usr/src/sys/netinet/tcp.h

/* tcp.h 1.21 82/01/18 */

typedef u_long  tcp_seq;
/*
 * TCP header.
 * Per RFC 793, September, 1981.
 */
struct tcphdr {
        u_short th_sport;               /* source port */
        u_short th_dport;               /* destination port */
        tcp_seq th_seq;                 /* sequence number */
        tcp_seq th_ack;                 /* acknowledgement number */
        u_char
                th_x2:4,                /* (unused) */
                th_off:4;               /* data offset */
        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 th_sum;                 /* checksum */
        u_short th_urp;                 /* urgent pointer */
};

#define TCPOPT_EOL      0
#define TCPOPT_NOP      1
#define TCPOPT_MAXSEG   2

#ifdef TCPTRUEOOB
/*
 * True out-of-band as value added option.
 * Advertise willingness with TCPOPT_WILOOB in
 * initial segment.  If peer is willing, will receive
 * such also.  Then can send TCPOPT_OOBDATA whenever oob data
 * exists; peer should ack with TCPOPT_OOBACK in segment.
 */
#define TCPOPT_WILLOOB  64              /* bytes: 64, 2 */
#define TCPOPT_OOBDATA  65              /* bytes: 65, 8, seq#, data, markseq */
#define TCPOPT_OOBACK   66              /* bytes: 66, 3, ack# */
#endif