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

@@ -10,6 +10,7 @@
 # mnc:                  Mobile Network Code
 # mme_addr:             IP address of MME for S1 connnection
 # gtp_bind_addr:        Local IP address to bind for GTP connection
+# gtp_advertise_addr:   IP address of eNB to advertise for DL GTP-U Traffic
 # s1c_bind_addr:        Local IP address to bind for S1AP connection
 # n_prb:                Number of Physical Resource Blocks (6,15,25,50,75,100)
 # tm:                   Transmission mode 1-4 (TM1 default)

srsenb/src/main.cc

@@ -71,6 +71,7 @@ void parse_args(all_args_t* args, int argc, char* argv[])
     ("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.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_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.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.n_prb",              bpo::value<uint32_t>(&args->enb.n_prb)->default_value(25),               "Number of PRB")
     ("enb.nof_ports",          bpo::value<uint32_t>(&args->enb.nof_ports)->default_value(1),            "Number of ports")

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];
+    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];
+  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];
   }