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AMWA BCP-004-01: NMOS Receiver Capabilities

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In IS-04, a Receiver resource expresses the capabilities of a Receiver through attributes that identify constraints on streams of compatible Senders.

Receivers indicate their transport and format. These attributes express constraints that can be evaluated against the related attributes of a Sender and its Flow.

The Receiver caps object is provided as an extensible mechanism to define finer-grained constraints.

IS-04 itself defines caps attributes for media_types (since v1.1) and, for data Receivers, also event_types (since v1.3). Both these attributes express constraints that can be evaluated against Flow attributes, as arrays whose elements define the alternatives that are acceptable. In each case, the constraint is satisfied when the target Flow attribute matches any of the enumerated alternatives.

When caps contains multiple attributes, i.e. both media_types and event_types, the Receiver indicates that it only accepts streams that satisfy all of (both!) the constraints.

This specification defines a new constraint_sets attribute for the Receiver caps object, which can also be combined with the existing ones. In common with the existing attributes, its value is an array of alternatives; this constraint is satisfied when any of its enumerated Constraint Sets are satisfied.

This specification defines a generic JSON syntax to express Constraint Sets made up of individual Parameter Constraints. The Constraint Set is satisfied if all of its Parameter Constraints are satisfied.

The representation of individual Parameter Constraints resembles the mechanism defined by IS-05 to constrain Sender and Receiver transport parameters at the /constraints endpoints, which Nodes and Controllers may also support.

This specification also defines a version attribute to indicate when changes to the caps object took place.

The constraints_set and version attributes are listed in the Capabilities parameter register in the NMOS Parameter Registers.

Use of Normative Language

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.

Defining Parameter Constraints

A specification for each Parameter Constraint is strongly RECOMMENDED to be listed in the Capabilities register in the NMOS Parameter Registers. Each specification defines a unique identifier, the constraint type, and the target parameter, as follows.

Parameter Constraint Identifiers

Each Parameter Constraint is given a unique identifier - a URN. Parameter Constraints defined by the AMWA have the form:

urn:x-nmos:cap:<category>:<constraint>

Two <category> names are initially defined, format for media-related constraints, and transport for transport-related constraints.

For example, urn:x-nmos:cap:format:grain_rate is a Parameter Constraint relating to the Grain rate of the stream, for example by targeting the grain_rate attribute of an IS-04 Flow. (The concept of Grains is defined in the JT-NM Reference Architecture.)

The <category> name meta is reserved for metadata related to Constraint Sets. These attributes are not constraints themselves.

Manufacturers MAY use their own namespaces to indicate Parameter Constraints which are not currently defined within the NMOS namespace (urn:x-nmos:cap:).

Parameter Constraint Types

The specification defines the JSON value type to which the constraint relates, which MUST be one of:

The type of the constraint defines which Constraint Keywords are allowed when the Parameter Constraint is instantiated.

Parameter Constraint Target

The specification defines the target parameter against which the constraint is to be evaluated, for example, the specific IS-04 Flow attribute or SDP format-specific parameter.

Especially in the case of parameters carried in non-JSON formats, such as a transport file, the specification MUST also describe how to map the parameter value to one of the supported JSON types.

Instantiating Parameter Constraints

The Receiver expresses its capabilities with respect to a particular Parameter Constraint by including the constraint’s unique identifier as an attribute in a Constraint Set with an object value, the attributes of which depend on the specified type.

For example:

"urn:x-nmos:cap:format:sample_depth": {
  "enum": [ 24, 20, 16 ]
}

Constraint Keywords

Each Parameter Constraint is instantiated as an object with attributes that are type-specific Constraint Keywords defining how the parameter is constrained.

The Parameter Constraint is satisfied if all of the constraints expressed by the Constraint Keywords are satisfied. This implies that if any of the constraints expressed by the Constraint Keywords are not satisfied, the Parameter Constraint is not satisfied. Equally, when there are no Constraint Keywords, the Parameter Constraint thus explicitly indicates that the target parameter is unconstrained.

Note that in some cases, the target parameter will allow future addition of new values, for example by inclusion in the Flow Attributes register in the NMOS Parameter Registers. It could therefore be better to explicitly constrain the target parameter to all supported values, if new values could cause an issue.

Common Constraint Keywords

The following attributes are allowed for all constraint types:

String Constraint Keywords

Nothing additional

Integer and Number Constraint Keywords

The following attributes are additionally allowed for integer and number constraints:

Boolean Constraint Keywords

Nothing additional

Rational Constraint Keywords

The following attributes are additionally allowed for rational constraints:

Note that comparison between two rational values, n1 / d1 and n2 / d2, SHOULD be performed by cross-multiplication and comparison of the products, n1 * d2 and n2 * d1, taking account of negative denominators.

Constraint Sets

Constraint Sets MUST be instantiated with one or more Parameter Constraints. The Constraint Set is satisfied if all of its Parameter Constraints are satisfied. This implies that if any of the Parameter Constraints are not satisfied, the Constraint Set as a whole is not satisfied.

The Constraint Set is represented as a JSON object with attributes that are the Parameter Constraints.

Constraint Set Metadata

Additional metadata about each Constraint Set MAY be included, using the metadata attributes listed in the Capabilities register in the NMOS Parameter Registers with the following unique identifiers:

urn:x-nmos:cap:meta:<attribute>

Constraint Set Label

The metadata attribute urn:x-nmos:cap:meta:label MAY be used to provide a human-readable name for the Constraint Set as a simple string value. If a Receiver uses this attribute, it SHOULD do so for all Constraint Sets.

A Controller MAY use this label to indicate to a user which Constraint Sets of a Receiver are satisfied by a Sender.

Constraint Set Preference

The metadata attribute urn:x-nmos:cap:meta:preference enables the Receiver to indicate its preference between the listed Constraint Sets.

A Controller MAY use Receiver preference, for example, to assist the user in choosing between multiple Senders to connect to a Receiver.

The preference is indicated as an integer value in the range from -100 to 100 inclusive.

If the Receiver has no preference, it MAY omit the attribute from all Constraint Sets.

When a Receiver wants to indicate preference, it adds the attribute either to all Constraint Sets or only to some, in which case the other Constraint Sets have an effective value of 0. The Receiver indicates its strongest preference for one or more Constraint Sets by assigning them its highest value. It indicates preference against, or weaker preference for, Constraint Sets by assigning them lower values.

For example:

Constraint Set Enabled

The metadata attribute urn:x-nmos:cap:meta:enabled MAY be used to indicate Constraint Sets which do not apply to the current operating configuration of a Receiver, but which can be enabled via some unspecified configuration mechanism.

A Controller MUST NOT take into consideration a Constraint Set that has this attribute set to false, unless the Controller is capable of making the required configuration changes. Controllers MAY use this attribute as a hint to users that a Sender and Receiver could be connected subject to a reconfiguration.

If a Constraint Set is enabled or the Receiver does not support offline capabilities then this attribute MAY be omitted.

Listing Constraint Sets

The Receiver advertises a list of Constraint Sets as a JSON array of these objects, using the key constraint_sets in the caps object.

The constraint_sets as a whole is satisfied if any of the listed Constraint Sets are satisfied. When the list is empty, or none of the Constraint Sets are satisfied, the constraint_sets as a whole is thus not satisfied.

Several worked examples are provided in the Examples section.

Validating Parameter Constraints and Constraint Sets

This specification includes a JSON Schema for each Parameter Constraint Type and for Constraint Sets and the constraint_sets attribute as a whole, in the APIs/schemas directory.

The Capabilities register in the NMOS Parameter Registers includes a supplementary schema that validates the specific requirements for every Parameter Constraint listed in the register.

Capabilities Version

IS-04 requires that the core resource version is updated when any attributes of the caps object or resource as a whole are changed.

Other attributes of the Receiver resource are likely to be updated more often than caps, for example the subscription attribute is updated to reflect a new connection. However, the capabilities of a Receiver could change over its lifetime, for example, as a result of reconfiguration by some other means. This specification therefore defines a version attribute for the caps object itself, which reflects only when that object last changed.

Behaviour: Receivers

In order to use the finer-grained constraints mechanism defined by this specification, Receivers MUST include both the constraint_sets and version attributes in the caps object.

Receivers SHOULD express their capabilities as precisely as possible, using the relevant Parameter Constraints listed in the Capabilities register in the NMOS Parameter Registers. However, this specification may not be sufficiently expressive to indicate every type of stream that a Receiver can or cannot consume successfully. It is entirely possible that a Receiver may fail to consume a stream even if the Receiver’s advertised Constraint Sets indicate that it can.

The value of the constraint_sets attribute MUST be valid according to this specification. The value of all the Constraint Set attributes MUST be valid according to the relevant specification in the Capabilities register in the NMOS Parameter Registers.

The Receiver MUST reflect any change in its capabilities by updating the caps object as appropriate and modifying the version attribute of that object as well as the core resource version.

Behaviour: Controllers

Controllers are strongly RECOMMENDED to support all Parameter Constraints listed in the Capabilities register in the NMOS Parameter Registers that are applicable for the kinds of Receiver with which they interact. However, Controllers MAY ignore individual Parameter Constraints whose unique identifiers they do not recognize. Some Parameter Constraints are only relevant to specific transport and format values or to particular IANA media types. When a Controller cannot evaluate any of the Parameter Constraints in a Constraint Set, that Constraint Set SHOULD be considered to be satisfied, but the Controller MAY distinguish this case for a user.

Controllers SHOULD provide an indication to a user whether a Sender satisfies a Constraint Set of a Receiver, for example in a cross-point matrix view. Controllers MAY allow a user to attempt to make a connection whether the constraint_sets are satisfied or not.

Controllers MAY use the version attribute of the caps object to avoid unnecessary re-evaluation of Receiver capabilities.

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