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;
+}
+
+