Section: 4.11.1.2.3 Handover Cancel
Content: Instead of completing the handover procedure, the source RAN node (NG-RAN, E-UTRAN) may at any time, during the handover procedure, up to the time when a handover command message is sent to the UE, cancel the handover. The reason for cancelling may be e.g. due to a timer expiration or due to other events within the source RAN node and is initiated by sending a handover cancel message to the source CN node (AMF or MME). A handover cancel message shall also be sent by the source RAN node after a handover command message is sent to the UE for the case where the handover fails and the UE returns to the old cell or radio contact with the UE is lost. This is done in order to release the resources reserved for the handover in the target system. During EPS to 5GS handover when the initial AMF has invoked a target AMF (as described in step 8a in clause 4.11.1.2.2), - when the source MME has received a N26 Forward Relocation Response from the target AMF, the MME sends N26 Cancel Relocation Request directly to the target AMF and the initial AMF is not involved at all in the Cancel Relocation procedure. - When the source MME has not received a response from the target AMF, the MME sends N26 Cancel Relocation Request to the initial AMF: - if the initial AMF has already invoked the target AMF, the initial AMF indicates the Cancel Relocation to the target AMF and the target AMF becomes responsible of the Handover Cancellation (e.g. the target AMF initiates Nsmf_PDUSession_UpdateSMContext request indicating Handover Cancel towards the SMF+PGW-C(s) unless the target AMF has already indicated a Handover failure due to prior target NG RAN rejection of the Handover); - otherwise the initial AMF handles the relocation cancellation. Figure 4.11.1.2.3-1: Handover Cancel procedure 1. When the source RAN (NG-RAN, E-UTRAN) decides to cancel the handover to the target system, the source RAN initiates handover cancel message to the source CN node (AMF or MME). 2. After receiving the handover cancel message from the source RAN, if the source CN node or the target CN node is MME, it sends a Relocation Cancel Request message to the target CN node (MME or AMF). If both the source CN node and target CN node are AMF, the source AMF invokes (via initial AMF if AMF re-allocation is performed during preparation phase) the Namf_Communication_ReleaseUEContext Request (UE Context ID) toward the target AMF. During EPS to 5GS handover with AMF reallocation if the initial AMF has invoked a target AMF, the initial AMF invokes the Namf_Communication_CancelRelocateUEContext Request (UE Context ID) toward the target AMF. 3. The target CN node (MME or AMF) triggers release of resources towards target RAN node. The target RAN node releases the AN resources allocated for the handover. 4. If the target CN node is MME, the MME sends the Delete Session Request to the SGW/SGW-C (see clause 5.5.2.5.2 of TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13]). 4a. [Conditional] The SGW-C releases the corresponding resource in the SGW-U if allocated during the handover preparation. 4b. If the target CN node is AMF, the AMF invokes the Nsmf_PDUSession_UpdateSMContext request (Relocation Cancel Indication) toward the SMF in non-roaming and local breakout roaming scenarios. For home-routed roaming scenario, AMF invokes the Nsmf_PDUSession_UpdateSMContext request (Relocation Cancel Indication) toward the (target) V-SMF and the V-SMF invokes the Nsmf_PDUSession_Update Request (Relocation Cancel Indication) towards the H-SMF. Based on the Relocation Cancel Indication, the SMF(s) deletes the session resources established during handover preparation phase in SMF(s) and UPF(s). 4c. [Conditional] The (target) V-SMF releases the corresponding resource in the (target) V-UPF if allocated during the handover preparation. 4d. [Conditional] The (H-)SMF+PGW-C releases the corresponding resource in the (H-)UPF+PGW-U if allocated during the handover preparation. 5. The target CN node (MME or AMF) sends Relocation Cancel Response towards the source CN node (AMF or MME). 6. The source CN node (AMF or MME) responds with handover cancel ACK towards the source RAN. 7. [Conditional] If target CN node is AMF and the source CN is MME and indirect forwarding tunnel had been set up during handover preparation phase then cancellation of handover triggers the MME to release the temporary resources used for indirect forwarding. 7b. [Conditional] If the source CN is AMF and the target CN node is MME and if indirect forwarding tunnel had been setup during handover preparation phase, then cancellation of handover triggers the AMF to release the session resources established during handover preparation phase in SMF(s) and UPF(s). - The AMF invokes the Nsmf_PDUSession_UpdateSMContext request (Relocation Cancel Indication) toward the SMF in non-roaming and local breakout roaming scenarios. - For home-routed roaming scenario, the AMF invokes the Nsmf_PDUSession_UpdateSMContext request (Relocation Cancel Indication) toward the V-SMF and the V-SMF invokes the Nsmf_PDUSession_Update Request (Relocation Cancel Indication) towards the H-SMF. Based on the Relocation Cancel Indication, the SMF(s) deletes the session resources established during handover preparation phase in SMF(s) and UPF(s). 7c. [Conditional] In home routed roaming case, the V-SMF releases the corresponding resource in the target V-UPF if allocated during the handover preparation. 7d. [Conditional] In non-roaming or LBO case, the SMF+PGW-C releases the corresponding resource in the UPF+PGW-U if allocated during the handover preparation. 8. [Conditional] If target CN node is MME and indirect forwarding tunnel is setup during handover preparation phase then cancellation of handover triggers the target MME to release the temporary resources used for indirect forwarding. 8a. [Conditional] The SGW-C releases the resources for indirect forwarding in the SGW-U if allocated during the handover preparation.