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Content for  TS 23.247  Word version:  18.7.0

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7.2.2 Multicast MBS Session leave and Multicast MBS Session release procedure  7.2.2.1 General  7.2.2.2 Multicast Session leave requested by the UE  7.2.2.3 Multicast session leave requested by the network or MBS session release  7.2.2.4 Release of shared delivery toward RAN node 
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7.2.2  Multicast MBS Session leave and Multicast MBS Session release procedure  p. 68

7.2.2.1  General  p. 68

This clause describes Multicast MBS Session leave requested by the UE or by the network. This clause also describes Multicast MBS Session release.

7.2.2.2  Multicast Session leave requested by the UE  p. 68

When the UE determines to leave the Multicast MBS session, it shall send PDU session Modification request to inform the 5GC the leaving operation. The Figure 7.2.2.2-1 describes the procedure.
Reproduction of 3GPP TS 23.247, Fig. 7.2.2.2-1: UE initiated Multicast MBS Session leave
Figure 7.2.2.2-1: UE initiated Multicast MBS Session leave
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Step 1.
The UE sends the PDU Session Modification Request when the UE determine to leave the multicast MBS Session. The PDU Session Modification Request carries leave indication and the MBS Session ID which the UE want to leave.
Step 2.
The AMF invokes Nsmf_PDUSession_UpdateSMContext (N1 SM container (PDU Session Modification Request)) to the SMF.
Step 3a.
[Conditional] If 5GC individual MBS traffic delivery is applied towards the UE, the SMF sends an N4 Session Modification Request to the UPF (PSA). The SMF reconfigures the UPF to terminate the distribution of multicast data via the PDU session.
Step 3b.
[Conditional] The UPF (PSA) sends an N4 Session Modification Response to the SMF.
If there are no PDU sessions to transmit the multicast MBS session data in the UPF, and unicast transport is used over N19mb, the UPF releases the DL N19mb tunnel endpoint and informs the SMF.
If there are no PDU sessions to transmit the multicast MBS session data in the UPF, and multicast transport is used over N19mb, the UPF leaves the multicast distribution tree of MB-UPF.
Step 4.
[Conditional] If the UPF indicates the tunnel release (i.e. unicast transport was used), the SMF invokes Nmbsmf_MBSSession_ContextUpdate Request (Release, MBS Session ID, tunnel information) to release the tunnel between UPF and MB-UPF for this multicast MBS session. The MB-SMF determines whether the context update is for tunnel release or create based on whether the tunnel information exists in the multicast MBS Session Context stored in the MB-SMF or not.
Step 5.
[Conditional] If the MB-SMF determines the context update is for tunnel release, the MB-SMF request to MB-UPF to release the tunnel between UPF and MB-UPF for the multicast MBS session.
Step 6.
[Conditional] The MB-SMF responds to the SMF for step 4.
Step 7.
The SMF invokes the Nsmf_PDUSession_UpdateSMContext Response (PDU Session ID, N2 SM information ([MBS Session ID], [leave indication]), N1 SM container) service operation. In the N2 SM information, the MBS Session ID and the leave indication are included for informing the NG-RAN to remove the UE from this MBS session if 5GC Shared MBS traffic delivery method is used towards the UE. If 5GC Individual MBS traffic delivery method is used towards the UE, the N2 SM information does not include MBS related information.
In the N2 SM information, the SMF also informs the NG-RAN to release the associated QoS Flow(s), which carry or intend to carry the multicast MBS session traffic for 5GC individual MBS traffic delivery.
The associated QoS Flow(s) are released as defined in clause 4.3.3.2 of TS 23.502.
Step 8.
The AMF send N2 message (N2 SM information, N1 SM container) to the NG-RAN
Step 9.
The NG-RAN node performs the necessary AN-specific resource modification procedure toward the UE and transports the N1 SM container received in step 7 to the UE.
Step 10.
The NG-RAN node removes the UE from this multicast MBS session and sends a N2 message to the AMF.
Step 11.
The AMF transfers the N2 message received in step 9 to the SMF via the Nsmf_PDUSession_UpdateSMContext service operation.
The SMF updates the associated PDU session context, e.g. remove the MBS Session ID from the associated PDU session context. In addition, if associated QoS flow is used for the multicast MBS session, the SMF also removes the associated QoS flow information associated with the indicated multicast MBS session from the associated PDU session context.
Step 12.
[Conditional] If the UE is the last joined one of the multicast MBS session in the SMF, The SMF also indicates that the last UE served by the SMF leaves the Multicast MBS Session, the SMF unsubscribes the notifications of the MBS Session Context status updates from the MB-SMF by invoking Nmbsmf_MBSSession_ContextStatusUnsubscribe service operation. The MB-SMF will no longer notify the SMF of the further context status updates of the multicast MBS session (e.g. activation, deactivation, update, release, etc.). For multicast transport between MB-UPF and content provider, if the SMF is the last remaining SMF that is subscribed for the MBS Session notification from the MB-SMF, i.e. if it is the last UE leaving the MBS session, the MB-SMF requests the MB-UPF to stop forwarding the multicast MBS session data and may request the MB-UPF to leave the multicast tree towards the AF/MBSF, if the MB-UPF joins the multicast tree when the first UE joins the MBS session.
Step 13.
[Conditional] If the UE is the last UE in this RAN node for this multicast MBS session, the NG-RAN release shared delivery between NG-RAN and MB-UPF as described in clause 7.2.2.4.
If release of the PDU Session associated with a multicast MBS session is triggered, corresponding procedures between UE, NG-RAN, AMF, and SMF are performed as described in clause 4.3.4 of TS 23.502, and SMF triggers the UE leave the multicast MBS session by performing steps 3-6 in Figure 7.2.2.2-1 for each multicast MBS session(s) associated with the PDU Session, and UE considers as left all the multicast MBS sessions associated with the PDU Session.
If the UE deregistration procedure is executed, corresponding procedures between UE, NG-RAN, AMF, and SMF are performed as described in clause 4.2.2.3 of TS 23.502, and SMF performs steps 3-6 in Figure 7.2.2.2-1 for all multicast MBS sessions joined by the UE. When the PDU Session Release procedure or UE deregistration procedure is executed, according to the UE context, NG-RAN performs step 10 for each multicast MBS session associated with the released PDU Session(s).
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7.2.2.3  Multicast session leave requested by the network or MBS session release  p. 70

This procedure applies to the following scenarios:
  1. When the MB-SMF decides to release an MBS Session:
    • based on a request from the AF (directly or via the NEF/MBSF);
    In this scenario, the MB-SMF notifies the SMF of multicast session release, and the SMF initiates procedures to remove all joined UEs from the MBS session.
  2. When the SMF decides to remove a UE from an MBS session:
    • based on a request from the UDM (subscription change); or
    • due to local and location dependent MBS service is described in clause 7.2.4; or
    • due to network internal reasons.
For the active MBS session, to release radio resources as early as possible, the MB-SMF may trigger Multicast Session Deactivation towards the NG-RAN as specified in steps 5-9 of clause 7.2.5.3, prior to or in parallel with triggering MBS Session Release to the SMF.
Reproduction of 3GPP TS 23.247, Fig. 7.2.2.3-1: MBS Session Release or Multicast session leave requested by the network
Figure 7.2.2.3-1: MBS Session Release or Multicast session leave requested by the network
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Step 1a.
For MB-SMF triggered MBS session release, the SMF receives Nmbsmf_MBSSession_ContextStatusNotify (MBS Session ID, multicast session release) from the MB-SMF with MBS Session ID. The SMF checks all joined UEs and perform step 2 to step 9 for each UE.
Step 1b.
The SMF decides to remove a UE from the MBS session without MBS session release (e.g. due to UE moving out of MBS service area for local or location dependent MBS service as described in clause 7.2.4).
Step 2.
For UEs without activated UP, the SMF may perform the same procedure as defined in step 3-7 in clause 7.2.5.2.
Alternatively, for UEs without activated UP, the SMF does not trigger message to the AMF, instead the SMF marks that the UE is to be informed of the MBS Session release. In this case, the SMF initiates PDU Session Modification to inform the UE of the MBS Session release at next UP activation of the associated PDU Session, if needed.
Step 3.
For the joined UEs with UP activated, the SMF invokes Namf_Communication_N1N2MessageTransfer to the AMF. The N1 SM container indicates UE removed from MBS session with appropriate cause (e.g. MBS session release, out of MBS service area, etc.). In N2 SM information, the SMF informs the NG-RAN to remove the UE from the MBS session. If there are associated QoS Flow(s) for individual delivery, the SMF also releases those QoS Flow(s) as specified in clause 4.3.3.2 of TS 23.502.
Step 4.
The AMF sends N2 Request to the NG-RAN.
Step 5.
The NG-RAN transports the N1 SM container (PDU Session Modification Command (MBS Session ID, UE removed from MBS session with appropriate cause)) to the UE.
Step 6.
The NG-RAN performs radio resource modification. If there are no joined UEs in the MBS session, the NG-RAN releases the radio resources.
Step 7.
If there are no joined UEs in the MBS session, for unicast transport of N3mb, the NG-RAN initiates the DL tunnel release towards MB-UPF via AMF and MB-SMF. For multicast transportation of N3mb, the NG-RAN performs IGMP/MLD Leave for the MBS session. See clause 7.2.2.4 for details.
Step 8.
The NG-RAN sends N2 Response to the AMF. If there are no joined UEs in the MBS session, the MBS Session Context is removed from the NG-RAN.
Step 9.
The AMF transfers the N2 message received in step 8 to the SMF via the Nsmf_PDUSession_UpdateSMContext service operation. The SMF removes the UE from the MBS Session.
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7.2.2.4  Release of shared delivery toward RAN node  p. 72

In the following case, the shared delivery tunnel may be released between NG-RAN and MB-UPF:
  • The last UE is excluded from the context of the multicast MBS session in the NG-RAN node;
  • Handover to the target NG-RAN when the UE is the last UE for this multicast MBS session in the source NG-RAN node during handover preparation phase known by the source NG-RAN node;
  • Handover to the target E-UTRAN when the UE is the last UE for this multicast MBS session in the source NG-RAN node;
  • MBS session deletion.
Reproduction of 3GPP TS 23.247, Fig. 7.2.2.4-1: Release of shared delivery toward RAN node
Figure 7.2.2.4-1: Release of shared delivery toward RAN node
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Step 1.
A RAN node decides to release shared delivery for a multicast MBS session, e.g. because it no longer serves at least one UE within the multicast MBS session. For location dependent services, the NG-RAN node may release shared delivery for the location dependent contents of a multicast MBS session if it no longer serves at least one UE assigned to an MBS Session ID and Area Session ID.
Step 2.
The NG-RAN node sends N2 MBS Session release request (MBS Session ID, [Area Session ID], [N2 SM information ([GTP tunnel info], release indication)])]) to the AMF. For location dependent services, the NG-RAN node also provides the Area Session ID. The RAN node includes the unicast DL tunnel info if unicast transport is used for the shared delivery. If the NG-RAN node was configured to use multicast transport for the 5GC Shared MBS traffic delivery, the NG-RAN node does not include the N2 SM information in the message.
Step 3.
[Conditional] If the N2 SM information is received or it is the last RAN node controlled by the AMF serving the multicast MBS session, the AMF invokes Nmbsmf_MBSSession_ContextUpdate request (MBS Session ID [Area Session ID], [leave indication], [N2 SM information ([DL GTP tunnel endpoint ID])], NG-RAN Node ID) to the MB-SMF corresponding to the MB-SMF ID stored in the AMF for the MBS Session ID. If it is the last RAN node controlled by the AMF serving the multicast MBS session identified by the MBS Session ID or both the MBS Session ID and Area Session ID (if exists), the leave indication is included.
Step 4.
[Conditional] If unicast transport was used towards the NG-RAN node, the MB-SMF determines whether the context update is for tunnel release or create based on the release indication in the N2 container. If the MB-SMF determines the context update is for tunnel release, it performs the following:
If the unicast transport was used towards the NG-RAN in a deployment where NG-RAN nodes share a common user plane entity, the MB-SMF removes the received NG-RAN Node ID and possibly DL GTP tunnel endpoint from the stored NG-RAN Node ID(s) for the DL GTP tunnel endpoint for the MBS session, and checks whether the DL GTP tunnel is in use by other NG-RAN nodes based on the stored NG-RAN Node ID(s) for DL GTP tunnel endpoint for the MBS session.
For the deployment where NG-RAN nodes do not share a common user plane entity, the DL GTP tunnel will not be used by other NG-RAN nodes.
If the related DL GTP tunnel is not in use by other NG-RAN nodes the MB-SMF sends N4mb Session Modification to the MB-UPF to release the N3mb tunnel used for the multicast MBS session (or location dependent content of the multicast MBS session if an Area Session ID was received) towards that NG-RAN node using the received GTP tunnel info.
Step 5.
[Conditional] The MB-SMF responses to the AMF with MBS Session ID and Area Session ID if received. If leave indication is received, the MB-SMF also removes the information of the AMF from the context of the multicast MBS session.
Step 6.
The AMF removes the information of the RAN node from the context of the multicast MBS session (or location dependent part of the multicast MBS session if an Area Session ID was received).
Step 7.
The AMF sends an N2 MBS Session release response (MBS Session ID, [Area Session ID]) to the RAN node. The NG-RAN node deletes the GTP tunnel info (if unicast transport is used for the shared delivery) or sends IGMP/MLD leave message to leave the multicast distribution tree (if multicast transport is used for the shared delivery). The NG-RAN node releases local resources for the multicast MBS session.
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