Function tcp_respond

freebsd/netinet/tcp_subr.c:1395–1682

* Send a single message to the TCP at address specified by * the given TCP/IP header. If m == NULL, then we make a copy * of the tcpiphdr at th and send directly to the addressed host. * This is used to force keep alive messages out using the TCP * template for a connection. If flags are given then we send * a message back to the TCP which originated the segment th, * and discard the mbuf

 * NOTE: If m != NULL, then th must point to *inside* the mbuf.
 */
void
tcp_respond(struct tcpcb *tp, void *ipgen, struct tcphdr *th, struct mbuf *m,
    tcp_seq ack, tcp_seq seq, int flags)
{
	struct tcpopt to;
	struct inpcb *inp;
	struct ip *ip;
	struct mbuf *optm;
	struct tcphdr *nth;
	u_char *optp;
#ifdef INET6
	struct ip6_hdr *ip6;
	int isipv6;
#endif /* INET6 */
	int optlen, tlen, win;
	bool incl_opts;

	KASSERT(tp != NULL || m != NULL, ("tcp_respond: tp and m both NULL"));
	NET_EPOCH_ASSERT();

#ifdef INET6
	isipv6 = ((struct ip *)ipgen)->ip_v == (IPV6_VERSION >> 4);
	ip6 = ipgen;
#endif /* INET6 */
	ip = ipgen;

	if (tp != NULL) {
		inp = tp->t_inpcb;
		KASSERT(inp != NULL, ("tcp control block w/o inpcb"));
		INP_WLOCK_ASSERT(inp);
	} else
		inp = NULL;

	incl_opts = false;
	win = 0;
	if (tp != NULL) {
		if (!(flags & TH_RST)) {
			win = sbspace(&inp->inp_socket->so_rcv);
			if (win > TCP_MAXWIN << tp->rcv_scale)
				win = TCP_MAXWIN << tp->rcv_scale;
		}
		if ((tp->t_flags & TF_NOOPT) == 0)
			incl_opts = true;
	}
	if (m == NULL) {
		m = m_gethdr(M_NOWAIT, MT_DATA);
		if (m == NULL)
			return;
		m->m_data += max_linkhdr;
#ifdef INET6
		if (isipv6) {
			bcopy((caddr_t)ip6, mtod(m, caddr_t),
			      sizeof(struct ip6_hdr));
			ip6 = mtod(m, struct ip6_hdr *);
			nth = (struct tcphdr *)(ip6 + 1);
		} else
#endif /* INET6 */
		{