From 0cc632988ff4c26547c06fa5cf46498a39830ce6 Mon Sep 17 00:00:00 2001
From: Pedro Alvarez <pedro.alvarez@softwareradiosystems.com>
Date: Thu, 7 Jul 2022 16:40:29 +0100
Subject: [PATCH] lib,rlc: tweaked some comments

---
 lib/src/rlc/rlc_am_nr.cc       | 25 +++++++++++++++++++++++--
 lib/test/rlc/rlc_am_nr_test.cc |  3 ++-
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/lib/src/rlc/rlc_am_nr.cc b/lib/src/rlc/rlc_am_nr.cc
index 7c0c1f2a6..a49b86a1d 100644
--- a/lib/src/rlc/rlc_am_nr.cc
+++ b/lib/src/rlc/rlc_am_nr.cc
@@ -784,8 +784,10 @@ void rlc_am_nr_tx::handle_control_pdu(uint8_t* payload, uint32_t nof_bytes)
 
   /*
    * Sanity check the received status report.
-   * Checking if the ACK_SN is inside the the TX_WINDOW makes sure we discard out of order status reports
-   * Checking if ACK_SN > Tx_Next makes sure we do not receive a ACK/NACK for something we did not TX
+   * Checking if the ACK_SN is inside the valid ACK_SN window (the TX window "off-by-one")
+   * makes sure we discard out of order status reports.
+   * Checking if ACK_SN > Tx_Next + 1 makes sure we do not receive a ACK/NACK for something we did not TX
+   * ACK_SN may be equal to TX_NEXT + 1, if not all SDU segments with SN=TX_NEXT have been transmitted.
    */
   if (not valid_ack_sn(status.ack_sn)) {
     RlcInfo("Received ACK with SN outside of TX_WINDOW, ignoring status report. ACK_SN=%d, TX_NEXT_ACK=%d.",
@@ -794,6 +796,7 @@ void rlc_am_nr_tx::handle_control_pdu(uint8_t* payload, uint32_t nof_bytes)
     info_state();
     return;
   }
+
   if (tx_mod_base_nr(status.ack_sn) > tx_mod_base_nr(st.tx_next + 1)) {
     RlcWarning("Received ACK with SN larger than TX_NEXT, ignoring status report.  SN=%d, TX_NEXT_ACK=%d, TX_NEXT=%d",
                status.ack_sn,
@@ -1309,6 +1312,15 @@ bool rlc_am_nr_tx::inside_tx_window(uint32_t sn) const
   return tx_mod_base_nr(sn) < tx_window_size();
 }
 
+/*
+ * This function is used to check if a received status report
+ * as a valid ACK_SN.
+ *
+ * ACK_SN may be equal to TX_NEXT + AM_Window_Size if the PDU
+ * with SN=TX_NEXT+AM_Window_Size has been received by the RX
+ * An ACK_SN == Tx_Next_Ack doesn't ACK or NACKs any PDUs, as
+ * such, such a status report can be discarded.
+ */
 bool rlc_am_nr_tx::valid_ack_sn(uint32_t sn) const
 {
   // Tx_Next_Ack < SN <= TX_Next + AM_Window_Size
@@ -1953,6 +1965,15 @@ bool rlc_am_nr_rx::inside_rx_window(uint32_t sn) const
   return rx_mod_base_nr(sn) < rx_window_size();
 }
 
+/*
+ * This function is used to check if the Rx_Highest_Status is
+ * valid when t-Reasseambly expires.
+ *
+ * ACK_SN may be equal to RX_NEXT + AM_Window_Size if the PDU
+ * with SN=RX_NEXT+AM_Window_Size has been received by the RX.
+ * An ACK_SN == Rx_Next should not update Rx_Highest_Status,
+ * it should be updated when Rx_Next is updated.
+ */
 bool rlc_am_nr_rx::valid_ack_sn(uint32_t sn) const
 {
   // RX_Next < SN <= RX_Next + AM_Window_Size
diff --git a/lib/test/rlc/rlc_am_nr_test.cc b/lib/test/rlc/rlc_am_nr_test.cc
index 24788e1d9..81f31932a 100644
--- a/lib/test/rlc/rlc_am_nr_test.cc
+++ b/lib/test/rlc/rlc_am_nr_test.cc
@@ -3286,6 +3286,7 @@ int lost_status_and_advanced_rx_window(rlc_am_nr_sn_size_t sn_size)
 
   return SRSRAN_SUCCESS;
 }
+
 int full_rx_window_t_reassembly_expiry(rlc_am_nr_sn_size_t sn_size)
 {
   rlc_am_tester tester(false, nullptr);
@@ -3326,7 +3327,7 @@ int full_rx_window_t_reassembly_expiry(rlc_am_nr_sn_size_t sn_size)
     unique_byte_buffer_t pdu_buf = srsran::make_byte_buffer();
     pdu_buf->N_bytes             = rlc1.read_pdu(pdu_buf->msg, 100);
 
-    // Write PDU into RLC 2
+    // Write PDUs into RLC 2
     // Do not write SN=0 to fill up the RX window
     if (sn != 0) {
       rlc2.write_pdu(pdu_buf->msg, pdu_buf->N_bytes);