OPSAWG                                                         B. Claise
Internet-Draft                                               J. Quilbeuf
Intended status: Standards Track                                  Huawei
Expires: 27 October 2022                                      P. Lucente
                                                                     NTT
                                                               P. Fasano
                                                               TIM S.p.A
                                                             T. Arumugam
                                                     Cisco Systems, Inc.
                                                           25 April 2022


                   YANG Modules for Service Assurance
              draft-ietf-opsawg-service-assurance-yang-04

Abstract

   This document specifies YANG modules representing assurance graphs.
   These graphs represent the assurance of a given service by
   decomposing it into atomic assurance elements called subservices.  A
   companion RFC, Service Assurance for Intent-based Networking
   Architecture, presents an architecture for implementing the assurance
   of such services.

   The YANG data models in this document conforms to the Network
   Management Datastore Architecture (NMDA) defined in RFC 8342.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 27 October 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  YANG Models Overview  . . . . . . . . . . . . . . . . . . . .   3
   3.  Base ietf-service-assurance YANG module . . . . . . . . . . .   4
     3.1.  Tree View . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Concepts  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Subservice Extension: ietf-service-assurance-device YANG
           module  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     4.1.  Tree View . . . . . . . . . . . . . . . . . . . . . . . .  15
     4.2.  Complete Tree View  . . . . . . . . . . . . . . . . . . .  15
     4.3.  Concepts  . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.4.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  17
   5.  Subservice Extension: ietf-service-assurance-interface YANG
           module  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     5.1.  Tree View . . . . . . . . . . . . . . . . . . . . . . . .  19
     5.2.  Complete Tree View  . . . . . . . . . . . . . . . . . . .  19
     5.3.  Concepts  . . . . . . . . . . . . . . . . . . . . . . . .  20
     5.4.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  21
   6.  Vendor-specific Subservice Extension:
           example-service-assurance-device-acme YANG module . . . .  23
     6.1.  Tree View . . . . . . . . . . . . . . . . . . . . . . . .  23
     6.2.  Complete Tree View  . . . . . . . . . . . . . . . . . . .  23
     6.3.  Concepts  . . . . . . . . . . . . . . . . . . . . . . . .  25
     6.4.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  25
   7.  Further Extensions: IP Connectivity and IS-IS subservices . .  27
     7.1.  IP Connectivity Tree View . . . . . . . . . . . . . . . .  27
     7.2.  IS-IS Tree View . . . . . . . . . . . . . . . . . . . . .  28
     7.3.  Global Tree View  . . . . . . . . . . . . . . . . . . . .  28
     7.4.  IP Connectivity YANG Module . . . . . . . . . . . . . . .  29
     7.5.  IS-IS YANG Module . . . . . . . . . . . . . . . . . . . .  32
   8.  Guidelines for Specific Subservice Extension  . . . . . . . .  33
     8.1.  Module Name . . . . . . . . . . . . . . . . . . . . . . .  34
     8.2.  Module Namespace  . . . . . . . . . . . . . . . . . . . .  34
     8.3.  Module Prefix . . . . . . . . . . . . . . . . . . . . . .  34
     8.4.  Subservice Specific Identity  . . . . . . . . . . . . . .  35
     8.5.  Parameters  . . . . . . . . . . . . . . . . . . . . . . .  35



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   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  35
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
     10.1.  The IETF XML Registry  . . . . . . . . . . . . . . . . .  36
     10.2.  The YANG Module Names Registry . . . . . . . . . . . . .  36
   11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  37
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  37
     12.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Appendix A.  Example of YANG instances  . . . . . . . . . . . . .  38
   Appendix B.  YANG Library for Service Assurance . . . . . . . . .  42
   Appendix C.  Changes between revisions  . . . . . . . . . . . . .  43
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  44
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

1.  Introduction

   The "Service Assurance for Intent-based Networking Architecture"
   [I-D.ietf-opsawg-service-assurance-architecture], specifies the
   architecture and all of its components for service assurance.  This
   document complements the architecture by providing open interfaces
   between components.  More specifically, the goal is to provide YANG
   modules for the purpose of service assurance in a format that is:

   *  machine readable

   *  vendor independent

   *  augmentable

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   13 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The terms used in this document are defined in
   [I-D.ietf-opsawg-service-assurance-architecture]

2.  YANG Models Overview

   The main YANG module, ietf-service-assurance, defines objects for
   assuring network services based on their decomposition into so-called
   subservices.  The subservices are hierarchically organised by
   dependencies.  The subservices, along with the dependencies,
   constitute an assurance graph.  This module should be supported by an
   agent, able to interact with the devices in order to produce a health



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   status and symptoms for each subservice in the assurance graph.  This
   module is intended for the following use cases:

   *  Assurance graph configuration:

      -  Subservices: configure a set of subservices to assure, by
         specifying their types and parameters.

      -  Dependencies: configure the dependencies between the
         subservices, along with their type.

   *  Assurance telemetry: export the health status of the subservices,
      along with the observed symptoms.

   The main module represents the configuration (subservice and
   dependencies) and operational data (health status and symptoms) in a
   single tree.  Other modules follows the same pattern.  Thus, the
   modules presented in this document conform to the Network Management
   Datastore Architecture defined in [RFC8342].

   The second YANG module, ietf-service-assurance-device, extends the
   ietf-service-assurance module to add support for the subservice
   DeviceHealthy.  Additional subservice types might be added the same
   way.

   The third YANG module, ietf-service-assurance-device, is another
   example that extends the ietf-service-assurance-device module.  This
   extension adds support for the subservice InterfaceHealthy.

   The fourth YANG module, example-service-assurance-device-acme,
   extends the ietf-service-assurance-device module as an example to add
   support for the subservice DeviceHealthy, with specifics for the
   fictional ACME Corporation.  Additional vendor-specific parameters
   might be added the same way.

   Finally, the modules example-service-assurance-ip-connectivity and
   example-service-assurance-is-is are provided to completely model the
   example from the SAIN architecture draft
   [I-D.ietf-opsawg-service-assurance-architecture].

3.  Base ietf-service-assurance YANG module

3.1.  Tree View

   The following tree diagram [RFC8340] provides an overview of the
   ietf-service-assurance data model.





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   module: ietf-service-assurance
     +--ro assurance-graph-version        yang:counter64
     +--ro assurance-graph-last-change    yang:date-and-time
     +--rw subservices
        +--rw subservice* [type id]
           +--rw type                                identityref
           +--rw id                                  string
           +--ro last-change?                        yang:date-and-time
           +--ro label?                              string
           +--rw under-maintenance?                  boolean
           +--rw maintenance-contact                 string
           +--rw (parameter)?
           |  +--:(service-instance-parameter)
           |     +--rw service-instance-parameter
           |        +--rw service          string
           |        +--rw instance-name    string
           +--ro health-score?                       union
           +--ro symptoms-history-start?             yang:date-and-time
           +--rw symptoms
           |  +--ro symptom* [start-date-time id]
           |     +--ro id                     string
           |     +--ro health-score-weight?   uint8
           |     +--ro description?           string
           |     +--ro start-date-time        yang:date-and-time
           |     +--ro stop-date-time?        yang:date-and-time
           +--rw dependencies
              +--rw dependency* [type id]
                 +--rw type
                 |       -> /subservices/subservice/type
                 +--rw id                 leafref
                 +--rw dependency-type?   identityref


3.2.  Concepts

   The ietf-service-assurance YANG model assumes an identified number of
   subservices, to be assured independently.  A subservice is a feature
   or a subpart of the network system that a given service instance
   might depend on.  Example of subservices include:

   *  DeviceHealthy: whether a device is healthy, and if not, what are
      the symptoms.  Potential symptoms are "CPU overloaded", "Out of
      RAM", or "Out of TCAM".

   *  ConnectivityHealthy: given two IP addresses owned by two devices,
      what is the quality of the connection between them.  Potential
      symptoms are "No route available" or "ECMP Imbalance".




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   The first example is a subservice representing a subpart of the
   network system, while the second is a subservice representing a
   feature of the network, In both cases, these subservices might depend
   on other subservices, for instance, the connectivity might depend on
   a subservice representing the routing mechanism and on a subservice
   representing ECMP.

   The status of each subservice contains a list of symptoms.  Each
   symptom is specified by a unique id and contains a health-score-
   weight (the impact to the health score incurred by this symptom), a
   label (text describing what the symptom is), and dates and times at
   which the symptom was detected and stopped being detected.  While the
   unique id is sufficient as an unique key list, the start-date-time
   second key help sorting and retrieving relevant symptoms.

   The relation between the health score and the health-score-weight of
   the currently active symptoms is not explicitly defined in this
   draft.  The only requirement is that a non-maximal score must be
   explained by at least one symptom.  A way to enforce that requirement
   is to first detect symptoms and then compute the health score based
   on the health-score-weight of the detected symptoms.  As an example,
   this computation could be to sum the health-score-weight of the
   active symptoms, subtract that value from 100 and change the value to
   0 if negative.  The relation between health-score and health-score-
   weight is left to the implementor (of an agent
   [I-D.ietf-opsawg-service-assurance-architecture]).  To consider for
   implementing this relation: the health-score is mostly for humans,
   the symptoms are what the closed loop automation can build on.

   The assurance of a given service instance can be obtained by
   composing the assurance of the subservices that it depends on, via
   the dependency relations.

   A subservice declaration MUST provide:

   *  A type: identity inheriting of the base identity for subservice,

   *  An id: string uniquely identifying the subservice among those with
      the same identity,

   *  One or more parameters, which should be specified in an augmenting
      model, as described in the next sections.

   The type and id uniquely identify a given subservice.  They are used
   to indicate the dependencies.  Dependencies have types as well.  Two
   types are specified in the model:





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   *  Impacting: such a dependency indicates an impact on the health of
      the dependent,

   *  Informational: such a dependency might explain why the dependent
      has issues but does not impact its health.

   To illustrate the difference between "impacting" and "informational",
   consider the subservice InterfaceHealthy, representing a network
   interface.  If the device to which the network interface belongs goes
   down, the network interface will transition to a down state as well.
   Therefore, the dependency of InterfaceHealthy towards DeviceHealthy
   is "impacting".  On the other hand, as a the dependency towards the
   ECMPLoad subservice, which checks that the load between ECMP remains
   ce remains stable throughout time, is only "informational".  Indeed,
   services might be perfectly healthy even if the load distribution
   between ECMP changed.  However, such an instability might be a
   relevant symptom for diagnosing the root cause of a problem.

   Service instances MUST be modeled as a particular type of subservice
   with two parameters, a type and an instance name.  The type is the
   name of the service defined in the network orchestrator, for instance
   "point-to-point-l2vpn".  The instance name is the name assigned to
   the particular instance that we are assuring, for instance the name
   of the customer using that instance.

   The "under-maintenance" and "maintenance-contact" flags inhibit the
   emission of symptoms for that subservice and subservices that depend
   on them.  See Section 3.7 of
   [I-D.ietf-opsawg-service-assurance-architecture] for a more detailed
   discussion.

   By specifying service instances and their dependencies in terms of
   subservices, one defines the whole assurance to apply for them.  An
   assurance agent supporting this model should then produce telemetry
   in return with, for each subservice: a health-status indicating how
   healthy the subservice is and when the subservice is not healthy, a
   list of symptoms explaining why the subservice is not healthy.

3.3.  YANG Module

   <CODE BEGINS> file "ietf-service-assurance@2022-04-07.yang"

module ietf-service-assurance {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-service-assurance";
  prefix sain;

  import ietf-yang-types {



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    prefix yang;
    reference
      "RFC 6991: Common YANG Data Types";
  }

  organization
    "IETF OPSAWG Working Group";
  contact
    "WG Web:   <https://datatracker.ietf.org/wg/opsawg/>
     WG List:  <mailto:opsawg@ietf.org>
     Author:   Benoit Claise  <mailto:benoit.claise@huawei.com>
     Author:   Jean Quilbeuf   <mailto:jean.quilbeu@huawei.com>";
  description
    "This module defines objects for assuring network services based on
     their decomposition into so-called subservices, according to the
     SAIN (Service Assurance for Intent-based Networking) architecture.

     The subservices hierarchically organised by dependencies constitute
     an assurance graph. This module should be supported by an assurance
     agent, able to interact with the devices in order to produce a
     health status and symptoms for each subservice in the assurance
     graph.

     This module is intended for the following use cases:
     * Assurance graph configuration:
       - subservices: configure a set of subservices to assure, by
         specifying their types and parameters.
       - dependencies: configure the dependencies between the
         subservices, along with their type.
     * Assurance telemetry: export the health status of the subservices,
       along with the observed symptoms.

     The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
     'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
     'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
     are to be interpreted as described in BCP 14 (RFC 2119)
     (RFC 8174) when, and only when, they appear in all
     capitals, as shown here.

     Copyright (c) 2022 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject
     to the license terms contained in, the Revised BSD License
     set forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents
     (https://trustee.ietf.org/license-info).



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     This version of this YANG module is part of RFC XXXX; see the
     RFC itself for full legal notices.  ";

  revision 2022-04-07 {
    description
      "Shorten prefix. Fix copyright.
       Fix module description";
    reference
      "RFC xxxx: YANG Modules for Service Assurance";
  }
  revision 2022-01-04 {
    description
      "Explicitely model a missing value";
    reference
      "RFC xxxx: YANG Modules for Service Assurance";
  }
  revision 2021-06-28 {
    description
      "Made service-instance parameters mandatory.";
    reference
      "RFC xxxx: YANG Modules for Service Assurance";
  }
  revision 2020-01-13 {
    description
      "Added the maintenance window concept.";
    reference
      "RFC xxxx: YANG Modules for Service Assurance";
  }
  revision 2019-11-16 {
    description
      "Initial revision.";
    reference
      "RFC xxxx: YANG Modules for Service Assurance";
  }

  identity subservice-idty {
    description
      "Root identity for all subservice types.";
  }

  identity service-instance-idty {
    base subservice-idty;
    description
      "Identity representing a service instance.";
  }

  identity dependency-type {
    description



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      "Base identity for representing dependency types.";
  }

  identity informational-dependency {
    base dependency-type;
    description
      "Indicates that symptoms of the dependency might be of interest
       for the dependent, but the status of the dependency should not
       have any impact on the dependent.";
  }

  identity impacting-dependency {
    base dependency-type;
    description
      "Indicates that the status of the dependency directly impacts the
       status of the dependent.";
  }

  grouping symptom {
    description
      "Contains the list of symptoms for a specific subservice.";
    leaf id {
      type string;
      description
        "A unique identifier for the symptom.";
    }
    leaf health-score-weight {
      type uint8 {
        range "0 .. 100";
      }
      description
        "The weight to the health score incurred by this symptom. The
         higher the value, the more of an impact this symptom has. If a
         subservice health score is not 100, there must be at least one
         symptom with a health score weight larger than 0.";
    }
    leaf description {
      type string;
      description
        "Description of the symptom, i.e. text describing what the
         symptom is, to be computer-consumable and be displayed on a
         human interface. ";
    }
    leaf start-date-time {
      type yang:date-and-time;
      description
        "Date and time at which the symptom was detected.";
    }



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    leaf stop-date-time {
      type yang:date-and-time;
      description
        "Date and time at which the symptom stopped being detected.";
    }
  }

  grouping subservice-dependency {
    description
      "Represent a dependency to another subservice.";
    leaf type {
      type leafref {
        path "/subservices/subservice/type";
      }
      description
        "The type of the subservice to refer to (e.g. DeviceHealthy).";
    }
    leaf id {
      type leafref {
        path "/subservices/subservice[type=current()/../type]/id";
      }
      description
        "The identifier of the subservice to refer to.";
    }
    leaf dependency-type {
      type identityref {
        base dependency-type;
      }
      description
        "Represents the type of dependency (i.e. informational,
         impacting).";
    }
    // Augment here to add parameters specific to a new dependency-type.
    // For instance, a specific dependency type could keep symptom
    // whose health-score-weight is larger than a given value.
  }

  leaf assurance-graph-version {
    type yang:counter64;
    config false;
    mandatory true;
    description
      "The assurance graph version, which increases by 1 for each new
       version, after the changes (dependencies and/or maintenance
       windows parameters) are applied to the subservice(s).";
  }
  leaf assurance-graph-last-change {
    type yang:date-and-time;



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    config false;
    mandatory true;
    description
      "Date and time at which the assurance graph last changed after the
       changes (dependencies and/or maintenance windows parameters) are
       applied to the subservice(s). These date and time must be more
       recent or equal compared to the more recent value of any changed
       subservices last-change";
  }
  container subservices {
    description
      "Root container for the subservices.";
    list subservice {
      key "type id";
      description
        "List of subservice configured.";
      leaf type {
        type identityref {
          base subservice-idty;
        }
        description
          "Name of the subservice, e.g. DeviceHealthy.";
      }
      leaf id {
        type string;
        description
          "Unique identifier of the subservice instance, for each
           type.";
      }
      leaf last-change {
        type yang:date-and-time;
        config false;
        description
          "Date and time at which the assurance graph for this
           subservice instance last changed, i.e. dependencies and/or
           maintenance windows parameters.";
      }
      leaf label {
        type string;
        config false;
        description
          "Label of the subservice, i.e. text describing what the
           subservice is to be displayed on a human interface.";
      }
      leaf under-maintenance {
        type boolean;
        default "false";
        description



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          "An optional flag indicating whether this particular
           subservice is under maintenance. Under this circumstance, the
           subservice symptoms and the symptoms of its dependencies in
           the assurance graph should not be taken into account.
           Instead, the subservice should send a 'Under Maintenance'
           single symptom.

           The operator changing the under-maintenance value must set
           the maintenance-contact variable.

           When the subservice is not under maintenance any longer, the
           under-maintenance flag must return to its default value and
           the under-maintenance-owner variable deleted.";
      }
      leaf maintenance-contact {
        when "../under-maintenance = 'true'";
        type string;
        mandatory true;
        description
          "A string used to model an administratively assigned name of
           the resource that changed the under-maintenance value to
           'true'.

           It is suggested that this name contain one or more of the
           following: IP address, management station name,
           network manager's name, location, or phone number. In some
           cases the agent itself will be the owner of an entry. In
           these cases, this string shall be set to a string starting
           with 'monitor'.";
      }
      choice parameter {
        description
          "Specify the required parameters per subservice type.";
        container service-instance-parameter {
          when "derived-from-or-self(../type,
                'sain:service-instance-idty')";
          description
            "Specify the parameters of a service instance.";
          leaf service {
            type string;
            mandatory true;
            description
              "Name of the service.";
          }
          leaf instance-name {
            type string;
            mandatory true;
            description



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              "Name of the instance for that service.";
          }
        }
        // Other modules can augment their own cases into here
      }
      leaf health-score {
        type union {
          type uint8 {
            range "0 .. 100";
          }
          type enumeration {
            enum missing {
              value -1;
              description
                "Explictly represent the fact that the health score is
                 missing. This could be used when metrics crucial to
                 establish the health score are not collected anymore.";
            }
          }
        }
        config false;
        description
          "Score value of the subservice health. A value of 100 means
           that subservice is healthy. A value of 0 means that the
           subservice is broken. A value between 0 and 100 means that
           the subservice is degraded.";
      }
      leaf symptoms-history-start {
        type yang:date-and-time;
        config false;
        description
          "Date and time at which the symptoms history starts for this
           subservice instance, either because the subservice instance
           started at that date and time or because the symptoms before
           that were removed due to a garbage collection process.";
      }
      container symptoms {
        description
          "Symptoms for the subservice.";
        list symptom {
          key "start-date-time id";
          config false;
          description
            "List of symptoms the subservice. While the start-date-time
             key is not necessary per se, this would get the entries
             sorted by start-date-time for easy consumption.";
          uses symptom;
        }



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      }
      container dependencies {
        description
          "configure the dependencies between the subservices, along
           with their types.";
        list dependency {
          key "type id";
          description
            "List of soft dependencies of the subservice.";
          uses subservice-dependency;
        }
      }
    }
  }
}


   <CODE ENDS>

4.  Subservice Extension: ietf-service-assurance-device YANG module

4.1.  Tree View

   The following tree diagram [RFC8340] provides an overview of the
   ietf-service-assurance-device data model.

   module: ietf-service-assurance-device

     augment /sain:subservices/sain:subservice/sain:parameter:
       +--rw parameters
          +--rw device    string


4.2.  Complete Tree View

   The following tree diagram [RFC8340] provides an overview of the
   ietf-service-assurance and ietf-service-assurance-device data models.














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   module: ietf-service-assurance
     +--ro assurance-graph-version        yang:counter64
     +--ro assurance-graph-last-change    yang:date-and-time
     +--rw subservices
        +--rw subservice* [type id]
           +--rw type                                identityref
           +--rw id                                  string
           +--ro last-change?                        yang:date-and-time
           +--ro label?                              string
           +--rw under-maintenance?                  boolean
           +--rw maintenance-contact                 string
           +--rw (parameter)?
           |  +--:(service-instance-parameter)
           |  |  +--rw service-instance-parameter
           |  |     +--rw service          string
           |  |     +--rw instance-name    string
           |  +--:(sain-device:parameters)
           |     +--rw sain-device:parameters
           |        +--rw sain-device:device    string
           +--ro health-score?                       union
           +--ro symptoms-history-start?             yang:date-and-time
           +--rw symptoms
           |  +--ro symptom* [start-date-time id]
           |     +--ro id                     string
           |     +--ro health-score-weight?   uint8
           |     +--ro description?           string
           |     +--ro start-date-time        yang:date-and-time
           |     +--ro stop-date-time?        yang:date-and-time
           +--rw dependencies
              +--rw dependency* [type id]
                 +--rw type
                 |       -> /subservices/subservice/type
                 +--rw id                 leafref
                 +--rw dependency-type?   identityref


4.3.  Concepts

   As the number of subservices will grow over time, the YANG module is
   designed to be extensible.  A new subservice type requires the
   precise specifications of its type and expected parameters.  Let us
   illustrate the example of the new DeviceHealthy subservice type.  As
   the name implies, it monitors and reports the device health, along
   with some symptoms in case of degradation.







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   For our DeviceHealthy subservice definition, the new identity device-
   idty is specified, as an inheritance from the base identity for
   subservices.  This indicates to the assurance agent that we are now
   assuring the health of a device.

   The typical parameter for the configuration of the DeviceHealthy
   subservice is the name of the device that we want to assure.  By
   augmenting the parameter choice from ietf-service-assurance YANG
   module for the case of the device-idty subservice type, this new
   parameter is specified.

4.4.  YANG Module

   <CODE BEGINS> file "ietf-service-assurance-device@2022-04-07.yang"

   module ietf-service-assurance-device {
     yang-version 1.1;
     namespace
       "urn:ietf:params:xml:ns:yang:ietf-service-assurance-device";
     prefix sain-device;

     import ietf-service-assurance {
       prefix sain;
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }

     organization
       "IETF OPSAWG Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/opsawg/>
        WG List:  <mailto:opsawg@ietf.org>
        Author:   Benoit Claise  <mailto:benoit.claise@huawei.com>
        Author:   Jean Quilbeuf   <mailto:jean.quilbeuf@huawei.com>";
     description
       "This module extends the ietf-service-assurance module to add
        support for the subservice DeviceHealthy.

        Checks whether a network device is healthy.

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
        'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
        'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
        are to be interpreted as described in BCP 14 (RFC 2119)
        (RFC 8174) when, and only when, they appear in all
        capitals, as shown here.

        Copyright (c) 2022 IETF Trust and the persons identified as



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        authors of the code.  All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject
        to the license terms contained in, the Revised BSD License
        set forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents
        (https://trustee.ietf.org/license-info).
        This version of this YANG module is part of RFC XXXX; see the
        RFC itself for full legal notices.  ";

     revision 2022-04-07 {
       description
         "Fix mandatory in augment error by moving when clause.
          Shorten prefix. Fix module description.
          Fix module description";
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }
     revision 2021-06-28 {
       description
         "Renamed the container for parameters.";
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }
     revision 2020-01-13 {
       description
         "Added the maintenance window concept.";
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }
     revision 2019-11-16 {
       description
         "Initial revision.";
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }

     identity device-idty {
       base sain:subservice-idty;
       description
         "Network Device is healthy.";
     }

     augment "/sain:subservices/sain:subservice/sain:parameter" {
       when "derived-from-or-self(sain:type, 'device-idty')";
       description
         "Specify the required parameters for a new subservice type";



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       container parameters {
         description
           "Specify the required parameters for the device-idty
            subservice type";
         leaf device {
           type string;
           mandatory true;
           description
             "The device to monitor.";
         }
       }
     }
   }


   <CODE ENDS>

5.  Subservice Extension: ietf-service-assurance-interface YANG module

5.1.  Tree View

   The following tree diagram [RFC8340] provides an overview of the
   ietf-service-assurance-interface data model.

   module: ietf-service-assurance-interface

     augment /sain:subservices/sain:subservice/sain:parameter:
       +--rw parameters
          +--rw device       string
          +--rw interface    string


5.2.  Complete Tree View

   The following tree diagram [RFC8340] provides an overview of the
   ietf-service-assurance, ietf-service-assurance-device, and ietf-
   service-assurance-interface data models.














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   module: ietf-service-assurance
     +--ro assurance-graph-version        yang:counter64
     +--ro assurance-graph-last-change    yang:date-and-time
     +--rw subservices
        +--rw subservice* [type id]
           +--rw type                                identityref
           +--rw id                                  string
           +--ro last-change?                        yang:date-and-time
           +--ro label?                              string
           +--rw under-maintenance?                  boolean
           +--rw maintenance-contact                 string
           +--rw (parameter)?
           |  +--:(service-instance-parameter)
           |  |  +--rw service-instance-parameter
           |  |     +--rw service          string
           |  |     +--rw instance-name    string
           |  +--:(sain-interface:parameters)
           |  |  +--rw sain-interface:parameters
           |  |     +--rw sain-interface:device       string
           |  |     +--rw sain-interface:interface    string
           |  +--:(sain-device:parameters)
           |     +--rw sain-device:parameters
           |        +--rw sain-device:device    string
           +--ro health-score?                       union
           +--ro symptoms-history-start?             yang:date-and-time
           +--rw symptoms
           |  +--ro symptom* [start-date-time id]
           |     +--ro id                     string
           |     +--ro health-score-weight?   uint8
           |     +--ro description?           string
           |     +--ro start-date-time        yang:date-and-time
           |     +--ro stop-date-time?        yang:date-and-time
           +--rw dependencies
              +--rw dependency* [type id]
                 +--rw type
                 |       -> /subservices/subservice/type
                 +--rw id                 leafref
                 +--rw dependency-type?   identityref


5.3.  Concepts

   For our InterfaceHealthy subservice definition, the new interface-
   idty is specified, as an inheritance from the base identity for
   subservices.  This indicates to the assurance agent that we are now
   assuring the health of an interface.





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   The typical parameters for the configuration of the InterfaceHealthy
   subservice are the name of the device and, on that specific device, a
   specific interface.  By augmenting the parameter choice from ietf-
   service-assurance YANG module for the case of the interface-idty
   subservice type, those two new parameters are specified.

5.4.  YANG Module

   <CODE BEGINS> file "ietf-service-assurance-interface@2022-04-07.yang"

   module ietf-service-assurance-interface {
     yang-version 1.1;
     namespace
       "urn:ietf:params:xml:ns:yang:ietf-service-assurance-interface";
     prefix sain-interface;

     import ietf-service-assurance {
       prefix sain;
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }

     organization
       "IETF OPSAWG Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/opsawg/>
        WG List:  <mailto:opsawg@ietf.org>
        Author:   Benoit Claise  <mailto:benoit.claise@huawei.com>
        Author:   Jean Quilbeuf   <mailto:jean.quilbeuf@huawei.com>";
     description
       "This module extends the ietf-service-assurance module to add
        support for the subservice InterfaceHealthy.

        Checks whether an interface is healthy.

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
        'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
        'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
        are to be interpreted as described in BCP 14 (RFC 2119)
        (RFC 8174) when, and only when, they appear in all
        capitals, as shown here.

        Copyright (c) 2022 IETF Trust and the persons identified as
        authors of the code.  All rights reserved.
        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject
        to the license terms contained in, the Revised BSD License
        set forth in Section 4.c of the IETF Trust's Legal Provisions



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        Relating to IETF Documents
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; see the
        RFC itself for full legal notices.  ";

     revision 2022-04-07 {
       description
         "Fix mandatory in augment error by moving when clause.
          Shorten prefix. Fix module description.
          Fix module description";
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }
     revision 2021-06-28 {
       description
         "Regroup parameters in a container.";
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }
     revision 2020-01-13 {
       description
         "Initial revision.";
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }

     identity interface-idty {
       base sain:subservice-idty;
       description
         "Checks whether an interface is healthy.";
     }

     augment "/sain:subservices/sain:subservice/sain:parameter" {
       when "derived-from-or-self(sain:type, 'interface-idty')";
       description
         "Specify the required parameters for the interface-idty
          subservice type";
       container parameters {
         description
           "Required parameters for the interface-idty subservice
            type";
         leaf device {
           type string;
           mandatory true;
           description
             "Device supporting the interface.";
         }



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         leaf interface {
           type string;
           mandatory true;
           description
             "Name of the interface.";
         }
       }
     }
   }


   <CODE ENDS>

6.  Vendor-specific Subservice Extension: example-service-assurance-
    device-acme YANG module

6.1.  Tree View

   The following tree diagram [RFC8340] provides an overview of the
   example-service-assurance-device-acme data model.

   module: example-service-assurance-device-acme

     augment /sain:subservices/sain:subservice/sain:parameter:
       +--rw parameters
          +--rw device                     string
          +--rw acme-specific-parameter    string


6.2.  Complete Tree View

   The following tree diagram [RFC8340] provides an overview of the
   ietf-service-assurance, ietf-service-assurance-device, and example-
   service-assurance-device-acme data models.

















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   module: ietf-service-assurance
     +--ro assurance-graph-version        yang:counter64
     +--ro assurance-graph-last-change    yang:date-and-time
     +--rw subservices
        +--rw subservice* [type id]
           +--rw type                                    identityref
           +--rw id                                      string
           +--ro last-change?
           |       yang:date-and-time
           +--ro label?                                  string
           +--rw under-maintenance?                      boolean
           +--rw maintenance-contact                     string
           +--rw (parameter)?
           |  +--:(service-instance-parameter)
           |  |  +--rw service-instance-parameter
           |  |     +--rw service          string
           |  |     +--rw instance-name    string
           |  +--:(sain-device:parameters)
           |  |  +--rw sain-device:parameters
           |  |     +--rw sain-device:device    string
           |  +--:(example-device-acme:parameters)
           |  |  +--rw example-device-acme:parameters
           |  |     +--rw example-device-acme:device
           |  |     |       string
           |  |     +--rw example-device-acme:acme-specific-parameter
           |  |             string
           |  +--:(sain-interface:parameters)
           |     +--rw sain-interface:parameters
           |        +--rw sain-interface:device       string
           |        +--rw sain-interface:interface    string
           +--ro health-score?                           union
           +--ro symptoms-history-start?
           |       yang:date-and-time
           +--rw symptoms
           |  +--ro symptom* [start-date-time id]
           |     +--ro id                     string
           |     +--ro health-score-weight?   uint8
           |     +--ro description?           string
           |     +--ro start-date-time        yang:date-and-time
           |     +--ro stop-date-time?        yang:date-and-time
           +--rw dependencies
              +--rw dependency* [type id]
                 +--rw type
                 |       -> /subservices/subservice/type
                 +--rw id                 leafref
                 +--rw dependency-type?   identityref





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6.3.  Concepts

   Under some circumstances, vendor-specific subservice types might be
   required.  As an example of this vendor-specific implementation, this
   section shows how to augment the ietf-service-assurance-device module
   to add support for the subservice DeviceHealthy, specific to the ACME
   Corporation.  The new parameter is acme-specific-parameter.

6.4.  YANG Module

 module example-service-assurance-device-acme {
   yang-version 1.1;
   namespace "urn:example:example-service-assurance-device-acme";
   prefix example-device-acme;

   import ietf-service-assurance {
     prefix sain;
     reference
       "RFC xxxx: YANG Modules for Service Assurance";
   }
   import ietf-service-assurance-device {
     prefix sain-device;
     reference
       "RFC xxxx: YANG Modules for Service Assurance";
   }

   organization
     "IETF OPSAWG Working Group";
   contact
     "WG Web:   <https://datatracker.ietf.org/wg/opsawg/>
      WG List:  <mailto:opsawg@ietf.org>
      Author:   Benoit Claise  <mailto:benoit.claise@huawei.com>
      Author:   Jean Quilbeuf   <mailto:jean.quilbeuf@huawei.com>";
   description
     "This module extends the ietf-service-assurance-device module to
      add support for the subservice DeviceHealthy, specific to the ACME
      Corporation.

      ACME Network Device is healthy.

      The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
      'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
      'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
      are to be interpreted as described in BCP 14 (RFC 2119)
      (RFC 8174) when, and only when, they appear in all
      capitals, as shown here.

      Copyright (c) 2022 IETF Trust and the persons identified as



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      authors of the code.  All rights reserved.

      Redistribution and use in source and binary forms, with or
      without modification, is permitted pursuant to, and subject
      to the license terms contained in, the Revised BSD License
      set forth in Section 4.c of the IETF Trust's Legal Provisions
      Relating to IETF Documents
      (https://trustee.ietf.org/license-info).

      This version of this YANG module is part of RFC XXXX; see the
      RFC itself for full legal notices.  ";

   revision 2022-04-07 {
     description
       "Fix mandatory in augment error by moving when clause.
        Shorten prefix.
        Fix module description";
     reference
       "RFC xxxx: YANG Modules for Service Assurance";
   }
   revision 2021-06-28 {
     description
       "Renamed the parameters container.";
     reference
       "RFC xxxx: YANG Modules for Service Assurance";
   }
   revision 2020-01-13 {
     description
       "Added the maintenance window concept.";
     reference
       "RFC xxxx: YANG Modules for Service Assurance";
   }
   revision 2019-11-16 {
     description
       "Initial revision.";
     reference
       "RFC xxxx: YANG Modules for Service Assurance";
   }

   identity device-acme-idty {
     base sain-device:device-idty;
     description
       "Network Device is healthy.";
   }

   augment "/sain:subservices/sain:subservice/sain:parameter" {
     when "derived-from-or-self(sain:type, 'device-acme-idty')";
     description



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       "Specify the required parameters for a new subservice type";
     container parameters {
       description
         "Specify the required parameters for the device-acme-idty
          subservice type";
       leaf device {
         type string;
         mandatory true;
         description
           "The device to monitor.";
       }
       leaf acme-specific-parameter {
         type string;
         mandatory true;
         description
           "The ACME Corporation sepcific parameter.";
       }
     }
   }
 }


7.  Further Extensions: IP Connectivity and IS-IS subservices

   In this section, we provide two additional YANG models to completely
   cover the example from Figure 2 in
   [I-D.ietf-opsawg-service-assurance-architecture].  The complete
   normalization of these modules is to be done in future work.

7.1.  IP Connectivity Tree View

   That subservice represents the unicast connectivity between two IP
   addresses located on to different devices.  Such a subservice could
   report symptoms such as "No route found".  The following tree diagram
   [RFC8340] provides an overview of the example-service-assurance-ip-
   connectivity data model.

   module: example-service-assurance-ip-connectivity

     augment /sain:subservices/sain:subservice/sain:parameter:
       +--rw parameters
          +--rw device1     string
          +--rw address1    inet:ip-address
          +--rw device2     string
          +--rw address2    inet:ip-address






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   To specify the connectivity that we are interested in, we specify two
   IP addresses and two devices.  The subservice assures that the
   connectivity between IP address 1 on device 1 and IP address 2 on
   device 2 is healthy.

7.2.  IS-IS Tree View

   The following tree diagram [RFC8340] provides an overview of the
   example-service-assurance-is-is data model.

   module: example-service-assurance-is-is

     augment /sain:subservices/sain:subservice/sain:parameter:
       +--rw parameters
          +--rw instance-name    string


   The parameter of this subservice is the name of the IS-IS instance to
   assure.

7.3.  Global Tree View

   The following tree diagram [RFC8340] provides an overview of the
   ietf-service-assurance, ietf-service-assurance-device, example-
   service-assurance-device-acme, example-service-assurance-ip-
   connectivity and example-service-assurance-is-is data models.

   module: ietf-service-assurance
     +--ro assurance-graph-version        yang:counter64
     +--ro assurance-graph-last-change    yang:date-and-time
     +--rw subservices
        +--rw subservice* [type id]
           +--rw type                                        identityref
           +--rw id                                          string
           +--ro last-change?
           |       yang:date-and-time
           +--ro label?                                      string
           +--rw under-maintenance?                          boolean
           +--rw maintenance-contact                         string
           +--rw (parameter)?
           |  +--:(service-instance-parameter)
           |  |  +--rw service-instance-parameter
           |  |     +--rw service          string
           |  |     +--rw instance-name    string
           |  +--:(example-ip-connectivity:parameters)
           |  |  +--rw example-ip-connectivity:parameters
           |  |     +--rw example-ip-connectivity:device1     string
           |  |     +--rw example-ip-connectivity:address1



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           |  |     |       inet:ip-address
           |  |     +--rw example-ip-connectivity:device2     string
           |  |     +--rw example-ip-connectivity:address2
           |  |             inet:ip-address
           |  +--:(example-is-is:parameters)
           |  |  +--rw example-is-is:parameters
           |  |     +--rw example-is-is:instance-name    string
           |  +--:(sain-device:parameters)
           |  |  +--rw sain-device:parameters
           |  |     +--rw sain-device:device    string
           |  +--:(example-device-acme:parameters)
           |  |  +--rw example-device-acme:parameters
           |  |     +--rw example-device-acme:device
           |  |     |       string
           |  |     +--rw example-device-acme:acme-specific-parameter
           |  |             string
           |  +--:(sain-interface:parameters)
           |     +--rw sain-interface:parameters
           |        +--rw sain-interface:device       string
           |        +--rw sain-interface:interface    string
           +--ro health-score?                               union
           +--ro symptoms-history-start?
           |       yang:date-and-time
           +--rw symptoms
           |  +--ro symptom* [start-date-time id]
           |     +--ro id                     string
           |     +--ro health-score-weight?   uint8
           |     +--ro description?           string
           |     +--ro start-date-time        yang:date-and-time
           |     +--ro stop-date-time?        yang:date-and-time
           +--rw dependencies
              +--rw dependency* [type id]
                 +--rw type
                 |       -> /subservices/subservice/type
                 +--rw id                 leafref
                 +--rw dependency-type?   identityref


7.4.  IP Connectivity YANG Module

   module example-service-assurance-ip-connectivity {
     yang-version 1.1;
     namespace "urn:example:example-service-assurance-ip-connectivity";
     prefix example-ip-connectivity;

     import ietf-inet-types {
       prefix inet;
       reference



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         "RFC 6991: Common YANG Data Types";
     }
     import ietf-service-assurance {
       prefix sain;
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }

     organization
       "IETF OPSAWG Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/opsawg/>
        WG List:  <mailto:opsawg@ietf.org>
        Author:   Benoit Claise  <mailto:benoit.claise@huawei.com>
        Author:   Jean Quilbeuf   <mailto:jean.quilbeuf@huawei.com>";
     description
       "This example module extends the ietf-service-assurance module to
        add support for the subservice ip-connectivity.

        Checks whether the ip connectivity between two ip addresses
        belonging to two network devices is healthy.

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
        'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
        'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
        are to be interpreted as described in BCP 14 (RFC 2119)
        (RFC 8174) when, and only when, they appear in all
        capitals, as shown here.

        Copyright (c) 2022 IETF Trust and the persons identified as
        authors of the code.  All rights reserved.
        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject
        to the license terms contained in, the Revised BSD License
        set forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; see the
        RFC itself for full legal notices.  ";

     revision 2022-04-07 {
       description
         "Fix mandatory in augment error by moving when clause.
          Shorten prefix. Fix module description.
          Fix module description";
       reference
         "RFC xxxx: YANG Modules for Service Assurance";



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     }
     revision 2021-06-28 {
       description
         "Initial revision.";
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }

     identity ip-connectivity-idty {
       base sain:subservice-idty;
       description
         "Checks connectivity between two IP addresses.";
     }

     augment "/sain:subservices/sain:subservice/sain:parameter" {
       when "derived-from-or-self(sain:type, 'ip-connectivity-idty')";
       description
         "Specify the required parameters for the ip-connectivity-idty
          subservice type";
       container parameters {
         description
           "Required parameters for the ip-connectivity-idty
            subservice type";
         leaf device1 {
           type string;
           mandatory true;
           description
             "Device at the first end of the connection.";
         }
         leaf address1 {
           type inet:ip-address;
           mandatory true;
           description
             "Address at the first end of the connection.";
         }
         leaf device2 {
           type string;
           mandatory true;
           description
             "Device at the second end of the connection.";
         }
         leaf address2 {
           type inet:ip-address;
           mandatory true;
           description
             "Address at the second end of the connection.";
         }
       }



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     }
   }


7.5.  IS-IS YANG Module

   module example-service-assurance-is-is {
     yang-version 1.1;
     namespace "urn:example:example-service-assurance-is-is";
     prefix example-is-is;

     import ietf-service-assurance {
       prefix sain;
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }

     organization
       "IETF OPSAWG Working Group";
     contact
       "WG Web:   <https://datatracker.ietf.org/wg/opsawg/>
        WG List:  <mailto:opsawg@ietf.org>
        Author:   Benoit Claise  <mailto:benoit.claise@huawei.com>
        Author:   Jean Quilbeuf  <mailto:jean.quilbeuf@huawei.com>";
     description
       "This module extends the ietf-service-assurance module to
        add support for the subservice is-is.

        Checks whether an IS-IS instance is healthy.

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
        'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
        'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
        are to be interpreted as described in BCP 14 (RFC 2119)
        (RFC 8174) when, and only when, they appear in all
        capitals, as shown here.

        Copyright (c) 2022 IETF Trust and the persons identified as
        authors of the code.  All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject
        to the license terms contained in, the Revised BSD License
        set forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents
        (https://trustee.ietf.org/license-info).
        This version of this YANG module is part of RFC XXXX; see the
        RFC itself for full legal notices.  ";



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     revision 2022-04-07 {
       description
         "Fix mandatory in augment error by moving when clause.
          Shorten prefix. Fix module description.
          Fix module description";
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }
     revision 2021-06-28 {
       description
         "Initial revision.";
       reference
         "RFC xxxx: YANG Modules for Service Assurance";
     }

     identity is-is-idty {
       base sain:subservice-idty;
       description
         "Health of IS-IS routing protocol.";
     }

     augment "/sain:subservices/sain:subservice/sain:parameter" {
       when "derived-from-or-self(sain:type, 'is-is-idty')";
       description
         "Specify the required parameters for a new subservice
          type";
       container parameters {
         description
           "Specify the required parameters for the IS-IS subservice
            type";
         leaf instance-name {
           type string;
           mandatory true;
           description
             "The instance to monitor.";
         }
       }
     }
   }


8.  Guidelines for Specific Subservice Extension

   The base YANG module defined in Section 3.3 only defines a single
   type of subservices that represent service instances.  As explained
   above, this model is meant to be augmented so that a variety of
   subservices can be used in the assurance graph.  In this section, we
   propose some guidelines in order to build theses extensions.



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   First, the specific subservice must be given an adequate unique short
   name that will be used to form longer names (e.g. module name, prefix
   ...) appearing in the YANG module.  The short name identifies the
   type of subpart of feature that the subservice will represent, for
   instance if the subservice will assure the health of a network
   interface then "interface" is an adequate short name.  If the
   subservice will assure the IS-IS routing protocol, then "is-is" is an
   adequate short name.  The short name must be in kebab-case.

   In this section, by subservice YANG module, we mean "YANG module that
   extends ietf-service-assurance in order to define a specific
   subservice".

8.1.  Module Name

   For subservice YANG modules vetted by the IETF, the module name
   should be "ietf-service-assurance-" followed by the short name.  For
   instance, "ietf-service-assurance-interface" or "ietf-service-
   assurance-is-is".

   For subservice YANG module that are directly provided by vendors, we
   propose that they use the company in the prefix.  For example, the
   prefix for the company "acme" would be "acme-assurance-" and the YANG
   modules would be "acme-assurance-interface", "acme-assurance-is-is",
   etc.

8.2.  Module Namespace

   For subservice YANG modules vetted by the IETF, the module namespace
   should be "urn:ietf:params:xml:ns:yang:ietf-service-assurance-"
   followed by the short name.  For instance,
   "urn:ietf:params:xml:ns:yang:ietf-service-assurance-interface" or
   "urn:ietf:params:xml:ns:yang:example-service-assurance-is-is".

   For subservice YANG module that are directly provided by vendors, a
   similar pattern can be used with the prefix being a namespace
   controlled by the vendor.

8.3.  Module Prefix

   For subservice YANG modules vetted by the IETF, the module prefix
   should be "sain-" followed by the short name.  For instance, "sain-
   interface" or "sain-is-is".

   For subservice YANG module that are directly provided by vendors, the
   same pattern can be used provided it does not conflict with an
   imported prefix.




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8.4.  Subservice Specific Identity

   Each augment specific to a subservice must define an identity
   representing the type of subpart or features of the network system
   that are assured by the subservice.  As required in the "ietf-
   service-assurance" module (see Section 3.3), that identity must be
   based on the "subservice-idty" identity.

   For subservice YANG modules vetted by the IETF, the subservice
   specific identity should be the short name of the subservice followed
   by "-idty".  For instance, "interface-idty" or "is-is-identity".

   For subservice YANG module that are directly provided by vendors, the
   same pattern can be used.

8.5.  Parameters

   For subservice YANG modules vetted by the IETF, the parameters
   specific to the subservice should be placed in a container named
   "parameters".  That container must be used to augment the "parameter"
   choice from the module "ietf-service-assurance" (see Section 3.3 and
   that augment must be guarded so that it is effective only for
   subservice instance whose type is the subservice specific identity
   from Section 8.4.

   For subservice YANG module that are directly provided by vendors, the
   same pattern can be used.

9.  Security Considerations

   The YANG module specified in this document defines a 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 Network Configuration Access Control Model (NACM) [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.









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   There are a number of data nodes defined in this YANG module that are
   writable/ creatable/deletable (i.e., config true, which is the
   default).  These data nodes may be considered sensitive or vulnerable
   in some network environments.  Write operations (e.g., edit-config)
   to these data nodes without proper protection can have a negative
   effect on network operations.  These are the subtrees and data nodes
   and their sensitivity/vulnerability:

   *  /subservices/subservice/type

   *  /subservices/subservice/id

   *  /subservices/subservice/under-maintenance

   *  /subservices/subservice/maintenance-contact

10.  IANA Considerations

10.1.  The IETF XML Registry

   This document registers two URIs in the IETF XML registry [RFC3688].
   Following the format in [RFC3688], the following registrations are
   requested:

   URI: urn:ietf:params:xml:ns:yang:ietf-service-assurance
   Registrant Contact: The NETCONF WG of the IETF.
   XML: N/A, the requested URI is an XML namespace.

   URI: urn:ietf:params:xml:ns:yang:ietf-service-assurance-device
   Registrant Contact: The NETCONF WG of the IETF.
   XML: N/A, the requested URI is an XML namespace.

   URI: urn:ietf:params:xml:ns:yang:ietf-service-assurance-interface
   Registrant Contact: The NETCONF WG of the IETF.
   XML: N/A, the requested URI is an XML namespace.

10.2.  The YANG Module Names Registry

   This document registers three YANG modules in the YANG Module Names
   registry [RFC7950].  Following the format in [RFC7950], the the
   following registrations are requested:










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name:       ietf-service-assurance
namespace:  urn:ietf:params:xml:ns:yang:ietf-service-assurance
prefix:     inc
reference:  RFC XXXX

name:       ietf-service-assurance-device
namespace:  urn:ietf:params:xml:ns:yang:ietf-service-assurance-device
prefix:     inc
reference:  RFC XXXX

name:       ietf-service-assurance-interface
namespace:  urn:ietf:params:xml:ns:yang:ietf-service-assurance-interface
prefix:     inc
reference:  RFC XXXX

11.  Open Issues

      -None

12.  References

12.1.  Normative References

   [I-D.ietf-opsawg-service-assurance-architecture]
              Claise, B.C., Quilbeuf, J.Q., Lopez, D.L., Voyer, D.V.,
              and T.A. Arumugam, "draft-claise-opsawg-service-assurance-
              architecture", 2020.

   [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>.







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   [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>.

   [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>.

   [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>.

12.2.  Informative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC7895]  Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module
              Library", RFC 7895, DOI 10.17487/RFC7895, June 2016,
              <https://www.rfc-editor.org/info/rfc7895>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

Appendix A.  Example of YANG instances

   This section contains examples of YANG instances that conform to the
   YANG modules.  The validity of these data instances has been checked
   using yangson (https://yangson.labs.nic.cz/).  Yangson requires a
   YANG library [RFC7895] to define the complete model against which the
   data instance must be validated.  We provide in Appendix B the JSON
   library file, named "ietf-service-assurance-library.json", that we
   used for validation.





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   We provide below the contents of the file
   "example_configuration_instance.json" which contains the
   configuration data that models the Figure 2 of
   [I-D.ietf-opsawg-service-assurance-architecture].  The instance can
   be validated with yangson by using the invocation "yangson -v
   example_configuration_instance.json ietf-service-assurance-
   library.json", assuming all the files (YANG and JSON) defined in this
   draft reside in the current folder.

{
  "ietf-service-assurance:subservices": {
    "subservice": [
      {
        "type": "service-instance-idty",
        "id": "simple-tunnel/example",
        "service-instance-parameter": {
          "service": "simple-tunnel",
          "instance-name": "example"
        },
        "dependencies": {
          "dependency": [
            {
              "type": "ietf-service-assurance-interface:interface-idty",
              "id": "interface/peer1/tunnel0",
              "dependency-type": "impacting-dependency"
            },
            {
              "type": "ietf-service-assurance-interface:interface-idty",
              "id": "interface/peer2/tunnel9",
              "dependency-type": "impacting-dependency"
            },
            {
              "type":
       "example-service-assurance-ip-connectivity:ip-connectivity-idty",
              "id": "connectivity/peer1/2001:db8::1/peer2/2001:db8::2",
              "dependency-type": "impacting-dependency"
            }
          ]
        }
      },
      {
        "type":
       "example-service-assurance-ip-connectivity:ip-connectivity-idty",
        "id": "connectivity/peer1/2001:db8::1/peer2/2001:db8::2",
        "example-service-assurance-ip-connectivity:parameters": {
          "device1": "Peer1",
          "address1": "2001:db8::1",
          "device2": "Peer2",



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          "address2": "2001:db8::2"
        },
        "dependencies": {
          "dependency": [
            {
              "type": "ietf-service-assurance-interface:interface-idty",
              "id": "interface/peer1/physical0",
              "dependency-type": "impacting-dependency"
            },
            {
              "type": "ietf-service-assurance-interface:interface-idty",
              "id": "interface/peer2/physical5",
              "dependency-type": "impacting-dependency"
            },
            {
              "type": "example-service-assurance-is-is:is-is-idty",
              "id": "is-is/instance1",
              "dependency-type": "impacting-dependency"
            }
          ]
        }
      },
      {
        "type": "example-service-assurance-is-is:is-is-idty",
        "id": "is-is/instance1",
        "example-service-assurance-is-is:parameters": {
          "instance-name": "instance1"
        }
      },
      {
        "type": "ietf-service-assurance-interface:interface-idty",
        "id": "interface/peer1/tunnel0",
        "ietf-service-assurance-interface:parameters": {
          "device": "Peer1",
          "interface": "tunnel0"
        },
        "dependencies": {
          "dependency": [
            {
              "type": "ietf-service-assurance-interface:interface-idty",
              "id": "interface/peer1/physical0",
              "dependency-type": "impacting-dependency"
            }
          ]
        }
      },
      {
        "type": "ietf-service-assurance-interface:interface-idty",



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        "id": "interface/peer1/physical0",
        "ietf-service-assurance-interface:parameters": {
          "device": "Peer1",
          "interface": "physical0"
        },
        "dependencies": {
          "dependency": [
            {
              "type": "ietf-service-assurance-device:device-idty",
              "id": "interface/peer1",
              "dependency-type": "impacting-dependency"
            }
          ]
        }
      },
      {
        "type": "ietf-service-assurance-device:device-idty",
        "id": "interface/peer1",
        "ietf-service-assurance-device:parameters": {
          "device": "Peer1"
        }
      },
      {
        "type": "ietf-service-assurance-interface:interface-idty",
        "id": "interface/peer2/tunnel9",
        "ietf-service-assurance-interface:parameters": {
          "device": "Peer2",
          "interface": "tunnel9"
        },
        "dependencies": {
          "dependency": [
            {
              "type": "ietf-service-assurance-interface:interface-idty",
              "id": "interface/peer2/physical5",
              "dependency-type": "impacting-dependency"
            }
          ]
        }
      },
      {
        "type": "ietf-service-assurance-interface:interface-idty",
        "id": "interface/peer2/physical5",
        "ietf-service-assurance-interface:parameters": {
          "device": "Peer2",
          "interface": "physical5"
        },
        "dependencies": {
          "dependency": [



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            {
              "type": "ietf-service-assurance-device:device-idty",
              "id": "interface/peer2",
              "dependency-type": "impacting-dependency"
            }
          ]
        }
      },
      {
        "type": "ietf-service-assurance-device:device-idty",
        "id": "interface/peer2",
        "ietf-service-assurance-device:parameters": {
          "device": "Peer2"
        }
      }
    ]
  }
}



Appendix B.  YANG Library for Service Assurance

   This section provides the JSON encoding of the YANG library [RFC7895]
   listing all modules defined in this draft and their dependencies.
   This library can be used to validate data instances using yangson, as
   explained in the previous section.

{
  "ietf-yang-library:modules-state": {
    "module-set-id": "ietf-service-assurance@2022-04-07",
    "module": [
      {
        "name": "ietf-service-assurance",
        "namespace":
          "urn:ietf:params:xml:ns:yang:ietf-service-assurance",
        "revision": "2022-04-07",
        "conformance-type": "implement"
      },
      {
        "name": "ietf-service-assurance-device",
        "namespace":
          "urn:ietf:params:xml:ns:yang:ietf-service-assurance-device",
        "revision": "2022-04-07",
        "conformance-type": "implement"
      },
      {
        "name": "ietf-service-assurance-interface",



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        "namespace":
         "urn:ietf:params:xml:ns:yang:ietf-service-assurance-interface",
        "revision": "2022-04-07",
        "conformance-type": "implement"
      },
      {
        "name": "example-service-assurance-device-acme",
        "namespace":
          "urn:example:example-service-assurance-device-acme",
        "revision": "2022-04-07",
        "conformance-type": "implement"
      },
      {
        "name": "example-service-assurance-is-is",
        "namespace": "urn:example:example-service-assurance-is-is",
        "revision": "2022-04-07",
        "conformance-type": "implement"
      },
      {
        "name": "example-service-assurance-ip-connectivity",
        "namespace":
          "urn:example:example-service-assurance-ip-connectivity",
        "revision": "2022-04-07",
        "conformance-type": "implement"
      },
      {
        "name": "ietf-yang-types",
        "namespace": "urn:ietf:params:xml:ns:yang:ietf-yang-types",
        "revision": "2021-04-14",
        "conformance-type": "import"
      },
      {
        "name": "ietf-inet-types",
        "namespace": "urn:ietf:params:xml:ns:yang:ietf-inet-types",
        "revision": "2021-02-22",
        "conformance-type": "import"
      }
    ]
  }
}


Appendix C.  Changes between revisions

   v03 - v04

   *  Fix YANG errors




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   *  Change is-is and ip-connectivity subservices from ietf to example.

   *  Mention that models are NMDA compliant

   *  Fix typos, reformulate for clarity

   v02 - v03

   *  Change counter32 to counter64 to avoid resetting too frequently

   *  Explain why relation between health-score and symptom's health-
      score-weight is not defined and how it could be defined

   v01 - v02

   *  Explicitly represent the fact that the health-score could not be
      computed (value -1)

   v00 - v01

   *  Added needed subservice to model example from architecture draft

   *  Added guideline section for naming models

   *  Added data instance examples and validation procedure

   *  Added the "parameters" container in the interface YANG module to
      correct a bug.

Acknowledgements

   The authors would like to thank Jan Lindblad for his help during the
   design of these YANG modules.  The authors would like to thank
   Stephane Litkowski, Charles Eckel, Mohamed Boucadair and Tom Petch
   for their reviews.

Authors' Addresses

   Benoit Claise
   Huawei
   Email: benoit.claise@huawei.com


   Jean Quilbeuf
   Huawei
   Email: jean.quilbeuf@huawei.com





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   Paolo Lucente
   NTT
   Siriusdreef 70-72
   2132 Hoofddorp
   Netherlands
   Email: paolo@ntt.net


   Paolo Fasano
   TIM S.p.A
   via G. Reiss Romoli, 274
   10148 Torino
   Italy
   Email: paolo2.fasano@telecomitalia.it


   Thangam Arumugam
   Cisco Systems, Inc.
   Milpitas (California),
   United States
   Email: tarumuga@cisco.com






























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