usr/src/sys/netinet/tcp.h

/* tcp.h 1.17 81/11/18 */

/*
 * TCP header.
 * Per RFC 793, September, 1981.
 */
struct tcphdr {
        u_short th_sport;               /* source port */
        u_short th_dport;               /* destination port */
        seq_t   th_seq;                 /* sequence number */
        seq_t   th_ackno;               /* acknowledgement number */
        u_char
                th_x2:4,                /* (unused) */
                th_off:4;               /* data offset */
        u_char  th_flags;
#define TH_FIN  001
#define TH_SYN  002
#define TH_RST  004
#define TH_EOL  010
#define TH_ACK  020
#define TH_URG  040
        u_short th_win;                 /* window */
        u_short th_sum;                 /* checksum */
        u_short th_urp;                 /* urgent pointer */
};