Function syncache_respond

freebsd/netinet/tcp_syncache.c:1862–2080

* Send SYN|ACK or ACK to the peer. Either in response to a peer's segment, * i.e. m0 != NULL, or upon 3WHS ACK timeout, i.e. m0 == NULL. */

 * i.e. m0 != NULL, or upon 3WHS ACK timeout, i.e. m0 == NULL.
 */
static int
syncache_respond(struct syncache *sc, const struct mbuf *m0, int flags)
{
	struct ip *ip = NULL;
	struct mbuf *m;
	struct tcphdr *th = NULL;
	int optlen, error = 0;	/* Make compiler happy */
	u_int16_t hlen, tlen, mssopt;
	struct tcpopt to;
#ifdef INET6
	struct ip6_hdr *ip6 = NULL;
#endif

	NET_EPOCH_ASSERT();

	hlen =
#ifdef INET6
	       (sc->sc_inc.inc_flags & INC_ISIPV6) ? sizeof(struct ip6_hdr) :
#endif
		sizeof(struct ip);
	tlen = hlen + sizeof(struct tcphdr);

	/* Determine MSS we advertize to other end of connection. */
	mssopt = max(tcp_mssopt(&sc->sc_inc), V_tcp_minmss);

	/* XXX: Assume that the entire packet will fit in a header mbuf. */
	KASSERT(max_linkhdr + tlen + TCP_MAXOLEN <= MHLEN,
	    ("syncache: mbuf too small"));

	/* Create the IP+TCP header from scratch. */
	m = m_gethdr(M_NOWAIT, MT_DATA);
	if (m == NULL)
		return (ENOBUFS);
#ifdef MAC
	mac_syncache_create_mbuf(sc->sc_label, m);
#endif
	m->m_data += max_linkhdr;
	m->m_len = tlen;
	m->m_pkthdr.len = tlen;
	m->m_pkthdr.rcvif = NULL;

#ifdef INET6
	if (sc->sc_inc.inc_flags & INC_ISIPV6) {
		ip6 = mtod(m, struct ip6_hdr *);
		ip6->ip6_vfc = IPV6_VERSION;
		ip6->ip6_nxt = IPPROTO_TCP;
		ip6->ip6_src = sc->sc_inc.inc6_laddr;
		ip6->ip6_dst = sc->sc_inc.inc6_faddr;
		ip6->ip6_plen = htons(tlen - hlen);
		/* ip6_hlim is set after checksum */
		/* Zero out traffic class and flow label. */
		ip6->ip6_flow &= ~IPV6_FLOWINFO_MASK;
		ip6->ip6_flow |= sc->sc_flowlabel;
		ip6->ip6_flow |= htonl(sc->sc_ip_tos << 20);

		th = (struct tcphdr *)(ip6 + 1);
	}
#endif