diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 679576a..9dcc0f1 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -20,7 +20,8 @@ set(HEADER_FILES
 		include/chiaki/audioreceiver.h
 		include/chiaki/video.h
 		include/chiaki/videoreceiver.h
-		include/chiaki/frameprocessor.h)
+		include/chiaki/frameprocessor.h
+		include/chiaki/seqnum.h)
 
 set(SOURCE_FILES
 		src/common.c
diff --git a/lib/include/chiaki/seqnum.h b/lib/include/chiaki/seqnum.h
new file mode 100644
index 0000000..16f2688
--- /dev/null
+++ b/lib/include/chiaki/seqnum.h
@@ -0,0 +1,56 @@
+/*
+ * This file is part of Chiaki.
+ *
+ * Chiaki is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Chiaki 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Chiaki.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef CHIAKI_SEQNUM_H
+#define CHIAKI_SEQNUM_H
+
+#include "common.h"
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef uint16_t ChiakiSeqNum16;
+
+#define CHIAKI_SEQ_NUM_16_MAX 0xffff
+
+static inline bool chiaki_seq_num_16_lt(ChiakiSeqNum16 a, ChiakiSeqNum16 b)
+{
+	if(a == b)
+		return false;
+	int32_t d = (int32_t)b - (int32_t)a;
+	return (a < b && d < (1 << 15))
+		|| ((a > b) && -d > (1 << 15));
+}
+
+static inline bool chiaki_seq_num_16_gt(ChiakiSeqNum16 a, ChiakiSeqNum16 b)
+{
+	if(a == b)
+		return false;
+	int32_t d = (int32_t)b - (int32_t)a;
+	return (a < b && d > (1 << 15))
+		   || ((a > b) && -d < (1 << 15));
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // CHIAKI_SEQNUM_H
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 4cd8ca9..c843621 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -7,7 +7,8 @@ add_executable(chiaki-unit
 		http.c
 		rpcrypt.c
 		gkcrypt.c
-		takion.c)
+		takion.c
+		seqnum.c)
 
 target_link_libraries(chiaki-unit chiaki-lib munit)
 
diff --git a/test/main.c b/test/main.c
index b453138..94e23ec 100644
--- a/test/main.c
+++ b/test/main.c
@@ -17,12 +17,20 @@
 
 #include <munit.h>
 
+extern MunitTest tests_seq_num[];
 extern MunitTest tests_http[];
 extern MunitTest tests_rpcrypt[];
 extern MunitTest tests_gkcrypt[];
 extern MunitTest tests_takion[];
 
 static MunitSuite suites[] = {
+	{
+		"/seq_num",
+		tests_seq_num,
+		NULL,
+		1,
+		MUNIT_SUITE_OPTION_NONE
+	},
 	{
 		"/http",
 		tests_http,
diff --git a/test/seqnum.c b/test/seqnum.c
new file mode 100644
index 0000000..d4a77d1
--- /dev/null
+++ b/test/seqnum.c
@@ -0,0 +1,64 @@
+/*
+ * This file is part of Chiaki.
+ *
+ * Chiaki is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Chiaki 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Chiaki.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include <munit.h>
+
+#include <chiaki/seqnum.h>
+
+
+static MunitResult test_seq_num_16(const MunitParameter params[], void *user)
+{
+	ChiakiSeqNum16 a = 0;
+	do
+	{
+		ChiakiSeqNum16 b = a + 1;
+		munit_assert(chiaki_seq_num_16_gt(b, a));
+		munit_assert(!chiaki_seq_num_16_gt(a, b));
+		munit_assert(chiaki_seq_num_16_lt(a, b));
+		munit_assert(!chiaki_seq_num_16_lt(b, a));
+		a = b;
+	} while(a);
+
+	a = 0;
+	do
+	{
+		ChiakiSeqNum16 b = a + 0xfff;
+		munit_assert(chiaki_seq_num_16_gt(b, a));
+		munit_assert(!chiaki_seq_num_16_gt(a, b));
+		munit_assert(chiaki_seq_num_16_lt(a, b));
+		munit_assert(!chiaki_seq_num_16_lt(b, a));
+		a++;
+	} while(a);
+
+	munit_assert(chiaki_seq_num_16_gt(1, 0xfff5));
+	munit_assert(!chiaki_seq_num_16_gt(0xfff5, 1));
+
+	return MUNIT_OK;
+}
+
+
+MunitTest tests_seq_num[] = {
+	{
+		"/seq_num_16",
+		test_seq_num_16,
+		NULL,
+		NULL,
+		MUNIT_TEST_OPTION_NONE,
+		NULL
+	},
+	{ NULL, NULL, NULL, NULL, MUNIT_TEST_OPTION_NONE, NULL }
+};
\ No newline at end of file