Network Working Group                                       M. Bjorklund
Internet-Draft                                            Tail-f Systems
Intended status: Standards Track                               L. Lhotka
Expires: February 8, April 20, 2019                                           CZ.NIC
                                                          August 7,
                                                        October 17, 2018

                           YANG Schema Mount


   This document defines a mechanism to add the schema trees defined by
   a set of YANG modules onto a mount point defined in the schema tree
   in some YANG module.

Status of This Memo

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

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   This Internet-Draft will expire on February 8, April 20, 2019.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   5
     2.1.  Tree Diagrams . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Namespace Prefixes  . . . . . . . . . . . . . . . . . . .   6
   3.  Schema Mount  . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Mount Point Definition  . . . . . . . . . . . . . . . . .   7
     3.2.  Data Model  . . . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  Specification of the Mounted Schema . . . . . . . . . . .   8
     3.4.  Multiple Levels of Schema Mount . . . . . . . . . . . . .   9
   4.  Referring to Data Nodes in the Parent Schema  . . . . . . . .   9
   5.  RPC operations and Notifications  . . . . . . . . . . . . . .  10  11
   6.  Network Management Datastore Architecture (NMDA)
       Considerations  . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Interaction with the Network Configuration Access Control
       Model (NACM)  . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Implementation Notes  . . . . . . . . . . . . . . . . . . . .  12
   9.  Schema Mount YANG Module  . . . . . . . . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  18
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18  19
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  18  19
     13.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  Example: Device Model with LNEs and NIs  . . . . . .  21
     A.1.  Physical Device . . . . . . . . . . . . . . . . . . . . .  21
     A.2.  Logical Network Elements  . . . . . . . . . . . . . . . .  23
     A.3.  Network Instances . . . . . . . . . . . . . . . . . . . .  26
     A.4.  Invoking an RPC Operation . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

   Modularity and extensibility were 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 and thus
   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], [I-D.ietf-netconf-rfc7895bis] 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 that an existing module (or a set of modules) is 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
   [I-D.ietf-rtgwg-lne-model], each of which has its own instantiation
   of "ietf-interfaces", and possibly other modules, but, 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:

     +--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 and,
   consequently, existing modules such as "ietf-interfaces" can be
   mounted without any modifications.

   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

   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].

2.  Terminology and Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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

   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

   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 [I-D.ietf-netconf-rfc7895bis] is not
   redefined here:

   o  YANG library checksum content identifier

   The following additional terms are used within 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" 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

   o  schema mount: The mechanism to combine data models defined in this

2.1.  Tree Diagrams

   Tree diagrams used in this document follow the notation defined in

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, as long as
   it is clear from the context in which YANG module each name is
   defined.  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],                     |
   |         |                        | [I-D.ietf-netconf-rfc7895bis]  |

                        Table 1: Namespace Prefixes

3.  Schema Mount

   The schema mount mechanism defined in this document provides a new
   extensibility mechanism for use with YANG 1.1.  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.  The procedure consists of two separate steps that are
   described in the following subsections.

3.1.  Mount Point Definition

   A "container" or "list" node becomes a mount point if the
   "mount-point" extension (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 is a YANG identifier that
   defines a label for the mount point.  A module MAY contain multiple
   "mount-point" 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" statement MUST NOT be used in a YANG version 1
   module [RFC6020].  The reason for this is that otherwise it is not
   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" statement does not define a new data

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)
              |  +--ro inline!
                 +--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
   and, if present, its data will generally have no relationship to the
   data of the parent.  Exceptions are possible and such needs to be
   defined in the model defining the exception.  For example,
   [I-D.ietf-rtgwg-lne-model] 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

   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 exactly as for 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 checksum. content identifier.
   In the "inline" case, if two instances have the same YANG library checksum
   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 "ietf-yang-library" and
   "ietf-yang-schema-mount" modules in the mounted schema, and
   specifying the schemas exactly as it is done in 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 augments) 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.

   However, this restriction is sometimes too severe.  A typical example
   is network instances (NI) [I-D.ietf-rtgwg-ni-model], where 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
           +--rw root
              +--rw routing

   Here, the "root" node 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

   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 nodeset (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
   nodesets is added to the accessible data tree.

   It is worth emphasizing that the nodes specified in
   "parent-reference" leaf-list are available in the mounted schema only
   for XPath evaluations.  In particular, they cannot be accessed there
   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.

   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

   Note, inline actions and notifications will not work when they are
   contained within a list node without a "key" statement (see section
   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.  Network Management Datastore Architecture (NMDA) Considerations

   The schema mount solution presented in this document is designed to
   work both with servers that implement the NMDA [RFC8342], and old
   servers that don't implement the NMDA.

   Note to RFC Editor: please update the date YYYY-MM-DD below with the
   revision of the ietf-yang-library in the published version of draft-
   ietf-netconf-rfc7895bis, and remove this note.

   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 YYYY-MM-DD of "ietf-yang-library"
   [I-D.ietf-netconf-rfc7895bis] under the mount points.

7.  Interaction with the Network Configuration Access Control Model

   If NACM [RFC8341] is implemented on a server, it can be 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
   [I-D.ietf-rtgwg-lne-model].  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

     <path xmlns:lne=

8.  Implementation Notes

   Network management of devices that use a data model with schema mount
   can be implemented in different ways.  However, the following
   implementations 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 but, in addition,
      an extra session exists for every instance of the mount point,
      having access only to the mounted data tree.

9.  Schema Mount YANG Module

   This module references [RFC6991].

   <CODE BEGINS> file "ietf-yang-schema-mount@2018-08-07" "ietf-yang-schema-mount@2018-10-16"

   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;
         "RFC 6991: Common YANG Data Types";

     import ietf-yang-types {
       prefix yang;
         "RFC 6991: Common YANG Data Types";

       "IETF NETMOD (NETCONF Data Modeling Language) Working Group";

       "WG Web:   <>
        WG List:  <>

        Editor:   Martin Bjorklund

        Editor:   Ladislav Lhotka

     // RFC Ed.: replace XXXX with actual RFC number and
     // remove this note.
       "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.

        Copyright (c) 2018 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

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
        'MAY', and 'OPTIONAL' in the module text are to be interpreted
        as described in RFC 2119 ( BCP 14 [RFC 2119] [RFC8174] when, and only when,
        they appear in all capitals, as shown here.

        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.
     revision 2018-08-07 2018-10-16 {
         "Initial revision.";
         "RFC XXXX: YANG Schema Mount";
      * Extensions

     extension mount-point {
       argument label;
         "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'

          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;
         "Contains information about the structure of the overall
          mounted data model implemented in the server.";
       list namespace {
         key "prefix";
           "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;
             "Namespace prefix.";
         leaf uri {
           type inet:uri;
             "Namespace URI reference.";
       list mount-point {
         key "module label";
           "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;
             "Name of a module containing the mount point.";
         leaf label {
           type yang:yang-identifier;
             "Label of the mount point defined using the 'mount-point'
         leaf config {
           type boolean;
           default "true";
             "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;
             "Alternatives for specifying the schema.";
           container inline {
               "A complete self-contained schema is mounted at the
                mount point.";
               "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 {
               "The mounted schema together with the 'parent-reference'
                make up the schema for this mount point.";
               "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;
                 "Entries of this leaf-list are XPath 1.0 expressions
                  that are evaluated in the following context:

                  - The context node is the node in the parent data tree
                    where the mount-point is defined.

                  - The accessible tree is the parent data tree
                    *without* any nodes defined in modules that are
                    mounted inside the parent schema.

                  - The context position and context size are both equal
                    to 1.

                  - The set of variable bindings is empty.

                  - The function library is the core function library
                    defined in [XPath] and the functions defined in
                    Section 10 of [RFC7950].

                  - The set of namespace declarations is defined by the
                    'namespace' list under 'schema-mounts'.

                  Each XPath expression MUST evaluate to a nodeset
                  (possibly empty). For the purposes of evaluating XPath
                  expressions whose context nodes are defined in the
                  mounted schema, the union of all these nodesets
                  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.";


10.  IANA Considerations

   This document registers a URI in the IETF XML registry [RFC3688].
   Following the format in RFC 3688, the following registration is
   requested to be made.

        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 XXXX

11.  Security Considerations

   YANG module "ietf-yang-schema-mount" 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 [RFC5246].

   The network configuration access control model [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:

   o  /schema-mounts: The schema defined by this state data provides
      detailed information about a server implementation 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 the security considerations for all nodes in
   the mounted schemas into account, and 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.  Contributors

   The idea of having some way to combine schemas from different YANG
   modules into one has been proposed independently by several groups of
   people: Alexander Clemm, Jan Medved, and Eric Voit
   ([I-D.clemm-netmod-mount]); and Lou Berger and Christian Hopps:

   o  Lou Berger, LabN Consulting, L.L.C., <>

   o  Alexander Clemm, Huawei, <>

   o  Christian Hopps, Deutsche Telekom, <>

   o  Jan Medved, Cisco, <>

   o  Eric Voit, Cisco, <>

13.  References

13.1.  Normative References

              Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
              and R. Wilton, "YANG Library", draft-ietf-netconf-
              rfc7895bis-06 (work in progress), April 2018.

   [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-

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004, <https://www.rfc-

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008, <https://www.rfc-

   [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-

   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,

   [RFC7895]  Bierman, A., Bjorklund, M., and K. Watsen, "YANG Module
              Library", RFC 7895, DOI 10.17487/RFC7895, June 2016,

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018, <https://www.rfc-

   [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,

   [XPATH]    Clark, J. and S. DeRose, "XML Path Language (XPath)
              Version 1.0", World Wide Web Consortium Recommendation
              REC-xpath-19991116, November 1999,

13.2.  Informative References

              Clemm, A., Voit, E., and J. Medved, "Mounting YANG-Defined
              Information from Remote Datastores", draft-clemm-netmod-
              mount-06 (work in progress), March 2017.

              Litkowski, S., Yeung, D., Lindem, A., Zhang, Z., and L.
              Lhotka, "YANG Data Model for IS-IS protocol", draft-ietf-
              isis-yang-isis-cfg-24 (work in progress), November 2017. August 2018.

              Lindem, A., Berger, L., Bogdanovic, D., and C. Hopps,
              "Network Device YANG Logical Organization", draft-ietf-
              rtgwg-device-model-02 (work in progress), March 2017.

              Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
              Liu, "YANG Model for Logical Network Elements", draft-
              ietf-rtgwg-lne-model-10 (work in progress), March 2018.

              Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
              Liu, "YANG Model for Network Instances", draft-ietf-rtgwg-
              ni-model-12 (work in progress), March 2018.

   [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,

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,

   [RFC8343]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,

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
   [I-D.ietf-rtgwg-device-model], using both logical network elements
   (LNE) and network instances (NI).

   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",
                "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" ],
                "name": "ietf-ip",
                "revision": "2018-02-22",
                "namespace": "urn:ietf:params:xml:ns:yang:ietf-ip"
                "name": "ietf-logical-network-element",
                "revision": "2016-10-21",
                "feature": [ "bind-lne-name" ],
                "name": "ietf-yang-library",
                "revision": "2018-02-21",
                "name": "ietf-yang-schema-mount",
                "revision": "2018-03-20",
            "import-only-module": [
                "name": "ietf-inet-types",
                "revision": "2013-07-15",
                "name": "ietf-yang-types",
                "revision": "2013-07-15",
        "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"

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:

     "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": {

   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": [
           "namespace": "urn:ietf:params:xml:ns:yang:ietf-interfaces",
           "conformance-type": "implement"
           "name": "ietf-ip",
           "revision": "2014-06-16",
           "feature": [
           "namespace": "urn:ietf:params:xml:ns:yang:ietf-ip",
           "conformance-type": "implement"
           "name": "ietf-network-instance",
           "revision": "2016-10-27",
           "feature": [
           "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": "2017-05-16",
           "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

     "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",
                 "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": [
                 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.

A.4.  Invoking an RPC Operation

   Assume that the mounted NI data model also implements the "ietf-isis"
   module [I-D.ietf-isis-yang-isis-cfg].  An RPC operation defined in
   this module, such as "clear-adjacency", can be invoked by a client
   session of a LNE's RESTCONF server as an action tied to a 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

Authors' Addresses

   Martin Bjorklund
   Tail-f Systems


   Ladislav Lhotka