<pre>Internet Engineering Task Force (IETF)                        A. Bierman
Request for Comments: 8525                                     YumaWorks
Obsoletes: <a href="./rfc7895">7895</a>                                             M. Bjorklund
Category: Standards Track                                 Tail-f Systems
ISSN: 2070-1721                                         J. Schoenwaelder
                                                       Jacobs University
                                                               K. Watsen
                                                         Watsen Networks
                                                               R. Wilton
                                                           Cisco Systems
                                                              March 2019


                              <span class="h1">YANG Library</span>

Abstract

   This document describes a YANG library that provides information
   about the YANG modules, datastores, and datastore schemas used by a
   network management server.  Simple caching mechanisms are provided to
   allow clients to minimize retrieval of this information.  This
   version of the YANG library supports the Network Management Datastore
   Architecture (NMDA) by listing all datastores supported by a network
   management server and the schema that is used by each of these
   datastores.

   This document obsoletes <a href="./rfc7895">RFC 7895</a>.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in <a href="./rfc7841#section-2">Section&nbsp;2 of RFC 7841</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="https://www.rfc-editor.org/info/rfc8525">https://www.rfc-editor.org/info/rfc8525</a>.










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Copyright Notice

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

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
      <a href="#section-1.1">1.1</a>. Terminology ................................................<a href="#page-4">4</a>
   <a href="#section-2">2</a>. Objectives ......................................................<a href="#page-5">5</a>
   <a href="#section-3">3</a>. YANG Library Data Model .........................................<a href="#page-6">6</a>
   <a href="#section-4">4</a>. YANG Library YANG Module ........................................<a href="#page-8">8</a>
   <a href="#section-5">5</a>. IANA Considerations ............................................<a href="#page-20">20</a>
   <a href="#section-6">6</a>. Security Considerations ........................................<a href="#page-21">21</a>
   <a href="#section-7">7</a>. References .....................................................<a href="#page-22">22</a>
      <a href="#section-7.1">7.1</a>. Normative References ......................................<a href="#page-22">22</a>
      <a href="#section-7.2">7.2</a>. Informative References ....................................<a href="#page-23">23</a>
   <a href="#appendix-A">Appendix A</a>.  Summary of Changes from <a href="./rfc7895">RFC 7895</a> .....................<a href="#page-25">25</a>
   <a href="#appendix-B">Appendix B</a>.  Example YANG Library Instance for a Basic Server .....<a href="#page-25">25</a>
   <a href="#appendix-C">Appendix C</a>.  Example YANG Library Instance for an Advanced Server .27
   Authors' Addresses ................................................<a href="#page-32">32</a>




















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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   There is a need for a standard mechanism to expose which YANG modules
   [<a href="./rfc7950" title="&quot;The YANG 1.1 Data Modeling Language&quot;">RFC7950</a>], datastores [<a href="./rfc8342" title="&quot;Network Management Datastore Architecture (NMDA)&quot;">RFC8342</a>], and datastore schemas [<a href="./rfc8342" title="&quot;Network Management Datastore Architecture (NMDA)&quot;">RFC8342</a>] are
   in use by a network management server.

   This document defines the YANG module "ietf-yang-library" that
   provides this information.  This version of the YANG library is
   compatible with the Network Management Datastore Architecture (NMDA)
   [<a href="./rfc8342" title="&quot;Network Management Datastore Architecture (NMDA)&quot;">RFC8342</a>].  The previous version of the YANG library, defined in
   [<a href="./rfc7895" title="&quot;YANG Module Library&quot;">RFC7895</a>], is not compatible with the NMDA since it assumes that all
   datastores have exactly the same schema.  This is not necessarily
   true in the NMDA since dynamic configuration datastores may have
   their own datastore schema.  Furthermore, the operational state
   datastore may support non-configurable YANG modules in addition to
   the YANG modules supported by conventional configuration datastores.

   The old YANG library definitions have been retained (for backwards-
   compatibility reasons), but the definitions have been marked as
   deprecated.  For backwards compatibility, an NMDA-supporting server
   SHOULD populate the deprecated "/modules-state" tree in a backwards-
   compatible manner.  The new "/yang-library" tree will be ignored by
   legacy clients but will provide all the data needed for NMDA-aware
   clients (which will ignore the "/modules-state" tree).  The
   recommended approach to populate "/modules-state" is to report the
   YANG modules with "config true" data nodes that are configurable via
   conventional configuration datastores and the YANG modules with
   "config false" data nodes that are returned via a Network
   Configuration Protocol (NETCONF) &lt;get&gt; operation or equivalent.

   The YANG library information can be different on every server, and it
   can change at runtime or across a server reboot.  If a server
   implements multiple network management protocols to access the
   server's datastores, then each such protocol may have its own
   conceptual instantiation of the YANG library.

   If a large number of YANG modules are utilized by a server, then the
   YANG library contents can be relatively large.  Since the YANG
   library contents change very infrequently, it is important that
   clients be able to cache the YANG library contents and easily
   identify whether their cache is out of date.










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<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Terminology</span>

   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
   <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a> [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>] [<a href="./rfc8174" title="&quot;Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words&quot;">RFC8174</a>] when, and only when, they appear in all
   capitals, as shown here.

   The following terms are defined in [<a href="./rfc7950" title="&quot;The YANG 1.1 Data Modeling Language&quot;">RFC7950</a>]:

   o  module

   o  submodule

   o  data node

   This document uses the phrase "implement a module" as defined in
   <a href="./rfc7950#section-5.6.5">Section&nbsp;5.6.5 of [RFC7950]</a>.

   The following terms are defined in [<a href="./rfc8342" title="&quot;Network Management Datastore Architecture (NMDA)&quot;">RFC8342</a>]:

   o  datastore

   o  datastore schema

   o  configuration

   o  conventional configuration datastore

   o  operational state

   o  operational state datastore

   o  dynamic configuration datastore

   o  client

   o  server

   The following terms are used within this document:

   o  YANG library: A collection of YANG modules, submodules,
      datastores, and datastore schemas used by a server.

   o  YANG library content identifier: A server-generated identifier of
      the contents of the YANG library.

   Tree diagrams in this document use the notation defined in [<a href="./rfc8340" title="&quot;YANG Tree Diagrams&quot;">RFC8340</a>].



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<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Objectives</span>

   The following information is needed by a client application (for each
   YANG module in the library) to fully utilize the YANG data modeling
   language:

   o  name: The name of the YANG module.

   o  revision: If defined in the YANG module or submodule, the revision
      is derived from the most recent revision statement within the
      module or submodule.

   o  submodule list: The name and (if defined) revision of each
      submodule used by the module must be identified.

   o  feature list: The name of each YANG feature supported by the
      server must be identified.

   o  deviation list: The name of each YANG module with deviation
      statements affecting a given YANG module must be identified.

   In addition, the following information is needed by a client
   application for each datastore supported by a server:

   o  identity: The YANG identity for the datastore.

   o  schema: The schema (i.e., the set of modules) implemented by the
      datastore.

   In order to select one out of several possible designs for the YANG
   library data model, the following criteria were used:

   1.  The information must be efficient for a client to consume.  Since
       the size of the YANG library can be quite large, it should be
       possible for clients to cache the YANG library information.

   2.  A dynamic configuration datastore must be able to implement a
       module or feature that is not implemented in the conventional
       configuration datastores.

   3.  It must be possible to not implement a module or feature in
       &lt;operational&gt;, even if it is implemented in some other datastore.
       This is required for transition purposes; a server that wants to
       implement &lt;operational&gt; should not have to implement all modules
       at once.






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   4.  A given module can only be implemented in one revision in all
       datastores.  If a module is implemented in more than one
       datastore, the same revision is implemented in all these
       datastores.

   5.  Multiple revisions can be used for import, if import-by revision
       is used.

   6.  It must be possible to use the YANG library by schema mount
       [<a href="./rfc8528" title="&quot;YANG Schema Mount&quot;">RFC8528</a>].

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  YANG Library Data Model</span>

   The "ietf-yang-library" YANG module provides information about the
   modules, submodules, datastores, and datastore schemas supported by a
   server.  All data nodes in the "ietf-yang-library" module are "config
   false" and thus only accessible in the operational state datastore.

   +-----------+
   | datastore |
   +-----------+
        |
        | has a
        V
   +-----------+            +--------+                +------------+
   | datastore |  union of  | module |  consists of   | modules +  |
   |  schema   |-----------&gt;|  set   |---------------&gt;| submodules |
   +-----------+            +--------+                +------------+

                                 Figure 1

   The conceptual model of the YANG library is depicted in Figure 1.
   Following the NMDA, every datastore has an associated datastore
   schema.  A datastore schema is a union of module sets, and every
   module set is a collection of modules and submodules, including the
   modules and submodules used for imports.  Note that multiple
   datastores may refer to the same datastore schema.  Furthermore, it
   is possible for individual datastore schemas to share module sets.  A
   common use case is the operational state datastore schema, which is a
   superset of the schema used by conventional configuration datastores.

   Below is the YANG tree diagram for the "ietf-yang-library" module,
   excluding the deprecated "/modules-state" tree:








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   module: ietf-yang-library
     +--ro yang-library
        +--ro module-set* [name]
        |  +--ro name                  string
        |  +--ro module* [name]
        |  |  +--ro name         yang:yang-identifier
        |  |  +--ro revision?    revision-identifier
        |  |  +--ro namespace    inet:uri
        |  |  +--ro location*    inet:uri
        |  |  +--ro submodule* [name]
        |  |  |  +--ro name        yang:yang-identifier
        |  |  |  +--ro revision?   revision-identifier
        |  |  |  +--ro location*   inet:uri
        |  |  +--ro feature*     yang:yang-identifier
        |  |  +--ro deviation*   -&gt; ../../module/name
        |  +--ro import-only-module* [name revision]
        |     +--ro name         yang:yang-identifier
        |     +--ro revision     union
        |     +--ro namespace    inet:uri
        |     +--ro location*    inet:uri
        |     +--ro submodule* [name]
        |        +--ro name        yang:yang-identifier
        |        +--ro revision?   revision-identifier
        |        +--ro location*   inet:uri
        +--ro schema* [name]
        |  +--ro name          string
        |  +--ro module-set*   -&gt; ../../module-set/name
        +--ro datastore* [name]
        |  +--ro name      ds:datastore-ref
        |  +--ro schema    -&gt; ../../schema/name
        +--ro content-id    string

     notifications:
       +---n yang-library-update
          +--ro content-id    -&gt; /yang-library/content-id

   The "/yang-library" container holds the entire YANG library.  The
   container has the following child nodes:

   o  The "/yang-library/module-set" contains entries representing
      module sets.  The list "/yang-library/module-set/module"
      enumerates the modules that belong to the module set.  A module is
      listed together with its submodules (if any), a set of features,
      and any deviation modules.  The list "/yang-library/module-set/
      import-only-module" lists all modules (and their submodules) used
      only for imports.  The assignment of a module to a module set is





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      at the server's discretion.  This revision of the YANG library
      attaches no semantics as to which module set a module is listed
      in.

   o  The "/yang-library/schema" list contains an entry for each
      datastore schema supported by the server.  All conventional
      configuration datastores use the same "schema" list entry.  A
      dynamic configuration datastore may use a different datastore
      schema from the conventional configuration datastores and hence
      may require a separate "schema" entry.  A "schema" entry has a
      leaf-list of references to entries in the "module-set" list.  The
      schema consists of the union of all modules in all referenced
      module sets.

   o  The "/yang-library/datastore" list contains one entry for each
      datastore supported by the server, and it identifies the datastore
      schema associated with a datastore via a reference to an entry in
      the "schema" list.  Each supported conventional configuration
      datastore has a separate entry, pointing to the same "schema" list
      element.

   o  The "/yang-library/content-id" leaf contains the YANG library
      content identifier, which is an implementation-specific identifier
      representing the current information in the YANG library on a
      specific server.  The value of this leaf MUST change whenever the
      information in the YANG library changes.  There is no requirement
      that the same information always result in the same "content-id"
      value.  This leaf allows a client to fetch all schema information
      once, cache it, and only refetch it if the value of this leaf has
      been changed.  If the value of this leaf changes, the server also
      generates a "yang-library-update" notification.

   Note that for a NETCONF server implementing the NETCONF extensions to
   support the NMDA [<a href="./rfc8526" title="&quot;NETCONF Extensions to Support the Network Management Datastore Architecture&quot;">RFC8526</a>], a change of the YANG library content
   identifier results in a new value for the :yang-library:1.1
   capability defined in [<a href="./rfc8526" title="&quot;NETCONF Extensions to Support the Network Management Datastore Architecture&quot;">RFC8526</a>].  Thus, if such a server implements
   NETCONF notifications [<a href="./rfc5277" title="&quot;NETCONF Event Notifications&quot;">RFC5277</a>] and the "netconf-capability-change"
   notification [<a href="./rfc6470" title="&quot;Network Configuration Protocol (NETCONF) Base Notifications&quot;">RFC6470</a>], a "netconf-capability-change" notification is
   generated whenever the YANG library content identifier changes.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  YANG Library YANG Module</span>

   The "ietf-yang-library" YANG module imports definitions from the
   "ietf-yang-types" and "ietf-inet-types" modules defined in [<a href="./rfc6991" title="&quot;Common YANG Data Types&quot;">RFC6991</a>]
   and from the "ietf-datastores" module defined in [<a href="./rfc8342" title="&quot;Network Management Datastore Architecture (NMDA)&quot;">RFC8342</a>].  While
   the YANG module is defined using YANG version 1.1, the YANG library
   supports YANG modules written in any version of YANG.




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   &lt;CODE BEGINS&gt; file "ietf-yang-library@2019-01-04.yang"

   module ietf-yang-library {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-yang-library";
     prefix yanglib;

     import ietf-yang-types {
       prefix yang;
       reference
         "<a href="./rfc6991">RFC 6991</a>: Common YANG Data Types";
     }
     import ietf-inet-types {
       prefix inet;
       reference
         "<a href="./rfc6991">RFC 6991</a>: Common YANG Data Types";
     }
     import ietf-datastores {
       prefix ds;
       reference
         "<a href="./rfc8342">RFC 8342</a>: Network Management Datastore Architecture
                    (NMDA)";
     }

     organization
       "IETF NETCONF (Network Configuration) Working Group";
     contact
       "WG Web:   &lt;<a href="https://datatracker.ietf.org/wg/netconf/">https://datatracker.ietf.org/wg/netconf/</a>&gt;
        WG List:  &lt;mailto:netconf@ietf.org&gt;

        Author:   Andy Bierman
                  &lt;mailto:andy@yumaworks.com&gt;

        Author:   Martin Bjorklund
                  &lt;mailto:mbj@tail-f.com&gt;

        Author:   Juergen Schoenwaelder
                  &lt;mailto:j.schoenwaelder@jacobs-university.de&gt;

        Author:   Kent Watsen
                  &lt;mailto:kent+ietf@watsen.net&gt;

        Author:   Robert Wilton
                  &lt;mailto:rwilton@cisco.com&gt;";
     description
       "This module provides information about the YANG modules,
        datastores, and datastore schemas used by a network
        management server.



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        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 <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a> (<a href="./rfc2119">RFC 2119</a>) (<a href="./rfc8174">RFC 8174</a>) when, and only when,
        they appear in all capitals, as shown here.

        Copyright (c) 2019 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 Simplified BSD License
        set forth in <a href="#section-4">Section 4</a>.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents
        (<a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a>).

        This version of this YANG module is part of <a href="./rfc8525">RFC 8525</a>; see
        the RFC itself for full legal notices.";

     revision 2019-01-04 {
       description
         "Added support for multiple datastores according to the
          Network Management Datastore Architecture (NMDA).";
       reference
         "<a href="./rfc8525">RFC 8525</a>: YANG Library";
     }
     revision 2016-04-09 {
       description
         "Initial revision.";
       reference
         "<a href="./rfc7895">RFC 7895</a>: YANG Module Library";
     }

     /*
      * Typedefs
      */

     typedef revision-identifier {
       type string {
         pattern '\d{4}-\d{2}-\d{2}';
       }
       description
         "Represents a specific date in YYYY-MM-DD format.";
     }

     /*
      * Groupings
      */



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     grouping module-identification-leafs {
       description
         "Parameters for identifying YANG modules and submodules.";
       leaf name {
         type yang:yang-identifier;
         mandatory true;
         description
           "The YANG module or submodule name.";
       }
       leaf revision {
         type revision-identifier;
         description
           "The YANG module or submodule revision date.  If no revision
            statement is present in the YANG module or submodule, this
            leaf is not instantiated.";
       }
     }

     grouping location-leaf-list {
       description
         "Common leaf-list parameter for the locations of modules and
          submodules.";
       leaf-list location {
         type inet:uri;
         description
           "Contains a URL that represents the YANG schema
            resource for this module or submodule.

            This leaf will only be present if there is a URL
            available for retrieval of the schema for this entry.";
       }
     }

     grouping module-implementation-parameters {
       description
         "Parameters for describing the implementation of a module.";
       leaf-list feature {
         type yang:yang-identifier;
         description
           "List of all YANG feature names from this module that are
            supported by the server, regardless whether they are defined
            in the module or any included submodule.";
       }
       leaf-list deviation {
         type leafref {
           path "../../module/name";
         }




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         description
           "List of all YANG deviation modules used by this server to
            modify the conformance of the module associated with this
            entry.  Note that the same module can be used for deviations
            for multiple modules, so the same entry MAY appear within
            multiple 'module' entries.

            This reference MUST NOT (directly or indirectly)
            refer to the module being deviated.

            Robust clients may want to make sure that they handle a
            situation where a module deviates itself (directly or
            indirectly) gracefully.";
       }
     }

     grouping module-set-parameters {
       description
         "A set of parameters that describe a module set.";
       leaf name {
         type string;
         description
           "An arbitrary name of the module set.";
       }
       list module {
         key "name";
         description
           "An entry in this list represents a module implemented by the
            server, as per <a href="./rfc7950#section-5.6.5">Section&nbsp;5.6.5 of RFC 7950</a>, with a particular
            set of supported features and deviations.";
         reference
           "<a href="./rfc7950">RFC 7950</a>: The YANG 1.1 Data Modeling Language";
         uses module-identification-leafs;
         leaf namespace {
           type inet:uri;
           mandatory true;
           description
             "The XML namespace identifier for this module.";
         }
         uses location-leaf-list;
         list submodule {
           key "name";
           description
             "Each entry represents one submodule within the
              parent module.";
           uses module-identification-leafs;
           uses location-leaf-list;
         }



<span class="grey">Bierman, et al.              Standards Track                   [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


         uses module-implementation-parameters;
       }
       list import-only-module {
         key "name revision";
         description
           "An entry in this list indicates that the server imports
            reusable definitions from the specified revision of the
            module but does not implement any protocol-accessible
            objects from this revision.

            Multiple entries for the same module name MAY exist.  This
            can occur if multiple modules import the same module but
            specify different revision dates in the import statements.";
         leaf name {
           type yang:yang-identifier;
           description
             "The YANG module name.";
         }
         leaf revision {
           type union {
             type revision-identifier;
             type string {
               length "0";
             }
           }
           description
             "The YANG module revision date.
              A zero-length string is used if no revision statement
              is present in the YANG module.";
         }
         leaf namespace {
           type inet:uri;
           mandatory true;
           description
             "The XML namespace identifier for this module.";
         }
         uses location-leaf-list;
         list submodule {
           key "name";
           description
             "Each entry represents one submodule within the
              parent module.";
           uses module-identification-leafs;
           uses location-leaf-list;
         }
       }
     }




<span class="grey">Bierman, et al.              Standards Track                   [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


     grouping yang-library-parameters {
       description
         "The YANG library data structure is represented as a grouping
          so it can be reused in configuration or another monitoring
          data structure.";
       list module-set {
         key "name";
         description
           "A set of modules that may be used by one or more schemas.

            A module set does not have to be referentially complete,
            i.e., it may define modules that contain import statements
            for other modules not included in the module set.";
         uses module-set-parameters;
       }
       list schema {
         key "name";
         description
           "A datastore schema that may be used by one or more
            datastores.

            The schema must be valid and referentially complete, i.e.,
            it must contain modules to satisfy all used import
            statements for all modules specified in the schema.";
         leaf name {
           type string;
           description
             "An arbitrary name of the schema.";
         }
         leaf-list module-set {
           type leafref {
             path "../../module-set/name";
           }
           description
             "A set of module-sets that are included in this schema.
              If a non-import-only module appears in multiple module
              sets, then the module revision and the associated features
              and deviations must be identical.";
         }
       }
       list datastore {
         key "name";
         description
           "A datastore supported by this server.

            Each datastore indicates which schema it supports.





<span class="grey">Bierman, et al.              Standards Track                   [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


            The server MUST instantiate one entry in this list per
            specific datastore it supports.
            Each datastore entry with the same datastore schema SHOULD
            reference the same schema.";
         leaf name {
           type ds:datastore-ref;
           description
             "The identity of the datastore.";
         }
         leaf schema {
           type leafref {
             path "../../schema/name";
           }
           mandatory true;
           description
             "A reference to the schema supported by this datastore.
              All non-import-only modules of the schema are implemented
              with their associated features and deviations.";
         }
       }
     }

     /*
      * Top-level container
      */

     container yang-library {
       config false;
       description
         "Container holding the entire YANG library of this server.";
       uses yang-library-parameters;
       leaf content-id {
         type string;
         mandatory true;
         description
           "A server-generated identifier of the contents of the
            '/yang-library' tree.  The server MUST change the value of
            this leaf if the information represented by the
            '/yang-library' tree, except '/yang-library/content-id', has
            changed.";
       }
     }

     /*
      * Notifications
      */

     notification yang-library-update {



<span class="grey">Bierman, et al.              Standards Track                   [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


       description
         "Generated when any YANG library information on the
          server has changed.";
       leaf content-id {
         type leafref {
           path "/yanglib:yang-library/yanglib:content-id";
         }
         mandatory true;
         description
           "Contains the YANG library content identifier for the updated
            YANG library at the time the notification is generated.";
       }
     }

     /*
      * Legacy groupings
      */

     grouping module-list {
       status deprecated;
       description
         "The module data structure is represented as a grouping
          so it can be reused in configuration or another monitoring
          data structure.";

       grouping common-leafs {
         status deprecated;
         description
           "Common parameters for YANG modules and submodules.";
         leaf name {
           type yang:yang-identifier;
           status deprecated;
           description
             "The YANG module or submodule name.";
         }
         leaf revision {
           type union {
             type revision-identifier;
             type string {
               length "0";
             }
           }
           status deprecated;
           description
             "The YANG module or submodule revision date.
              A zero-length string is used if no revision statement
              is present in the YANG module or submodule.";
         }



<span class="grey">Bierman, et al.              Standards Track                   [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


       }

       grouping schema-leaf {
         status deprecated;
         description
           "Common schema leaf parameter for modules and submodules.";
         leaf schema {
           type inet:uri;
           description
             "Contains a URL that represents the YANG schema
              resource for this module or submodule.

              This leaf will only be present if there is a URL
              available for retrieval of the schema for this entry.";
         }
       }
       list module {
         key "name revision";
         status deprecated;
         description
           "Each entry represents one revision of one module
            currently supported by the server.";
         uses common-leafs {
           status deprecated;
         }
         uses schema-leaf {
           status deprecated;
         }
         leaf namespace {
           type inet:uri;
           mandatory true;
           status deprecated;
           description
             "The XML namespace identifier for this module.";
         }
         leaf-list feature {
           type yang:yang-identifier;
           status deprecated;
           description
             "List of YANG feature names from this module that are
              supported by the server, regardless of whether they are
              defined in the module or any included submodule.";
         }
         list deviation {
           key "name revision";
           status deprecated;





<span class="grey">Bierman, et al.              Standards Track                   [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


           description
             "List of YANG deviation module names and revisions
              used by this server to modify the conformance of
              the module associated with this entry.  Note that
              the same module can be used for deviations for
              multiple modules, so the same entry MAY appear
              within multiple 'module' entries.

              The deviation module MUST be present in the 'module'
              list, with the same name and revision values.
              The 'conformance-type' value will be 'implement' for
              the deviation module.";
           uses common-leafs {
             status deprecated;
           }
         }
         leaf conformance-type {
           type enumeration {
             enum implement {
               description
                 "Indicates that the server implements one or more
                  protocol-accessible objects defined in the YANG module
                  identified in this entry.  This includes deviation
                  statements defined in the module.

                  For YANG version 1.1 modules, there is at most one
                  'module' entry with conformance type 'implement' for a
                  particular module name, since YANG 1.1 requires that
                  at most one revision of a module is implemented.

                  For YANG version 1 modules, there SHOULD NOT be more
                  than one 'module' entry for a particular module
                  name.";
             }
             enum import {
               description
                 "Indicates that the server imports reusable definitions
                  from the specified revision of the module but does
                  not implement any protocol-accessible objects from
                  this revision.

                  Multiple 'module' entries for the same module name MAY
                  exist.  This can occur if multiple modules import the
                  same module but specify different revision dates in
                  the import statements.";
             }
           }
           mandatory true;



<span class="grey">Bierman, et al.              Standards Track                   [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


           status deprecated;
           description
             "Indicates the type of conformance the server is claiming
              for the YANG module identified by this entry.";
         }
         list submodule {
           key "name revision";
           status deprecated;
           description
             "Each entry represents one submodule within the
              parent module.";
           uses common-leafs {
             status deprecated;
           }
           uses schema-leaf {
             status deprecated;
           }
         }
       }
     }

     /*
      * Legacy operational state data nodes
      */

     container modules-state {
       config false;
       status deprecated;
       description
         "Contains YANG module monitoring information.";
       leaf module-set-id {
         type string;
         mandatory true;
         status deprecated;
         description
           "Contains a server-specific identifier representing
            the current set of modules and submodules.  The
            server MUST change the value of this leaf if the
            information represented by the 'module' list instances
            has changed.";
       }
       uses module-list {
         status deprecated;
       }
     }

     /*
      * Legacy notifications



<span class="grey">Bierman, et al.              Standards Track                   [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


      */

     notification yang-library-change {
       status deprecated;
       description
         "Generated when the set of modules and submodules supported
          by the server has changed.";
       leaf module-set-id {
         type leafref {
           path "/yanglib:modules-state/yanglib:module-set-id";
         }
         mandatory true;
         status deprecated;
         description
           "Contains the module-set-id value representing the
            set of modules and submodules supported at the server
            at the time the notification is generated.";
       }
     }
   }


   &lt;CODE ENDS&gt;

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  IANA Considerations</span>

   <a href="./rfc7895">RFC 7895</a> previously registered one URI in the "IETF XML Registry"
   [<a href="./rfc3688" title="&quot;The IETF XML Registry&quot;">RFC3688</a>].  This document takes over this registration entry made by
   <a href="./rfc7895">RFC 7895</a> and changes the Registrant Contact to the IESG according to
   <a href="./rfc3688#section-4">Section&nbsp;4 of [RFC3688]</a>.

     URI: urn:ietf:params:xml:ns:yang:ietf-yang-library

     Registrant Contact: The IESG.

     XML: N/A, the requested URI is an XML namespace.

   This document registers a YANG module in the "YANG Module Names"
   registry [<a href="./rfc6020" title="&quot;YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)&quot;">RFC6020</a>]:

     name:         ietf-yang-library
     namespace:    urn:ietf:params:xml:ns:yang:ietf-yang-library
     prefix:       yanglib
     reference:    <a href="./rfc8525">RFC 8525</a>







<span class="grey">Bierman, et al.              Standards Track                   [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Security Considerations</span>

   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 [<a href="./rfc6241" title="&quot;Network Configuration Protocol (NETCONF)&quot;">RFC6241</a>] or RESTCONF [<a href="./rfc8040" title="&quot;RESTCONF Protocol&quot;">RFC8040</a>].  The lowest NETCONF layer
   is the secure transport layer, and the mandatory-to-implement secure
   transport is Secure Shell (SSH) [<a href="./rfc6242" title="&quot;Using the NETCONF Protocol over Secure Shell (SSH)&quot;">RFC6242</a>].  The lowest RESTCONF layer
   is HTTPS, and the mandatory-to-implement secure transport is TLS
   [<a href="./rfc8446" title="&quot;The Transport Layer Security (TLS) Protocol Version 1.3&quot;">RFC8446</a>].

   The Network Configuration Access Control Model (NACM) [<a href="./rfc8341" title="&quot;Network Configuration Access Control Model&quot;">RFC8341</a>]
   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.

   Some of the readable data nodes in this YANG module may be considered
   sensitive or vulnerable in some network environments.  It is thus
   important to control read access (e.g., via get, get-config, or
   notification) to these data nodes.  These are the subtrees and data
   nodes and their sensitivity/vulnerability:

   The "/yang-library" subtree of the YANG library may help an attacker
   identify the server capabilities and server implementations with
   known bugs since the set of YANG modules supported by a server may
   reveal the kind of device and the manufacturer of the device.
   Although some of this information may be available to all NETCONF
   users via the NETCONF &lt;hello&gt; message (or similar messages in other
   management protocols), this YANG module potentially exposes
   additional details that could be of some assistance to an attacker.
   Server vulnerabilities may be specific to particular modules, module
   revisions, module features, or even module deviations.  For example,
   if a particular operation on a particular data node is known to cause
   a server to crash or significantly degrade device performance, then
   the "module" list information will help an attacker identify server
   implementations with such a defect, in order to launch a denial-of-
   service attack on the device.















<span class="grey">Bierman, et al.              Standards Track                   [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  References</span>

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  Normative References</span>

   [<a id="ref-RFC2119">RFC2119</a>]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
              DOI 10.17487/RFC2119, March 1997,
              &lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;.

   [<a id="ref-RFC3688">RFC3688</a>]  Mealling, M., "The IETF XML Registry", <a href="https://www.rfc-editor.org/bcp/bcp81">BCP 81</a>, <a href="./rfc3688">RFC 3688</a>,
              DOI 10.17487/RFC3688, January 2004,
              &lt;<a href="https://www.rfc-editor.org/info/rfc3688">https://www.rfc-editor.org/info/rfc3688</a>&gt;.

   [<a id="ref-RFC6020">RFC6020</a>]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", <a href="./rfc6020">RFC 6020</a>,
              DOI 10.17487/RFC6020, October 2010,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6020">https://www.rfc-editor.org/info/rfc6020</a>&gt;.

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

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

   [<a id="ref-RFC6991">RFC6991</a>]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              <a href="./rfc6991">RFC 6991</a>, DOI 10.17487/RFC6991, July 2013,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6991">https://www.rfc-editor.org/info/rfc6991</a>&gt;.

   [<a id="ref-RFC7950">RFC7950</a>]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              <a href="./rfc7950">RFC 7950</a>, DOI 10.17487/RFC7950, August 2016,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7950">https://www.rfc-editor.org/info/rfc7950</a>&gt;.

   [<a id="ref-RFC8040">RFC8040</a>]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", <a href="./rfc8040">RFC 8040</a>, DOI 10.17487/RFC8040, January 2017,
              &lt;<a href="https://www.rfc-editor.org/info/rfc8040">https://www.rfc-editor.org/info/rfc8040</a>&gt;.

   [<a id="ref-RFC8174">RFC8174</a>]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in <a href="./rfc2119">RFC</a>
              <a href="./rfc2119">2119</a> Key Words", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc8174">RFC 8174</a>, DOI 10.17487/RFC8174,
              May 2017, &lt;<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>&gt;.

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




<span class="grey">Bierman, et al.              Standards Track                   [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


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

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

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  Informative References</span>

   [<a id="ref-RFC5277">RFC5277</a>]  Chisholm, S. and H. Trevino, "NETCONF Event
              Notifications", <a href="./rfc5277">RFC 5277</a>, DOI 10.17487/RFC5277, July 2008,
              &lt;<a href="https://www.rfc-editor.org/info/rfc5277">https://www.rfc-editor.org/info/rfc5277</a>&gt;.

   [<a id="ref-RFC6470">RFC6470</a>]  Bierman, A., "Network Configuration Protocol (NETCONF)
              Base Notifications", <a href="./rfc6470">RFC 6470</a>, DOI 10.17487/RFC6470,
              February 2012, &lt;<a href="https://www.rfc-editor.org/info/rfc6470">https://www.rfc-editor.org/info/rfc6470</a>&gt;.

   [<a id="ref-RFC7895">RFC7895</a>]  Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module
              Library", <a href="./rfc7895">RFC 7895</a>, DOI 10.17487/RFC7895, June 2016,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7895">https://www.rfc-editor.org/info/rfc7895</a>&gt;.

   [<a id="ref-RFC8340">RFC8340</a>]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              <a href="https://www.rfc-editor.org/bcp/bcp215">BCP 215</a>, <a href="./rfc8340">RFC 8340</a>, DOI 10.17487/RFC8340, March 2018,
              &lt;<a href="https://www.rfc-editor.org/info/rfc8340">https://www.rfc-editor.org/info/rfc8340</a>&gt;.

   [<a id="ref-RFC8343">RFC8343</a>]  Bjorklund, M., "A YANG Data Model for Interface
              Management", <a href="./rfc8343">RFC 8343</a>, DOI 10.17487/RFC8343, March 2018,
              &lt;<a href="https://www.rfc-editor.org/info/rfc8343">https://www.rfc-editor.org/info/rfc8343</a>&gt;.

   [<a id="ref-RFC8344">RFC8344</a>]  Bjorklund, M., "A YANG Data Model for IP Management",
              <a href="./rfc8344">RFC 8344</a>, DOI 10.17487/RFC8344, March 2018,
              &lt;<a href="https://www.rfc-editor.org/info/rfc8344">https://www.rfc-editor.org/info/rfc8344</a>&gt;.

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

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







<span class="grey">Bierman, et al.              Standards Track                   [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


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

   [<a id="ref-RFC8526">RFC8526</a>]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "NETCONF Extensions to Support the Network
              Management Datastore Architecture", <a href="./rfc8526">RFC 8526</a>,
              DOI 10.17487/RFC8526, March 2019,
              &lt;<a href="https://www.rfc-editor.org/info/rfc8526">https://www.rfc-editor.org/info/rfc8526</a>&gt;.

   [<a id="ref-RFC8528">RFC8528</a>]  Bjorklund, M. and L. Lhotka, "YANG Schema Mount",
              <a href="./rfc8528">RFC 8528</a>, DOI 10.17487/RFC8528, March 2019,
              &lt;<a href="https://www.rfc-editor.org/info/rfc8528">https://www.rfc-editor.org/info/rfc8528</a>&gt;.





































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<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Summary of Changes from <a href="./rfc7895">RFC 7895</a></span>

   This document changes [<a href="./rfc7895" title="&quot;YANG Module Library&quot;">RFC7895</a>] in the following ways:

   o  Renamed document title from "YANG Module Library" to "YANG
      Library".

   o  Added a new top-level "/yang-library" container to hold the entire
      YANG library providing information about module sets, schemas, and
      datastores.

   o  Refactored the "/modules-state" container into a new
      "/yang-library/module-set" list.

   o  Added a new "/yang-library/schema" list and a new "/yang-library/
      datastore" list.

   o  Added a set of new groupings as replacements for the deprecated
      groupings.

   o  Added a "yang-library-update" notification as a replacement for
      the deprecated "yang-library-change" notification.

   o  Deprecated the "/modules-state" tree.

   o  Deprecated the "/module-list" grouping.

   o  Deprecated the "/yang-library-change" notification.

<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Example YANG Library Instance for a Basic Server</span>

   The following example shows the YANG library of a basic server
   implementing the "ietf-interfaces" [<a href="./rfc8343" title="&quot;A YANG Data Model for Interface Management&quot;">RFC8343</a>] and "ietf-ip" [<a href="./rfc8344" title="&quot;A YANG Data Model for IP Management&quot;">RFC8344</a>]
   modules in the &lt;running&gt;, &lt;startup&gt;, and &lt;operational&gt; datastores and
   the "ietf-hardware" [<a href="./rfc8348" title="&quot;A YANG Data Model for Hardware Management&quot;">RFC8348</a>] module in the &lt;operational&gt; datastore.

   Newline characters in leaf values are added for formatting reasons.

   &lt;yang-library
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
       xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores"&gt;

     &lt;module-set&gt;
       &lt;name&gt;config-modules&lt;/name&gt;
       &lt;module&gt;
         &lt;name&gt;ietf-interfaces&lt;/name&gt;
         &lt;revision&gt;2018-02-20&lt;/revision&gt;
         &lt;namespace&gt;



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<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


           urn:ietf:params:xml:ns:yang:ietf-interfaces
         &lt;/namespace&gt;
       &lt;/module&gt;
       &lt;module&gt;
         &lt;name&gt;ietf-ip&lt;/name&gt;
         &lt;revision&gt;2018-02-22&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-ip
         &lt;/namespace&gt;
       &lt;/module&gt;
       &lt;import-only-module&gt;
         &lt;name&gt;ietf-yang-types&lt;/name&gt;
         &lt;revision&gt;2013-07-15&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-yang-types
         &lt;/namespace&gt;
       &lt;/import-only-module&gt;
       &lt;import-only-module&gt;
         &lt;name&gt;ietf-inet-types&lt;/name&gt;
         &lt;revision&gt;2013-07-15&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-inet-types
         &lt;/namespace&gt;
       &lt;/import-only-module&gt;
     &lt;/module-set&gt;

     &lt;module-set&gt;
       &lt;name&gt;state-modules&lt;/name&gt;
       &lt;module&gt;
         &lt;name&gt;ietf-hardware&lt;/name&gt;
         &lt;revision&gt;2018-03-13&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-hardware
         &lt;/namespace&gt;
       &lt;/module&gt;
       &lt;import-only-module&gt;
         &lt;name&gt;ietf-inet-types&lt;/name&gt;
         &lt;revision&gt;2013-07-15&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-inet-types
         &lt;/namespace&gt;
       &lt;/import-only-module&gt;
       &lt;import-only-module&gt;
         &lt;name&gt;ietf-yang-types&lt;/name&gt;
         &lt;revision&gt;2013-07-15&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-yang-types
         &lt;/namespace&gt;



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<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


       &lt;/import-only-module&gt;
       &lt;import-only-module&gt;
         &lt;name&gt;iana-hardware&lt;/name&gt;
         &lt;revision&gt;2018-03-13&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:iana-hardware
         &lt;/namespace&gt;
       &lt;/import-only-module&gt;
     &lt;/module-set&gt;

     &lt;schema&gt;
       &lt;name&gt;config-schema&lt;/name&gt;
       &lt;module-set&gt;config-modules&lt;/module-set&gt;
     &lt;/schema&gt;
     &lt;schema&gt;
       &lt;name&gt;state-schema&lt;/name&gt;
       &lt;module-set&gt;config-modules&lt;/module-set&gt;
       &lt;module-set&gt;state-modules&lt;/module-set&gt;
     &lt;/schema&gt;

     &lt;datastore&gt;
       &lt;name&gt;ds:startup&lt;/name&gt;
       &lt;schema&gt;config-schema&lt;/schema&gt;
     &lt;/datastore&gt;
     &lt;datastore&gt;
       &lt;name&gt;ds:running&lt;/name&gt;
       &lt;schema&gt;config-schema&lt;/schema&gt;
     &lt;/datastore&gt;
     &lt;datastore&gt;
       &lt;name&gt;ds:operational&lt;/name&gt;
       &lt;schema&gt;state-schema&lt;/schema&gt;
     &lt;/datastore&gt;

     &lt;content-id&gt;75a43df9bd56b92aacc156a2958fbe12312fb285&lt;/content-id&gt;
   &lt;/yang-library&gt;

<span class="h2"><a class="selflink" id="appendix-C" href="#appendix-C">Appendix C</a>.  Example YANG Library Instance for an Advanced Server</span>

   The following example extends the example in <a href="#appendix-B">Appendix B</a> by using
   modules from [<a href="./rfc8345" title="&quot;A YANG Data Model for Network Topologies&quot;">RFC8345</a>] and [<a href="./rfc8349" title="&quot;A YANG Data Model for Routing Management (NMDA Version)&quot;">RFC8349</a>] to illustrate a slightly more
   advanced server that:

   o  Has a module with features only enabled in &lt;operational&gt;; the
      "ietf-routing" module is supported in &lt;running&gt;, &lt;startup&gt;, and
      &lt;operational&gt;, but the "multiple-ribs" and "router-id" features
      are only enabled in &lt;operational&gt;.  Hence, the "router-id" leaf
      may be read but not configured.




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<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


   o  Supports a dynamic configuration datastore "example-ds-ephemeral",
      with only the "ietf-network" and "ietf-network-topology" modules
      configurable via a notional dynamic configuration protocol.

   o  Shows an example of datastore-specific deviations.  The
      "example-vendor-hardware-deviations" module is included in the
      schema for &lt;operational&gt; to remove data nodes that cannot be
      supported by the server.

   o  Shows how module-sets can be used to organize related modules
      together.

   Newline characters in leaf values are added for formatting reasons.

   &lt;yang-library
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
       xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores"
       xmlns:ex-ds-eph="urn:example:ds-ephemeral"&gt;

     &lt;module-set&gt;
       &lt;name&gt;config-state-modules&lt;/name&gt;
       &lt;module&gt;
         &lt;name&gt;ietf-interfaces&lt;/name&gt;
         &lt;revision&gt;2018-02-20&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-interfaces
         &lt;/namespace&gt;
       &lt;/module&gt;
       &lt;module&gt;
         &lt;name&gt;ietf-ip&lt;/name&gt;
         &lt;revision&gt;2018-02-22&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-ip
         &lt;/namespace&gt;
       &lt;/module&gt;
       &lt;module&gt;
         &lt;name&gt;ietf-routing&lt;/name&gt;
         &lt;revision&gt;2018-03-13&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-routing
         &lt;/namespace&gt;
       &lt;/module&gt;
       &lt;import-only-module&gt;
         &lt;name&gt;ietf-yang-types&lt;/name&gt;
         &lt;revision&gt;2013-07-15&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-yang-types
         &lt;/namespace&gt;



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<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


       &lt;/import-only-module&gt;
       &lt;import-only-module&gt;
         &lt;name&gt;ietf-inet-types&lt;/name&gt;
         &lt;revision&gt;2013-07-15&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-inet-types
         &lt;/namespace&gt;
       &lt;/import-only-module&gt;
     &lt;/module-set&gt;

     &lt;module-set&gt;
       &lt;name&gt;config-only-modules&lt;/name&gt;
       &lt;module&gt;
         &lt;name&gt;ietf-routing&lt;/name&gt;
         &lt;revision&gt;2018-03-13&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-routing
         &lt;/namespace&gt;
       &lt;/module&gt;
     &lt;/module-set&gt;

     &lt;module-set&gt;
       &lt;name&gt;dynamic-config-state-modules&lt;/name&gt;
       &lt;module&gt;
         &lt;name&gt;ietf-network&lt;/name&gt;
         &lt;revision&gt;2018-02-26&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-network
         &lt;/namespace&gt;
       &lt;/module&gt;
       &lt;module&gt;
         &lt;name&gt;ietf-network-topology&lt;/name&gt;
         &lt;revision&gt;2018-02-26&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-network-topology
         &lt;/namespace&gt;
       &lt;/module&gt;
       &lt;import-only-module&gt;
         &lt;name&gt;ietf-inet-types&lt;/name&gt;
         &lt;revision&gt;2013-07-15&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-inet-types
         &lt;/namespace&gt;
       &lt;/import-only-module&gt;
     &lt;/module-set&gt;

     &lt;module-set&gt;
       &lt;name&gt;state-only-modules&lt;/name&gt;



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<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


       &lt;module&gt;
         &lt;name&gt;ietf-hardware&lt;/name&gt;
         &lt;revision&gt;2018-03-13&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-hardware
         &lt;/namespace&gt;
         &lt;deviation&gt;example-vendor-hardware-deviations&lt;/deviation&gt;
       &lt;/module&gt;
       &lt;module&gt;
         &lt;name&gt;ietf-routing&lt;/name&gt;
         &lt;revision&gt;2018-03-13&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-routing
         &lt;/namespace&gt;
         &lt;feature&gt;multiple-ribs&lt;/feature&gt;
         &lt;feature&gt;router-id&lt;/feature&gt;
       &lt;/module&gt;
       &lt;module&gt;
         &lt;name&gt;example-vendor-hardware-deviations&lt;/name&gt;
         &lt;revision&gt;2018-01-31&lt;/revision&gt;
         &lt;namespace&gt;
           urn:example:example-vendor-hardware-deviations
         &lt;/namespace&gt;
       &lt;/module&gt;
       &lt;import-only-module&gt;
         &lt;name&gt;ietf-inet-types&lt;/name&gt;
         &lt;revision&gt;2013-07-15&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-inet-types
         &lt;/namespace&gt;
       &lt;/import-only-module&gt;
       &lt;import-only-module&gt;
         &lt;name&gt;ietf-yang-types&lt;/name&gt;
         &lt;revision&gt;2013-07-15&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:ietf-yang-types
         &lt;/namespace&gt;
       &lt;/import-only-module&gt;
       &lt;import-only-module&gt;
         &lt;name&gt;iana-hardware&lt;/name&gt;
         &lt;revision&gt;2018-03-13&lt;/revision&gt;
         &lt;namespace&gt;
           urn:ietf:params:xml:ns:yang:iana-hardware
         &lt;/namespace&gt;
       &lt;/import-only-module&gt;
     &lt;/module-set&gt;

     &lt;schema&gt;



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<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


       &lt;name&gt;config-schema&lt;/name&gt;
       &lt;module-set&gt;config-state-modules&lt;/module-set&gt;
       &lt;module-set&gt;config-only-modules&lt;/module-set&gt;
     &lt;/schema&gt;
     &lt;schema&gt;
       &lt;name&gt;dynamic-config-schema&lt;/name&gt;
       &lt;module-set&gt;dynamic-config-state-modules&lt;/module-set&gt;
     &lt;/schema&gt;
     &lt;schema&gt;
       &lt;name&gt;state-schema&lt;/name&gt;
       &lt;module-set&gt;config-state-modules&lt;/module-set&gt;
       &lt;module-set&gt;dynamic-config-state-modules&lt;/module-set&gt;
       &lt;module-set&gt;state-only-modules&lt;/module-set&gt;
     &lt;/schema&gt;

     &lt;datastore&gt;
       &lt;name&gt;ds:startup&lt;/name&gt;
       &lt;schema&gt;config-schema&lt;/schema&gt;
     &lt;/datastore&gt;
     &lt;datastore&gt;
       &lt;name&gt;ds:running&lt;/name&gt;
       &lt;schema&gt;config-schema&lt;/schema&gt;
     &lt;/datastore&gt;
     &lt;datastore&gt;
       &lt;name&gt;ex-ds-eph:ds-ephemeral&lt;/name&gt;
       &lt;schema&gt;dynamic-config-schema&lt;/schema&gt;
     &lt;/datastore&gt;
     &lt;datastore&gt;
       &lt;name&gt;ds:operational&lt;/name&gt;
       &lt;schema&gt;state-schema&lt;/schema&gt;
     &lt;/datastore&gt;

     &lt;content-id&gt;14782ab9bd56b92aacc156a2958fbe12312fb285&lt;/content-id&gt;
   &lt;/yang-library&gt;

















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<span class="grey"><a href="./rfc8525">RFC 8525</a>                      YANG Library                    March 2019</span>


Authors' Addresses

   Andy Bierman
   YumaWorks

   Email: andy@yumaworks.com


   Martin Bjorklund
   Tail-f Systems

   Email: mbj@tail-f.com


   Juergen Schoenwaelder
   Jacobs University

   Email: j.schoenwaelder@jacobs-university.de


   Kent Watsen
   Watsen Networks

   Email: kent+ietf@watsen.net


   Robert Wilton
   Cisco Systems

   Email: rwilton@cisco.com





















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