Add feature to provide custom GTP-U (S1) address to be advertised inside S1AP messages

This feature is useful in scenarios where eNB is behind a NAT and EPC on public cloud
Since eNB's GTP-U can only bind to IP address visible in it's host (usually a private IP beind a NAT), it advertises the same to UE in S1AP message
resulting in traffic from EPC to eNB (DL traffic) not able to reach eNB. This commit solves this issue by providing a means to advertise a different IP address than the one GTP-U UDP socket is bound to.
This IP is usually advertised in InitialContextSetupResponse and E-RABSetupResponse.

In order to make use of this feature, use the following parameter in enb.conf

Example:

gtp_advertise_addr = 213.61.141.82

lib/include/srsran/interfaces/enb_s1ap_interfaces.h

@@ -26,6 +26,7 @@ struct s1ap_args_t {
   uint16_t    mnc;     // BCD-coded with 0xF filler
   std::string mme_addr;
   std::string gtp_bind_addr;
+  std::string gtp_advertise_addr;
   std::string s1c_bind_addr;
   std::string enb_name;
 };

srsenb/enb.conf.example

@@ -5,15 +5,16 @@
 #####################################################################
 # eNB configuration
 #
-# enb_id:         20-bit eNB identifier.
+# enb_id:               20-bit eNB identifier.
-# mcc:            Mobile Country Code
+# mcc:                  Mobile Country Code
-# mnc:            Mobile Network Code
+# mnc:                  Mobile Network Code
-# mme_addr:       IP address of MME for S1 connnection
+# mme_addr:             IP address of MME for S1 connnection
-# gtp_bind_addr:  Local IP address to bind for GTP connection
+# gtp_bind_addr:        Local IP address to bind for GTP connection
-# s1c_bind_addr:  Local IP address to bind for S1AP connection
+# gtp_advertise_addr:   IP address of eNB to advertise for DL GTP-U Traffic
-# n_prb:          Number of Physical Resource Blocks (6,15,25,50,75,100)
+# s1c_bind_addr:        Local IP address to bind for S1AP connection
-# tm:             Transmission mode 1-4 (TM1 default)
+# n_prb:                Number of Physical Resource Blocks (6,15,25,50,75,100)
-# nof_ports:      Number of Tx ports (1 port default, set to 2 for TM2/3/4)
+# tm:                   Transmission mode 1-4 (TM1 default)
+# nof_ports:            Number of Tx ports (1 port default, set to 2 for TM2/3/4)
 #
 #####################################################################
 [enb]

srsenb/src/main.cc

@@ -64,18 +64,19 @@ void parse_args(all_args_t* args, int argc, char* argv[])
   bpo::options_description common("Configuration options");
   common.add_options()
 
-    ("enb.stack", bpo::value<string>(&args->stack.type)->default_value("lte"), "Type of the upper stack [lte, nr]")
+    ("enb.stack",              bpo::value<string>(&args->stack.type)->default_value("lte"), "Type of the upper stack [lte, nr]")
-    ("enb.enb_id",        bpo::value<string>(&enb_id)->default_value("0x0"),                       "eNodeB ID")
+    ("enb.enb_id",             bpo::value<string>(&enb_id)->default_value("0x0"),                       "eNodeB ID")
-    ("enb.name",          bpo::value<string>(&args->stack.s1ap.enb_name)->default_value("srsenb01"), "eNodeB Name")
+    ("enb.name",               bpo::value<string>(&args->stack.s1ap.enb_name)->default_value("srsenb01"), "eNodeB Name")
-    ("enb.mcc",           bpo::value<string>(&mcc)->default_value("001"),                          "Mobile Country Code")
+    ("enb.mcc",                bpo::value<string>(&mcc)->default_value("001"),                          "Mobile Country Code")
-    ("enb.mnc",           bpo::value<string>(&mnc)->default_value("01"),                           "Mobile Network Code")
+    ("enb.mnc",                bpo::value<string>(&mnc)->default_value("01"),                           "Mobile Network Code")
-    ("enb.mme_addr",      bpo::value<string>(&args->stack.s1ap.mme_addr)->default_value("127.0.0.1"),"IP address of MME for S1 connection")
+    ("enb.mme_addr",           bpo::value<string>(&args->stack.s1ap.mme_addr)->default_value("127.0.0.1"),"IP address of MME for S1 connection")
-    ("enb.gtp_bind_addr", bpo::value<string>(&args->stack.s1ap.gtp_bind_addr)->default_value("192.168.3.1"), "Local IP address to bind for GTP connection")
+    ("enb.gtp_bind_addr",      bpo::value<string>(&args->stack.s1ap.gtp_bind_addr)->default_value("192.168.3.1"), "Local IP address to bind for GTP connection")
-    ("enb.s1c_bind_addr", bpo::value<string>(&args->stack.s1ap.s1c_bind_addr)->default_value("192.168.3.1"), "Local IP address to bind for S1AP connection")
+    ("enb.gtp_advertise_addr", bpo::value<string>(&args->stack.s1ap.gtp_advertise_addr)->default_value(""), "IP address of eNB to advertise for DL GTP-U Traffic")
-    ("enb.n_prb",         bpo::value<uint32_t>(&args->enb.n_prb)->default_value(25),               "Number of PRB")
+    ("enb.s1c_bind_addr",      bpo::value<string>(&args->stack.s1ap.s1c_bind_addr)->default_value("192.168.3.1"), "Local IP address to bind for S1AP connection")
-    ("enb.nof_ports",     bpo::value<uint32_t>(&args->enb.nof_ports)->default_value(1),            "Number of ports")
+    ("enb.n_prb",              bpo::value<uint32_t>(&args->enb.n_prb)->default_value(25),               "Number of PRB")
-    ("enb.tm",            bpo::value<uint32_t>(&args->enb.transmission_mode)->default_value(1),    "Transmission mode (1-8)")
+    ("enb.nof_ports",          bpo::value<uint32_t>(&args->enb.nof_ports)->default_value(1),            "Number of ports")
-    ("enb.p_a",           bpo::value<float>(&args->enb.p_a)->default_value(0.0f),                  "Power allocation rho_a (-6, -4.77, -3, -1.77, 0, 1, 2, 3)")
+    ("enb.tm",                 bpo::value<uint32_t>(&args->enb.transmission_mode)->default_value(1),    "Transmission mode (1-8)")
+    ("enb.p_a",                bpo::value<float>(&args->enb.p_a)->default_value(0.0f),                  "Power allocation rho_a (-6, -4.77, -3, -1.77, 0, 1, 2, 3)")
 
     ("enb_files.sib_config", bpo::value<string>(&args->enb_files.sib_config)->default_value("sib.conf"), "SIB configuration files")
     ("enb_files.rr_config",  bpo::value<string>(&args->enb_files.rr_config)->default_value("rr.conf"),   "RR configuration files")

srsenb/src/stack/upper/s1ap.cc

@@ -1176,7 +1176,11 @@ bool s1ap::send_ho_req_ack(const asn1::s1ap::ho_request_s&                msg,
     container.erab_admitted_list.value[i].load_info_obj(ASN1_S1AP_ID_ERAB_ADMITTED_ITEM);
     auto& c                   = container.erab_admitted_list.value[i].value.erab_admitted_item();
     c                         = admitted_bearers[i];
-    c.transport_layer_address = addr_to_asn1(args.gtp_bind_addr.c_str());
+    if (!args.gtp_advertise_addr.empty()) {
+      c.transport_layer_address = addr_to_asn1(args.gtp_advertise_addr.c_str());
+    } else {
+      c.transport_layer_address = addr_to_asn1(args.gtp_bind_addr.c_str());
+    }
 
     // If E-RAB is proposed for forward tunneling
     if (c.dl_g_tp_teid_present) {
@@ -1669,7 +1673,11 @@ void s1ap::ue::get_erab_addr(uint16_t erab_id, transp_addr_t& transp_addr, asn1:
   // Note: RRC does not yet update correctly gtpu transp_addr
   transp_addr.resize(32);
   uint8_t addr[4];
-  inet_pton(AF_INET, s1ap_ptr->args.gtp_bind_addr.c_str(), addr);
+  if (!s1ap_ptr->args.gtp_advertise_addr.empty()) {
+    inet_pton(AF_INET, s1ap_ptr->args.gtp_advertise_addr.c_str(), addr);
+  } else {
+    inet_pton(AF_INET, s1ap_ptr->args.gtp_bind_addr.c_str(), addr);
+  }
   for (uint32_t j = 0; j < 4; ++j) {
     transp_addr.data()[j] = addr[3 - j];
   }