cu_cp: Release drbs at cu-up first

Author: FabianEckermann

lib/cu_cp/routines/pdu_session_resource_release_routine.cpp

@@ -50,24 +50,8 @@ void pdu_session_resource_release_routine::operator()(
     next_config = rrc_ue_up_resource_manager.calculate_update(release_cmd);
   }
 
-  // Release DRB resources at DU
-  {
-    // prepare UeContextModificationRequest and call F1 notifier
-    ue_context_mod_request.ue_index = release_cmd.ue_index;
-    for (const auto& drb_id : next_config.drb_to_remove_list) {
-      ue_context_mod_request.drbs_to_be_released_list.push_back(drb_id);
-    }
-
-    CORO_AWAIT_VALUE(ue_context_modification_response,
-                     f1ap_ue_ctxt_notifier.on_ue_context_modification_request(ue_context_mod_request));
-
-    // Handle UE Context Modification Response
-    if (not ue_context_modification_response.success) {
-      logger.error("ue={}: \"{}\" failed to modify UE context at DU.", release_cmd.ue_index, name());
-    }
-  }
-
   // Inform CU-UP about the release of a bearer
+  // Note: The Bearers must be released at CU-UP before the DRBs are released at DU.
   if (next_config.context_removal_required) {
     // Remove bearer context.
     bearer_context_release_command.ue_index = release_cmd.ue_index;
@@ -98,6 +82,23 @@ void pdu_session_resource_release_routine::operator()(
     }
   }
 
+  // Release DRB resources at DU
+  {
+    // prepare UeContextModificationRequest and call F1 notifier
+    ue_context_mod_request.ue_index = release_cmd.ue_index;
+    for (const auto& drb_id : next_config.drb_to_remove_list) {
+      ue_context_mod_request.drbs_to_be_released_list.push_back(drb_id);
+    }
+
+    CORO_AWAIT_VALUE(ue_context_modification_response,
+                     f1ap_ue_ctxt_notifier.on_ue_context_modification_request(ue_context_mod_request));
+
+    // Handle UE Context Modification Response
+    if (not ue_context_modification_response.success) {
+      logger.error("ue={}: \"{}\" failed to modify UE context at DU.", release_cmd.ue_index, name());
+    }
+  }
+
   CORO_RETURN(generate_pdu_session_resource_release_response(true));
 }