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-rw-r--r--isisd/iso_checksum.c60
1 files changed, 60 insertions, 0 deletions
diff --git a/isisd/iso_checksum.c b/isisd/iso_checksum.c
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+// 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;
+}