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Network Working Group                                           A. Clemm
Internet-Draft                                                    Huawei
Intended status: Standards Track                               J. Medved
Expires: August 20, 2017                                           Cisco
                                                                R. Varga
                                               Pantheon Technologies SRO
                                                              N. Bahadur
                                                       Bracket Computing
                                                      H. Ananthakrishnan
                                                           Packet Design
                                                                  X. Liu
                                                                Ericsson
                                                       February 16, 2017


                  A Data Model for Network Topologies
                draft-ietf-i2rs-yang-network-topo-11.txt

Abstract

   This document defines an abstract (generic) YANG data model for
   network/service topologies and inventories.  The model serves as a
   base model which is augmented with technology-specific details in
   other, more specific topology and inventory models.

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 August 20, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Key Words . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Definitions and Acronyms  . . . . . . . . . . . . . . . . . .   7
   4.  Model Structure Details . . . . . . . . . . . . . . . . . . .   8
     4.1.  Base Network Model  . . . . . . . . . . . . . . . . . . .   8
     4.2.  Base Network Topology Model . . . . . . . . . . . . . . .  10
     4.3.  Extending the model . . . . . . . . . . . . . . . . . . .  12
     4.4.  Discussion and selected design decisions  . . . . . . . .  12
       4.4.1.  Container structure . . . . . . . . . . . . . . . . .  12
       4.4.2.  Underlay hierarchies and mappings . . . . . . . . . .  13
       4.4.3.  Dealing with changes in underlay networks . . . . . .  13
       4.4.4.  Use of groupings  . . . . . . . . . . . . . . . . . .  14
       4.4.5.  Cardinality and directionality of links . . . . . . .  14
       4.4.6.  Multihoming and link aggregation  . . . . . . . . . .  14
       4.4.7.  Mapping redundancy  . . . . . . . . . . . . . . . . .  15
       4.4.8.  Typing  . . . . . . . . . . . . . . . . . . . . . . .  15
       4.4.9.  Representing the same device in multiple networks . .  15
       4.4.10. Supporting client-configured and server-provided
               network topology  . . . . . . . . . . . . . . . . . .  16
       4.4.11. Identifiers of string or URI type . . . . . . . . . .  17
   5.  Interactions with Other YANG Modules  . . . . . . . . . . . .  18
   6.  YANG Modules  . . . . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  Defining the Abstract Network: network.yang . . . . . . .  18
     6.2.  Creating Abstract Network Topology: network-topology.yang  23
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  31
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  32
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     11.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33







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1.  Introduction

   This document introduces an abstract (base) YANG [RFC7950] [RFC6991]
   data model to represent networks and topologies.  The data model is
   divided into two parts.  The first part of the model defines a
   network model that allows to define network hierarchies (i.e. network
   stacks) and to maintain an inventory of nodes contained in a network.
   The second part of the model augments the basic network model with
   information to describe topology information.  Specifically, it adds
   the concepts of links and termination points to describe how nodes in
   a network are connected to each other.  Moreover the model introduces
   vertical layering relationships between networks that can be
   augmented to cover both network inventories and network/service
   topologies.

   While it would be possible to combine both parts into a single model,
   the separation facilitates integration of network topology and
   network inventory models, by allowing to augment network inventory
   information separately and without concern for topology into the
   network model.

   The model can be augmented to describe specifics of particular types
   of networks and topologies.  For example, an augmenting model can
   provide network node information with attributes that are specific to
   a particular network type.  Examples of augmenting models include
   models for Layer 2 network topologies, Layer 3 network topologies,
   such as Unicast IGP, IS-IS [RFC1195] and OSPF [RFC2328], traffic
   engineering (TE) data [RFC3209], or any of the variety of transport
   and service topologies.  Information specific to particular network
   types will be captured in separate, technology-specific models.

   The basic data models introduced in this document are generic in
   nature and can be applied to many network and service topologies and
   inventories.  The models allow applications to operate on an
   inventory or topology of any network at a generic level, where
   specifics of particular inventory/topology types are not required.
   At the same time, where data specific to a network type does comes
   into play and the model is augmented, the instantiated data still
   adheres to the same structure and is represented in consistent
   fashion.  This also facilitates the representation of network
   hierarchies and dependencies between different network components and
   network types.

   The abstract (base) network YANG module introduced in this document,
   entitled "network.yang", contains a list of abstract network nodes
   and defines the concept of network hierarchy (network stack).  The
   abstract network node can be augmented in inventory and topology
   models with inventory and topology specific attributes.  Network



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   hierarchy (stack) allows any given network to have one or more
   "supporting networks".  The relationship of the base network model,
   the inventory models and the topology models is shown in the
   following figure (dotted lines in the figure denote possible
   augmentations to models defined in this document).

                  +------------------------+
                  |                        |
                  | Abstract Network Model |
                  |                        |
                  +------------------------+
                               |
                       +-------+-------+
                       |               |
                       V               V
                +------------+  ..............
                |  Abstract  |  : Inventory  :
                |  Topology  |  :  Model(s)  :
                |   Model    |  :            :
                +------------+  ''''''''''''''
                       |
         +-------------+-------------+-------------+
         |             |             |             |
         V             V             V             V
   ............  ............  ............  ............
   :    L1    :  :    L2    :  :    L3    :  :  Service :
   : Topology :  : Topology :  : Topology :  : Topology :
   :   Model  :  :   Model  :  :   Model  :  :   Model  :
   ''''''''''''  ''''''''''''  ''''''''''''  ''''''''''''

                   Figure 1: The network model structure

   The network-topology YANG module introduced in this document,
   entitled "network-topology.yang", defines a generic topology model at
   its most general level of abstraction.  The module defines a topology
   graph and components from which it is composed: nodes, edges and
   termination points.  Nodes (from the network.yang module) represent
   graph vertices and links represent graph edges.  Nodes also contain
   termination points that anchor the links.  A network can contain
   multiple topologies, for example topologies at different layers and
   overlay topologies.  The model therefore allows to capture
   relationships between topologies, as well as dependencies between
   nodes and termination points across topologies.  An example of a
   topology stack is shown in the following figure.







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          +---------------------------------------+
         /            _[X1]_          "Service"  /
        /           _/  :   \_                  /
       /          _/     :    \_               /
      /         _/        :     \_            /
     /         /           :      \          /
    /       [X2]__________________[X3]      /
   +---------:--------------:------:-------+
              :              :     :
          +----:--------------:----:--------------+
         /      :              :   :        "L3" /
        /        :              :  :            /
       /         :               : :           /
      /         [Y1]_____________[Y2]         /
     /           *               * *         /
    /            *              *  *        /
   +--------------*-------------*--*-------+
                   *           *   *
          +--------*----------*----*--------------+
         /     [Z1]_______________[Z1] "Optical" /
        /         \_         *   _/             /
       /            \_      *  _/              /
      /               \_   * _/               /
     /                  \ * /                /
    /                    [Z]                /
   +---------------------------------------+

               Figure 2: Topology hierarchy (stack) example

   The figure shows three topology levels.  At top, the "Service"
   topology shows relationships between service entities, such as
   service functions in a service chain.  The "L3" topology shows
   network elements at Layer 3 (IP) and the "Optical" topology shows
   network elements at Layer 1.  Service functions in the "Service"
   topology are mapped onto network elements in the "L3" topology, which
   in turn are mapped onto network elements in the "Optical" topology.
   The figure shows two Service Functions - X1 and X2 - mapping onto a
   single L3 network element; this could happen, for example, if two
   service functions reside in the same VM (or server) and share the
   same set of network interfaces.  The figure shows a single "L3"
   network element mapped onto multiple "Optical" network elements.
   This could happen, for example, if a single IP router attaches to
   multiple ROADMs in the optical domain.

   Another example of a service topology stack is shown in the following
   figure.





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                           VPN1                       VPN2
         +---------------------+    +---------------------+
        /   [Y5]...           /    / [Z5]______[Z3]      /
       /    /  \  :          /    /  : \_       / :     /
      /    /    \  :        /    /   :   \_    /  :    /
     /    /      \  :      /    /   :      \  /   :   /
    /   [Y4]____[Y1] :    /    /   :       [Z2]   :  /
   +------:-------:---:--+    +---:---------:-----:-+
          :        :   :         :          :     :
          :         :   :       :           :     :
          :  +-------:---:-----:------------:-----:-----+
          : /       [X1]__:___:___________[X2]   :     /
          :/         / \_  : :       _____/ /   :     /
          :         /    \_ :  _____/      /   :     /
         /:        /       \: /           /   :     /
        / :       /        [X5]          /   :     /
       /   :     /       __/ \__        /   :     /
      /     :   /    ___/       \__    /   :     /
     /       : / ___/              \  /   :     /
    /        [X4]__________________[X3]..:     /
   +------------------------------------------+
                                  L3 Topology

               Figure 3: Topology hierarchy (stack) example

   The figure shows two VPN service topologies (VPN1 and VPN2)
   instantiated over a common L3 topology.  Each VPN service topology is
   mapped onto a subset of nodes from the common L3 topology.

   There are multiple applications for such a data model.  For example,
   within the context of I2RS, nodes within the network can use the data
   model to capture their understanding of the overall network topology
   and expose it to a network controller.  A network controller can then
   use the instantiated topology data to compare and reconcile its own
   view of the network topology with that of the network elements that
   it controls.  Alternatively, nodes within the network could propagate
   this understanding to compare and reconcile this understanding either
   among themselves or with help of a controller.  Beyond the network
   element and the immediate context of I2RS itself, a network
   controller might even use the data model to represent its view of the
   topology that it controls and expose it to applications north of
   itself.  Further use cases that the data model can be applied to are
   described in [I-D.draft-ietf-i2rs-usecase-reqs-summary].

   In this data model, a network is categorized as either server-
   provided or not.  If a network is server-provided, then it is
   dynamically learned information that can be read from the operational
   data-store.  For example, as mentioned above, when a network



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   controller reads a router's topology, that network is server-
   provided.  This data model can also be used to create or modify
   network topologies such as might be associated with an inventory or
   with an overlay network.  Such a network is not server-provided but
   configured.  This data model allows a network to refer to a
   supporting-network, supporting-nodes, supporting-links, etc.

   The model does allow to layer a network that is configured on top of
   one that is server-provided.  This permits the configuration of
   overlay networks on top of networks that are discovered.
   Specifically, this data model is structured to support being
   implemented as part of the ephemeral data-store
   [I-D.draft-ietf-netmod-revised-datastores], defined as requirement
   Ephemeral-REQ-03 in [I-D.draft-ietf-i2rs-ephemeral-state].  This
   allows a written (e.g. not server-provided) network topology to refer
   to a dynamically learned server-provided network.  A simple use case
   might involve creating an overlay network that is supported by the
   dynamically discovered IP routed network topology.  When an
   implementation places written data for this data model in the
   ephemeral data store, then such a network MAY refer to another
   network that is server-provided.

   An implementation's security policy MAY further restrict what
   information the server-provided model is allowed to access in
   standard configuration data-stores, or what server-provided network
   an ephemeral data store may access.  These security policies are
   outside the scope of the standardization of this model.

   Finally, it should be noted that the model is still subject to update
   per ongoing discussions that are related to design decisions
   regarding the fact that some layers of the network topology may be
   server provided while others may be configured.  These issues are
   outlined in section 4.4.10.

2.  Key Words

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

3.  Definitions and Acronyms

   Datastore: A conceptual store of instantiated management information,
   with individual data items represented by data nodes which are
   arranged in hierarchical manner.

   Data subtree: An instantiated data node and the data nodes that are
   hierarchically contained within it.



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   HTTP: Hyper-Text Transfer Protocol

   IGP: Interior Gateway Protocol

   IS-IS: Intermediate System to Intermediate System protocol

   NETCONF: Network Configuration Protocol

   OSPF: Open Shortest Path First, a link state routing protocol

   URI: Uniform Resource Identifier

   ReST: Representational State Transfer, a style of stateless interface
   and protocol that is generally carried over HTTP

   YANG: A data definition language for NETCONF

4.  Model Structure Details

4.1.  Base Network Model

   The abstract (base) network model is defined in the network.yang
   module.  Its structure is shown in the following figure.  Brackets
   enclose list keys, "rw" means configuration data, "ro" means
   operational state data, and "?" designates optional nodes.  A "+"
   indicates a line break.


            module: ietf-network
      +--rw networks
         +--rw network* [network-id]
            +--rw network-types
            +--rw network-id            network-id
            +--ro server-provided?      boolean
            +--rw supporting-network* [network-ref]
            |  +--rw network-ref    -> /networks/network/network-id
            +--rw node* [node-id]
               +--rw node-id            node-id
               +--rw supporting-node* [network-ref node-ref]
                  +--rw network-ref    -> ../../../supporting-network/ +
                  |                    network-ref
                  +--rw node-ref       -> /networks/network/node/node-id


       Figure 4: The structure of the abstract (base) network model

   The model contains a container with a list of networks.  Each network
   is captured in its own list entry, distinguished via a network-id.



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   A network has a certain type, such as L2, L3, OSPF or IS-IS.  A
   network can even have multiple types simultaneously.  The type, or
   types, are captured underneath the container "network-types".  In
   this module it serves merely as an augmentation target; network-
   specific modules will later introduce new data nodes to represent new
   network types below this target, i.e. insert them below "network-
   types" by ways of YANG augmentation.

   When a network is of a certain type, it will contain a corresponding
   data node.  Network types SHOULD always be represented using presence
   containers, not leafs of empty type.  This allows to represent
   hierarchies of network subtypes within the instance information.  For
   example, an instance of an OSPF network (which, at the same time, is
   a layer 3 unicast IGP network) would contain underneath "network-
   types" another container "l3-unicast-igp-network", which in turn
   would contain a container "ospf-network".

   A network can in turn be part of a hierarchy of networks, building on
   top of other networks.  Any such networks are captured in the list
   "supporting-network".  A supporting network is in effect an underlay
   network.

   Furthermore, a network contains an inventory of nodes that are part
   of the network.  The nodes of a network are captured in their own
   list.  Each node is identified relative to its containing network by
   a node-id.

   It should be noted that a node does not exist independently of a
   network; instead it is a part of the network that it is contained in.
   In cases where the same entity takes part in multiple networks, or at
   multiple layers of a networking stack, the same entity will be
   represented by multiple nodes, one for each network.  In other words,
   the node represents an abstraction of the device for the particular
   network that it a is part of.  To represent that the same entity or
   same device is part of multiple topologies or networks, it is
   possible to create one "physical" network with a list of nodes for
   each of the devices or entities.  This (physical) network,
   respectively the (entities) nodes in that network, can then be
   referred to as underlay network and nodes from the other (logical)
   networks and nodes, respectively.  Note that the model allows to
   define more than one underlay network (and node), allowing for
   simultaneous representation of layered network- and service
   topologies and physical instantiation.

   Similar to a network, a node can be supported by other nodes, and map
   onto one or more other nodes in an underlay network.  This is
   captured in the list "supporting-node".  The resulting hierarchy of
   nodes allows also to represent device stacks, where a node at one



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   level is supported by a set of nodes at an underlying level.  For
   example, a "router" node might be supported by a node representing a
   route processor and separate nodes for various line cards and service
   modules, a virtual router might be supported or hosted on a physical
   device represented by a separate node, and so on.

   Finally, there is an object "server-provided".  This object is state
   that indicates how the network came into being.  Network data can
   come into being in one of two ways.  In one way, network data is
   configured by client applications, for example in case of overlay
   networks that are configured by an SDN Controller application.  In
   annother way, it is populated by the server, in case of networks that
   can be discovered.

   If server-provided is set to false, the network was configured by a
   client application, for example in the case of an overlay network
   that is configured by a controller application.  If server-provided
   is set to true, the network was populated by the server itself,
   respectively an application on the server that is able to discover
   the network.  Client applications SHOULD NOT modify configurations of
   networks for which "server-provided" is true.  When they do, they
   need to be aware that any modifications they make are subject to be
   reverted by the server.  For servers that support NACM (Netconf
   Access Control Model), data node rules should ideally prevent write
   access by other clients to network instances for which server-
   provided is set to true.

4.2.  Base Network Topology Model

   The abstract (base) network topology model is defined in the
   "network-topology.yang" module.  It builds on the network model
   defined in the "network.yang" module, augmenting it with links
   (defining how nodes are connected) and termination-points (which
   anchor the links and are contained in nodes).  The structure of the
   network topology module is shown in the following figure.  Brackets
   enclose list keys, "rw" means configuration data, "ro" means
   operational state data, and "?" designates optional nodes.  A "+"
   indicates a line break.













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module: ietf-network-topology
augment /nd:networks/nd:network:
   +--rw link* [link-id]
      +--rw source
      |  +--rw source-node?   -> ../../../nd:node/node-id
      |  +--rw source-tp?     -> ../../../nd:node[nd:node-id=current()/+
      |                       ../source-node]/termination-point/tp-id
      +--rw destination
      |  +--rw dest-node?   -> ../../../nd:node/node-id
      |  +--rw dest-tp?     -> ../../../nd:node[nd:node-id=current()/+
      |                     ../dest-node]/termination-point/tp-id
      +--rw link-id            link-id
      +--rw supporting-link* [network-ref link-ref]
         +--rw network-ref    -> ../../../nd:supporting-network/+
         |                    network-ref
         +--rw link-ref       -> /nd:networks/network+
                              [nd:network-id=current()/../network-ref]/+
                              link/link-id
augment /nd:networks/nd:network/nd:node:
   +--rw termination-point* [tp-id]
      +--rw tp-id                           tp-id
      +--rw supporting-termination-point* [network-ref node-ref tp-ref]
         +--rw network-ref    -> ../../../nd:supporting-node/network-ref
         +--rw node-ref       -> ../../../nd:supporting-node/node-ref
         +--rw tp-ref         -> /nd:networks/network[nd:network-id=+
                              current()/../network-ref]/node+
                              [nd:node-id=current()/../node-ref]/+
                              termination-point/tp-id

   Figure 5: The structure of the abstract (base) network topology model

   A node has a list of termination points that are used to terminate
   links.  An example of a termination point might be a physical or
   logical port or, more generally, an interface.

   Like a node, a termination point can in turn be supported by an
   underlying termination point, contained in the supporting node of the
   underlay network.

   A link is identified by a link-id that uniquely identifies the link
   within a given topology.  Links are point-to-point and
   unidirectional.  Accordingly, a link contains a source and a
   destination.  Both source and destination reference a corresponding
   node, as well as a termination point on that node.  Similar to a
   node, a link can map onto one or more links in an underlay topology
   (which are terminated by the corresponding underlay termination
   points).  This is captured in the list "supporting-link".




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4.3.  Extending the model

   In order to derive a model for a specific type of network, the base
   model can be extended.  This can be done roughly as follows: for the
   new network type, a new YANG module is introduced.  In this module, a
   number of augmentations are defined against the network and network-
   topology YANG modules.

   We start with augmentations against the network.yang module.  First,
   a new network type needs to be defined.  For this, a presence
   container that resembles the new network type is defined.  It is
   inserted by means of augmentation below the network-types container.
   Subsequently, data nodes for any network-type specific node
   parameters are defined and augmented into the node list.  The new
   data nodes can be defined as conditional ("when") on the presence of
   the corresponding network type in the containing network.  In cases
   where there are any requirements or restrictions in terms of network
   hierarchies, such as when a network of a new network-type requires a
   specific type of underlay network, it is possible to define
   corresponding constraints as well and augment the supporting-network
   list accordingly.  However, care should be taken to avoid excessive
   definitions of constraints.

   Subsequently, augmentations are defined against network-
   topology.yang.  Data nodes are defined both for link parameters, as
   well as termination point parameters, that are specific to the new
   network type.  Those data nodes are inserted by way of augmentation
   into the link and termination-point lists, respectively.  Again, data
   nodes can be defined as conditional on the presence of the
   corresponding network-type in the containing network, by adding a
   corresponding "when"-statement.

   It is possible, but not required, to group data nodes for a given
   network-type under a dedicated container.  Doing so introduces
   further structure, but lengthens data node path names.

   In cases where a hierarchy of network types is defined, augmentations
   can in turn against augmenting modules, with the module of a network
   "sub-type" augmenting the module of a network "super-type".

4.4.  Discussion and selected design decisions

4.4.1.  Container structure

   Rather than maintaining lists in separate containers, the model is
   kept relatively flat in terms of its containment structure.  Lists of
   nodes, links, termination-points, and supporting-nodes, supporting-
   links, and supporting-termination-points are not kept in separate



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   containers.  Therefore, path specifiers used to refer to specific
   nodes, be it in management operations or in specifications of
   constraints, can remain relatively compact.  Of course, this means
   there is no separate structure in instance information that separates
   elements of different lists from one another.  Such structure is
   semantically not required, although it might enhance human
   readability in some cases.

4.4.2.  Underlay hierarchies and mappings

   To minimize assumptions of what a particular entity might actually
   represent, mappings between networks, nodes, links, and termination
   points are kept strictly generic.  For example, no assumptions are
   made whether a termination point actually refers to an interface, or
   whether a node refers to a specific "system" or device; the model at
   this generic level makes no provisions for that.

   Where additional specifics about mappings between upper and lower
   layers are required, those can be captured in augmenting modules.
   For example, to express that a termination point in a particular
   network type maps to an interface, an augmenting module can introduce
   an augmentation to the termination point which introduces a leaf of
   type ifref that references the corresponding interface [RFC7223].
   Similarly, if a node maps to a particular device or network element,
   an augmenting module can augment the node data with a leaf that
   references the network element.

   It is possible for links at one level of a hierarchy to map to
   multiple links at another level of the hierarchy.  For example, a VPN
   topology might model VPN tunnels as links.  Where a VPN tunnel maps
   to a path that is composed of a chain of several links, the link will
   contain a list of those supporting links.  Likewise, it is possible
   for a link at one level of a hierarchy to aggregate a bundle of links
   at another level of the hierarchy.

4.4.3.  Dealing with changes in underlay networks

   It is possible for a network to undergo churn even as other networks
   are layered on top of it.  When a supporting node, link, or
   termination point is deleted, the supporting leafrefs in the overlay
   will be left dangling.  To allow for this possibility, the model
   makes use of the "require-instance" construct of YANG 1.1 [RFC7950].

   It is the responsibility of the application maintaining the overlay
   to deal with the possibility of churn in the underlay network.  When
   a server receives a request to configure an overlay network, it
   SHOULD validate whether supporting nodes/links/tps refer to nodes in
   the underlay are actually in existence.  Configuration requests in



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   which supporting nodes/links/tps refer to objects currently not in
   existence SHOULD be rejected.  It is the responsibility of the
   application to update the overlay when a supporting node/link/tp is
   deleted at a later point in time.  For this purpose, an application
   might subscribe to updates when changes to the underlay occur, for
   example using mechanisms defined in
   [I-D.draft-ietf-netconf-yang-push].

4.4.4.  Use of groupings

   The model makes use of groupings, instead of simply defining data
   nodes "in-line".  This allows to more easily include the
   corresponding data nodes in notifications, which then do not need to
   respecify each data node that is to be included.  The tradeoff for
   this is that it makes the specification of constraints more complex,
   because constraints involving data nodes outside the grouping need to
   be specified in conjunction with a "uses" statement where the
   grouping is applied.  This also means that constraints and XPath-
   statements need to specified in such a way that they navigate "down"
   first and select entire sets of nodes, as opposed to being able to
   simply specify them against individual data nodes.

4.4.5.  Cardinality and directionality of links

   The topology model includes links that are point-to-point and
   unidirectional.  It does not directly support multipoint and
   bidirectional links.  While this may appear as a limitation, it does
   keep the model simple, generic, and allows it to very easily be
   subjected to applications that make use of graph algorithms.  Bi-
   directional connections can be represented through pairs of
   unidirectional links.  Multipoint networks can be represented through
   pseudo-nodes (similar to IS-IS, for example).  By introducing
   hierarchies of nodes, with nodes at one level mapping onto a set of
   other nodes at another level, and introducing new links for nodes at
   that level, topologies with connections representing non-point-to-
   point communication patterns can be represented.

4.4.6.  Multihoming and link aggregation

   Links are terminated by a single termination point, not sets of
   termination points.  Connections involving multihoming or link
   aggregation schemes need to be represented using multiple point-to-
   point links, then defining a link at a higher layer that is supported
   by those individual links.







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4.4.7.  Mapping redundancy

   In a hierarchy of networks, there are nodes mapping to nodes, links
   mapping to links, and termination points mapping to termination
   points.  Some of this information is redundant.  Specifically, if the
   link-to-links mapping known, and the termination points of each link
   known, termination point mapping information can be derived via
   transitive closure and does not have to be explicitly configured.
   Nonetheless, in order to not constrain applications regarding which
   mappings they want to configure and which should be derived, the
   model does provide for the option to configure this information
   explicitly.  The model includes integrity constraints to allow for
   validating for consistency.

4.4.8.  Typing

   A network's network types are represented using a container which
   contains a data node for each of its network types.  A network can
   encompass several types of network simultaneously, hence a container
   is used instead of a case construct, with each network type in turn
   represented by a dedicated presence container itself.  The reason for
   not simply using an empty leaf, or even simpler, do away even with
   the network container and just use a leaf-list of network-type
   instead, is to be able to represent "class hierarchies" of network
   types, with one network type refining the other.  Network-type
   specific containers are to be defined in the network-specific
   modules, augmenting the network-types container.

4.4.9.  Representing the same device in multiple networks

   One common requirement concerns the ability to represent that the
   same device can be part of multiple networks and topologies.
   However, the model defines a node as relative to the network that it
   is contained in.  The same node cannot be part of multiple
   topologies.  In many cases, a node will be the abstraction of a
   particular device in a network.  To reflect that the same device is
   part of multiple topologies, the following approach might be chosen:
   A new type of network to represent a "physical" (or "device") network
   is introduced, with nodes representing devices.  This network forms
   an underlay network for logical networks above it, with nodes of the
   logical network mapping onto nodes in the physical network.

   This scenario is depicted in the following figure.  It depicts three
   networks with two nodes each.  A physical network P consists of an
   inventory of two nodes, D1 and D2, each representing a device.  A
   second network, X, has a third network, Y, as its underlay.  Both X
   and Y also have the physical network P as underlay.  X1 has both Y1
   and D1 as underlay nodes, while Y1 has D1 as underlay node.



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   Likewise, X2 has both Y2 and D2 as underlay nodes, while Y2 has D2 as
   underlay node.  The fact that X1 and Y1 are both instantiated on the
   same physical node D1 can be easily derived.


                         +---------------------+
                        /   [X1]____[X2]      /  X(Service Overlay)
                       +----:--:----:--------+
                         ..:    :..: :
                ........:     ....: : :....
         +-----:-------------:--+    :     :...
        /   [Y1]____[Y2]....:  /      :..      :
       +------|-------|-------+          :..    :...
        Y(L3) |       +---------------------:-----+ :
              |                         +----:----|-:----------+
              +------------------------/---[D1]  [D2]         /
                                      +----------------------+
                                        P (Physical network)

         Figure 6: Topology hierarchy example - multiple underlays

   In the case of a physical network, nodes represent physical devices
   and termination points physical ports.  It should be noted that it is
   also conceivable to augment the model for a physical network-type,
   defining augmentations that have nodes reference system information
   and termination points reference physical interfaces, in order to
   provide a bridge between network and device models.

4.4.10.  Supporting client-configured and server-provided network
         topology

   YANG requires data nodes to be designated as either configuration or
   operational data, but not both, yet it is important to have all
   network information, including vertical cross-network dependencies,
   captured in one coherent model.  In most cases, network topology
   information is discovered about a network; the topology is considered
   a property of the network that is reflected in the model.  That said,
   it is conceivable that certain types of topology need to also be
   configurable by an application.  The model needs to support both
   cases.

   There are several alternatives in which this can be addressed.  The
   alternative chosen in this draft does not restrict network topology
   information as read-only, but includes a state "server-provided" that
   indicates for each network whether it is populated by the server or
   by a client application.  Client applications that do attempt to
   modify network topology may simply see their actions reverted, not
   unlike other client applications that compete with one another, each



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   wanting to "own" the same data.  When Netconf Access Control Model
   [RFC6536] is supported, node access rules SHOULD be automatically
   maintained by a server to deny client write access to network and
   topology instances for which "server-provided" is true.

   It should be noted that this solution stretches its use of the
   configuration concept slightly.  Configuration information in general
   is subject to backup and restore, which is not applicable to server-
   provided information.  Perhaps more noteworthy is the potential
   ability of a client to lock a configuration and thus prevent changes
   to server-provided network topology while the lock is in effect.  As
   a result it would potentially incur a time lag until topology changes
   that occur in the meantime are reflected, unless implementations
   choose to provide special treatment for network topology information.

   Other alternatives had been considered.  In one alternative, all
   information about network topology is in effect is represented as
   network state, i.e. as read-only information, regardless of how it
   came into being.  For cases where network topology needs to be
   configured, a second branch for configurable topology information is
   introduced.  Any network topology configuration is mirrored by
   network state information.  A configurable network will thus be
   represented twice: once in the read-only list of all networks, a
   second time in a configuration sandbox.  One implication of this
   solution would have been significantly increased complexity of
   augmentations due to multiple target branches.

   Another alternative would make use of a YANG extension to tag
   specific network instances as "server-provided" instead of defining a
   leaf object, or rely on the concept of YANG metadata [RFC7952] for
   the same effect.  The tag would be automatically applied to any
   topology data that comes into being (respectively is configured) by
   an embedded application on the network, as opposed to e.g. a
   controller application.

4.4.11.  Identifiers of string or URI type

   The current model defines identifiers of nodes, networks, links, and
   termination points as URIs.  An alternative would define them as
   string.

   The case for strings is that they will be easier to implement.  The
   reason for choosing URIs is that the topology/node/tp exists in a
   larger context, hence it is useful to be able to correlate
   identifiers across systems.  While strings, being the universal data
   type, are easier for human beings (a string is a string is a string),
   they also muddle things.  What typically happens is that strings have
   some structure which is magically assigned and the knowledge of this



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   structure has to be communicated to each system working with the
   data.  A URI makes the structure explicit and also attaches
   additional semantics: the URI, unlike a free-form string, can be fed
   into a URI resolver, which can point to additional resources
   associated with the URI.  This property is important when the
   topology data is integrated into a larger, more complex system.

5.  Interactions with Other YANG Modules

   The model makes use of data types that have been defined in
   [RFC6991].

   This is a protocol independent yang model with topology information.
   It is separate from and not linked with data models that are used to
   configure routing protocols or routing information.  This includes
   e.g. model "ietf-routing" [RFC8022].

   The model obeys the requirements for the ephemeral state found in the
   document [I-D.draft-ietf-i2rs-ephemeral-state].  For ephemeral
   topology data that is server provided, the process tasked with
   maintaining topology information will load information from the
   routing process (such as OSPF) into the data model without relying on
   a configuration datastore.

6.  YANG Modules

6.1.  Defining the Abstract Network: network.yang

   <CODE BEGINS> file "ietf-network@2017-02-16.yang"
   module ietf-network {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-network";
     prefix nd;

     import ietf-inet-types {
       prefix inet;
     }

     organization
       "IETF I2RS (Interface to the Routing System) Working Group";

     contact
       "WG Web:    <http://tools.ietf.org/wg/i2rs/>
        WG List:   <mailto:i2rs@ietf.org>

        WG Chair:  Susan Hares
                   <mailto:shares@ndzh.com>




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        WG Chair:  Russ White
                   <mailto:russ@riw.us>

        Editor:    Alexander Clemm
                   <mailto:ludwig@clemm.org>

        Editor:    Jan Medved
                   <mailto:jmedved@cisco.com>

        Editor:    Robert Varga
                   <mailto:robert.varga@pantheon.sk>

        Editor:    Nitin Bahadur
                   <mailto:nitin_bahadur@yahoo.com>

        Editor:    Hariharan Ananthakrishnan
                   <mailto:hari@packetdesign.com>

        Editor:    Xufeng Liu
                   <mailto:xliu@kuatrotech.com>";

     description
       "This module defines a common base model for a collection
        of nodes in a network. Node definitions are further used
        in network topologies and inventories.

        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
        (http://trustee.ietf.org/license-info).

        This version of this YANG module is part of
        draft-ietf-i2rs-yang-network-topo-11;
        see the RFC itself for full legal notices.

        NOTE TO RFC EDITOR: Please replace above reference to
        draft-ietf-i2rs-yang-network-topo-11 with RFC
        number when published (i.e. RFC xxxx).";

     revision 2017-02-16 {
       description
         "Initial revision.
          NOTE TO RFC EDITOR: Please replace the following reference



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          to draft-ietf-i2rs-yang-network-topo-11 with
          RFC number when published (i.e. RFC xxxx).";
       reference
         "draft-ietf-i2rs-yang-network-topo-11";
     }

     typedef node-id {
       type inet:uri;
       description
         "Identifier for a node.  The precise structure of the node-id
          will be up to the implementation.  Some implementations MAY
          for example, pick a uri that includes the network-id as
          part of the path. The identifier SHOULD be chosen such that
          the same node in a real network topology will always be
          identified through the same identifier, even if the model is
          instantiated in separate datastores. An implementation MAY
          choose to capture semantics in the identifier, for example to
          indicate the type of node.";
     }

     typedef network-id {
       type inet:uri;
       description
         "Identifier for a network.  The precise structure of the
         network-id will be up to an implementation.
         The identifier SHOULD be chosen such that the same network
         will always be identified through the same identifier,
         even if the model is instantiated in separate datastores.
         An implementation MAY choose to capture semantics in the
         identifier, for example to indicate the type of network.";
     }

     grouping network-ref {
       description
         "Contains the information necessary to reference a network,
          for example an underlay network.";
       leaf network-ref {
         type leafref {
           path "/nd:networks/nd:network/nd:network-id";
         require-instance false;
         }
         description
           "Used to reference a network, for example an underlay
            network.";
       }
     }

     grouping node-ref {



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       description
         "Contains the information necessary to reference a node.";
       leaf node-ref {
         type leafref {
           path "/nd:networks/nd:network[nd:network-id=current()/../"+
             "network-ref]/nd:node/nd:node-id";
           require-instance false;
         }
         description
           "Used to reference a node.
            Nodes are identified relative to the network they are
            contained in.";
       }
       uses network-ref;
     }

     container networks {
       description
         "Serves as top-level container for a list of networks.";
       list network {
         key "network-id";
         description
           "Describes a network.
            A network typically contains an inventory of nodes,
            topological information (augmented through
            network-topology model), as well as layering
            information.";
         container network-types {
           description
             "Serves as an augmentation target.
              The network type is indicated through corresponding
              presence containers augmented into this container.";
         }
         leaf network-id {
           type network-id;
           description
             "Identifies a network.";
         }
         leaf server-provided {
           type boolean;
           config false;
           description
             "Indicates whether the information concerning this
              particular network is populated by the server
              (server-provided true, the general case for network
              information discovered from the server),
              or whether it is configured by a client
              (server-provided true, possible e.g. for



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              service overlays managed through a controller).
              Clients should not attempt to make modifications
              to network instances with server-provided set to
              true; when they do, they need to be aware that
              any modifications they make are subject to be
              reverted by the server.
              For servers that support NACM (Netconf Access Control
              Model), data node rules should ideally prevent
              write access by other clients to the network instance
              when server-provided is set to true.";
         }
         list supporting-network {
           key "network-ref";
           description
             "An underlay network, used to represent layered network
              topologies.";
           leaf network-ref {
             type leafref {
               path "/networks/network/network-id";
             require-instance false;
             }
             description
               "References the underlay network.";
           }
         }
         list node {
           key "node-id";
           description
             "The inventory of nodes of this network.";
           leaf node-id {
             type node-id;
             description
               "Identifies a node uniquely within the containing
                network.";
           }
           list supporting-node {
             key "network-ref node-ref";
             description
               "Represents another node, in an underlay network, that
                this node is supported by.  Used to represent layering
                structure.";
             leaf network-ref {
               type leafref {
                 path "../../../supporting-network/network-ref";
               require-instance false;
               }
               description
                 "References the underlay network that the



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                  underlay node is part of.";
             }
             leaf node-ref {
               type leafref {
                 path "/networks/network/node/node-id";
               require-instance false;
               }
               description
                 "References the underlay node itself.";
             }
           }
         }
       }
     }
   }

   <CODE ENDS>

6.2.  Creating Abstract Network Topology: network-topology.yang

 <CODE BEGINS> file "ietf-network-topology@2017-02-16.yang"
 module ietf-network-topology {
   yang-version 1.1;
   namespace "urn:ietf:params:xml:ns:yang:ietf-network-topology";
   prefix lnk;

   import ietf-inet-types {
     prefix inet;
   }
   import ietf-network {
     prefix nd;
   }

   organization
     "IETF I2RS (Interface to the Routing System) Working Group";

   contact
     "WG Web:    <http://tools.ietf.org/wg/i2rs/>
      WG List:   <mailto:i2rs@ietf.org>

      WG Chair:  Susan Hares
                 <mailto:shares@ndzh.com>

      WG Chair:  Russ White
                 <mailto:russ@riw.us>

      Editor:    Alexander Clemm
                 <mailto:ludwig@clemm.org>



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      Editor:    Jan Medved
                 <mailto:jmedved@cisco.com>

      Editor:    Robert Varga
                 <mailto:robert.varga@pantheon.sk>

      Editor:    Nitin Bahadur
                 <mailto:nitin_bahadur@yahoo.com>

      Editor:    Hariharan Ananthakrishnan
                 <mailto:hari@packetdesign.com>

      Editor:    Xufeng Liu
                 <mailto:xliu@kuatrotech.com>";

   description
     "This module defines a common base model for network topology,
      augmenting the base network model with links to connect nodes,
      as well as termination points to terminate links on nodes.

      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
      (http://trustee.ietf.org/license-info).

      This version of this YANG module is part of
      draft-ietf-i2rs-yang-network-topo-11;
      see the RFC itself for full legal notices.

      NOTE TO RFC EDITOR: Please replace above reference to
      draft-ietf-i2rs-yang-network-topo-11 with RFC
      number when published (i.e. RFC xxxx).";

   revision 2017-02-16 {
     description
       "Initial revision.
        NOTE TO RFC EDITOR: Please replace the following reference
        to draft-ietf-i2rs-yang-network-topo-11 with
        RFC number when published (i.e. RFC xxxx).";
     reference
       "draft-ietf-i2rs-yang-network-topo-11";
   }




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   typedef link-id {
     type inet:uri;
     description
       "An identifier for a link in a topology.
        The precise structure of the link-id
        will be up to the implementation.
        The identifier SHOULD be chosen such that the same link in a
        real network topology will always be identified through the
        same identifier, even if the model is instantiated in
            separate datastores. An implementation MAY choose to capture
        semantics in the identifier, for example to indicate the type
        of link and/or the type of topology that the link is a part
        of.";
   }

   typedef tp-id {
     type inet:uri;
     description
       "An identifier for termination points (TPs) on a node.
        The precise structure of the tp-id
        will be up to the implementation.
        The identifier SHOULD be chosen such that the same termination
        point in a real network topology will always be identified
        through the same identifier, even if the model is instantiated
        in separate datastores. An implementation MAY choose to
        capture semantics in the identifier, for example to indicate
        the type of termination point and/or the type of node
        that contains the termination point.";
   }

   grouping link-ref {
     description
       "References a link in a specific network.";
     leaf link-ref {
       type leafref {
         path "/nd:networks/nd:network[nd:network-id=current()/../"+
           "network-ref]/lnk:link/lnk:link-id";
         require-instance false;
       }
       description
         "A type for an absolute reference a link instance.
          (This type should not be used for relative references.
          In such a case, a relative path should be used instead.)";
     }
     uses nd:network-ref;
   }

   grouping tp-ref {



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     description
       "References a termination point in a specific node.";
     leaf tp-ref {
       type leafref {
         path "/nd:networks/nd:network[nd:network-id=current()/../"+
           "network-ref]/nd:node[nd:node-id=current()/../"+
           "node-ref]/lnk:termination-point/lnk:tp-id";
         require-instance false;
       }
       description
         "A type for an absolute reference to a termination point.
          (This type should not be used for relative references.
          In such a case, a relative path should be used instead.)";
     }
     uses nd:node-ref;
   }

   augment "/nd:networks/nd:network" {
     description
       "Add links to the network model.";
     list link {
       key "link-id";
       description
         "A network link connects a local (source) node and
          a remote (destination) node via a set of
          the respective node's termination points.
          It is possible to have several links between the same
          source and destination nodes.  Likewise, a link could
          potentially be re-homed between termination points.
          Therefore, in order to ensure that we would always know
          to distinguish between links, every link is identified by
          a dedicated link identifier.  Note that a link models a
          point-to-point link, not a multipoint link.";
       container source {
         description
           "This container holds the logical source of a particular
            link.";
         leaf source-node {
           type leafref {
             path "../../../nd:node/nd:node-id";
             require-instance false;
           }
           description
             "Source node identifier, must be in same topology.";
         }
         leaf source-tp {
           type leafref {
             path "../../../nd:node[nd:node-id=current()/../"+



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               "source-node]/termination-point/tp-id";
             require-instance false;
           }
           description
             "Termination point within source node that terminates
              the link.";
         }
       }
       container destination {
         description
           "This container holds the logical destination of a
            particular link.";
         leaf dest-node {
           type leafref {
             path "../../../nd:node/nd:node-id";
           require-instance false;
           }
           description
             "Destination node identifier, must be in the same
              network.";
         }
         leaf dest-tp {
           type leafref {
             path "../../../nd:node[nd:node-id=current()/../"+
               "dest-node]/termination-point/tp-id";
             require-instance false;
           }
           description
             "Termination point within destination node that
              terminates the link.";
         }
       }
       leaf link-id {
         type link-id;
         description
           "The identifier of a link in the topology.
            A link is specific to a topology to which it belongs.";
       }
       list supporting-link {
         key "network-ref link-ref";
         description
           "Identifies the link, or links, that this link
            is dependent on.";
         leaf network-ref {
           type leafref {
             path "../../../nd:supporting-network/nd:network-ref";
           require-instance false;
           }



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           description
             "This leaf identifies in which underlay topology
              the supporting link is present.";
         }
         leaf link-ref {
           type leafref {
             path "/nd:networks/nd:network[nd:network-id=current()/"+
               "../network-ref]/link/link-id";
             require-instance false;
           }
           description
             "This leaf identifies a link which is a part
              of this link's underlay. Reference loops in which
              a link identifies itself as its underlay, either
              directly or transitively, are not allowed.";
         }
       }
     }
   }
   augment "/nd:networks/nd:network/nd:node" {
     description
       "Augment termination points which terminate links.
        Termination points can ultimately be mapped to interfaces.";
     list termination-point {
       key "tp-id";
       description
         "A termination point can terminate a link.
          Depending on the type of topology, a termination point
          could, for example, refer to a port or an interface.";
       leaf tp-id {
         type tp-id;
         description
           "Termination point identifier.";
       }
       list supporting-termination-point {
         key "network-ref node-ref tp-ref";
         description
           "This list identifies any termination points that
            the termination point is dependent on, or maps onto.
            Those termination points will themselves be contained
            in a supporting node.
            This dependency information can be inferred from
            the dependencies between links.  For this reason,
            this item is not separately configurable.  Hence no
            corresponding constraint needs to be articulated.
            The corresponding information is simply provided by the
            implementing system.";
         leaf network-ref {



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           type leafref {
             path "../../../nd:supporting-node/nd:network-ref";
           require-instance false;
           }
           description
             "This leaf identifies in which topology the
              supporting termination point is present.";
         }
         leaf node-ref {
           type leafref {
             path "../../../nd:supporting-node/nd:node-ref";
           require-instance false;
           }
           description
             "This leaf identifies in which node the supporting
              termination point is present.";
         }
         leaf tp-ref {
           type leafref {
             path "/nd:networks/nd:network[nd:network-id=current()/"+
               "../network-ref]/nd:node[nd:node-id=current()/../"+
               "node-ref]/termination-point/tp-id";
             require-instance false;
           }
           description
             "Reference to the underlay node, must be in a
              different topology";
         }
       }
     }
   }
 }

 <CODE ENDS>

7.  IANA Considerations

   This document registers the following namespace URIs in the "IETF XML
   Registry" [RFC3688]:

   URI: urn:ietf:params:xml:ns:yang:ietf-network
   Registrant Contact: The IESG.
   XML: N/A; the requested URI is an XML namespace.

   URI:urn:ietf:params:xml:ns:yang:ietf-network-topology
   Registrant Contact: The IESG.
   XML: N/A; the requested URI is an XML namespace.




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   This document registers the following YANG modules in the "YANG
   Module Names" registry [RFC6020]:

   Name: ietf-network
   Namespace: urn:ietf:params:xml:ns:yang:ietf-network
   Prefix: nd
   Reference: draft-ietf-i2rs-yang-network-topo-11.txt (RFC form)

   Name: ietf-network-topology
   Namespace: urn:ietf:params:xml:ns:yang:ietf-network-topology
   Prefix: lnk
   Reference: draft-ietf-i2rs-yang-network-topo-11.txt (RFC form)

8.  Security Considerations

   The YANG module defined in this memo is independent of a particular
   protocol and can be accessed via a number of protocols that need to
   access YANG-defined data.  This includes but is not limited to the
   NETCONF protocol [RFC6241].  The lowest NETCONF layer is the secure
   transport layer and the mandatory-to-implement secure transport is
   Secure Shell (SSH) [RFC6242].

   The NETCONF access control model (NACM) [RFC6536] provides the means
   to restrict access for particular NETCONF users to a pre-configured
   subset of all available NETCONF protocol operations and content.
   However, NACM can be applied analogously also to other protocols that
   attempt access to YANG-defined data.  In fact, it needs to be applied
   in the same way and should, like YANG, thus be considered independent
   of any particular protocol that is used to access YANG-defined data.
   Otherwise, access control rules defined by NACM could be very easily
   circumvented simply by using another access mechanism which does not
   enforce NACM.  The alternative of mandating the introduction of
   mechanisms parallel to NACM that specify the same access control
   rules for other transports is clearly undesirable, as this would not
   only inhibit ease-of-use of systems that implement multiple protocols
   to access YANG data, but also open the specter of security holes due
   to inconsistencies in articulation and enforcement of rules across
   mechanisms that are essentially redundant.

   The YANG module defines information that can be configurable in
   certain instances, for example in the case of overlay topologies that
   can be created by client applications.  In such cases, a malicious
   client could introduce topologies that are undesired.  Specifically,
   a malicious client could attempt to do the following:

   o  Remove or add a node, a link, a termination point, by creating or
      deleting corresponding elements in the node, link, and termination
      point lists, respectively.  In the case of a topology that is



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      server-provided, the server will automaticaly correct such
      misconfiguration attempts.  In the case of a topology that is
      configured, the services provided via this topology might be
      disrupted.  For example, the topology could be "cut" or be
      configured in a suboptimal way, leading to degradation of service
      levels and possibly disruption of service.

   o  Modify the underlay information, i.e. the configuration of
      supporting-node, supporting-link, and supporting-termination-
      point, respectively.  Again, in the case of a topology that is
      server-provided, the server will automaticaly correct such
      misconfiguration attempts.  However, in the case of a topology
      that is configured, this will affect the vertical layering and the
      way in which the overlay maps onto an overlay.  This could be
      exploited to severely disrupt the overlay network by degrading
      service levels.  In addition, it could be exploited to result in
      increased consumption of resources in the underlay network, for
      example by disrupting congruence between overlay and underlay
      nodes which would result in routing and bandwidth utilization
      inefficiencies.

   For those reasons, it is important that the NETCONF access control
   model is vigorously applied to prevent topology misconfiguration by
   unauthorized clients.

   Topologies that are server-provided and that provide ephemeral
   topology information are less vulnerable, as they provide read-only
   access to clients.

9.  Contributors

   The model presented in this paper was contributed to by more people
   than can be listed on the author list.  Additional contributors
   include:

   o  Vishnu Pavan Beeram, Juniper

   o  Ken Gray, Cisco Systems

   o  Tom Nadeau, Brocade

   o  Tony Tkacik

   o  Kent Watsen, Juniper

   o  Aleksandr Zhdankin, Cisco





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10.  Acknowledgements

   We wish to acknowledge the helpful contributions, comments, and
   suggestions that were received from Alia Atlas, Andy Bierman, Martin
   Bjorklund, Igor Bryskin, Benoit Claise, Susan Hares, Ladislav Lhotka,
   Carlos Pignataro, Juergen Schoenwaelder, and Xian Zhang.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to indicate
              requirement levels", RFC 2119, March 1997.

   [RFC3688]  Mealling, M., "The IETF XML Registry", RFC 3688, January
              2004.

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

   [RFC6241]  Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
              Bierman, "Network Configuration Protocol (NETCONF)",
              RFC 6241, June 2011.

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

   [RFC6536]  Bierman, A. and M. Bjorklund, "Network Configuration
              Protocol (NETCONF) Access Control Model", RFC 6536, March
              2012.

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

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

11.2.  Informative References

   [I-D.draft-ietf-i2rs-ephemeral-state]
              Haas, J. and S. Hares, "I2RS Ephemeral State
              Requirements", I-D draft-ietf-i2rs-ephemeral-state-23,
              November 2016.







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   [I-D.draft-ietf-i2rs-usecase-reqs-summary]
              Hares, S. and M. Chen, "Summary of I2RS Use Case
              Requirements", I-D draft-ietf-i2rs-usecase-reqs-summary-
              30, November 2016.

   [I-D.draft-ietf-netconf-yang-push]
              Clemm, A., Voit, E., Gonzalez Prieto, A., Tripathy, A.,
              Nilsen-Nygaard, E., Bierman, A., and B. Lengyel,
              "Subscribing to YANG datastore push updates", I-D draft-
              ietf-netconf-yang-push-04, October 2016.

   [I-D.draft-ietf-netmod-revised-datastores]
              Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "A Revised Conceptual Model for YANG
              Datastores", I-D draft-ietf-netmod-revised-datastores-00,
              December 2016.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and
              Dual Environments", RFC 1195, December 1990.

   [RFC2328]  Moy, J., "OSPF Version 2", RFC 2328, April 1998.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, December 2001.

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

   [RFC7952]  Lhotka, L., "Defining and Using Metadata with YANG",
              RFC 7952, August 2016.

   [RFC8022]  Lhotka, L. and A. Lindem, "A YANG Data Model for Routing
              Management", RFC 8022, November 2016.

Authors' Addresses

   Alexander Clemm
   Huawei

   EMail: ludwig@clemm.org


   Jan Medved
   Cisco

   EMail: jmedved@cisco.com




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   Robert Varga
   Pantheon Technologies SRO

   EMail: robert.varga@pantheon.sk


   Nitin Bahadur
   Bracket Computing

   EMail: nitin_bahadur@yahoo.com


   Hariharan Ananthakrishnan
   Packet Design

   EMail: hari@packetdesign.com


   Xufeng Liu
   Ericsson

   EMail: xliu@kuatrotech.com





























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