See the definition of TCP header in /netinet/tcp.h:
struct tcphdr { u_int16_t th_sport; /* source port */ u_int16_t th_dport; /* destin
It may help to know that this is code is only run if "# ifdef __FAVOR_BSD". It is from /usr/include/netinet/tcp.h
# ifdef __FAVOR_BSD typedef u_int32_t tcp_seq; /* * TCP header. * Per RFC 793, September, 1981. */ struct tcphdr