Add NGAP Handover Failure message

This message is sent from the Target NG-RAN to the AMF.

The commit introduces:
* NGAP encoding function
* N2 callback for the target gNB
* handle the failure in RRC, inform NGAP
* send NGAP message to AMF via SCTP

Note: UE context in NGAP is fetched from the amf_ue_ngap_id

Co-authored-by: batuhan duyuler <batuhan.duyuler@firecell.io>

openair2/COMMON/ngap_messages_def.h

@@ -72,6 +72,7 @@ MESSAGE_DEF(NGAP_PDUSESSION_SETUP_RESP          , MESSAGE_PRIORITY_MED, ngap_pdu
 MESSAGE_DEF(NGAP_PDUSESSION_MODIFY_RESP          , MESSAGE_PRIORITY_MED, ngap_pdusession_modify_resp_t          , ngap_pdusession_modify_resp)
 MESSAGE_DEF(NGAP_PDUSESSION_RELEASE_RESPONSE    , MESSAGE_PRIORITY_MED, ngap_pdusession_release_resp_t        , ngap_pdusession_release_resp)
 MESSAGE_DEF(NGAP_HANDOVER_REQUIRED, MESSAGE_PRIORITY_MED, ngap_handover_required_t, ngap_handover_required)
+MESSAGE_DEF(NGAP_HANDOVER_FAILURE, MESSAGE_PRIORITY_MED, ngap_handover_failure_t, ngap_handover_failure)
 
 /* NGAP -> RRC messages */
 MESSAGE_DEF(NGAP_DOWNLINK_NAS              , MESSAGE_PRIORITY_MED, ngap_downlink_nas_t              , ngap_downlink_nas )

openair2/COMMON/ngap_messages_types.h

@@ -66,6 +66,8 @@
 #define NGAP_PDUSESSION_MODIFY_REQ(mSGpTR)              (mSGpTR)->ittiMsg.ngap_pdusession_modify_req
 #define NGAP_PAGING_IND(mSGpTR)                 (mSGpTR)->ittiMsg.ngap_paging_ind
 #define NGAP_HANDOVER_REQUIRED(mSGpTR)           (mSGpTR)->ittiMsg.ngap_handover_required
+#define NGAP_HANDOVER_FAILURE(mSGpTR) (mSGpTR)->ittiMsg.ngap_handover_failure
+
 #define NGAP_UE_CONTEXT_RELEASE_REQ(mSGpTR)     (mSGpTR)->ittiMsg.ngap_ue_release_req
 #define NGAP_PDUSESSION_RELEASE_COMMAND(mSGpTR)      (mSGpTR)->ittiMsg.ngap_pdusession_release_command
 #define NGAP_PDUSESSION_RELEASE_RESPONSE(mSGpTR)     (mSGpTR)->ittiMsg.ngap_pdusession_release_resp
@@ -548,6 +550,14 @@ typedef struct {
   source_to_target_transparent_container_t *source2target;
 } ngap_handover_required_t;
 
+/* 3GPP TS 38.413 9.2.3.6 */
+typedef struct {
+  // AMF UE NGAP ID (M)
+  uint64_t amf_ue_ngap_id;
+  // Cause (M)
+  ngap_cause_t cause;
+} ngap_handover_failure_t;
+
 typedef struct ngap_ue_cap_info_ind_s {
   uint32_t  gNB_ue_ngap_id;
   byte_array_t ue_radio_cap;

openair2/RRC/NR/rrc_gNB_NGAP.c

@@ -1063,7 +1063,15 @@ int rrc_gNB_process_NGAP_UE_CONTEXT_RELEASE_REQ(MessageDef *msg_p, instance_t in
   }
 }
 
-//-----------------------------------------------------------------------------
+/** @brief Sends the NG Handover Failure from the Target NG-RAN to the AMF */
+void rrc_gNB_send_NGAP_HANDOVER_FAILURE(gNB_RRC_INST *rrc, ngap_handover_failure_t *msg)
+{
+  LOG_I(NR_RRC, "Send NG Handover Failure message (amf_ue_ngap_id %ld) with cause %d \n ", msg->amf_ue_ngap_id, msg->cause.value);
+  MessageDef *msg_p = itti_alloc_new_message(TASK_RRC_GNB, 0, NGAP_HANDOVER_FAILURE);
+  NGAP_HANDOVER_FAILURE(msg_p) = *msg;
+  itti_send_msg_to_task(TASK_NGAP, rrc->module_id, msg_p);
+}
+
 /*
 * Process the NG command NGAP_UE_CONTEXT_RELEASE_COMMAND, sent by AMF.
 * The gNB should remove all pdu session, NG context, and other context of the UE.

openair2/RRC/NR/rrc_gNB_NGAP.h

@@ -94,4 +94,6 @@ void rrc_gNB_send_NGAP_HANDOVER_REQUIRED(gNB_RRC_INST *rrc,
                                          const nr_neighbour_cell_t *neighbourCellConfiguration,
                                          const byte_array_t hoPrepInfo);
 
+void rrc_gNB_send_NGAP_HANDOVER_FAILURE(gNB_RRC_INST *rrc, ngap_handover_failure_t *msg);
+
 #endif

openair2/RRC/NR/rrc_gNB_mobility.c

@@ -406,6 +406,19 @@ static byte_array_t rrc_gNB_generate_HandoverPreparationInformation(gNB_RRC_INST
   return hoPrepInfo;
 }
 
+/** @brief Callback function to trigger NG Handover Failure on the target gNB, to inform the AMF
+ * that the preparation of resources has failed (e.g. unsatisfied criteria, gNB is already loaded).
+ * This message represents an Unsuccessful Outcome of the Handover Resource Allocation */
+void nr_rrc_n2_ho_failure(gNB_RRC_INST *rrc, uint32_t gnb_ue_id, ngap_handover_failure_t *msg)
+{
+  LOG_I(NR_RRC, "Triggering N2 Handover Failure\n");
+  rrc_gNB_send_NGAP_HANDOVER_FAILURE(rrc, msg);
+  LOG_I(NR_RRC, "Send UE Context Release for gnb_ue_id %d\n", gnb_ue_id);
+  rrc_gNB_ue_context_t *ue_context_p = rrc_gNB_get_ue_context(rrc, gnb_ue_id);
+  rrc_gNB_send_NGAP_UE_CONTEXT_RELEASE_REQ(rrc->module_id, ue_context_p, msg->cause);
+  return;
+}
+
 /** @brief Trigger N2 handover on source gNB:
  *         1) Prepare RRC Container with HandoverPreparationInformation message
  *         2) send NGAP Handover Required message */

openair2/RRC/NR/rrc_gNB_mobility.h

@@ -53,6 +53,7 @@ typedef struct nr_ho_source_cu {
 /* acknowledgement of handover request. buf+len is the RRC Reconfiguration */
 typedef void (*ho_req_ack_t)(gNB_RRC_INST *rrc, gNB_RRC_UE_t *ue);
 typedef void (*ho_success_t)(gNB_RRC_INST *rrc, gNB_RRC_UE_t *ue);
+
 typedef struct nr_ho_target_cu {
   /// pointer to the (target) DU structure
   const nr_rrc_du_container_t *du;
@@ -78,6 +79,7 @@ nr_handover_context_t *alloc_ho_ctx(ho_ctx_type_t type);
 
 void nr_rrc_trigger_f1_ho(gNB_RRC_INST *rrc, gNB_RRC_UE_t *ue, nr_rrc_du_container_t *source_du, nr_rrc_du_container_t *target_du);
 void nr_rrc_finalize_ho(gNB_RRC_UE_t *ue);
+void nr_rrc_n2_ho_failure(gNB_RRC_INST *rrc, uint32_t gnb_ue_id, ngap_handover_failure_t *msg);
 
 void nr_rrc_trigger_n2_ho(gNB_RRC_INST *rrc,
                           gNB_RRC_UE_t *ue,