rlcmac: Mark received BSNs falling out of the V(N)/RBB when V(R) is raised

Problem this patch is fixing:
The current RLCMAC window code ported from osmo-pcu is never
invalidating the BSNs which have been received after they are not
needed.
As a result, when the DL TBF keeps sending data for a long time, and
finally BSN wraps around 127->0, when this implementation receives the
BSN=0, it will find it is already received and hence will discard it,
and then keep asking for BSN=0 nevertheless in PKT DL ACK, causing an
endless loop where PCU stays submitting the same block forever.

Explanation of the solution:
The V(N) array contains the status of the previous WS (64 in GPRS) data
blocks. This array is used to construct the RRB signaled to the peer
during PKT DL ACK/NACK messages together with the SSN (start sequence
number), which in our case is mainly V(R), aka one block higher than the
highest received block in the rx window.
Hence, whenever PKT DL ACK/NACK is transmitted, it contains an RRB
ranging [V(R)-1,...V(R)-WS)] mod SNS (SNS=128 in GPRS).

The receive window is basically [V(Q) <= BSN < V(R)] mod SNS, as is of
size 64.
The V(R) is increased whenever a highest new block arrives which is in the
receive window (guaranteeing it will be increased to at most V(Q)+64.

Since we are only announcing state of blocks from V(R)..V(R)-WS, and
blocks received which are before that BSN are dropped since don't fall
inside the rx window, we can securely mark as invalid those blocks
falling behind V(R)-WS whenever we increase V(R).

Related: OS#6102

Change-Id: I962111995e741a7e9c230b2dd4904c2fa9a828e9

3 files changed, 15 insertions, 11 deletions

diff --git a/include/osmocom/gprs/rlcmac/rlc_window_dl.h b/include/osmocom/gprs/rlcmac/rlc_window_dl.h
index 58faaa4..93ccbe4 100644
--- a/include/osmocom/gprs/rlcmac/rlc_window_dl.h
+++ b/include/osmocom/gprs/rlcmac/rlc_window_dl.h
@@ -62,7 +62,6 @@ void gprs_rlcmac_rlc_dl_window_raise_v_r(struct gprs_rlcmac_rlc_dl_window *dlw,
 uint16_t gprs_rlcmac_rlc_dl_window_raise_v_q(struct gprs_rlcmac_rlc_dl_window *dlw);
 
 void gprs_rlcmac_rlc_dl_window_receive_bsn(struct gprs_rlcmac_rlc_dl_window *dlw, uint16_t bsn);
-bool gprs_rlcmac_rlc_dl_window_invalidate_bsn(struct gprs_rlcmac_rlc_dl_window *dlw, uint16_t bsn);
 
 static inline uint16_t gprs_rlcmac_rlc_dl_window_ssn(const struct gprs_rlcmac_rlc_dl_window *dlw)
 {
diff --git a/src/rlcmac/rlc_window_dl.c b/src/rlcmac/rlc_window_dl.c
index 77153b6..eab01cd 100644
--- a/src/rlcmac/rlc_window_dl.c
+++ b/src/rlcmac/rlc_window_dl.c
@@ -69,6 +69,11 @@ void gprs_rlcmac_rlc_v_n_mark_missing(struct gprs_rlcmac_rlc_v_n *v_n, int bsn)
 	return gprs_rlcmac_rlc_v_n_mark(v_n, bsn, GPRS_RLCMAC_RLC_DL_BSN_MISSING);
 }
 
+void gprs_rlcmac_rlc_v_n_mark_invalid(struct gprs_rlcmac_rlc_v_n *v_n, int bsn)
+{
+	return gprs_rlcmac_rlc_v_n_mark(v_n, bsn, GPRS_RLCMAC_RLC_DL_BSN_INVALID);
+}
+
 /*************
  * UL WINDOW
 *************/
@@ -220,8 +225,14 @@ void gprs_rlcmac_rlc_dl_window_raise_v_r(struct gprs_rlcmac_rlc_dl_window *dlw,
 	/* Positive offset, so raise. */
 	if (offset_v_r < (gprs_rlcmac_rlc_window_sns(w) >> 1)) {
 		while (offset_v_r--) {
-			if (offset_v_r) /* all except the received block */
-				gprs_rlcmac_rlc_v_n_mark_missing(&dlw->v_n, gprs_rlcmac_rlc_dl_window_v_r(dlw));
+			const uint16_t v_r = gprs_rlcmac_rlc_dl_window_v_r(dlw);
+			const uint16_t bsn_no_longer_in_ws = gprs_rlcmac_rlc_window_mod_sns_bsn(w, v_r - gprs_rlcmac_rlc_window_ws(w));
+			LOGRLCMAC(LOGL_DEBUG, "- Mark BSN %u as INVALID\n", bsn_no_longer_in_ws);
+			gprs_rlcmac_rlc_v_n_mark_invalid(&dlw->v_n, bsn_no_longer_in_ws);
+			if (offset_v_r) {/* all except the received block */
+				LOGRLCMAC(LOGL_DEBUG, "- Mark BSN %u as MISSING\n", v_r);
+				gprs_rlcmac_rlc_v_n_mark_missing(&dlw->v_n, v_r);
+			}
 			gprs_rlcmac_rlc_dl_window_raise_v_r_to(dlw, 1);
 		}
 		LOGRLCMAC(LOGL_DEBUG, "- Raising V(R) to %d\n", gprs_rlcmac_rlc_dl_window_v_r(dlw));
@@ -255,11 +266,3 @@ void gprs_rlcmac_rlc_dl_window_receive_bsn(struct gprs_rlcmac_rlc_dl_window *dlw
 	gprs_rlcmac_rlc_v_n_mark_received(&dlw->v_n, bsn);
 	gprs_rlcmac_rlc_dl_window_raise_v_r(dlw, bsn);
 }
-
-bool gprs_rlcmac_rlc_dl_window_invalidate_bsn(struct gprs_rlcmac_rlc_dl_window *dlw, uint16_t bsn)
-{
-	bool was_valid = gprs_rlcmac_rlc_v_n_is_received(&dlw->v_n, bsn);
-	gprs_rlcmac_rlc_v_n_mark_missing(&dlw->v_n, bsn);
-
-	return was_valid;
-}
diff --git a/tests/rlcmac/rlcmac_prim_test.err b/tests/rlcmac/rlcmac_prim_test.err
index ec707d2..006efc3 100644
--- a/tests/rlcmac/rlcmac_prim_test.err
+++ b/tests/rlcmac/rlcmac_prim_test.err
@@ -885,6 +885,7 @@ DLGLOBAL DEBUG TBF(DL:NR-0:TLLI-00000001) DL DATA TFI=0 received (V(Q)=0 .. V(R)
 DLGLOBAL DEBUG TBF(DL:NR-0:TLLI-00000001) Got CS-1 RLC data block: FBI=1, BSN=0, SPB=0, S/P=1 RRBP=1, E=0, bitoffs=24
 DLGLOBAL DEBUG TBF(DL:NR-0:TLLI-00000001) BSN 0 storing in window (0..63)
 DLGLOBAL DEBUG TBF(DL:NR-0:TLLI-00000001) data_length=20, data=19 43 c0 01 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
+DLGLOBAL DEBUG - Mark BSN 64 as INVALID
 DLGLOBAL DEBUG - Raising V(R) to 1
 DLGLOBAL DEBUG - Taking block 0 out, raising V(Q) to 1
 DLGLOBAL DEBUG TBF(DL:NR-0:TLLI-00000001) Assembling frames: (len=20)
@@ -923,6 +924,7 @@ DLGLOBAL DEBUG TBF(DL:NR-0:TLLI-00000001) DL DATA TFI=0 received (V(Q)=0 .. V(R)
 DLGLOBAL DEBUG TBF(DL:NR-0:TLLI-00000001) Got CS-1 RLC data block: FBI=1, BSN=0, SPB=0, S/P=1 RRBP=1, E=0, bitoffs=24
 DLGLOBAL DEBUG TBF(DL:NR-0:TLLI-00000001) BSN 0 storing in window (0..63)
 DLGLOBAL DEBUG TBF(DL:NR-0:TLLI-00000001) data_length=20, data=19 43 c0 01 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 
+DLGLOBAL DEBUG - Mark BSN 64 as INVALID
 DLGLOBAL DEBUG - Raising V(R) to 1
 DLGLOBAL DEBUG - Taking block 0 out, raising V(Q) to 1
 DLGLOBAL DEBUG TBF(DL:NR-0:TLLI-00000001) Assembling frames: (len=20)