4.3.4 PDU Session Release

23.5023GPPProcedures for the 5G System (5GS)Release 18TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

4.3.4.1 General

The PDU Session Release procedure is used to release all the resources associated with a PDU Session, including:

– The IP address/Prefixes allocated for an IP-based PDU Session; this may include the release of multiple Prefixes in the case of Multi-homing (as defined in TS 23.501 [2]).

– Any UPF resource (including N3/N9/N19 termination) that was used by the PDU Session. For N19 termination, the UPF resource may be released if all the PDU Sessions associated with the 5G VN group are released.

– Any access resource that was used by the PDU Session.

The SMF takes care to notify any entity associated with PDU Session: PCF, DN (e.g. when DN authorization has taken place at PDU Session establishment), etc. of a PDU Session Release.

4.3.4.2 UE or network requested PDU Session Release for Non-Roaming and Roaming with Local Breakout

Figure 4.3.4.2-1 captures both the UE Requested PDU Session Release procedure and the network requested PDU Session Release procedure. The procedure allows the UE to request the release of one PDU Session. The procedure also allows the AMF, the SMF or the PCF to initiate the release of a PDU Session. In the case of LBO, the procedure is as in the case of non-roaming with the difference that the AMF, the SMF, the UPF and the PCF are located in the visited network.

Figure 4.3.4.2-1: UE or network requested PDU Session Release for non-roaming and roaming with local breakout

1. The procedure is triggered by one of the following events:

1a. (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. The NAS message is forwarded by the (R)AN to the AMF with an indication of User Location Information. This message is relayed to the SMF corresponding to the PDU Session ID via N2 and the AMF. The AMF invokes the Nsmf_PDUSession_UpdateSMContext service operation and provides the N1 SM container to the SMF together with User Location Information (ULI) received from the (R)AN.

NOTE 1: Depending on the Access Type, when the UE is in CM-IDLE state, the UE can trigger a Service Request procedure before being able to release the PDU Session.

1b. (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. 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. (R)AN may decide to indicate to the SMF that the PDU Session related resource is released, e.g. when all the QoS Flow(s) of the PDU Session are released.

NOTE 2: In this case, it’s up to SMF to decide whether to keep the PDU Session with user plane connection deactivated or release the PDU Session.

1e. (PDU Session Release initiated by the SMF).

The SMF may decide to release a PDU Session under the following scenarios:

– Based on a request from the DN (cancelling the UE authorization to access to the DN);

– Based on a request from the UDM (subscription change) or from the CHF;

– If the SMF received an event notification from the AMF that the UE is out of LADN service area

– Based on locally configured policy (e.g. the release procedure may be related with the UPF re-allocation for SSC mode 2 / mode 3);

– If the SMF is notified by the (R)AN that the PDU Session resource establishment has failed during mobility procedure; or

– The SMF initiates release of an emergency PDU Session when the UPF reports Detection of PDU Session Inactivity for a specified period as specified in clause 4.4.2.2.

1f. 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 (e.g. due to a change of the set of network slices for a UE where a network slice instance is no longer available as described in clause 5.15.5.2.2 of TS 23.501 [2] or the AAA Server triggered Network Slice-Specific Re-authentication and Re-authorization procedure fails as specified in clause 4.2.9.2 or the AAA Server triggered Slice-Specific Authorization Revocation takes place as specified in clause 4.2.9.4 or AMF determines that Control Plane Only indication associated with PDU Session is not applicable any longer as described in clause 5.31.4.1 of TS 23.501 [2]).

If the SMF receives one of the triggers in step 1a, 1b, 1c, 1e, or 1f, the SMF starts PDU Session Release procedure.

2. The SMF releases the IP address / Prefix(es) that were allocated to the PDU Session and releases the corresponding User Plane resources:

2a. The SMF sends an N4 Session Release Request (N4 Session ID) message to the UPF(s) of the PDU Session. The UPF(s) shall drop any remaining packets of the PDU Session and release all tunnel resource and contexts associated with the N4 Session.

2b. 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.

The UPF includes Small Data Rate Control Status if the PDU Session used Small Data Rate Control.

If a NEF has been selected as anchor of the Control Plane CIoT 5GS Optimisation enabled PDU session which is Unstructured PDU Session Type as described in clause 4.3.2.2 and the SMF-NEF Connection is released for this PDU Session.

NOTE 3: If there are multiple UPFs associated with the PDU Session (e.g. due to the insertion of UL CL or Branching Point, or redundant I-UPFs if the redundant I-UPFs are used for URLLC), the Session Release Request procedure (steps 2a and 2b) is done for each UPF. In order to avoid charging of PDUs that later get dropped, the SMF performs the N4 Session Release first with the UPF(s) performing usage reporting, before releasing the other UPF(s) that forward traffic for the same user plane resources.

3 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, e.g. due to the UE is in MICO mode or periodical registration failure, the procedure continues in step 11 by SMF notifying the AMF that the PDU Session is released by invoking the Nsmf_PDUSession_SMContextStatusNotify. The rest of step 3 and the steps 4-10 are skipped.

If the PDU Session Release procedure was triggered by steps 1a, 1b, 1d or 1e above, the SMF creates an N1 SM including PDU Session Release Command message (PDU Session ID, Cause). The Cause may indicate a trigger to establish a new PDU Session with the same characteristics (e.g. when procedures related with SSC mode 2 are invoked).

If the User Plane connection of the PDU Session is activated, the message sent by the SMF to the AMF shall include N2 SM Resource Release request. If the User Plane connection of the PDU Session is not activated, the message sent by the SMF to the AMF shall not include N2 SM Resource Release request.

NOTE 4: SSC modes are defined in clause 5.6.9 of TS 23.501 [2].

3a. (If the PDU Session Release is initiated by the UE in step 1a or has been triggered by (R)AN in step 1d) The SMF responds to the AMF with the Nsmf_PDUSession_UpdateSMContext response (N2 SM Resource Release request, N1 SM container (PDU Session Release Command)). N2 SM Resource Release request is included if the PDU Session Release is initiated by the UE and if the UP connection of the PDU Session is active. Neither N2 SM Resource Release request nor N1 SM container is included if the PDU Session Release is triggered by (R)AN and the SMF decides to keep the PDU session with user plane connection deactivated and the subsequent steps are skipped.

3b. If the PDU Session Release is initiated by the SMF or the PCF, the SMF invokes the Namf_Communication_N1N2MessageTransfer service operation (N1 SM container (PDU Session Release Command), skip indicator).

If the UP connection of the PDU Session is active, the SMF shall also include the N2 Resource Release request (PDU Session ID) in the Namf_Communication_N1N2MessageTransfer, to release the (R)AN resources associated with the PDU Session.

The "skip indicator" tells the AMF whether it may skip sending the N1 SM container to the UE (e.g. when the UE is in CM-IDLE state). SMF includes the "skip indicator" in the Namf_Communication_N1N2MessageTransfer except when the procedure is triggered to change PDU Session Anchor of a PDU Session with SSC mode 2.

If the UE is in CM-IDLE state and "skip indicator" is included in the Namf_Communication_N1N2MessageTransfer service operation or if the UE is in CM-IDLE state and "skip indicator" is not included in the Namf_Communication_N1N2MessageTransfer service operation but the AMF detects that the UE context contains Paging Restriction Information indicating that all paging is restricted, the AMF acknowledges the step 3b by sending an Namf_Communication_N1N2MessageTransfer Response message ("N1 SM Message Not Transferred") to SMF and steps 4 to 10 are skipped.

3c. 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, optionally including the Small Data Rate Control Status and APN Rate Control Status.

If the UPF included APN Rate Control Status and/or Small Data Rate Control Status in step 2 then the SMF includes APN Rate Control and/or Small Data Rate Control Status and the AMF stores the Small Data Rate Control Status and/or the APN Rate Control Status in the UE context in AMF.

The AMF and SMF shall remove all contexts (including the PDU Session ID) associated with the PDU Session which are indicated as released at the UE. The SMF shall remove any event subscriptions on the AMF by the SMF that becomes no more needed due to the PDU Session Release. The steps 4 to 11 are skipped.

3d. 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, the SMF responds to the AMF with the Nsmf_PDUSession_UpdateSMContext Response which may contain the N1 SM container (PDU Session Release Command) to release the PDU session at the UE.

If the UP connection of the PDU Session is active, the Nsmf_PDUSession_UpdateSMContext Response shall also include the N2 Resource Release request (PDU Session ID) to release the (R)AN resources associated with the PDU Session.

4. If the UE is in CM-IDLE state and "N1 SM delivery can be skipped" is not indicated, 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.

If the message received from the SMF in step 3 does not include N2 SM Resource Release request, the AMF transmits the NAS message (PDU Session ID, N1 SM container) to the UE and the steps 6, 7 are skipped.

If the PDU Session is Control Plane CIoT 5GS Optimisation enabled, the SMF shall not include N2 SM Resource Release request in the message sent to the AMF, the AMF transmits the NAS message (PDU Session ID, N1 SM container) to the UE and the steps 6, 7 are skipped.

If the UE is in CM-CONNECTED state and the received message from the SMF in step 3 includes N2 SM Resource Release request, the AMF transfers the SM information received from the SMF in step 4 (N2 SM Resource Release request, N1 SM container) to the (R)AN.

If the message from the SMF includes Small Data Rate Control Status then the AMF stores it in the UE Context in AMF.

5. 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.

In the case of a NG-RAN, the NAS message is sent to the UE in an RRC message which may take place with the UE releasing the NG-RAN resources related to the PDU Session. If NG-RAN resources do not need to be released (i.e. the User Plane of the PDU Session is deactivated), the NAS message is sent to the UE in an RRC message which does not release the NG-RAN resources related to the PDU Session.

During this procedure, the (R)AN sends any NAS message (N1 SM container (PDU Session Release Command)) received from the AMF in step 5.

6. [Conditional] If the (R)AN had received a N2 SM request to release the AN resources, the (R)AN acknowledges the N2 SM Resource Release Request by sending an N2 SM Resource Release Ack (User Location Information, Secondary RAT usage data) Message to the AMF.

If the PLMN has configured secondary RAT usage reporting, the NG-RAN node may provide RAN Usage Data Report.

7a. The AMF invokes the Nsmf_PDUSession_UpdateSMContext (N2 SM Resource Release Ack (Secondary RAT usage data), User Location Information) to the SMF.

7b. The SMF responds to the AMF with an Nsmf_PDUSession_UpdateSMContext response.

8. 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. [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. The AMF invokes the Nsmf_PDUSession_UpdateSMContext (N1 SM container (PDU Session Release Ack, User Location Information) to the SMF.

10b. The SMF responds to the AMF with an Nsmf_PDUSession_UpdateSMContext response.

Steps 8-10 may happen before steps 6-7.

11. If steps 3a, 3b or 3d were performed, the SMF waits until it has received replies to the N1 and N2 information provided in step 3, as needed.

The SMF invokes Nsmf_PDUSession_SMContextStatusNotify to notify AMF that the SM context for this PDU Session is released. If the UPF included Small Data Rate Control Status and/or APN Rate Control Status in step 2 then the SMF includes Small Data Rate Control Status and/or APN Rate Control Status in its request to the AMF in this step. The AMF releases the association between the SMF ID and the PDU Session ID, DNN, as well as S-NSSAI and stores the Small Data Rate Control Status and/or the APN Rate Control Status in the UE context in AMF. The SMF shall remove any event subscriptions on the AMF that becomes no more needed due to the PDU Session Release.

NOTE 5: The UE and the 5GC will get synchronized about the status of the (released) PDU Session at the next Service Request or Registration procedure.

12. 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.

13. SMF notifies any entity that has subscribed to User Location Information related with PDU Session change.

14. If it is the last PDU Session the SMF is handling for the UE for the associated (DNN, S-NSSAI), the SMF unsubscribes from Session Management Subscription data changes notification with the UDM by means of the Nudm_SDM_Unsubscribe (SUPI, DNN, S-NSSAI) service operation. The UDM may unsubscribe the subscription notification from UDR by Nudr_DM_Unsubscribe (SUPI, Subscription Data, Session Management Subscription data, DNN, S-NSSAI).

15. The SMF invokes the Nudm_UECM_Deregistration service operation including the DNN and the PDU Session Id. The UDM removes the association it had stored between the SMF identity and the associated DNN and PDU Session Id. The UDM may update this information by Nudr_DM_Update (SUPI, Subscription Data, UE context in SMF data).

4.3.4.3 UE or network requested PDU Session Release for Home-routed Roaming

This procedure is used in the case of home-routed roaming scenarios.

Figure 4.3.4.3-1: UE or network requested PDU Session Release for home-routed roaming

1. The procedure is triggered by one of the following events:

1a. (UE initiated release) As in step 1a of clause 4.3.4.2 with the addition that:

– the V-SMF initiates N4 Session Modification to instruct the V-UPF to stop forwarding uplink traffic; and

– the V-SMF invokes the Nsmf_PDUSession_Update Request (SM Context ID, information from the SM message from the UE e.g. PCO, "Trigger PDU Session Release" indication, Timezone, User Location Information) service operation to request the H-SMF to release the PDU Session. The H-SMF responds to the request immediately.

1b. (Serving network initiated release) The serving network initiates the PDU Session Release during UE or serving network initiated Deregistration procedure as specified in clause 4.2.2.3. There is no NAS SM message between the UE and the V-SMF in this case. This step is the same as step 1c in clause 4.3.4.2, with the addition that:

– the V-SMF initiates N4 Session Modification to instruct the V-UPF to stop forwarding uplink traffic; and

– the V-SMF initiates the release of the PDU Session at the H-SMF by invoking the Nsmf_PDUSession_Release request.

The serving network also initiates the PDU Session Release where neither N1 nor N2 SM signalling is needed before releasing the SM context as in step 1c in clause 4.3.4.2 if e.g. due to the set of network slices for a UE changes where a network slice instance is no longer available (e.g. as described in clause 5.15.5.2.2 of TS 23.501 [2], or the AAA Server triggered Network Slice-Specific Re-authentication and Re-authorization procedure fails as specified in clause 4.2.9.2 or the AAA Server triggered Slice-Specific Authorization Revocation takes place as specified in clause 4.2.9.4).

1c. (HPLMN initiated release) This step is the same as step 1b in clause 4.3.4.2, with the addition that:

– the H-SMF initiates N4 Session Modification to instruct the H-UPF to stop forwarding downlink traffic.

1d. This step is the same as step 1d in clause 4.3.4.2, with the addition that:

– the V-SMF initiates N4 Session Modification to instruct the V-UPF to stop forwarding uplink traffic; and

– the V-SMF invokes the Nsmf_PDUSession_Update Request towards H-SMF.

1e. (HPLMN initiated release) This step is the same as step 1e in clause 4.3.4.2, with the addition that:

– the H-SMF initiates N4 Session Modification to instruct the H-UPF to stop forwarding downlink traffic.

1f. This step is the same as step 1f in clause 4.3.4.2, with the addition that:

– the V-SMF initiates N4 Session Modification to instruct the V-UPF to stop forwarding uplink traffic; and

– the V-SMF invokes the Nsmf_PDUSession_Update Request towards H-SMF.

If the SMF receives one of the triggers in step 1a, 1c, 1e or 1f, the H-SMF starts PDU Session Release procedure.

When a SMF above initiates N4 Session Modification to instruct the UPF to stop forwarding traffic this means also to stop taking into account the traffic for usage monitoring.

2a-2b. (UE-, (R)AN- or Serving network initiated) This step is performed in case the PDU Session Release is triggered by a message from V-SMF. These steps are the same as steps 2a-2b in clause 4.3.4.2. The SMF is the SMF in HPLMN.

NOTE 1: This step 2a-2b can correspond to steps 1a, 1b, 1d, 1f.

3a. (UE or HPLMN initiated release) The H-SMF prepares the SM Release PDU Session Command message and initiates the PDU Session Release towards the UE by invoking the Nsmf_PDUSession_Update Request service operation towards the V-SMF. The Nsmf_PDUSession_Update Request contains necessary information to build the SM Release PDU Session Command by the V-SMF towards the UE (for example a Release Cause or PCO).

3b. (Serving network initiated release) The H-SMF responds to the PDU release request from the V-SMF with a Nsmf_PDUSession_Release response.

If the UPF included Small Data Rate Control Status in step 2 then the SMF includes Small Data Rate Control Status in the request to the AMF.

If the Control Plane CIoT 5GS Optimisation is enabled for this PDU Session, the steps 4a and 4b are skipped.

4a-4b. The V-SMF releases the corresponding User Plane resources. This includes the same procedure in step 2 but controlled from the SMF in VPLMN.

5-13. These steps are the same as steps 3-10 in clause 4.3.4.2.

14. (UE or HPLMN initiated release) The V-SMF responds to the Nsmf_PDUSession_Update Request invoked at step 3a and confirms the PDU Session Release. The Nsmf_PDUSession_Update response may carry information such as PCO received from the UE in SM PDU Session Release Accept. as well as User Location Information, Time Zone and Secondary RAT Usage Data.

15a. (UE or HPLMN or Serving network initiated release) The H-SMF releases the SM policy control association with the PCF by invoking the SM Policy Association Termination procedure defined in clause 4.16.6. For serving network initiated PDU Session Release case, this step happens between step 1b and step 3b.

15b-15c. (HPLMN initiated release) In case the PDU Session Release is HPLMN-initiated (i.e. triggers in 1c, 1e), the H-SMF releases the corresponding User Plane resources. This includes the same procedure as in step 2.

15d. As in step 15 of clause 4.3.4.2, the SMF invokes the Nudm_UECM_Deregistration service operation.

NOTE 2: Step 15d does not necessarily take place after step 15c.

16. (UE or HPLMN initiated release) The H-SMF shall remove all contexts associated with the PDU Session:

16a. The H-SMF requests the V-SMF to release all contexts associated with the PDU Session by invoking the Nsmf_PDUSession_StatusNotify (Release) operation.

16b. The V-SMF requests the AMF to release all contexts associated with the PDU Session by invoking the Nsmf_PDUSession_SMContexStatusNotify (Release). The AMF releases the association between the SMF ID and the PDU Session ID.