Network Working Group                                         A. Bierman
Internet-Draft                                                 YumaWorks
Obsoletes: rfc7895 (if approved)                            M. Bjorklund
Updates: rfc7950, rfc8040 (if approved)                   Tail-f Systems
Intended status: Standards Track                               K. Watsen                          Tail-f Systems
Expires: February 25, March 4, 2018                                         K. Watsen
                                                        Juniper Networks
                                                         August 24, 31, 2017

                              YANG Library
                    draft-ietf-netconf-rfc7895bis-00
                    draft-ietf-netconf-rfc7895bis-01

Abstract

   This document describes a YANG library that provides information
   about all the YANG modules used by a network management server (e.g.,
   a Network Configuration Protocol (NETCONF) server).  Simple caching
   mechanisms are provided to allow clients to minimize retrieval of
   this information.

Status of This Memo

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

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

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

   This Internet-Draft will expire on February 25, March 4, 2018.

Copyright Notice

   Copyright (c) 2017 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
   (http://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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Tree Diagrams . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Motivation for rfc7895bis . . . . . . . . . . . . . . . .   4
     1.4.  Summary of Changes from RFC 7895  . . . . . . . . . . . .   5
     1.5.  Summary of Updates to RFC 7950  . . . . . . . . . . . . .   6
     1.6.  Summary of Updates to RFC 8040  . . . . . . . . . . . . .   6
     1.7.  Open Issues . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  YANG Library  . . . . . . . . . . . . . . . . . . . . . . . .   7   5
     2.1.  yang-library  . . . . . . . . . . . . . . . . . . . . . .   9   6
       2.1.1.  yang-library/modules/module . . . . . . . . . . . . .   9   6
       2.1.2.  yang-library/module-sets/module-set . . . . . . . . .   9   7
       2.1.3.  yang-library/datastores/datastore . . . . . . . . . .   9   7
     2.2.  modules-state . . . . . . . . . . . . . . . . . . . . . .   9
       2.2.1.  modules-state/module-set-id . . . . . . . . . . . . .   9
       2.2.2.  modules-state/module  . . . . . . . . . . . . . . . .  10
     2.3.  YANG Library Module . . . . . . . . . . . . . . . . . . .  10   7
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20  18
     3.1.  YANG Module Registry  . . . . . . . . . . . . . . . . . .  20  18
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  21  18
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21  19
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21  19
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  21  19
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  23  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23  21

1.  Introduction

   There is a need for standard mechanisms to provide the operational
   state of the server.  This includes, for instance, identifying the
   YANG modules and datastores that are in use by a server and how they
   relate to each other.

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

   YANG library information can be different on every server and can
   change at runtime or across a server reboot.

   If the server implements multiple protocols to access the YANG-
   defined data, each such protocol has its own conceptual instantiation
   of the YANG library.

   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  identifier: a unique identifier for the module that includes the
      module's name, revision, submodules, features, and deviations.

   o  name: The name of the YANG module.

   o  revision: Each YANG module and submodule within the library has a
      revision.  This is derived from the most recent revision statement
      within the module or submodule.  If no such revision statement
      exists, the module's or submodule's revision is the zero-length
      string.

   o  submodule list: The name and revision of each submodule used by
      the module MUST be identified.

   o  feature list: The name of each YANG feature supported by the
      server, in a given context, MUST be identified.

   o  deviation list: The name of each YANG module used for deviation
      statements, in a given context, MUST be identified.

   The following information is needed by a client application (for each
   datastore supported by the server) to fully access all the YANG-
   modelled data available on the server:

   o  identity: the YANG identity for the datastore.

   o  properties: properties supported by the datastore.

   o  modules: modules supported by the datastore, including any
      features and deviations.

1.1.  Terminology

   The keywords "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].

   The following terms are defined in [RFC6241]:

   o  client

   o  server

   The following terms are defined in [RFC7950]:

   o  module

   o  submodule

   The following terms are used within this document:

   o  YANG library: A collection of metadata describing the server's
      operational state.

1.2.  Tree Diagrams

   A simplified graphical representation of the data model is used in
   this document.  The meaning of the symbols in these diagrams is as
   follows:

   o  Brackets "[" and "]" enclose list keys.

   o  Abbreviations before data node names: "rw" means configuration
      data (read-write) and "ro" state data (read-only).

   o  Symbols after data node names: "?" means an optional node, "!"
      means a presence container, and "*" denotes a list and leaf-list.

   o  Parentheses enclose choice and case nodes, and case nodes are also
      marked with a colon (":").

   o  Ellipsis ("...") stands for contents of subtrees that are not
      shown.

1.3.  Motivation for rfc7895bis

   RFC Ed.: delete this section, including this note, at time of
   publication.

   All NETCONF servers supporting YANG 1.1 [RFC7950] MUST support YANG
   Library (see Section 5.6.4 of RFC 7950).  Similarly, all RESTCONF
   servers MUST support YANG Library (see Section 10 of RFC 8040).
   These requirements are independent of if the server supports NMDA or
   not.

   RFC 7895 has a mandatory to implement 'modules-state' tree that a
   server uses to advertise all the modules it supports.  However, this
   module was designed assuming the all modules would be in all
   datastores, and with the same number of features and deviations.
   However, this is not the case with NMDA-compatible servers that may
   have some modules that only appear in <operational> (e.g., ietf-
   network-topo) or only also appear in a dynamic datastore (e.g., i2rs-
   ephemeral-rib).  It is also possible that a server only implements a
   module in <running>, at it hasn't yet coded support for returning the
   module's opstate yet.  Presumably, an NMDA-supporting server would
   return all modules implemented in every datastore, but this would be
   misleading to existing clients and unhelpful to NMDA-aware clients.

   In the end, it appears that the 'modules-state' node should be for
   non-NMDA aware clients.  For backwards compatability, an NMDA-
   supporting server SHOULD populate 'modules-state' in a backwards-
   compatible manner.  The new 'yang-library' node would be ignored by
   legacy clients, while providing all the data needed for NMDA-aware
   clients, which would themselves ignore the 'modules-state' tree.

   In addition to resolving the 'modules-state' node NMDA-
   incompatibility issue described above, the solution presented in this
   document is further motivated by the following desires:

   o  leverage Section 5.6.4 of RFC 7950 and Section 10 of RFC 8040.

   o  indicate which modules are supported by each datastore

   o  enable the features and deviations to vary by datastore

   o  structure extensible to support schema-mount

   o  provide a top-level container for all server metadata

1.4.  Summary of Changes from RFC 7895

   This document updates [RFC7895] in the following ways:

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

   o  Adds a new top-level "yang-library" container to hold many types
      of server metadata: modules supported, datastores supported,
      relationships between datastores and modules, etc.

   o  Deprecates the modules-state "modules-state" tree.

1.5.  Summary of Updates to RFC 7950

   This document updates [RFC7950] in

   o  Deprecates the following ways:

   1.  Section 5.6.4 says:

      A NETCONF server MUST announce "module-list" grouping.

   o  Deprecates the "yang-library-change" notification.

2.  YANG Library

   The "ietf-yang-library" module provides information about the modules it implements (see
      Section 5.6.5)
   used by implementing the YANG a server.  This module "ietf-yang-library" is defined in [RFC7895] and listing all implemented modules in using YANG version 1, but
   it supports the
      "/modules-state/module" list.

   This should be updated to allow for also listing all implemented description of YANG modules written in any revision
   of YANG.

   Following is the "/yang-library/modules/module" list or, more
   generally, use the entire YANG Library.

   2.  Section 5.6.4 also says:

      The parameter "module-set-id" has the same value as the leaf
      "/modules-state/module-set-id" from "ietf-yang-library".  This
      parameter MUST be present.

   This should be updated to say that, for NMDA-capable servers, this
   parameter has the same value as the leaf
   "/yang-library/module-sets/module-set/id", for the module-set that is
   used by <running>.

1.6.  Summary of Updates to RFC 8040

   This document updates [RFC8040] in the following ways:

   1.  Section 10.1 says that the "modules-state/module" list is
   mandatory.  This should be updated to allow for also listing all
   supported modules in the "/yang-library/modules/module" list or, more
   generally, use the entire YANG Library.

1.7.  Open Issues

   o  The per-datastore 'properties' idea is still being discussed.
      It's included here so as to provide something to point at.

   o  There's debate if there should be a list of module-sets or if
      instead each 'module-set' should be embeded into the datastore
      definition.  This discussion goes into if a datastore can
      reference more than one module-set.

2.  YANG Library

   The "ietf-yang-library" module provides information about the modules
   used by a server.  This module is defined using YANG version 1, but
   it supports the description of YANG modules written in any revision
   of YANG.

   Following is the YANG Tree Diagram Tree Diagram for the "ietf-yang-library"
   module, including excluding the deprecated 'modules-state' tree:

   module: ietf-yang-library
       +--ro yang-library
    |
          +--ro modules
          |  |  +--ro module* [id]
          |  |     +--ro id                  string
          |  |     +--ro name? name                yang:yang-identifier
          |  |     +--ro revision? revision            union
          |  |     +--ro schema?             inet:uri
          |  |     +--ro namespace           inet:uri
          |  |     +--ro feature*            yang:yang-identifier
          |  |     +--ro deviation* [name revision]
          |     |     |  +--ro name        yang:yang-identifier
          |     |     |  +--ro revision    union
          |  |     +--ro conformance-type    enumeration
          |  |     +--ro submodule* [name revision]
          |  |        +--ro name        yang:yang-identifier
          |  |        +--ro revision    union
          |  |        +--ro schema?     inet:uri
    |
          +--ro module-sets
          |  |  +--ro module-set*
    | [id]
          |     +--ro id? id        string
          |  |     +--ro module*   -> /yang-library/modules/module/id
    | ../../../modules/module/id
          +--ro datastores
          |  +--ro datastore* [name]
          |     +--ro name          identityref
          |     +--ro properties
    |        |  +--ro property*   identityref
    |        +--ro module-set?
          |             -> /yang-library/module-sets/module-set/id
    x--ro modules-state ../../../module-sets/module-set/id
          +--ro module-set-id checksum       string
       +--ro module* [name revision]
          +--ro name                yang:yang-identifier
          +--ro revision            union
          +--ro schema?             inet:uri
          +--ro namespace           inet:uri
          +--ro feature*            yang:yang-identifier
          +--ro deviation* [name revision]
          |  +--ro name        yang:yang-identifier
          |  +--ro revision    union
          +--ro conformance-type    enumeration
          +--ro submodule* [name revision]
             +--ro name        yang:yang-identifier
             +--ro revision    union
             +--ro schema?     inet:uri

     notifications:
       +---n yang-library-update

2.1.  yang-library

   This mandatory container holds all of the server's metadata.

2.1.1.  yang-library/modules/module

   This mandatory list contains one entry for each unique instance of a
   module in use by the server.  Each entry is distiguished by the
   module's name, revisions, features, and deviations.

2.1.2.  yang-library/module-sets/module-set

   This mandatory list contains one entry for each module-set in use by
   the server (e.g., presented by a datastore).

2.1.3.  yang-library/datastores/datastore

   This mandatory list contains one entry for each datastore supported
   by the server.  Each datastore entry both identifies any special
   propoerties it has and any module-sets it uses.

2.2.  modules-state

   This mandatory container holds the identifiers for the YANG data
   model modules supported by the server.

2.2.1.  modules-state/module-set-id

   This mandatory leaf contains a unique implementation-specific
   identifier representing the current set of modules and submodules on
   a specific server.  The value of this leaf MUST change whenever the
   set of modules and submodules in the YANG library changes.  There is
   no requirement that the same set always results in the same
   "module-set-id" value.

   This leaf allows a client to fetch the module list 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-change" notification, with the new value of
   "module-set-id".

   Note that for a NETCONF server that implements YANG 1.1 [RFC7950], a
   change of the "module-set-id" value results in a new value for the
   :yang-library capability defined in [RFC7950].  Thus, if such a
   server implements NETCONF notifications [RFC5277], and the
   notification "netconf-capability-change" [RFC6470], a
   "netconf-capability-change" notification is generated whenever the
   "module-set-id" changes.

2.2.2.  modules-state/module

   This mandatory list contains one entry for each YANG data model
   module supported by the server.  There MUST be an entry in this list
   for each revision of each YANG module that is used by the server.  It
   is possible for multiple revisions of the same module to be imported,
   in addition to an entry for the revision that is implemented by the
   server.

2.3. datastore entry both identifies any special
   propoerties it has and any module-sets it uses.

2.2.  YANG Library Module

   The "ietf-yang-library" module defines monitoring information for the
   YANG modules used by a server.

   The modules "ietf-yang-types" and "ietf-inet-types" from [RFC6991]
   and the module "ietf-datastores" from
   [I-D.ietf-netmod-revised-datastores] are used by this module for some
   type definitions.

   RFC Ed.: update the date below with the date of RFC publication and
   remove this note.

   <CODE BEGINS> file "ietf-yang-library@2017-07-03.yang" "ietf-yang-library@2017-08-17.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 "I-D.ietf-revised-datastoes: "I-D.ietf-revised-datastores:
                  Network Management Datastore Architecture.";
     }

     organization
       "IETF NETCONF (Network Configuration) Working Group";

     contact
       "WG Web:   <http://tools.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:   Kent Watsen
                  <mailto:kwatsen@juniper.net>";

     description
       "This module contains monitoring information about the YANG server
        instance, including the modules and submodules that datastores the
        server supports, and which modules are used within a YANG-based
      server. present in
        which datastores.

        Copyright (c) 2016 2017 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
        (http://trustee.ietf.org/license-info).

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

     // RFC Ed.: update the date below with the date of RFC publication
     // and remove this note.
     // RFC Ed.: replace XXXX with actual RFC number and remove this
     // note.
     revision 2017-07-03 2017-08-17 {
       description
         "Updated revision.";
       reference
         "RFC XXXX: 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
      */

     grouping common-leafs2 module-identification-leafs {
       description
       "Common parameters
         "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 union {
           type revision-identifier;
           type string { length 0; }
         }
         mandatory true;
         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.";
       }
     }

     grouping schema-leaf2 schema-leaf {
       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.";
       }
     }

   /*
    * Top-level container
    */
   container yang-library

     grouping implementation-parameters {
     config false;
       description
       "Top-level resource providing all the meta information
         "Parameters for describing the
        server possesses.";

     container modules {
       description
         "A container holding a list of modules.  Note, modules being
          listed here does not mean that they are supported by any
          particular datastore.";

       list module {
         key "id";

         description
           "Each entry represents one revision implementation of one module
            currently supported by the server.";

         leaf id {
           type string;
           description
             "A system-generated value that uniquely represents the
              module listing, including its name, revision, features,
              and deviations.";
         }

         uses common-leafs2;
         uses schema-leaf2;

         leaf namespace {
           type inet:uri;
           mandatory true;
           description
             "The XML namespace identifier for this a module.";
         }

       leaf-list feature {
         type yang:yang-identifier;
         description
           "List of YANG feature names from this module that are
            supported by the server, regardless whether they are defined
            in the module or any included submodule.";
       }
       list deviation {
         key "name revision";
         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-leafs2; module-identification-leafs;
       }
       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;
         description
           "Indicates the type of conformance the server is claiming
            for the YANG module identified by the YANG module identified by this entry.";
       }
     }

     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.";

       container modules {
         description
           "A container holding a list of modules.  Note, modules being
            listed here does not mean that they are supported by any
            particular datastore.";

         list module {
           key "id";
           description
             "Each entry represents one revision of one module
              currently supported by the server.";

           leaf id {
             type string;
             description
               "A stable identifier, independent of any other part
                of this module instance.";
           }

           uses module-identification-leafs;
           uses schema-leaf;
           leaf namespace {
             type inet:uri;
             mandatory true;
             description
               "The XML namespace identifier for this entry."; module.";
           }

           uses implementation-parameters;

           list submodule {
             key "name revision";
             description
               "Each entry represents one submodule within the
                parent module.";
             uses common-leafs2; module-identification-leafs;
             uses schema-leaf2; schema-leaf;
           }
         }
       }

       container module-sets {
         description
           "A container for a list of module-sets.  Module-sets being
            listed here does not mean that they are used by any
            particular datastore.";
         list module-set {
           key "id";
           description
             "An arbitrary module-set definition provided by the
              server.";

           leaf id {
             type string;
             description
               "A server-supplied identifier for this system-generated value that uniquely represents the
                referenced set of modules."; modules.  Any change to the number
                of modules referenced, or to the modules themselves,
                generates a different value.";
           }
           leaf-list module {
             type leafref {
               path "/yang-library/modules/module/id"; "../../../modules/module/id";
             }
             description
               "A module-instance supported by the server, including its
                features and deviations.";
           }
         }

       }

       container datastores {
         description
           "A container for a list of datastores supported by the
            server.  Each datastore indicates which module-sets it
            supports.";

         list datastore {
           key name; "name";

           leaf name {
             type identityref {
               base ds:datastore;
             }
             description
               "The identity of the datastore.";
           }
         container properties {
           leaf-list property {
             type identityref {
               base ds:property;
             }
             description
               "A property of the datastore.";
           }
           description
             "A list of properties supported by this datastore.";
         }
           leaf module-set {
             type leafref {
               path "/yang-library/module-sets/module-set/id"; "../../../module-sets/module-set/id";
             }
             description
               "A reference to a module-set supported by this
                datastore";
           }
           description
             "A datastore supported by this server.";
         }
       } // end 'datastores'
     } // end

     /*
      * Top-level container
      */

     container yang-library {
       config false;
       description
         "Container providing all the YANG meta information the
          server possesses.";

       uses yang-library-parameters;

       leaf checksum {
         type string;
         config false;
         mandatory true;
         description
           "A server-generated checksum 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/checksum, has
            changed.";
       }
     }

     /*
      * Notifications
      */

     notification yang-library-update {
       description
         "Generated when any YANG library information on the
          server has changed.";
     }

     /*
      * 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.";
         }
       }

     grouping schema-leaf {
       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; common-leafs {
           status deprecated;
         }
         uses schema-leaf; 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 whether they are
              defined in the module or any included submodule.";
         }
         list deviation {
           key "name revision";
           status deprecated;
           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; 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;
           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; common-leafs {
             status deprecated;
           }
           uses schema-leaf; 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; module-list {
         status deprecated;
       }
     }

     /*
      * Notifications Legacy notifications
      */

     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>

3.  IANA Considerations

3.1.  YANG Module Registry

   RFC 7895 previously registered one URI in the IETF XML registry
   [RFC3688].  Following the format in RFC 3688, the following
   registration was made:

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

   This document takes over this registration entry made by RFC 7895.

   RFC 7895 previously registered one YANG module in the "YANG Module
   Names" registry [RFC6020] as follows:

     name:         ietf-yang-library
     namespace:    urn:ietf:params:xml:ns:yang:ietf-yang-library
     prefix:       yanglib
     reference:    RFC 7895

   This document takes over this registration entry made by RFC 7895.

4.  Security Considerations

   The YANG module defined in this memo is designed to be accessed via
   the NETCONF protocol [RFC6241].  The lowest NETCONF layer is the
   secure transport layer and the mandatory-to-implement secure
   transport is SSH [RFC6242].  The NETCONF access control model
   [RFC6536] provides the means to restrict access for particular
   NETCONF users to a pre-configured subset of all available NETCONF
   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:

   o  /modules-state/module: The module list used in a server
      implementation may help an attacker identify the server
      capabilities and server implementations with known bugs.  Although
      some of this information may be available to all 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.  This
      information is included in each module entry.  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.

5.  Acknowledgements

   Contributions to this material by Andy Bierman are based upon work
   supported by the The Space & Terrestrial Communications Directorate
   (S&TCD) under Contract No.  W15P7T-13-C-A616.  Any opinions, findings
   and conclusions or recommendations expressed in this material are
   those of the author(s) and do not necessarily reflect the views of
   The Space & Terrestrial Communications Directorate (S&TCD).

6.  References

6.1.  Normative References

   [I-D.ietf-netmod-revised-datastores]
              Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore
              Architecture", draft-ietf-netmod-revised-datastores-03
              (work in progress), July 2017.

   [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>.

   [RFC6536]  Bierman, A. and M. Bjorklund, "Network Configuration
              Protocol (NETCONF) Access Control Model", RFC 6536,
              DOI 10.17487/RFC6536, March 2012, <https://www.rfc-
              editor.org/info/rfc6536>.

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

   [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>.

   [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>.

6.2.  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>.

Authors' Addresses

   Andy Bierman
   YumaWorks

   Email: andy@yumaworks.com

   Martin Bjorklund
   Tail-f Systems

   Email: mbj@tail-f.com

   Kent Watsen
   Juniper Networks

   Email: kwatsen@juniper.net