Handle reestablishment from different PCI, simplified implementation

This implementation might work, but it simply triggers RRC setup on the new
DU when we might send a RRC reestablishment speeding up the whole
connection setup. For this:

- we should send the F1 UE context setup to the new DU
- we should still do the reestablishment, containing the reestablishment message

The latter should be sent once we received the F1 UE context setup
response. We don't need a new context. send a release to the old DU (and
don't release our own context!!).

We might also send the reestablishment to the UE encapsulated with the
F1 UE context setup response. Need to investigate if this is feasible
(because if the DU rejects [can that happen here?], the UE will still
send reestablishment complete -- what then?).

Also, we would need to send the E1 bearer context modification to the
CU-UP.

openair2/RRC/NR/rrc_gNB.c

@@ -1263,6 +1263,11 @@ static void rrc_handle_RRCSetupRequest(gNB_RRC_INST *rrc, sctp_assoc_t assoc_id,
 
 static void rrc_handle_RRCReestablishmentRequest(gNB_RRC_INST *rrc, sctp_assoc_t assoc_id, const NR_RRCReestablishmentRequest_IEs_t *req, const f1ap_initial_ul_rrc_message_t *msg)
 {
+  /* in case we need to do RRC Setup, give the UE a new random identity */
+  uint64_t random_value;
+  fill_random(&random_value, sizeof(random_value));
+  random_value = random_value & 0x7fffffffff; /* random value is 39 bits */
+
   const NR_ReestablishmentCause_t cause = req->reestablishmentCause;
   const long physCellId = req->ue_Identity.physCellId;
   LOG_I(NR_RRC,
@@ -1278,25 +1283,7 @@ static void rrc_handle_RRCReestablishmentRequest(gNB_RRC_INST *rrc, sctp_assoc_t
     LOG_E(RRC, "received CCCH message, but no corresponding DU found\n");
     return;
   }
-  const f1ap_served_cell_info_t *cell_info = &du->setup_req->cell[0].info;
-  if (physCellId != cell_info->nr_pci) {
-    /* UE was moving from previous cell so quickly that RRCReestablishment for previous cell was received in this cell */
-    LOG_E(NR_RRC,
-          " NR_RRCReestablishmentRequest ue_Identity.physCellId(%ld) is not equal to current physCellId(%d), fallback to RRC "
-          "establishment\n",
-          physCellId,
-          cell_info->nr_pci);
-    /* 38.401 8.7: "If the UE accessed from a gNB-DU other than the
-     * original one, the gNB-CU should trigger the UE Context Setup
-     * procedure" we did not implement this yet; TBD for Multi-DU */
-    AssertFatal(false, "not implemented\n");
-    return;
-  }
 
-  /* in case we need to do RRC Setup, give the UE a new random identity */
-  uint64_t random_value;
-  fill_random(&random_value, sizeof(random_value));
-  random_value = random_value & 0x7fffffffff; /* random value is 39 bits */
   if (du->mib == NULL || du->sib1 == NULL) {
     /* we don't have MIB/SIB1 of the DU, and therefore cannot generate the
      * Reestablishment (as we would need the SSB's ARFCN, which we cannot
@@ -1317,6 +1304,23 @@ static void rrc_handle_RRCReestablishmentRequest(gNB_RRC_INST *rrc, sctp_assoc_t
     return;
   }
 
+  const f1ap_served_cell_info_t *cell_info = &du->setup_req->cell[0].info;
+  if (physCellId != cell_info->nr_pci) {
+    /* UE was moving from previous cell so quickly that RRCReestablishment for previous cell was received in this cell */
+    LOG_I(NR_RRC,
+          "RRC Reestablishment Request from different physCellId (%ld) than current physCellId (%d), fallback to RRC setup\n",
+          physCellId,
+          cell_info->nr_pci);
+    /* 38.401 8.7: "If the UE accessed from a gNB-DU other than the original
+     * one, the gNB-CU should trigger the UE Context Setup procedure". Let's
+     * assume that the old DU will trigger a release request, also freeing the
+     * ongoing context at the CU. Hence, create new below */
+    rrc_gNB_ue_context_t *new = rrc_gNB_create_ue_context(assoc_id, msg->crnti, rrc, random_value, msg->gNB_DU_ue_id);
+    new->ue_context.Srb[1].Active = 1;
+    rrc_gNB_generate_RRCSetup(0, msg->crnti, new, msg->du2cu_rrc_container, msg->du2cu_rrc_container_length);
+    return;
+  }
+
   gNB_RRC_UE_t *UE = &ue_context_p->ue_context;
   // 3GPP TS 38.321 version 15.13.0 Section 7.1 Table 7.1-1: RNTI values
   if (req->ue_Identity.c_RNTI < 0x1 || req->ue_Identity.c_RNTI > 0xffef) {