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.

4 files changed, 76 insertions, 1 deletions

diff --git a/openbsc/tests/Makefile.am b/openbsc/tests/Makefile.am
index 64667c9ad..73e8b89e6 100644
--- a/openbsc/tests/Makefile.am
+++ b/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
diff --git a/openbsc/tests/gprs/Makefile.am b/openbsc/tests/gprs/Makefile.am
new file mode 100644
index 000000000..e44125967
--- /dev/null
+++ b/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
diff --git a/openbsc/tests/gprs/gprs_test.c b/openbsc/tests/gprs/gprs_test.c
new file mode 100644
index 000000000..7b0e64113
--- /dev/null
+++ b/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;
+}
diff --git a/openbsc/tests/gprs/gprs_test.ok b/openbsc/tests/gprs/gprs_test.ok
new file mode 100644
index 000000000..39d37c120
--- /dev/null
+++ b/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.