#include....
#define MSG_LENGTH 1024


struct hdr{
struct iphdr ip;
struct tcphdr tcp;
char Data[MSG_LENGTH];

};

int main(){
  
  struct hdr paket;
  
  struct pseudohdr Pseudo;
 
  struct sockaddr_in addr;
 
  int rawsock;
 
  const int on=1;
 
  int se;
  
  /**
   *create a raw socket
   *
   *transport protokol: TCP Protokol
   */
  rawsock = create_socket(AF_INET, SOCK_RAW, IPPROTO_TCP);
  printf("create a Raw Socket\n");
  /**
   *set a socket option 
   */
  if(setsockopt(rawsock, IPPROTO_IP, IP_HDRINCL, &on, sizeof(on)) < 0){
    perror("setsockopt()");
    exit(1);
  }
  /**
   *initialize paket
   */
  memset(&paket, 0, sizeof(struct hdr));
  /**
   *create IP Header
   */
  ....

  paket.ip.check = compute_checksum((uint16*)&paket.ip, sizeof(struct iphdr));///checksum
  /**
   *create TCP Header
  
   */
 ....

  /**
   *create Pseudo Header for computing TCP checksum
   *(quelle rfc 793)
   */
  Pseudo.saddr = paket.ip.saddr;
  Pseudo.daddr = paket.ip.daddr;
  Pseudo.reserved = 0;
  Pseudo.protocol = paket.ip.protocol;
  Pseudo.tcp_seg_len = htons(sizeof(struct tcphdr));
  bcopy((char*)&paket.tcp, (char*)&Pseudo.tcp, sizeof(struct tcphdr));
 
  
  printf("give your Data:\t");
  fgets(paket.Data, MSG_LENGTH, stdin);

  bcopy((char*)&paket.Data, (char*)&Pseudo.buf, MSG_LENGTH);


  paket.tcp.check = compute_checksum((uint16*)&Pseudo, MSG_LENGTH +
             sizeof(struct tcphdr) + sizeof(struct pseudohdr));

  /**
   *now the paket is sent
   */

    se = sendto(rawsock, &paket, sizeof(struct hdr), 0, 
    (struct sockaddr*)&addr, sizeof(struct sockaddr_in));
   
  printf("the paket is sent\n");
  printf("Sent %d byte paket to %s\n",paket.ip.tot_len, DES_IP);
  

  close_socket(rawsock);

  return 0;
}
/**
 *hier ist checksum
 */
uint16
compute_checksum(uint16 *ptr,
     int byte){
  register int sum=0;
  u_short odd_byte=0;
  /**
   *we add sequential 16 bit words to a 32 bit accumulator(sum)
   *
   *and at the end, fold back all the carry bit from the top 16 bits
   *into the lower 16 bits
   */
  while(byte >1){
    sum += *ptr++;
    byte -=2;
  }
  /**
   *mod up and add byte, if necessary
   */
  if(byte ==1){
    *((u_char*)&odd_byte) = *(u_char*)ptr;
    sum += odd_byte;
  }
  /**
   *add back carry outs from top 16 bits to low 16 bits
   */
  sum = (sum >> 16) + (sum & 0xffff);///add hight to low
  sum += (sum >>16);///add carry
  odd_byte =~sum;
  return (odd_byte);
}