Instance Information for SDF

1.1. Conventions and Definitions

The definitions of [RFC6690], [RFC8288], and [I-D.ietf-asdf-sdf] apply.¶

Terminology may need to be imported from [LAYERS].¶

Representation:

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.¶

Message:

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:

Instantiation is a process that takes a Model, some Context Information, and possibly information from a Device and creates an Instance.¶

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.¶

Instance-related Message:

A message that describes the state or a state change of a specific instance. (TBC -- also: do we need this additional term?)¶

Message Archetype:

In the context of instance-related messages: A message with specific content and effect, covering a wider set of different use cases. In this document, we are observing a total of four instance-related message archetypes: Snapshot Messages, Construction Messages, Delta Messages, and Patch Messages.¶

Proofshot:

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.¶

Construction:

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.¶

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.¶

2.1. Axioms for instance-related messages

Instance-related messages can be messages that relate to a property, action, or event (input or output data), or they can be "proofshots" (extracted state information, either in general or in a specific form such as a context snapshot etc.).¶

Instance-related messages are controlled by a model (class-level information), which normally is the interaction model of the device. That interaction model may provide a model of the interaction during which the instance-related message is interchanged (at least conceptually), or it may be a "built-in" interaction (such as a proofshot, a context snapshot, ...) that is implicitly described by the entirety of the interaction model. This may need to be augmented/composed in some way, as device modeling may be separate from e.g. asset management system modeling or digital twin modeling. Instance-related messages use JSON pointers into the model in order to link the instance-related information to the model.¶

Instance-related messages are conceptual and will often be mapped into ecosystem-specific protocol messages (e.g., a bluetooth command). It is still useful to be able to represent them in a neutral ("red-star") format, which we build here as an adaption of the JSON-based format of the models themselves. An ecosystem might even decide to use the neutral format as its ecosystem-specific format (or as an alternative format).¶

Instance-related messages may be plain messages, or they may be deltas (from a previous state) and/or patches (leading from a previous or the current state to a next state). Several media types can be defined for deltas/patches; JSON merge-patch [RFC7396] is already in use in SDF (for sdfRef) and therefore is a likely candidate. (Assume that some of the models will be using Conflict-free replicated data types (CRDTs) to facilitate patches.)¶

2.2. Context Information

Messages always have context, typically describing the "me" and the "you" of the interaction, the "now" and "here", allowing deictic statements such as "the temperature here" or "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"). Information that enables interactions via application-layer protocols (such as an IP address) can also be considered context information.¶

For this purpose, we are using the sdfContext keyword introduced by [I-D.ietf-asdf-sdf-nonaffordance]. Note that sdfContext could also be modelled via sdfProperty.¶

TODO: explain how [RFC9039] could be used to obtain device names (using urn:dev:org in the example).¶

Note that one interesting piece of context information is the model itself, including the information block and the default namespace. 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. Also note that multiple models may apply to the same device (including but not only revisions of the same models).¶

3.1. Information Block

The information block contains the same qualities as an SDF model and, additionally, a mandatory messageId to uniquely identify the message. Furthermore, Delta messages can utilize the previousMessageId in order to link two messages and indicate the state change.¶

3.2. Namespaces Block

Similar to SDF models, instance-related messages contain a namespaces block with a namespace map and the defaultNamespace setting. In constrast to models, including a namespace quality is mandatory as at least one namespace reference is needed to be able to refer to the SDF model the instance-related message corresponds with.¶

3.3. Instance-of Block

Distinct from SDF models are two instance-specific blocks, the first of which is identified via the sdfInstanceOf keyword. Via the model keyword, this quality defines the entry point the sdfInstance quality from the next section is referring to. Furthermore, via the patchMethod field, a patch algorithm different from JSON Merge Patch can be specified.¶

3.4. Instance Block

In the instance block, state information for properties, actions, and events as well as context information can be included. Depending on the archetype, this information will either be used to report a Thing's current state, to report state changes, or to update state via a patch or reconfiguration.¶

In addition to the messageId and previousMessageId from the info block, we are able to refer to¶

• the device identity (via the identifier qualitity) and¶

• the point in time when the information regarding the device state has been captured (via the timestamp quality).¶

Since we are using the sdfInstance keyword as an entry point at the location pointed to via the model specfied in sdfInstanceOf, the instance-related message has to follow the structure of this part of the model (although, depending on the archetype, information that has not changed or will not be updated can be left out.)¶

The alternating structure of the SDF model (e. g., sdfObject/envSensor/sdfProperty/temperature) is repeated within the instance-related message, with the top-level sdfObject or sdfThing being replaced by sdfInstance at the entry point. Note that we also have to replicate a nested structure via sdfThing and/or sdfObject if present in the referenced SDF model.¶

4.1. Snapshot Messages

This instance-related message contains information on a Thing's state, both in terms of context information and the state of individual affordances. In the message, the previousMessageId field in the information block MUST NOT be present. Furthermore, when transmitting this message in its JSON format, the content type application/sdf-snapshot+json MUST be indicated if supported by the protocol used for transmission.¶

Snapshot messages MAY only contain values for a subset of all possible affordances and context information exposed by a Thing. Security-related aspects, e.g. regarding authentication and authorization, MUST be taken into account when issueing a state report for a requesting party.¶

In practical use, we can at least differentiate two use cases for snapshot messages.¶

First, we have context snapshots, which only contain context information related to a Thing (indicated via the sdfContext quality). Figure 1 gives an example for this kind of instance-related message.¶

{
  "info": {
    "messageId": "75532020-8f64-4daf-a241-fcb0b6dc4a42"
  },
  "namespace": {
    "models": "https://example.com/models",
    "sensors": "https://example.com/sensors"
  },
  "defaultNamespace": "models",
  "sdfInstanceOf": {
    "model": "sensors:#/sdfObject/envSensor"
  },
  "sdfInstance": {
    "identifier": "envSensor:abc123",
    "timestamp": "2025-07-01T12:00:00Z",
    "sdfContext": {
      "installationInfo": {
        "floor": 3,
        "mountType": "ceiling",
        "indoorOutdoor": "indoor"
      }
    }
  }
}

Second, we have proofshot messages which are supersets of context snapshots that may also include state information associated with a Thing's interaction affordances (properties, actions, and events).¶

Note that while the format for describing the state of properties is clearly governed by the schema information from the corresponding sdfProperty definition, it is still unclear how to best model the state of sdfActions and sdfEvents.¶

Figure 2 shows a proofshot that captures the state of a sensor. Here, every property and context definition of the corresponding SDF model (see Figure 10) is mapped to a concrete value that satisfies the associated schema.¶

{
  "info": {
    "messageId": "75532020-8f64-4daf-a241-fcb0b6dc4a42"
  },
  "namespace": {
    "models": "https://example.com/models",
    "sensors": "https://example.com/sensor"
  },
  "defaultNamespace": "models",
  "sdfInstanceOf": {
    "model": "sensors:#/sdfObject/envSensor"
  },
  "sdfInstance": {
    "identifier": "envSensor:abc123",
    "timestamp": "2025-07-01T12:00:00Z",
    "sdfContext": {
      "installationInfo": {
        "mountType": "ceiling"
      }
    },
    "sdfProperty": {
      "temperature": 23.124
    }
  }
}

=============== NOTE: '\' line wrapping per RFC 8792 ================

{
  "namespace": {
    "models": "https://example.com/models",
    "sensors": "https://example.com/sensor"
  },
  "defaultNamespace": "models",
  "sdfObject": {
    "sensor": {
      "sdfContext": {
        "ipAddress": {
          "type": "string"
        }
      },
      "sdfProperty": {
        "temperature": {
          "type": "number",
          "sdfProtocolMap": {
            "coap": {
              "sdfParameters": {
                "ipAddress": "#/sdfObject/sensor/sdfContext/\
                                                           ipAddress"
              },
              "read": {
                "method": "GET",
                "href": "/temperature",
                "contentType": 60
              }
            }
          }
        }
      }
    }
  }
}

{
  "info": {
    "messageId": "75532020-8f64-4daf-a241-fcb0b6dc4a47"
  },
  "namespace": {
    "models": "https://example.com/models",
    "sensors": "https://example.com/sensor"
  },
  "defaultNamespace": "models",
  "sdfInstanceOf": {
    "model": "sensors:#/sdfObject/sensor"
  },
  "sdfInstance": {
    "sdfContext": {
      "ipAddress": "192.168.1.5"
    }
  }
}

4.2. Construction Messages

Construction messages are structurally equivalent to snapshot messages but may only contain context information. Furthermore, the recipient of a construction message is supposed to initiate a configuration or comissioning process upon recption. Construction messages MUST be indicated by the media type application/sfd-construction+json if possible.¶

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) which can be described via instance-related messages such as snapshot messages.¶

The creation of construction messages is linked to the invocation of a constructor that starts the actual construction process. In practice, these constructors are going to be modeled as an sdfAction, although the way the sdfAction is going to be used exactly is not entirely clear yet.¶

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.¶

Figure 5 shows a potential SDF construction message that initializes a device, setting its manufacturer and firmwareVersion as context information. The construction message also assigns an identifier as well as an initial ipAddress that can be used with the interaction affordances that may be present in the corresponding SDF model.¶

{
  "info": {
    "messageId": "75532020-8f64-4daf-a241-fcb0b6dc4a42"
  },
  "namespace": {
    "models": "https://example.com/models",
    "sensors": "https://example.com/sensor"
  },
  "defaultNamespace": "models",
  "sdfInstanceOf": {
    "model": "sensors:#/sdfObject/envSensor"
  },
  "sdfInstance": {
    "identifier": "envSensor:unit42",
    "sdfContext": {
      "ipAddress": "192.168.1.5",
      "deviceIdentity": {
        "manufacturer": "HealthTech Inc.",
        "firmwareVersion": "1.4.3"
      }
    }
  }
}

A special type of construction message that only contains identity-related information may be called an Identity Manifest. Figure 6 shows an example of an identity manifest that is structurally identical to the construction message from Figure 5, with the non-identity-related information left out.¶

Via sdfRequired, an SDF model can indicate which context information must be present and therefore initialized within an instance. All definitions included in sdfRequired MUST also be present in a construction message, while other sdfContext definitions could be left out.¶

{
  "info": {
    "messageId": "75532020-8f64-4daf-a241-fcb0b6dc4a42"
  },
  "namespace": {
    "models": "https://example.com/models",
    "sensors": "https://example.com/sensor"
  },
  "defaultNamespace": "models",
  "sdfInstanceOf": {
    "model": "sensors:#/sdfObject/envSensor"
  },
  "sdfInstance": {
    "identifier": "envSensor:unit42",
    "sdfContext": {
      "deviceIdentity": {
        "manufacturer": "HealthTech Inc.",
        "firmwareVersion": "1.4.3"
      }
    }
  }
}

{
  "namespace": {
    "models": "https://example.com/models",
    "sensors": "https://example.com/sensor"
  },
  "defaultNamespace": "models",
  "sdfObject": {
    "sensor": {
      "sdfContext": {
        "ipAddress": {
          "type": "string"
        },
        "unit": {
          "type": "string"
        }
      },
      "sdfProperty": {
        "temperature": {
          "type": "number",
          "sdfParameters": {
            "unit": "#/sdfObject/sensor/sdfContext/unit"
          }
        }
      }
    }
  }
}

4.3. Delta Messages

Delta messages describe updates to a Thing's state relative to a previous message. For this purpose, a previousMessageId MUST be present in the info block. When transmitting delta messages, the media type application/sdf-delta+json MUST be used if possible.¶

By default, the values contained in the message are applied to the preceding message(s) via the JSON Merge Patch algorithm. Via the patchMethod quality, different patch algorithms MAY be indicated.¶

Figure 8 shows an example Delta message that reports state changes compared to the ones reported in the previous message (identified via its previousMessageId). In this example, only the temperature that has been measured by the sensor has changed, which is why it is the only piece of information that is included.¶

Delta messages could be used in the Series Transfer Pattern [STP], which may be one way to model a telemetry stream from a device.¶

{
  "info": {
    "title": "Example SDF delta message",
    "previousMessageId": "026c1f58-7bb9-4927-81cf-1ca0c25a857b",
    "messageId": "75532020-8f64-4daf-a241-fcb0b6dc4a42"
  },
  "namespace": {
    "cap": "https://example.com/capability/cap",
    "models": "https://example.com/models"
  },
  "defaultNamespace": "cap",
  "sdfInstanceOf": {
    "model": "models:/sdfObject/envSensor"
  },
  "sdfInstance": {
    "sdfProperty": {
      "temperature": 24
    }
  }
}

4.4. Patch Messages

Patch messages are structurally equivalent to delta messages, but once again are only allowed to contain context information. They utilize a patch mechanism (which may be explicitly indicated via the patchMethod quality) to alter the state of a Thing instead of reporting state changes. Since patch messages are not referring to a preceding message, a previosMessageId MUST NOT be present in the information block. When transmitting state patches, the media type application/sdf-patch+json MUST be used if possible.¶

An example Patch Message is shown in Figure 9, where a change of the device's mountType is signalled.¶

{
  "info": {
    "messageId": "75532020-8f64-4daf-a241-fcb0b6dc4a42"
  },
  "namespace": {
    "models": "https://example.com/models",
    "sensors": "https://example.com/sensor"
  },
  "defaultNamespace": "models",
  "sdfInstanceOf": {
    "model": "sensors:#/sdfObject/envSensor",
    "patchMethod": "merge-patch"
  },
  "sdfInstance": {
    "sdfContext": {
      "installationInfo": {
        "mountType": "wall"
      }
    }
  }
}

Practical uses for patch message include digital twins [I-D.lee-asdf-digital-twin], where changes to physical attributes (such as the location) need to be reflected in the digital representation of a Thing.¶

8.1. Normative References

[BCP14]

Best Current Practice 14, <https://www.rfc-editor.org/info/bcp14>.
At the time of writing, this BCP comprises the following:

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>.

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>.

[I-D.ietf-asdf-sdf]

Koster, M., Bormann, C., and A. Keränen, "Semantic Definition Format (SDF) for Data and Interactions of Things", Work in Progress, Internet-Draft, draft-ietf-asdf-sdf-25, 13 October 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-asdf-sdf-25>.

[I-D.ietf-asdf-sdf-nonaffordance]

Hong, J. and H. Lee, "Semantic Definition Format (SDF) Extension for Non-Affordance Information", Work in Progress, Internet-Draft, draft-ietf-asdf-sdf-nonaffordance-02, 20 October 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-asdf-sdf-nonaffordance-02>.

[RFC8288]

Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017, <https://www.rfc-editor.org/rfc/rfc8288>.

[STD97]

Internet Standard 97, <https://www.rfc-editor.org/info/std97>.
At the time of writing, this STD comprises the following:

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/info/rfc9110>.

8.2. Informative References

[I-D.amsuess-t2trg-raytime]

Amsüss, C., "Raytime: Validating token expiry on an unbounded local time interval", Work in Progress, Internet-Draft, draft-amsuess-t2trg-raytime-03, 19 October 2024, <https://datatracker.ietf.org/doc/html/draft-amsuess-t2trg-raytime-03>.

[I-D.bormann-asdf-sdf-mapping]

Bormann, C. and J. Romann, "Semantic Definition Format (SDF): Mapping files", Work in Progress, Internet-Draft, draft-bormann-asdf-sdf-mapping-07, 20 July 2025, <https://datatracker.ietf.org/doc/html/draft-bormann-asdf-sdf-mapping-07>.

[I-D.ietf-asdf-sdf-protocol-mapping]

Mohan, R., Brinckman, B., and L. Corneo, "Protocol Mapping for SDF", Work in Progress, Internet-Draft, draft-ietf-asdf-sdf-protocol-mapping-02, 2 December 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-asdf-sdf-protocol-mapping-02>.

[I-D.ietf-iotops-7228bis]

Bormann, C., Ersue, M., Keränen, A., and C. Gomez, "Terminology for Constrained-Node Networks", Work in Progress, Internet-Draft, draft-ietf-iotops-7228bis-03, 4 November 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-iotops-7228bis-03>.

[I-D.lee-asdf-digital-twin]

Lee, H., Hong, J., Youn, J., and Y. Hong, "Semantic Definition Format (SDF) modeling for Digital Twin", Work in Progress, Internet-Draft, draft-lee-asdf-digital-twin-09, 4 July 2025, <https://datatracker.ietf.org/doc/html/draft-lee-asdf-digital-twin-09>.

[LAYERS]

"Terminology for Layers", WISHI Wiki, <https://github.com/t2trg/wishi/wiki/NOTE:-Terminology-for-Layers>.

[REST]

Fielding, R., "Architectural Styles and the Design of Network-based Software Architectures", Ph.D. Dissertation, University of California, Irvine, 2000, <http://www.ics.uci.edu/~fielding/pubs/dissertation/fielding_dissertation.pdf>.

[RFC6690]

Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, <https://www.rfc-editor.org/rfc/rfc6690>.

[RFC7396]

Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396, DOI 10.17487/RFC7396, October 2014, <https://www.rfc-editor.org/rfc/rfc7396>.

[RFC9039]

Arkko, J., Jennings, C., and Z. Shelby, "Uniform Resource Names for Device Identifiers", RFC 9039, DOI 10.17487/RFC9039, June 2021, <https://www.rfc-editor.org/rfc/rfc9039>.

[STP]

Bormann, C. and K. Hartke, "The Series Transfer Pattern (STP)", Work in Progress, Internet-Draft, draft-bormann-t2trg-stp-03, 7 April 2020, <https://datatracker.ietf.org/doc/html/draft-bormann-t2trg-stp-03>.