IP Header Checksum Code

         
        /* 
         * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994
         *     The Regents of the University of California.  All rights reserved.
         *
         * Redistribution and use in source and binary form, with or without
         * modifications, are permitted provided that the following conditions
         * are met:
         * 1. Redistributions of source code must retain the above copyright
         *    notice, this list of conditions and the following disclaimer.
         * 2. Redistributions in binary form must reproduce the above copyright
         *    notice, this list of conditions and the following disclaimer in the
         *    documentation and/or other materials provided with the distribution.
         * 3. All advertising materials mentioning features or use of this software
         *    must display the following acknowledgement:
         *      This product includes software developed by the University of 
         *      California, Berkeley and its contributors.
         * 4. Neither the name of the University nor the names of its contributors
         *    may be used to endorse or promote products derived from this software
         *    without specific prior written permission.
         *
         * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ''AS IS'' AND
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         * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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         * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
         * SUCH DAMAGE.
         */

         unsigned short cksum(struct ip *ip, int len){
           long sum = 0;  /* assume 32 bit long, 16 bit short */

           while(len > 1){
             sum += *((unsigned short*) ip)++;
             if(sum & 0x80000000)   /* if high order bit set, fold */
               sum = (sum & 0xFFFF) + (sum >> 16);
             len -= 2;
           }

           if(len)       /* take care of left over byte */
             sum += (unsigned short) *(unsigned char *)ip;
          
           while(sum>>16)
             sum = (sum & 0xFFFF) + (sum >> 16);

           return ~sum;
         }

         /* taken from TCP/IP Illustrated Vol. 2(1995) by Gary R. Wright and W. Richard
            Stevens. Page 236 */