[MB-SMF] Handle mbsSession.activityStatus for Multicast MBS Sessions. #27
Description
The mbsSession.activityStatus is only relevant when the MBS Session is for Multicast and is used to indicate the Active or Inactive state of the MBS Session multicast when the Session is in an established state.
TS 23.247 Figures 4.3-1 and 4.3-2 show the state machine for a Multicast MBS Session.
When first created the Multicast MBS Session will be in a Configured state. In this state a change in the activityStatus has no effect other than to change the state the Multicast MBS Session will enter when it becomes established. In this state no packets are sent via the shared MBS traffic delivery method to the gNodeB.
When the first UE joins an MBS Session multicast, the Multicast MBS Session will become established. At this point the state transitions to either Active or Inactive. In both established states the MB-UPF will be instructed to forward the multicast packets it may be receiving via the UDP tunnel to the gNodeB via the shared MBS traffic delivery method.
In the Active state the MB-SMF will instruct the AMF to configure the gNodeB to transmit the packets it is now receiving via the shared MBS delivery to the UE.
In the Inactive state the MB-SMF will instruct the AMF to configure the gNodeB not to deliver any packets to the UE.
The transition from Active to Inactive and vice-versa is controlled by the NF (usually MBSF or other AF). It may be transitioned into the Active state by the reception of data at the MB-UPF UDP tunnel from the MBSTF. However for this initial implementation the automatic transition to Active will not be implemented and can be added later.
If the last UE subscribed to a Multicast MBS Session unsubscribes, then the Multicast MBS Session will transition back to the Configured state. At this transition the MB-UPF will be instructed to stop forwarding packets for the MBS Session via the shared MBS traffic deliver method. The AMF will also be instructed to configure the gNodeB not to deliver any packets to the UE for this MBS Session.
If the MBS Session is deleted then the session will be removed from the MB-UPF and the AMF will also be instructed to configure the gNodeB not to deliver any packets to the UE for this MBS Session.