    Internet Engineering Task Force (IETF)                        A. Bierman
    Request for Comments: 8525                                     YumaWorks
    Obsoletes: 7895                                             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

                                  YANG Library

    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 RFC 7895.

    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 Section 2 of RFC 7841.

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

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    RFC 8525                      YANG Library                    March 2019

    Copyright Notice

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

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       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

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

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    1.  Introduction

       There is a need for a standard mechanism to expose which YANG modules
       [RFC7950], datastores [RFC8342], and datastore schemas [RFC8342] 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)
       [RFC8342].  The previous version of the YANG library, defined in
       [RFC7895], 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) <get> 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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    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 14 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

       The following terms are defined in [RFC7950]:

       o  module

       o  submodule

       o  data node

       This document uses the phrase "implement a module" as defined in
       Section 5.6.5 of [RFC7950].

       The following terms are defined in [RFC8342]:

       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 [RFC8340].

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    2.  Objectives

       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
           <operational>, even if it is implemented in some other datastore.
           This is required for transition purposes; a server that wants to
           implement <operational> should not have to implement all modules
           at once.

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    RFC 8525                      YANG Library                    March 2019

       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
           [RFC8528].

    3.  YANG Library Data Model

       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   |----------->|  set   |--------------->| 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*   -> ../../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*   -> ../../module-set/name
            +--ro datastore* [name]
            |  +--ro name      ds:datastore-ref
            |  +--ro schema    -> ../../schema/name
            +--ro content-id    string

         notifications:
           +---n yang-library-update
              +--ro content-id    -> /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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    RFC 8525                      YANG Library                    March 2019

          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 [RFC8526], a change of the YANG library content
       identifier results in a new value for the :yang-library:1.1
       capability defined in [RFC8526].  Thus, if such a server implements
       NETCONF notifications [RFC5277] and the "netconf-capability-change"
       notification [RFC6470], a "netconf-capability-change" notification is
       generated whenever the YANG library content identifier changes.

    4.  YANG Library YANG Module

       The "ietf-yang-library" YANG module imports definitions from the
       "ietf-yang-types" and "ietf-inet-types" modules defined in [RFC6991]
       and from the "ietf-datastores" module defined in [RFC8342].  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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       <CODE BEGINS> 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
             "RFC 6991: Common YANG Data Types";
         }
         import ietf-inet-types {
           prefix inet;
           reference
             "RFC 6991: Common YANG Data Types";
         }
         import ietf-datastores {
           prefix ds;
           reference
             "RFC 8342: Network Management Datastore Architecture
                        (NMDA)";
         }

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

            Author:   Andy Bierman
                      <mailto:andy@yumaworks.com>

            Author:   Martin Bjorklund
                      <mailto:mbj@tail-f.com>

            Author:   Juergen Schoenwaelder
                      <mailto:j.schoenwaelder@jacobs-university.de>

            Author:   Kent Watsen
                      <mailto:kent+ietf@watsen.net>

            Author:   Robert Wilton
                      <mailto:rwilton@cisco.com>";
         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 BCP 14 (RFC 2119) (RFC 8174) 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 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 8525; 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
             "RFC 8525: YANG Library";
         }
         revision 2016-04-09 {
           description
             "Initial revision.";
           reference
             "RFC 7895: 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 Section 5.6.5 of RFC 7950, with a particular
                set of supported features and deviations.";
             reference
               "RFC 7950: 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;
             }

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             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;
             }
           }
         }

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         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.

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                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 {

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           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.";
             }

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           }

           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;

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               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;

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               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

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          */

         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.";
           }
         }
       }

       <CODE ENDS>

    5.  IANA Considerations

       RFC 7895 previously registered one URI in the "IETF XML Registry"
       [RFC3688].  This document takes over this registration entry made by
       RFC 7895 and changes the Registrant Contact to the IESG according to
       Section 4 of [RFC3688].

         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 [RFC6020]:

         name:         ietf-yang-library
         namespace:    urn:ietf:params:xml:ns:yang:ietf-yang-library
         prefix:       yanglib
         reference:    RFC 8525

    Bierman, et al.              Standards Track                   [Page 20]

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    6.  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.

       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 <hello> 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.

    Bierman, et al.              Standards Track                   [Page 21]

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    7.  References

    7.1.  Normative 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>.

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

       [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>.

       [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
                  RFC 6991, DOI 10.17487/RFC6991, July 2013,
                  <https://www.rfc-editor.org/info/rfc6991>.

       [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>.

       [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>.

       [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>.

    Bierman, et al.              Standards Track                   [Page 22]

    RFC 8525                      YANG Library                    March 2019

       [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>.

    7.2.  Informative References

       [RFC5277]  Chisholm, S. and H. Trevino, "NETCONF Event
                  Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008,
                  <https://www.rfc-editor.org/info/rfc5277>.

       [RFC6470]  Bierman, A., "Network Configuration Protocol (NETCONF)
                  Base Notifications", RFC 6470, DOI 10.17487/RFC6470,
                  February 2012, <https://www.rfc-editor.org/info/rfc6470>.

       [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>.

       [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>.

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

       [RFC8344]  Bjorklund, M., "A YANG Data Model for IP Management",
                  RFC 8344, DOI 10.17487/RFC8344, March 2018,
                  <https://www.rfc-editor.org/info/rfc8344>.

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

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

    Bierman, et al.              Standards Track                   [Page 23]

    RFC 8525                      YANG Library                    March 2019

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

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

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

    Bierman, et al.              Standards Track                   [Page 24]

    RFC 8525                      YANG Library                    March 2019

    Appendix A.  Summary of Changes from RFC 7895

       This document changes [RFC7895] 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.

    Appendix B.  Example YANG Library Instance for a Basic Server

       The following example shows the YANG library of a basic server
       implementing the "ietf-interfaces" [RFC8343] and "ietf-ip" [RFC8344]
       modules in the <running>, <startup>, and <operational> datastores and
       the "ietf-hardware" [RFC8348] module in the <operational> datastore.

       Newline characters in leaf values are added for formatting reasons.

       <yang-library
           xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library"
           xmlns:ds="urn:ietf:params:xml:ns:yang:ietf-datastores">

         <module-set>
           <name>config-modules</name>
           <module>
             <name>ietf-interfaces</name>
             <revision>2018-02-20</revision>
             <namespace>

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

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

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           </import-only-module>
           <import-only-module>
             <name>iana-hardware</name>
             <revision>2018-03-13</revision>
             <namespace>
               urn:ietf:params:xml:ns:yang:iana-hardware
             </namespace>
           </import-only-module>
         </module-set>

         <schema>
           <name>config-schema</name>
           <module-set>config-modules</module-set>
         </schema>
         <schema>
           <name>state-schema</name>
           <module-set>config-modules</module-set>
           <module-set>state-modules</module-set>
         </schema>

         <datastore>
           <name>ds:startup</name>
           <schema>config-schema</schema>
         </datastore>
         <datastore>
           <name>ds:running</name>
           <schema>config-schema</schema>
         </datastore>
         <datastore>
           <name>ds:operational</name>
           <schema>state-schema</schema>
         </datastore>

         <content-id>75a43df9bd56b92aacc156a2958fbe12312fb285</content-id>
       </yang-library>

    Appendix C.  Example YANG Library Instance for an Advanced Server

       The following example extends the example in Appendix B by using
       modules from [RFC8345] and [RFC8349] to illustrate a slightly more
       advanced server that:

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

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       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 <operational> 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.

       <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">

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

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           </import-only-module>
           <import-only-module>
             <name>ietf-inet-types</name>
             <revision>2013-07-15</revision>
             <namespace>
               urn:ietf:params:xml:ns:yang:ietf-inet-types
             </namespace>
           </import-only-module>
         </module-set>

         <module-set>
           <name>config-only-modules</name>
           <module>
             <name>ietf-routing</name>
             <revision>2018-03-13</revision>
             <namespace>
               urn:ietf:params:xml:ns:yang:ietf-routing
             </namespace>
           </module>
         </module-set>

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

         <module-set>
           <name>state-only-modules</name>

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

         <schema>

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           <name>config-schema</name>
           <module-set>config-state-modules</module-set>
           <module-set>config-only-modules</module-set>
         </schema>
         <schema>
           <name>dynamic-config-schema</name>
           <module-set>dynamic-config-state-modules</module-set>
         </schema>
         <schema>
           <name>state-schema</name>
           <module-set>config-state-modules</module-set>
           <module-set>dynamic-config-state-modules</module-set>
           <module-set>state-only-modules</module-set>
         </schema>

         <datastore>
           <name>ds:startup</name>
           <schema>config-schema</schema>
         </datastore>
         <datastore>
           <name>ds:running</name>
           <schema>config-schema</schema>
         </datastore>
         <datastore>
           <name>ex-ds-eph:ds-ephemeral</name>
           <schema>dynamic-config-schema</schema>
         </datastore>
         <datastore>
           <name>ds:operational</name>
           <schema>state-schema</schema>
         </datastore>

         <content-id>14782ab9bd56b92aacc156a2958fbe12312fb285</content-id>
       </yang-library>

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    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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