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.
changes/88/3188/1
Holger Hans Peter Freyther 12 years ago
parent 26d0fe3c89
commit faf1f64a2d
  1. 1
      openbsc/.gitignore
  2. 1
      openbsc/configure.ac
  3. 16
      openbsc/include/openbsc/gprs_llc.h
  4. 5
      openbsc/src/gprs/gprs_llc.c
  5. 2
      openbsc/tests/Makefile.am
  6. 8
      openbsc/tests/gprs/Makefile.am
  7. 51
      openbsc/tests/gprs/gprs_test.c
  8. 16
      openbsc/tests/gprs/gprs_test.ok

@ -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

@ -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)

@ -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

@ -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;

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

@ -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

@ -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;
}

@ -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.
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