uint16_t Icmp::create_checksum(uint16_t type_code) {
    unsigned long checksum = 0;

    // Cast the data pointer to one that can be indexed.
    unsigned char* data = raw_data.data();
    size_t length = raw_data.size();

    // Initialise the accumulator.
    uint32_t acc=0xffff;
    acc += type_code;

    // Handle complete 16-bit blocks.
    for (size_t i=0;i+1<length;i+=2) {
        uint16_t word;
        memcpy(&word,data+i,2);
        acc+=(word);
        if (acc>0xffff) {
            acc-=0xffff;
        }
    }

    // Handle any partial block at the end of the data.
    if (length&1) {
        uint16_t word=0;
        memcpy(&word,data+length-1,1);
        acc+=(word);
        if (acc>0xffff) {
            acc-=0xffff;
        }
    }

    // Return the checksum in network byte order.
    return (~acc);
}