Initial revision
diff --git a/isisd/iso_checksum.c b/isisd/iso_checksum.c
new file mode 100644
index 0000000..d0bb8a7
--- /dev/null
+++ b/isisd/iso_checksum.c
@@ -0,0 +1,192 @@
+/*
+ * 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
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public Licenseas published by the Free
+ * Software Foundation; either version 2 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 General Public License for
+ * more details.
+
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ */
+
+#include <zebra.h>
+#include "iso_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
+ * FIXME: Check for overflow
+ */
+
+int
+iso_csum_verify (u_char *buffer, int len, uint16_t *csum)
+{
+ u_int8_t *p;
+ u_int32_t c0;
+ u_int32_t c1;
+ u_int16_t checksum;
+ int i;
+
+ p = buffer;
+ checksum = 0;
+ 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;
+
+ /*
+ * Otherwise initialize to zero and calculate...
+ */
+ c0 = 0;
+ c1 = 0;
+
+ for (i = 0; i < len; i++) {
+ c0 = c0 + *(p++);
+ c1 += c0;
+ }
+
+ c0 = c0 % 255;
+ c1 = c1 % 255;
+
+ if ( c0 == 0 && c1 == 0)
+ return 0;
+
+ return 1;
+}
+
+
+/*
+ * Creates the checksum. *csum points to the position of the checksum in the
+ * PDU.
+ * Based on Annex C.4 of ISO/IEC 8473
+ * we will not overflow until about length of 6000,
+ * which is the answer to (255+255n)*n/2 > 2^32
+ * so if we have a length of over 5000 we will return zero (for now)
+ */
+#define FIXED_CODE
+u_int16_t
+iso_csum_create (u_char *buffer, int len, u_int16_t n)
+{
+
+ u_int8_t *p;
+ int x;
+ int y;
+ u_int32_t mul;
+ u_int32_t c0;
+ u_int32_t c1;
+ u_int16_t checksum;
+ u_int16_t *csum;
+ int i;
+
+ checksum = 0;
+
+ /*
+ * Zero the csum in the packet.
+ */
+ csum = (u_int16_t*)(buffer + n);
+ *(csum) = checksum;
+
+ /* for the limitation of our implementation */
+ if (len > 5000) {
+ return 0;
+ }
+
+ p = buffer;
+ c0 = 0;
+ c1 = 0;
+
+ for (i = 0; i < len; i++) {
+ c0 = c0 + *(p++);
+ c1 += c0;
+ }
+
+ c0 = c0 % 255;
+ c1 = c1 % 255;
+
+ mul = (len - n)*(c0);
+
+#ifdef FIXED_CODE
+ x = mul - c0 - c1;
+ y = c1 - mul - 1;
+
+ if ( y >= 0 ) y++;
+ if ( x < 0 ) x--;
+
+ x %= 255;
+ y %= 255;
+
+ if (x == 0) x = 255;
+ if (y == 0) y = 255;
+
+ x &= 0x00FF;
+
+ checksum = ((y << 8) | x);
+
+#else
+ x = mul - c0 - c1;
+ x %= 255;
+
+ y = c1 - mul - 1;
+ y %= 255;
+
+ if (x == 0) x = 255;
+ if (y == 0) y = 255;
+
+ checksum = ((y << 8) | x);
+#endif
+
+ /*
+ * Now we write this to the packet
+ */
+ *(csum) = checksum;
+
+ /* return the checksum for user usage */
+ return checksum;
+}
+
+
+int
+iso_csum_modify (u_char *buffer, int len, uint16_t *csum)
+{
+
+ return 0;
+}
+
+