对面的高手请近来。（TCP）

请教高人，我该怎样填充下面这个TCP协议头结构，急，不胜感激。
typedef u_int tcp_seq;
typedef struct udp_hdr {
u_short src_portno; /* source port */
u_short dst_portno; /* destination port */
tcp_seq th_seq; /* sequence number */
tcp_seq th_ack; /* acknowledgement number */
#if BYTE_ORDER == LITTLE_ENDIAN
u_char th_x2:4, /* (unused) */
th_off:4; /* data offset */
#endif
#if BYTE_ORDER == BIG_ENDIAN
u_char th_off:4, /* data offset */
th_x2:4; /* (unused) */
#endif
u_char th_flags;
#define TH_FIN 0x01
#define TH_SYN 0x02
#define TH_RST 0x04
#define TH_PUSH 0x08
#define TH_ACK 0x10
#define TH_URG 0x20
u_short th_win; /* window */
u_short tcp_checksum; /* checksum */
u_short th_urp; /* urgent pointer */
}TCP_HDR;