C - TCP/IP checksum function (RFC1071)
======================================
    
Public domain
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###  Computing the Internet Checksum
    
    #include <stdio.h>
    
    unsigned short in_cksum(unsigned short *addr, int len)
    {
            /* This is from RFC1071 */
            unsigned short checksum;
            register long sum = 0;
    
            /* Adding all 2-byte words */
            while (len > 1) {
                    sum += *addr++;
                    len -= 2;
            }
    
            /* Add left-over byte, if any */
            if (len > 0 )
                    sum += *(unsigned char*) addr;
    
            /* Fold 32-bit sum to 16 bits */
            while (sum >> 16)
                    sum = (sum & 0xFFFF) + (sum >> 16);
    
            /* Inverting the result */
            checksum = ~sum;
            return checksum;
    }
    
    main()
    {
            unsigned short buffer[] = {
                    0x4500, 0x0052, 0xefc0, 0x0000, 0x4311,
                    0xDA77, 0x42DC, 0x9e7f, 0x2598, 0xa66f
            };
    
            printf("Checksum: 0x%X \n", in_cksum(buffer, 20));
            return 0;
    }

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_BY: Pejman Moghadam_  
_TAG: checksum, tcpip, rfc_  
_DATE: 2014-01-10 23:59:28_