6.3.1.0 Principles for Binding, Selection and Reselection

Binding can be used to indicate suitable target NF producer instance(s) for NF service instance selection, reselection and routing of subsequent requests associated with a specific NF producer resource (context) and NF service. This allows the NF producer to indicate that the NF consumer, for a particular context, should be bound to an NF service instance, NF instance, NF service set or NF set depending on local policies and other criteria (e.g. at what point it is in the middle of a certain procedure, considering performance aspects etc).

Binding can also be used by the NF consumer to indicate suitable NF consumer instance(s) for notification target instance reselection and routing of subsequent notification requests associated with a specific notification subscription and for providing Binding Indication for service(s) that the NF consumer produces for the same data context and the NF service producer is subsequently likely to invoke.

The Binding Indication contains the information in Table 6.3.1.0-1.

The Routing Binding Indication may be included in Request, Subscribe or Notification messages (see clause 7.1.2). It may be used in the case of indirect communication by the SCP to route the message. The Routing Binding Indication is a copy of the information in the Binding Indication and also contains the information in Table 6.3.1.0-1.

NOTE 1: Subscription request messages can contain both a Binding Indication and a Routing Binding Indication.

The NF service producer may provide a Binding Indication to the NF service consumer as part of the Direct or Indirect Communication procedures, to be used in subsequent related service requests. The level of Binding Indication provided by the NF service producer to the NF consumer indicates if the resource in the NF service producer is either bound to NF service instance, NF instance, NF Service Set or NF set as specified in Table 6.3.1.0-1. The Binding Indication may include NF Service Set ID, NF Set ID, NF instance ID, or NF service instance ID, for use by the NF consumer or SCP for NF Service Producer (re-)selection. If the resource is created in the NF Service Producer, the NF Service Producer provides resource information which includes the endpoint address of the NF service producer. For indirect communication, the NF service consumer copies the Binding Indication into the Routing Binding Indication in Request or Subscribe message, unless the NF service consumer performs a reselection for indirect communication without delegated discovery.

During explicit or implicit notification subscription, a Binding Indication may be provided by the NF service consumer to NF service producer; the NF service consumer will also provide a Notification Endpoint. The NF service consumer may also provide a Binding Indication in response to notification requests. The level of Binding Indication provided by the NF service consumer to the NF service provider indicates if the Notification Endpoint is either bound to NF service instance, NF instance, NF Service Set or NF set as specified in Table 6.3.1.0-1. The Binding Indication shall include at least one of NF Set ID, NF instance ID, NF Service Set ID and/or NF service instance ID and may also include the service name. The NF Service Set ID, NF service instance ID and service name relate to the service of the NF service consumer that will handle the notification.

NOTE 2: The NF service can either be a standardised service as per this specification or a custom service. The custom service can be used for the sole purpose of registering endpoint address(es) to receive notifications at the NRF.

The Binding Indication is used by the NF service producer as notification sender to reselect an endpoint address and construct the Notification Endpoint, i.e. the URI where the notification is to be sent, e.g. if the provided Notification Endpoint of the NF service consumer included in the subscription cannot be reached, according to the following:

• If the service name in the Binding Indication is omitted and the binding for notification is on NF Set or NF Instance level, the endpoint address registered in the NRF at NF Profile level of the NF(s) selected according to the Binding Indication shall be used to construct a new Notification Endpoint.
• If the service name is included in the Binding Indication, an endpoint address registered in the NRF for that service in the NF profile(s) selected according to the Binding Indication shall be used to construct a new Notification Endpoint.

For indirect communication, the NF service producer copies the Binding Indication into the Routing Binding Indication that is included in the Notification request, to be used by the SCP to discover an alternative endpoint address and construct a Notification Endpoint e.g. if the Notification Endpoint that the request targets cannot be reached, according to the following:

• If the service name in the Routing Binding Indication is omitted and the binding for notification is on NF Set or NF Instance level, the endpoint address registered in the NRF at NF Profile level of the NF(s) selected according to the Binding Indication shall be used to construct a new Notification Endpoint.
• If the service name is included in the Routing Binding Indication, an endpoint address registered in the NRF for that service in the NF profile(s) selected according to the Binding Indication shall be used to construct a new Notification Endpoint.

For subscription to notifications via another network function, a separate Binding Indication for subscription related events may be provided by the NF service consumer (see clause 4.17.12.4 of TS 23.502 [3]) and if provided shall be associated with an applicability indicating notification for subscription related events.

If the NF as an NF consumer provides a Binding Indication for services that the NF produces in service requests, the Binding Indication shall be associated with an applicability indicating other service and may contain the related service name(s), in addition to the other parameters listed in Table 6.3.1.0-1. If no service name(s) are provided, the Binding Indication relates to all services that the NF produces.

For NF Set or NF Instance level of binding, a Binding Indication for notifications and other services may be combined if it relates to the same service and that combined Binding Indication shall then be associated with an applicability indicating all scenarios that the Binding Indication relates to (For this purpose, the applicability can indicate a combination of values).

If no applicability is indicated in a request or subscribe messages, a Binding Indication in that messages is applicable for notification to all events except for the subscription related event (see clause 4.17.12.4 of TS 23.502 [3]).

NOTE 3: Such a request message can be used for implicit subscription.

NOTE 4: Request messages can contain both the Binding Indications for services and for notifications and in addition, the Routing Binding Indication in the case of indirect communication.

A Binding Indication may be shared by a group of resources (e.g. contexts) identified by a group identifier. This enables a NF Service consumer or producer to update the binding indication for this group of resources in a single request or notification towards a given peer NF service instance, e.g. when a group of resources need to be taken over by a different NF within an NF set. See clause 6.12.1 of TS 29.500 [49].

Table 6.3.1.0-1 defines the selection and reselection behaviour of NF services consumers and SCPs depending on the Binding Indication provided by an NF service producer. The detailed procedures refer to clause 4.17.11 and 4.17.12 of TS 23.502 [3].

Table 6.3.1.0-1: Binding, selection and reselection