among the various architecture
      layers thus promoting a network-wise provisioning instead of
      device-specific configuration is eased.

   *  Dynamically feeding a decision-making process (e.g., Controllers,
      Orchestrators) with notifications that will trigger appropriate
      actions, allowing that decision-making process to continuously
      adjust a network (and thus the involved resources) to deliver the
      service that conforms to the intended parameters (service
      objectives) is allowed.

   This framework is drawn from a network operator perspective
   irrespective of the origin of a data model; it can also accommodate
   YANG modules that are developed outside the IETF.  The document
   covers service models that are used by an operator to expose its
   services and capture service requirements from the customers
   (including other operators).  Nevertheless, the document does not
   elaborate on the communication protocol(s) that makes use of these
   service models in order to request and deliver a service.  Such
   considerations are out of scope.

   The document identifies a list of use cases to exemplify the proposed
   approach (Section 5), but it does not claim nor aim to be exhaustive.
   Appendix A lists some examples to illustrate the layered YANG modules
   view.

2.  Terminology and Abbreviations


2.1.  Terminology

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

   *  Network Operator

   *  Customer

   *  Service

   *  Data Model

   *  Service Model

   *  Network Element Model

   In addition, the document makes use of the following terms:

   Network Model:
      Describes a network-level abstraction (or a subset of aspects of a
      network infrastructure), including devices and their subsystems,
      and relevant protocols operating at the link and network layers
      across multiple devices.  This model corresponds to the network
      configuration model discussed in [RFC8309].

      It can be used by a network operator to allocate resources (e.g.,
      tunnel resource, topology resource) for the service or schedule
      resources to meet the service requirements defined in a service
      model.

   Network Domain:
      Refers to a network partitioning that is usually followed by
      network operators to delimit parts of their network. "access
      network" and "core network" are examples of network domains.

   Device Model:
      Refers to the Network Element YANG data model described in
      [RFC8199] or the device configuration model discussed in
      [RFC8309].

      Device models are also used to refer to model a function embedded
      in a device (e.g., Network Address Translation (NAT) [RFC8512],
      Access Control Lists (ACLs) [RFC8519]).

   Pipe:
      Refers to a communication scope where only one-to-one (1:1)
      communications are allowed.  The scope can be identified between
      ingress and egress nodes, two service sites, etc.

   Hose:
      Refers to a communication scope where one-to-many (1:N)
      communications are allowed (e.g., one site to multiple sites).

   Funnel:
      Refers to a communication scope where many-to-one (N:1)
      communications are allowed.

2.2.  Abbreviations

   The following abbreviations are used in the document:

   ACL     Access Control List
   AS      Autonomous System
   AP      Access Point
   CE      Customer Edge
   DBE     Data Border Element
   E2E     End-to-End
   ECA     Event Condition Action
   L2VPN   Layer 2 Virtual Private Network
   L3VPN   Layer 3 Virtual Private Network
   L3SM    L3VPN Service Model
   L3NM    L3VPN Network Model
   NAT     Network Address Translation
   OAM     Operations, Administration, and Maintenance
   OWD     One-Way Delay
   PE      Provider Edge
   PM      Performance Monitoring
   QoS     Quality of Service
   RD      Route Distinguisher
   RT      Route Target
   SBE     Session Border Element
   SDN     Software-Defined Networking
   SP      Service Provider
   TE      Traffic Engineering
   VN      Virtual Network
   VPN     Virtual Private Network
   VRF     Virtual Routing and Forwarding


3.  Architectural Concepts and Goals

3.1.  Data Models: Layering and Representation

   As described in Section 2 of [RFC8199], layering of modules allows
   for better reusability of lower-layer modules by higher-level modules
   while limiting duplication of features across layers.

   Data models in the context of network management can be classified
   into service, network, and device models.  Different service models
   may rely on the same set of network and/or device models.

   Service models traditionally follow a top-down approach and are
   mostly customer-facing YANG modules providing a common model
   construct for higher-level network services (e.g., Layer 3 Virtual
   Private Network (L3VPN)).  Such modules can be mapped to network
   technology-specific modules at lower layers (e.g., tunnel, routing,
   Quality of Service (QoS), security).  For example, service models can
   be used to characterize the network service(s) to be ensured between
   service nodes (ingress/egress) such as:

   *  the communication scope (pipe, hose, funnel, etc.),
   *  the directionality (inbound/outbound),
   *  the traffic performance guarantees expressed using metrics such as
      One-Way Delay (OWD) [RFC7679] or One-Way Loss [RFC7680]; a summary
      of performance metrics maintained by IANA can be found in [IPPM],
   *  link capacity [RFC5136] [METRIC-METHOD],
   *  etc.

   Figure 1 depicts the example of a Voice over IP (VoIP) service that
   relies upon connectivity services offered by a network operator.  In
   this example, the VoIP service is offered to the network operator's
   customers by Service Provider 1 (SP1).  In order to provide global
   VoIP reachability, SP1 Service Site interconnects with other Service
   Providers service sites typically by interconnecting Session Border
   Elements (SBEs) and Data Border Elements (DBEs) [RFC5486][RFC6406].
   For other VoIP destinations, sessions are forwarded over the
   Internet.  These connectivity services can be captured in a YANG
   service model that reflects the service attributes that are shown in
   Figure 2.  This example follows the IP Connectivity Provisioning
   Profile template defined in [RFC7297].

                     ,--,--,--.              ,--,--,--.
                  ,-'    SP1   `-.        ,-'   SP2     `-.
                 ( Service Site   )      ( Service Site    )
                  `-.          ,-'        `-.          ,-'
                     `--'--'--'              `--'--'--'
                      x  | o *                  * |
                   (2)x  | o *                  * |
                     ,x-,--o-*-.    (1)     ,--,*-,--.
                  ,-' x    o  * * * * * * * * *       `-.
                 (    x    o       +----(     Internet    )
          User---(x x x      o o o o o o o o o o o o o o o o o o
                  `-.          ,-'       `-.          ,-'   (3)
                     `--'--'--'             `--'--'--'
                  Network Operator

          **** (1) Inter-SP connectivity
          xxxx (2) Customer-to-SP connectivity
          oooo (3) SP to any destination connectivity

          Figure 1: An Example of Service Connectivity Components

   In reference to Figure 2, "Full traffic performance guarantees class"
   refers to a service class where all traffic performance metrics
   included in the service model (OWD, loss, delay variation) are
   guaranteed, while "Delay traffic performance guarantees class" refers
   to a service class where only OWD is guaranteed.

   Connectivity: Scope and Guarantees
      (1) Inter-SP connectivity
         - Pipe scope from the local to the remote SBE/DBE
         - Full traffic performance guarantees class
      (2) Customer-to-SP connectivity
         - Hose/Funnel scope connecting the local SBE/DBE
           to the customer access points
         - Full traffic performance guarantees class
      (3) SP to any destination connectivity
         - Hose/Funnel scope from the local SBE/DBE to the
           Internet gateway
         - Delay traffic performance guarantees class
   Flow Identification
      * Destination IP address (SBE, DBE)
      * DSCP marking
   Traffic Isolation
      * VPN
   Routing & Forwarding
      * Routing rule to exclude some ASes from the inter-domain
        paths
   Notifications (including feedback)
      * Statistics on aggregate traffic to adjust capacity
      * Failures
      * Planned maintenance operations
      * Triggered by thresholds

          Figure 2: Sample Attributes Captured in a Service Model

   Network models are mainly network-resource-facing modules; they
   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) models usually follow a bottom-up approach and
   are mostly technology-specific modules used to realize a service
   (e.g., BGP, ACL).

   Each level maintains a view of the supported YANG modules provided by
   lower levels (see for example, Appendix A).  Mechanisms such as the
   YANG library [RFC8525] can be used to expose which YANG modules are
   supported by nodes in lower levels.

   Figure 3 illustrates the overall layering model.  The reader may
   refer to Section 4 of [RFC8309] for an overview of "Orchestrator" and
   "Controller" elements.  All these elements (i.e., Orchestrator(s),
   Controller(s), device(s)) are under the responsibility of the same
   operator.


    +-----------------------------------------------------------------+
    |                                         Hierarchy Abstraction   |
    |                                                                 |
    | +-----------------------+                    Service Model      |
    | |    Orchestrator       |                 (Customer Oriented)   |
    | |+---------------------+|               Scope: "1:1" Pipe model |
    | ||  Service Modeling   ||                                       |
    | |+---------------------+|                                       |
    | |                       |                   Bidirectional       |
    | |+---------------------+|              +-+  Capacity, OWD +-+   |
    | ||Service Orchestration||              | +----------------+ |   |
    | |+---------------------+|              +-+                +-+   |
    | +-----------------------+            Ingress             Egress |
    |                                                                 |
    |                                                                 |
    | +-----------------------+                Network Model          |
    | |   Controller          |             (Operator Oriented)       |
    | |+---------------------+|           +-+    +--+    +---+   +-+  |
    | || Network Modeling    ||           | |    |  |    |   |   | |  |
    | |+---------------------+|           | o----o--o----o---o---o |  |
    | |                       |           +-+    +--+    +---+   +-+  |
    | |+---------------------+|           src                    dst  |
    | ||Network Orchestration||                L3VPN over TE          |
    | |+---------------------+|        Instance Name/Access Interface |
    | +-----------------------+      Protocol Type/Capacity/RD/RT/... |
    |                                                                 |
    |                                                                 |
    | +-----------------------+                 Device Model          |
    | |    Device             |                                       |
    | |+--------------------+ |                                       |
    | || Device Modeling    | |           Interface add, BGP Peer,    |
    | |+--------------------+ |              Tunnel ID, QoS/TE, ...   |
    | +-----------------------+                                       |
    +-----------------------------------------------------------------+

      Figure 3: Layering and Representation within a Network Operator


   A composite service offered by a network operator may rely on
   services from other operators.  In such a case, the network operator
   acts as a customer to request services from other networks.  The
   operators providing these services will then follow the layering
   depicted in Figure 3.  The mapping between a composite service and a
   third-party service is maintained at the orchestration level.  From a
   data-plane perspective, appropriate traffic steering policies (e.g.,
   Service Function Chaining [RFC7665]) are managed by the network
   controllers to guide how/when a third-party service is invoked for
   flows bound to a composite service.

   The layering model depicted in Figure 3 does not make any assumption
   about the location of the various entities (e.g., Controller,
   Orchestrator) within the network.  As such, the architecture does not
   preclude deployments where, for example, the Controller is embedded
   on a device that hosts other functions that are controlled via YANG
   modules.

   In order to ease the mapping between layers and data reuse, this
   document focuses on service models that are modeled using YANG.
   Nevertheless, fully compliant with Section 3 of [RFC8309], Figure 3
   does not preclude service models to be modeled using data modeling
   languages other than YANG.

3.2.  Automation of Service Delivery Procedures

   Service models can be used by a network operator to expose its
   services to its customers.  Exposing such models allows automation of
   the activation of service orders and thus the service delivery.  One
   or more monolithic service models can be used in the context of a
   composite service activation request (e.g., delivery of a caching
   infrastructure over a VPN).  Such models are used to feed a decision-
   making intelligence to adequately accommodate customer needs.

   Also, such models 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
   Distributed Denial-of-Service (DDoS) attacks [RFC8783].  The service
   filtering request modeled using [RFC8783] will be translated into
   device-specific filtering (e.g., ACLs defined in [RFC8519]) that
   fulfills the service request.

   Network models can be derived from service models and used to
   provision, monitor, and instantiate the service.  Also, they are used
   to provide life-cycle management of network resources.  Doing so is
   meant to:

   *  expose network resources to customers (including other network
      operators) to provide service fulfillment and assurance.

   *  allow customers (or network operators) to dynamically adjust the
      network resources based on service requirements as described in
      service models (e.g., Figure 2) and the current network
      performance information described in the telemetry modules.

   Note that it is out of the scope of this document to elaborate on the
   communication protocols that are used to implement the interface
   between the service ordering (customer) and service order handling
   (provider).

3.3.  Service Fulfillment Automation

   To operate a service, the settings of the parameters in the device
   models are derived from service models and/or network models and are
   used to:

   *  Provision each involved network function/device with the proper
      configuration information.

   *  Operate the network based on service requirements as described in
      the service model(s) and local operational guidelines.

   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.

   Filters are enforced on the notifications that are communicated to
   Service layers.  The type and frequency of notifications may be
   agreed upon in the service model.

   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 Module Integration

   To support top-down service delivery, YANG modules at different
   levels or at the same level need to be integrated for proper service
   delivery (including proper network setup).  For example, the service
   parameters captured in service models 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 or network models can
   be further integrated with each other using the schema mount
   mechanism [RFC8528] to provision each involved network function/
   device or each involved network domain to support newly added modules
   or features.  A collection of integrated device models can be loaded
   and validated during implementation.

   High-level policies can be defined at service or network models
   (e.g., "Autonomous System Number (ASN) Exclude" in the example
   depicted in Figure 2).  Device models will be tweaked accordingly to
   provide policy-based management.  Policies can also be used for
   telemetry automation, e.g., policies that contain conditions to
   trigger the generation and pushing of new telemetry data.

4.  Functional Blocks and Interactions

   The architectural considerations described in Section 3 lead to the
   life-cycle management architecture illustrated in Figure 4 and
   described in the following subsections.

                   +------------------+
   ............... |                  |
    Service level  |                  |
                   V                  |
     E2E          E2E                E2E                    E2E
   Service --> Service --------->  Service  ------------>  Service
   Exposure    Creation     ^    Optimization   ^          Diagnosis
              /Modification |                   |              |
                 ^ |        |Diff               |              |
       E2E       | |        |         E2E       |              |
     Service ----+ |        |        Service    |              |
    Decommission   |        +------ Assurance --+              |
                   |                     ^                     |
    Multi-layer    |                     |                     |
    Multi-domain   |                     |                     |
    Service Mapping|                     |                     |
   ............... |<-----------------+  |                     |
    Network level  |                  |  +-------+             v
                   V                  |          |         Specific
               Specific           Specific       |          Service
               Service  -------->  Service <--+  |         Diagnosis
               Creation     ^    Optimization |  |             |
             /Modification  |                 |  |             |
                   |        |Diff             |  |             |
                   |        |      Specific --+  |             |
          Service  |        |       Service      |             |
     Decomposition |        +----- Assurance ----+             |
                   |                  ^                        |
   ............... |                  |  Aggregation           |
    Device level   |                  +------------+           |
                   V                               |           |
   Service      Intent                             |           v
   Fulfillment  Config  ----> Config  ----> Performance ----> Fault
                Provision     Validation    Monitoring        Diagnostic

            Figure 4: Service and Network Life-Cycle Management

4.1.  Service Life-Cycle Management Procedure

   Service life-cycle management includes end-to-end service life-cycle
   management at the service level and technology-specific network life-
   cycle management at the network level.

   The end-to-end service life-cycle management is technology-
   independent service management and spans across multiple network
   domains and/or multiple layers while technology-specific service
   life-cycle management is technology domain-specific or layer-specific
   service life-cycle management.

4.1.1.  Service Exposure

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

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

   Service model catalogs can be created 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 filled using a service model.

   Upon receiving a service request, and assuming that appropriate
   authentication and authorization checks have been made with success,
   the service Orchestrator/management system should verify whether the
   service requirements in the service request can be met (i.e., whether
   there are sufficient resources that can be allocated with the
   requested guarantees).

   If the request is accepted, the service Orchestrator/management
   system maps such a service request to its view.  This view can be
   described as a technology-specific network model or a set of
   technology-specific device models, 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 a change in the underlying
   network infrastructure to adapt to new customers' needs and service
   requirements (e.g., service a new customer site, add a new access
   link, or provide disjoint paths).  This service modification can be
   issued following the same service model used by the service request.

   Withdrawing a service is discussed in Section 4.1.6.

4.1.3.  Service Assurance

   The performance measurement telemetry (Section 4.2.3) can be used to
   provide service assurance at service and/or network levels.  The
   performance measurement telemetry model can tie with service or
   network models to monitor network performance or Service Level
   Agreements.

4.1.4.  Service Optimization

   Service optimization is a technique that gets the configuration of
   the network updated due to network changes, incident mitigation, or
   new service requirements.  One example is once a tunnel or a VPN is
   set up, 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.

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

   The overall service optimization is managed at the service level,
   while the network level is responsible for the optimization of the
   specific network services it provides.

4.1.5.  Service Diagnosis

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

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

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

   *  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 recommendations (or instructions)
   for network recovery.

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

   Refer to Section 4.2.4 for the device-specific side.

4.1.6.  Service Decommission

   Service decommission allows a customer to stop the service by
   removing the service from active status, thus releasing the network
   resources that were allocated to the service.  Customers can also use
   the service model to withdraw the subscription to a service.

4.2.  Service Fulfillment Management Procedure

4.2.1.  Intended Configuration Provision

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

   *  Virtual Routing and Forwarding (VRF) definition, including VPN
      policy expression

   *  Physical Interface(s)

   *  IP layer (IPv4, IPv6)

   *  QoS features such as classification, profiles, etc.

   *  Routing protocols: support of configuration of all protocols
      listed in a service request, as well as routing policies
      associated with those protocols

   *  Multicast support

   *  Address sharing

   *  Security (e.g., access control, authentication, encryption)

   These specific configuration models can be used to configure Provider
   Edge (PE) and Customer Edge (CE) devices within a site, e.g., a BGP
   policy model can be used to establish VPN membership between sites
   and VPN service topology.

   Note that in networks with legacy devices (that support proprietary
   modules or do not support YANG at all), an adaptation layer is likely
   to be required at the network level so that these devices can be
   involved in the delivery of the network services.

   This interface is also used to handle service withdrawal
   (Section 4.1.6).

4.2.2.  Configuration Validation

   Configuration validation is used to validate intended configuration
   and ensure the configuration takes effect.

   For example, if a customer creates an interface "eth-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 the
   <operational> datastore.  More details about <intended> and
   <operational> datastores can be found in Section 5.1 of [RFC8342].

4.2.3.  Performance Monitoring

   When a configuration is in effect in a device, the <operational>
   datastore holds the complete operational state of the device,
   including learned, system, default configuration, and system state.
   However, the configurations and state of a particular device do not
   have visibility on the whole network, nor can they show how packets
   are going to be forwarded through the entire network.  Therefore, it
   becomes more difficult to operate the entire network without
   understanding the current status of the network.

   The management system should subscribe to updates of a YANG datastore
   in all the network devices for performance monitoring purposes and
   build a full topological visibility of the network by aggregating
   (and filtering) these operational states from different sources.

4.2.4.  Fault Diagnostic

   When configuration is in effect in a device, some devices may be
   misconfigured (e.g., device links are not consistent in both sides of
   the network connection) or network resources might be misallocated.
   Therefore, services may be negatively affected without knowing the
   root cause in the network.

   Technology-dependent nodes and RPC commands are defined in
   technology-specific YANG data models, which can use and extend the
   base model described in Section 4.1.5 to deal with these issues.

   These RPC commands received in the technology-dependent node can be
   used to trigger technology-specific OAM message exchanges for fault
   verification and fault isolation.  For example, Transparent
   Interconnection of Lots of Links (TRILL) Multi-destination Tree
   Verification (MTV) RPC command [TRILL-YANG-OAM] can be used to
   trigger Multi-Destination Tree Verification Messages (MTVMs) defined
   in [RFC7455] to verify TRILL distribution tree integrity.

4.3.  Multi-layer/Multi-domain Service Mapping

   Multi-layer/Multi-domain Service Mapping allows the mapping of an
   end-to-end abstract view of the service segmented at different layers
   and/or different network domains into domain-specific views.

   One example is to map service parameters in the L3SM into
   configuration parameters such as Route Distinguisher (RD), Route
   Target (RT), and VRF in the L3VPN Network Model (L3NM).

   Another example is to map service parameters in the L3SM into Traffic
   Engineered (TE) tunnel parameters (e.g., Tunnel ID) in TE model and
   Virtual Network (VN) parameters (e.g., Access Point (AP) list and VN
   members) in the YANG data model for VN operation [ACTN-VN-YANG].

4.4.  Service Decomposition

   Service Decomposition allows to decompose service models at the
   service level or network models at the network level into a set of
   device models at the device level.  These device models may be tied
   to specific device types or classified into a collection of related
   YANG modules based on service types and features offered, and they
   may load at the implementation time before configuration is loaded
   and validated.

5.  YANG Data Model Integration Examples

   The following subsections provide some YANG data model integration
   examples.

5.1.  L2VPN/L3VPN Service Delivery

   In reference to Figure 5, the following steps are performed to
   deliver the L3VPN service within the network management automation
   architecture defined in Section 4:

   1.  The Customer requests to create two sites (as per Service
       Creation in Section 4.1.2) relying upon L3SM with each site
       having one network access connectivity, for example:

       *  Site A: network-access A, link-capacity = 20 Mbps, class
          "foo", guaranteed-capacity-percent = 10, average-one-way-delay
          = 70 ms.

       *  Site B: network-access B, link-capacity = 30 Mbps, class
          "foo1", guaranteed-capacity-percent = 15, average-one-way-
          delay = 60 ms.

   2.  The Orchestrator extracts the service parameters from the L3SM.
       Then, it uses them as input to the Service Mapping in Section 4.3
       to translate them into orchestrated configuration parameters
       (e.g., RD, RT, and VRF) that are part of the L3NM specified in
       [OPSAWG-L3SM-L3NM].

   3.  The Controller takes the orchestrated configuration parameters in
       the L3NM and translates them into an orchestrated (Service
       Decomposition in Section 4.4) configuration of network elements
       that are part of, e.g., BGP, QoS, Network Instance, IP
       management, and interface models.

   [UNI-TOPOLOGY] can be used for representing, managing, and
   controlling the User Network Interface (UNI) topology.

                             L3SM    |
                           Service   |
                            Model    |
            +------------------------+------------------------+
            |               +--------V--------+               |
            |               | Service Mapping |               |
            |               +--------+--------+               |
            | Orchestrator           |                        |
            +------------------------+------------------------+
                             L3NM    |     ^ UNI Topology Model
                            Network  |     |
                             Model   |     |
            +------------------------+------------------------+
            |            +-----------V-----------+            |
            |            | Service Decomposition |            |
            |            +--++---------------++--+            |
            |               ||               ||               |
            | Controller    ||               ||               |
            +---------------++---------------++---------------+
                            ||               ||
                            ||     BGP,      ||
                            ||     QoS,      ||
                            ||   Interface,  ||
               +------------+|      NI,      |+------------+
               |             |      IP       |             |
            +--+--+       +--+--+         +--+--+       +--+--+
            | CE1 +-------+ PE1 |         | PE2 +-------+ CE2 |
            +-----+       +-----+         +-----+       +-----+

             Figure 5: L3VPN Service Delivery Example (Current)

   L3NM inherits some of the data elements from the L3SM.  Nevertheless,
   the L3NM as designed in [OPSAWG-L3SM-L3NM] does not expose some
   information to the above layer such as the capabilities of an
   underlying network (which can be used to drive service order
   handling) or notifications (to notify subscribers about specific
   events or degradations as per agreed SLAs).  Some of this information
   can be provided using, e.g., [OPSAWG-YANG-VPN].  A target overall
   model is depicted in Figure 6.

                             L3SM    |     ^
                           Service   |     |  Notifications
                            Model    |     |
            +------------------------+------------------------+
            |               +--------V--------+               |
            |               | Service Mapping |               |
            |               +--------+--------+               |
            | Orchestrator           |                        |
            +------------------------+------------------------+
                               L3NM  |     ^ UNI Topology Model
                              Network|     | L3NM Notifications
                               Model |     | L3NM Capabilities
            +------------------------+------------------------+
            |            +-----------V-----------+            |
            |            | Service Decomposition |            |
            |            +--++---------------++--+            |
            |               ||               ||               |
            | Controller    ||               ||               |
            +---------------++---------------++---------------+
                            ||               ||
                            ||     BGP,      ||
                            ||     QoS,      ||
                            ||   Interface,  ||
               +------------+|      NI,      |+------------+
               |             |      IP       |             |
            +--+--+       +--+--+         +--+--+       +--+--+
            | CE1 +-------+ PE1 |         | PE2 +-------+ CE2 |
            +-----+       +-----+         +-----+       +-----+

             Figure 6: L3VPN Service Delivery Example (Target)

   Note that a similar analysis can be performed for Layer 2 VPNs
   (L2VPNs).  An L2VPN Service Model (L2SM) is defined in [RFC8466],
   while the YANG L2VPN Network Model (L2NM) is specified in
   [OPSAWG-L2NM].

5.2.  VN Life-Cycle Management

   In reference to Figure 7, the following steps are performed to
   deliver the VN service within the network management automation
   architecture defined in Section 4:

   1.  A customer makes a request (Service Exposure in Section 4.1.1) to
       create a VN.  The association between the VN, APs, and VN members
       is defined in the VN YANG model [ACTN-VN-YANG].

   2.  The Orchestrator creates the single abstract node topology based
       on the information captured in the request.

   3.  The customer exchanges with the Orchestrator the connectivity
       matrix on the abstract node topology and explicit paths using the
       TE topology model [RFC8795].  This information can be used to
       instantiate the VN and set up tunnels between source and
       destination endpoints (Service Creation in Section 4.1.2).

   4.  In order to provide service assurance (Service Optimization in
       Section 4.1.4), the telemetry model that augments the VN model
       and corresponding TE tunnel model can be used by the Orchestrator
       to subscribe to performance measurement data.  The Controller
       will then notify the Orchestrator with all the parameter changes
       and network performance changes related to the VN topology and
       the tunnels [TEAS-ACTN-PM].

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

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

                  Figure 7: A VN Service Delivery Example

5.3.  Event-Based Telemetry in the Device Self Management

   In reference to Figure 8, the following steps are performed to
   monitor state changes of managed resources in a network device and
   provide device self management within the network management
   automation architecture defined in Section 4:

   1.  To control which state a network device should be in or is
       allowed to be in at any given time, a set of conditions and
       actions are defined and correlated with network events (e.g.,
       allow the NETCONF server to send updates only when the value
       exceeds a certain threshold for the first time, but not again
       until the threshold is cleared), which constitute an Event
       Condition Action (ECA) policy or an event-driven policy control
       logic that can be executed on the device (e.g., [EVENT-YANG]).

   2.  To provide a rapid autonomic response that can exhibit self-
       management properties, the Controller pushes the ECA policy to
       the network device and delegates the network control logic to the
       network device.

   3.  The network device uses the ECA model to subscribe to the event
       source, e.g., an event stream or datastore state data conveyed to
       the server via YANG-Push subscription [RFC8641], monitors state
       parameters, and takes simple and instant actions when an
       associated event condition on state parameters is met.  ECA
       notifications can be generated as the result of actions based on
       event stream subscription or datastore subscription (model-driven
       telemetry operation discussed in Section 4.2.3).

                      +----------------+
                      |                <----+
                      |   Controller   |    |
                      +-------+--------+    |
                              |             |
                              |             |
                          ECA |             | ECA
                        Model |             | Notification
                              |             |
                              |             |
                 +------------V-------------+-----+
                 |Device                    |     |
                 | +-------+ +---------+ +--+---+ |
                 | | Event +-> Event   +->Event | |
                 | | Source| |Condition| |Action| |
                 | +-------+ +---------+ +------+ |
                 +--------------------------------+

                      Figure 8: Event-Based Telemetry

6.  Security Considerations

   Many of the YANG modules cited in this document define schema for
   data that is designed to be accessed via network management protocols
   such as NETCONF [RFC6241] or RESTCONF [RFC8040].  The lowest NETCONF
   layer is the secure transport layer, and the mandatory-to-implement
   secure transport is Secure Shell (SSH) [RFC6242].  The lowest
   RESTCONF layer is HTTPS, and the mandatory-to-implement secure
   transport is TLS [RFC8446].

   The NETCONF access control model [RFC8341] provides the means to
   restrict access for particular NETCONF or RESTCONF users to a
   preconfigured subset of all available NETCONF or RESTCONF protocol
   operations and content.

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

   In order to prevent leaking sensitive information and the "confused
   deputy" problem [Hardy] in general, special care should be considered
   when translating between the various layers in Section 4 or when
   aggregating data retrieved from various sources.  Authorization and
   authentication checks should be performed to ensure that data is
   available to an authorized entity.  The network operator must enforce
   means to protect privacy-related information included in customer-
   facing models.

   To detect misalignment between layers that might be induced by
   misbehaving nodes, upper layers should continuously monitor the
   perceived service (Section 4.1.4) and should proceed with checks to
   assess that the provided service complies with the expected service
   and that the data reported by an underlying layer is matching the
   perceived service by the above layer.  Such checks are the
   responsibility of the service diagnosis (Section 4.1.5).

   When a YANG module includes security-related parameters, it is
   recommended to include the relevant information as part of the
   service assurance to track the correct functioning of the security
   mechanisms.

   Additional considerations are discussed in the following subsections.

6.1.  Service Level

   A provider may rely on services offered by other providers to build
   composite services.  Appropriate mechanisms should be enabled by the
   provider to monitor and detect a service disruption from these
   providers.  The characterization of a service disruption (including
   mean time between failures and mean time to repair), the escalation
   procedure, and penalties are usually documented in contractual
   agreements (e.g., as described in Section 2.1 of [RFC4176]).
   Misbehaving peer providers will thus be identified and appropriate
   countermeasures will be applied.

   The communication protocols that make use of a service model between
   a customer and an operator are out of scope.  Relevant security
   considerations should be discussed in the specification documents of
   these protocols.

6.2.  Network Level

   Security considerations specific to the network level are listed
   below:

   *  A controller may create forwarding loops by misconfiguring the
      underlying network nodes.  It is recommended to proceed with tests
      to check the status of forwarding paths regularly or whenever
      changes are made to routing or forwarding processes.  Such checks
      may be triggered from the service level owing to the means
      discussed in Section 4.1.5.

   *  Some service models may include a traffic isolation clause that is
      passed down to the network level so that appropriate technology-
      specific actions must be enforced at the underlying network (and
      thus involved network devices) to avoid that such traffic is
      accessible to non-authorized parties.  In particular, network
      models may indicate whether encryption is enabled and, if so,
      expose a list of supported encryption schemes and parameters.
      Refer, for example, to the encryption feature defined in
      [OPSAWG-VPN-COMMON] and its use in [OPSAWG-L3SM-L3NM].

6.3.  Device Level

   Network operators should monitor and audit their networks to detect
   misbehaving nodes and abnormal behaviors.  For example, OAM, as
   discussed in Section 4.1.5, can be used for that purpose.

   Access to some data requires specific access privilege levels.
   Devices must check that a required access privilege is provided
   before granting access to specific data or performing specific
   actions.

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  Normative References

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
              <https://www.rfc-editor.org/info/rfc6242>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,
              <https://www.rfc-editor.org/info/rfc8341>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

8.2.  Informative References

   [ACTN-VN-YANG]
              Lee, Y., Dhody, D., Ceccarelli, D., Bryskin, I., and B. Y.
              Yoon, "A YANG Data Model for VN Operation", Work in
              Progress, Internet-Draft, draft-ietf-teas-actn-vn-yang-10,
              2 November 2020, <https://tools.ietf.org/html/draft-ietf-
              teas-actn-vn-yang-10>.

   [BFD-YANG] Rahman, R., Zheng, L., Jethanandani, M., Pallagatti, S.,
              and G. Mirsky, "YANG Data Model for Bidirectional
              Forwarding Detection (BFD)", Work in Progress, Internet-
              Draft, draft-ietf-bfd-yang-17, 2 August 2018,
              <https://tools.ietf.org/html/draft-ietf-bfd-yang-17>.

   [DOTS-DDOS]
              Boucadair, M., Shallow, J., and T. Reddy.K, "Distributed
              Denial-of-Service Open Threat Signaling (DOTS) Signal
              Channel Specification", Work in Progress, Internet-Draft,
              draft-ietf-dots-rfc8782-bis-04, 3 December 2020,
              <https://tools.ietf.org/html/draft-ietf-dots-rfc8782-bis-
              04>.

   [EVENT-YANG]
              Wu, Q., Bryskin, I., Birkholz, H., Liu, X., and B. Claise,
              "A YANG Data model for ECA Policy Management", Work in
              Progress, Internet-Draft, draft-wwx-netmod-event-yang-10,
              1 November 2020, <https://tools.ietf.org/html/draft-wwx-
              netmod-event-yang-10>.

   [EVPN-YANG]
              Brissette, P., Shah, H., Hussain, I., Tiruveedhula, K.,
              and J. Rabadan, "Yang Data Model for EVPN", Work in
              Progress, Internet-Draft, draft-ietf-bess-evpn-yang-07, 11
              March 2019, <https://tools.ietf.org/html/draft-ietf-bess-
              evpn-yang-07>.

   [Hardy]    Hardy, N., "The Confused Deputy: (or why capabilities
              might have been invented)", DOI 10.1145/54289.871709,
              October 1988,
              <https://dl.acm.org/doi/10.1145/54289.871709>.

   [IDR-BGP-MODEL]
              Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP
              YANG Model for Service Provider Networks", Work in
              Progress, Internet-Draft, draft-ietf-idr-bgp-model-10, 15
              November 2020,
              <https://tools.ietf.org/html/draft-ietf-idr-bgp-model-10>.

   [IPPM]     IANA, "Performance Metrics", March 2020,
              <https://www.iana.org/assignments/performance-metrics/
              performance-metrics.xhtml>.

   [L2VPN-YANG]
              Shah, H., Brissette, P., Chen, I., Hussain, I., Wen, B.,
              and K. Tiruveedhula, "YANG Data Model for MPLS-based
              L2VPN", Work in Progress, Internet-Draft, draft-ietf-bess-
              l2vpn-yang-10, 2 July 2019, <https://tools.ietf.org/html/
              draft-ietf-bess-l2vpn-yang-10>.

   [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", Work in Progress, Internet-Draft,
              draft-ietf-bess-l3vpn-yang-04, 19 October 2018,
              <https://tools.ietf.org/html/draft-ietf-bess-l3vpn-yang-
              04>.

   [METRIC-METHOD]
              Morton, A., Geib, R., and L. Ciavattone, "Metrics and
              Methods for One-way IP Capacity", Work in Progress,
              Internet-Draft, draft-ietf-ippm-capacity-metric-method-04,
              10 September 2020, <https://tools.ietf.org/html/draft-
              ietf-ippm-capacity-metric-method-04>.

   [MVPN-YANG]
              Liu, Y., Guo, F., Litkowski, S., Liu, X., Kebler, R., and
              M. Sivakumar, "Yang Data Model for Multicast in MPLS/BGP
              IP VPNs", Work in Progress, Internet-Draft, draft-ietf-
              bess-mvpn-yang-04, 30 June 2020,
              <https://tools.ietf.org/html/draft-ietf-bess-mvpn-yang-
              04>.

   [NETMOD-MODEL]
              Clarke, J. and B. Claise, "YANG module for
              yangcatalog.org", Work in Progress, Internet-Draft, draft-
              clacla-netmod-model-catalog-03, 3 April 2018,
              <https://tools.ietf.org/html/draft-clacla-netmod-model-
              catalog-03>.

   [OPSAWG-L2NM]
              Barguil, S., Dios, O. G. D., Boucadair, M., Munoz, L. A.,
              Jalil, L., and J. Ma, "A Layer 2 VPN Network YANG Model",
              Work in Progress, Internet-Draft, draft-ietf-opsawg-l2nm-
              01, 2 November 2020,
              <https://tools.ietf.org/html/draft-ietf-opsawg-l2nm-01>.

   [OPSAWG-L3SM-L3NM]
              Barguil, S., Dios, O. G. D., Boucadair, M., Munoz, L. A.,
              and A. Aguado, "A Layer 3 VPN Network YANG Model", Work in
              Progress, Internet-Draft, draft-ietf-opsawg-l3sm-l3nm-05,
              16 October 2020, <https://tools.ietf.org/html/draft-ietf-
              opsawg-l3sm-l3nm-05>.

   [OPSAWG-VPN-COMMON]
              Barguil, S., Dios, O. G. D., Boucadair, M., and Q. Wu, "A
              Layer 2/3 VPN Common YANG Model", Work in Progress,
              Internet-Draft, draft-ietf-opsawg-vpn-common-03, 14
              January 2021, <https://tools.ietf.org/html/draft-ietf-
              opsawg-vpn-common-03>.

   [OPSAWG-YANG-VPN]
              Wu, B., Wu, Q., Boucadair, M., Dios, O. G. D., Wen, B.,
              Liu, C., and H. Xu, "A YANG Model for Network and VPN
              Service Performance Monitoring", Work in Progress,
              Internet-Draft, draft-www-opsawg-yang-vpn-service-pm-03,
              21 January 2021, <https://tools.ietf.org/html/draft-www-
              opsawg-yang-vpn-service-pm-03>.

   [PIM-YANG] Liu, X., McAllister, P., Peter, A., Sivakumar, M., Liu,
              Y., and F. Hu, "A YANG Data Model for Protocol Independent
              Multicast (PIM)", Work in Progress, Internet-Draft, draft-
              ietf-pim-yang-17, 19 May 2018,
              <https://tools.ietf.org/html/draft-ietf-pim-yang-17>.

   [QOS-MODEL]
              Choudhary, A., Jethanandani, M., Strahle, N., Aries, E.,
              and I. Chen, "YANG Model for QoS", Work in Progress,
              Internet-Draft, draft-ietf-rtgwg-qos-model-02, 9 July
              2020, <https://tools.ietf.org/html/draft-ietf-rtgwg-qos-
              model-02>.

   [RFC4176]  El Mghazli, Y., Ed., Nadeau, T., Boucadair, M., Chan, K.,
              and A. Gonguet, "Framework for Layer 3 Virtual Private
              Networks (L3VPN) Operations and Management", RFC 4176,
              DOI 10.17487/RFC4176, October 2005,
              <https://www.rfc-editor.org/info/rfc4176>.

   [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>.

   [RFC5136]  Chimento, P. and J. Ishac, "Defining Network Capacity",
              RFC 5136, DOI 10.17487/RFC5136, February 2008,
              <https://www.rfc-editor.org/info/rfc5136>.

   [RFC5486]  Malas, D., Ed. and D. Meyer, Ed., "Session Peering for
              Multimedia Interconnect (SPEERMINT) Terminology",
              RFC 5486, DOI 10.17487/RFC5486, March 2009,
              <https://www.rfc-editor.org/info/rfc5486>.

   [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>.

   [RFC6406]  Malas, D., Ed. and J. Livingood, Ed., "Session PEERing for
              Multimedia INTerconnect (SPEERMINT) Architecture",
              RFC 6406, DOI 10.17487/RFC6406, November 2011,
              <https://www.rfc-editor.org/info/rfc6406>.

   [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>.

   [RFC7224]  Bjorklund, M., "IANA Interface Type YANG Module",
              RFC 7224, DOI 10.17487/RFC7224, May 2014,
              <https://www.rfc-editor.org/info/rfc7224>.

   [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>.

   [RFC7317]  Bierman, A. and M. Bjorklund, "A YANG Data Model for
              System Management", RFC 7317, DOI 10.17487/RFC7317, August
              2014, <https://www.rfc-editor.org/info/rfc7317>.

   [RFC7455]  Senevirathne, T., Finn, N., Salam, S., Kumar, D., Eastlake
              3rd, D., Aldrin, S., and Y. Li, "Transparent
              Interconnection of Lots of Links (TRILL): Fault
              Management", RFC 7455, DOI 10.17487/RFC7455, March 2015,
              <https://www.rfc-editor.org/info/rfc7455>.

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,
              <https://www.rfc-editor.org/info/rfc7665>.

   [RFC7679]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Delay Metric for IP Performance Metrics
              (IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
              2016, <https://www.rfc-editor.org/info/rfc7679>.

   [RFC7680]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Loss Metric for IP Performance Metrics
              (IPPM)", STD 82, RFC 7680, DOI 10.17487/RFC7680, January
              2016, <https://www.rfc-editor.org/info/rfc7680>.

   [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>.

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>.

   [RFC8343]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
              <https://www.rfc-editor.org/info/rfc8343>.

   [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>.

   [RFC8348]  Bierman, A., Bjorklund, M., Dong, J., and D. Romascanu, "A
              YANG Data Model for Hardware Management", RFC 8348,
              DOI 10.17487/RFC8348, March 2018,
              <https://www.rfc-editor.org/info/rfc8348>.

   [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>.

   [RFC8525]  Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
              and R. Wilton, "YANG Library", RFC 8525,
              DOI 10.17487/RFC8525, March 2019,
              <https://www.rfc-editor.org/info/rfc8525>.

   [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>.

   [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>.

   [RFC8632]  Vallin, S. and M. Bjorklund, "A YANG Data Model for Alarm
              Management", RFC 8632, DOI 10.17487/RFC8632, September
              2019, <https://www.rfc-editor.org/info/rfc8632>.

   [RFC8641]  Clemm, A. and E. Voit, "Subscription to YANG Notifications
              for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641,
              September 2019, <https://www.rfc-editor.org/info/rfc8641>.

   [RFC8652]  Liu, X., Guo, F., Sivakumar, M., McAllister, P., and A.
              Peter, "A YANG Data Model for the Internet Group
              Management Protocol (IGMP) and Multicast Listener
              Discovery (MLD)", RFC 8652, DOI 10.17487/RFC8652, November
              2019, <https://www.rfc-editor.org/info/rfc8652>.

   [RFC8675]  Boucadair, M., Farrer, I., and R. Asati, "A YANG Data
              Model for Tunnel Interface Types", RFC 8675,
              DOI 10.17487/RFC8675, November 2019,
              <https://www.rfc-editor.org/info/rfc8675>.

   [RFC8676]  Farrer, I., Ed. and M. Boucadair, Ed., "YANG Modules for
              IPv4-in-IPv6 Address plus Port (A+P) Softwires", RFC 8676,
              DOI 10.17487/RFC8676, November 2019,
              <https://www.rfc-editor.org/info/rfc8676>.

   [RFC8783]  Boucadair, M., Ed. and T. Reddy.K, Ed., "Distributed
              Denial-of-Service Open Threat Signaling (DOTS) Data
              Channel Specification", RFC 8783, DOI 10.17487/RFC8783,
              May 2020, <https://www.rfc-editor.org/info/rfc8783>.

   [RFC8791]  Bierman, A., Björklund, M., and K. Watsen, "YANG Data
              Structure Extensions", RFC 8791, DOI 10.17487/RFC8791,
              June 2020, <https://www.rfc-editor.org/info/rfc8791>.

   [RFC8795]  Liu, X., Bryskin, I., Beeram, V., Saad, T., Shah, H., and
              O. Gonzalez de Dios, "YANG Data Model for Traffic
              Engineering (TE) Topologies", RFC 8795,
              DOI 10.17487/RFC8795, August 2020,
              <https://www.rfc-editor.org/info/rfc8795>.

   [RFC8819]  Hopps, C., Berger, L., and D. Bogdanovic, "YANG Module
              Tags", RFC 8819, DOI 10.17487/RFC8819, January 2021,
              <https://www.rfc-editor.org/info/rfc8819>.

   [RFC8944]  Dong, J., Wei, X., Wu, Q., Boucadair, M., and A. Liu, "A
              YANG Data Model for Layer 2 Network Topologies", RFC 8944,
              DOI 10.17487/RFC8944, November 2020,
              <https://www.rfc-editor.org/info/rfc8944>.

   [RFC8960]  Saad, T., Raza, K., Gandhi, R., Liu, X., and V. Beeram, "A
              YANG Data Model for MPLS Base", RFC 8960,
              DOI 10.17487/RFC8960, December 2020,
              <https://www.rfc-editor.org/info/rfc8960>.

   [RTGWG-POLICY]
              Qu, Y., Tantsura, J., Lindem, A., and X. Liu, "A YANG Data
              Model for Routing Policy", Work in Progress, Internet-
              Draft, draft-ietf-rtgwg-policy-model-27, 10 January 2021,
              <https://tools.ietf.org/html/draft-ietf-rtgwg-policy-
              model-27>.

   [SNOOPING-YANG]
              Zhao, H., Liu, X., Liu, Y., Sivakumar, M., and A. Peter,
              "A Yang Data Model for IGMP and MLD Snooping", Work in
              Progress, Internet-Draft, draft-ietf-pim-igmp-mld-
              snooping-yang-18, 14 August 2020,
              <https://tools.ietf.org/html/draft-ietf-pim-igmp-mld-
              snooping-yang-18>.

   [SPRING-SR-YANG]
              Litkowski, S., Qu, Y., Lindem, A., Sarkar, P., and J.
              Tantsura, "YANG Data Model for Segment Routing", Work in
              Progress, Internet-Draft, draft-ietf-spring-sr-yang-29, 8
              December 2020, <https://tools.ietf.org/html/draft-ietf-
              spring-sr-yang-29>.

   [STAMP-YANG]
              Mirsky, G., Min, X., and W. S. Luo, "Simple Two-way Active
              Measurement Protocol (STAMP) Data Model", Work in
              Progress, Internet-Draft, draft-ietf-ippm-stamp-yang-06, 7
              October 2020, <https://tools.ietf.org/html/draft-ietf-
              ippm-stamp-yang-06>.

   [TEAS-ACTN-PM]
              Lee, Y., Dhody, D., Karunanithi, S., Vilalta, R., King,
              D., and D. Ceccarelli, "YANG models for VN/TE Performance
              Monitoring Telemetry and Scaling Intent Autonomics", Work
              in Progress, Internet-Draft, draft-ietf-teas-actn-pm-
              telemetry-autonomics-04, 2 November 2020,
              <https://tools.ietf.org/html/draft-ietf-teas-actn-pm-
              telemetry-autonomics-04>.

   [TEAS-YANG-PATH-COMP]
              Busi, I., Belotti, S., Lopez, V., Sharma, A., and Y. Shi,
              "Yang model for requesting Path Computation", Work in
              Progress, Internet-Draft, draft-ietf-teas-yang-path-
              computation-11, 16 November 2020,
              <https://tools.ietf.org/html/draft-ietf-teas-yang-path-
              computation-11>.

   [TEAS-YANG-RSVP]
              Beeram, V. P., Saad, T., Gandhi, R., Liu, X., Bryskin, I.,
              and H. Shah, "A YANG Data Model for RSVP-TE Protocol",
              Work in Progress, Internet-Draft, draft-ietf-teas-yang-
              rsvp-te-08, 9 March 2020, <https://tools.ietf.org/html/
              draft-ietf-teas-yang-rsvp-te-08>.

   [TEAS-YANG-TE]
              Saad, T., Gandhi, R., Liu, X., Beeram, V. P., and I.
              Bryskin, "A YANG Data Model for Traffic Engineering
              Tunnels, Label Switched Paths and Interfaces", Work in
              Progress, Internet-Draft, draft-ietf-teas-yang-te-25, 27
              July 2020,
              <https://tools.ietf.org/html/draft-ietf-teas-yang-te-25>.

   [TRILL-YANG-OAM]
              Kumar, D., Senevirathne, T., Finn, N., Salam, S., Xia, L.,
              and W. Hao, "YANG Data Model for TRILL Operations,
              Administration, and Maintenance (OAM)", Work in Progress,
              Internet-Draft, draft-ietf-trill-yang-oam-05, 31 March
              2017, <https://tools.ietf.org/html/draft-ietf-trill-yang-
              oam-05>.

   [TWAMP-DATA-MODEL]
              Civil, R., Morton, A., Rahman, R., Jethanandani, M., and
              K. Pentikousis, "Two-Way Active Measurement Protocol
              (TWAMP) Data Model", Work in Progress, Internet-Draft,
              draft-ietf-ippm-twamp-yang-13, 2 July 2018,
              <https://tools.ietf.org/html/draft-ietf-ippm-twamp-yang-
              13>.

   [UNI-TOPOLOGY]
              Dios, O. G. D., Barguil, S., Wu, Q., and M. Boucadair, "A
              YANG Model for User-Network Interface (UNI) Topologies",
              Work in Progress, Internet-Draft, draft-ogondio-opsawg-
              uni-topology-01, 2 April 2020,
              <https://tools.ietf.org/html/draft-ogondio-opsawg-uni-
              topology-01>.

Appendix A.  Layered YANG Module Examples Overview

   This appendix lists a set of YANG data models that can be used for
   the delivery of connectivity services.  These models can be
   classified as service, network, or device models.

   It is not the intent of this appendix 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].

   The reader may refer to the YANG Catalog
   (<https://www.yangcatalog.org>) or the public Github YANG repository
   (<https://github.com/YangModels/yang>) to query existing YANG models.
   The YANG Catalog includes some metadata to indicate the module type
   ('module-classification') [NETMOD-MODEL].  Note that the mechanism
   defined in [RFC8819] allows to associate tags with YANG modules in
   order to help classifying the modules.

A.1.  Service Models: Definition and Samples

   As described in [RFC8309], the 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".  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, for example, in terms of QoS, performance, and
   availability) to properly forward traffic to the said "Destination"
   [RFC7297].  The "Scope" denotes the network perimeter (e.g., between
   Provider Edge (PE) routers or Customer Nodes) where the said
   guarantees need to be provided.

   For example:

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

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

   *  The Virtual Network (VN) model [ACTN-VN-YANG] provides a YANG data
      model applicable to any mode of VN operation.

   L2SM and L3SM are customer service models as per [RFC8309].

A.2.  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.

A.3.  Network Models: Samples

   L2NM [OPSAWG-L2NM] and L3NM [OPSAWG-L3SM-L3NM] are examples of YANG
   network models.

   Figure 9 depicts a set of additional network models such as topology
   and tunnel models:

     +-------------------------------+-------------------------------+
     |      Topology YANG modules    |     Tunnel YANG modules       |
     +-------------------------------+-------------------------------+
     |  +------------------+         |                               |
     |  |Network Topologies|         | +------+  +-----------+       |
     |  |       Model      |         | |Other |  | TE Tunnel |       |
     |  +--------+---------+         | |Tunnel|  +----+------+       |
     |           |   +---------+     | +------+       |              |
     |           +---+Service  |     |     +----------+---------+    |
     |           |   |Topology |     |     |          |         |    |
     |           |   +---------+     |     |          |         |    |
     |           |   +---------+     |+----+---+ +----+---+ +---+---+|
     |           +---+Layer 3  |     ||MPLS-TE | |RSVP-TE | | SR-TE ||
     |           |   |Topology |     || Tunnel | | Tunnel | |Tunnel ||
     |           |   +---------+     |+--------+ +--------+ +-------+|
     |           |   +---------+     |                               |
     |           +---+TE       |     |                               |
     |           |   |Topology |     |                               |
     |           |   +---------+     |                               |
     |           |   +---------+     |                               |
     |           +---+Layer 3  |     |                               |
     |               |Topology |     |                               |
     |               +---------+     |                               |
     +-------------------------------+-------------------------------+

              Figure 9: Sample Resource-Facing Network Models


   Examples of topology YANG modules are listed below:

   Network Topologies Model:
      [RFC8345] defines a base model for network topology and
      inventories.  Network topology data includes link, node, and
      terminate-point resources.

   TE Topology Model:
      [RFC8795] defines a YANG data model for representing and
      manipulating TE topologies.

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

   Layer 3 Topology Model:
      [RFC8346] defines a YANG data model for representing and
      manipulating Layer 3 topologies.  This model is extended from the
      network topology model defined in [RFC8345] and includes content
      related to Layer 3 topology specifics.

   Layer 2 Topology Model:
      [RFC8944] defines a YANG data model for representing and
      manipulating Layer 2 topologies.  This model is extended from the
      network topology model defined in [RFC8345] and includes content
      related to Layer 2 topology specifics.

   Examples of tunnel YANG modules are provided below:

   Tunnel Identities:
      [RFC8675] defines a collection of YANG identities used as
      interface types for tunnel interfaces.

   TE Tunnel Model:
      [TEAS-YANG-TE] defines a YANG module for the configuration and
      management of TE interfaces, tunnels, and LSPs.

   Segment Routing (SR) Traffic Engineering (TE) Tunnel Model:
      [TEAS-YANG-TE] augments the TE generic and MPLS-TE model(s) and
      defines a YANG module for SR-TE-specific data.

   MPLS-TE Model:
      [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.

   RSVP-TE MPLS Model:
      [TEAS-YANG-RSVP] augments the RSVP-TE generic module with
      parameters to configure and manage signaling of MPLS RSVP-TE LSPs.

   Other sample network models are listed hereafter:

   Path Computation API Model:
      [TEAS-YANG-PATH-COMP] defines a YANG module for a stateless RPC
      that complements the stateful solution defined in [TEAS-YANG-TE].

   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 connectionless 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.
      [RFC8632] defines a YANG module for alarm management.

A.4.  Device Models: Samples

   Network Element models (listed in Figure 10) 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.  For
   example, the L3VPN service will involve many PEs and require
   manipulating the following modules:

   *  Routing management [RFC8349]

   *  BGP [IDR-BGP-MODEL]

   *  PIM [PIM-YANG]

   *  NAT management [RFC8512]

   *  QoS management [QOS-MODEL]

   *  ACLs [RFC8519]

   Figure 10 uses IETF-defined data models as an example.

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

                 Figure 10: Network Element Models Overview

A.4.1.  Model Composition

   Logical Network Element Model:
      [RFC8530] defines a logical network element model that 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.

   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 VRF instances and Virtual
      Switch Instances (VSIs).

A.4.2.  Device Management

   The following list enumerates some YANG modules that can be used for
   device management:

   *  [RFC8348] defines a YANG module for the management of hardware.

   *  [RFC7317] defines the "ietf-system" YANG module that provides many
      features such as the configuration and the monitoring of system or
      system control operations (e.g., shutdown, restart, and setting
      time) identification.

   *  [RFC8341] defines a network configuration access control YANG
      module.

A.4.3.  Interface Management

   The following provides some YANG modules that can be used for
   interface management:

   *  [RFC7224] defines a YANG module for interface type definitions.

   *  [RFC8343] defines a YANG module for the management of network
      interfaces.

A.4.4.  Some Device Model Examples

   The following provides an overview of some device models that can be
   used within a network.  This list is not comprehensive.

   L2VPN:
      [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
      Virtual Private Wire Service (VPWS) and Multipoint Virtual Private
      LAN Service (VPLS).  These services use signaling of Pseudowires
      across MPLS networks using LDP [RFC8077][RFC4762] or BGP
      [RFC4761].

   EVPN:
      [EVPN-YANG] defines a YANG module for Ethernet VPN services.  The
      model is agnostic of the underlay.  It applies to MPLS as well as
      to Virtual eXtensible Local Area Network (VxLAN) encapsulation.
      The module is also agnostic to the services, including E-LAN,
      E-LINE, and E-TREE services.

   L3VPN:
      [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.

   Core Routing:
      [RFC8349] defines the core routing YANG 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, or OSPF
      models) will augment the Core Routing base YANG module.

   MPLS:
      [RFC8960] defines a base model for MPLS that 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.

   BGP:
      [IDR-BGP-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.

   Routing Policy:
      [RTGWG-POLICY] defines a YANG module for configuring and managing
      routing policies based on operational practice.  The module
      provides a generic policy framework that can be augmented with
      protocol-specific policy configuration.

   SR/SRv6:
      [SPRING-SR-YANG] defines a YANG module for segment routing
      configuration and operation.


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

   Multicast:
      [PIM-YANG] defines a YANG module that can be used to configure and
      manage Protocol Independent Multicast (PIM) devices.

      [RFC8652] defines a YANG module that can be used to configure and
      manage Internet Group Management Protocol (IGMP) and Multicast
      Listener Discovery (MLD) devices.

      [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.

      [MVPN-YANG] defines a YANG data model to configure and manage
      Multicast in MPLS/BGP IP VPNs (MVPNs).

   PM:
      [TWAMP-DATA-MODEL] defines a YANG data model for client and server
      implementations of the Two-Way Active Measurement Protocol
      (TWAMP).

      [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.

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

   ACL:
      An 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 YANG data model of ACL basic building
      blocks.

   QoS:
      [QOS-MODEL] describes a YANG module of Differentiated Services for
      configuration and operations.

   NAT:
      For the sake of network automation and the need for programming
      the Network Address Translation (NAT) function in particular, a
      YANG 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 (EAMs) for SIIT, IPv6-to-IPv6 Network Prefix
      Translation (NPTv6), and Destination NAT.

      [RFC8513] specifies a Dual-Stack Lite (DS-Lite) YANG module.

   Stateless Address Sharing:
      [RFC8676] specifies a YANG module for Address plus Port (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.

Acknowledgements

   Thanks to Joe Clark, Greg Mirsky, Shunsuke Homma, Brian Carpenter,
   Adrian Farrel, Christian Huitema, Tommy Pauly, Ines Robles, and
   Olivier Augizeau for the review.

   Many thanks to Robert Wilton for the detailed AD review.

   Thanks to Éric Vyncke, Roman Danyliw, Erik Kline, and Benjamin Kaduk
   for the IESG review.

Contributors

   Christian Jacquenet
   Orange
   Rennes, 35000
   France

   Email: Christian.jacquenet@orange.com


   Luis Miguel Contreras Murillo
   Telefonica

   Email: luismiguel.contrerasmurillo@telefonica.com


   Oscar Gonzalez de Dios
   Telefonica
   Madrid
   Spain

   Email: oscar.gonzalezdedios@telefonica.com


   Weiqiang Cheng
   China Mobile

   Email: chengweiqiang@chinamobile.com


   Young Lee
   Sung Kyun Kwan University

   Email: younglee.tx@gmail.com


Authors' Addresses

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

   Email: bill.wu@huawei.com


   Mohamed Boucadair (editor)
   Orange
   Rennes 35000
   France

   Email: mohamed.boucadair@orange.com


   Diego R. Lopez
   Telefonica I+D
   Spain

   Email: diego.r.lopez@telefonica.com


   Chongfeng Xie
   China Telecom
   Beijing
   China

   Email: xiechf@chinatelecom.cn


   Liang Geng
   China Mobile

   Email: gengliang@chinamobile.com
