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PDU Session Release procedure is used to release all the resources associated with a PDU Session.
1a. PDU Session Release Request
-> (UE requested) The UE initiates the UE Requested PDU Session Release procedure by the transmission of an NAS message (N1 SM container (PDU Session Release Request (PDU session ID)), PDU Session ID) message.
1b. PDU Session Release initiated by the PCF
-> (PDU Session Release initiated by the PCF) The PCF may invoke an SM Policy Association Termination procedure as defined in clause 4.16.6 to request the release of the PDU Session.
1c. Nsmf_PDUSession_ReleaseSMContext
-> The AMF may invoke the Nsmf_PDUSession_ReleaseSMContext service operation to request the release of the PDU Session in the case of mismatch of PDU Session status between UE and AMF or other cases where neither N1 nor N2 SM signalling is needed before the releasing of SM context.
1d. N2 message(PDU Session ID, SM information)
-> (R)AN may decide to indicate to the SMF that the PDU Session related resource is released.
1e. PDU Session Release Trigger
-> The SMF may decide to release a PDU Session under the following scenarios
-> Based on a request from the DN
-> Based on a request from the UDM
1f. Nsmf_PDUSession_ReleaseSMContext
-> The AMF may invoke the Nsmf_PDUSession_UpdateSMContext service operation with a release indication to request the release of the PDU Session where N1 or N2 SM signalling may be needed before releasing the SM context.
2. N4 interface
-> The SMF releases the IP address / Prefix(es) that were allocated to the PDU Session and releases the corresponding User Plane resources:
2a. N4 Session Release Request
-> The SMF sends an N4 Session Release Request (N4 Session ID) message to the UPF(s) of the PDU Session.
2b. N4 Session Release Response
-> The UPF(s) acknowledges the N4 Session Release Request by the transmission of an N4 Session Release Response (N4 Session ID, [Small Data Rate Control Status], [APN Rate Control Status]) message to the SMF.
3. If PDU session release is initiated by PCF and SMF
-> If the PDU Session Release is initiated by the PCF and SMF, and the SMF has been notified by the AMF that UE is unreachable.
3a. Nsmf_PDUSession_UpdateSMContext
-> The SMF responds to the AMF with the Nsmf_PDUSession_UpdateSMContext response.
3b. Namf_Communication_N1N2MessageTransfer
-> If the PDU Session Release is initiated by the SMF or the PCF, the SMF invokes the Namf_Communication_N1N2MessageTransfer service operation.
3c. Nsmf_PDUSession_ReleaseSMContext Response
-> If the PDU Session Release is initiated by the AMF in step 1c, i.e. the SMF received the Nsmf_PDUSession_ReleaseSMContext Request from the AMF, the SMF responds to the AMF with the Nsmf_PDUSession_ReleaseSMContext response.
3d. Nsmf_PDUSession_UpdateSMContext Response
-> If the PDU Session Release is initiated by the AMF in step 1f, i.e. the SMF received the Nsmf_PDUSession_UpdateSMContext Request from the AMF with a release indication to request the release of the PDU Session.
4. N2 Resource Release Request
-> the AMF initiates the network triggered Service Request procedure to transmit the NAS message (PDU Session ID, N1 SM container) to the UE and the steps 6, 7 are skipped.
5. AN-specific response modification(including PDU Session Release Command)
-> When the (R)AN has received an N2 SM request to release the AN resources associated with the PDU Session it issues AN specific signalling exchange(s) with the UE to release the corresponding AN resources.
6. N2 Resource Release Ack [Conditional]
-> the (R)AN acknowledges the N2 SM Resource Release Request by sending an N2 SM Resource Release Ack
7a. Nsmf_PDUSession_UpdateSMContext(N2 SM info)
-> The AMF invokes the Nsmf_PDUSession_UpdateSMContext (N2 SM Resource Release Ack (Secondary RAT usage data), User Location Information) to the SMF.
7b. Nsmf_PDUSession_UpdateSMContext
-> The SMF responds to the AMF with an Nsmf_PDUSession_UpdateSMContext response.
8. PDU Session Release Ack
-> The UE acknowledges the PDU Session Release Command by sending a NAS message (PDU Session ID, N1 SM container (PDU Session Release Ack)) message over the (R)AN.
9. N2 Uplink NSA transport [Conditional]
-> [Conditional] The (R)AN forwards the NAS message from the UE by sending a N2 NAS uplink transport (NAS message (PDU Session ID, N1 SM container (PDU Session Release Ack)), User Location Information) to the AMF.
10a. Nsmf_PDUSession_UpdateSMContext
-> The AMF invokes the Nsmf_PDUSession_UpdateSMContext (N1 SM container (PDU Session Release Ack, User Location Information) to the SMF.
10b. Nsmf_PDUSession_UpdateSMContext Response
-> The SMF responds to the AMF with an Nsmf_PDUSession_UpdateSMContext response.
11. Nsmf_PDUSession_UpdateSMStatusNotify(Release)
-> If step 3a, 3b or 3d is performed, the SMF waits until it receives the required replies to the N1 and N2 information provided in step 3.
12. SM Policy Association Termination
-> If Dynamic PCC applied to this session the SMF invokes an SM Policy Association Termination procedure as defined in clause 4.16.6 to delete the PDU Session.
Source TS 23.502