1 files changed, 60 insertions, 0 deletions
diff --git a/isisd/iso_checksum.c b/isisd/iso_checksum.c
new file mode 100644
index 0000000..f12c195
--- /dev/null
+++ b/isisd/iso_checksum.c
@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * IS-IS Rout(e)ing protocol - iso_checksum.c
+ *                             ISO checksum related routines
+ *
+ * Copyright (C) 2001,2002   Sampo Saaristo
+ *                           Tampere University of Technology
+ *                           Institute of Communications Engineering
+ */
+
+#include <zebra.h>
+#include "iso_checksum.h"
+#include "checksum.h"
+
+/*
+ * Calculations of the OSI checksum.
+ * ISO/IEC 8473 defines the sum as
+ *
+ *     L
+ *  sum  a (mod 255) = 0
+ *     1  i
+ *
+ *     L
+ *  sum (L-i+1)a (mod 255) = 0
+ *     1        i
+ *
+ */
+
+/*
+ * Verifies that the checksum is correct.
+ * Return 0 on correct and 1 on invalid checksum.
+ * Based on Annex C.4 of ISO/IEC 8473
+ */
+
+int iso_csum_verify(uint8_t *buffer, int len, uint16_t csum, int offset)
+{
+	uint16_t checksum;
+	uint32_t c0;
+	uint32_t c1;
+
+	c0 = csum & 0xff00;
+	c1 = csum & 0x00ff;
+
+	/*
+	 * If both are zero return correct
+	 */
+	if (c0 == 0 && c1 == 0)
+		return 0;
+
+	/*
+	 * If either, but not both are zero return incorrect
+	 */
+	if (c0 == 0 || c1 == 0)
+		return 1;
+
+	checksum = fletcher_checksum(buffer, len, offset);
+	if (checksum == htons(csum))
+		return 0;
+	return 1;
+}