gprs: Honor GSM 04.64 8.4.2 Receipt of unacknowledged information

GSM 04.64 8.4.2 asks to ignore UI frames if the DLCI is not known,
or if the "(V(UR)- 32) <= N(U) < V(UR)". E.g. if we want to have
V(UR) == 511 and this frame is dropped, we would ignore N(U)'s
0 to 510. Calculate the delta.

The code is based on Jonathan Santos's "LLC UI window" fix but the
issue was discovered independly.

openbsc/.gitignore

@@ -51,6 +51,7 @@ tests/mgcp/mgcp_test
 tests/sccp/sccp_test
 tests/sms/sms_test
 tests/timer/timer_test
+tests/gprs/gprs_test
 
 tests/atconfig
 tests/package.m4

openbsc/configure.ac

@@ -112,6 +112,7 @@ AC_OUTPUT(
     tests/channel/Makefile
     tests/bsc-nat/Makefile
     tests/mgcp/Makefile
+    tests/gprs/Makefile
     doc/Makefile
     doc/examples/Makefile
     Makefile)

openbsc/include/openbsc/gprs_llc.h

@@ -179,4 +179,20 @@ int gprs_llgmm_assign(struct gprs_llc_llme *llme,
 int gprs_llc_init(const char *cipher_plugin_path);
 int gprs_llc_vty_init(void);
 
+/**
+ * \short Check if N(U) should be considered a retransmit
+ *
+ * Implements the range check as of GSM 04.64 8.4.2
+ * Receipt of unacknowledged information.
+ *
+ * @returns Returns 1 if  (V(UR)-32) <= N(U) < V(UR)
+ * @param nu N(U) unconfirmed sequence number of the UI frame
+ * @param vur V(UR) unconfirmend received state variable
+ */
+static inline int gprs_llc_is_retransmit(uint16_t nu, uint16_t vur)
+{
+	int delta = (vur - nu) & 0x1ff;
+	return 0 < delta && delta < 32;
+}
+
 #endif

openbsc/src/gprs/gprs_llc.c

@@ -518,8 +518,9 @@ static int gprs_llc_hdr_rx(struct gprs_llc_hdr_parsed *gph,
 		rx_llc_xid(lle, gph);
 		break;
 	case GPRS_LLC_UI:
-		if (gph->seq_tx < lle->vu_recv) {
-			LOGP(DLLC, LOGL_NOTICE, "TLLI=%08x dropping UI, vurecv %u <= %u\n",
+		if (gprs_llc_is_retransmit(gph->seq_tx, lle->vu_recv)) {
+			LOGP(DLLC, LOGL_NOTICE,
+				"TLLI=%08x dropping UI, N(U=%d) not in window V(URV(UR:%d).\n",
 				lle->llme ? lle->llme->tlli : -1,
 				gph->seq_tx, lle->vu_recv);
 			return -EIO;

openbsc/tests/Makefile.am

@@ -1,4 +1,4 @@
-SUBDIRS = debug gsm0408 db channel mgcp
+SUBDIRS = debug gsm0408 db channel mgcp gprs
 
 if BUILD_NAT
 SUBDIRS += bsc-nat

openbsc/tests/gprs/Makefile.am

@@ -0,0 +1,8 @@
+INCLUDES = $(all_includes) -I$(top_srcdir)/include
+AM_CFLAGS=-Wall -ggdb3 $(LIBOSMOCORE_CFLAGS) $(LIBOSMOGSM_CFLAGS)
+
+EXTRA_DIST = gprs_test.ok
+
+noinst_PROGRAMS = gprs_test
+
+gprs_test_SOURCES = gprs_test.c

openbsc/tests/gprs/gprs_test.c

@@ -0,0 +1,51 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <inttypes.h>
+
+#include <openbsc/gprs_llc.h>
+
+#define ASSERT_FALSE(x) if (x)  { printf("Should have returned false.\n"); abort(); }
+#define ASSERT_TRUE(x)  if (!x) { printf("Should have returned true.\n"); abort(); }
+
+/**
+ * GSM 04.64 8.4.2 Receipt of unacknowledged information
+ */
+static int nu_is_retransmission(uint16_t nu, uint16_t vur)
+{
+	int ret = gprs_llc_is_retransmit(nu, vur);
+	printf("N(U) = %d, V(UR) = %d => %s\n", nu, vur,
+	       ret == 1 ? "retransmit" : "new");
+	return ret;
+}
+
+static void test_8_4_2()
+{
+	printf("Testing gprs_llc_is_retransmit.\n");
+
+	ASSERT_FALSE(nu_is_retransmission(0, 0));
+	ASSERT_TRUE (nu_is_retransmission(0, 1));
+
+	/* expect 1... check for retransmissions */
+	ASSERT_TRUE (nu_is_retransmission(0, 1));
+	ASSERT_TRUE (nu_is_retransmission(511, 1));
+	ASSERT_TRUE (nu_is_retransmission(483, 1));
+	ASSERT_TRUE (nu_is_retransmission(482, 1));
+	ASSERT_FALSE(nu_is_retransmission(481, 1));
+
+	/* expect 511... check for retransmissions */
+	ASSERT_FALSE(nu_is_retransmission(0, 240)); // ahead
+	ASSERT_FALSE(nu_is_retransmission(0, 511)); // ahead
+	ASSERT_FALSE(nu_is_retransmission(1, 511)); // ahead
+	ASSERT_FALSE(nu_is_retransmission(511, 511)); // same
+	ASSERT_TRUE (nu_is_retransmission(510, 511)); // behind
+	ASSERT_TRUE (nu_is_retransmission(481, 511)); // behind
+	ASSERT_FALSE(nu_is_retransmission(479, 511)); // wrapped
+}
+
+int main(int argc, char **argv)
+{
+	test_8_4_2();
+
+	printf("Done.\n");
+	return EXIT_SUCCESS;
+}

openbsc/tests/gprs/gprs_test.ok

@@ -0,0 +1,16 @@
+Testing gprs_llc_is_retransmit.
+N(U) = 0, V(UR) = 0 => new
+N(U) = 0, V(UR) = 1 => retransmit
+N(U) = 0, V(UR) = 1 => retransmit
+N(U) = 511, V(UR) = 1 => retransmit
+N(U) = 483, V(UR) = 1 => retransmit
+N(U) = 482, V(UR) = 1 => retransmit
+N(U) = 481, V(UR) = 1 => new
+N(U) = 0, V(UR) = 240 => new
+N(U) = 0, V(UR) = 511 => new
+N(U) = 1, V(UR) = 511 => new
+N(U) = 511, V(UR) = 511 => new
+N(U) = 510, V(UR) = 511 => retransmit
+N(U) = 481, V(UR) = 511 => retransmit
+N(U) = 479, V(UR) = 511 => new
+Done.