A Network Function instance can be deployed such that several network function instances are present within an NF Set to provide distribution, redundancy and scalability together as a Set of NF instances. The same is also supported for NF Services. This can be achieved when the equivalent NFs and NF Services share the same context data or by Network Function/NF Service Context Transfer procedures as specified in clause 4.26 of TS 23.502
: A NF can be replaced by an alternative NF within the same NF Set in case of scenarios such as failure, load balancing, load re-balancing.
Such a network reliability design shall work in both communication modes, i.e. Direct Communication and Indirect Communication. In the Direct Communication mode, the NF Service consumer is involved in the reliability related procedures. In Indirect Communication mode, the SCP is involved in the reliability related procedures.
Equivalent Control Plane NFs may be grouped into NF Sets, e.g. several SMF instances are grouped into an SMF Set. NFs within a NF Set are interchangeable because they share the same context data, and may be deployed in different locations, e.g. different data centers.
In the case of SMF, multiple instances of SMFs within an SMF Set need to be connected to the same UPF:
If the N4 association is established between a SMF instance and an UPF, each N4 association is only managed by the related SMF instance.
If only one N4 association is established between a SMF Set and an UPF, any SMF in the SMF Set should be able to manage the N4 association with the UPF.
Furthermore, for a given UE and PDU Session any SMF in the SMF Set should be able to control the N4 session with the UPF (however, at any given time, only one SMF in the SMF Set will control the UPF for a given UE's PDU Session).
A Control Plane NF is composed of one or multiple NF Services. Within a NF a NF service may have multiple instances. These multiple NF Service instances can be grouped into NF Service Set if they are interchangeable with each other because they share the same context data.
: The actual mapping of instances to a given Set is up to deployment.
The NF producer instance is the NF instance which host the NF Service Producer. When the NF producer instance is not available, another NF producer instance within the same NF Set is selected.
For Direct Communication mode, the NF Service consumer may subscribe to status change notifications of NF instance from the NRF. If the NF Service consumer is notified by the NRF or detects by itself (e.g. request is not responded) that the NF producer instance is not available anymore, another available NF producer instance within the same NF Set is selected by the NF Service consumer.
For Indirect Communication mode, the SCP or NF Service consumer may subscribe to status change notifications of NF instance from the NRF and selects another NF producer instance within the same NF Set if the original NF producer instance serving the UE is not available anymore.
: It is up to the implementation on how the SCP knows a NF producer instance is not available anymore.
When multiple NF Service instances within a NF Service Set are exposed to the NF Service consumer or SCP and the failure of NF Service instance is detected or notified by the NRF, i.e. it is not available anymore, the NF Service consumer or SCP selects another NF Service instance of the same NF Service Set within the NF instance, if available. Otherwise the NF Service consumer or SCP selects a different NF instance within the same NF Set.
: The NF Producer instance can change the NF Service instance in the response to the service request.
When multiple NF Service instances within a NF Service Set are exposed to the NF Service consumer or SCP as a single NF Service, the reliability, i.e. the selection of an alternative NF Service instance is handled within the NF instance.