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.
diff --git a/openbsc/tests/gprs/gprs_test.c b/openbsc/tests/gprs/gprs_test.c
new file mode 100644
index 0000000..7b0e641
--- /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;
+}