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Applications ASDF Internet-Draft 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 note will be removed by the RFC editor:) The present revision (–19) addresses comments from the AD review and from the IESG directorate reviews. 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 (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 note will be removed by the RFC editor:) The present revision (–19) addresses comments from the AD review and from the IESG directorate reviews. SDF is designed to be an extensible format. The present document constitutes the base specification for SDF: we speak of "base SDF" for short. In addition, SDF extensions can be defined, some of which may make use of extension points specifically defined for this in base SDF. One area for such extensions would be refinements of SDF's abstract interaction models into protocol bindings for specific ecosystems (e.g., ). For the use of certain other extensions, it may be necessary to indicate in the SDF document using them that a specific extension is in effect; see for details of the features quality that can be used for such indications. With extension points and feature indications available, base SDF does not define a "version" concept for the SDF format itself (as opposed to version indications within SDF documents indicating their own evolution, see ).

Structure of This Document After introductory material and an overview () over the elements of the model and over the different kinds of names used, introduces the main components of an SDF model. revisits names and structures them into namespaces. discusses the inner structure of the Objects defined by SDF, the sdfObjects, in further detail. discusses how SDF supports composition. Conventional Sections (, , , and ) follow. The normative defines the syntax of SDF in terms of its JSON structures, employing the Concise Data Definition Language (CDDL) . This is followed by the informative , a rendition of the SDF syntax in a "JSON Schema" format as they are defined by json-schema.org (collectively called JSO). The normative defines certain terms ("data qualities") used at the SDF data model level that were inspired by JSO. The informative provides a few examples for the use of composition in SDF. Finally, provides some historical information that can be useful in upgrading earlier, pre-standard SDF models and implementations to SDF base.

Terminology and Conventions Terms introduced in this section are capitalized when used in this section; to maintain readability, capitalization is only done when needed where they are used in the body of this document.

Programming Platform Terms The following definitions mention terms that are used with specific meanings in various programming platforms, but often have an independent definition for this document, which can be found further below in this section.

Element:

A generic term used here in its English sense. Exceptionally, in , used explicitly in accordance with its meaning in the JSON ecosystem, i.e., the elements of JSON arrays.

Entry:

A key-value pair in a map. (In JSON maps, sometimes also called "member".)

Map:

A collection of entries (key-value pairs), where there are no two entries with equivalent keys. (Also known as associative array, dictionary, or symbol table.)

Object:

An otherwise very generic term that JavaScript (and thus JSON) uses for the kind of maps that were part of the original languages from the outset. In this document, Object is used exclusively in its general English meaning or as the colloquial shorthand for sdfObject, even if the type name "object" is imported with JSON-related semantics from a data definition language.

Property:

Certain environments use the term "property" for a JSON concept that JSON calls "member" and we call "entry", or sometimes just for the map key of these. In this document, the term Property is specifically reserved for a certain kind of Affordance, even if the map key "properties" is imported with JSON-related semantics from a data definition language.

Byte:

This document uses the term "byte" in its now-customary sense as a synonym for "octet".

Conceptual Terms

Thing:

A physical item that is also available for interaction over a network.

Element:

A part or an aspect of something abstract; i.e., the term is used here in its usual English definition.

Affordance:

An element of an interface offered for interaction. Such an element becomes an Affordance when information is available (directly or indirectly) that indicates how it can or should be used. In the present document, the term is used for the digital (network-directed) interfaces of a Thing only; as it is a physical object as well, the Thing might also have physical affordances such as buttons, dials, and displays.

Property:

An Affordance that can potentially be used to read, write, and/or observe state (current/stored information) on a Grouping.

Action:

An Affordance that can potentially be used to perform a named operation on a Grouping.

Event:

An Affordance that can potentially be used to obtain information about what happened to a Grouping.

Specification Language Terms

SDF Document:

Container for SDF Definitions, together with data about the SDF Document itself (information block). Represented as a JSON text representing a single JSON map, which is built from nested maps.

SDF Model:

Definitions and declarations that model the digital interaction opportunities offered by one or more kinds of Things, represented by Groupings (sdfObjects and sdfThings). An SDF Model can be fully contained in a single SDF Document, or it can be built from an SDF Document that references definitions and declarations from additional SDF documents.

Block:

One or more entries in a JSON map that is part of an SDF specification; these entries can be described as a Block to emphasize that they together serve a specific function.

Group:

An entry in the main JSON map that represents the SDF document, and in certain nested definitions. A group has a Class Name Keyword as its key and a map of named definition entries (Definition Group) as a value.

Class Name Keyword:

One of sdfThing, sdfObject, sdfProperty, sdfAction, sdfEvent, or sdfData; the Classes for these type keywords are capitalized and prefixed with sdf.

Class:

Abstract term for the information that is contained in groups identified by a Class Name Keyword.

Quality:

A metadata item in a definition or declaration which says something about that definition or declaration. A quality is represented in SDF as an entry in a JSON map (JSON object) that serves as a definition or declaration. (The term "Quality" is used because another popular term, "Property", already has a different meaning.)

Definition:

An entry in a Definition Group. The entry creates a new semantic term for use in SDF models and associates it with a set of qualities. Unless the Class Name Keyword of the Group also makes it a Declaration (see ), a definition just defines a term, it does not create a component item within the enclosing definition.

Declaration:

A definition within an enclosing definition that is intended to create a component item within that enclosing definition. Every declaration can also be used as a definition for reference elsewhere.

Grouping:

An sdfThing or sdfObject, i.e., (directly or indirectly) a description for a combination of Affordances.

Object, sdfObject:

A Grouping that contains Affordance declarations (Property, Action, and Event declarations) only. It serves as the main "atom" of reusable semantics for model construction, representing the interaction model for a Thing that is simple enough to not require nested structure. sdfObjects are therefore similar to sdfThings but do not allow nesting, i.e., they cannot contain other Groupings (sdfObjects or sdfThings).

sdfThing:

A Grouping that can contain nested Groupings (sdfThings and sdfObjects). Like sdfObject, it can also contain Affordance declarations (Property, Action, and Event declarations). (Note that "Thing" has a different meaning from sdfThing and therefore is not available as a colloquial shorthand of sdfThing.)

Augmentation Mechanism:

A companion document to a base SDF Model that provides additional information ("augments" the base specification). The information may be for use in a specific ecosystem or with a specific protocol ("Protocol Binding"). No specific Augmentation Mechanisms are defined in base SDF. A simple mechanism for such augmentations has been discussed as a "mapping file" .

Protocol Binding:

A companion document to an SDF Model that defines how to map the abstract concepts in the model into the protocols in use in a specific ecosystem. The Protocol Binding might supply URL components, numeric IDs, and similar details. Protocol Bindings are one case of an Augmentation Mechanism.

Conventions Regular expressions that are used in the text as a "pattern" for some string are interpreted as per . (Note that a form of regular expressions is also used as values of the quality pattern; see .) The singular form is chosen as the preferred one for the keywords defined in this specification. 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.

Overview

Example Definition We start with an example for the SDF definition of a simple sdfObject called "Switch" ().

A simple example of an SDF document

This is a model of a switch. The state value declared in the sdfProperty group, represented by a Boolean, will be true for "on" and will be false for "off". The actions on or off declared in the sdfAction group are redundant with setting the value and are in the example to illustrate that there are often different ways of achieving the same effect. The action toggle will invert the value of the sdfProperty value, so that 2-way switches can be created; having such action will avoid the need for first retrieving the current value and then applying/setting the inverted value. The sdfObject group lists the affordances of Things modeled by this sdfObject. The sdfProperty group lists the property affordances described by the model; these represent various perspectives on the state of the sdfObject. Properties can have additional qualities to describe the state more precisely. Properties can be annotated to be read, write or read/write; how this is actually done by the underlying transfer protocols is not described in the SDF model but left to companion protocol bindings. Properties are often used with RESTful paradigms , describing state. The sdfAction group is the mechanism to describe other interactions in terms of their names, input, and output data (no data are used in the example), as in a POST method in REST or in a remote procedure call. The example toggle is an Action that changes the state based on the current state of the Property named value. (The third type of affordance is Events, which are not described in this example.) In the JSON representation, the info group is an exception in that this group's map has keys taken from the SDF vocabulary. All other groups (such as namespace, sdfObject) have maps with keys that are freely defined by the model writer (Switch, value, on, etc.); we speak of given names. The groups made up of entries with given names as keys usually use the named<> production in the formal syntax of SDF. Where the values of these entries are maps, these again use SDF vocabulary keys, and so on, generally alternating in further nesting. The SDF-defined vocabulary items used in the hierarchy of such groups are often, but not always, called quality names or qualities. See for more information about naming in SDF.

Elements of an SDF model The SDF language uses six predefined Class Name Keywords for modeling connected Things which are illustrated in .

Main classes used in SDF models

The six main Class Name Keywords are discussed below.

sdfObject sdfObjects, the items listed in an sdfObject definition group, are the main "atom" of reusable semantics for model construction. The concept aligns in scope with common definition items from many IoT modeling systems, for example ZigBee Clusters , OMA SpecWorks LwM2M Objects , and OCF Resource Types . An sdfObject definition contains a set of sdfProperty, sdfAction, and sdfEvent definitions that describe the interaction affordances associated with some scope of functionality. For the granularity of definition, sdfObject definitions are meant to be kept narrow enough in scope to enable broad reuse and interoperability. For example, defining a light bulb using separate sdfObject definitions for on/off control, dimming, and color control affordances will enable interoperable functionality to be configured for diverse product types. An sdfObject definition for a common on/off control may be used to control many different kinds of Things that require on/off control. The presence of one or both of the optional qualities "minItems" and "maxItems" defines the sdfObject as an array, i.e., all the affordances defined for the sdfObject exist a number of times, indexed by a number constrained to be between minItems and maxItems, inclusive, if given. (Note: Setting "minItems" to zero and leaving out "maxItems" puts the minimum constraints on that array.)

sdfProperty sdfProperty is used to model elements of state within Things modeled by the enclosing grouping. A named definition entry in an sdfProperty may be associated with some protocol affordance to enable the application to obtain the state variable and, optionally, modify the state variable. Additionally, some protocols provide for in-time reporting of state changes. (These three aspects are described by the qualities readable, writable, and observable defined for an sdfProperty.) Definitions in sdfProperty groups look like the definitions in sdfData groups. However, they actually also declare that a Property with the given qualities potentially is present in the containing sdfObject. For definitions in sdfProperty and sdfData, SDF provides qualities that can constrain the structure and values of data allowed in the interactions modeled by them. It also provides qualities that associate semantics to these data, such as engineering units and unit scaling information. For the data definition within sdfProperty or sdfData, SDF borrows some vocabulary proposed for the drafts 4 and 7 of the json-schema.org "JSON Schema" format (collectively called JSO here), enhanced by qualities that are specific to SDF. Details about the JSO-inspired vocabulary are in . For base SDF, data are constrained to be of simple types (number, string, Boolean), JSON maps composed of named data, and arrays of these types. Syntax extension points are provided that can be used to provide richer types in a future extension of this specification (possibly more of which can be borrowed from json-schema.org). Note that sdfProperty definitions (and sdfData definitions in general) are not intended to constrain the formats of data used for communication over network interfaces. Where needed, data definitions for payloads of protocol messages are expected to be part of the protocol binding.

sdfAction The sdfAction group contains declarations of Actions, model affordances that, when triggered, have more effect than just reading, updating, or observing Thing state, often resulting in some outward physical effect (which, itself, cannot be modeled in SDF). From a programmer's perspective, they might be considered to be roughly analogous to method calls. Actions may have data parameters: these are modeled as a single item of input data and output data, each. Where multiple parameters need to be modeled, an "object" type can be used to combine these parameters into one; for an example see in . Actions may be long-running, that is to say that the effects may not take place immediately as would be expected for an update to an sdfProperty; the effects may play out over time and emit action results. Actions may also not always complete and may result in application errors, such as an item blocking the closing of an automatic door. One idiom for giving an action initiator status and control about the ongoing action is to provide a URI for an ephemeral "action resource" in the sdfAction output data, allowing the action to deliver immediate feedback (including errors that prevent the action from starting) and the action initiator to use the action resource for further observation or modification of the ongoing action (including canceling it). Base SDF does not provide any tailored support for describing such action resources; an extension for modeling links in more detail (for instance, ) may be all that is needed to fully enable modeling them. Actions may have (or lack) qualities of idempotence and side-effect safety (see Section of RFC 9110 for more on these terms). Base SDF only provides data constraint modeling and semantics for the input and output data of definitions in sdfAction groups. Again, data definitions for payloads of protocol messages, and detailed protocol settings for invoking the action, are expected to be part of the protocol binding.

sdfEvent The sdfEvent group contains declarations of Events, which can model affordances that inform about "happenings" associated with a Thing modeled by the enclosing sdfObject; these may result in a signal being stored or emitted as a result. Note that there is a trivial overlap with sdfProperty state changes, which may also be defined as events but are not generally required to be defined as such. However, Events may exhibit certain ordering, consistency, and reliability requirements that are expected to be supported in various implementations of sdfEvent that do distinguish sdfEvent from sdfProperty. For instance, while a state change may simply be superseded by another state change, some events are "precious" and need to be preserved even if further events follow. Base SDF only provides data constraint modeling and semantics for the output data of Event affordances. Again, data definitions for payloads of protocol messages, and detailed protocol settings for soliciting the event, are expected to be part of the protocol binding.

sdfData Definitions in sdfData groups do not themselves specify affordances. These definitions are provided separately from those in sdfProperty groups to enable common modeling patterns, data constraints, and semantic anchor concepts to be factored out for data items that make up sdfProperty items and serve as input and output data for sdfAction and sdfEvent items. The data types defined in sdfData definitions only spring to life by being referenced in one of these contexts (directly or indirectly via some other sdfData definitions). It is a common use case for such a data definition to be shared between an sdfProperty item and input or output parameters of an sdfAction or output data provided by an sdfEvent. sdfData definitions also enable factoring out extended application data types such as mode and machine state enumerations to be reused across multiple definitions that have similar basic characteristics and requirements.

sdfThing Back at the top level, the sdfThing group enables definition of models for complex devices that will use one or more sdfObject definitions. Like sdfObject, sdfThing groups allow for the inclusion of interaction affordances, sdfData, as well as "minItems" and "maxItems" qualities. Therefore, they can be seen as a superset of sdfObject groups, additionally allowing for composition. As a result, an sdfThing directly or indirectly contains a set of sdfProperty, sdfAction, and sdfEvent definitions that describe the interaction affordances associated with some scope of functionality. A definition in an sdfThing group can refine the metadata of the definitions it is composed of: other definitions in sdfThing groups or definitions in sdfObject groups.

Member names: Given Names and Quality Names SDF documents are JSON maps that mostly employ JSON maps as member values, which in turn mostly employ JSON maps as their member values, and so on. This nested structure of JSON maps creates a tree, where the edges are the member names (map keys) used in these JSON maps. (In certain cases, where member names are not needed, JSON arrays may be interspersed in this tree.)

Given Names and Quality Names For any particular JSON map in an SDF document, the set of member names that are used is either of:

• A set of "Quality Names", where the entries in the map are Qualities. Quality Names are defined by the present specification and its extensions, together with specific semantics to be associated with the member value given with a certain Quality Name.
• A set of "Given Names", where the entries in the map are separate entities (definitions, declarations, etc.) that each have names that are chosen by the SDF document author in order that these names can be employed by a user of that model.

In a path from the root of the tree to any leaf, Quality Names and Given Names roughly alternate (with the information block, , as a prominent exception). The meaning of the JSON map that is the member value associated with a Given Name is derived from the Quality Name that was used as the member name associated to the parent. In the CDDL grammar given in , JSON maps with member names that are Given Names are defined using the CDDL generic rule reference named<membervalues>, where membervalues is in turn the structure of the member values of the JSON map, i.e., the value of the member named by the Given Name. As quality-named maps and given-named maps roughly alternate in a path down the tree, membervalues is usually a map built from Quality Names as keys.

Hierarchical Names From the outside of a specification, Given Names are usually used as part of a hierarchical name that looks like a JSON pointer , itself generally rooted in (used as the fragment identifier in) an outer namespace that looks like an https:// URL (see ). As Quality Names and Given Names roughly alternate in a path into the model, the JSON pointer part of the hierarchical name also alternates between Quality Names and Given Names. Note that the actual Given Names may need to be encoded when specified via the JSON pointer fragment identifier syntax, and that there are two layers of such encoding: tilde encoding of ~ and / as per , and then percent encoding of the tilde-encoded name into a valid URI fragment as per . For example, when a model is using the Given Name (with an embedded slash and a space) for an sdfObject, that sdfObject may need to be referenced as To sidestep potential interoperability problems, it is probably wise to avoid characters in Given Names that need such encoding (Quality Names are already defined in such a way that they never do).

Extensibility of Given Names and Quality Names In SDF, both Quality Names and Given Names are extension points. This is more obvious for Quality Names: Extending SDF is mostly done by defining additional qualities. To enable non-conflicting third party extensions to SDF, qualified names (names with an embedded colon) can be used as Quality Names. A nonqualified Quality Name is composed of ASCII letters, digits, and $ signs, starting with a lower case letter or a $ sign (i.e., using a pattern of "⁠[a-z$][A-Za-z$0-9]*"). Names with $ signs are intended to be used for functions separate from most other names; for instance, in this specification $comment is used for the comment quality (the presence or absence of a $comment quality does not change the meaning of the SDF model). Names that are composed of multiple English words can use the "lowerCamelCase" convention for indicating the word boundaries; no other use is intended for upper case letters in quality names. A qualified Quality Name is composed of a Quality Name Prefix, a : (colon) character, and a nonqualified Quality Name. Quality Name Prefixes are registered in the "Quality Name Prefixes" registry in the "SDF Parameters" registry group (). They are composed of lower case ASCII letters and digits, starting with a lower case ASCII letter (i.e., using a pattern of "⁠[a-z][a-z0-9]*"). Given Names are not restricted by the formal SDF syntax. To enable non-surprising name translations in tools, combinations of ASCII alphanumeric characters and - (ASCII hyphen/minus) are preferred, typically employing kebab-case for names constructed out of multiple words . ASCII hyphen/minus can then unambiguously be translated to an ASCII _ underscore character and back depending on the programming environment. Some styles also allow a dot (".") in given names. Given Names are often sufficiently self-explanatory that they can be used in place of the label quality if that is not given. In turn, if a given name turns out too complicated, a more elaborate label can be given and the given name kept simple. Base SDF does not address internationalization of given names. Further, to enable Given Names to have a more powerful role in building global hierarchical names, an extension is planned that makes use of qualified names for Given Names. So, until that extension is defined, Given Names with one or more embedded colons are reserved and MUST NOT be used in an SDF document. All names in SDF are case-sensitive.

SDF structure SDF definitions are contained in SDF documents, together with data about the SDF document itself (information block). Definitions and declarations from additional SDF documents can be referenced; together with the definitions and declarations in the referencing SDF document they build the SDF model expressed by that SDF document. Each SDF document is represented as a single JSON map. This map can be thought of as having three blocks: the information block, the namespaces block, and the definitions block. These blocks contain zero or more JSON name/value pairs, the names of which are quality names and the values of which mostly are (nested) maps (the exception defined in SDF base is the defaultNamespace quality, the value of which is a text string). An empty nested map of this kind is equivalent to not having the quality included at all.

Information block The information block contains generic metadata for the SDF document itself and all included definitions. To enable tool integration, the information block is optional in the grammar of SDF; most processes for working with SDF documents will have policies that only SDF documents with an info block can be processed. It is therefore RECOMMENDED that SDF validator tools emit a warning when no information block is found. The keyword (map key) that defines an information block is "info". Its value is a JSON map in turn, with a set of entries that represent qualities that apply to the included definition. Qualities of this map are shown in . None of these qualities are required or have default values that are assumed if the quality is absent. 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
features	array of strings	List of extension features used
$comment	string	Source code comments only, no semantics

The version quality is used to indicate version information about the set of definitions in the SDF document. The version is RECOMMENDED to be lexicographically increasing over the life of a model: a newer model always has a version string that string-compares higher than all previous versions. This is easily achieved by following the convention to start the version with an date-time or, if new versions are generated less frequently than once a day, just the full-date (i.e., YYYY-MM-DD); in many cases, that will be all that is needed (see for an example). This specification does not give a strict definition for the format of the version string but each using system or organization should define internal structure and semantics to the level needed for their use. If no further details are provided, a date-time or full-date in this field can be assumed to indicate the latest update time of the definitions in the SDF document. The modified quality can be used with a value using date-time (with Z for time-zone) or full-date format to express time of the latest revision of the definitions. The license string is preferably either a URI that points to a web page with an unambiguous definition of the license, or an license identifier. (As an example, for models to be handled by the One Data Model liaison group, this license identifier will typically be "BSD-3-Clause".) The features quality can be used to list names of critical (i.e., cannot be safely ignored) SDF extension features that need to be understood for the definitions to be properly processed. Extension feature names will be specified in extension documents.

Namespaces block The namespaces block contains the namespace map and the defaultNamespace setting; none of these qualities are required or have default values that are assumed if the quality is absent. The namespace map is a map from short names for URIs to the namespace URIs themselves. The defaultNamespace setting selects one of the entries in the namespace map by giving its short name. The associated URI (value of this entry) becomes the default namespace for the SDF document. 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

The following example declares a set of namespaces and defines cap as the default namespace. By convention, the values in the namespace map contain full URIs without a fragment identifier, and the fragment identifier is then added, if needed, where the namespace entry is used. Multiple SDF documents can contribute to the same namespace by using the same namespace URI for the default namespace across the documents. If no defaultNamespace setting is given, the SDF document does not contribute to a global namespace (all definitions remain local to the model and are not accessible for re-use by other models). As the defaultNamespace is set by supplying a namespace short name, its presence requires a namespace map that contains a mapping for that namespace short name. If no namespace map is given, no short names for namespace URIs are set up, and no defaultNamespace can be given.

Definitions block The Definitions block contains one or more groups, each identified by a Class Name Keyword such as sdfObject or sdfProperty. There can only be one group per keyword at this level; putting all the individual definitions in the group under that keyword is just a shortcut for identifying the class name keyword that applies to each of them, without repeating it for each definition. The value of each group is a JSON map, the keys of which serve for naming the individual definitions in this group, and the corresponding values provide a set of qualities (name-value pairs) for the individual definition. (In short, we speak of the map entries as "named sets of qualities".) Each group may contain zero or more definitions. Each identifier defined creates a new type and term in the target namespace. Declarations have a scope of the definition block they are directly contained in. A definition may in turn contain other definitions. Each definition is a named set of qualities, i.e., it consists of the newly defined identifier and a set of key-value pairs that represent the defined qualities and contained definitions. An example for an sdfObject definition is given in :

Example sdfObject definition

This example defines an sdfObject "foo" that is defined in the default namespace (full address: #/sdfObject/foo), containing a property that can be addressed as #/sdfObject/foo/sdfProperty/bar, with data of type boolean. Often, definitions are also declarations: the definition of the entry "bar" in the property "foo" means that data corresponding to the "foo" property in a property interaction offered by Thing can have zero or one components modeled by "bar". Entries within sdfProperty, sdfAction, and sdfEvent that are in turn within sdfObject or sdfThing entries, are also declarations; entries within sdfData are not. Similarly, sdfObject or sdfThing entries within an sdfThing definition specify that the interactions offered by a Thing modeled by this sdfThing include the interactions modeled by the nested sdfObject or sdfThing.

Top-level Affordances and sdfData Besides their placement within an sdfObject or sdfThing, affordances (i.e., sdfProperty, sdfAction, and sdfEvent) as well as sdfData can also be placed at the top level of an SDF document. Since they are not associated with an sdfObject or sdfThing, these kinds of definitions are intended to be re-used via the sdfRef mechanism (see ).

Names and namespaces SDF documents may contribute to a global namespace, and may reference elements from that global namespace. (An SDF document that does not set a defaultNamespace does not contribute to a global namespace.)

Structure Global names look exactly like https:// URIs with attached fragment identifiers. There is no intention to require that these URIs can be dereferenced. (However, as future extensions of SDF might find a use for dereferencing global names, the URI should be chosen in such a way that this may become possible in the future. See also for a discussion of dereferenceable identifiers.) The absolute URI of a global name should be a URI as per Section of RFC 3986 , with a scheme of "https" and a path (hier-part in ). For base SDF, the query part should not be used (it might be used in extensions). The fragment identifier is constructed as per .

Contributing global names The fragment identifier part of a global name defined in an SDF document is constructed from a JSON pointer that selects the element defined for this name in the SDF document. The absolute URI part is a copy of the default namespace. As a result, the default namespace is always the target namespace for a name for which a definition is contributed. When we want to emphasize that name definitions are contributed to the default namespace, we therefore also call it the "target namespace" of the SDF document. For instance, in , definitions for the following global names are contributed:

• https://example.com/capability/cap#/sdfObject/Switch
• https://example.com/capability/cap#/sdfObject/Switch/sdfProperty/value
• https://example.com/capability/cap#/sdfObject/Switch/sdfAction/on
• https://example.com/capability/cap#/sdfObject/Switch/sdfAction/off

Note the #, which separates the absolute-URI part (Section of RFC 3986 ) from the fragment identifier part (including the #, a JSON Pointer as in ).

Referencing global names A name reference takes the form of the production curie in Section 3 of , but limiting the IRIs involved in that grammar to URIs as per and the prefixes to ASCII characters . (Note that this definition does not make use of the production safe-curie in .) A name that is contributed by the current SDF document can be referenced by a Same-Document Reference as per Section of RFC 3986 . As there is little point in referencing the entire SDF document, this will be a # followed by a JSON pointer. This is the only kind of name reference to itself that is possible in an SDF document that does not set a default namespace. Name references that point outside the current SDF document need to contain curie prefixes. These then reference namespace declarations in the namespaces block. For example, if a namespace prefix is defined: Then this reference to that namespace: references the global name: Note that there is no way to provide a URI scheme name in a curie, so all references to outside of the document need to go through the namespace map. Name references occur only in specific elements of the syntax of SDF:

• copying elements via sdfRef values
• pointing to elements via sdfRequired value elements

sdfRef In a JSON map establishing a definition, the keyword sdfRef is used to copy the qualities and enclosed definitions of the referenced definition, indicated by the included name reference, into the newly formed definition. (This can be compared to the processing of the $ref keyword in .) The referenced definition should be such that, after copying and applying the additional qualities in the referencing definition, the newly built definition is also valid SDF (e.g., the copied qualities and definitions are valid in the context of the new definition). For example, this reference: creates a new definition "temperatureProperty" that contains all of the qualities defined in the definition at /sdfData/temperatureData. The sdfRef member need not be the only member of a map. Additional members may be present with the intention to override parts of the referenced map or to add new qualities or definitions. When processing sdfRef, if the target definition contains also sdfRef (i.e., is based on yet another definition), that MUST be processed as well. More formally, for a JSON map that contains an sdfRef member, the semantics is defined to be as if the following steps were performed:

1. The JSON map that contains the sdfRef member is copied into a variable named "patch".
2. The sdfRef member of the copy in "patch" is removed.
3. the JSON pointer that is the value of the sdfRef member is dereferenced and the result is copied into a variable named "original".
4. The JSON Merge Patch algorithm is applied to patch the contents of "original" with the contents of "patch".
5. The result of the Merge Patch is used in place of the value of the original JSON map.

Note that the formal syntaxes given in Appendices and generally describe the result of applying a merge-patch: the notations are not powerful enough to describe, for instance, how the merge-patch algorithm causes null values within the sdfRef to remove members of JSON maps from the referenced target. Nonetheless, the syntaxes also give the syntax of the sdfRef itself, which vanishes during the resolution; in many cases therefore even merge-patch inputs will validate with these formal syntaxes. Given the example (), and the following definition: The resulting definition of the "BasicSwitch" sdfObject would be identical to the definition of the "Switch" sdfObject except it would not contain the "toggle" Action.

Resolved models A model where all sdfRef references are processed as described in is called a resolved model. For example, given the following sdfData definitions: After resolving the definitions would look as follows:

sdfRequired The keyword sdfRequired is provided to apply a constraint that defines for which declarations the corresponding data are mandatory in a grouping (sdfThing or sdfObject) modeled by the current definition. The value of sdfRequired is an array of references, each indicating one or more declarations that are mandatory to be represented. References in this array can be SDF names (JSON Pointers), or one of two abbreviated reference formats:

• a text string with a "referenceable-name", namely an affordance name or a grouping name:
  • All affordance declarations that are directly in the same grouping (i.e., not nested further in another grouping) and that carry this name are declared to be mandatory to be represented. Note that there can be multiple such affordance declarations, one per affordance type.
  • The same applies to groupings made mandatory within groupings containing them.
• the Boolean value true. The affordance or grouping itself that carries the sdfRequired keyword is declared to be mandatory to be represented.

Note that referenceable-names are not subject to the encoding JSON pointers require as discussed in . To ensure that referenceable-names are reliably distinguished from JSON pointers, they are defined such that they cannot contain ":" or "#" characters (see rule referenceable-name in ). (If these characters are indeed contained in a Given Name, a JSON pointer needs to be formed instead in order to reference it in "sdfRequired", potentially requiring further path elements as well as JSON pointer encoding. The need for this is best avoided by choosing Given Names without these characters.) The example in shows two required elements in the sdfObject definition for "temperatureWithAlarm", the sdfProperty "currentTemperature", and the sdfEvent "overTemperatureEvent". The example also shows the use of JSON pointer with "sdfRef" to use a pre-existing definition for the sdfProperty "currentTemperature" and for the sdfOutputData produced by the sdfEvent "overTemperatureEvent".

Using sdfRequired

In , the same sdfRequired can also be represented in short form: Or, for instance "overTemperatureEvent" could carry

Common Qualities Definitions in SDF share a number of qualities that provide metadata for them. These are listed in . None of these qualities are required or have default values that are assumed if the quality is absent. If a short textual description is required for an application and no label is given in the SDF model, in its place applications could use the last part (the last reference-token, ) of the JSON pointer to the definition. Common Qualities

Quality	Type	Description
description	string	long text (no constraints)
label	string	short text (no constraints)
$comment	string	source code comments only, no semantics
sdfRef	sdf-pointer	(see )
sdfRequired	pointer-list	(see , used in affordances or groupings)

Data Qualities Data qualities are used in sdfData and sdfProperty definitions, which are named sets of data qualities (abbreviated as named-sdq). These qualities include the common qualities, JSO-inspired qualities (see below), and data qualities defined specifically for the present specification; the latter are shown in . None of these qualities are required or have default values that are assumed if the quality is absent. lists data qualities inspired by the various proposals at json-schema.org; the intention is that these (information model level) qualities are compatible with the (data model) semantics from the versions of the json-schema.org proposal they were imported from. SDF-defined Qualities of sdfData and sdfProperty

Quality	Type	Description	Default
(common)
unit	string	unit name (note 1)	N/A
nullable	boolean	indicates a null value is available for this type	true
contentFormat	string	content type (IANA media type string plus parameters), encoding (note 2)	N/A
sdfType	string ()	sdfType enumeration (extensible)	N/A
sdfChoice	named set of data qualities ()	named alternatives	N/A
enum	array of strings	abbreviation for string-valued named alternatives	N/A

1. The unit name SHOULD be as per the registry or the registry in as specified by Sections and of and , respectively. Exceptionally, if a registration in these registries cannot be obtained or would be inappropriate, the unit name can also be a URI that is pointing to a definition of the unit. Note that SDF processors are not expected to, and normally SHOULD NOT, dereference these URIs; the definition pointed to may be useful to humans, though. (See for a more extensive discussion of dereferenceable identifiers). A URI unit name is distinguished from a registered unit name by the presence of a colon; any registered unit names that contain a colon (at the time of writing, none) can therefore not be used in SDF. For use by translators into ecosystems that require URIs for unit names, the URN sub-namespace "urn:ietf:params:unit" is provided (); URNs from this sub-namespace MUST NOT be used in a unit quality, in favor of simply notating the unit name (such as kg instead of urn:ietf:params:unit:kg).
2. The contentFormat quality follows the Content-Format-Spec as defined in , allowing for expressing both numeric and string based Content-Formats.

sdfType SDF defines a number of basic types beyond those provided by JSON or JSO. These types are identified by the sdfType quality, which is a text string from a set of type names defined by the "sdfType values" registry in the "SDF Parameters" registry group (). The sdfType name is composed of lower case ASCII letters, digits, and - (ASCII hyphen/minus) characters, starting with a lower case ASCII letter (i.e., using a pattern of "⁠[a-z][-a-z0-9]*"), typically employing kebab-case for names constructed out of multiple words . To aid interworking with JSO implementations, it is RECOMMENDED that sdfType is always used in conjunction with the type quality inherited from , in such a way as to yield a common representation of the type's values in JSON. Values for sdfType that are defined in this specification are shown in . This table also gives a description of the semantics of the sdfType, the conventional value for type to be used with the sdfType value, and a conventional JSON representation for values of the type. Values Defined in Base SDF for the sdfType Quality

Name	Description	type	JSON Representation	Reference
byte-string	A sequence of zero or more bytes	string	base64url without padding	Section of RFC 8949
unix-time	A point in civil time (note 1)	number	POSIX time	Section of RFC 8949

(1) Note that the definition of unix-time does not imply the capability to represent points in time that fall on leap seconds. More date/time-related sdfTypes are likely to be added in the sdfType value registry.

sdfChoice Data can be a choice of named alternatives, called sdfChoice. Each alternative is identified by a name (string, key in the outer JSON map used to represent the overall choice) and a set of dataqualities (each in an inner JSON map, the value used to represent the individual alternative in the outer JSON map). Dataqualities that are specified at the same level as the sdfChoice apply to all choices in the sdfChoice, except those specific choices where the dataquality is overridden at the choice level. sdfChoice merges the functions of two constructs found in :

• enum What could be expressed as in JSO, is often best represented as: This allows the placement of other dataqualities such as description in the example. If an enum needs to use a data type different from text string, what would for instance have been: in JSO, is represented as: where the string names obviously would be chosen in a way that is descriptive for what these numbers actually stand for; sdfChoice also makes it easy to add number ranges into the mix. (Note that const can also be used for strings as in the previous example, for instance, if the actual string value is indeed a crucial element for the data model.)
• anyOf JSO provides a type union called anyOf, which provides a choice between anonymous alternatives. What could have been in JSO: can be more descriptively notated in SDF as:

Note that there is no need in SDF for the type intersection construct allOf or the peculiar type-xor construct oneOf found in . As a simplification for users of SDF models who are accustomed to the JSO enum keyword, this is retained, but limited to a choice of text string values, such that is syntactic sugar for In a single definition, the keyword enum cannot be used at the same time as the keyword sdfChoice, as the former is just syntactic sugar for the latter.

Keywords for definition groups The following SDF keywords are used to create definition groups in the target namespace. All these definitions share some common qualities as discussed in .

sdfObject The sdfObject keyword denotes a group of zero or more sdfObject definitions. sdfObject definitions may contain or include definitions of named Properties, Actions, Events declared for the sdfObject, as well as named data types (sdfData group) to be used in this or other sdfObjects. The qualities of an sdfObject include the common qualities; additional qualities are shown in . None of these qualities are required or have default values that are assumed if the quality is absent. Qualities of sdfObject

Quality	Type	Description
(common)
sdfProperty	property	zero or more named property definitions for this sdfObject
sdfAction	action	zero or more named action definitions for this sdfObject
sdfEvent	event	zero or more named event definitions for this sdfObject
sdfData	named-sdq	zero or more named data type definitions that might be used in the above
minItems	number	(array) Minimum number of multiplied affordances in array
maxItems	number	(array) Maximum number of multiplied affordances in array

sdfProperty The sdfProperty keyword denotes a group of zero or more Property definitions. Properties are used to model elements of state. The qualities of a Property definition include the data qualities (and thus the common qualities), see , additional qualities are shown in . Qualities of sdfProperty

Quality	Type	Description	Default
(data)
readable	boolean	Reads are allowed	true
writable	boolean	Writes are allowed	true
observable	boolean	flag to indicate asynchronous notification is available	true

sdfAction The sdfAction keyword denotes a group of zero or more Action definitions. Actions are used to model commands and methods which are invoked. Actions may have parameter data that are supplied upon invocation and output data that is provided as a direct result of the invocation of the action (note that "action objects" may also be created to furnish ongoing information during a long-running action; these would be pointed to by the output data). The qualities of an Action definition include the common qualities, additional qualities are shown in . Qualities of sdfAction

Quality	Type	Description
(common)
sdfInputData	map	data qualities of the input data for an Action
sdfOutputData	map	data qualities of the output data for an Action
sdfData	named-sdq	zero or more named data type definitions that might be used in the above

sdfInputData defines the input data of the action. sdfOutputData defines the output data of the action. As discussed in , a set of data qualities with type "object" can be used to substructure either data item, with optionality indicated by the data quality required.

sdfEvent The sdfEvent keyword denotes zero or more Event definitions. Events are used to model asynchronous occurrences that may be communicated proactively. Events have data elements which are communicated upon the occurrence of the event. The qualities of sdfEvent include the common qualities, additional qualities are shown in . Qualities of sdfEvent

Quality	Type	Description
(common)
sdfOutputData	map	data qualities of the output data for an Event
sdfData	named-sdq	zero or more named data type definitions that might be used in the above

sdfOutputData defines the output data of the action. As discussed in , a set of data qualities with type "object" can be used to substructure the output data item, with optionality indicated by the data quality required.

sdfData The sdfData keyword denotes a group of zero or more named data type definitions (named-sdq). An sdfData definition provides a reusable semantic identifier for a type of data item and describes the constraints on the defined type. sdfData is not itself a declaration, so it does not cause any of these data items to be included in an affordance definition. The qualities of sdfData include the data qualities (and thus the common qualities), see .

High Level Composition The requirements for high level composition include the following:

• The ability to represent products, standardized product types, and modular products while maintaining the atomicity of sdfObjects.
• The ability to compose a reusable definition block from sdfObjects. Example: a single plug unit of an outlet strip with sdfObjects for on/off control, energy monitor, and optional dimmer, while retaining the atomicity of the individual sdfObjects.
• The ability to compose sdfObjects and other definition blocks into a higher level sdfThing that represents a product, while retaining the atomicity of sdfObjects.
• The ability to enrich and refine a base definition to have product-specific qualities and quality values, such as unit, range, and scale settings.
• The ability to reference items in one part of a complex definition from another part of the same definition. Example: summarizing the energy readings from all plugs in an outlet strip.

Paths in the model namespaces The model namespace is organized according to terms that are defined in the SDF documents that contribute to the namespace. For example, definitions that originate from an organization or vendor are expected to be in a namespace that is specific to that organization or vendor. The structure of a path in a namespace is defined by the JSON Pointers to the definitions in the SDF documents in that namespace. For example, if there is an SDF document defining an sdfObject "Switch" with an action "on", then the reference to the action would be "ns:/sdfObject/Switch/sdfAction/on" where ns is the namespace prefix (short name for the namespace).

Modular Composition Modular composition of definitions enables an existing definition (could be in the same or another SDF document) to become part of a new definition by including a reference to the existing definition within the model namespace.

Use of the "sdfRef" keyword to re-use a definition An existing definition may be used as a template for a new definition, that is, a new definition is created in the target namespace which uses the defined qualities of some existing definition. This pattern uses the keyword sdfRef as a quality of a new definition with a value consisting of a reference to the existing definition that is to be used as a template. In the definition that uses sdfRef, new qualities may be added and existing qualities from the referenced definition may be overridden. (Note that JSON maps do not have a defined order, so the SDF processor may see these overrides before seeing the sdfRef.) Note that the definition referenced by sdfRef might contain qualities or definitions that are not valid in the context where the sdfRef is used. In this case, the resulting model, when resolved, may be invalid. Example: an sdfRef adds an sdfThing definition in an sdfObject definition. As a convention, overrides are intended to be used only for further restricting the allowable set of data values. Such a usage is shown in : any value allowable for a cable-length also is an allowable value for a length, with the additional restriction that the length cannot be smaller than 5 cm. (This is labeled as a convention as it cannot be checked in the general case. A quality of implementation consideration for a tool might be to provide at least some form of checking.) Note that the example provides a description that overrides the description of the referenced definition; as this quality is intended for human consumption there is no conflict with the intended goal.

Using an Override to Further Restrict the Set of Data Values

sdfThing An sdfThing is a set of declarations and qualities that may be part of a more complex model. For example, the sdfObject declarations that make up the definition of a single socket of an outlet strip could be encapsulated in an sdfThing, which itself could be used in a declaration in the sdfThing definition for the outlet strip. (See in for parts of an SDF model for this example). sdfThing definitions carry semantic meaning, such as a defined refrigerator compartment and a defined freezer compartment, making up a combination refrigerator-freezer product. An sdfThing may be composed of sdfObjects and other sdfThings. It can also contain sdfData definitions, as well as declarations of interaction affordances itself, such as a status (on/off) for the refrigerator-freezer as a whole (see in for an example SDF model illustrating these aspects). The qualities of sdfThing are shown in . Analogous to sdfObject, the presence of one or both of the optional qualities "minItems" and "maxItems" defines the sdfThing as an array. Qualities of sdfThing

Quality	Type	Description
(common)
sdfThing	thing
sdfObject	object
sdfProperty	property	zero or more named property definitions for this thing
sdfAction	action	zero or more named action definitions for this thing
sdfEvent	event	zero or more named event definitions for this thing
sdfData	named-sdq	zero or more named data type definitions that might be used in the above
minItems	number	(array) Minimum number of multiplied affordances in array
maxItems	number	(array) Maximum number of multiplied affordances in array

IANA Considerations RFC Ed.: throughout this section, please replace RFC XXXX with this RFC number, and remove this note.

Media Type IANA is requested to add the following Media-Type to the "Media Types" registry. Media Type Registration for SDF

Name	Template	Reference
sdf+json	application/sdf+json	RFC XXXX,

Type name:

application

Subtype name:

sdf+json

Required parameters:

none

Optional parameters:

none

Encoding considerations:

binary (JSON is UTF-8-encoded text)

Security considerations:

of RFC XXXX

Interoperability considerations:

none

Published specification:

of RFC XXXX

Applications that use this media type:

Tools for data and interaction modeling in the Internet of Things and related environments

Fragment identifier considerations:

A JSON Pointer fragment identifier may be used, as defined in .

Additional information:

Magic number(s):

n/a

File extension(s):

.sdf.json

Windows Clipboard Name:

"Semantic Definition Format (SDF) for Data and Interactions of Things"

Macintosh file type code(s):

n/a

Macintosh Universal Type Identifier code:

org.ietf.sdf-json
conforms to public.text

Person & email address to contact for further information:

ASDF WG mailing list (asdf@ietf.org), or IETF Applications and Real-Time Area (art@ietf.org)

Intended usage:

COMMON

Restrictions on usage:

none

Author/Change controller:

IETF

Provisional registration:

no

Content-Format This document adds the following Content-Format to the "CoAP Content-Formats" registry, within the "Constrained RESTful Environments (CoRE) Parameters" registry group, where 434 comes from the "IETF Review" 256-999 range. SDF Content-format Registration

Content Type	Content Coding	ID	Reference
application/sdf+json	-	434	RFC XXXX

// RFC Ed.: 434 was earmarked in https://mailarchive.ietf.org/arch/msg/core-parameters/iLDsdxk80YO9IsLMXMAgcx5S8Ak/; please replace 434 with the assigned ID, remove the requested range, and remove this note.
// RFC Ed.: please replace RFC XXXX with this RFC number and remove this note.

IETF URN Sub-namespace for Unit Names (urn:ietf:params:unit) IANA is requested to register the following value in the "" registry in , following the template in :

Registry name:

unit

Specification:

RFC XXXX

Repository:

combining the symbol values from the registry and the registry in as specified by Sections and of and , respectively (which by the registration policy are guaranteed to be non-overlapping).

Index value:

Percent-encoding (Section of RFC 3986 ) is required of any characters in unit names as required by ABNF rule "pchar" in Section of RFC 3986 , specifically at the time of writing for the unit names "%" (deprecated in favor of "/"), "%RH", "%EL".

Registries IANA is requested to create an "SDF Parameters" registry group, with the registries defined in this Section.

Quality Names IANA is requested to create a "Quality Names" registry in the "SDF Parameters" registry group, with the following template:

Name:

A quality name composed of ASCII letters, digits, and dollar signs, starting with a lower case ASCII letter or a dollar sign (i.e., using a pattern of "⁠[a-z$][A-Za-z$0-9]*").

Brief Description:

A brief description.

Reference:

A pointer to a specification.

Change Controller:

(see Section of RFC 8126 )

Quality Names in this registry are intended to be registered in conjunction with RFCs and activities of the IETF. The registration policy is Specification Required as per Section of RFC 8126 . (Note that the policy is not "RFC Required" or "IETF Review" Sections and of RFC 8126 so that registrations can be made earlier in the process, even earlier than foreseen in .) The instructions to the Experts are: * to ascertain that the specification is available in an immutable reference and has achieved a good level of review in conjunction with RFCs or activities of the IETF, and * to be frugal in the allocation of quality names that are suggestive of generally applicable semantics, keeping them in reserve for qualities that are likely to enjoy wide use and can make good use of their conciseness. The "Quality Name Prefixes" registry starts out as in ; all references for these initial entries are to RFC XXXX and all change controllers are given as "IETF"". Initial Content of Quality Names Registry

Name	Brief Description
$comment	source code comments only, no semantics
const	constant value
contentFormat	content format
default	default value
description	long description text
enum	sdfChoice limited to text strings
exclusiveMaximum	exclusive maximum for a number
exclusiveMinimum	exclusive minimum for a number
format	specific format for a text string
items	items of an array
label	short text (no constraints); defaults to key
maxItems	maximum number of items in an array
maxLength	maximum length (in characters) for a text string
maximum	maximum for a number
minItems	minimum number of items in an array
minLength	minimum length (in characters) for a text string
minimum	minimum for a number
multipleOf	step size of number
nullable	boolean: can the item be left out?
observable	boolean: can the item be observed?
pattern	regular expression pattern for a text string
properties	named dataqualities for type="object"
readable	boolean: can the item be read?
required	which data items are required?
sdfChoice	named dataqualities for a choice
sdfData	named dataqualities for an independent data type definition
sdfInputData	input data to an action
sdfOutputData	output data of an action or event (sdfRequired applies here)
sdfRef	sdf-pointer to definition being referenced
sdfRequired	pointer-list to declarations of required components
sdfRequiredInputData	pointer-list to declarations of required input data for an action
sdfType	more detailed information about the type of a string
type	general category of data type
uniqueItems	boolean: do the items of the array need to be all different?
unit	engineering unit and scale (per SenML registry)
writable	boolean: can the item be written to?

Quality Name Prefixes IANA is requested to create a "Quality Name Prefixes" registry in the "SDF Parameters" registry group, with the following template:

Prefix:

A quality name prefix composed of lower case ASCII letters and digits, starting with a lower case ASCII letter (i.e., using a pattern of "⁠[a-z][a-z0-9]*").

Contact:

A contact point for the organization that assigns quality names with this prefix.

Reference:

A pointer to additional information, if available.

Quality Name Prefixes are intended to be registered by organizations that plan to define quality names constructed with an organization-specifix prefix (). The registration policy is Expert Review as per Section of RFC 8126 . The instructions to the Expert are to ascertain that the organization will handle quality names constructed using their prefix in a way that roughly achieves the objectives for an IANA registry that support interoperability of SDF models employing these quality names, including:

• Stability, "stable and permanent";
• Transparency, "readily available", "in sufficient detail" (Section of RFC 8126 ).

The Expert will take into account that other organizations operate in different ways than the IETF, and that as a result some of these overall objectives will be achieved in a different way and to a different level of comfort. The "Quality Name Prefixes" registry starts out empty.

sdfType Values IANA is requested to create a "sdfType values" registry in the "SDF Parameters" registry group, with the following template:

Name:

A name composed of lower case ASCII letters, digits and - (ASCII hyphen/minus) characters, starting with a lower case ASCII letter (i.e., using a pattern of "⁠[a-z][-a-z0-9]*").

Description:

A short description of the information model level structure and semantics

type:

The value of the quality "type" to be used with this sdfType

JSON Representation

A short description of a JSON representation that can be used for this sdfType. This MUST be consistent with the type.

Reference:

A more detailed specification of meaning and use of sdfType.

sdfType values are intended to be registered to enable modeling additional SDF-specific types (see ). The registration policy is Specification Required as per Section of RFC 8126 . The instructions to the Expert are to ascertain that the specification provides enough detail to enable interoperability between implementations of the sdfType being registered, and that names are chosen with enough specificity that ecosystem-specific sdfTypes will not be confused with more generally applicable ones. The initial set of registrations is described in .

Security Considerations Some wider security considerations applicable to Things are discussed in . gives an overview over security considerations that arise when formal description techniques are used to govern interoperability; analogs of these security considerations can apply to SDF. The security considerations of underlying building blocks such as those detailed in Section of RFC 3629 apply. SDF uses JSON as a representation language. For a number of cases, indicates that implementation behavior for certain constructs allowed by the JSON grammar is unpredictable. Implementations need to be robust against invalid or unpredictable cases on input, preferably by rejecting input that is invalid or that would lead to unpredictable behavior, and need to avoid generating these cases on output. Implementations of model languages may also exhibit performance-related availability issues when the attacker can control the input, see for a brief discussion and for considerations specific to the use of pattern. SDF may be used in two processes that are often security relevant: model-based validation of data that is intended to be described by SDF models, and model-based augmentation of these data with information obtained from the SDF models that apply. Implementations need to ascertain the provenance (and thus authenticity and integrity) and applicability of the SDF models they employ operationally in such security relevant ways. Implementations that make use of the composition mechanisms defined in this document need to do this for each of the components they combine into the SDF models they employ. Essentially, this process needs to undergo the same care and scrutiny as any other introduction of source code into a build environment; the possibility of supply-chain attacks on the modules imported needs to be considered. Specifically, implementations might rely on model-based input validation for enforcing certain properties of the data structure ingested (which, if not validated, could lead to malfunctions such as crashes and remote code execution). These implementations need to be particularly careful about the data models they apply, including their provenance and potential changes of these properties that upgrades to the referenced modules may (inadvertently or as part of an attack) cause. More generally speaking, implementations should strive to be robust against expected and unexpected limitations of the model-based input validation mechanisms and their implementations. Similarly, implementations that rely on model-based augmentation may generate false data from their inputs; this is an integrity violation in any case but also can possibly be exploited for further attacks. In applications that dynamically acquire models and obtain modules from module references in these, the security considerations of dereferenceable identifiers apply (see for a more extensive discussion of dereferenceable identifiers). There may be confidentiality requirements on SDF models, both on their content and on the fact that a specific model is used in a particular Thing or environment. The present specification does not discuss model discovery or define an access control model for SDF models, nor does it define a way to obtain selective disclosure where that may be required. It is likely that these definitions require additional context from underlying ecosystems and the characteristics of the protocols employed there; this is therefore left as future work (e.g., for documents such as ).

References Normative References IANA IANA This document defines a date and time format for use in Internet protocols that is a profile of the ISO 8601 standard for representation of dates and times using the Gregorian calendar. This specification defines a format for representing simple sensor measurements and device parameters in Sensor Measurement Lists (SenML). Representations are defined in JavaScript Object Notation (JSON), Concise Binary Object Representation (CBOR), Extensible Markup Language (XML), and Efficient XML Interchange (EXI), which share the common SenML data model. A simple sensor, such as a temperature sensor, could use one of these media types in protocols such as HTTP or the Constrained Application Protocol (CoAP) to transport the measurements of the sensor or to be configured. The Sensor Measurement Lists (SenML) media type supports the indication of units for a quantity represented. This short document registers a number of additional unit names in the IANA registry for units in SenML. It also defines a registry for secondary units that cannot be in SenML's main registry, as they are derived by linear transformation from units already in that registry. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] This specification defines UUIDs (Universally Unique IDentifiers) -- also known as GUIDs (Globally Unique IDentifiers) -- and a Uniform Resource Name namespace for UUIDs. A UUID is 128 bits long and is intended to guarantee uniqueness across space and time. UUIDs were originally used in the Apollo Network Computing System (NCS), later in the Open Software Foundation's (OSF's) Distributed Computing Environment (DCE), and then in Microsoft Windows platforms. This specification is derived from the OSF DCE specification with the kind permission of the OSF (now known as "The Open Group"). Information from earlier versions of the OSF DCE specification have been incorporated into this document. This document obsoletes RFC 4122. JSON Pointer defines a string syntax for identifying a specific value within a JavaScript Object Notation (JSON) document. 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. ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. The Sensor Measurement Lists (SenML) media types support multiple types of values, from numbers to text strings and arbitrary binary Data Values. In order to facilitate processing of binary Data Values, this document specifies a pair of new SenML fields for indicating the content format of those binary Data Values, i.e., their Internet media type, including parameters as well as any content codings applied. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. This document describes a new sub-delegation for the 'ietf' URN namespace for registered protocol items. The 'ietf' URN namespace is defined in RFC 2648 as a root for persistent URIs that refer to IETF- defined resources. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Concise Data Definition Language (CDDL), standardized in RFC 8610, provides "control operators" as its main language extension point. The present document defines a number of control operators that were not yet ready at the time RFC 8610 was completed:.plus,.cat, and.det for the construction of constants;.abnf/.abnfb for including ABNF (RFC 5234 and RFC 7405) in CDDL specifications; and.feature for indicating the use of a non-basic feature in an instance. 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 Cloudflare, Inc. JSON Schema defines the media type "application/schema+json", a JSON- based format for describing the structure of JSON data. JSON Schema asserts what a JSON document must look like, ways to extract information from it, and how to interact with it. The "application/ schema-instance+json" media type provides additional feature-rich integration with "application/schema+json" beyond what can be offered for "application/json" documents. This is the base specification for json-schema.org "draft 7". Cloudflare, Inc. JSON Schema (application/schema+json) has several purposes, one of which is JSON instance validation. This document specifies a vocabulary for JSON Schema to describe the meaning of JSON documents, provide hints for user interfaces working with JSON data, and to make assertions about what a valid document must look like. This is the validation specification for json-schema.org "draft 7". SitePen (USA) JSON Schema defines the media type "application/schema+json", a JSON based format for defining the structure of JSON data. JSON Schema provides a contract for what JSON data is required for a given application and how to interact with it. JSON Schema is intended to define validation, documentation, hyperlink navigation, and interaction control of JSON data. This is the base specification for json-schema.org "draft 4". SitePen (USA) JSON Schema (application/schema+json) has several purposes, one of which is instance validation. The validation process may be interactive or non interactive. For instance, applications may use JSON Schema to build a user interface enabling interactive content generation in addition to user input checking, or validate data retrieved from various sources. This specification describes schema keywords dedicated to validation purposes. This is the validation specification for json-schema.org "draft 4". Ericsson Siemens This document gives guidance for designing Internet of Things (IoT) systems that follow the principles of the Representational State Transfer (REST) architectural style. This document is a product of the IRTF Thing-to-Thing Research Group (T2TRG). Elsevier The Internet of Things (IoT) concept refers to the usage of standard Internet protocols to allow for human-to-thing and thing-to-thing communication. The security needs for IoT systems are well recognized, and many standardization steps to provide security have been taken -- for example, the specification of the Constrained Application Protocol (CoAP) secured with Datagram Transport Layer Security (DTLS). However, security challenges still exist, not only because there are some use cases that lack a suitable solution, but also because many IoT devices and systems have been designed and deployed with very limited security capabilities. In this document, we first discuss the various stages in the lifecycle of a thing. Next, we document the security threats to a thing and the challenges that one might face to protect against these threats. Lastly, we discuss the next steps needed to facilitate the deployment of secure IoT systems. This document can be used by implementers and authors of IoT specifications as a reference for details about security considerations while documenting their specific security challenges, threat models, and mitigations. This document is a product of the IRTF Thing-to-Thing Research Group (T2TRG). Ecma International Universität Bremen TZI This document defines and registers an sdfType "link" for the Semantic Definition Format (SDF) for Data and Interactions of Things (draft-ietf-asdf-sdf). Universität Bremen TZI Hochschule Emden/Leer 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 In a protocol or an application environment, it is often important to be able to create unambiguous identifiers for some meaning (concept or some entity). Due to the simplicity of creating URIs, these have become popular for this purpose. Beyond the purpose of identifiers to be uniquely associated with a meaning, some of these URIs are in principle _dereferenceable_, so something can be placed that can be retrieved when encountering such a URI. // The present revision -04 includes a few clarifications. This document specifies I-Regexp, a flavor of regular expression that is limited in scope with the goal of interoperation across many different regular expression libraries. JSONPath defines a string syntax for selecting and extracting JSON (RFC 8259) values from within a given JSON value. 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. This memo describes the process for early allocation of code points by IANA from registries for which "Specification Required", "RFC Required", "IETF Review", or "Standards Action" policies apply. This process can be used to alleviate the problem where code point allocation is needed to facilitate desired or required implementation and deployment experience prior to publication of an RFC, which would normally trigger code point allocation. The procedures in this document are intended to apply only to IETF Stream documents.

Formal Syntax of SDF This normative appendix describes the syntax of SDF using CDDL . This appendix shows the framework syntax only, i.e., a syntax with liberal extension points. Since this syntax is nearly useless in finding typos in an SDF specification, a second syntax, the validation syntax, is defined that does not include the extension points. The validation syntax can be generated from the framework syntax by leaving out all lines containing the string EXTENSION-POINT; as this is trivial, the result is not shown here. This appendix makes use of CDDL "features" as defined in . Features whose names end in "-ext" indicate extension points for further evolution. ? defaultNamespace: text ; Thing is a composition of objects that work together in some way ? sdfThing: named ; Object is a set of Properties, Actions, and Events that together ; perform a particular function ? sdfObject: named ; Includes Properties, Actions, and Events as well as sdfData paedataqualities EXTENSION-POINT<"top-ext"> } sdfinfo = { ? title: text ? description: text ? version: text ? copyright: text ? license: text ? modified: modified-date-time ? features: [ * (any .feature "feature-name") ; EXTENSION-POINT ] optional-comment EXTENSION-POINT<"info-ext"> } ; Shortcut for a map that gives names to instances of X ; (has keys of type text and values of type X) named = { * text => X } ; EXTENSION-POINT is only used in framework syntax EXTENSION-POINT = ( * (quality-name .feature f) => any ) quality-name = text .regexp "([a-z][a-z0-9]*:)?[a-z$][A-Za-z$0-9]*" sdf-pointer = global / same-object / true global = text .regexp ".*[:#].*" ; rough CURIE or JSON Pointer syntax same-object = referenceable-name referenceable-name = text .regexp "[^:#]*" ; per se no point in having an empty list, but used for sdfRequired ; in odmobject-multiple_axis_joystick.sdf.json pointer-list = [* sdf-pointer] optional-comment = ( ? $comment: text ; source code comments only, no semantics ) commonqualities = ( ? description: text ; long text (no constraints) ? label: text ; short text (no constraints); default to key optional-comment ? sdfRef: sdf-pointer ; applies to qualities of properties, of data: ? sdfRequired: pointer-list ) arraydefinitionqualities = ( ? "minItems" => uint ? "maxItems" => uint ) paedataqualities = ( ; Property represents the state of an instance of an object ? sdfProperty: named ; Action invokes an application layer verb associated with an object ? sdfAction: named ; Event represents an occurrence of event associated with an object ? sdfEvent: named ; Data represents a piece of information that can be the state of a ; property or a parameter to an action or a signal in an event ? sdfData: named ) ; for building hierarchy thingqualities = { commonqualities ? sdfObject: named ? sdfThing: named paedataqualities arraydefinitionqualities EXTENSION-POINT<"thing-ext"> } ; for single objects, or for arrays of objects objectqualities = { commonqualities paedataqualities arraydefinitionqualities EXTENSION-POINT<"object-ext"> } parameter-list = dataqualities actionqualities = { commonqualities ? sdfInputData: parameter-list ; sdfRequiredInputData applies here ? sdfOutputData: parameter-list ; sdfRequired applies here ; zero or more named data type definitions that might be used above ? sdfData: named EXTENSION-POINT<"action-ext"> } eventqualities = { commonqualities ? sdfOutputData: parameter-list ; sdfRequired applies here ; zero or more named data type definitions that might be used above ? sdfData: named EXTENSION-POINT<"event-ext"> } sdftype-name = text .regexp "[a-z][-a-z0-9]*" ; EXTENSION-POINT dataqualities = { commonqualities jsonschema ? "unit" => text ? nullable: bool ? "sdfType" => "byte-string" / "unix-time" / (sdftype-name .feature "sdftype-ext") ; EXTENSION-POINT ? contentFormat: text EXTENSION-POINT<"data-ext"> } propertyqualities = { ? observable: bool ? readable: bool ? writable: bool ~dataqualities } allowed-types = number / text / bool / null / [* number] / [* text] / [* bool] / {* text => any} / (any .feature "allowed-ext") ; EXTENSION-POINT compound-type = ( "type" => "object" ? required: [+text] ? properties: named ) optional-choice = ( ? (("sdfChoice" => named) // ("enum" => [+ text])) ; limited to text strings ) jsonschema = ( ? (("type" => "number" / "string" / "boolean" / "integer" / "array") // compound-type // (type: text .feature "type-ext") ; EXTENSION-POINT ) ; if present, all other qualities apply to all choices: optional-choice ; the next three should validate against type: ? const: allowed-types ? default: allowed-types ; number/integer constraints ? minimum: number ? maximum: number ? exclusiveMinimum: number ? exclusiveMaximum: number ? multipleOf: number ; text string constraints ? minLength: uint ? maxLength: uint ? pattern: text ; regexp ? format: "date-time" / "date" / "time" / "uri" / "uri-reference" / "uuid" / (text .feature "format-ext") ; EXTENSION-POINT ; array constraints ? minItems: uint ? maxItems: uint ? uniqueItems: bool ? items: jso-items ) jso-items = { ? sdfRef: sdf-pointer ; import limited to subset allowed here... ? description: text ; long text (no constraints) optional-comment ; leave commonqualities out for non-complex data types, ; but need the above three. ; no further nesting: no "array" ? ((type: "number" / "string" / "boolean" / "integer") // compound-type // (type: text .feature "itemtype-ext") ; EXTENSION-POINT ) ; if present, all other qualities apply to all choices optional-choice ; jso subset ? minimum: number ? maximum: number ? format: text ? minLength: uint ? maxLength: uint EXTENSION-POINT<"items-ext"> } 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"] ' ]]>

json-schema.org Rendition of SDF Syntax This informative appendix describes the syntax of SDF defined in , but using a version of the description techniques advertised on json-schema.org . The appendix shows both the validation and the framework syntax. Since most of the lines are the same between these two files, those lines are shown only once, with a leading space, in the form of a unified diff. Lines leading with a - are part of the validation syntax, and lines leading with a + are part of the framework syntax.

Data Qualities inspired by json-schema.org This appendix is normative. Data qualities define data used in SDF affordances at an information model level. A popular way to describe JSON data at a data model level is proposed by a number of drafts on json-schema.org (which collectively are abbreviated JSO here); for reference to a popular version we will point here to and . As the vocabulary used by JSO is familiar to many JSON modelers, the present specification borrows some of the terms and ports their semantics to the information model level needed for SDF. The main data quality imported is the "type". In SDF, this can take one of six (text string) values, which are discussed in the following subsections (note that the JSO type "null" is not supported as a value of this data quality in SDF). The additional quality "const" restricts the data to one specific value (given as the value of the const quality). Similarly, the additional quality "default" provides data that can be used in the absence of the data (given as the value of the const quality); this is mainly documentary and not very well-defined for SDF as no process is defined that would add default values to an instance of some interaction data.

type "number", type "integer" The types "number" and "integer" are associated with floating point and integer numbers, as they are available in JSON. A type value of integer means that only integer values of JSON numbers can be used (note that 10.0 is an integer value, even if it is in a notation that would also allow non-zero decimal fractions). The additional data qualities "minimum", "maximum", "exclusiveMinimum", "exclusiveMaximum" provide number values that serve as inclusive/exclusive lower/upper bounds for the number. (Note that the Boolean form of "exclusiveMinimum"/"exclusiveMaximum" found in earlier JSO drafts is not used.) The data quality "multipleOf" gives a positive number that constrains the data value to be an integer multiple of the number given. (Type "integer" can also be expressed as a "multipleOf" quality of value 1, unless another "multipleOf" quality is present.)

type "string" The type "string" is associated with Unicode text string values as they can be represented in JSON. The length (as measured in characters) can be constrained by the additional data qualities "minLength" and "maxLength", which are inclusive bounds. (More specifically, Unicode text strings as defined in this specification are sequences of Unicode scalar values, the number of which is taken as the length of such a text string. The data quality "pattern" takes a string value that is interpreted as an regular expression in Unicode mode that constrains the string (note that these are not anchored by default, so unless ^ and $ anchors are employed, ECMA-262 regular expressions match any string that contains a match). The JSO proposals acknowledge that regular expression support is rather diverse in various platforms, so the suggestion is to limit them to:

• characters;
• character classes in square brackets, including ranges; their complements;
• simple quantifiers *, +, ?, and range quantifiers {n}, {n,m}, and {n,};
• grouping parentheses;
• the choice operator |;
• and anchors (beginning-of-input ^ and end-of-input $).

Note that this subset is somewhat similar to the subset introduced by I-Regexps , which however are anchored regular expressions, and which include certain backslash escapes for characters and character classes. The additional data quality "format" can take one of the following values. Note that, at an information model level, the presence of this data quality changes the type from being a simple text string to the abstract meaning of the format given (i.e., the format "date-time" is less about the specific syntax employed in than about the usage as an absolute point in civil time).

• "date-time", "date", "time": An date-time, full-date, or full-time, respectively.
• "uri", "uri-reference": An URI or URI Reference, respectively.
• "uuid": An UUID.

type "boolean" The type "boolean" can take the values "true" or "false".

type "array" The type "array" is associated with arrays as they are available in JSON. The additional quality "items" gives the type that each of the elements of the array must match. The number of elements in the array can be constrained by the additional data qualities "minItems" and "maxItems", which are inclusive bounds. The additional data quality "uniqueItems" gives a Boolean value that, if true, requires the elements to be all different.

type "object" The type "object" is associated with maps, from strings to values, as they are available in JSON. The additional quality "properties" is a map the entries of which describe entries in the specified JSON map: The key gives an allowable map key for the specified JSON map, and the value is a map with a named set of data qualities giving the type for the corresponding value in the specified JSON map. All entries specified this way are optional, unless they are listed in the value of the additional quality "required", which is an array of string values that give the key names of required entries. Note that the term "properties" as an additional quality for defining map entries is unrelated to sdfProperty. For example, to include information about the type of the event in the "overTemperatureEvent" of , the sdfOutputData there could be defined as follows:

Using object type with sdfOutputData

Implementation notes JSO-based keywords are also used in the specification techniques of a number of ecosystems, but some adjustments may be required. For instance, is based on Swagger 2.0 which appears to be based on "draft-4" (also called draft-5, but semantically intended to be equivalent to draft-4). The "exclusiveMinimum" and "exclusiveMaximum" keywords use the Boolean form there, so on import to SDF their values have to be replaced by the values of the respective "minimum"/"maximum" keyword, which are themselves then removed; the reverse transformation applies on export.

Composition Examples This informative appendix contains two examples illustrating different composition approaches using the sdfThing quality.

Outlet Strip Example

Outlet Strip Example

Refrigerator-Freezer Example

Refrigerator-Freezer Example

Some Changes From Earlier Drafts This appendix is informative. The present document provides the SDF base definition. Previous revisions of SDF have been in use for several years, and both significant collections of older SDF models and older SDF conversion tools are available today. This appendix provides a brief checklist that can aid in upgrading these to the standard.

• The quality unit was previously called units.
• sdfType was developed out of a concept previously called subtype.
• sdfChoice is the preferred way to represent JSO enum (only a limited form of which is retained), and also the way to represent JSO anyOf.
• the length of text strings (as used with minLength/maxLength constraints) was previously defined in bytes. It now is defined as the number of characters (Unicode scalar values, to be exact); a length in bytes is not meaningful unless bound to a specific encoding, which might differ from UTF-8 in some ecosystem mappings and protocol bindings.

Acknowledgements This specification is based on work by the One Data Model group.

Contributors Hochschule Emden/Leer

Germany jan.romann@hs-emden-leer.de

Cascoda Ltd.

Threefield House Threefield Lane Southampton United Kingdom w.vanderbeek@cascoda.com