wdiff rfc8345v1.txt rfc8345.txt

Internet Engineering Task Force (IETF) A. Clemm
Request for Comments: 8345 Huawei
Category: Standards Track J. Medved
ISSN: 2070-1721 Cisco
R. Varga
Pantheon Technologies SRO
N. Bahadur
Bracket Computing
H. Ananthakrishnan
Packet Design
X. Liu
Jabil
February
March 2018

An Abstract

A YANG Data Model for Network Topologies

Abstract

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

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8345.

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

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Definitions and Abbreviations . . . . . . . . . . . . . . . . 7
4. Model Structure Details . . . . . . . . . . . . . . . . . . . 8
4.1. Base Network Model . . . . . . . . . . . . . . . . . . . 8
4.2. Base Network Topology Data Model . . . . . . . . . . . . 10
4.3. Extending the Data Model . . . . . . . . . . . . . . . . 12
4.4. Discussion and Selected Design Decisions . . . . . . . . 12
4.4.1. Container Structure . . . . . . . . . . . . . . . . . 13
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 . . . . . . . . . . 15
4.4.7. Mapping Redundancy . . . . . . . . . . . . . . . . . 15
4.4.8. Typing . . . . . . . . . . . . . . . . . . . . . . . 15
4.4.9. Representing the Same Device in Multiple Networks . . 16
4.4.10. Supporting Client-Configured and System-Controlled
Network Topologies . . . . . . . . . . . . . . . . . 17
4.4.11. Identifiers of String or URI Type . . . . . . . . . . 18
5. Interactions with Other YANG Modules . . . . . . . . . . . . 18
6. YANG Modules . . . . . . . . . . . . . . . . . . . . . . . . 18
6.1. Defining the Abstract Network: ietf-network.yang ietf-network . . . . . . . 18
6.2. Creating Abstract Network Topology:
ietf-network-topology.yang . . . . . . . . . . . . . . . ietf-network-topology 23
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
8. Security Considerations . . . . . . . . . . . . . . . . . . . 30
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. Normative References . . . . . . . . . . . . . . . . . . 32
9.2. Informative References . . . . . . . . . . . . . . . . . 33
Appendix A. Model Use Cases . . . . . . . . . . . . . . . . . . 34
A.1. Fetching Topology from a Network Element . . . . . . . . 34
A.2. Modifying TE Topology Imported from an Optical Controller 35
A.3. Annotating Topology for Local Computation . . . . . . . . 35
A.4. SDN Controller-Based Configuration of Overlays on Top of
Underlays . . . . . . . . . . . . . . . . . . . . . . . . 35
Appendix B. Companion YANG Data Models for Non-NMDA-Compliant Implementations . . Not
Compliant with NMDA . . . . . . . . . . . . . . . . 36
B.1. YANG Model Module for Network State . . . . . . . . . . . . . . 37
B.2. YANG Data Model Module for Network Topology State . . . . . . . . . 40
Appendix C. An Example . . . . . . . . . . . . . . . . . . . . . 46
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 50
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50

1. Introduction

This document introduces an abstract (base) YANG [RFC7950] data model
[RFC3444] to represent networks and topologies. The data model is
divided into two parts: The first part of the data model defines a
network data model that enables (1) the definition of network
hierarchies (i.e.,
hierarchies, or network stacks of (i.e., networks that are layered on
top of each other) and (2) maintenance of an inventory of nodes contained
in a network. The second part of the data model augments the basic
network data 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 data model introduces vertical layering
relationships between networks that can be augmented to cover both
network inventories and network/service topologies.

Although it would be possible to combine both parts into a single
data model, the separation facilitates integration of network
topology and network inventory data models, because it allows network
inventory information to be augmented separately, and without concern
for topology, into the network data model.

The data model can be augmented to describe the specifics of
particular types of networks and topologies. For example, an
augmenting data model can provide network node information with
attributes that are specific to a particular network type. Examples
of augmenting models include data 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 data 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 data models allow applications to operate on an
inventory or topology of any network at a generic level, where the
specifics of particular inventory/topology types are not required.
At the same time, where data specific to a network type comes into
play and the data model is augmented, the instantiated data still
adheres to the same structure and is represented in a 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 "ietf-network.yang" "ietf-network" (Section 6.1), 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 data models with inventory inventory-specific and topology-
specific topology-specific
attributes. The network hierarchy (stack) allows any given network
to have one or more "supporting networks". The relationship between
the base network data model, the inventory data models, and the
topology data models is shown in Figure 1 (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 Data Model Structure

The network-topology YANG module introduced in this document,
entitled "ietf-network-topology.yang" "ietf-network-topology" (Section 6.2), defines a generic
topology data 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
ietf-network.yang
"ietf-network" 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 data
model therefore allows relationships between topologies, as well as
dependencies between nodes and termination points across topologies,
to be captured. An example of a topology stack is shown in Figure 2.

+---------------------------------------+
/ _[X1]_ "Service" /
/ _/ : \_ /
/ _/ : \_ /
/ _/ : \_ /
/ / : \ /
/ [X2]__________________[X3] /
+---------:--------------:------:-------+
: : :
+----:--------------:----:--------------+
/ : : : "L3" /
/ : : : /
/ : : : /
/ [Y1]_____________[Y2] /
/ * * * /
/ * * * /
+--------------*-------------*--*-------+
* * *
+--------*----------*----*--------------+
/ [Z1]_______________[Z1] [Z1]_______________[Z2] "Optical" /
/ \_ * _/ /
/ \_ * _/ /
/ \_ * _/ /
/ \ * / /
/ [Z] /
+---------------------------------------+

Figure 2: Topology Hierarchy (Stack) Example

Figure 2 shows three topology levels. At the 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
Two service functions (X1 and X3) mapping are mapped onto a single L3 network
element (Y2); this could happen, for example, if two service
functions reside in the same Virtual Machine (VM) (or server) and
share the same set of network interfaces. The figure
shows a A single "L3" network
element (Y2) is mapped onto multiple two "Optical" network elements (Z (Z2 and Z1).
Z). This could happen, for example, if a single IP router attaches
to multiple Reconfigurable Optical Add/Drop Multiplexers (ROADMs) in
the optical domain.

Another example of a service topology stack is shown in Figure 3.

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

Figure 3: Topology Hierarchy (Stack) Example

Figure 3 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 Interface to the Routing System (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 the 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 where the data model can be applied are
described in [USECASE-REQS].

In this data model, a network is categorized as either system
controlled or not. If a network is system controlled, then it is
automatically populated by the server and represents dynamically
learned information that can be read from the operational state
datastore. The data model can also be used to create or modify
network topologies that might be associated with an inventory model
or with an overlay network. Such a network is not system controlled;
rather, it is configured by a client.

The data model allows a network to refer to a supporting-network,
supporting-nodes, supporting-links, supporting network,
supporting nodes, supporting links, etc. The data model also allows
the layering of a network that is configured on top of a network that
is system controlled. 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 datastore [RFC8342], the requirements for which are defined as requirement
Ephemeral-REQ-03
in Section 3 of [RFC8242]. This allows network topology data that is
written, i.e., configured by a client and not system controlled, to
refer to dynamically learned data that is controlled by the system,
not configured by a client. 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 datastore, such a network
MAY refer to another network that is system controlled.

2. Key Words

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Definitions and Abbreviations

Datastore: A conceptual place to store and access information. A
datastore might be implemented, for example, using files, a
database, flash memory locations, or combinations thereof. A
datastore maps to an instantiated YANG data tree (definition from
[RFC8342]).

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

IGP: Interior Gateway Protocol.

IS-IS: Intermediate System to Intermediate System.

OSPF: Open Shortest Path First (a link-state routing protocol).

SDN: Software-Defined Networking.

URI: Uniform Resource Identifier.

VM: Virtual Machine.

4. Model Structure Details

4.1. Base Network Model

The abstract (base) network data model is defined in the
ietf-network.yang
"ietf-network" module. Its structure is shown in Figure 4. The
notation syntax follows the syntax used in [RFC8340].

module: ietf-network
+--rw networks
+--rw network* [network-id]
+--rw network-id network-id
+--rw network-types
+--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 -> ../../../supporting-network/network-ref
+--rw node-ref -> /networks/network/node/node-id

Figure 4: The Structure of the Abstract (Base) Network Data Model

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

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,
model, 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., will insert them below "network-types"
via 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 type "empty". This allows the
representation of 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 presence container
"l3-unicast-igp-network", which in turn would contain a presence
container "ospf-network". Actual examples of this pattern can be
found in [RFC8346].

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 contains it. In
cases where the same device or entity takes part in multiple
networks, or at multiple layers of a networking stack, the same
device or 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 of which it is a part. To indicate
that the same entity or 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 -- the nodes (entities) in that network -- can then be
referred to as an underlay network and as nodes from the other
(logical) networks and nodes, respectively. Note that the data model
allows for the definition of more than one underlay network (and
node), allowing for simultaneous representation of layered network
topologies and service topologies, and their 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 also allows for the representation of device stacks, where a
node at one level is supported by a set of nodes at an underlying
level. For example, example:

o a "router" node might be supported by a node representing a route
processor and separate nodes for various line cards and service
modules,

o a virtual router might be supported or hosted on a physical device
represented by a separate node,
and so on.

Network data of a network at a particular layer can come into being
in one of two ways: (1) the network data is configured by client
applications -- for example, in the case of overlay networks that are
configured by an SDN Controller application, or (2) the network data
is automatically controlled by the system, in the case of networks
that can be discovered. It is possible for a configured (overlay)
network to refer to a (discovered) underlay network.

The revised datastore architecture [RFC8342] is used to account for
those possibilities. Specifically, for each network, the origin of
its data is indicated per the "origin" metadata [RFC7952] annotation
(as defined in [RFC8342]) -- "intended" for data that was configured
by a client application and "learned" for data that is discovered.
Network data that is discovered is automatically populated as part of
the operational state datastore. Network data that is configured is
part of the configuration and intended datastores, respectively.
Configured network data that is actually in effect is, in addition,
reflected in the operational state datastore. Data in the
operational state datastore will always have complete referential
integrity. Should a configured data item (such as a node) have a
dangling reference that refers to a non-existing data item (such as a
supporting node), the configured data item will automatically be
removed from the operational state datastore and thus only appear in
the intended datastore. It will be up to the client application
(such as an SDN controller) Controller) to resolve the situation and ensure that
the reference to the supporting resources is configured properly.

4.2. Base Network Topology Data Model

The abstract (base) network topology data model is defined in the
"ietf-network-topology.yang"
"ietf-network-topology" module. It builds on the network data model
defined in the "ietf-network.yang" "ietf-network" module, augmenting it with links
(defining how nodes are connected) and termination-points termination points (which
anchor the links and are contained in nodes). The structure of the
network topology module is shown in Figure 5. The notation syntax
follows the syntax used in [RFC8340].

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

Figure 5: The Structure of the Abstract (Base) Network Topology
Data 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 (which are terminated by
the corresponding underlay termination points) in an underlay
topology. This is captured in the list "supporting-link".

4.3. Extending the Data Model

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

We start with augmentations against the ietf-network.yang "ietf-network" module.
First, a new network type needs to be defined; this is done by
defining a presence container that represents the new network type.
The new network type is inserted, by means of augmentation, below the
network-types container. Subsequently, data nodes for any node
parameters that are specific to a network type 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 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 the
ietf-network-topology.yang
"ietf-network-topology" module. Data nodes are defined for link
parameters, as well as termination point parameters, that are
specific to the new network type. Those data nodes are inserted via
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 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
network type under a dedicated container. Doing so introduces
additional structure but lengthens data node path names.

In cases where a hierarchy of network types is defined, augmentations
can in turn be applied against augmenting modules, with the module of
a network whose type is more specific augmenting the module of a
network whose type is more general.

4.4. Discussion and Selected Design Decisions

4.4.1. Container Structure

Rather than maintaining lists in separate containers, the data model
is kept relatively flat in terms of its containment structure. Lists
of nodes, links, termination-points, termination points, and supporting-nodes;
supporting-links; supporting nodes; supporting
links; and supporting-termination-points supporting termination points are not kept in separate
containers. Therefore, path identifiers that are used to refer to
specific nodes, be it nodes -- in management operations or in specifications of constraints,
constraints -- can remain relatively compact. Of course, this means
that there is no separate structure in instance information that
separates elements of different lists from one another. Such a
structure is semantically not required, but it might provide enhanced
"human readability" in some cases.

4.4.2. Underlay Hierarchies and Mappings

To minimize assumptions regarding 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 regarding whether a termination point actually
refers to an interface or whether a node refers to a specific
"system" or device; the data model at this generic level makes no
provisions for these.

Where additional specifics about mappings between upper and lower
layers are required, the information 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 point. The
augmentation introduces a leaf of type "interface-ref" that "interface-ref". That leaf
references the corresponding interface [RFC8343]. 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 data model
makes use of the "require-instance" construct of YANG 1.1 [RFC7950].

A dangling leafref of a configured object leaves the corresponding
instance in a state in which it lacks referential integrity,
effectively rendering it nonoperational. Any corresponding object
instance is therefore removed from the operational state datastore
until the situation has been resolved, i.e., until either (1) the
supporting object is added to the operational state datastore or
(2) the instance is reconfigured to refer to another object that is
actually reflected in the operational state datastore. It will
remain part of the intended datastore.

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 / termination points
refer to nodes in the underlay that actually exist, i.e., verify that
the nodes are reflected in the operational state datastore.
Configuration requests in which supporting nodes / links /
termination points 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 / termination point 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 [YANG-Push].

4.4.4. Use of Groupings

The data model makes use of groupings instead of simply defining data
nodes "inline". This makes it easier to include the corresponding
data nodes in notifications, which then do not need to respecify each
data node that is to be included. The trade-off 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 XML Path Language (XPath)
statements need to be 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 data model includes links that are point-to-point and
unidirectional. It does not directly support multipoint and
bidirectional links. Although this may appear as a limitation, it the
decision to do so keeps the data model simple and generic, and it
allows it to be very easily subjected to applications that make use
of graph algorithms. Bidirectional connections can be represented
through pairs of unidirectional links. Multipoint networks can be
represented through pseudonodes (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 by introducing new links
for nodes at that level, topologies with connections representing non-point-to-
point
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 and then defining a link at a higher layer that is
supported by those individual links.

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 of a link to one or more other links is known and the
termination points of each link are known, the mapping information
for the termination points 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 data model provides the
option to configure this information explicitly. The data model
includes integrity constraints to allow for validating for
consistency.

4.4.8. Typing

A network's network types are represented using a container that
contains a data node for each of its network types. A network can
encompass several types of networks simultaneously; hence, a
container is used instead of a case construct, with each network type
in turn represented by a dedicated presence container itself. container. The reason
for not simply using an empty leaf, or (even more simply) even doing
away with the network container and just using a leaf-list of
"network-types"
"network-type" instead, is to be able to represent "class
hierarchies" of network types, with one network type "refining" the
other. Containers specific to a network type 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 indicate that the same
device can be part of multiple networks and topologies. However, the
data model defines a node as relative to the network that contains
it. 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 Figure 6. This figure depicts three
networks with two nodes each. A physical network ("P" in the figure)
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 their
underlay. X1 has both Y1 and D1 as underlay nodes, while Y1 has D1
as its underlay node. Likewise, X2 has both Y2 and D2 as underlay
nodes, while Y2 has D2 as its underlay node. The fact that X1 and Y1
are both instantiated on the same physical node (D1) can be
easily seen.

+---------------------+
/ [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 represent physical ports. It should be noted
that it is also possible to augment the data model for a physical
network-type,
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 System-Controlled Network
Topologies

YANG requires data nodes to be designated as either configuration
data ("config true") or operational data ("config false"), but not
both, yet it is important to have all network information, including
vertical cross-network dependencies, captured in one coherent data
model. In most cases, network topology information about a network
is discovered; the topology is considered a property of the network
that is reflected in the data model. That said, certain types of
topologies need to also be configurable by an application, e.g., in
the case of overlay topologies.

The YANG data model for network topologies designates all data as
"config true". The distinction between data that is actually
configured and data that is in effect, including network data that is
discovered, is provided through the datastores introduced as part of
the Network Management Datastore Architecture (NMDA) [RFC8342].
Network topology data that is discovered is automatically populated
as part of the operational state datastore, i.e., <operational>. It
is "system controlled". Network topology that is configured is
instantiated as part of a configuration datastore, e.g., <intended>.
Only when it has actually taken effect will it also be instantiated
as part of the operational state datastore, i.e., <operational>.

In general, a configured network topology will refer to an underlay
topology and include layering information, such as the supporting
node(s) underlying a node, supporting link(s) underlying a link, and
supporting termination point(s) underlying a termination point. The
supporting objects must be instantiated in the operational state
datastore in order for the dependent overlay object to be reflected
in the operational state datastore. Should a configured data item
(such as a node) have a dangling reference that refers to a
nonexistent data item (such as a supporting node), the configured
data item will automatically be removed from <operational> and show
up only in <intended>. It will be up to the client application to
resolve the situation and ensure that the reference to the supporting
resources is configured properly.

For each network, the origin of its data is indicated per the
"origin" metadata [RFC7952] annotation defined in [RFC8342]. In
general, the origin of discovered network data is "learned"; the
origin of configured network data is "intended".

4.4.11. Identifiers of String or URI Type

The current data model defines identifiers of nodes, networks, links,
and termination points as URIs. An alternative would define them Alternatively, they could have been
defined as strings.

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

5. Interactions with Other YANG Modules

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

This is a protocol-independent YANG data 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, for example, the "ietf-routing" data model YANG module [RFC8022].

The data model obeys the requirements for the ephemeral state as
specified in [RFC8242]. For ephemeral topology data that is system
controlled, the process tasked with maintaining topology information
will load information from the routing process (such as OSPF) into
the operational state datastore without relying on a configuration
datastore.

6. YANG Modules

6.1. Defining the Abstract Network: ietf-network.yang ietf-network

<CODE BEGINS> file "ietf-network@2018-02-26.yang"

module ietf-network {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-network";
prefix nw;
import ietf-inet-types {
prefix inet;
reference
"RFC 6991: Common YANG Data Types";
}

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

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

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

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

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

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

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

Editor: Xufeng Liu
<mailto:Xufeng_Liu@jabil.com>";

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

Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(https://trustee.ietf.org/license-info).

This version of this YANG module is part of RFC 8345;
see the RFC itself for full legal notices.";
revision 2018-02-26 {
description
"Initial revision.";
reference
"RFC 8345: An Abstract A YANG Data Model for Network Topologies";
}

typedef node-id {
type inet:uri;
description
"Identifier for a node. The precise structure of the node-id
will be up to the implementation. For example, some
implementations MAY 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 data 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 the implementation. The identifier
SHOULD be chosen such that the same network will always be
identified through the same identifier, even if the data 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 "/nw:networks/nw:network/nw:network-id";
require-instance false;
}
description
"Used to reference a network -- for example, an underlay
network.";
}
}
grouping node-ref {
description
"Contains the information necessary to reference a node.";
leaf node-ref {
type leafref {
path "/nw:networks/nw:network[nw:network-id=current()/../"+
"network-ref]/nw:node/nw:node-id";
require-instance false;
}
description
"Used to reference a node.
Nodes are identified relative to the network that
contains them.";
}
uses network-ref;
}

container networks {
description
"Serves as a 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 the
network-topology data model), and layering information.";
leaf network-id {
type network-id;
description
"Identifies a network.";
}
container network-types {
description
"Serves as an augmentation target.
The network type is indicated through corresponding
presence containers augmented into this container.";
}
list supporting-network {
key "network-ref";
description
"An underlay network, used to represent layered network
topologies.";
leaf network-ref {
type leafref {
path "/nw:networks/nw:network/nw: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
"Uniquely identifies a node within the containing
network.";
}
list supporting-node {
key "network-ref node-ref";
description
"Represents another node that is in an underlay network
and that supports this node. Used to represent layering
structure.";
leaf network-ref {
type leafref {
path "../../../nw:supporting-network/nw:network-ref";
require-instance false;
}
description
"References the underlay network of which the
underlay node is a part.";
}
leaf node-ref {
type leafref {
path "/nw:networks/nw:network/nw:node/nw:node-id";
require-instance false;
}
description
"References the underlay node itself.";
}
}
}
}
}
}

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

<CODE BEGINS> file "ietf-network-topology@2018-02-26.yang"

module ietf-network-topology {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-network-topology";
prefix nt;

import ietf-inet-types {
prefix inet;
reference
"RFC 6991: Common YANG Data Types";
}
import ietf-network {
prefix nw;
reference
"RFC 8345: An Abstract A YANG Data Model for Network Topologies";
}

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

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

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

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

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

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

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

Editor: Xufeng Liu
<mailto:Xufeng_Liu@jabil.com>";

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

This version of this YANG module is part of RFC 8345;
see the RFC itself for full legal notices.";

revision 2018-02-26 {
description
"Initial revision.";
reference
"RFC 8345: An Abstract A YANG Data Model for Network Topologies";
}

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 data 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 of which the link is
a part.";
}

typedef tp-id {
type inet:uri;
description
"An identifier for termination points 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 data 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
"This grouping can be used to reference a link in a specific
network. Although it is not used in this module, it is
defined here for the convenience of augmenting modules.";
leaf link-ref {
type leafref {
path "/nw:networks/nw:network[nw:network-id=current()/../"+
"network-ref]/nt:link/nt:link-id";
require-instance false;
}
description
"A type for an absolute reference to a link instance.
(This type should not be used for relative references.
In such a case, a relative path should be used instead.)";
}
uses nw:network-ref;
}

grouping tp-ref {
description
"This grouping can be used to reference a termination point
in a specific node. Although it is not used in this module,
it is defined here for the convenience of augmenting
modules.";
leaf tp-ref {
type leafref {
path "/nw:networks/nw:network[nw:network-id=current()/../"+
"network-ref]/nw:node[nw:node-id=current()/../"+
"node-ref]/nt:termination-point/nt: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 nw:node-ref;
}

augment "/nw:networks/nw:network" {
description
"Add links to the network data 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.";
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.";
}
container source {
description
"This container holds the logical source of a particular
link.";
leaf source-node {
type leafref {
path "../../../nw:node/nw:node-id";
require-instance false;
}
description
"Source node identifier. Must be in the same topology.";
}
leaf source-tp {
type leafref {
path "../../../nw:node[nw:node-id=current()/../"+
"source-node]/termination-point/tp-id";
require-instance false;
}
description
"This termination point is located within the source node
and terminates the link.";
}
}
container destination {
description
"This container holds the logical destination of a
particular link.";
leaf dest-node {
type leafref {
path "../../../nw:node/nw:node-id";
require-instance false;
}
description
"Destination node identifier. Must be in the same
network.";
}
leaf dest-tp {
type leafref {
path "../../../nw:node[nw:node-id=current()/../"+
"dest-node]/termination-point/tp-id";
require-instance false;
}
description
"This termination point is located within the
destination node and terminates the link.";
}
}
list supporting-link {
key "network-ref link-ref";
description
"Identifies the link or links on which this link depends.";
leaf network-ref {
type leafref {
path "../../../nw:supporting-network/nw:network-ref";
require-instance false;
}
description
"This leaf identifies in which underlay topology
the supporting link is present.";
}
leaf link-ref {
type leafref {
path "/nw:networks/nw:network[nw:network-id=current()/"+
"../network-ref]/link/link-id";
require-instance false;
}
description
"This leaf identifies a link that 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 "/nw:networks/nw:network/nw:node" {
description
"Augment
"Augments termination points that 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 on which a
given termination point depends or onto which it maps.
Those termination points will themselves be contained
in a supporting node. This dependency information can be
inferred from the dependencies between links. Therefore,
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 {
type leafref {
path "../../../nw:supporting-node/nw:network-ref";
require-instance false;
}
description
"This leaf identifies in which topology the
supporting termination point is present.";
}
leaf node-ref {
type leafref {
path "../../../nw:supporting-node/nw:node-ref";
require-instance false;
}
description
"This leaf identifies in which node the supporting
termination point is present.";
}
leaf tp-ref {
type leafref {
path "/nw:networks/nw:network[nw:network-id=current()/"+
"../network-ref]/nw:node[nw:node-id=current()/../"+
"node-ref]/termination-point/tp-id";
require-instance false;
}
description
"Reference to the underlay node. Must node (the underlay node must
be in a different topology."; topology).";
}
}
}
}
}

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.

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

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

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: nw
Reference: RFC 8345

Name: ietf-network-topology
Namespace: urn:ietf:params:xml:ns:yang:ietf-network-topology
Prefix: nt
Reference: RFC 8345

Name: ietf-network-state
Namespace: urn:ietf:params:xml:ns:yang:ietf-network-state
Prefix: nw-s
Reference: RFC 8345

Name: ietf-network-topology-state
Namespace: urn:ietf:params:xml:ns:yang:ietf-network-topology-state
Prefix: nt-s
Reference: RFC 8345

8. Security Considerations

The YANG module modules specified in this document defines define a schema for data
that is designed to be accessed via network management protocols such
as NETCONF [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer
is the secure transport layer, and the mandatory-to-implement secure
transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer
is HTTPS, and the mandatory-to-implement secure transport is TLS
[RFC5246].

The NETCONF access control model [RFC8341] provides the means to
restrict access for particular NETCONF or RESTCONF users to a
preconfigured subset of all available NETCONF or RESTCONF protocol
operations and content.

The network topology and inventory created by this module these modules reveal
information about the structure of networks that could be very
helpful to an attacker. As a privacy consideration, although there
is no personally identifiable information defined in this module, these modules,
it is possible that some node identifiers may be associated with
devices that are in turn associated with specific users.

The YANG modules define 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 remove or add a
node, a link, or a termination point by creating or deleting
corresponding elements in node, link, or termination point lists,
respectively. In the case of a topology that is learned, the server
will automatically prohibit such misconfiguration attempts. In the
case of a topology that is configured, i.e., whose origin is
"intended", the undesired configuration could become effective and be
reflected in the operational state datastore, leading to disruption
of services provided via this topology. For example, the topology
could be "cut" or could be configured in a suboptimal way, leading to
increased consumption of resources in the underlay network due to the
routing and bandwidth utilization inefficiencies that would result.
Likewise, it could lead to degradation of service levels as well as
possible disruption of service. For those reasons, it is important
that the NETCONF access control model be vigorously applied to
prevent topology misconfiguration by unauthorized clients.

There are a number of data nodes defined in this these YANG module modules that
are writable/creatable/deletable (i.e., config true, which is the
default). These data nodes may be considered sensitive or vulnerable
in some network environments. Write operations (e.g., edit-config)
to these data nodes without proper protection can have a negative
effect on network operations. These are the subtrees and data nodes
and their sensitivity/vulnerability:

In the ietf-network "ietf-network" module:

o network: A malicious client could attempt to remove or add a
network in an effort to remove an overlay topology or to create an
unauthorized overlay.

o supporting-network: supporting network: A malicious client could attempt to disrupt
the logical structure of the model, resulting in a lack of overall
data integrity and making it more difficult to, for example,
troubleshoot problems rooted in the layering of network
topologies.

o node: A malicious client could attempt to remove or add a node
from the network -- for example, in order to sabotage the topology
of a network overlay.

o supporting-node: supporting node: A malicious client could attempt to change the
supporting-node
supporting node in order to sabotage the layering of an overlay.

In the ietf-network-topology "ietf-network-topology" module:

o link: A malicious client could attempt to remove a link from a
topology, add a new link, manipulate the way the link is layered
over supporting links, or modify the source or destination of the
link. In each case, the structure of the topology would be
sabotaged, and this scenario could, for example, result in an
overlay topology that is less than optimal.

o termination-point: termination point: A malicious client could attempt to remove
termination points from a node, add "phantom" termination points
to a node, or change the layering dependencies of termination
points, again in an effort to sabotage the integrity of a topology
and potentially disrupt orderly operations of an overlay.

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

[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/info/rfc3688>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<https://www.rfc-editor.org/info/rfc6020>.

[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.

[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
<https://www.rfc-editor.org/info/rfc6242>.

[RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC6991, July 2013,
<https://www.rfc-editor.org/info/rfc6991>.

[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.

[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8341] Bierman, A. and M. Bjorklund, "Network Configuration
Access Control Model", STD 91, RFC 8341,
DOI 10.17487/RFC8341, February March 2018,
<https://www.rfc-editor.org/info/rfc8341>.

[RFC8342] Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
and R. Wilton, "Network Management Datastore Architecture
(NMDA)", RFC 8342, DOI 10.17487/RFC8342, February March 2018,
<https://www.rfc-editor.org/info/rfc8342>.

9.2. Informative References

[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, <https://www.rfc-editor.org/info/rfc1195>.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.

[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.

[RFC3444] Pras, A. and J. Schoenwaelder, "On the Difference between
Information Models and Data Models", RFC 3444,
DOI 10.17487/RFC3444, January 2003,
<https://www.rfc-editor.org/info/rfc3444>.

[RFC7951] Lhotka, L., "JSON Encoding of Data Modeled with YANG",
RFC 7951, DOI 10.17487/RFC7951, August 2016,
<https://www.rfc-editor.org/info/rfc7951>.

[RFC7952] Lhotka, L., "Defining and Using Metadata with YANG",
RFC 7952, DOI 10.17487/RFC7952, August 2016,
<https://www.rfc-editor.org/info/rfc7952>.

[RFC8022] Lhotka, L. and A. Lindem, "A YANG Data Model for Routing
Management", RFC 8022, DOI 10.17487/RFC8022, November
2016, <https://www.rfc-editor.org/info/rfc8022>.

[RFC8242] Haas, J. and S. Hares, "Interface to the Routing System
(I2RS) Ephemeral State Requirements", RFC 8242,
DOI 10.17487/RFC8242, September 2017,
<https://www.rfc-editor.org/info/rfc8242>.

[RFC8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
BCP 215, RFC 8340, DOI 10.17487/RFC8340, February March 2018,
<https://www.rfc-editor.org/info/rfc8340>.

[RFC8343] Bjorklund, M., "A YANG Data Model for Interface
Management", RFC 8343, DOI 10.17487/RFC8343, February March 2018,
<https://www.rfc-editor.org/info/rfc8343>.

[RFC8346] Clemm, A., Medved, J., Varga, R., Liu, X.,
Ananthakrishnan, H., and N. Bahadur, "A YANG Data Model
for Layer 3 Topologies", RFC 8346, DOI 10.17487/RFC8346,
February
March 2018, <https://www.rfc-editor.org/info/rfc8346>.

[USECASE-REQS]
Hares, S. and M. Chen, "Summary of I2RS Use Case
Requirements", Work in Progress, draft-ietf-i2rs-usecase-
reqs-summary-03, November 2016.

[YANG-Push]
Clemm, A., Voit, E., Prieto, A., Tripathy, A., Nilsen-
Nygaard, E., Bierman, A., and B. Lengyel, "YANG Datastore
Subscription", Work in Progress, draft-ietf-netconf-yang-
push-14,
push-15, February 2018.

Appendix A. Model Use Cases

A.1. Fetching Topology from a Network Element

In its simplest form, topology is learned by a network element (e.g.,
a router) through its participation in peering protocols (IS-IS, BGP,
etc.). This learned topology can then be exported (e.g., to a
Network Management System) for external utilization. Typically, any
network element in a domain can be queried for its topology and be
expected to return the same result.

In a slightly more complex form, the network element may be a
controller, either by nature of it having
controller. It could be a network element with satellite or
subtended devices hanging off of it, or it could be a controller in
the more classical sense, such as sense -- that is, a special device designated to
orchestrate the activities of a number of other devices (e.g., an optical controller).
Optical Controller). In this case, the controller device is
logically a singleton and must be queried distinctly.

It is worth noting that controllers can be built on top of other
controllers to establish a topology incorporating all of the domains
within an entire network.

In all of the cases above, the topology learned by the network
element is considered to be operational state data. That is, the
data is accumulated purely by the network element's interactions with
other systems and is subject to change dynamically without input or
consent.

A.2. Modifying TE Topology Imported from an Optical Controller

Consider a scenario where an Optical Transport Controller presents its
topology, in abstract TE terms, to a client packet controller. This
customized topology (which gets merged into the client's native
topology) contains sufficient information for the path-computing
client to select paths across the optical domain according to its
policies. If the client determines (at any given point in time) that
this imported topology does not cater exactly to its requirements, it
may decide to request modifications to the topology. Such
customization requests may include the addition or deletion of
topological elements or the modification of attributes associated
with existing topological elements. From the perspective of the
Optical Controller, these requests translate into configuration
changes to the exported abstract topology.

A.3. Annotating Topology for Local Computation

In certain scenarios, the topology learned by a controller needs to
be augmented with additional attributes before running a computation
algorithm on it. Consider the case where a path-computation
application on the controller needs to take the geographic
coordinates of the nodes into account while computing paths on the
learned topology. If the learned topology does not contain these
coordinates, then these additional attributes must be configured on
the corresponding topological elements.

A.4. SDN Controller-Based Configuration of Overlays on Top of Underlays

In this scenario, an SDN controller Controller (for example, Open Daylight)
maintains a view of the topology of the network that it controls
based on information that it discovers from the network. In
addition, it provides an application in which it configures and
maintains an overlay topology.

The SDN Controller thus maintains two roles:

o It is a client to the network.

o It is a server to its own northbound applications and clients,
e.g., an Operations Support System (OSS).

In other words, one system's client (or controller, in this case) may
be another system's server (or managed system).

In this scenario, the SDN controller Controller maintains a consolidated data
model of multiple layers of topology. This includes the lower layers
of the network topology, built from information that is discovered
from the network. It also includes upper layers of topology overlay,
configurable by the controller's client, i.e., the OSS. To the OSS,
the lower topology layers constitute "read-only" information. The
upper topology layers need to be read-writable.

Appendix B. Companion YANG Data Models for Non-NMDA-Compliant Implementations Not
Compliant with NMDA

The YANG modules defined in this document are designed to be used in
conjunction with implementations that support the Network Management
Datastore Architecture (NMDA) as defined in [RFC8342]. In order to
allow implementations to use the data model even in cases when NMDA
is not supported, the following two companion modules --
ietf-network-state
"ietf-network-state" and ietf-network-topology-state "ietf-network-topology-state" -- are
defined; they represent the operational state of networks and network
topologies, respectively. These modules mirror the ietf-network "ietf-network"
and
ietf-network-topology "ietf-network-topology" modules (defined in Sections 6.1 and 6.2
of this document); however, in the case of these modules, all data
nodes are non-configurable. They represent state that comes into
being by either (1) learning topology information from the network or
(2) applying configuration from the mirrored modules.

The ietf-network-state "ietf-network-state" and ietf-network-topology-state "ietf-network-topology-state" companion
modules are redundant and SHOULD NOT be supported by implementations
that support NMDA; therefore, we define these modules in
Appendices B.1 and B.2 (below) instead of the main body of this
document.

As the structure of both modules mirrors that of their underlying
modules, the YANG tree is not depicted separately.

B.1. YANG Model Module for Network State

<CODE BEGINS> file "ietf-network-state@2018-02-26.yang"

module ietf-network-state {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-network-state";
prefix nw-s;

import ietf-network {
prefix nw;
reference
"RFC 8345: An Abstract A YANG Data Model for Network Topologies";
}

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

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

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

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

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

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

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

Editor: Xufeng Liu
<mailto:Xufeng_Liu@jabil.com>";

description
"This module defines a common base data model for a collection
of nodes in a network. Node definitions are further used
in network topologies and inventories. It represents
information that either (1) is learned and automatically
populated or (2) results from applying network information
that has been configured per the ietf-network 'ietf-network' data model,
mirroring the corresponding data nodes in this data model.

The data model mirrors ietf-network 'ietf-network' but contains only
read-only state data. The data model is not needed when the
underlying implementation infrastructure supports the Network
Management Datastore Architecture (NMDA).

This version of this YANG module is part of RFC 8345;
see the RFC itself for full legal notices.";

revision 2018-02-26 {
description
"Initial revision.";
reference
"RFC 8345: An Abstract A YANG Data Model for Network Topologies";
}

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

container networks {
config false;
description
"Serves as a 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 the
network-topology data model), and 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 nw:network-id;
description
"Identifies a network.";
}
list supporting-network {
key "network-ref";
description
"An underlay network, used to represent layered network
topologies.";
leaf network-ref {
type leafref {
path "/nw-s:networks/nw-s:network/nw-s: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 nw:node-id;
description
"Uniquely identifies a node within the containing
network.";
}
list supporting-node {
key "network-ref node-ref";
description
"Represents another node that is in an underlay network
and that supports this node. Used to represent layering
structure.";
leaf network-ref {
type leafref {
path "../../../nw-s:supporting-network/nw-s:network-ref";
require-instance false;
}
description
"References the underlay network of which the
underlay node is a part.";
}
leaf node-ref {
type leafref {
path "/nw-s:networks/nw-s:network/nw-s:node/nw-s:node-id";
require-instance false;
}
description
"References the underlay node itself.";
}
}
}
}
}
}

B.2. YANG Data Model Module for Network Topology State

<CODE BEGINS> file "ietf-network-topology-state@2018-02-26.yang"

module ietf-network-topology-state {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-network-topology-state";
prefix nt-s;

import ietf-network-state {
prefix nw-s;
reference
"RFC 8345: An Abstract A YANG Data Model for Network Topologies";
}
import ietf-network-topology {
prefix nt;
reference
"RFC 8345: An Abstract A YANG Data Model for Network Topologies";
}

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

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

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

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

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

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

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

Editor: Xufeng Liu
<mailto:Xufeng_Liu@jabil.com>";

description
"This module defines a common base data model for network
topology state, representing topology that either (1) is learned
or (2) results from applying topology that has been configured
per the ietf-network-topology 'ietf-network-topology' data model, mirroring the
corresponding data nodes in this data model. It augments the
base network state data model with links to connect nodes, as
well as termination points to terminate links on nodes.

The data model mirrors ietf-network-topology 'ietf-network-topology' but contains only
read-only state data. The data model is not needed when the
underlying implementation infrastructure supports the Network
Management Datastore Architecture (NMDA).

This version of this YANG module is part of RFC 8345;
see the RFC itself for full legal notices.";

revision 2018-02-26 {
description
"Initial revision.";
reference
"RFC 8345: An Abstract A YANG Data Model for Network Topologies";
}

grouping link-ref {
description
"References a link in a specific network. Although this
grouping is not used in this module, it is defined here for
the convenience of augmenting modules.";
leaf link-ref {
type leafref {
path "/nw-s:networks/nw-s:network[nw-s:network-id=current()"+
"/../network-ref]/nt-s:link/nt-s:link-id";
require-instance false;
}
description
"A type for an absolute reference to a link instance.
(This type should not be used for relative references.
In such a case, a relative path should be used instead.)";
}
uses nw-s:network-ref;
}

grouping tp-ref {
description
"References a termination point in a specific node. Although
this grouping is not used in this module, it is defined here
for the convenience of augmenting modules.";
leaf tp-ref {
type leafref {
path "/nw-s:networks/nw-s:network[nw-s:network-id=current()"+
"/../network-ref]/nw-s:node[nw-s:node-id=current()/../"+
"node-ref]/nt-s:termination-point/nt-s: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 nw-s:node-ref;
}

augment "/nw-s:networks/nw-s:network" {
description
"Add links to the network data 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 "../../../nw-s:node/nw-s:node-id";
require-instance false;
}
description
"Source node identifier. Must be in the same topology.";
}
leaf source-tp {
type leafref {
path "../../../nw-s:node[nw-s:node-id=current()/../"+
"source-node]/termination-point/tp-id";
require-instance false;
}
description
"This termination point is located within the source node
and terminates the link.";
}
}
container destination {
description
"This container holds the logical destination of a
particular link.";
leaf dest-node {
type leafref {
path "../../../nw-s:node/nw-s:node-id";
require-instance false;
}
description
"Destination node identifier. Must be in the same
network.";
}
leaf dest-tp {
type leafref {
path "../../../nw-s:node[nw-s:node-id=current()/../"+
"dest-node]/termination-point/tp-id";
require-instance false;
}
description
"This termination point is located within the
destination node and terminates the link.";
}
}
leaf link-id {
type nt: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 on which this link depends.";
leaf network-ref {
type leafref {
path "../../../nw-s:supporting-network/nw-s:network-ref";
require-instance false;
}
description
"This leaf identifies in which underlay topology
the supporting link is present.";
}
leaf link-ref {
type leafref {
path "/nw-s:networks/nw-s:network[nw-s:network-id="+
"current()/../network-ref]/link/link-id";
require-instance false;
}
description
"This leaf identifies a link that 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 "/nw-s:networks/nw-s:network/nw-s:node" {
description
"Augment
"Augments termination points that 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 nt:tp-id;
description
"Termination point identifier.";
}
list supporting-termination-point {
key "network-ref node-ref tp-ref";
description
"This list identifies any termination points on which a
given termination point depends or onto which it maps.
Those termination points will themselves be contained
in a supporting node. This dependency information can be
inferred from the dependencies between links. Therefore,
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 {
type leafref {
path "../../../nw-s:supporting-node/nw-s:network-ref";
require-instance false;
}
description
"This leaf identifies in which topology the
supporting termination point is present.";
}
leaf node-ref {
type leafref {
path "../../../nw-s:supporting-node/nw-s:node-ref";
require-instance false;
}
description
"This leaf identifies in which node the supporting
termination point is present.";
}
leaf tp-ref {
type leafref {
path "/nw-s:networks/nw-s:network[nw-s:network-id="+
"current()/../network-ref]/nw-s:node[nw-s:node-id="+
"current()/../node-ref]/termination-point/tp-id";
require-instance false;
}
description
"Reference to the underlay node. Must node (the underlay node must
be in a different topology."; topology).";
}
}
}
}
}

Appendix C. An Example

This section contains an example of an instance data tree in JSON
encoding [RFC7951]. The example instantiates ietf-network-topology "ietf-network-topology"
(and ietf-network, "ietf-network", which ietf-network-topology "ietf-network-topology" augments) for the
topology depicted in Figure 7. There are three nodes: D1, D2, and
D3. D1 has three termination points (1-0-1, 1-2-1, and 1-3-1).
D2 has three termination points as well (2-1-1, 2-0-1, and 2-3-1).
D3 has two termination points (3-1-1 and 3-2-1). In addition, there
are six links, two between each pair of nodes with one going in each
direction.

+------------+ +------------+
| D1 | | D2 |
/-\ /-\ /-\ /-\
| | 1-0-1 | |---------------->| | 2-1-1 | |
| | 1-2-1 | |<----------------| | 2-0-1 | |
\-/ 1-3-1 \-/ \-/ 2-3-1 \-/
| /----\ | | /----\ |
+---| |---+ +---| |---+
\----/ \----/
A | A |
| | | |
| | | |
| | +------------+ | |
| | | D3 | | |
| | /-\ /-\ | |
| +----->| | 3-1-1 | |-------+ |
+---------| | 3-2-1 | |<---------+
\-/ \-/
| |
+------------+

Figure 7: A Network Topology Example

The corresponding instance data tree is depicted in Figure 8:

{
"ietf-network:networks": {
"network": [
{
"network-types": {
},
"network-id": "otn-hc",
"node": [
{
"node-id": "D1",
"termination-point": [
{
"tp-id": "1-0-1"
},
{
"tp-id": "1-2-1"
},
{
"tp-id": "1-3-1"
}
]
},
{
"node-id": "D2",
"termination-point": [
{
"tp-id": "2-0-1"
},
{
"tp-id": "2-1-1"
},
{
"tp-id": "2-3-1"
}
]
},
{
"node-id": "D3",
"termination-point": [
{
"tp-id": "3-1-1"
},
{
"tp-id": "3-2-1"
}
]
}
],
"ietf-network-topology:link": [
{
"link-id": "D1,1-2-1,D2,2-1-1",
"destination":
"source": {
"source-node": "D1",
"source-tp": "1-2-1"
}
"destination": {
"dest-node": "D2",
"dest-tp": "2-1-1"
}
},
{
"link-id": "D2,2-1-1,D1,1-2-1",
"destination":
"source": {
"source-node": "D2",
"source-tp": "2-1-1"
}
"destination": {
"dest-node": "D1",
"dest-tp": "1-2-1"
}
},
{
"link-id": "D1,1-3-1,D3,3-1-1",
"destination":
"source": {
"source-node": "D1",
"source-tp": "1-3-1"
}
"destination": {
"dest-node": "D3",
"dest-tp": "3-1-1"
}
},
{
"link-id": "D3,3-1-1,D1,1-3-1",
"destination":
"source": {
"source-node": "D3",
"source-tp": "3-1-1"
}
"destination": {
"dest-node": "D1",
"dest-tp": "1-3-1"
}
},
{
"link-id": "D2,2-3-1,D3,3-2-1",
"destination":
"source": {
"source-node": "D2",
"source-tp": "2-3-1"
}
"destination": {
"dest-node": "D3",
"dest-tp": "3-2-1"
}
},
{
"link-id": "D3,3-2-1,D2,2-3-1",
"destination":
"source": {
"source-node": "D3",
"source-tp": "3-2-1"
}
"destination": {
"dest-node": "D2",
"dest-tp": "2-3-1"
}
}
]
}
]
}
}

Figure 8: Instance Data Tree

Acknowledgments

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, Robert Wilton, Qin Wu, and
Xian Zhang.

Contributors

More people contributed to the data model presented in this paper
than can be listed in the "Authors' Addresses" section. Additional
contributors include:

o Vishnu Pavan Beeram, Juniper

o Ken Gray, Cisco

o Tom Nadeau, Brocade

o Tony Tkacik

o Kent Watsen, Juniper

o Aleksandr Zhdankin, Cisco

Authors' Addresses

Alexander Clemm
Huawei USA - Futurewei Technologies Inc.
Santa Clara, CA
United States of America

Email: ludwig@clemm.org ludwig@clemm.org, alexander.clemm@huawei.com

Jan Medved
Cisco

Email: jmedved@cisco.com
Robert Varga
Pantheon Technologies SRO

Email: robert.varga@pantheon.tech

Nitin Bahadur
Bracket Computing

Email: nitin_bahadur@yahoo.com

Hariharan Ananthakrishnan
Packet Design

Email: hari@packetdesign.com

Xufeng Liu
Jabil

Email: Xufeng_Liu@jabil.com