BTS: Convert relative frame numbers to absolute frame numbers

The implementation of the method rcv_rach() in class BTS, restores
the absolute frame number of the incoming RACH-Request by using
the relative frame number (RFn = Fn mod 42432) from the rach
request and the already known internal absolute frame number
m_cur_fn, which is continusly updated by the CCU interface.

In some rare cases, a RACH request might be received by the BTS,
a very short time before the frame number wraps in its 42432.
Depending on the PCU location, RACH request might be received
by the BSC, which forwards it to the PCU. It is then likely
that, while the RACH request is being forwarded to the PCU, the
PCU internal absolute frame number wraps before the RACH
can be processed. The relative frame number from the rach
request would then be interpreted as if it were received after
the wrapping of the internal frame number modulos.

This commit adds logic to detect and resolve this race condition.
Also a unit test is added to check some cornercases.

Change-Id: I74f00c11e5739d49f370ce6c357149e81d9aa759

2 files changed, 225 insertions, 0 deletions

diff --git a/tests/fn/FnTest.cpp b/tests/fn/FnTest.cpp
new file mode 100644
index 00000000..279903c8
--- /dev/null
+++ b/tests/fn/FnTest.cpp
@@ -0,0 +1,175 @@
+/* Frame number calculation test */
+
+/* (C) 2016 by sysmocom s.f.m.c. GmbH <info@sysmocom.de>
+ * All Rights Reserved
+ *
+ * Author: Philipp Maier
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "bts.h"
+#include <string.h>
+#include <stdio.h>
+
+extern "C" {
+#include <osmocom/core/application.h>
+#include <osmocom/gsm/gsm_utils.h>
+#include <osmocom/core/talloc.h>
+#include <osmocom/core/utils.h>
+}
+
+#define RFN_MODULUS 42432
+
+/* globals used by the code */ void *tall_pcu_ctx;
+int16_t spoof_mnc = 0, spoof_mcc = 0;
+
+static uint32_t calc_fn(BTS * bts, uint32_t rfn)
+{
+	uint32_t fn;
+	fn = bts->rfn_to_fn(rfn);
+	printf("rfn=%i ==> fn=%i\n", rfn, fn);
+	return fn;
+}
+
+static void set_fn(BTS * bts, uint32_t fn)
+{
+	printf("\n");
+	bts->set_current_frame_number(fn);
+	printf("bts: fn=%i\n", fn);
+}
+
+static void run_test()
+{
+	BTS bts;
+	uint32_t fn;
+
+	printf("RFN_MODULUS=%i\n",RFN_MODULUS);
+	printf("GSM_MAX_FN=%i\n",GSM_MAX_FN);
+
+
+	/* Test with a collection of real world examples,
+	 * all all of them are not critical and do not
+	 * assume the occurence of any race contions */
+	set_fn(&bts, 1320462);
+	fn = calc_fn(&bts, 5066);
+	OSMO_ASSERT(fn == 1320458);
+
+	set_fn(&bts, 8246);
+	fn = calc_fn(&bts, 8244);
+	OSMO_ASSERT(fn == 8244);
+
+	set_fn(&bts, 10270);
+	fn = calc_fn(&bts, 10269);
+	OSMO_ASSERT(fn == 10269);
+
+	set_fn(&bts, 311276);
+	fn = calc_fn(&bts, 14250);
+	OSMO_ASSERT(fn == 311274);
+
+
+	/* Now lets assume a case where the frame number
+	 * just wrapped over a little bit above the
+	 * modulo 42432 raster, but the rach request
+	 * occurred before the wrapping */
+	set_fn(&bts, RFN_MODULUS + 30);
+	fn = calc_fn(&bts, RFN_MODULUS - 10);
+	OSMO_ASSERT(fn == 42422);
+
+	set_fn(&bts, RFN_MODULUS + 1);
+	fn = calc_fn(&bts, RFN_MODULUS - 1);
+	OSMO_ASSERT(fn == 42431);
+
+	set_fn(&bts, RFN_MODULUS * 123 + 16);
+	fn = calc_fn(&bts, RFN_MODULUS - 4);
+	OSMO_ASSERT(fn == 5219132);
+
+	set_fn(&bts, RFN_MODULUS * 123 + 451);
+	fn = calc_fn(&bts, RFN_MODULUS - 175);
+	OSMO_ASSERT(fn == 5218961);
+
+
+	/* Lets check a special cornercase. We assume that
+	 * the BTS just wrapped its internal frame number
+	 * but we still get rach requests with high relative
+	 * frame numbers. */
+	set_fn(&bts, 0);
+	fn = calc_fn(&bts, RFN_MODULUS - 13);
+	OSMO_ASSERT(fn == 2715635);
+
+	set_fn(&bts, 453);
+	fn = calc_fn(&bts, RFN_MODULUS - 102);
+	OSMO_ASSERT(fn == 2715546);
+
+	set_fn(&bts, 10);
+	fn = calc_fn(&bts, RFN_MODULUS - 10);
+	OSMO_ASSERT(fn == 2715638);
+
+	set_fn(&bts, 23);
+	fn = calc_fn(&bts, RFN_MODULUS - 42);
+	OSMO_ASSERT(fn == 2715606);
+
+
+	/* Also check with some corner case
+	 * values where Fn and RFn reach its
+	 * maximum/minimum valid range */
+	set_fn(&bts, GSM_MAX_FN);
+	fn = calc_fn(&bts, RFN_MODULUS-1);
+	OSMO_ASSERT(fn == GSM_MAX_FN-1);
+
+	set_fn(&bts, 0);
+	fn = calc_fn(&bts, RFN_MODULUS-1);
+	OSMO_ASSERT(fn == GSM_MAX_FN-1);
+
+	set_fn(&bts, GSM_MAX_FN);
+	fn = calc_fn(&bts, 0);
+	OSMO_ASSERT(fn == GSM_MAX_FN);
+
+	set_fn(&bts, 0);
+	fn = calc_fn(&bts, 0);
+	OSMO_ASSERT(fn == 0);
+}
+
+int main(int argc, char **argv)
+{
+	tall_pcu_ctx = talloc_named_const(NULL, 1, "fn test context");
+	if (!tall_pcu_ctx)
+		abort();
+
+	msgb_talloc_ctx_init(tall_pcu_ctx, 0);
+	osmo_init_logging(&gprs_log_info);
+	log_set_use_color(osmo_stderr_target, 0);
+	log_set_print_filename(osmo_stderr_target, 0);
+	log_set_log_level(osmo_stderr_target, LOGL_DEBUG);
+
+	run_test();
+	return EXIT_SUCCESS;
+}
+
+/*
+ * stubs that should not be reached
+ */
+extern "C" {
+	void l1if_pdch_req() {
+		abort();
+	} void l1if_connect_pdch() {
+		abort();
+	}
+	void l1if_close_pdch() {
+		abort();
+	}
+	void l1if_open_pdch() {
+		abort();
+	}
+}
diff --git a/tests/fn/FnTest.ok b/tests/fn/FnTest.ok
new file mode 100644
index 00000000..be6400fe
--- /dev/null
+++ b/tests/fn/FnTest.ok
@@ -0,0 +1,50 @@
+RFN_MODULUS=42432
+GSM_MAX_FN=2715648
+
+bts: fn=1320462
+rfn=5066 ==> fn=1320458
+
+bts: fn=8246
+rfn=8244 ==> fn=8244
+
+bts: fn=10270
+rfn=10269 ==> fn=10269
+
+bts: fn=311276
+rfn=14250 ==> fn=311274
+
+bts: fn=42462
+rfn=42422 ==> fn=42422
+
+bts: fn=42433
+rfn=42431 ==> fn=42431
+
+bts: fn=5219152
+rfn=42428 ==> fn=5219132
+
+bts: fn=5219587
+rfn=42257 ==> fn=5218961
+
+bts: fn=0
+rfn=42419 ==> fn=2715635
+
+bts: fn=453
+rfn=42330 ==> fn=2715546
+
+bts: fn=10
+rfn=42422 ==> fn=2715638
+
+bts: fn=23
+rfn=42390 ==> fn=2715606
+
+bts: fn=2715648
+rfn=42431 ==> fn=2715647
+
+bts: fn=0
+rfn=42431 ==> fn=2715647
+
+bts: fn=2715648
+rfn=0 ==> fn=2715648
+
+bts: fn=0
+rfn=0 ==> fn=0