5G to EPS Handover Cancellation Call Flow

When the Source Radio Access Network (RAN) triggers a handover cancellation after the preparation phase, the AMF invokes the "Nsmf_PDUSession_UpdateSMContext request (SUPI, Relocation Cancel Indication) toward the SMF. Based on the Relocation Cancel Indication, the SMF deletes the session resources established during the handover preparation phase. That is, the SMF removes all the Packet Detection Rules (PDRs), Forwarding Action Rules (FARs), and other rules that were allocated in preparation of handoff for indirect tunnel and the 5G session.

The following call flow depicts the 5GS to EPS handover cancellation procedure.

5GS to EPS Handover Cancellation Call Flow
5GS to EPS Handover Cancellation Call Flow Description

Step

Description

1

The AMF requests the SMF to cancel the handover of an existing PDU session by sending a POST request for Sm Context Update service, with the following information:

  • updating the hoState attribute of the individual SM Context resource in the SMF to CANCELLED

  • cause information

2

The SMF returns a 200 OK response message including the following information:

  • hoState attribute set to CANCELLED

The SMF cancels the execution of the handover, for example, releases the resources reserved for the handover to the target RAN. Then, the SMF sets the hoState to NONE and deletes any stored targetServingNfId.