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rfc:rfc8525

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.

Bierman, et al. Standards Track [Page 1] 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

Bierman, et al. Standards Track [Page 2] RFC 8525 YANG Library March 2019

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.

Bierman, et al. Standards Track [Page 3] RFC 8525 YANG Library March 2019

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].

Bierman, et al. Standards Track [Page 4] RFC 8525 YANG Library March 2019

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.

Bierman, et al. Standards Track [Page 5] 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:

Bierman, et al. Standards Track [Page 6] RFC 8525 YANG Library March 2019

 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

Bierman, et al. Standards Track [Page 7] 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.

Bierman, et al. Standards Track [Page 8] RFC 8525 YANG Library March 2019

 <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.

Bierman, et al. Standards Track [Page 9] RFC 8525 YANG Library March 2019

      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
    */

Bierman, et al. Standards Track [Page 10] RFC 8525 YANG Library March 2019

   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";
       }

Bierman, et al. Standards Track [Page 11] RFC 8525 YANG Library March 2019

       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;
       }

Bierman, et al. Standards Track [Page 12] RFC 8525 YANG Library March 2019

       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;
       }
     }
   }

Bierman, et al. Standards Track [Page 13] RFC 8525 YANG Library March 2019

   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.

Bierman, et al. Standards Track [Page 14] RFC 8525 YANG Library March 2019

          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 {

Bierman, et al. Standards Track [Page 15] RFC 8525 YANG Library March 2019

     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.";
       }

Bierman, et al. Standards Track [Page 16] RFC 8525 YANG Library March 2019

     }
     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;

Bierman, et al. Standards Track [Page 17] RFC 8525 YANG Library March 2019

         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;

Bierman, et al. Standards Track [Page 18] RFC 8525 YANG Library March 2019

         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

Bierman, et al. Standards Track [Page 19] RFC 8525 YANG Library March 2019

  • /
   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] RFC 8525 YANG Library March 2019

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] RFC 8525 YANG Library March 2019

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>

Bierman, et al. Standards Track [Page 25] RFC 8525 YANG Library March 2019

         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>

Bierman, et al. Standards Track [Page 26] RFC 8525 YANG Library March 2019

     </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.

Bierman, et al. Standards Track [Page 27] RFC 8525 YANG Library March 2019

 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>

Bierman, et al. Standards Track [Page 28] RFC 8525 YANG Library March 2019

     </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>

Bierman, et al. Standards Track [Page 29] RFC 8525 YANG Library March 2019

     <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>

Bierman, et al. Standards Track [Page 30] RFC 8525 YANG Library March 2019

     <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>

Bierman, et al. Standards Track [Page 31] RFC 8525 YANG Library March 2019

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

Bierman, et al. Standards Track [Page 32]

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