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rfc:rfc8528

Internet Engineering Task Force (IETF) M. Bjorklund Request for Comments: 8528 Tail-f Systems Category: Standards Track L. Lhotka ISSN: 2070-1721 CZ.NIC

                                                            March 2019
                         YANG Schema Mount

Abstract

 This document defines a mechanism that adds the schema trees defined
 by a set of YANG modules onto a mount point defined in the schema
 tree in another YANG module.

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/rfc8528.

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.

Bjorklund & Lhotka Standards Track [Page 1] RFC 8528 YANG Schema Mount March 2019

Table of Contents

 1. Introduction ....................................................3
 2. Terminology and Notation ........................................6
    2.1. Tree Diagrams ..............................................7
    2.2. Namespace Prefixes .........................................7
 3. Schema Mount ....................................................8
    3.1. Mount Point Definition .....................................8
    3.2. Data Model .................................................9
    3.3. Specification of the Mounted Schema ........................9
    3.4. Multiple Levels of Schema Mount ...........................10
 4. Referring to Data Nodes in the Parent Schema ...................10
 5. RPC Operations and Notifications ...............................11
 6. NMDA Considerations ............................................12
 7. Interaction with NACM ..........................................12
 8. Implementation Notes ...........................................13
 9. Schema Mount YANG Module .......................................13
 10. IANA Considerations ...........................................18
 11. Security Considerations .......................................18
 12. References ....................................................19
    12.1. Normative References .....................................19
    12.2. Informative References ...................................21
 Appendix A.  Example: Device Model with LNEs and NIs ..............22
   A.1.  Physical Device ...........................................22
   A.2.  Logical Network Elements ..................................24
   A.3.  Network Instances .........................................27
   A.4.  Invoking an RPC Operation .................................28
 Contributors ......................................................28
 Authors' Addresses ................................................28

Bjorklund & Lhotka Standards Track [Page 2] RFC 8528 YANG Schema Mount March 2019

1. Introduction

 Modularity and extensibility are among the leading design principles
 of the YANG data modeling language.  As a result, the same YANG
 module can be combined with various sets of other modules to form a
 data model that is tailored to meet the requirements of a specific
 use case.  Server implementors are only required to specify all YANG
 modules comprising the data model (together with their revisions and
 other optional choices) in the YANG library data ([RFC7895],
 [RFC8525], and Section 5.6.4 of [RFC7950]) implemented by the server.
 Such YANG modules appear in the data model "side by side", i.e., top-
 level data nodes of each module (if there are any) are also top-level
 nodes of the overall data model.
 YANG has two mechanisms for contributing a schema hierarchy defined
 elsewhere to the contents of an internal node of the schema tree.
 These mechanisms are realized through the following YANG statements:
 o  The "uses" statement explicitly incorporates the contents of a
    grouping defined in the same or another module.  See Section 4.2.6
    of [RFC7950] for more details.
 o  The "augment" statement explicitly adds contents to a target node
    defined in the same or another module.  See Section 4.2.8 of
    [RFC7950] for more details.
 With both mechanisms, the YANG module with the "uses" or "augment"
 statement explicitly defines the exact location in the schema tree
 where the new nodes are placed.
 In some cases, these mechanisms are not sufficient; it is sometimes
 necessary for an existing module (or a set of modules) to be added to
 the data model starting at locations other than the root.  For
 example, YANG modules such as "ietf-interfaces" [RFC8343] are defined
 so as to be used in a data model of a physical device.  Now suppose
 we want to model a device that supports multiple logical devices
 [RFC8530], each of which has its own instantiation of
 "ietf-interfaces", and possibly other modules; at the same time, we
 want to be able to manage all these logical devices from the master
 device.  Hence, we would like to have a schema tree like this:

Bjorklund & Lhotka Standards Track [Page 3] RFC 8528 YANG Schema Mount March 2019

   +--rw interfaces
   |  +--rw interface* [name]
   |     ...
   +--rw logical-network-element* [name]
      +--rw name
      |   ...
      +--rw interfaces
        +--rw interface* [name]
           ...
 With the "uses" approach, the complete schema tree of
 "ietf-interfaces" would have to be wrapped in a grouping, and then
 this grouping would have to be used at the top level (for the master
 device) and then also in the "logical-network-element" list (for the
 logical devices).  This approach has several disadvantages:
 o  It is not scalable because every time there is a new YANG module
    that needs to be added to the logical device model, we have to
    update the model for logical devices with another "uses" statement
    pulling in contents of the new module.
 o  Absolute references to nodes defined inside a grouping may break
    if the grouping is used in different locations.
 o  Nodes defined inside a grouping belong to the namespace of the
    module where it is used, which makes references to such nodes from
    other modules difficult or even impossible.
 o  It would be difficult for vendors to add proprietary modules when
    the "uses" statements are defined in a standard module.
 With the "augment" approach, "ietf-interfaces" would have to augment
 the "logical-network-element" list with all its nodes and, at the
 same time, define all its nodes at the top level.  The same hierarchy
 of nodes would thus have to be defined twice, which is clearly not
 scalable either.
 This document introduces a new mechanism, denoted as "schema mount",
 that allows for mounting one data model consisting of any number of
 YANG modules at a specified location of another (parent) schema.
 Unlike the "uses" and "augment" approaches discussed above, the
 mounted modules needn't be specially prepared for mounting;
 consequently, existing modules such as "ietf-interfaces" can be
 mounted without any modifications.

Bjorklund & Lhotka Standards Track [Page 4] RFC 8528 YANG Schema Mount March 2019

 The basic idea of schema mount is to label a data node in the parent
 schema as the mount point and then define a complete data model to be
 attached to the mount point so that the labeled data node effectively
 becomes the root node of the mounted data model.
 In principle, the mounted schema can be specified at three different
 phases of the data model life cycle:
 1.  Design time: The mounted schema is defined along with the mount
     point in the parent YANG module.  In this case, the mounted
     schema has to be the same for every implementation of the parent
     module.
 2.  Implementation time: The mounted schema is defined by a server
     implementor and is as stable as the YANG library information of
     the server.
 3.  Run time: The mounted schema is defined by instance data that is
     part of the mounted data model.  If there are multiple instances
     of the same mount point (e.g., in multiple entries of a list),
     the mounted data model may be different for each instance.
 The schema mount mechanism defined in this document provides support
 only for the latter two cases.  Design-time mounts are outside the
 scope of this document and could be possibly dealt with in a future
 revision of the YANG data modeling language.
 Schema mount applies to the data model and specifically does not
 assume anything about the source of instance data for the mounted
 schemas.  It may be implemented using the same instrumentation as the
 rest of the system, or it may be implemented by querying some other
 system.  Future specifications may define mechanisms to control or
 monitor the implementation of specific mount points.
 How and when specific mount points are instantiated by the server is
 out of scope for this document.  Such mechanisms may be defined in
 future specifications.
 This document allows mounting of complete data models only.  Other
 specifications may extend this model by defining additional
 mechanisms such as mounting sub-hierarchies of a module.
 The YANG modules in this document conform to the Network Management
 Datastore Architecture (NMDA) [RFC8342].

Bjorklund & Lhotka Standards Track [Page 5] RFC 8528 YANG Schema Mount March 2019

2. Terminology and Notation

 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] and are not redefined
 here:
 o  action
 o  container
 o  data node
 o  list
 o  RPC operation
 o  schema node
 o  schema tree
 The following terms are defined in [RFC8342] and are not redefined
 here:
 o  client
 o  notification
 o  operational state
 o  server
 The following term is defined in [RFC8343] and is not redefined here:
 o  system-controlled interface
 The following term is defined in [RFC8525] and is not redefined here:
 o  YANG library content identifier

Bjorklund & Lhotka Standards Track [Page 6] RFC 8528 YANG Schema Mount March 2019

 The following additional terms are used in this document:
 o  mount point: A container or a list node whose definition contains
    the "mount-point" extension statement.  The argument of the
    "mount-point" extension statement defines a label for the mount
    point.
 o  schema: A collection of schema trees with a common root.
 o  top-level schema: A schema rooted at the root node.
 o  mounted schema: A schema rooted at a mount point.
 o  parent schema (of a mounted schema): A schema containing the mount
    point.
 o  schema mount: The mechanism to combine data models defined in this
    document.

2.1. Tree Diagrams

 Tree diagrams used in this document follow the notation defined in
 [RFC8340].

2.2. Namespace Prefixes

 In this document, names of data nodes, YANG extensions, actions, and
 other data model objects are often used without a prefix when the
 YANG module in which they are defined is clear from the context.
 Otherwise, names are prefixed using the standard prefix associated
 with the corresponding YANG module, as shown in Table 1.
      +---------+------------------------+----------------------+
      | Prefix  | YANG module            | Reference            |
      +---------+------------------------+----------------------+
      | yangmnt | ietf-yang-schema-mount | Section 9            |
      | inet    | ietf-inet-types        | [RFC6991]            |
      | yang    | ietf-yang-types        | [RFC6991]            |
      | yanglib | ietf-yang-library      | [RFC7895], [RFC8525] |
      +---------+------------------------+----------------------+
                      Table 1: Namespace Prefixes

Bjorklund & Lhotka Standards Track [Page 7] RFC 8528 YANG Schema Mount March 2019

3. Schema Mount

 The schema mount mechanism defined in this document provides a new
 extensibility mechanism for use with YANG 1.1 [RFC7950].  In contrast
 to the existing mechanisms described in Section 1, schema mount
 defines the relationship between the source and target YANG modules
 outside these modules.

3.1. Mount Point Definition

 A container or list node becomes a mount point if the "mount-point"
 extension statement (defined in the "ietf-yang-schema-mount" module)
 is used in its definition.  This extension can appear only as a
 substatement of "container" and "list" statements.
 The argument of the "mount-point" extension statement is a YANG
 identifier that defines a label for the mount point.  A module MAY
 contain multiple "mount-point" extension statements having the same
 argument.
 It is therefore up to the designer of the parent schema to decide
 about the placement of mount points.  A mount point can also be made
 conditional by placing "if-feature" and/or "when" as substatements of
 the "container" or "list" statement that represents the mount point.
 The "mount-point" extension statement MUST NOT be used in a YANG
 version 1 module [RFC6020].  If used in a YANG version 1 module, it
 would not be possible to invoke mounted RPC operations and receive
 mounted notifications.  See Section 5 for details.  Note, however,
 that modules written in any YANG version, including version 1, can be
 mounted under a mount point.
 Note that the "mount-point" extension statement does not define a new
 data node.

Bjorklund & Lhotka Standards Track [Page 8] RFC 8528 YANG Schema Mount March 2019

3.2. Data Model

 This document defines the YANG 1.1 module [RFC7950]
 "ietf-yang-schema-mount", which has the following structure:
 module: ietf-yang-schema-mount
   +--ro schema-mounts
      +--ro namespace* [prefix]
      |  +--ro prefix    yang:yang-identifier
      |  +--ro uri?      inet:uri
      +--ro mount-point* [module label]
         +--ro module                 yang:yang-identifier
         +--ro label                  yang:yang-identifier
         +--ro config?                boolean
         +--ro (schema-ref)
            +--:(inline)
            |  +--ro inline!
            +--:(shared-schema)
               +--ro shared-schema!
                  +--ro parent-reference*   yang:xpath1.0

3.3. Specification of the Mounted Schema

 Mounted schemas for all mount points in the parent schema are
 determined from state data in the "/schema-mounts" container.
 Generally, the modules that are mounted under a mount point have no
 relation to the modules in the parent schema; specifically, if a
 module is mounted, it may or may not be present in the parent schema;
 if present, its data will generally have no relationship to the data
 of the parent.  Exceptions are possible and need to be defined in the
 model itself.  For example, [RFC8530] defines a mechanism to bind
 interfaces to mounted logical network elements.
 The "/schema-mounts" container has the "mount-point" list as one of
 its children.  Every entry of this list refers (through its key) to a
 mount point and specifies the mounted schema.
 If a mount point is defined in the parent schema but does not have an
 entry in the "mount-point" list, then the mounted schema is void,
 i.e., instances of that mount point MUST NOT contain any data except
 those that are defined in the parent schema.
 If multiple mount points with the same name are defined in the same
 module -- either directly or because the mount point is defined in a
 grouping and the grouping is used multiple times -- then the
 corresponding "mount-point" entry applies equally to all such mount
 points.

Bjorklund & Lhotka Standards Track [Page 9] RFC 8528 YANG Schema Mount March 2019

 The "config" property of mounted schema nodes is overridden and all
 nodes in the mounted schema are read-only ("config false") if at
 least one of the following conditions is satisfied for a mount point:
 o  The mount point is itself defined as "config false".
 o  The "config" leaf in the corresponding entry of the "mount-point"
    list is set to "false".
 An entry of the "mount-point" list can specify the mounted schema in
 two different ways: "inline" or "shared-schema".
 The mounted schema is determined at run time: every instance of the
 mount point that exists in the operational state MUST contain a copy
 of YANG library data that defines the mounted schema in exactly the
 same way as a top-level schema.  A client is expected to retrieve
 this data from the instance tree.  In the "inline" case, instances of
 the same mount point MAY use different mounted schemas, whereas in
 the "shared-schema" case, all instances MUST use the same mounted
 schema.  This means that in the "shared-schema" case, all instances
 of the same mount point MUST have the same YANG library content
 identifier.  In the "inline" case, if two instances have the same
 YANG library content identifier, it is not guaranteed that the YANG
 library contents are equal for these instances.
 Examples of "inline" and "shared-schema" can be found in Appendix A.2
 and Appendix A.3, respectively.

3.4. Multiple Levels of Schema Mount

 YANG modules in a mounted schema MAY again contain mount points under
 which other schemas can be mounted.  Consequently, it is possible to
 construct data models with an arbitrary number of mounted schemas.  A
 schema for a mount point contained in a mounted module can be
 specified by implementing the "ietf-yang-library" and
 "ietf-yang-schema-mount" modules in the mounted schema and specifying
 the schemas in exactly the same way as the top-level schema.

4. Referring to Data Nodes in the Parent Schema

 A fundamental design principle of schema mount is that the mounted
 schema works exactly as a top-level schema, i.e., it is confined to
 the "mount jail".  This means that all paths in the mounted schema
 (in leafrefs, instance-identifiers, XPath [XPATH] expressions, and
 target nodes of "augment" statements) are interpreted with the mount
 point as the root node.  YANG modules of the mounted schema as well
 as corresponding instance data thus cannot refer to schema nodes or
 instance data outside the "mount jail".

Bjorklund & Lhotka Standards Track [Page 10] RFC 8528 YANG Schema Mount March 2019

 However, this restriction is sometimes too severe.  A typical example
 is network instances (NIs) [RFC8529] in which each NI has its own
 routing engine but the list of interfaces is global and shared by all
 NIs.  If we want to model this organization with the NI schema
 mounted using schema mount, the overall schema tree would look
 schematically as follows:
   +--rw interfaces
   |  +--rw interface* [name]
   |     ...
   +--rw network-instances
      +--rw network-instance* [name]
         +--rw name
         +--mp root
            +--rw routing
               ...
 Here, the "root" container is the mount point for the NI schema.
 Routing configuration inside an NI often needs to refer to interfaces
 (at least those that are assigned to the NI), which is impossible
 unless such a reference can point to a node in the parent schema
 (interface name).
 Therefore, schema mount also allows for such references.  For every
 mount point in the "shared-schema" case, it is possible to specify a
 leaf-list named "parent-reference" that contains zero or more XPath
 1.0 expressions.  Each expression is evaluated with the node in the
 parent data tree where the mount point is defined as the context
 node.  The result of this evaluation MUST be a node-set (see the
 description of the "parent-reference" node for a complete definition
 of the evaluation context).  For the purposes of evaluating XPath
 expressions within the mounted data tree, the union of all such node-
 sets is added to the accessible data tree.
 It is worth emphasizing that the nodes specified in the
 "parent-reference" leaf-list are available in the mounted schema only
 for XPath evaluations.  In particular, they cannot be accessed in the
 mounted schema via network management protocols such as NETCONF
 [RFC6241] or RESTCONF [RFC8040].

5. RPC Operations and Notifications

 If a mounted YANG module defines an RPC operation, clients can invoke
 this operation as if it were defined as an action for the
 corresponding mount point; see Section 7.15 of [RFC7950].  An example
 of this is given in Appendix A.4.

Bjorklund & Lhotka Standards Track [Page 11] RFC 8528 YANG Schema Mount March 2019

 Similarly, if the server emits a notification defined at the top
 level of any mounted module, it MUST be represented as if the
 notification was connected to the mount point; see Section 7.16 of
 [RFC7950].
 Note that inline actions and notifications will not work when they
 are contained within a list node without a "key" statement (see
 Sections 7.15 and 7.16 of [RFC7950]).  Therefore, to be useful, mount
 points that contain modules with RPCs, actions, and notifications
 SHOULD NOT have any ancestor node that is a list node without a "key"
 statement.  This requirement applies to the definition of modules
 using the "mount-point" extension statement.

6. NMDA Considerations

 The schema mount solution presented in this document is designed to
 work with both servers that implement the NMDA [RFC8342] and old
 servers that don't implement the NMDA.
 Specifically, a server that doesn't support the NMDA MAY implement
 revision 2016-06-21 of "ietf-yang-library" [RFC7895] under a mount
 point.  A server that supports the NMDA MUST implement at least
 revision 2019-01-04 of "ietf-yang-library" [RFC8525] under a mount
 point.

7. Interaction with NACM

 If the Network Configuration Access Control Model (NACM) [RFC8341] is
 implemented on a server, it is used to control access to nodes
 defined by the mounted schema in the same way as for nodes defined by
 the top-level schema.
 For example, suppose the module "ietf-interfaces" is mounted in the
 "root" container in the "logical-network-element" list defined in
 [RFC8530].  Then, the following NACM path can be used to control
 access to the "interfaces" container (where the character '\' is used
 where a line break has been inserted for formatting reasons):
   <path xmlns:lne=
           "urn:ietf:params:xml:ns:yang:ietf-logical-network-element"
         xmlns:if="urn:ietf:params:xml:ns:yang:ietf-interfaces">
     /lne:logical-network-elements\
       /lne:logical-network-element/lne:root/if:interfaces
   </path>

Bjorklund & Lhotka Standards Track [Page 12] RFC 8528 YANG Schema Mount March 2019

8. Implementation Notes

 Network management of devices that use a data model with schema mount
 can be implemented in different ways.  However, the following
 implementation options are envisioned as typical:
 o  shared management: Instance data of both parent and mounted
    schemas are accessible within the same management session.
 o  split management: One (master) management session has access to
    instance data of both parent and mounted schemas; in addition, an
    extra session that has access only to the mounted data tree exists
    for every instance of the mount point.

9. Schema Mount YANG Module

 This module references [RFC6991] and [RFC7950].
 <CODE BEGINS> file "ietf-yang-schema-mount@2019-01-14.yang"
 module ietf-yang-schema-mount {
   yang-version 1.1;
   namespace "urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount";
   prefix yangmnt;
   import ietf-inet-types {
     prefix inet;
     reference
       "RFC 6991: Common YANG Data Types";
   }
   import ietf-yang-types {
     prefix yang;
     reference
       "RFC 6991: Common YANG Data Types";
   }
   organization
     "IETF NETMOD (NETCONF Data Modeling Language) Working Group";
   contact
     "WG Web:   <https://datatracker.ietf.org/wg/netmod/>
      WG List:  <mailto:netmod@ietf.org>
      Editor:   Martin Bjorklund
                <mailto:mbj@tail-f.com>

Bjorklund & Lhotka Standards Track [Page 13] RFC 8528 YANG Schema Mount March 2019

      Editor:   Ladislav Lhotka
                <mailto:lhotka@nic.cz>";
   description
     "This module defines a YANG extension statement that can be used
      to incorporate data models defined in other YANG modules in a
      module.  It also defines operational state data that specify the
      overall structure of the data model.
      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 8528;
      see the RFC itself for full legal notices.";
   revision 2019-01-14 {
     description
       "Initial revision.";
     reference
       "RFC 8528: YANG Schema Mount";
   }
   /*
    * Extensions
    */
   extension mount-point {
     argument label;
     description
       "The argument 'label' is a YANG identifier, i.e., it is of the
        type 'yang:yang-identifier'.
        The 'mount-point' statement MUST NOT be used in a YANG
        version 1 module, neither explicitly nor via a 'uses'
        statement.

Bjorklund & Lhotka Standards Track [Page 14] RFC 8528 YANG Schema Mount March 2019

        The 'mount-point' statement MAY be present as a substatement
        of 'container' and 'list' and MUST NOT be present elsewhere.
        There MUST NOT be more than one 'mount-point' statement in a
        given 'container' or 'list' statement.
        If a mount point is defined within a grouping, its label is
        bound to the module where the grouping is used.
        A mount point defines a place in the node hierarchy where
        other data models may be attached.  A server that implements a
        module with a mount point populates the
        '/schema-mounts/mount-point' list with detailed information on
        which data models are mounted at each mount point.
        Note that the 'mount-point' statement does not define a new
        data node.";
   }
   /*
    * State data nodes
    */
   container schema-mounts {
     config false;
     description
       "Contains information about the structure of the overall
        mounted data model implemented in the server.";
     list namespace {
       key "prefix";
       description
         "This list provides a mapping of namespace prefixes that are
          used in XPath expressions of 'parent-reference' leafs to the
          corresponding namespace URI references.";
       leaf prefix {
         type yang:yang-identifier;
         description
           "Namespace prefix.";
       }
       leaf uri {
         type inet:uri;
         description
           "Namespace URI reference.";
       }
     }
     list mount-point {
       key "module label";

Bjorklund & Lhotka Standards Track [Page 15] RFC 8528 YANG Schema Mount March 2019

       description
         "Each entry of this list specifies a schema for a particular
          mount point.
          Each mount point MUST be defined using the 'mount-point'
          extension in one of the modules listed in the server's
          YANG library instance with conformance type 'implement'.";
       leaf module {
         type yang:yang-identifier;
         description
           "Name of a module containing the mount point.";
       }
       leaf label {
         type yang:yang-identifier;
         description
           "Label of the mount point defined using the 'mount-point'
            extension.";
       }
       leaf config {
         type boolean;
         default "true";
         description
           "If this leaf is set to 'false', then all data nodes in the
            mounted schema are read-only ('config false'), regardless
            of their 'config' property.";
       }
       choice schema-ref {
         mandatory true;
         description
           "Alternatives for specifying the schema.";
         container inline {
           presence
             "A complete self-contained schema is mounted at the
              mount point.";
           description
             "This node indicates that the server has mounted at least
              the module 'ietf-yang-library' at the mount point, and
              its instantiation provides the information about the
              mounted schema.
              Different instances of the mount point may have
              different schemas mounted.";
         }
         container shared-schema {
           presence
             "The mounted schema together with the 'parent-reference'
              make up the schema for this mount point.";

Bjorklund & Lhotka Standards Track [Page 16] RFC 8528 YANG Schema Mount March 2019

           description
             "This node indicates that the server has mounted at least
              the module 'ietf-yang-library' at the mount point, and
              its instantiation provides the information about the
              mounted schema.  When XPath expressions in the mounted
              schema are evaluated, the 'parent-reference' leaf-list
              is taken into account.
              Different instances of the mount point MUST have the
              same schema mounted.";
           leaf-list parent-reference {
             type yang:xpath1.0;
             description
               "Entries of this leaf-list are XPath 1.0 expressions
                that are evaluated in the following context:
  1. The context node is the node in the parent data tree

where the mount-point is defined.

  1. The accessible tree is the parent data tree
    • without* any nodes defined in modules that are

mounted inside the parent schema.

  1. The context position and context size are both equal

to 1.

  1. The set of variable bindings is empty.
  1. The function library is the core function library

defined in the W3C XPath 1.0 document

                  (http://www.w3.org/TR/1999/REC-xpath-19991116) and
                  the functions defined in Section 10 of RFC 7950.
  1. The set of namespace declarations is defined by the

'namespace' list under 'schema-mounts'.

                Each XPath expression MUST evaluate to a node-set
                (possibly empty).  For the purposes of evaluating
                XPath expressions whose context nodes are defined in
                the mounted schema, the union of all these node-sets
                together with ancestor nodes are added to the
                accessible data tree.
                Note that in the case 'ietf-yang-schema-mount' is
                itself mounted, a 'parent-reference' in the mounted
                module may refer to nodes that were brought into the
                accessible tree through a 'parent-reference' in the
                parent schema.";

Bjorklund & Lhotka Standards Track [Page 17] RFC 8528 YANG Schema Mount March 2019

           }
         }
       }
     }
   }
 }
 <CODE ENDS>

10. IANA Considerations

 This document registers a URI in the "IETF XML Registry" [RFC3688].
      URI: urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount
      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-schema-mount
   namespace:   urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount
   prefix:      yangmnt
   reference:   RFC 8528

11. 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:

Bjorklund & Lhotka Standards Track [Page 18] RFC 8528 YANG Schema Mount March 2019

 o  /schema-mounts: The schema defined by this state data provides
    detailed information about a server implementation that may help
    an attacker identify the server capabilities and server
    implementations with known bugs.  Server vulnerabilities may be
    specific to particular modules included in the schema, 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 schema information will help an attacker
    identify server implementations with such a defect, in order to
    launch a denial-of-service attack on the device.
 It is important to take into account the security considerations for
 all nodes in the mounted schemas and to control access to these nodes
 by using the mechanism described in Section 7.
 Care must be taken when the "parent-reference" XPath expressions are
 constructed, since the result of the evaluation of these expressions
 is added to the accessible tree for any XPath expression found in the
 mounted schema.

12. References

12.1. 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>.

Bjorklund & Lhotka Standards Track [Page 19] RFC 8528 YANG Schema Mount March 2019

 [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>.
 [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>.
 [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>.
 [RFC8525]  Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
            and R. Wilton, "YANG Library", RFC 8525,
            DOI 10.17487/RFC8525, March 2019,
            <https://www.rfc-editor.org/info/rfc8525>.
 [XPATH]    Clark, J. and S. DeRose, "XML Path Language (XPath)
            Version 1.0", World Wide Web Consortium Recommendation
            REC-xpath-19991116, November 1999,
            <http://www.w3.org/TR/1999/REC-xpath-19991116>.

Bjorklund & Lhotka Standards Track [Page 20] RFC 8528 YANG Schema Mount March 2019

12.2. Informative References

 [DEVICE-YANG]
            Lindem, A., Ed., Berger, L., Ed., Bogdanovic, D., and C.
            Hopps, "Network Device YANG Logical Organization", Work in
            Progress, draft-ietf-rtgwg-device-model-02, March 2017.
 [IS-IS-YANG]
            Litkowski, S., Yeung, D., Lindem, A., Zhang, J., and L.
            Lhotka, "YANG Data Model for IS-IS protocol", Work in
            Progress, draft-ietf-isis-yang-isis-cfg-34, January 2019.
 [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>.
 [RFC8529]  Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and
            X. Liu, "YANG Data Model for Network Instances", RFC 8529,
            DOI 10.17487/RFC8529, March 2019,
            <https://www.rfc-editor.org/info/rfc8529>.
 [RFC8530]  Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and
            X. Liu, "YANG Model for Logical Network Elements",
            RFC 8530, DOI 10.17487/RFC8530, March 2019,
            <https://www.rfc-editor.org/info/rfc8530>.
 [YANG-MOUNT]
            Clemm, A., Voit, E., and J. Medved, "Mounting YANG-Defined
            Information from Remote Datastores", Work in Progress,
            draft-clemm-netmod-mount-06, March 2017.

Bjorklund & Lhotka Standards Track [Page 21] RFC 8528 YANG Schema Mount March 2019

Appendix A. Example: Device Model with LNEs and NIs

 This non-normative example demonstrates an implementation of the
 device model as specified in Section 2 of [DEVICE-YANG], using both
 logical network elements (LNEs) and network instances (NIs).
 In these examples, the character '\' is used where a line break has
 been inserted for formatting reasons.

A.1. Physical Device

 The data model for the physical device may be described by this YANG
 library content, assuming the server supports the NMDA:
 {
    "ietf-yang-library:yang-library": {
      "content-id": "14e2ab5dc325f6d86f743e8d3ade233f1a61a899",
      "module-set": [
        {
          "name": "physical-device-modules",
          "module": [
            {
              "name": "ietf-datastores",
              "revision": "2018-02-14",
              "namespace":
                "urn:ietf:params:xml:ns:yang:ietf-datastores"
            },
            {
              "name": "iana-if-type",
              "revision": "2015-06-12",
              "namespace": "urn:ietf:params:xml:ns:yang:iana-if-type"
            },
            {
              "name": "ietf-interfaces",
              "revision": "2018-02-20",
              "feature": ["arbitrary-names", "pre-provisioning" ],
              "namespace":
                "urn:ietf:params:xml:ns:yang:ietf-interfaces"
            },
            {
              "name": "ietf-ip",
              "revision": "2018-02-22",
              "namespace": "urn:ietf:params:xml:ns:yang:ietf-ip"
            },
            {
              "name": "ietf-logical-network-element",
              "revision": "2018-03-20",
              "feature": [ "bind-lne-name" ],

Bjorklund & Lhotka Standards Track [Page 22] RFC 8528 YANG Schema Mount March 2019

              "namespace":
                "urn:ietf:params:xml:ns:yang:\
                ietf-logical-network-element"
            },
            {
              "name": "ietf-yang-library",
              "revision": "2019-01-04",
              "namespace":
                "urn:ietf:params:xml:ns:yang:ietf-yang-library"
            },
            {
              "name": "ietf-yang-schema-mount",
              "revision": "2019-01-14",
              "namespace":
                "urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount"
            }
          ],
          "import-only-module": [
            {
              "name": "ietf-inet-types",
              "revision": "2013-07-15",
              "namespace":
                "urn:ietf:params:xml:ns:yang:ietf-inet-types"
            },
            {
              "name": "ietf-yang-types",
              "revision": "2013-07-15",
              "namespace":
                "urn:ietf:params:xml:ns:yang:ietf-yang-types"
            }
          ]
        }
      ],
      "schema": [
        {
          "name": "physical-device-schema",
          "module-set": [ "physical-device-modules" ]
        }
      ],
      "datastore": [
        {
          "name": "ietf-datastores:running",
          "schema": "physical-device-schema"
        },
        {
          "name": "ietf-datastores:operational",
          "schema": "physical-device-schema"
        }

Bjorklund & Lhotka Standards Track [Page 23] RFC 8528 YANG Schema Mount March 2019

      ]
    }
 }

A.2. Logical Network Elements

 Each LNE can have a specific data model that is determined at run
 time, so it is appropriate to mount it using the "inline" method.
 Hence, the following "schema-mounts" data is used:
 {
   "ietf-yang-schema-mount:schema-mounts": {
     "mount-point": [
       {
         "module": "ietf-logical-network-element",
         "label": "root",
         "inline": {}
       }
     ]
   }
 }
 An administrator of the host device has to configure an entry for
 each LNE instance, for example:
 {
   "ietf-interfaces:interfaces": {
     "interface": [
       {
         "name": "eth0",
         "type": "iana-if-type:ethernetCsmacd",
         "enabled": true,
         "ietf-logical-network-element:bind-lne-name": "eth0"
       }
     ]
   },
   "ietf-logical-network-element:logical-network-elements": {
     "logical-network-element": [
       {
         "name": "lne-1",
         "managed": true,
         "description": "LNE with NIs",
         "root": {
           ...
         }
       }
       ...
     ]

Bjorklund & Lhotka Standards Track [Page 24] RFC 8528 YANG Schema Mount March 2019

   }
 }
 and then also place necessary state data as the contents of the
 "root" instance, which should include at least:
 o  YANG library data specifying the LNE's data model, for example,
    assuming the server does not implement the NMDA:
 {
   "ietf-yang-library:modules-state": {
     "module-set-id": "9358e11874068c8be06562089e94a89e0a392019",
     "module": [
       {
         "name": "iana-if-type",
         "revision": "2014-05-08",
         "namespace": "urn:ietf:params:xml:ns:yang:iana-if-type",
         "conformance-type": "implement"
       },
       {
         "name": "ietf-inet-types",
         "revision": "2013-07-15",
         "namespace": "urn:ietf:params:xml:ns:yang:ietf-inet-types",
         "conformance-type": "import"
       },
       {
         "name": "ietf-interfaces",
         "revision": "2014-05-08",
         "feature": [
           "arbitrary-names",
           "pre-provisioning"
         ],
         "namespace": "urn:ietf:params:xml:ns:yang:ietf-interfaces",
         "conformance-type": "implement"
       },
       {
         "name": "ietf-ip",
         "revision": "2014-06-16",
         "feature": [
           "ipv6-privacy-autoconf"
         ],
         "namespace": "urn:ietf:params:xml:ns:yang:ietf-ip",
         "conformance-type": "implement"
       },
       {
         "name": "ietf-network-instance",
         "revision": "2018-03-20",
         "feature": [

Bjorklund & Lhotka Standards Track [Page 25] RFC 8528 YANG Schema Mount March 2019

           "bind-network-instance-name"
         ],
         "namespace":
           "urn:ietf:params:xml:ns:yang:ietf-network-instance",
         "conformance-type": "implement"
       },
       {
         "name": "ietf-yang-library",
         "revision": "2016-06-21",
         "namespace": "urn:ietf:params:xml:ns:yang:ietf-yang-library",
         "conformance-type": "implement"
       },
       {
         "name": "ietf-yang-schema-mount",
         "revision": "2019-01-14",
         "namespace":
           "urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount",
         "conformance-type": "implement"
       },
       {
         "name": "ietf-yang-types",
         "revision": "2013-07-15",
         "namespace": "urn:ietf:params:xml:ns:yang:ietf-yang-types",
         "conformance-type": "import"
       }
     ]
   }
 }
 o  state data for interfaces assigned to the LNE instance (that
    effectively become system-controlled interfaces for the LNE), for
    example:
 {
   "ietf-interfaces:interfaces": {
     "interface": [
       {
         "name": "eth0",
         "type": "iana-if-type:ethernetCsmacd",
         "oper-status": "up",
         "statistics": {
           "discontinuity-time": "2016-12-16T17:11:27+02:00"
         },
         "ietf-ip:ipv6": {
           "address": [
             {
               "ip": "fe80::42a8:f0ff:fea8:24fe",
               "origin": "link-layer",

Bjorklund & Lhotka Standards Track [Page 26] RFC 8528 YANG Schema Mount March 2019

               "prefix-length": 64
             }
           ]
         }
       }
     ]
   }
 }

A.3. Network Instances

 Assuming that network instances share the same data model, it can be
 mounted using the "shared-schema" method as follows:
 {
   "ietf-yang-schema-mount:schema-mounts": {
     "namespace": [
       {
           "prefix": "if",
           "uri": "urn:ietf:params:xml:ns:yang:ietf-interfaces"
       },
       {
           "prefix": "ni",
           "uri": "urn:ietf:params:xml:ns:yang:ietf-network-instance"
       }
     ],
     "mount-point": [
       {
         "module": "ietf-network-instance",
         "label": "root",
           "shared-schema": {
             "parent-reference": [
               "/if:interfaces/if:interface[\
               ni:bind-network-instance-name = current()/../ni:name]"
             ]
           }
       }
     ]
   }
 }
 Note also that the "ietf-interfaces" module appears in the
 "parent-reference" leaf-list for the mounted NI schema.  This means
 that references to LNE interfaces, such as "outgoing-interface" in
 static routes, are valid despite the fact that "ietf-interfaces"
 isn't part of the NI schema.

Bjorklund & Lhotka Standards Track [Page 27] RFC 8528 YANG Schema Mount March 2019

A.4. Invoking an RPC Operation

 Assume that the mounted NI data model also implements the "ietf-isis"
 module [IS-IS-YANG].  An RPC operation defined in this module, such
 as "clear-adjacency", can be invoked by a client session of an LNE's
 RESTCONF server as an action tied to the mount point of a particular
 network instance using a request URI like this (all on one line):
   POST /restconf/data/ietf-network-instance:network-instances/
       network-instance=rtrA/root/ietf-isis:clear-adjacency HTTP/1.1

Contributors

 The idea of having some way to combine schemas from different YANG
 modules into one has been proposed independently by others:
 o  Authors of [YANG-MOUNT]:
 o  Jan Medved, Cisco, <jmedved@cisco.com>
 o  Eric Voit, Cisco, <evoit@cisco.com>

Authors' Addresses

 Martin Bjorklund
 Tail-f Systems
 Email: mbj@tail-f.com
 Ladislav Lhotka
 CZ.NIC
 Email: lhotka@nic.cz

Bjorklund & Lhotka Standards Track [Page 28]

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