gnb,cu_up: add option to set external address of N3 interface

Author: robertfalkenberg

apps/gnb/gnb_appconfig.h

@@ -649,6 +649,7 @@ struct amf_appconfig {
   std::string n2_bind_interface      = "auto";
   std::string n3_bind_addr           = "auto";
   std::string n3_bind_interface      = "auto";
+  std::string n3_ext_addr            = "auto";
   int         sctp_rto_initial       = 120;
   int         sctp_rto_min           = 120;
   int         sctp_rto_max           = 500;

apps/gnb/gnb_appconfig_translators.cpp

@@ -417,6 +417,7 @@ srs_cu_up::cu_up_configuration srsran::generate_cu_up_config(const gnb_appconfig
   } else {
     out_cfg.net_cfg.n3_bind_addr = config.amf_cfg.n3_bind_addr;
   }
+  out_cfg.net_cfg.n3_ext_addr       = config.amf_cfg.n3_ext_addr;
   out_cfg.net_cfg.n3_bind_interface = config.amf_cfg.n3_bind_interface;
   out_cfg.net_cfg.n3_rx_max_mmsg    = config.amf_cfg.udp_rx_max_msgs;
   out_cfg.net_cfg.f1u_bind_addr = config.amf_cfg.bind_addr; // FIXME: check if this can be removed for co-located case

include/srsran/cu_up/cu_up_configuration.h

@@ -35,6 +35,10 @@ struct network_interface_config {
   /// Local IP address to bind for connection from UPF to receive downlink user-plane traffic (N3 interface).
   std::string n3_bind_addr = "127.0.1.1";
 
+  /// External IP address that is advertised to receive GTP-U packets from UPF via N3 interface.
+  /// It defaults to \c n3_bind_addr but may differ in case the CU-UP is behind a NAT.
+  std::string n3_ext_addr = "auto";
+
   /// Interface name to bind the N3. `auto` does not force a specific interface and uses a normal `bind()`.
   std::string n3_bind_interface = "auto";
 

lib/cu_up/pdu_session_manager_impl.cpp

@@ -260,8 +260,12 @@ pdu_session_setup_result pdu_session_manager_impl::setup_pdu_session(const e1ap_
   // Allocate local TEID
   new_session->local_teid = allocate_local_teid(new_session->pdu_session_id);
 
-  pdu_session_result.gtp_tunnel = up_transport_layer_info(
-      transport_layer_address::create_from_string(net_config.n3_bind_addr), new_session->local_teid);
+  // Advertise either local or external IP address of N3 interface
+  const std::string& n3_addr = net_config.n3_ext_addr.empty() || net_config.n3_ext_addr == "auto"
+                                   ? net_config.n3_bind_addr
+                                   : net_config.n3_ext_addr;
+  pdu_session_result.gtp_tunnel =
+      up_transport_layer_info(transport_layer_address::create_from_string(n3_addr), new_session->local_teid);
 
   // Create SDAP entity
   sdap_entity_creation_message sdap_msg = {ue_index, session.pdu_session_id, &new_session->sdap_to_gtpu_adapter};