rlc, nr: inform upper layer when max retransmissions is exceeded

lib/include/srsran/rlc/rlc_am_nr.h

@@ -76,7 +76,7 @@ struct rlc_amd_tx_pdu_nr {
   uint32_t               pdcp_sn    = INVALID_RLC_SN;
   rlc_am_nr_pdu_header_t header     = {};
   unique_byte_buffer_t   sdu_buf    = nullptr;
-  uint32_t               retx_count = 0;
+  uint32_t               retx_count = RETX_COUNT_NOT_STARTED;
   struct pdu_segment {
     uint32_t so          = 0;
     uint32_t retx_count  = 0;
@@ -135,6 +135,7 @@ private:
 
   uint32_t        mod_nr = 4096;
   inline uint32_t tx_mod_base_nr(uint32_t sn) const;
+  void            check_sn_reached_max_retx(uint32_t sn);
 
   /****************************************************************************
    * Configurable parameters

lib/include/srsran/rlc/rlc_am_nr_packing.h

@@ -22,6 +22,7 @@ namespace srsran {
 const uint32_t RLC_AM_NR_WINDOW_SIZE_12BIT = 4096;
 const uint32_t RLC_AM_NR_WINDOW_SIZE_18BIT = 262144;
 const uint32_t INVALID_RLC_SN              = 0xFFFFFFFF;
+const uint32_t RETX_COUNT_NOT_STARTED      = 0xFFFFFFFF;
 
 ///< AM NR PDU header
 struct rlc_am_nr_pdu_header_t {

lib/src/rlc/rlc_am_nr.cc

@@ -747,14 +747,23 @@ void rlc_am_nr_tx::handle_control_pdu(uint8_t* payload, uint32_t nof_bytes)
               retx.so_end          = segm->so + segm->payload_len;
             }
           }
+
+          // TODO: Handle retx_count for segments
+
         } else {
           // NACK'ing full SDU.
           // add to retx queue if it's not already there
           if (not retx_queue.has_sn(nack_sn)) {
-            // increment Retx counter and inform upper layers if needed
+            // Increment retx_count and inform upper layers if needed
+            if (pdu.retx_count == RETX_COUNT_NOT_STARTED) {
+              // Set retx_count = 0 on first RE-transmission (38.322 Sec. 5.3.2)
+              pdu.retx_count = 0;
+            } else {
+              // Increment otherwise
               pdu.retx_count++;
+            }
 
-            // check_sn_reached_max_retx(nack_sn);
+            check_sn_reached_max_retx(nack_sn);
             rlc_amd_retx_t& retx = retx_queue.push();
             retx.sn              = nack_sn;
             retx.is_segment      = false;
@@ -776,6 +785,30 @@ void rlc_am_nr_tx::handle_control_pdu(uint8_t* payload, uint32_t nof_bytes)
    */
 }
 
+/**
+ * Helper to check if a SN has reached the max reTx threshold
+ *
+ * Caller _must_ hold the mutex when calling the function.
+ * If the retx has been reached for a SN the upper layers (i.e. RRC/PDCP) will be informed.
+ * The SN is _not_ removed from the Tx window, so retransmissions of that SN can still occur.
+ *
+ *
+ * @param  sn The SN of the PDU to check
+ */
+void rlc_am_nr_tx::check_sn_reached_max_retx(uint32_t sn)
+{
+  if (tx_window[sn].retx_count == cfg.max_retx_thresh) {
+    RlcWarning("Signaling max number of reTx=%d for SN=%d", tx_window[sn].retx_count, sn);
+    parent->rrc->max_retx_attempted();
+    srsran::pdcp_sn_vector_t pdcp_sns;
+    pdcp_sns.push_back(tx_window[sn].pdcp_sn);
+    parent->pdcp->notify_failure(parent->lcid, pdcp_sns);
+
+    std::lock_guard<std::mutex> lock(parent->metrics_mutex);
+    parent->metrics.num_lost_pdus++;
+  }
+}
+
 uint32_t rlc_am_nr_tx::get_buffer_state()
 {
   uint32_t tx_queue      = 0;

lib/test/rlc/rlc_am_nr_test.cc

@@ -883,6 +883,74 @@ int retx_segment_test()
   return SRSRAN_SUCCESS;
 }
 
+// This test checks whether RLC informs upper layer when max retransmission has been reached
+int max_retx_test()
+{
+  rlc_am_tester tester;
+  timer_handler timers(8);
+  int           len = 0;
+
+  auto&  test_logger = srslog::fetch_basic_logger("TESTER  ");
+  rlc_am rlc1(srsran_rat_t::nr, srslog::fetch_basic_logger("RLC_AM_1"), 1, &tester, &tester, &timers);
+  srslog::fetch_basic_logger("RLC_AM_1").set_hex_dump_max_size(100);
+  srslog::fetch_basic_logger("RLC").set_hex_dump_max_size(100);
+
+  const rlc_config_t rlc_cfg = rlc_config_t::default_rlc_am_nr_config();
+  if (not rlc1.configure(rlc_cfg)) {
+    return SRSRAN_ERROR;
+  }
+
+  // Push 2 SDUs into RLC1
+  const uint32_t       n_sdus = 2;
+  unique_byte_buffer_t sdu_bufs[n_sdus];
+  for (uint32_t i = 0; i < n_sdus; i++) {
+    sdu_bufs[i]             = srsran::make_byte_buffer();
+    sdu_bufs[i]->msg[0]     = i; // Write the index into the buffer
+    sdu_bufs[i]->N_bytes    = 1; // Give each buffer a size of 1 byte
+    sdu_bufs[i]->md.pdcp_sn = i; // PDCP SN for notifications
+    rlc1.write_sdu(std::move(sdu_bufs[i]));
+  }
+
+  // Read 2 PDUs from RLC1 (1 byte each)
+  const uint32_t n_pdus = 2;
+  byte_buffer_t  pdu_bufs[n_pdus];
+  for (uint32_t i = 0; i < n_pdus; i++) {
+    len                 = rlc1.read_pdu(pdu_bufs[i].msg, 3); // 2 byte header + 1 byte payload
+    pdu_bufs[i].N_bytes = len;
+  }
+
+  TESTASSERT(0 == rlc1.get_buffer_state());
+
+  // Fake status PDU that ack SN=1 and nack SN=0
+  rlc_am_nr_status_pdu_t fake_status = {};
+  fake_status.ack_sn                 = 2; // delivered up to SN=1
+  fake_status.N_nack                 = 1; // one SN was lost
+  fake_status.nacks[0].nack_sn       = 0; // it was SN=0 that was lost
+
+  // pack into PDU
+  byte_buffer_t status_pdu;
+  rlc_am_nr_write_status_pdu(fake_status, rlc_cfg.am_nr.tx_sn_field_length, &status_pdu);
+
+  // Exceed the number of tolerated retransmissions by one additional retransmission
+  // to trigger notification of the higher protocol layers. Note that the initial transmission
+  // (before starting retransmissions) does not count. See TS 38.322 Sec. 5.3.2
+  for (uint32_t retx_count = 0; retx_count < rlc_cfg.am_nr.max_retx_thresh + 1; ++retx_count) {
+    // we've not yet reached max attempts
+    TESTASSERT(tester.max_retx_triggered == false);
+
+    // Write status PDU to RLC1
+    rlc1.write_pdu(status_pdu.msg, status_pdu.N_bytes);
+
+    byte_buffer_t pdu_buf;
+    len = rlc1.read_pdu(pdu_buf.msg, 3); // 2 byte header + 1 byte payload
+  }
+
+  // Now maxRetx should have been triggered
+  TESTASSERT(tester.max_retx_triggered == true);
+
+  return SRSRAN_SUCCESS;
+}
+
 // This test checks the correct functioning of RLC discard functionality
 int discard_test()
 {
@@ -1016,6 +1084,7 @@ int main()
   TESTASSERT(basic_segmentation_test() == SRSRAN_SUCCESS);
   TESTASSERT(segment_retx_test() == SRSRAN_SUCCESS);
   TESTASSERT(retx_segment_test() == SRSRAN_SUCCESS);
+  TESTASSERT(max_retx_test() == SRSRAN_SUCCESS);
   TESTASSERT(discard_test() == SRSRAN_SUCCESS);
   return SRSRAN_SUCCESS;
 }