struct tcp_header {
       unsigned short int th_sport;
       unsigned short int th_dport;
       unsigned int th_seq;
       unsigned int th_ack;
       unsigned char th_x2:4, th_off:4;
       unsigned char th_flags;
       unsigned short int th_win;
       unsigned short int th_sum;
       unsigned short int th_urp;
       } TCPHDR;