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/.gitignore b/openbsc/.gitignore
index 37495cc..04ebc2f 100644
--- a/openbsc/.gitignore
+++ b/openbsc/.gitignore
@@ -51,6 +51,7 @@
tests/sccp/sccp_test
tests/sms/sms_test
tests/timer/timer_test
+tests/gprs/gprs_test
tests/atconfig
tests/package.m4
diff --git a/openbsc/configure.ac b/openbsc/configure.ac
index 90e3654..636416b 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -112,6 +112,7 @@
tests/channel/Makefile
tests/bsc-nat/Makefile
tests/mgcp/Makefile
+ tests/gprs/Makefile
doc/Makefile
doc/examples/Makefile
Makefile)
diff --git a/openbsc/include/openbsc/gprs_llc.h b/openbsc/include/openbsc/gprs_llc.h
index f905473..e3fc82e 100644
--- a/openbsc/include/openbsc/gprs_llc.h
+++ b/openbsc/include/openbsc/gprs_llc.h
@@ -179,4 +179,20 @@
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
diff --git a/openbsc/src/gprs/gprs_llc.c b/openbsc/src/gprs/gprs_llc.c
index 77fa879..708c4c6 100644
--- a/openbsc/src/gprs/gprs_llc.c
+++ b/openbsc/src/gprs/gprs_llc.c
@@ -518,8 +518,9 @@
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;
diff --git a/openbsc/tests/Makefile.am b/openbsc/tests/Makefile.am
index 64667c9..73e8b89 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 0000000..e441259
--- /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 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;
+}
diff --git a/openbsc/tests/gprs/gprs_test.ok b/openbsc/tests/gprs/gprs_test.ok
new file mode 100644
index 0000000..39d37c1
--- /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.