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ASDF IoT Link Information Model Interaction Model Data Model 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 –06 takes into account the discussion that happened during IETF 123 and 124 as well as the interim meetings inbetween, and intends to harmonize different instance-related message concepts via a common format. About This Document Status information for this document may be found at . Discussion of this document takes place on the »A Semantic Definition Format for Data and Interactions of Things« (ASDF) Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction The Semantic Definition Format for Data and Interactions of Things (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 Instance Information in different types and roles. It defines an abstraction of this, as an eco-system independent way to reason about this information. This abstraction can be used at a conceptual level, e.g., to define models that govern the instance information. However, where this is desired, it also can be used as the basis for a concrete neutral representation (Format) that can actually be used for interchange to exchange information and parameters for interactions to be performed. In either case, the structure and semantics of this information are governed by SDF Models. This document is truly work in progress. It freely copies examples from the document that evolves in parallel, with a goal of further synchronizing the development where that hasn't been fully achieved yet. After the discussion stabilizes, we'll need to discuss how the information should be distributed into the different documents and/or how documents should be merged.

Conventions and Definitions The definitions of , , and apply. Terminology may need to be imported from .

Representation:

As defined in Section of RFC 9110 , but understood to analogously apply to other interaction styles than Representational State Transfer 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 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.

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. Discuss Proofshots of a Thing (device) and of other components. Discuss concurrency problems with getting and setting Proofshots. Discuss Timestamps appropriate for Things (, ). TODO: Also mention the other message types we had so far (context snapshot, context patch, identity manifest) here?

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. 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 () () when, and only when, they appear in all capitals, as shown here.

Terms we are trying not to use

Non-affordance:

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. In this draft version, we are trying to use a new keyword called sdfContext that is supposed to be slightly more accurate, replacing the $context concept that was used in previous draft versions.

Instance Information and SDF The instantiation of an SDF model does not directly express an instance, which is, for example, a physical device or a digital twin. Instead, the instantiation produces an instance-related message, which adheres to a uniform message format and is always controlled by the corresponding SDF model. Depending on the recipient and its purpose, a message can be interpreted as a report regarding the state of a Thing or the instruction to change it when consumed by the recipient. Taking into account previous revisions of this document as well as , we identified two main dimensions for covering the potential use cases for instance-related messages:

1. the intended effect of a message, which can either be a report or an update of a Thing's state, and
2. the actual content of the message, which may be freestanding (without a reference to a previous message or state) or relative (with such a reference).

Based on these considerations (as illustrated by the systematization in ), we can identify the following four message archetypes:

1. State reports that may contain contain both affordance-related and context information, including information about a Thing's identity,
2. Construction messages, which trigger a Thing's initial configuration process or its commissioning,
3. State report updates that indicate changes that have occurred since a reference state report, and
4. State patches that update the Thing's state.

Systematization of instance-related messages along the dimensions "Content" and "(Intended) Effect".

		Content
		Freestanding	Relative
(Intended) Effect	State Exposure	Status Report	Status Report Update
	State Change	Construction	State Patch

The uniform message format can be used for all four message archetypes. specifies the formal syntax of instance-related messages that all normative statements as well as the examples in this document will adhere to. This syntax can serve to describe both the abstract structure and the concrete shape of the messages that can be used as a neutral form in interchange. In the following, we will first outline a number of general principles for instance-related messages, before detailing the specific archetypes we define in this document. The specification text itself will be accompanied by examples that have been inspired by and that each correspond with one of the four archetypes.

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 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.) To identify the reference state for a delta/patch, we need

• device identity (thingId?)
• state info (timestamp? state/generation identifier?)

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 . Note that sdfContext could also be modelled via sdfProperty. TODO: explain how could be used to obtain device names (using urn:dev:org in the example).

Message Format The data model of instance-related messages makes use of the structural features of SDF models (e.g., when it comes to metadata and namespace information), but is also different in crucial aspects. TODO: Decide where we want to keep this: One interesting piece of offDevice information is the model itself, including 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. Multiple models may apply to the same device (including but not only revisions of the same models).

Information Block The information block contains the same qualities as an SDF model and, additionally, a mandatory messageId to uniquely identify the message. Furthermore, "status report update" messages can utilize the previousMessageId in order to link two messages and indicate the state change. Qualities of the Information Block

Quality	Type	Description
title	string	A short summary to be displayed in search results, etc.
description	string	Long-form text description (no constraints)
version	string	The incremental version of the definition
modified	string	Time of the latest modification
copyright	string	Link to text or embedded text containing a copyright notice
license	string	Link to text or embedded text containing license terms
messageId	string	Unique identifier of this instance-related message
previousMessageId	string	Identifier used to connect this instance-related message to a previous one
features	array of strings	List of extension features used
$comment	string	Source code comments only, no semantics

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. Namespaces Block

Quality	Type	Description
namespace	map	Defines short names mapped to namespace URIs, to be used as identifier prefixes
defaultNamespace	string	Identifies one of the prefixes in the namespace map to be used as a default in resolving identifiers

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. Instance-of Block

Quality	Type	Description
model	string	Defines the entry point for sdfInstance by pointing to an sdfObject or an sdfThing. Has to be based on a namespace identifier from the namespaces map.
patchMethod	string	Allows for overriding the default patch method (JSON Merge Patch) by providing a registered value.
$comment	string	Source code comments only, no semantics

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. 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. Instance Block

Quality	Type	Description
sdfThing	map	Values for the thing entries in the referenced SDF definition
sdfObject	map	Values for the object entries in the referenced SDF definition
sdfContext	map	Values for the context entries in the referenced SDF definition
sdfProperty	map	Values for the properties in the referenced SDF definition
sdfAction	map	Values for the actions in the referenced SDF definition
sdfEvent	map	Values for the events in the referenced SDF definition

Message Archetypes Based on the common message format defined in and the systematization from , we can derive a set of four archetypes that serve different use cases and recipients. TODO: Decide whether we want to add specific CDDL schemas for the four archetypes via extension points in the "base schema" TODO: The description of the individual messages probably has to be expanded. Maybe some of the content from the six example messages should be moved here.

State Reports 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-state-report+json MUST be indicated if supported by the protocol used for transmission. State reports 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.

Construction Messages (These might not be covered here but via dedicated actions.) Construction messages are structurally equivalent to state reports, with the main difference being that the recipient is supposed to initiate a configuration or comissioning process upon when receiving it. Furthermore, construction messages MUST be indicated by a different media type, namely application/sfd-construction+json.

State Report Updates State report updates are messages that only describe updates relative to a previous message. For this purpose, a previousMessageId MUST be present in the info block. When transmitting state report updates, the media type application/sdf-state-report-update+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.

State Patches State patches are structurally equivalent to state report updates. However, they utilize the patch mechanism (using the provided patchMethod) to alter the state of a Thing instead of reporting state changes. Since they 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-state-patch+json MUST be used if possible.

Message Purposes and Usecases The four archetypes can be further subdivided into (at least) six kinds of messages that all deal with different use cases. While the archetypes each have their own media type that can be used to identity them during a message exchange, the six concete messages in this section are may only be identified by their content. TODO: Consider only describing the different kinds of state reports State Reports can be used as

• Context snapshots that only report context information about a Thing,
• Proofshots that report a Thing's state (or parts of it), which may include context information, or
• Identity manifests that report information related to a Thing's identity.

In the case of state report updates, we have Deltas that indicate state changes compared to a previous context snapshot, proofshot message, or identity manifest. State patches can appear as Patch messages that indicate state changes that should be applied to a Thing. And finally, we have the Construction Messages that initiate a Thing's (re)configuration or its comissioning As we can see, the great amount of variation within the state report archetype in the case of messages 1 to 3 comes from the different kinds and the characteristic of the information that is the reported in the eventual message. However, the message format stays identical across the three manifestations of the archetype. In the remainder of this section, we will discuss the differences of these three messages in particular and will also deal with the potential modelling of construction messages.

Context Snapshots Context snapshots are state reports that only include context information via the sdfContext keyword. gives an example for this kind of instance-related message by showing a status report message that only contains context information.

Example of an SDF context snapshot.

This kind of message may become especially relevant later in conjunction with the sdfProtocolMap introduced in for complementing protocol-specific information at the model-level with instance-related context information such as IP addresses.

Proofshots (See defn above.) Proofshots are similar to context snapshots, with the important difference that they are not only reporting the context information associated with an entity but also state information associated with its interaction affordances (properties, actions, and events). As in the case of the Context Snapshot, the Proofshot may also contain concrete values that reflect context information associated with a device via the sdfContext keyword . TODO: 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. The following examples are based on and . shows a proofshot that captures the state of a sensor. Here, every property and context definition of the corresponding SDF model (see ) is mapped to a concrete value that satisfies the associated schema.

SDF proofshot example.

Construction Messages Construction messages enable the creation of the digital instance, e.g., initialization/commissioning of a device or creation of its digital twins. Construction messages 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). This output -- which can once again be modeled as an instance-related message -- may be referred to as an identity manifest when it primarily contains identity-related context information. Construction messages need to refer to some kind of constructor in order to be able to start 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. As the device that is being constructed will not be initialized before the construction has finished, the sdfAction has to be modeled as an external or "off-device" action. This raises the question whether the sdfAction still belongs into the SDF model that corresponds with the class the resulting device instance belongs to. (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.) shows a potential SDF construction message that initializes a device, setting its manufacturer and firmwareVersion as context information.

Example for an SDF construction message

Delta Messages TODO: Reword When the state of a device at a given point in time is known (e.g., due to a previous instance-related message), an external entity might only be interested in the changes since that point in time. Or it might want to adjust its state and/or context the device operates in. For both purposes, instance-related messages can be used. shows an example that contains an instance-related message reporting a "proofshot delta", that is the state changes that occured compared to the ones reported in the previous message (identified via its previousMessageId). In this example, only the temperature as measured by the sensor has changed, so only that information is included. Delta messages could be used in the Series Transfer Pattern , which may be one way to model a telemetry stream from a device.

Example of an SDF instance-related message that serves as a delta.

Patch Messages Yet another purpose for instance-related messages is the application of updates to a device's configuration via a so-called patch message. Such a message is shown in , where a change of the device's mountType is reflected. This message type might be especially relevant for digital twins , where changes to physical attributes (such as the location) need to be reflected somehow.

Example of an SDF context patch message that uses the common instance-related message format.

TODO: Maybe the following can be shortened or even removed When comparing and , we can see that the main difference between the messages is the purpose these message are being used for. This purpose could be implicitly reflected by the nature of the resource that accepts or returns the respective message type. It would also be possible to indicate the purpose more explicitly by using a different content format when transferring the messages over the wire. Another difference, however, lays in the fact that the context patch is not including a previousMessageId, which might be sufficient to distinguish the two message types. Despite their different purpose, both messages will apply some kind of patch algorithm. JSON Merge Patch is probably a strong contender for the default algorithm that will be used a little bit differently depending on the message type (the context patch will be applied "internally" by the device, while the delta message will be processed together with its predecessor by a consumer). As there might be cases where the Merge Patch algorithm is not sufficient, different algorithms (that can be IANA registered) are going to be settable via the patchMethod field within the sdfInstanceOf quality.

Identity Manifest Identity manifests belong like proofshots and context snapshots to the Status Report archetype. However, their use case is tied more strongly to identity information which may be modeled as context information. shows an example of an identity manifest, that is structurally identical to the construction message shown in . What makes qualifies the message as an identity manifest is its media type, which differs from the construction message, as well as the circumstances under which the message might be emitted -- for instance, as the result of a construction.

Example for an SDF construction message

Examples for SDF Constructors TODO: This section needs to be updated/reworked/removed shows a potential approach for describing constructors via the sdfAction keyword with a set of construction parameters contained in its sdfInputData. As the constructor action is modeled as being detached from the device and performed by an external constructor in this example, both the resulting model and the initial instance description (which can be considered an identity manifest) are returned. The schema information that governs the shape of both the model and the instance message are referred to via the sdfRef keyword. DISCUSS: Note that the action may also return a pointer to an external SDF model and provide the additional information from the constructor via an SDF Mapping File. These are aspects that still require discussion, examples, and implementation experience.

Example for SDF model with constructors

Discussion (TODO)

Security Considerations

• Pieces of instance-related information might only be available in certain scopes, e.g. certain security-related configuration parameters

(TODO)

IANA Considerations TODO: Add media type registrations

References Normative References KTC Control AB Universität Bremen TZI Ericsson The Semantic Definition Format (SDF) is concerned with Things, namely physical objects that are available for interaction over a network. SDF is a format for domain experts to use in the creation and maintenance of data and interaction models that describe Things. An SDF specification describes definitions of SDF Objects/SDF Things and their associated interactions (Events, Actions, Properties), as well as the Data types for the information exchanged in those interactions. Tools convert this format to database formats and other serializations as needed. This specification defines a model for the relationships between resources on the Web ("links") and the type of those relationships ("link relation types"). It also defines the serialisation of such links in HTTP headers with the Link header field. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. Electronics and Telecommunications Research Institute Electronics and Telecommunications Research Institute This document describes an extension to the Semantic Definition Format (SDF) for representing non-affordance information of Things, such as physical, contextual, and descriptive metadata. This extension introduces a new class keyword, sdfContext, that enables comprehensive modeling of Things and improves semantic clarity. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References University of California, Irvine This specification defines Web Linking using a link format for use by constrained web servers to describe hosted resources, their attributes, and other relationships between links. Based on the HTTP Link Header field defined in RFC 5988, the Constrained RESTful Environments (CoRE) Link Format is carried as a payload and is assigned an Internet media type. "RESTful" refers to the Representational State Transfer (REST) architecture. A well-known URI is defined as a default entry point for requesting the links hosted by a server. [STANDARDS-TRACK] This specification defines the JSON merge patch format and processing rules. The merge patch format is primarily intended for use with the HTTP PATCH method as a means of describing a set of modifications to a target resource's content. Universität Bremen TZI Universität Bremen The Semantic Definition Format (SDF) is a format for domain experts to use in the creation and maintenance of data and interaction models that describe Things, i.e., physical objects that are available for interaction over a network. It was created as a common language for use in the development of the One Data Model liaison organization (OneDM) models. Tools convert this format to database formats and other serializations as needed. An SDF specification often needs to be augmented by additional information that is specific to its use in a particular ecosystem or application. SDF mapping files provide a mechanism to represent this augmentation. Universität Bremen TZI Ericsson Universitat Politecnica de Catalunya The Internet Protocol Suite is increasingly used on small devices with severe constraints on power, memory, and processing resources, creating constrained-node networks. This document provides a number of basic terms that have been useful in research and standardization work for constrained-node networks. This document obsoletes RFC 7228. When devices are deployed in locations with no real-time access to the Internet, obtaining a trusted time for validation of time limited tokens and certificates is sometimes not possible. This document explores the options for deployments in which the trade-off between availability and security needs to be made in favor of availability. While considerations are general, terminology and examples primarily focus on the ACE framework. Electronics and Telecommunications Research Institute Electronics and Telecommunications Research Institute DONG-EUI University Daejeon University 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. WISHI Wiki Universität Bremen TZI Ericsson Many applications make use of Series of data items, i.e., an array of data items where new items can be added over time. Where such Series are to be made available using REST protocols such as CoAP or HTTP, the Series has to be mapped into a structure of one or more resources and a protocol for a client to obtain the Series and to learn about new items. Various protocols have been standardized that make Series-shaped data available, with rather different properties and objectives. The present document is an attempt to extract a common underlying pattern and to define media types and an access scheme that can be used right away for further protocols that provide Series-shaped data. This document describes a new Uniform Resource Name (URN) namespace for hardware device identifiers. A general representation of device identity can be useful in many applications, such as in sensor data streams and storage or in equipment inventories. A URN-based representation can be passed along in applications that need the information. Cisco Systems Cisco Systems Ericsson This document defines protocol mapping extensions for the Semantic Definition Format (SDF) to enable mapping of protocol-agnostic SDF affordances to protocol-specific operations. The protocol mapping mechanism allows SDF models to specify how properties, actions, and events should be accessed using specific IP and non-IP protocols such as Bluetooth Low Energy, Zigbee or HTTP and CoAP.

Example SDF Model shows the model all of the examples for instance-related messages are pointing to in this document. Note how the namespace is managed here to export the envSensor component into models:#/sdfObject/envSensor, which is the "entry point" used in the instance messages within the main document.

SDF Model that serves as a reference point for the instance-related messages in this draft

Formal Syntax of Instance-related Messages ? defaultNamespace: text ? sdfInstanceOf: sdf-instance-of ? sdfInstance: sdf-instance } sdfinfo = { ? title: text ? description: text ? version: text ? copyright: text ? license: text ? messageId: text ; Identifier used to connect this instance message to a previous ; one: ; Allows this instance message to only contain values that have ; actually changed, turning it into a "Delta" or a "Patch", ; depending on the purpose of the message. ? previousMessageId: text ? modified: modified-date-time ? features: [ ] optional-comment } sdf-instance-of = { model: text ? patchMethod: text ; default is merge-patch optional-comment } optional-comment = ( ? $comment: text ; source code comments only, no semantics ) ; Shortcut for a map that gives names to instances of X ; (has keys of type text and values of type X) named = { * text => X } commonqualities = ( optional-comment ) ; For describing the state of instances at a given point in time ; ; An sdfInstance can refer to either an sdfThing or an sdfObject. ; Structurally, it is equivalent to that of an sdfThing. sdf-instance = thingqualities objectqualities = { commonqualities cpaedataqualities } thingqualities = { sdfThing: named sdfObject: named commonqualities cpaedataqualities } cpaedataqualities = ( ? sdfContext: named ; Models the current state of the instance's properties ? sdfProperty: named ; Models the current state of the instance's action affordances ; ; DISCUSS: How should the state of actions be modeled? ? sdfAction: named ; Models an history for every event affordance ? sdfEvent: named ) eventhistory = [* eventqualities] eventqualities = { outputValue: allowed-types timestamp: modified-date-time } allowed-types = number / text / bool / null / [* number] / [* text] / [* bool] / {* text => any} modified-date-time = text .abnf modified-dt-abnf modified-dt-abnf = "modified-dt" .det rfc3339z ; RFC 3339 sans time-numoffset, slightly condensed rfc3339z = ' date-fullyear = 4DIGIT date-month = 2DIGIT ; 01-12 date-mday = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on ; month/year time-hour = 2DIGIT ; 00-23 time-minute = 2DIGIT ; 00-59 time-second = 2DIGIT ; 00-58, 00-59, 00-60 based on leap sec ; rules time-secfrac = "." 1*DIGIT DIGIT = %x30-39 ; 0-9 partial-time = time-hour ":" time-minute ":" time-second [time-secfrac] full-date = date-fullyear "-" date-month "-" date-mday modified-dt = full-date ["T" partial-time "Z"] ' ]]>

Roads Not Taken This appendix documents previous modelling approaches that we eventually decided against pursuing further. Its main purpose is to illustrate our development process, showing which kind of alternatives we considered before choosing a particular way to describe instance information. We will remove this appendix as soon as this document is about to be finished.

Using SDF Models as Proofshots As shown in , the proofshot format could have also been modeled via SDF models where all sdfProperty definitions are given constvalues. However, this concept is not capable of capturing actions and events.

SDF instance proposal for Figure 2 in [I-D.lee-asdf-digital-twin-09]

Alternative Instance Keys Below you can see an alternative instance modelling approach with IDs as (part of the) instance keys.

SDF instance proposal (with IDs as part of the instance keys) for Figure 2 in [I-D.lee-asdf-digital-twin-09]

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Acknowledgments (TODO)