Network Working Group                                       M. Bjorklund
Internet-Draft                                            Tail-f Systems
Intended status: Standards Track                               L. Lhotka
Expires: November 17, 2017 January 6, 2018                                          CZ.NIC
                                                            May 16,
                                                            July 5, 2017

                           YANG Schema Mount


   This document defines a mechanism to combine YANG modules into the
   schema defined in other YANG modules.

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   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 November 17, 2017. January 6, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   5
     2.1.  Glossary of New Terms . . . . . . . . . . . . . . . . . .   6
     2.2.  Tree Diagrams . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Namespace Prefixes  . . . . . . . . . . . . . . . . . . .   6
   3.  Schema Mount  . . . . . . . . . . . . . . . . . . . . . . . .   7   6
     3.1.  Mount Point Definition  . . . . . . . . . . . . . . . . .   7
     3.2.  Specification of the Mounted Schema . . . . . . . . . . .   7
     3.3.  Multiple Levels of Schema Mount . . . . . . . . . . . . .   9
   4.  Referring to Data Nodes in the Parent Schema  . . . . . . . .   9
   5.  RPC operations and Notifications  . . . . . . . . . . . . . .  10
   6.  Implementation Notes  . . . . . . . . . . . . . . . . . . . .  11
   7.  Data Model  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Schema Mount YANG Module  . . . . . . . . . . . . . . . . . .  13
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  18
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     12.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Appendix A.  Example: Device Model with LNEs and NIs  . . . . . .  20
     A.1.  Physical Device . . . . . . . . . . . . . . . . . . . . .  20  21
     A.2.  Logical Network Elements  . . . . . . . . . . . . . . . .  22
     A.3.  Network Instances . . . . . . . . . . . . . . . . . . . .  25
     A.4.  Invoking an RPC Operation . . . . . . . . . . . . . . . .  27
   Appendix B.  Open Issues  . . . . . . . . . . . . . . . . . . . .  27
     B.1.  RPC Operations and Notifications in Mounted Modules . . .  27
     B.2.  Tree Representation . . . . . . . . . . . . . . . . . . .  27
     B.3.  Design-Time Mounts  . . . . . . . . . . . . . . . . . . .  28  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28  27

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], 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.

   Furthermore, 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 source or target YANG module explicitly
   defines the exact location in the schema tree where the new nodes are

   In some cases these mechanisms are not sufficient; it is often
   necessary that an existing module (or a set of modules) is added to
   the data model starting at a non-root location.  For example, YANG
   modules such as "ietf-interfaces" [RFC7223] are often 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 like this:

     +--rw interfaces
     |  +--rw interface* [name]
     |     ...
     +--rw logical-device* [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-device" 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-device" 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

   This document introduces a new generic 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 YANG library information, i.e.,
       it may change after an upgrade of server software but not after
       rebooting the server.  Also, a client can learn the entire schema
       together with YANG library data.

   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 because design-time definition of cases.  Design-time mounts are outside the
   mounted schema doesn't play well
   scope of this document, and could be possibly dealt with in a future
   revision of the existing YANG modularity
   mechanisms.  For example, it would be impossible to augment the
   mounted data model. 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.

   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.

2.  Terminology and Notation

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14, [RFC2119].

   The following terms are defined in [RFC6241] and are not redefined

   o  client

   o  notification

   o  server

   The following terms are defined in [RFC7950] and are not redefined

   o  action

   o  configuration data

   o  container

   o  list

   o  operation

   The following terms are defined in [RFC7223] and are not redefined

   o  system-controlled interface
   Tree diagrams used in this document follow the notation defined in

2.1.  Glossary of New Terms

   o  inline schema: a mounted schema whose definition is provided as
      part of the mounted data, using YANG library [RFC7895].

   o  mount point: container or list node whose definition contains the
      "mount-point" extension statement.  The argument of the
      "mount-point" statement defines the name of the mount point.

   o  parent schema (of a particular mounted schema): the schema that
      contains the mount point for the mounted schema.

   o  top-level schema: a schema according to [RFC7950] in which schema
      trees of each module (except augments) start at the root node.

2.2.  Tree Diagrams

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

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

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

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

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

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

2.3.  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 8 |
             | inet    | ietf-inet-types        | [RFC6991] |
             | yang    | ietf-yang-types        | [RFC6991] |
             | yanglib | ietf-yang-library      | [RFC7895] |

                        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 the name of 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.  Note, however, that modules written in any YANG version,
   including version 1, can be mounted under a mount point.

3.2.  Specification of the Mounted Schema

   Mounted schemas for all mount points in the parent schema are
   determined from state data in the "yangmnt:schema-mounts" container.
   Data in this container is intended to be as stable as data in the
   top-level YANG library [RFC7895].  In particular, it SHOULD NOT
   change during the same management session.

   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

   Generally, the modules that are mounted schema.

   If under a mount point is defined 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, e.g., the interface
   module in [I-D.ietf-rtgwg-lne-model].

   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 above
   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:

   1.  by stating that the schema is available inline, i.e., in run-time
       instance data; or

   2.  by referring to one or more entries of the "schema" list in the
       same instance of "schema-mounts".

   In case 1, the mounted schema is determined at run time: every
   instance of the mount point that exists in the parent tree MUST
   contain a copy of YANG library data [RFC7895] that defines the
   mounted schema exactly as for a top-level data model.  A client is
   expected to retrieve this data from the instance tree, possibly after
   creating the mount point.  Instances of the same mount point MAY use
   different mounted schemas.

   In case 2, the mounted schema is defined by the combination of all
   "schema" entries referred to in the "use-schema" list.  In this case,
   the mounted schema is specified as implementation-time data that can
   be retrieved together with YANG library data for the parent schema,
   i.e., even before any instances of the mount point exist.  However,
   the mounted schema has to be the same for all instances of the mount
   point.  Note, that in this case a mount point may include a mounted
   YANG library module and the data contained in the mounted module MUST
   exactly match the data contained in the "schema" entries associated
   with the mount point.

   Each entry of the "schema" list contains contains:

   o  a list in the YANG library format specifying all YANG modules (and
      revisions etc.) that are implemented or imported in the mounted
      schema.  Note that this includes modules that solely augment other
      listed modules;

   o  (optionally) a new "mount-point" list that applies to mount points
      defined within the mounted schema.

3.3.  Multiple Levels of Schema Mount

   YANG modules in a mounted schema MAY again contain mount points under
   which subschemas can be mounted.  Consequently, it is possible to
   construct data models with an arbitrary number of schema levels.  A
   subschema for a mount point contained in a mounted module can be
   specified in one of the following ways:

   o  by implementing "ietf-yang-library" and "ietf-yang-schema-mount"
      modules in the mounted schema, and specifying the subschemas
      exactly as it is done in the top-level schema

   o  by using the "mount-point" list inside the corresponding "schema"

   The former method is applicable to both "inline" and "use-schema"
   cases whereas the latter requires the "use-schema" case.  On the
   other hand, the latter method allows for a compact representation of
   a multi-level schema the does not rely on the presence of any
   instance data.

4.  Referring to Data Nodes in the Parent Schema

   A fundamental design principle of schema mount is that the mounted
   data model works exactly as a top-level data model, i.e., it is
   confined to the "mount jail".  This means that all paths in the
   mounted data model (in leafrefs, instance-identifiers, 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
   schema mounted using the "use-schema" method, 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
   root of the parent data tree as the context node and the result 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

   o  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].

   o  The mechanism of referencing nodes in the parent schema is not
      available for schemas mounted using the "inline" method.

5.  RPC operations and Notifications

   If a mounted YANG module defines an RPC operation, clients can invoke
   this operation by representing it as an action defined 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
   which 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.  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.

7.  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 ns-uri? uri?      inet:uri
          +--ro mount-point* [module name]
          |  +--ro module        yang:yang-identifier
          |  +--ro name          yang:yang-identifier
          |  +--ro config?       boolean
          |  +--ro (schema-ref)?
          |     +--:(inline)
          |     |  +--ro inline?       empty
          |     +--:(use-schema)
          |        +--ro use-schema* [name]
          |           +--ro name
          |           |       -> /schema-mounts/schema/name
          |           +--ro parent-reference*   yang:xpath1.0
          +--ro schema* [name]
             +--ro name           string
             +--ro module* [name revision]
             |  +--ro name                yang:yang-identifier
             |  +--ro revision            union
             |  +--ro schema?             inet:uri
             |  +--ro namespace           inet:uri
             |  +--ro feature*            yang:yang-identifier
             |  +--ro deviation* [name revision]
             |  |  +--ro name        yang:yang-identifier
             |  |  +--ro revision    union
             |  +--ro conformance-type    enumeration
             |  +--ro submodule* [name revision]
             |     +--ro name        yang:yang-identifier
             |     +--ro revision    union
             |     +--ro schema?     inet:uri
             +--ro mount-point* [module name]
                +--ro module        yang:yang-identifier
                +--ro name          yang:yang-identifier
                +--ro config?       boolean
                +--ro (schema-ref)?
                   |  +--ro inline?       empty
                      +--ro use-schema* [name]
                         +--ro name
                         |       -> /schema-mounts/schema/name
                         +--ro parent-reference*   yang:xpath1.0

8.  Schema Mount YANG Module

   This module references [RFC6991] and [RFC7895].

   <CODE BEGINS> file "ietf-yang-schema-mount@2017-05-16.yang" "ietf-yang-schema-mount@2017-06-16.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;
         "RFC 6991: Common YANG Data Types";

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

     import ietf-yang-library {
       prefix yanglib;
         "RFC 7895: YANG Module Library";

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

       "WG Web:   <>
        WG List:  <>

        Editor:   Martin Bjorklund

        Editor:   Ladislav Lhotka

       "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) 2017 IETF Trust and the persons identified as
        authors of the code. All rights reserved.

        Redistribution and use in source and binary forms, with or
        without modification, is permitted pursuant to, and subject to
        the license terms contained in, the Simplified BSD License set
        forth in Section 4.c of the IETF Trust's Legal Provisions
        Relating to IETF Documents

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
        'OPTIONAL' in the module text are to be interpreted as described
        in RFC 2119 (

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

     revision 2017-05-16 2017-06-16 {
         "Initial revision.";
         "RFC XXXX: YANG Schema Mount";

      * Extensions

     extension mount-point {
       argument name;
         "The argument 'name' 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.

          If a mount point is defined in a grouping, its name 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.";

      * Groupings

     grouping mount-point-list {
         "This grouping is used inside the 'schema-mounts' container and
          inside the 'schema' list.";
       list mount-point {
         key "module name";
           "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 corresponding
            YANG library instance with conformance type 'implement'. The
            corresponding YANG library instance is:

            - standard YANG library state data as defined in RFC 7895,
              if the 'mount-point' list is a child of 'schema-mounts',

            - the contents of the sibling 'yanglib:modules-state'
              container, if the 'mount-point' list is a child of
         leaf module {
           type yang:yang-identifier;
             "Name of a module containing the mount point.";
         leaf name {
           type yang:yang-identifier;
             "Name 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 {
             "Alternatives for specifying the schema.";
           leaf inline {
             type empty;
               "This leaf indicates that the server has mounted
                'ietf-yang-library' and 'ietf-schema-mount' at the mount
                point, and their instantiation (i.e., state data
                containers 'yanglib:modules-state' and 'schema-mounts')
                provides the information about the mounted schema.";
           list use-schema {
             key "name";
               "Each entry of this list contains a reference to a schema
                defined in the /schema-mounts/schema list.";
             leaf name {
               type leafref {
                 path "/schema-mounts/schema/name";
                 "Name of the referenced schema.";
             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 root node of the parent data

                  - 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.";

      * 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 ns-uri uri {
           type inet:uri;
             "Namespace URI reference.";
       uses mount-point-list;
       list schema {
         key "name";
           "Each entry specifies a schema that can be mounted at a mount
            point.  The schema information consists of two parts:

            - an instance of YANG library that defines YANG modules used
              in the schema,

            - mount-point list with content identical to the top-level
              mount-point list (this makes the schema structure
         leaf name {
           type string;
             "Arbitrary name of the schema entry.";
         uses yanglib:module-list;
         uses mount-point-list;


9.  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

10.  Security Considerations


11.  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, <>

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, 10.17487/
              RFC2119, March 1997,

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,

   [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,

12.2.  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-17 (work in progress), March 2017.

              Bjorklund, M. and L. Berger, "YANG Tree Diagrams", draft-
              ietf-netmod-yang-tree-diagrams-01 (work in progress), June

              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., and D. Bogdanovic, D., and X.
              Liu, "YANG Logical Network Elements", draft-ietf-rtgwg-lne-
              model-02 draft-ietf-rtgwg-
              lne-model-03 (work in progress), March July 2017.

              Berger, L., Hopps, C., Lindem, A., and D. Bogdanovic, D., and X.
              Liu, "YANG Network Instances", draft-ietf-rtgwg-ni-model-02 draft-ietf-rtgwg-ni-
              model-03 (work in progress), March July 2017.

   [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,

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,

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

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).

A.1.  Physical Device

   The data model for the physical device may be described by this YANG
   library content:

   "ietf-yang-library:modules-state": {
     "module-set-id": "14e2ab5dc325f6d86f743e8d3ade233f1a61a899",
     "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",
         "namespace": "urn:ietf:params:xml:ns:yang:ietf-ip",
         "conformance-type": "implement"
         "name": "ietf-logical-network-element",
         "revision": "2016-10-21",
         "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"

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",
         "name": "root",
         "inline": [null]

   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:

   "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-state": {
     "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 "use-schema" method as follows:

   "ietf-yang-schema-mount:schema-mounts": {
     "namespace": [
           "prefix": "if",
           "uri": "urn:ietf:params:xml:ns:yang:ietf-interfaces"
     "mount-point": [
         "module": "ietf-network-instance",
         "name": "root",
           "parent-reference": ["ietf-interfaces"],
         "use-schema": [
             "name": "ni-schema" "ni-schema",
             "parent-reference": ["/if:interfaces"]

     "schema": [
         "name": "ni-schema",
         "module": [
             "name": "ietf-ipv4-unicast-routing",
             "revision": "2016-11-04",
             "conformance-type": "implement"
             "name": "ietf-ipv6-unicast-routing",
             "revision": "2016-11-04",
             "conformance-type": "implement"
             "name": "ietf-routing",
             "revision": "2016-11-04",
             "feature": [
             "namespace": "urn:ietf:params:xml:ns:yang:ietf-routing",
             "conformance-type": "implement"

   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

Appendix B.  Open Issues

B.1.  RPC Operations and Notifications in Mounted Modules

   Turning RPC operations defined in mounted modules into actions tied
   to the corresponding mount point (see Section 5, and similarly for
   notifications) is not possible if the path to the mount point in the
   parent schema contains a keyless list (Section 7.15 of [RFC7950]).
   The solutions for this corner case are possible:

   1.  any mount point MUST NOT have a keyless list among its ancestors

   2.  any mounted module MUST NOT contain RPC operations and/or

   3.  specifically for each mount point, at least one of the above
       conditions MUST be satisfied.

   4.  treat such actions and notifications as non-existing, i.e.,
       ignore them.

   The first two requirements seem rather restrictive.  On the other
   hand, the last one is difficult to guarantee - for example, things
   can break after an augment within the mounted schema.

B.2.  Tree Representation

   Need to decide how/if mount points are represented in trees.

B.3.  Design-Time Mounts

   The document currently doesn't provide explicit support for design-
   time mounts.  Design-time mounts have been identified as possibly for
   multiple cases, and it may be worthwhile to identify a minimum or
   complete set of modules that must be supported under a mount point.
   This could be used in service modules that want to allow for
   configuration of device-specific information.  One option could be to
   add an extension that specify that a certain module is required to be

   Also, if design-time mounts are supported, it could be possible to
   represent both mounts points and their required modules in tree
   representations and support for such would need to be defined.

Authors' Addresses

   Martin Bjorklund
   Tail-f Systems


   Ladislav Lhotka