Angaza Nexus Channel Core


Interoperability for off-grid devices

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Nexus Channel Core - Resource Type Specification

Version: 0.9.0


This document outlines the subset of CBOR functionality that is required for Nexus Channel Core devices to understand data payloads representing valid resource models.

This document also outlines the structure and format of resource type definitions.

From a developer’s perspective, this document provides the information required to implement (or select) a CBOR serializer or deserializer, as well as design a Nexus Channel Core resource type/model that will be understood by other devices.

CBOR Restrictions

Nexus Channel Core resources may consist of the following CBOR data types:

Note that an array in Nexus Channel Core must contain only the following types, and may not contain other nested arrays:

This implies that a valid Nexus Channel Core CBOR parsing layer must be capable of handling any of the above CBOR data types, and that no resource type may specify a property that cannot be represented by one of the above CBOR data types.

Floating-point numbers (IEEE 754) are not supported.

The Content-Type of any valid Nexus Channel Core resource when represented as CBOR is always application/vnd.ocf+cbor (value 10000).

Defining a New Resource: Resource Type (rt and rte)

All resources must have a “Resource Type” (rt) property, which follows the format specified in [OCF Core Specification V2.1.1

Regardless of the URI that a resource instance is hosted on, the resource type precises defines what properties that resource must expose, and how those properties may be interacted with (RETRIEVE/GET and/or UPDATE/POST).

Additionally, once a resource is accepted to the Nexus Channel Core resource model registry it will be assigned an arbitrary “Resource Type Enumerator” (‘rte’) value, an unsigned integer value that is unique to that resource. This is meant to reduce the need to store static string “Resource Types” in devices with limited memory.

Devices hosting a resource that has an assigned ‘rte’ must expose it as part of the resource’s properties on a “GET” request. Devices may also expose the ‘rt’ string value if available.

A resource must have exactly one Resource Type (‘rt’).

Defining a New Resource: Properties

The properties of a resource define values that are read from or written to the resource. Each property has a name, and may be ‘required’ by every implementation of the resource, or may be ‘optional’.

For example, the Nexus Channel Core Battery resource only requires the ‘charge’ (cg) percentage 0-100% to be implemented, but defines a number of other optional properties to expose if available.

The OpenAPI specification document must include the following for each property in a resource:

For array properties, the following properties must additionally be defined:

Any property may include the readOnly or writeOnly attribute if desired.

Other OpenAPI properties may be used to clarify the specification.

Typically, resource properties are defined as objects in the OpenAPI spec file within the ‘definitions’ section, and referenced elsewhere in the specification using the $ref tag.

Defining a New Resource: Total Size

It is recommended that all Nexus Channel Core resources do not return more than 100 bytes (in CBOR representation) when responding to a GET or POST request, for maximum compatibility on networks with smaller packet transmission size.

100 bytes allows room for lower-layer protocol overhead (headers, check fields), the CoAP header, and addressing information (source, destination) while still remaining below 127 bytes total, the maximum payload size of 802.15.4 networks.

To remain below this recommended limit, specifying short property names to aid implementers (e.g. ‘lv’ instead of ‘level’) may be helpful, as long as the entire unabbreviated property name is clearly indicated in the resource type specification.

Defining a New Resource: Path

The path indicate thes recommended URI for the resource (e.g. ‘/my/resource/path’) and whether the resource supports GET (retrieve values), POST (update values), or both. A given resource has exactly one path.

It is recommended to avoid URIs beginning with nx/, as these will overlap with Nexus Channel security-specific resources.

However, as a device may host multiple instances of a specific resource type, and there may be conflicting recommended resource URIs, there is no guarantee that the recommended URI will be used on all implementing devices.

For both GET and POST requests, the possible responses (CoAP status code and content) must be specified. For example, a simple read-only resource might always return a status code ‘200’ while including all property values in the response body.

If the resource can return an error code (e.g. 405, 401, 400) the OpenAPI spec file must indicate an description of what is required to succeed after seeing this error.

The required payload must be specified for POST requests.

Note that a device might host multiple instances of a specific resource type, so there is no guarantee that the recommended URI will be used for every instance of a given resource in field devices.

Instead, devices rely on resource discovery to determine what resources another device is hosting, and what URIs to use to access those resource instances.

Defining a New Resource: Resource Interface (if)

OCF specifies an interface property (if) which is required for every compliant resource. This interface allows for different ‘views’ of the same resource, so that a GET request might return different results if a different interface string was provided in a CoAP query parameter.

However, Nexus Channel Core encourages use of a limited subset of interfaces, and does not require implementations to support this query parameter feature.

In other words, it is acceptable to define one ‘interface’ that is always used by a resource type, and implement the resource such that no ‘interface selection’ by query parameters is supported.

Nexus Channel Core requires that the if (interface) property is present in the YAML resource model, and must specify at least one of the two interfaces below:

If a resource type uses one of the above two interface types, implementing devices may exclude the if property from the on-device instance of the resource.

If a resource type does not use one of the above two interface types, implementing devices must always include the if property from the on-device instance of the resource.

Other OCF interface types, like oic.if.baseline, may be specified, but there is no guarantee that other Nexus Channel Core devices will understand or honor that interface, and one of the above ‘primary’ interface types must also be defined.

Resource Discovery

Note: This section describes specified functionality that is not yet available in the reference implementation, but is committed for a future release.

A device may discover resources on another device by making a CoAP GET request to the special URI nx/res, which must elicit a response with an array, one element for each resource hosted by the device. Each element in the array is a map containing the following keys:

Each resource entry in the array may also contain:

No other properties/keys may be present.

An example response in human-readable CBOR diagnostic format is:

  {"href": "/batt1", "rtr": 763},
  {"href": "/batt2", "rtr": 763},
  {"href": "/tamper", "rtr": 628},
  {"href": "/grinder", "rtr": 902, "rt": "", "if": ["", "oic.if.baseline"]}

The above response indicates to the requester that the device is hosting four total resources. There are two resources of rtr type 763 at URIs batt1 and batt2 (likely a system with multiple batteries being monitored separately), a resource of rtr type 628 at URI tamper, and a resource of type (rtr 902) at URI grinder, which also supports OIC interface queries (an optional OCF feature not required by Nexus Channel Core devices).

A typical use of the nx/res discovery resource is:

  1. New device enters the local network
  2. New device determines what other nodes are present (using lower layer protocol)
  3. New device sends CoAP GET to nx/res at each other node (multicast or unicast)
  4. Each other node sends a reply, indicating their resources to the new device
  5. New device determines whether to further interact with any other device resource based on application logic.

The nx/res resource is functionally similar to to the OCF standard oic/res discovery resource, except providing a minimal subset of information.

Nexus Channel Core devices are not required to implement the oic/res resource.

Reference: Nexus Channel Core Resource Type Registry

A complete registry of all Nexus Channel Core resource types (and a mapping to the associated rtr value and specification) can be found here.

The ‘Resource Type Registry’ is a list of validated, publicly available resource type definitions to promote interoperability between Nexus Channel Core devices.

Anyone may contribute new resource types to this repository, as long as they meet the requirements listed in this document, and are not ‘functional duplicates’ of existing resources (in which case, proposing updates to the existing resource may be more appropriate).

Contributing a resource type makes it openly, publicly available for other parties to use. Please see the Contributor License Agreement for more details.

Example: Compliant CBOR Payload

Below is an example of the CBOR payload received in response to a CoAP “GET” request to a compliant Nexus Channel Core resource.

This 99-byte response is larger than a typical resource might provide, but is used to illustrate a variety of supported data types and format.

0xbf, 0x62, 0x72, 0x74, 0x9f, 0x78, 0x18, 0x61, 0x6e, 0x67, 0x61, 0x7a,
0x61, 0x2e, 0x63, 0x6f, 0x6d, 0x2e, 0x6e, 0x65, 0x78, 0x75, 0x73, 0x2e,
0x6c, 0x69, 0x6e, 0x6b, 0x2e, 0x68, 0x73, 0xff, 0x63, 0x72, 0x74, 0x65,
0x19, 0x02, 0x0a, 0x62, 0x69, 0x66, 0x9f, 0x69, 0x6f, 0x69, 0x63, 0x2e,
0x69, 0x66, 0x2e, 0x72, 0x77, 0xff, 0x62, 0x63, 0x44, 0x50, 0x40, 0xe2,
0x01, 0x00, 0x40, 0xe2, 0x01, 0x00, 0x8d, 0xd0, 0x80, 0xd0, 0x8e, 0x18,
0x38, 0xc4, 0x62, 0x72, 0x44, 0x40, 0x62, 0x74, 0x49, 0x00, 0x62, 0x74,
0x54, 0x19, 0x01, 0x2c, 0x62, 0x73, 0x4c, 0x81, 0x00, 0x62, 0x73, 0x43,
0x81, 0x00, 0xff

The above list of bytes can be pasted into, which will display the human-readable representation. Below, we will also step through the meaning of each portion of this payload.

First, the payload begins with 0xbf, opening an indefinite map (a list of key/value pairs). Every OCF resource consists of a map of key/value pairs (one for each ‘property’ that the resource contains).

Common Property: rt

627274 indicates the first key, a property named rt. This is the ‘resource type’ of this resource.

9F7818616E67617A612E636F6D2E6E657875732E6C696E6B2E6873FF is the value associated with this key. It is an array containing one element, the text string “”. (If present, the rt field is contained within an array to remain compatible with the OCF specifications).

All resources of rt “” must provide a defined set of properties, which we should expect to see as we continue decoding this payload. This particular resource is used to enable Nexus Channel linking features (on devices which support Nexus Channel secure links, e.g. devices that have this resource).

Common Property: rtr

63727465 is the key for property rtr. This is the ‘resource type registry’ value of this resource. It is an integer assigned to the specific rt for this resource. In most cases, devices will only expose the rtr property (and not the rt string property), for a given resource.

19020A is the value. This decodes to integer value ‘522’.

Common Property: if

626966 is the key for property if. This is the OCF-mandated array of supported interfaces for this resource.

9F696F69632E69662E7277FF is the value, which is an array containing the a single element - the text string This indicates that the resource has properties that are writeable.

Property cD

626344 is the key for property cD, the ‘challenge data’ used during link handshakes.

5040E2010040E201008DD080D08E1838C4 is the value. 50 begins a 16-byte long bytestring, and the remainder bytes are the bytestring itself.

(as a string: “@\xE2\x01\x00@\xE2\x01\x00\x8D\xD0\x80\xD0\x8E\x188\xC4”)

Property rD

627244 is the key for property rD, the ‘response data’ used during link handshakes.

40 is the value, indicating an empty bytestring.

Property tI

627449 is the key for property tI, the time since the handshake started.

00 is the value, an integer 0.

Property tT

627454 is the key for property tT, the fixed timeout value for handshakes.

19012C is the value, an integer 300.

Property sL

62734C is the key for property sL, the supported Nexus Channel link security modes.

8100 is an array containing one value, 0. Note that in CBOR, short arrays (up to 23 elements long) carry very little overhead (81 indicates ‘an array with 1 data item’, and the only remaining information is the single value in this array).

Property sC

727343 is the key for property sC, the supported Nexus Channel link handshake modes.

8100 is an array containing one value, 0.

The following FF terminates the indefinite map that was opened with the very first byte, BF, and ends the payload.