YANG Schema Mount
draft-ietf-netmod-schema-mount-03

Network Working Group                                       M. Bjorklund
Internet-Draft                                            Tail-f Systems
Intended status: Standards Track                               L. Lhotka
Expires: May 4, 2017                                              CZ.NIC
                                                        October 31, 2016

                           YANG Schema Mount
                   draft-ietf-netmod-schema-mount-03

Abstract

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

Status of This Memo

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

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

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

   This Internet-Draft will expire on May 4, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Bjorklund & Lhotka         Expires May 4, 2017                  [Page 1]
Internet-Draft              YANG Schema Mount               October 2016

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   2
       1.1.1.  Tree Diagrams . . . . . . . . . . . . . . . . . . . .   2
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Schema Mount  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Augment and Validation in Mounted Data  . . . . . . . . .   4
     3.2.  Top-level RPCs  . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Top-level Notifications . . . . . . . . . . . . . . . . .   5
   4.  Data Model  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Schema Mount YANG Module  . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Example: Logical Devices . . . . . . . . . . . . . .  14
   Appendix B.  Example: Network Manager with Fixed Device Models  .  16
   Appendix C.  Example: Network Manager with Arbitrary Device
                Models . . . . . . . . . . . . . . . . . . . . . . .  19
     C.1.  Invoking an RPC . . . . . . . . . . . . . . . . . . . . .  23
   Appendix D.  Open Issues  . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

1.1.  Terminology

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

1.1.1.  Tree Diagrams

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

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

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

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

Bjorklund & Lhotka         Expires May 4, 2017                  [Page 2]
Internet-Draft              YANG Schema Mount               October 2016

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

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

2.  Background

   YANG has two mechanisms for extending a data model with additional
   nodes; "uses" and "augment".  The "uses" statement explicitly
   incorporates the contents of a "grouping" defined in some other
   module.  The "augment" statement explicitly adds contents to a target
   node defined in some other module.  In both these cases, the source
   and/or target model explicitly defines the relationship between the
   models.

   In some cases these mechanisms are not sufficient.  For example,
   suppose we have a model like ietf-interfaces [RFC7223] that is
   defined to be implemented in a device.  Now suppose we want to model
   a device that supports multiple logical devices
   [I-D.rtgyangdt-rtgwg-device-model], where each such logical device
   has its own instantiation of ietf-interfaces (and other models), but
   at the same time, we'd like to be able to manage all these logical
   devices from the main device.  We would like something like this:

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

   With the "uses" approach, ietf-interfaces would have to define a
   grouping with all its nodes, and the new model for logical devices
   would have to use this grouping.  This is a not a scalable solution,
   since every time there is a new model defined, we would have to
   update our model for logical devices to use a grouping from the new
   model.  Another problem is that this approach cannot handle vendor-
   specific modules.

   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 on the top-level.  This approach is also not
   scalable, since there may be other models to which we would like to
   add the interface list.

Bjorklund & Lhotka         Expires May 4, 2017                  [Page 3]
Internet-Draft              YANG Schema Mount               October 2016

3.  Schema Mount

   The schema mount mechanism defined in this document takes a different
   approach to the extensibility problem described in the previous
   section.  It decouples the definition of the relation between the
   source and target models from the definitions of the models
   themselves.

   This is accomplished with a YANG extension statement that is used to
   specify a mount point in a data model.  The purpose of a mount point
   is to define a place in the node hierarchy where other YANG data
   models may be attached, without any special notation in the other
   YANG data models.  Only "anydata" nodes can be used as mount points.

   For each mount point supported by a server, the server populates an
   operational state node hierarchy with information about which models
   it has mounted.  This node hierarchy can be read by a client in order
   to learn what is implemented on a server.

   Schema mount applies to the data model, and specifically does not
   assume anything about how the mounted data is implemented.  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, for example mounting of sub-hierarchies of a module.

3.1.  Augment and Validation in Mounted Data

   All paths (in leafrefs, instance-identifiers, XPath expressions, and
   target nodes of augments) in the data models mounted at a mount point
   are interpreted with the mount point as the root node, and the
   mounted data nodes as its children.  This means that data within a
   mounted subtree can never refer to data outside of this subtree.

3.2.  Top-level RPCs

   If any mounted data model defines RPCs, these RPCs can be invoked by
   clients by treating them as actions defined where the mount point is
   specified.  An example of this is given in Appendix C.1.

Bjorklund & Lhotka         Expires May 4, 2017                  [Page 4]
Internet-Draft              YANG Schema Mount               October 2016

3.3.  Top-level Notifications

   If the server emits a notification defined at the top-level in any
   mounted data model, it is treated as if the notification was attached
   to the data node where the mount point is specified.

4.  Data Model

   This document defines the YANG 1.1 module [RFC7950]
   "ietf-yang-schema-mount", which has the following structure:

Bjorklund & Lhotka         Expires May 4, 2017                  [Page 5]
Internet-Draft              YANG Schema Mount               October 2016

   module: ietf-yang-schema-mount
       +--ro schema-mounts
          +--ro namespace* [prefix]
          |  +--ro prefix    yang:yang-identifier
          |  +--ro ns-uri?   inet:uri
          +--ro mount-point* [module name]
          |  +--ro module        yang:yang-identifier
          |  +--ro name          yang:yang-identifier
          |  +--ro (subschema-ref)?
          |     +--:(inline)
          |     |  +--ro inline?       empty
          |     +--:(use-schema)
          |        +--ro use-schema* [name]
          |           +--ro name    -> /schema-mounts/schema/name
          |           +--ro when?   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 (subschema-ref)?
                   +--:(inline)
                   |  +--ro inline?       empty
                   +--:(use-schema)
                      +--ro use-schema* [name]
                         +--ro name    -> /schema-mounts/schema/name
                         +--ro when?   yang:xpath1.0

5.  Schema Mount YANG Module

   This module references [RFC6991] and [RFC7895].

   <CODE BEGINS> file "ietf-yang-schema-mount@2016-04-05.yang"

Bjorklund & Lhotka         Expires May 4, 2017                  [Page 6]
Internet-Draft              YANG Schema Mount               October 2016

  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";
    }

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

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

    contact
      "WG Web:   <https://tools.ietf.org/wg/netmod/>
       WG List:  <mailto:netmod@ietf.org>

       WG Chair:  Lou Berger
                  <mailto:lberger@labn.net>

       WG Chair:  Kent Watsen
                  <mailto:kwatsen@juniper.net>

       Editor:    Martin Bjorklund
                  <mailto:mbj@tail-f.com>

       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.

Bjorklund & Lhotka         Expires May 4, 2017                  [Page 7]
Internet-Draft              YANG Schema Mount               October 2016

       Copyright (c) 2016 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).

       The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
       NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and
       'OPTIONAL' in the module text are to be interpreted as described
       in RFC 2119 (https://tools.ietf.org/html/rfc2119).

       This version of this YANG module is part of RFC XXXX
       (https://tools.ietf.org/html/rfcXXXX); see the RFC itself for
       full legal notices.";

    revision 2016-10-26 {
      description
        "Initial revision.";
      reference
        "RFC XXXX: YANG Schema Mount";
    }

    /*
     * Extensions
     */

    extension mount-point {
      argument name;
      description
        "The argument 'name' is a yang-identifier. The name of the
         mount point MUST be unique within the module where it is
         defined.

         The 'mount-point' statement can only be present as a
         substatement of 'anydata'.

         If a mount point is defined in a grouping, its name is bound
         to the module where the grouping is used. Note that this
         implies that such a grouping can be used at most once in a
         module.

         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

Bjorklund & Lhotka         Expires May 4, 2017                  [Page 8]
Internet-Draft              YANG Schema Mount               October 2016

         /schema-mounts/mount-point list with detailed information on
         which data models are mounted at each mount point.";
    }

    /*
     * Groupings
     */

    grouping mount-point-list {
      description
        "This grouping is used inside the 'schema-mounts' container and
         inside the 'schema' list.";
      list mount-point {
        key "module name";
        description
          "Each entry of this list specifies a subschema 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
             'schema'.";
        leaf module {
          type yang:yang-identifier;
          description
            "Name of a module containing the mount point.";
        }
        leaf name {
          type yang:yang-identifier;
          description
            "Name of the mount point defined using the 'mount-point'
             extension.";
        }
        choice subschema-ref {
          description
            "Alternative way for specifying the subschema.";
          leaf inline {
            type empty;
            description
              "This leaf indicates that the server has mounted
               'ietf-yang-library' and 'ietf-schema-mount' at the mount

Bjorklund & Lhotka         Expires May 4, 2017                  [Page 9]
Internet-Draft              YANG Schema Mount               October 2016

               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";
            description
              "Each entry of this list contains a reference to a
               subschema defined in the /schema-mounts/schema list. The
               entry can be made conditional by specifying an XPath
               expression in the 'when' leaf.";
            leaf name {
              type leafref {
                path "/schema-mounts/schema/name";
              }
              description
                "Name of the referenced schema.";
            }
            leaf when {
              type yang:xpath1.0;
              description
                "This leaf contains an XPath expression. If it is
                 present, then the current entry applies if and only if
                 the expression evaluates to true.

                 The XPath expression is evaluated once for each
                 instance of the anydata node containing the mount
                 point for which the 'when' leaf is defined.

                 The XPath expression is evaluated using the rules
                 specified in sec. 6.4 of RFC 7950, with these
                 modifications:

                 - The context node is the anydata instance containing
                   the corresponding 'mount-point' statement.

                 - The accessible tree contains only data belonging to
                   the parent schema, i.e., all instances of anydata
                   nodes containing the mount points are considered
                   empty.

                 - The set of namespace declarations is the set of all
                   prefix/namespace pairs defined in the
                   /schema-mounts/namespace list. Names without a
                   namespace prefix belong to the same namespace as the
                   context node.";
            }
          }

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 10]
Internet-Draft              YANG Schema Mount               October 2016

        }
      }
    }

    /*
     * State data nodes
     */

    container schema-mounts {
      config "false";
      description
        "Contains information about the structure of the overall 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 'when' leafs to the
           corresponding namespace URI references.";
        leaf prefix {
          type yang:yang-identifier;
          description
            "Namespace prefix.";
        }
        leaf ns-uri {
          type inet:uri;
          description
            "Namespace URI reference.";
        }
      }
      uses mount-point-list;
      list schema {
        key "name";
        description
          "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
             recursive).";
        leaf name {
          type string;
          description
            "Arbitrary name of the entry.";
        }

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 11]
Internet-Draft              YANG Schema Mount               October 2016

        uses yanglib:module-list;
        uses mount-point-list;
      }
    }
  }

   <CODE ENDS>

6.  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

7.  Security Considerations

   TBD

8.  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]); Ladislav Lhotka
   ([I-D.lhotka-netmod-ysdl]); and Lou Berger and Christian Hopps.

9.  References

9.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 12]
Internet-Draft              YANG Schema Mount               October 2016

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <http://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,
              <http://www.rfc-editor.org/info/rfc6020>.

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7950>.

9.2.  Informative References

   [I-D.clemm-netmod-mount]
              Clemm, A., Medved, J., and E. Voit, "Mounting YANG-Defined
              Information from Remote Datastores", draft-clemm-netmod-
              mount-05 (work in progress), September 2016.

   [I-D.lhotka-netmod-ysdl]
              Lhotka, L., "YANG Schema Dispatching Language", draft-
              lhotka-netmod-ysdl-00 (work in progress), November 2015.

   [I-D.rtgyangdt-rtgwg-device-model]
              Lindem, A., Berger, L., Bogdanovic, D., and C. Hopps,
              "Network Device YANG Organizational Models", draft-
              rtgyangdt-rtgwg-device-model-05 (work in progress), August
              2016.

   [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,
              <http://www.rfc-editor.org/info/rfc6241>.

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
              <http://www.rfc-editor.org/info/rfc7223>.

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 13]
Internet-Draft              YANG Schema Mount               October 2016

   [RFC7277]  Bjorklund, M., "A YANG Data Model for IP Management",
              RFC 7277, DOI 10.17487/RFC7277, June 2014,
              <http://www.rfc-editor.org/info/rfc7277>.

   [RFC7317]  Bierman, A. and M. Bjorklund, "A YANG Data Model for
              System Management", RFC 7317, DOI 10.17487/RFC7317, August
              2014, <http://www.rfc-editor.org/info/rfc7317>.

Appendix A.  Example: Logical Devices

   Logical devices within a device typically use the same set of data
   models in each instance.  This can be modelled with a mount point:

   module example-logical-devices {
     yang-version 1.1;
     namespace "urn:example:logical-devices";
     prefix exld;

     import ietf-yang-schema-mount {
       prefix yangmnt;
     }

     container logical-devices {
       list logical-device {
         key name;
         leaf name {
           type string;
         }

         anydata root {
           yangmnt:mount-point logical-device;
         }
       }
     }
   }

   A server with two logical devices that both implement
   "ietf-interfaces" [RFC7223], "ietf-ip" [RFC7277], and "ietf-system"
   [RFC7317] YANG modules might populate the "schema-mounts" container
   with:

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 14]
Internet-Draft              YANG Schema Mount               October 2016

   <schema-mounts
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount">
     <mount-point>
       <module>example-logical-devices</module>
       <name>logical-device</name>
       <use-schema>
         <name>logical-device</name>
       </use-schema>
     </mount-point>
     <schema>
       <name>logical-device</name>
       <module>
         <name>ietf-interface</name>
         <revision>2014-05-08</revision>
         <namespace>
           urn:ietf:params:xml:ns:yang:ietf-interfaces
         </namespace>
         <conformance-type>implement</conformance-type>
       </module>
       <module>
         <name>ietf-ip</name>
         <revision>2014-06-16</revision>
         <namespace>
           urn:ietf:params:xml:ns:yang:ietf-ip
         </namespace>
         <conformance-type>implement</conformance-type>
       </module>
       <module>
         <name>ietf-system</name>
         <revision>2014-08-06</revision>
         <namespace>
           urn:ietf:params:xml:ns:yang:ietf-system
         </namespace>
         <conformance-type>implement</conformance-type>
       </module>
       <module>
         <name>ietf-yang-types</name>
         <revision>2013-07-15</revision>
         <namespace>
           urn:ietf:params:xml:ns:yang:ietf-yang-types
         </namespace>
         <conformance-type>import</conformance-type>
       </module>
     </schema>
   </schema-mounts>

   and the "logical-devices" container might have:

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 15]
Internet-Draft              YANG Schema Mount               October 2016

   <logical-devices xmlns="urn:example:logical-devices">
     <logical-device>
       <name>vrtrA</name>
       <root>
         <interfaces
             xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
           <interface>
             <name>eth0</name>
               <ipv6 xmlns="urn:ietf:params:xml:ns:yang:ietf-ip">
                 <enabled>true</enabled>
                 ...
               </ipv6>
             ...
           </interface>
         </interfaces>
         <system xmlns="urn:ietf:params:xml:ns:yang:ietf-system">
           ...
         </system>
       </root>
     </logical-device>
     <logical-device>
       <name>vrtrB</name>
       <root>
         <interfaces
             xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
           <interface>
             <name>eth0</name>
               <ipv6 xmlns="urn:ietf:params:xml:ns:yang:ietf-ip">
                 <enabled>true</enabled>
                 ...
               </ipv6>
             ...
           </interface>
         </interfaces>
         <system xmlns="urn:ietf:params:xml:ns:yang:ietf-system">
           ...
         </system>
       </root>
     </logical-device>
   </logical-devices>

Appendix B.  Example: Network Manager with Fixed Device Models

   This example shows how a Network Manager application can use schema
   mount to define a data model for a network consisting of devices
   whose data models are known a priori and fixed.

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 16]
Internet-Draft              YANG Schema Mount               October 2016

   Assume for simplicity that only two device types are used (switch and
   router), and they are identified by identities defined in the module
   "example-device-types":

   module example-device-types {
     namespace "http://example.org/device-types";
     prefix edt;
     identity device-type;
     identity switch-device {
       base device-type;
     }
     identity router-device {
       base device-type;
     }
   }

   Schema mount is used to mount the device data models conditionally,
   depending on the "type" leaf that is a sibling of the mount point.
   This approach is similar to "ietf-interfaces" [RFC7223] where the
   same effect is achieved via conditional augments.

   The top-level module may look as follows:

   module example-network-manager-fixed {
     yang-version 1.1;
     namespace "urn:example:network-manager-fixed";
     prefix exf;

     import ietf-inet-types {
       prefix inet;
     }
     import ietf-yang-schema-mount {
       prefix yangmnt;
     }
     import example-device-types {
       prefix edt;
     }

     container managed-devices {
       description
         "The managed devices and device communication settings.";

       list device {
         key name;
         leaf name {
           type string;
         }
         leaf type {

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 17]
Internet-Draft              YANG Schema Mount               October 2016

           type identityref {
             base edt:device-type;
           }
         }
         container transport {
           choice protocol {
             mandatory true;
             container netconf {
               leaf address {
                 type inet:ip-address;
                 mandatory true;
               }
               container authentication {
                 // ...
               }
             }
             container restconf {
               leaf address {
                 type inet:ip-address;
                 mandatory true;
               }
               // ...
             }
           }
         }
         anydata root {
           yangmnt:mount-point managed-device;
         }
       }
     }
   }

   The "schema-mounts" container may have the following data:

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 18]
Internet-Draft              YANG Schema Mount               October 2016

   <data-model
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount">
     <namespace>
       <prefix>edt</prefix>
       <ns-uri>http://example.org/device-types</ns-uri>
     </namespace>
     <mount-point>
       <module>example-network-manager</module>
       <name>managed-device</name>
       <use-schema>
         <name>switch</name>
         <when>derived-from-or-self(../type, 'edt:switch-device')</when>
       </use-schema>
       <use-schema>
         <name>router</name>
         <when>derived-from-or-self(../type, 'edt:router-device')</when>
       </use-schema>
     </mount-point>
     <schema>
       <name>switch</name>
       <module>
         ...
       </module>
       ...
     </schema>
     <schema>
       <name>router</name>
       <module>
         ...
       </module>
       ...
     </schema>
   </data-model>

   The "devices" list may contain any number of instances of either
   type.

Appendix C.  Example: Network Manager with Arbitrary Device Models

   This example shows how a Network Manager application can use schema
   mount to define a data model for a network consisting of devices
   whose data models are not known in advance -- each device is expected
   to provide its data model dynamically.

   Schema mount is used to mount the data models that each device
   supports, and these data models can be discovered by inspecting state
   data under the corresponding mount point.  Every such device must

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 19]
Internet-Draft              YANG Schema Mount               October 2016

   therefore implement "ietf-yang-library" and optionally
   "ietf-schema-mount".

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 20]
Internet-Draft              YANG Schema Mount               October 2016

   module example-network-manager-arbitrary {
     yang-version 1.1;
     namespace "urn:example:network-manager-arbitrary";
     prefix exa;

     import ietf-inet-types {
       prefix inet;
     }
     import ietf-yang-schema-mount {
       prefix yangmnt;
     }

     container managed-devices {
       description
         "The managed devices and device communication settings.";

       list device {
         key name;
         leaf name {
           type string;
         }
         container transport {
           choice protocol {
             mandatory true;
             container netconf {
               leaf address {
                 type inet:ip-address;
                 mandatory true;
               }
               container authentication {
                 // ...
               }
             }
             container restconf {
               leaf address {
                 type inet:ip-address;
                 mandatory true;
               }
               // ...
             }
           }
         }
         anydata root {
           yangmnt:mount-point managed-device;
         }
       }
     }
   }

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 21]
Internet-Draft              YANG Schema Mount               October 2016

   The "schema-mounts" container may have the following data:

   <data-model
       xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-schema-mount">
     <mount-point>
       <module>example-network-manager</module>
       <name>managed-device</name>
       <inline/>
     </mount-point>
   </data-model>

   The "devices" container might have:

   <devices xmlns="urn:example:network-manager">
     <device>
       <name>rtrA</name>
       <transport>
         <netconf>
           <address>2001:db8::2</address>
           <authentication>
             ...
           </authentication>
           ...
         </netconf>
       </transport>
       <root>
         <modules-state
             xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
           <module>
             <name>ietf-system</name>
             ...
           </module>
         </modules-state>
         <system xmlns="urn:ietf:params:xml:ns:yang:ietf-system">
           ...
         </system>
       </root>
     </device>
     <device>
       <name>rtrB</name>
       <transport>
         <restconf>
           <address>2001:db8::3</address>
           <authentication>
             ...
           </authentication>
           ...
         </restconf>

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 22]
Internet-Draft              YANG Schema Mount               October 2016

       </transport>
       <root>
         <modules-state
             xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
           <module>
             <name>ietf-interfaces</name>
             ...
           </module>
         </modules-state>
         <interfaces
             xmlns="urn:ietf:params:xml:ns:yang:ietf-interfaces">
           ...
         </interfaces>
       </root>
     </device>
   </devices>

C.1.  Invoking an RPC

   A client that wants to invoke the "restart" operation [RFC7317] on
   the managed device "rtrA" over NETCONF [RFC6241] can send:

   <rpc message-id="101"
        xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
     <action xmlns="urn:ietf:params:xml:ns:yang:1">
       <managed-devices xmlns="urn:example:network-manager">
         <device>
           <name>rtrA</name>
           <root>
             <system xmlns="urn:ietf:params:xml:ns:yang:ietf-system">
               <restart/>
             </system>
           </root>
         </device>
       </managed-devices>
     </action>
   </rpc>

Appendix D.  Open Issues

   o  Is the 'mount-point' extension really needed?  Now that mount
      points can only appear under anydata nodes, there seems to be
      little need to otherwise restrict mount point locations.  In the
      'mount-point' list, schema node identifiers (as in 'augment'
      statements) can be used instead of the (module, name) pair for
      identifying mount points.  As a useful side effect, a grouping
      containing mount points could be used any number of times in the
      same module.  OTOH, by using this extension, the intention of the

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 23]
Internet-Draft              YANG Schema Mount               October 2016

      data modeller is clear, and it provides a formal machine readable
      instruction about where mounts are allowed to occur.

Authors' Addresses

   Martin Bjorklund
   Tail-f Systems

   Email: mbj@tail-f.com

   Ladislav Lhotka
   CZ.NIC

   Email: mbj@lhotka@nic.cz

Bjorklund & Lhotka         Expires May 4, 2017                 [Page 24]