How it Works
This section describes the call flows related to the roaming feature in UPF.
PDU Session Establishment Procedure
This call flow describes the PDU Session Establishment procedure for the UPF.

Step |
Description |
---|---|
1 |
The UE initiates a PDU Session Establishment Request to the AMF. |
2 |
The AMF selects an SMF. |
3a |
The AMF invokes the Nsmf_PDUSession_CreateSMContext Request and sends it to the vSMF. |
3b |
The vSMF sends a Nsmf_PDUSession_CreateSMContext Response to the AMF. |
4 |
The vSMF selects a UPF in VPLMN. |
5a |
The vSMF sends an N4 Session Establishment Request with dummy PDRs, FARs, QERs, or URRs to the vUPF. In this request, the vSMF informs the vUPF to allocate the CN Tunnel information for the N3 and N9 interfaces. The vSMF sends Source Interface Type IE in Access PDR as N3 3GPP Access (11), and in Core PDR as N9 (21) for roaming. |
5b |
The vUPF creates the N3 and N9 CN tunnel information and acknowledges by sending this information in an N4 Session Establishment Response. |
6 |
The vSMF sends an Nsmf_PDUSession_Create Request to the hSMF. |
7 |
The hSMF registers with the UDM for a given PDU session. |
8 |
The SMF performs a secondary authorization or authentication during the establishment of the PDU session by a DN-AAA server. |
9a |
The hSMF selects the PCF. |
9b |
The SMF performs an SM Policy Association Establishment procedure to establish an SM Policy Association with the PCF and get the default PCC rules for the PDU session. |
10 |
The hSMF selects the UPF. |
11 |
The hSMF initiates an SM Policy Association Modification procedure. |
12a |
The hSMF initiates an N4 Session Establishment procedure with the selected UPF. The N9 CN tunnel information from the vUPF is transferred to hUPF in FAR. The hSMF sends Source Interface Type in Access PDR as N9(21) or N3 3GPP ACCESS (11) or S5u interface depending on the RAT and roaming status where it registers the call. |
12b |
The UPF acknowledges by sending an N4 Session Establishment Response. The hUPF provides the N9 CN tunnel information to the vUPF. |
12c |
The downlink path is established between the hUPF and vUPF and data packets are sent to the vUPF where it gets buffered. |
13 |
The hSMF sends an Nsmf_PDUSession_Create Response message to the vSMF. |
14 |
The vSMF sends an Namf_Communication_N1N2MessageTransfer message to the AMF. |
15 |
The AMF sends the NAS message containing PDU Session ID and PDU Session Establishment Accept targeted to the UE and the N2 SM information received from the SMF within the N2 PDU Session Request to the RAN. |
16 |
The RAN issues AN specific signalling exchange with the UE that is related with the information received from SMF. |
17 |
The RAN sends an N2 PDU Session Response message to the AMF. |
18 |
The AMF sends the Nsmf_PDUSession_UpdateSMContext Request message to the vSMF. The AMF forwards the N2 SM information received from RAN to the vSMF. |
19a |
The vSMF initiates an N4 Session Modification procedure with the vUPF. The vSMF provides packet detection, enforcement, and reporting rules to be installed on the vUPF for this PDU session, including AN Tunnel Information, H-CN Tunnel Information and V-CN Tunnel Information. |
19b |
The vUPF provides an N4 Session Modification Response to the vSMF. After this step, the vUPF delivers any down-link packets to the UE that might have been buffered for this PDU Session. |
20 |
The vSMF sends an Nsmf_PDUSession_UpdateSMContext Response message to the AMF. |
21 |
The vSMF sends an Nsmf_PDUSession_SMContextStatusNotify message to the AMF. |
22 |
If it's a PDU session of type IPv6 or IPv4v6, the hSMF generates an IPv6 Router Advertisement and sends it to the UE through the N4 interface, hUPF, and vUPF. |
23 |
If the vSMF received in step 18 is an indication that the RAN has rejected some QFIs the vSMF notifies the hSMF through an Nsmf_PDUSession_Update Request. The hSMF is responsible for updating the QoS rules and QoS Flow level QoS parameters for the QoS Flow(s) associated with the QoS rule(s) in the UE accordingly. |
24 |
If the PDU Session establishment failed after step 4, the hSMF performs the following steps:
|
PDU Session Modification Procedure
This call flow describes the PDU Session Modification procedure for the UPF.

Step |
Description |
---|---|
1a |
|
1b |
The PCF performs a PCF initiated SM Policy Association Modification procedure to notify SMF about the modification of policies. |
1c |
The UDM updates the subscription data of hSMF by Nudm_SDM_Notification (SUPI, Session Management Subscription Data). The hSMF updates the Session Management Subscription Data and acknowledges the UDM by returning an Ack with (SUPI). |
1d |
The SMF might modify the PDU session. This procedure can also be triggered based on locally configured policy or triggered from the RAN. |
1e |
RAN indicates to the SMF when the AN resources onto which a QoS Flow is mapped are released irrespective of whether notification control is configured. RAN sends the N2 message to the AMF. The AMF invokes Nsmf_PDUSession_UpdateSMContext procedure. The vSMF sends an Nsmf_PDUSession_Update Request message to the hSMF. The hSMF acknowledges and sends an Nsmf_PDUSession_Update Response message to the vSMF. |
2 |
The SMF reports some subscribed event to the PCF by performing an SMF initiated SM Policy Association Modification procedure. |
3 |
The hSMF invokes the Nsmf_PDUSession_Update Request service operation to the vSMF. |
3a |
The hSMF initiates an N4 Session Modification procedure with the selected hUPF. |
3b |
The hUPF acknowledges and sends an N4 Session Modification Response message to the hSMF. |
4a |
The vSMF sends an Nsmf_PDUSession_UpdateSMContext Response message to the AMF. |
4b |
The vSMF sends an Nsmf_PDUSession_SMContextStatusNotify message to the AMF. |
5 |
The AMF sends the NAS message containing PDU Session ID and PDU Session Establishment Accept targeted to the UE and the N2 SM information received from the SMF within the N2 PDU Session Request to the RAN. |
6 |
The RAN issues AN specific signalling exchange with the UE that is related with the information received from SMF. |
7 |
The AMF sends the Nsmf_PDUSession_UpdateSMContext Request message to the vSMF. The AMF forwards the N2 SM information received from RAN to the vSMF. |
8 |
The AMF sends the Nsmf_PDUSession_UpdateSMContext Request message to the vSMF. The AMF forwards the N2 SM information received from RAN to the vSMF. |
9a |
The vSMF initiates an N4 Session Modification procedure with the vUPF. The vSMF provides packet detection, enforcement, and reporting rules to be installed on the vUPF for this PDU session, including AN Tunnel Information, H-CN Tunnel Information and V-CN Tunnel Information. |
9b |
The vUPF provides an N4 Session Modification Response to the vSMF. After this step, the vUPF delivers any down-link packets to the UE that might have been buffered for this PDU Session. |
10 |
The vSMF sends an Nsmf_PDUSession_UpdateSMContext Response message to the AMF. |
11 |
The UE sends a PDU Session Modification Command Ack message to the RAN. |
12 |
The RAN initiates an N2 NAS Uplink Transfer with the AMF. |
13 |
The vSMF sends an Nsmf_PDUSession_UpdateSMContext Response message to the AMF. |
14 |
The vSMF sends an Nsmf_PDUSession_SMContextStatusNotify message to the AMF. |
15 |
The vSMF responds to the hSMF with an Nsmf_PDUSession_Update response carrying the information like PCO provided by the UE in the SM PDU Session Modification Command Ack message from the UE to the vSMF. The hSMF modifies the PDU session context. |
16a |
The vSMF initiates an N4 Session Modification procedure with the vUPF. The vSMF provides packet detection, enforcement, and reporting rules to be installed on the vUPF for this PDU session, including AN Tunnel Information, H-CN Tunnel Information and V-CN Tunnel Information. |
16b |
The vUPF provides an N4 Session Modification Response to the vSMF. After this step, the vUPF delivers any down-link packets to the UE that might have been buffered for this PDU Session. |
17 |
The hSMF initiates an SM Policy Association Modification procedure. |
PDU Session Release Procedure
This call flow describes the PDU Session Release procedure for the UPF.

Step |
Description |
---|---|
1a |
|
1b |
|
1c |
|
1d |
|
1e |
The SMF decides to release a PDU session. The hSMF initiates N4 Session Modification to instruct the hUPF to stop forwarding downlink traffic. |
1f |
The AMF invokes the Nsmf_PDUSession_UpdateSMContext service operation with a release indication to request the release of the PDU session. The vSMF initiates N4 Session Modification to instruct the vUPF to stop forwarding uplink traffic. The vSMF invokes the Nsmf_PDUSession_Update Request toward the hSMF. |
2a |
The hSMF sends an N4 Session Release Request (N4 Session ID) message to the hUPFs of the PDU session. The hUPFs drop any remaining packets of the PDU session and release all tunnel resource and contexts associated with the N4 Session. |
2b |
The hUPF(s) acknowledges the N4 Session Release Request by the transmission of an N4 Session Release Response message to the hSMF. |
3a |
The SMF responds to the AMF with the Nsmf_PDUSession_UpdateSMContext response. |
4a |
The vSMF sends an N4 Session Release request to the vUPF. |
4b |
The vUPF acknowledges and sends an N4 Session Release response to the hSMF. |
5a |
The vSMF sends an Nsmf_PDUSession_ReleaseSMContext Response message to the AMF. |
5b |
The vSMF sends an Nsmf_PDUSession_UpdateSMContext Response message to the AMF. |
5c |
The N1N2 Message Transfer procedure occurs between the AMF and vSMF. |
6 |
The hSMF includes the N2 SM Resource Release request in the message sent to the AMF, then the AMF transmits the NAS message to the UE. |
7 |
When the RAN receives an N2 SM request to release the AN resources associated with the PDU session, it issues AN specific signalling exchanges with the UE to release the corresponding AN resources. |
8 |
If the RAN receives an N2 SM request to release the AN resources, it acknowledges the N2 SM Resource Release Request by sending an N2 SM Resource Release Ack message to the AMF. |
9 |
The AMF sends an Nsmf_PDUSession_UpdateSMContext Response message to the vSMF. |
10 |
The vSMF responds to the AMF with an Nsmf_PDUSession_UpdateSMContext response. |
11 |
The UE acknowledges the PDU Session Release Command by sending a NAS message over the RAN. |
12 |
The AMF invokes the Nsmf_PDUSession_UpdateSMContext to the vSMF. |
13 |
The SMF responds to the AMF with an Nsmf_PDUSession_UpdateSMContext response. |
14 |
The vSMF responds to the hSMF with an Nsmf_PDUSession_Update Request invoked at step 3a and confirms the PDU session release. |
15 |
The vSMF initiates an N4 Session Modification procedure with the vUPF. The vSMF provides packet detection, enforcement, and reporting rules to be installed on the vUPF for this PDU session, including AN Tunnel Information, H-CN Tunnel Information and V-CN Tunnel Information. |
15a |
The hSMF releases the SM policy control association with the PCF by invoking the SM Policy Association Termination procedure. |
15b - 15c |
In case the PDU Session Release is HPLMN-initiated, the hSMF releases the corresponding User Plane resources. |
15d |
The hSMF invokes the Nudm_UECM_Deregistration service operation. |
16a |
The hSMF requests the vSMF to release all contexts associated with the PDU session by invoking the Nsmf_PDUSession_StatusNotify (Release) operation. |
16b |
The vSMF 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 vSMF ID and the PDU Session ID. |
5G to 4G Handover
This call flow describes the 5G to 4G handover.

Step |
Description |
---|---|
1 |
After the 5G session is established, the NG-RAN initiates the handover process by sending the Handover Required message to the AMF. |
2 |
The AMF invokes the Nsmf_PDUSession_Context Request and sends it to the vSMF. |
3 |
The vSMF sends a Nsmf_PDUSession_Context Response to the AMF. |
4 |
The AMF sends a Relocation Request to the MME. |
5 |
The MME sends Create Session Request to the S-GW. |
6 |
The S-GW sends a Create Session Response message back to the MME. |
7 |
The MME sends a Handover Request message to E-UTRAN. |
8 |
The E-UTRAN acknowledges and sends a Handover Request ACK message back to the MME. |
9 |
The MME sends the Relocation Response message to the AMF. |
10 |
The AMF sends a Handover Command message to the NG-RAN. |
11 |
The NG-RAN commands the UE to handover to the target access network by sending the Handover Command. |
12 |
The UE responds to the E-UTRAN with a Handover Complete message, and the uplink data path is established. |
13 |
The E-UTRAN notifies the MME that the UE is handed over to the NG-RAN. |
14 |
The MME sends a Relocation Complete Notification message to the AMF. |
15 |
The AMF acknowledges and sends a Relocation Complete Ack message to the MME. |
16 |
The MME sends a Modify Bearer Request message to the S-GW. |
17 |
The S-GW forwards the Modify Bearer Request message to the hSMF. |
18 |
The hSMF initiates an N4 Session Modification procedure with the hUPF. |
19 |
The hSMF responds to the S-GW with a Modify Bearer Response message, and the downlink data path is established. |
20 |
The S-GW sends the Modify Bearer Response message to the MME. |
4G to 5G Handover
This call flow describes the 4G to 5G handover.

Steps |
Description |
---|---|
1 |
After the 4G session is established, the E-UTRAN initiates the handover process by sending the Handover Required message to the MME. |
2 |
The MME sends a Forward Relocation Request to the AMF. |
3 |
The AMF invokes the Nsmf_PDUSession_CreateSMContext Request and sends it to the vSMF. |
4 |
The vSMF sends a Nsmf_PDUSession_CreateSMContext Response to the AMF. |
5 |
The vSMF sends a Nsmf_PDUSession_Create Service message to create a new PDU Session in the hSMF. |
6 |
The hSMF responds with a Nsmf_PDUSession_Create Response message. |
7 |
The vSMF sends an N4 Establishment Request to the vUPF. |
8 |
The vUPF acknowledges by sending an N4 Establishment Response. |
9 |
The vSMF sends an Namf_Communication_N1N2MessageTransfer message to the AMF. |
10 |
The AMF sends the Nsmf_PDUSession_UpdateSMContext Request message to the vSMF. |
11 |
The AMF sends a Forward Relocation Response message to the MME. |
12 |
The MME sends the Handover Command to the E-UTRAN. |
13 |
The NG-RAN notifies the AMF that the UE is handed over to the NG-RAN. |
14 |
The AMF sends the Nsmf_PDUSession_UpdateSMContext Request message to the vSMF. |
15 |
The vSMF initiates the Nsmf_PDUSession_Update Request toward the hSMF. |
16 |
The hSMF sends an N4 Modification Request with PDRs, FARs, QERs, or URRs to the hUPF. |
17 |
The hUPF responds with an N4 Modification Response message. |
18 |
The hSMF invokes the Nsmf_PDUSession_Update Request service operation to the vSMF. |
19 |
The vSMF sends an Nsmf_PDUSession_UpdateSMContext Response message to the AMF. |
20 |
The vSMF initiates an N4 Modification procedure with the vUPF. |
Note | To support the roaming status functionality, the Roaming Status IE is sent in Subscriber Params during PDU Session Establishment or during handover. |
Inter-PLMN 5G HO from Homer to Roamer
This call flow describes the inter-PLMN 5G handover (HO) procedure with v-SMF insertion.
The UE moves from a homer in H-PLMN to a roamer in V-PLMN. When V-SMF is newly inserted, SMF transitions to H-SMF.

Steps |
Description |
---|---|
1 |
UE moves from H-PLMN (5G Homer) to V-PLMN (5G Roamer). |
2 |
V-SMF sends Nsmf_PDUSession_Create Request with HO Preparation Ind info to the H-SMF. |
3a |
H-SMF sends the following Create PDRs in an N4 Session Modification Request to the H-UPF:
|
3b |
H-UPF sends N4 Session Modification Response with created N9 UL Tunnel info to the H-SMF. |
4 |
H-SMF sends Nsmf_PDUSession_Create Response with N9 UL Tunnel info to the V-SMF. |
5a |
V-SMF sends N4 Session Modification Request with N9 UL Tunnel info to the V-UPF. |
5b |
V-UPF sends N4 Session Modification Response back to the V-SMF. |
6a |
V-SMF sends N4 Session Modification Request with N3 DL Tunnel Info of RAN to the V-UPF. |
6b |
V-UPF sends N4 Session Modification Response back to the V-SMF. |
7 |
V-SMF sends Nsmf_PDUSession_UpdateSMContext Request with DL Tunnel info of V-UPF for N9 to the H-SMF. |
8a |
H-SMF deletes N3 UL PDR (if created), updates N9 DL tunnel Info in an N4 Session Modification Request and sends to the H-UPF. H-SMF sends Create N9 DL FAR, Remove old N3 PDR, Remove old S5 PDR, Remove FAR, Create Core QER, and Update Core PDR in Sx-Modify. |
8b |
H-UPF sends N4 Session Modification Response back to the H-SMF. |
9 |
H-SMF sends Nsmf_PDUSession_UpdateSMContext Response back to the V-SMF. |
Inter-PLMN HO from 5G Roamer to 4G Homer
This call flow describes the inter-PLMN HO from 5G roamer to 4G homer.

Steps |
Description |
---|---|
1 |
UE moves from H-PLMN (5G Roamer) to V-PLMN (4G Homer). |
2 |
S-GW sends Modify Bearer Request to the H-SMF. Here, if N3/N9 UserPlane Separation is enabled then create N3 secondary tunnel. |
3a |
H-SMF sends N4 Session Modification Request to H-UPF with the following IEs: Create S5 PDR (old S5-U FTEID), Create N3 PDR (CH bit set for New FTEID), Create S5 DL FAR, Remove old N9 PDR, Remove old S5 PDR, Remove FAR, and Update Core QER. |
3b |
H-UPF sends N4 Session Modification Response back to the H-SMF. |
4 |
H-SMF sends Modify Bearer Response back to the S-GW. |
5 |
S-AMF sends Nsmf_PDUSession_ReleaseSMContext Request with Release PDU Session to the V-SMF. |
6a |
V-SMF sends N4 Session Release Request with Delete N3,N9 tunnels to the V-UPF. |
6b |
V-UPF sends N4 Session Release Response back to the V-SMF. |
7 |
V-SMF sends Nsmf_PDUSession_ReleaseSMContext Response back to the S-AMF. |