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 March 2018 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.
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Table of Contents 1. Introduction ....................................................4 2. Key Words .......................................................8 3. Definitions and Abbreviations ...................................9 4. Model Structure Details .........................................9 4.1. Base Network Model .........................................9 4.2. Base Network Topology Data Model ..........................12 4.3. Extending the Data Model ..................................13 4.4. Discussion and Selected Design Decisions ..................14 4.4.1. Container Structure ................................14 4.4.2. Underlay Hierarchies and Mappings ..................14 4.4.3. Dealing with Changes in Underlay Networks ..........15 4.4.4. Use of Groupings ...................................15 4.4.5. Cardinality and Directionality of Links ............16 4.4.6. Multihoming and Link Aggregation ...................16 4.4.7. Mapping Redundancy .................................16 4.4.8. Typing .............................................17 4.4.9. Representing the Same Device in Multiple Networks ..17 4.4.10. Supporting Client-Configured and System-Controlled Network Topologies ..............18 4.4.11. Identifiers of String or URI Type .................19 5. Interactions with Other YANG Modules ...........................19 6. YANG Modules ...................................................20 6.1. Defining the Abstract Network: ietf-network ...............20 6.2. Creating Abstract Network Topology: ietf-network-topology .....................................25 7. IANA Considerations ............................................32 8. Security Considerations ........................................33 9. References .....................................................35 9.1. Normative References ......................................35 9.2. Informative References ....................................36 Appendix A. Model Use Cases .......................................38 A.1. Fetching Topology from a Network Element ...................38 A.2. Modifying TE Topology Imported from an Optical Controller ..38 A.3. Annotating Topology for Local Computation ..................39 A.4. SDN Controller-Based Configuration of Overlays on Top of Underlays ..................................................39 Appendix B. Companion YANG Data Models for Implementations Not Compliant with NMDA ...................................39 B.1. YANG Module for Network State ..............................40 B.2. YANG Module for Network Topology State .....................45 Appendix C. An Example ............................................52 Acknowledgments ...................................................56 Contributors ......................................................56 Authors' Addresses ................................................57
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 the definition of network hierarchies, or network stacks (i.e., networks that are layered on top of each other) and 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" (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-specific and 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" (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" 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]_______________[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. Two service functions (X1 and X3) 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. A single "L3" network element (Y2) is mapped onto two "Optical" network elements (Z2 and 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, 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 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" 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 +--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 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: 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) 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" module. It builds on the network data model defined in the "ietf-network" 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 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? leafref +--rw destination | +--rw dest-node? -> ../../../nw:node/node-id | +--rw dest-tp? leafref +--rw supporting-link* [network-ref link-ref] +--rw network-ref | -> ../../../nw:supporting-network/network-ref +--rw link-ref 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 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" and "ietf-network-topology" modules. We start with augmentations against the "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 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" 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 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 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, and supporting nodes; supporting links; and supporting termination points are not kept in separate containers. Therefore, path identifiers that are used to refer to specific nodes -- in management operations or in specifications of 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. The augmentation introduces a leaf of type "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, 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 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 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. 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-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, 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. 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.