Reject TAU to force UE to reconnect

srsepc/hdr/mme/nas.h

@@ -247,6 +247,7 @@ public:
   bool pack_identity_request(srslte::byte_buffer_t* nas_buffer);
   bool pack_emm_information(srslte::byte_buffer_t* nas_buffer);
   bool pack_service_reject(srslte::byte_buffer_t* nas_buffer, uint8_t emm_cause);
+  bool pack_tracking_area_update_reject(srslte::byte_buffer_t* nas_buffer, uint8_t emm_cause);
   bool pack_attach_accept(srslte::byte_buffer_t* nas_buffer);
 
   /* Security functions */

srsepc/src/mme/nas.cc

@@ -776,6 +776,8 @@ bool nas::handle_tracking_area_update_request(uint32_t                m_tmsi,
                                               nas_if_t                itf,
                                               srslte::log*            nas_log)
 {
+  srslte::byte_buffer_pool* pool = srslte::byte_buffer_pool::get_instance();
+
   nas_log->info("Tracking Area Update Request -- S-TMSI 0x%x\n", m_tmsi);
   nas_log->console("Tracking Area Update Request -- S-TMSI 0x%x\n", m_tmsi);
   nas_log->info("Tracking Area Update Request -- eNB UE S1AP Id %d\n", enb_ue_s1ap_id);
@@ -789,20 +791,17 @@ bool nas::handle_tracking_area_update_request(uint32_t                m_tmsi,
   hss_interface_nas*  hss  = itf.hss;
   gtpc_interface_nas* gtpc = itf.gtpc;
 
-  uint64_t imsi = s1ap->find_imsi_from_m_tmsi(m_tmsi);
+  // TODO don't search for NAS ctxt, just send that reject
-  if (imsi == 0) {
+  // with context we could enable integrity protection
-    nas_log->console("Could not find IMSI from M-TMSI. M-TMSI 0x%x\n", m_tmsi);
-    nas_log->error("Could not find IMSI from M-TMSI. M-TMSI 0x%x\n", m_tmsi);
-    return true;
-  }
-
-  nas*       nas_ctx = s1ap->find_nas_ctx_from_imsi(imsi);
-  emm_ctx_t* emm_ctx = &nas_ctx->m_emm_ctx;
-  ecm_ctx_t* ecm_ctx = &nas_ctx->m_ecm_ctx;
 
-  sec_ctx_t* sec_ctx = &nas_ctx->m_sec_ctx;
+  nas nas_tmp(args, itf, nas_log);
+  nas_tmp.m_ecm_ctx.enb_ue_s1ap_id = enb_ue_s1ap_id;
+  nas_tmp.m_ecm_ctx.mme_ue_s1ap_id = s1ap->get_next_mme_ue_s1ap_id();
 
-  sec_ctx->ul_nas_count++; // Increment the NAS count, not to break the security ctx
+  srslte::byte_buffer_t* nas_tx    = pool->allocate();
+  nas_tmp.pack_tracking_area_update_reject(nas_tx, LIBLTE_MME_EMM_CAUSE_IMPLICITLY_DETACHED);
+  s1ap->send_downlink_nas_transport(enb_ue_s1ap_id, nas_tmp.m_ecm_ctx.mme_ue_s1ap_id, nas_tx, *enb_sri);
+  pool->deallocate(nas_tx);
   return true;
 }
 
@@ -1150,6 +1149,18 @@ bool nas::handle_tracking_area_update_request(srslte::byte_buffer_t* nas_rx)
   m_nas_log->console("Warning: Tracking Area Update Request messages not handled yet.\n");
   m_nas_log->warning("Warning: Tracking Area Update Request messages not handled yet.\n");
 
+  srslte::byte_buffer_pool* pool = srslte::byte_buffer_pool::get_instance();
+  srslte::byte_buffer_t*    nas_tx;
+
+  /* TAU handling unsupported, therefore send TAU reject with cause IMPLICITLY DETACHED.
+   * this will trigger full re-attach by the UE, instead of going to a TAU request loop */
+  nas_tx = pool->allocate();
+  // TODO we could enable integrity protection in some cases, but UE should comply anyway
+  pack_tracking_area_update_reject(nas_tx, LIBLTE_MME_EMM_CAUSE_IMPLICITLY_DETACHED);
+  // Send reply
+  m_s1ap->send_downlink_nas_transport(m_ecm_ctx.enb_ue_s1ap_id, m_ecm_ctx.mme_ue_s1ap_id, nas_tx, m_ecm_ctx.enb_sri);
+  pool->deallocate(nas_tx);
+
   return true;
 }
 
@@ -1561,6 +1572,28 @@ bool nas::pack_service_reject(srslte::byte_buffer_t* nas_buffer, uint8_t emm_cau
   return true;
 }
 
+bool nas::pack_tracking_area_update_reject(srslte::byte_buffer_t* nas_buffer, uint8_t emm_cause)
+{
+  LIBLTE_MME_TRACKING_AREA_UPDATE_REJECT_MSG_STRUCT tau_rej;
+  tau_rej.t3446_present = false;
+  tau_rej.t3446         = 0;
+  tau_rej.emm_cause     = emm_cause;
+
+  if (emm_cause == LIBLTE_MME_EMM_CAUSE_CONGESTION) {
+    // Standard would want T3446 set in this case
+    m_nas_log->error("Tracking Area Update Reject EMM Cause set to \"CONGESTION\", but back-off timer not set.\n");
+  }
+
+  LIBLTE_ERROR_ENUM err = liblte_mme_pack_tracking_area_update_reject_msg(&tau_rej, LIBLTE_MME_SECURITY_HDR_TYPE_PLAIN_NAS, 0,
+                                                                         (LIBLTE_BYTE_MSG_STRUCT*)nas_buffer);
+  if (err != LIBLTE_SUCCESS) {
+    m_nas_log->error("Error packing Tracking Area Update Reject\n");
+    m_nas_log->console("Error packing Tracking Area Update Reject\n");
+    return false;
+  }
+  return true;
+}
+
 /************************
  *
  * Security Functions