| Internet-Draft | SDF modeling for digital twin | June 2025 |
| Lee, et al. | Expires 16 December 2025 | [Page] |
This memo specifies SDF modeling for a digital twin, i.e. a digital twin system, and its Things. An SDF is a format that is used to create and maintain data and interaction, and to represent the various kinds of data that is exchanged for these interactions. The SDF format can be used to model the characteristics, behavior and interactions of Things, i.e. physical objects, in a digital twin that contain Things as components.¶
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A digital twin is defined as a digital representation of an object of interest and may require different capabilities, for example, synchronization and real-time support, according to the specific domain of application. [Y.4600]. Digital twin help organizations improve important functional objectives, including real-time control, off-line analytics, and predictive maintenance, by modeling and simulating objects in the real world. Therefore, it is important for a digital twin to represent as much real-world information about the object as possible when digitally representing the object.¶
Nowadays, digital twin technologies are applied in various domains including manufacturing, energy, medical, farm, transportation, etc. And a common format is needed to represent the objects in the domains as digital twins. SDF [I-D.ietf-asdf-sdf] can be used for modeling objects as digital twins.¶
This document specifies the modeling and guidance on how to use SDF to represent objects as digital twins.¶
This specification uses the terminology specified in [I-D.ietf-asdf-sdf] in particular "Class Name Keyword", "Object", and "Affordance".¶
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.¶
This section describes SDF structure with the new Class Name Keyword, sdfNonAffordance, to represent a thing or an object as a digital twin. The architecture of a digital twin based on the SDF model is illustrated in Figure 1, , following the guidelines of [ISO23247-3].¶
The Physical Layer comprises affordance and non-affordance objects. From the real-world objects, only those deemed relevant are selected for representation as digital twins. The Digital Twin Layer is structured into three sublayers: the Device Communication Sublayer, the Digital Twin Sublayer, and the Application Sublayer. The Device Communication Sublayer is responsible for monitoring and collecting data from both affordance and non-affordance objects. This sublayer provides the necessary data to synchronize the physical objects with their digital twin counterparts. The Digital Twin Sublayer ensures synchronization between the affordance and non-affordance objects and their respective digital twins using the data provided by the Device Communication Sublayer. The Application Sublayer presents the synchronized values of the digital twins to users, facilitating informed decision-making.¶
+---------------------------------------------+ - - - - - - - - - - -
| Application Sublayer |
| +----------+ +------+ +--------+ +--------+ |
| | Human | | HMI | | Apps | | Peers | |
| +----------+ +------+ +--------+ +--------+ |
+---------------------------------------------+
| Digital Twin Sublayer |
| +----------+ +-------------+ +------------+ |
| | Operation| | Application | | Resource | |
| | and | | and | | access and | |
| |management| | service | |interchange | |
| +----------+ +-------------+ +------------+ |
| +-----------------------------------------+ | Digital twin Layer
| | Digital representation of objects | |
| | +-------------+ +----------------+ | |
| | | Affordance | | NonAffordance | | |
| | | objects | | objects | | |
| | +-------------+ +----------------+ | |
| +-----------------------------------------+ |
+---------------------------------------------+
| Device Communication Sublayer |
| +-------------+ +----------------+ |
| | Data | | Object | |
| | collection | | control | |
| +-------------+ +----------------+ |
+---------------------------------------------+ - - - - - - - - - - -
| +-------------+ +----------------+ |
| | Affordance | | sdfNonAffordance | |
| | objects | | objects | | Physical Layer
| +-------------+ +----------------+ |
+---------------------------------------------+ - - - - - - - - - - -
The document is based on the underlying structure defined in [I-D.ietf-asdf-sdf], which which standardizes the semantic definition format (SDF) for representing IoT affordance. This specification provides a strong basis for representing individual devices and their features (sdfProperty, sdfAction, sdfEvent, etc.), but additional mechanisms are needed to address the unique requirements of digital twin modeling.¶
Digital twin systems defined in [ISO23247-3] often have to describe virtual representations of various physical assets, including metadata, identity, contextual relationships, historical data, as well as device interfaces.¶
A new SDF keyword sdfNonAffordance described in [I-D.draft-hong-asdf-sdf-nonaffordance] is introduced to represent non-functional or metadata elements that describe a device or component without implying direct interaction:¶
Identifier (e.g., UUID, URN)¶
Location (e.g. site, zone, GPS tag)¶
Owner (e.g., representative, ,anufacturer)¶
These field can appear in both sdfObject and sdfThing contexts and follow the same structural pattern as sdfData and is designed for scalability.¶
To support hierarchical representations (e.g., a boat composed of heater, GPS, and battery subsystems), this document encourages use of sdfThing to aggregate related sdfObject components, along with metadata.¶
The mapping of digital twin attributes to sdf qualities are shown in Table 1.¶
| Attribute | Recommended Mapping | Description |
|---|---|---|
| Identifier | sdfNonAffordance | Globally unique digital twin ID (e.g., URN) |
| Characteristic | sdfProperty or sdfData | General description or domain properties |
| Schedule | sdfEvent or sdfData | Time-based actions, availability, or maintenance |
| Status | sdfAction or sdfProperty | Actual or calculated operating conditions |
| Location | sdfNonAffordance | Physical or logical location information |
| Report | sdfData | Measurement summaries, analytics, or logs |
| owner | sdfNonAffordance | Organization or entity responsible for the digital twin |
| Relationship | [TBD] | Inter-object/inter-twin relationships |
The example of boat007 Figure 2illustrates how a Digital Twin representation can be constructed for a heater component (heater1) installed on a specific vessel (boat007). The Digital Twin is modeled using the sdfThing structure, which references the heater object defined in the sdfObject section.¶
{
"info": {
"title": "An example of the heater #1 in the boat #007",
"version": "2025-01-27",
"copyright": "Copyright 2025. All rights reserved.",
},
"namespace": {
"heater1": "https://example.com/boat007/heater1"
},
"defaultNamespace": "heater1",
"sdfThing": {
"boat007": {
"label": "Digital Twin of Boat #007",
"description" : "A ship equipped with heating and navigation systems",
"sdfRequired": {
"heater1": "#/sdfObject/heater"
},
"sdfNonAffordance": {
"identifier": {
"type": "string",
"const": "urn:boat:007:heater:1"
},
location:{
"wgs84": {
"latitude": "35.2988233791372",
"longitude": "129.25478376484913",
"altitude": "0.0"
},
"postal": {
"city": "Ulsan",
"post-code": "44110",
"country": "South Korea"
},
"w3w": {
"what3words":"toggle.mopped.garages"
}
},
"owner": {
"type": "string",
"default": "ExamTech Ltd."
}
},
"sdfObject": {
"heater1": {
"label": "Cabin Heater",
"description": "Temperature control system for cabin heating",
"sdfProperty": {
"characteristic": {
"description": "Technical summary of the heater",
"type": "string",
"default": "12V electric heater, 800W, automatic cutoff"
},
"status": {
"description": "Current operational status",
"type": "string",
"enum": ["on", "off", "error"],
"default": "off"
}
},
"sdfEvent": {
"maintenanceSchedule": {
"description": "Next scheduled maintenance date",
"type": "string",
"format": "date-time"
}
}
"sdfData" : {
report: {
"On February 24, 2025, the boat #007's heater #1 was on from 9 a.m. to 6 p.m."
}
}
}
A digital twin is a partial representation of sdfThing or sdfObject that contains attributes such as sdfProperty, sdfAction and sdfEvent[ISO23247-1]. By representing sdfThing as a digital twin, crucial events that require appropriate action can be quickly detected and controlled. The requirements defined in [ISO23247-1] are applied to represent sdfThings and sdfObjects as digital twins.¶
Data related to sdfThing and sdfObject, such as sdfProperty, sdfEvent, and sdfAction, should be collected from IP and non-IP devices.¶
The collected data needs to be analyzed to understand the state of sdfThing and sdfObject.¶
The sdfThings and sdfObjects should contain data that uniquely identifies them as digital twins.¶
The sdfThings and sdfObjects should be represented as digital twins with appropriate levels of detail and accuracy, depending on the application.¶
The sdfThings and sdfObjects should be synchronized with their digital twins in real-time as appropriate for the application. Newly added or removed sdfThings and sdfObjects should be recognized and reflected in the digital twin.¶
Only authorized users should have the authority to manage sdfThings and sdfObjects.¶
This document has no IANA actions.¶