| Internet-Draft | SDF Instance Information | April 2025 |
| Bormann & Romann | Expires 12 October 2025 | [Page] |
This document discusses types of Instance Information to be used in conjunction with the Semantic Definition Format (SDF) for Data and Interactions of Things (draft-ietf-asdf-sdf) and will ultimately define Representation Formats for them as well as ways to use SDF Models to describe them.¶
The present revision –01 has been slightly updated from the discussion at the 2025-04-09 ASDF meeting.¶
This note is to be removed before publishing as an RFC.¶
Status information for this document may be found at https://datatracker.ietf.org/doc/draft-bormann-asdf-instance-information/.¶
Discussion of this document takes place on the »A Semantic Definition Format for Data and Interactions of Things« (ASDF) Working Group mailing list (mailto:asdf@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/asdf/. Subscribe at https://www.ietf.org/mailman/listinfo/asdf/.¶
Source for this draft and an issue tracker can be found at https://github.com/ietf-wg-asdf/instance-information.¶
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The Semantic Definition Format for Data and Interactions of Things (SDF, [I-D.ietf-asdf-sdf]) is a format for domain experts to use in the creation and maintenance of data and interaction models in the Internet of Things.¶
SDF is an Interaction Modeling format, enabling a modeler to describe the digital interactions that a class of Things (devices) offers, including the abstract data types of messages used in these interactions.¶
SDF is designed to be independent of specific ecosystems that specify conventions for performing these interactions, e.g., over Internet protocols or over ecosystem-specific protocol stacks.¶
SDF does not define representation formats for the Instance Information that is exchanged in, or the subject of such, interactions; this is left to the specific ecosystems, which tend to have rather different ways to represent this information.¶
This document discusses types of Instance Information and will ultimately define Abstract (eco-system independent) Representation Formats for them as well as ways to use SDF Models to describe them.¶
The definitions of [RFC6690], [RFC8288], and [I-D.ietf-asdf-sdf] apply.¶
Terminology may need to be imported from [LAYERS].¶
As defined in Section 3.2 of RFC 9110 [STD97], but understood to analogously apply to other interaction styles than Representational State Transfer [REST] as well.¶
A Representation that is exchanged in, or is the subject of, an Interaction. Messages are "data in flight", not instance "data at rest" (the latter are called "Instance" and are modeled by the interaction model).¶
Depending on the specific message, an abstract data model for the
message may be provided by the sdfData definitions (or of
declarations that look like these, such as sdfProperty) of an SDF
model.¶
Deriving an ecosystem specific representation of a message may be aided by mapping files [I-D.bormann-asdf-sdf-mapping] that apply to the SDF model providing the abstract data model.¶
Instantiation is a process that takes a Model, some Context Information, and possibly information from a Device and creates an Instance.¶
Anything that can be interacted with based on the SDF model. E.g., the Thing itself (device), a Digital Twin, an Asset Management system... Instances are modeled as "data at rest", not "data in flight" (the latter are called "Message" and actually are/have a Representation). Instances that relate to a single Thing are bound together by some form of identity. Instances become useful if they are "situated", i.e., with a physical or digital "address" that they can be found at and made the subject of an interaction.¶
A message that attempts to describe the state of an Instance at a particular moment (which may be part of the context). We are not saying that the Proofshot is the instance because there may be different ways to make one from an Instance (or to consume one in updating the state of the Instance), and because the proofshot, being a message, is not situated.¶
Proofshots are snapshots, and they are "proofs" in the photographic sense, i.e., they may not be of perfect quality. Not all state that is characteristic of an Instance may be included in a Proofshot (e.g., information about an active action that is not embedded in an action resource). Proofshots may depend on additional context (such as the identity of the Instance and a Timestamp).¶
An interaction affordance to obtain a Proofshot may not be provided by every Instance. An Instance may provide separate Construction affordances instead of simply setting a Proofshot.¶
Discuss Proofshots of a Thing (device) and of other components.¶
Discuss concurrency problems with getting and setting Proofshots.¶
Discuss Timestamps appropriate for Things (Section 4.4 of [I-D.ietf-iotops-7228bis], [I-D.amsuess-t2trg-raytime]).¶
Construction messages enable the creation of a digital Instance, e.g., initialization/commissioning of a device or creation of its digital twins. They are like proofshots, in that they embody a state, however this state needs to be precise so the construction can actually happen.¶
Discuss YANG config=true approach.¶
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 [BCP14] (RFC2119) (RFC8174) when, and only when, they appear in all capitals, as shown here.¶
Originally a term for information that is the subject of interactions with other Instances than the Thing (called "offDevice" now), this term is now considered confusing as it would often just be an affordance of another Instance than the Thing.¶
Instantiation doesn't produce an instance (ouch), which is the device, twin, etc., but a message.¶
Pre-structured types of messages are those that relate to an SDF model in a way that, together with context and model, they are fully self-describing.¶
Messages always have context, typically describing the "me" and the "you" of the interaction, the "now" and "here", allowing deictic statements ("the temperature here", "my current draw")...¶
Messages may have to be complemented by this context for interpretation, i.e., the context needed may need to be reified in the message (compare the use of SenML "n").¶
TODO: Use NIPC as an example how this could be used, including SCIM as a source of context information.¶
TODO: explain how [RFC9039] could be used to obtain device names (using urn:dev:org in the example).¶
(Describe how protocol bindings can be used to convert these messages to/from concrete serializations...)¶
{
"namespace": {
"models": "https://example.com/models",
"boats": "https://example.com/boats"
},
"defaultNamespace": "boats",
"sdfInstance": {
"$context": {
"$comment": "Potential contents for the SDF context",
"deviceName": "urn:dev:org:30810-boat007",
"deviceEui64Address": "50:32:5F:FF:FE:E7:67:28",
"scimObjectId": "8988be82-50dc-4249-bed2-60c9c8797677",
"parentInstance": "TODO -- addressing instance in data tree"
}
}
}
(See defn above.)¶
The following examples are based on Figure 2 of [I-D.lee-asdf-digital-twin-07], separated into an SDF instance and an SDF model.¶
A proofshot that captures the state of an instance can be modelled as shown in Figure 2. Here, every property of the corresponding SDF model (see Figure 3) is mapped to a concrete value that corresponds with the associated schema information.¶
As in any instance message, information from the model is not repeated but
referenced via a pointer into the model tree (sdfInstanceOf); the
namespace needed for this is set up in the usual namespace section that we
also have in model files.¶
Note that in this example, the proofshot also contains values for the implied (offDevice) properties that are static (e.g., the physical location assigned to the instance) but still part of the instance's proofshot as the location is known.¶
=============== NOTE: '\' line wrapping per RFC 8792 ================
{
"info": {
"title": "An example of the heater #1 in the boat #007",
"version": "2025-04-08",
"copyright": "Copyright 2025. All rights reserved."
},
"namespace": {
"models": "https://example.com/models",
"boats": "https://example.com/boats"
},
"defaultNamespace": "boats",
"sdfInstance": {
"boat007": {
"sdfInstanceOf": "models:#/sdfThing/boat",
"$comment": "TODO: How to deal with wrapped instances..?",
"sdfInstance": {
"heater01": {
"sdfInstanceOf": "models:#/sdfThing/boat/sdfObject/heater",
"$context": {
"scimObjectId": "a2e06d16-df2c-4618-aacd-490985a3f763"
},
"isHeating": true,
"location": {
"wgs84": {
"latitude": 35.2988233791372,
"longitude": 129.25478376484912,
"altitude": 0.0
},
"postal": {
"city": "Ulsan",
"post-code": "44110",
"country": "South Korea"
},
"w3w": {
"what3words": "toggle.mopped.garages"
}
},
"report": {
"value": "On February 24, 2025, the boat #007's heater #\
1 was on from 9 a.m. to 6 p.m."
}
}
}
}
}
}
Figure 3 shows a model like the one that could have
been pointed to by the sdfInstanceOf pointers in the instance message.
Note how the namespace is managed here to export the model components into
models:#/sdfThing/boat and models:#/sdfThing/boat/sdfObject/heater.¶
(This example model only specifies structure; it also could come with
semantic information such as the units that are used for wgs84 etc.
In practice, the definition of wgs84 etc. probably would come from a common
library and just be referenced via sdfRef.)¶
=============== NOTE: '\' line wrapping per RFC 8792 ================
{
"info": {
"title": "An example model of a heater on a boat",
"version": "2025-04-08",
"copyright": "Copyright 2025. All rights reserved."
},
"namespace": {
"models": "https://example.com/models"
},
"defaultNamespace": "models",
"sdfThing": {
"boat": {
"sdfObject": {
"heater": {
"sdfProperty": {
"isHeating": {
"offDevice": true,
"description": "The state of the heater on a boat; \
false for off and true for on.",
"type": "boolean"
},
"location": {
"offDevice": true,
"sdfRef": "#/sdfData/location"
}
},
"report": {
"type": "object",
"properties": {
"value": {
"type": "string"
}
}
}
}
}
}
},
"sdfData": {
"location": {
"type": "object",
"properties": {
"wgs84": {
"type": "object",
"properties": {
"latitude": {
"type": "number"
},
"longitude": {
"type": "number"
},
"altitude": {
"type": "number"
}
}
},
"postal": {
"type": "object",
"properties": {
"city": {
"type": "string"
},
"post-code": {
"type": "string"
},
"country": {
"type": "string"
}
}
},
"w3w": {
"type": "object",
"properties": {
"what3words": {
"type": "string",
"format": "..."
}
}
}
}
}
}
}
Construction messages enable the creation of the digital instance, e.g., initialization/commissioning of a device or creation of its digital twins. They are like proofshots, in that they embody a state, however this state needs to be precise so the construction can actually happen.¶
A construction message for a temperature sensor might assign an identity and/or complement it by temporary identity information (e.g., an IP address); its processing might also generate construction output (e.g., a public key or an IP address if those are generated on device).¶
(Note that it is not quite clear what a destructor would be for a physical instance -- apart from a scrap metal press, but according to RFC 8576 we would want to move a system to a re-usable initial state, which is pretty much a constructor.)¶
=============== NOTE: '\' line wrapping per RFC 8792 ================
{
"info": {
"title": "Example document for SDF (Semantic Definition Format) \
with constructors for instantiation",
"version": "2019-04-24",
"copyright": "Copyright 2019 Example Corp. All rights reserved.",
"license": "https://example.com/license"
},
"namespace": {
"cap": "https://example.com/capability/cap"
},
"defaultNamespace": "cap",
"sdfObject": {
"temperatureSensor": {
"sdfProperty": {
"temperature": {
"description": "Temperature value measure by this Thing's \
temperature sensor.",
"type": "number",
"sdfParameters": [
"minimum",
{
"targetQuality": "minimum",
"parameterName": "minimum",
"constructorName": "construct"
},
"maximum",
{
"targetQuality": "unit",
"parameterName": "#/sdfObject/Switch/sdfConstructors/\
construct/temperatureUnit"
}
]
}
},
"sdfConstructors": {
"$comment": "TODO: Dicuss whether this should be assumed to \
be the default constructor",
"construct": {
"parameters": {
"minimum": {
"required": true
},
"maximum": {
"required": false,
"$comment": "Constructors could allow for further \
restricting values that can be assigned to affordances",
"type": "integer"
},
"temperatureUnit": {
"required": false
}
}
}
}
}
}
}
=============== NOTE: '\' line wrapping per RFC 8792 ================
{
"info": {
"title": "Example SDF construction message",
"$comment": "TODO: What kind of metadata do we need here?"
},
"namespace": {
"cap": "https://example.com/capability/cap"
},
"defaultNamespace": "cap",
"sdfConstruction": {
"sdfConstructor": "cap:#/sdfObject/temperatureSensor/\
sdfConstructors/construct",
"arguments": {
"minimum": 42,
"temperatureUnit": "Cel"
}
}
}
What changed since the last proofshot?¶
What is different from the base status of the device?¶
Can I get the same (equivalent, not identical) coffee I just ordered but with 10 % more milk?¶
(Think merge-patch.)¶
A construction message may be a delta, or it may have parameters that algorithmically influence the elements of state that one would find in a proofshot.¶
=============== NOTE: '\' line wrapping per RFC 8792 ================
{
"info": {
"title": "Example SDF delta construction message",
"$comment": "TODO: What kind of metadata do we need here?"
},
"namespace": {
"cap": "https://example.com/capability/cap"
},
"defaultNamespace": "cap",
"sdfConstruction": {
"sdfConstructor": "cap:#/sdfObject/temperatureSensor/\
sdfConstructors/construct",
"previousProofshot": "???",
"arguments": {
"currentTemperature": 24
}
}
}
Deltas and Default/Base messages could be used in the Series Transfer Pattern [STP], which may be one way to model a telemetry stream from a device.¶
One interesting piece of offDevice information is the model itself, including sdfinfo and the defaultnamespace. This is of course not about the device or its twin (or even its asset management), because models and devices may want to associate freely. Multiple models may apply to the same device (including but not only revisions of the models).¶
(TODO)¶
(TODO)¶
(TODO)¶
(TODO)¶