Solution to the problem that the called party cannot be connected due to the INVITE message not being forwarded

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During a VoLTE test, an operator discovered that calls could not be connected, and conducted a packet capture analysis to locate the problem.

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1. The SBC analyzes the user's VoLTE signaling and finds that the SBC sends an INVITE message to the called user and resends it four times, but does not receive a response from the called terminal, as shown in Figure 1.

Figure 1 VoLTE signaling

2. After analyzing the user plane packet capture on UPF, it was found that UPF received 5 INVITE packets, but they were not encapsulated into GTP and forwarded to the base station, as shown in Figure 2.

Figure 2 User plane analysis

3. Analyzing the user signaling, it is found that the UPF of CTNET first triggers the Session_ Report_ Request process to SMF due to the downlink data packet. SMF sends an N1N2MessageTransfer Request message to AMF to request the establishment of a user plane connection. This action triggers the paging process of AMF, as shown in Figure 3.

Figure 3 Paging process

4. When the INVITE downlink data packet triggers the Session_Report_Request process from IMS UPF to SMF, SMF sends an N1N2 message to AMF (226) to request the establishment of a user plane connection. Since AMF is currently paging the user, AMF responds to SMF with a HIGHER_PRIORITY_REQUEST_ONGOING process conflict failure, as shown in Figure 4.

5. When the user is paged, AMF notifies SMF to establish a user plane connection for the corresponding PDU session through the PDUSession_UpdateSMContext Request message (238). SMF sends a Session_Modification_Request message to notify the UPF of CTNET to establish a user plane link.

6. Since AMF has previously responded with a rejection to the N1N2 process triggered by IMS UPF, AMF will no longer trigger the PDUSession_UpdateSMContext Request message to establish a user plane session for the IMS session. As a result, the INVITE message of the IMS session on UPF cannot be forwarded to the base station, as shown in Figure 5.

Figure 5 INVITE message cannot be forwarded

7. After AMF receives the N1N2 request from SMF, there are two processing actions:

a. If the UE is in idle state, the AMF sends a paging message to all gNBs in the registration area where the UE is located.

b. If the UE is in a connected state, AMF does not need to initiate a paging process. AMF directly calls the SMF service interface Nsmf_PDU Session_Update SM Context Request to request SMF to establish a user plane connection for the corresponding PDU session.

8. Since the user status on AMF is at the user level (idle/connected), the second N1N2 request sent by SMF during the process of changing the user status through the paging process is a conflicting process. However, SMF needs to have protection measures for the conflicting session process to ensure that the downlink data packets can be forwarded normally.

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The subsequent versions of SMF will optimize the processing flow of this problem. For the N1N2MessageTransfer process that reports a conflict failure, it is necessary to resend the N1N2MessageTransfer Request request after a certain time interval, so that after the user becomes connected, the downlink data packets of the previous conflicting process can continue to be forwarded. This optimization function is first implemented through the SMF hot patch method.

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