C Programming TCP Checksum

I found a fairly good example on the winpcap-users mailing list which should address Greg's comment about odd length data and give you something to compare your code against.

USHORT CheckSum(USHORT *buffer, int size)
{
    unsigned long cksum=0;
    while(size >1)
    {
        cksum+=*buffer++;
        size -=sizeof(USHORT);
    }
    if(size)
        cksum += *(UCHAR*)buffer;

    cksum = (cksum >> 16) + (cksum & 0xffff);
    cksum += (cksum >>16);
    return (USHORT)(~cksum);
}

I see a couple of things:

• You are not accounting for odd length data by padding with zero.
• You will need to account for network byte order when reading each word from the packet.
• Your use of htonl(0x0000ffff) seems suspicious. Why are you converting a constant to network byte order to combine it with data in host byte order?

RFC 793 says "If a segment contains an odd number of header and text octets to be checksummed, the last octet is padded on the right with zeros to form a 16 bit word for checksum purposes." Your code above does not handle that case. I think the loop conditional should be i > 1 and then check for i == 1 outside the loop and do the special handling for the last octet.