Example #1
0
/*
 * Tcp output routine: figure out what should be sent and send it.
 */
int
tcp_output(struct tcpcb *tp)
{
	struct inpcb * const inp = tp->t_inpcb;
	struct socket *so = inp->inp_socket;
	long len, recvwin, sendwin;
	int nsacked = 0;
	int off, flags, error;
#ifdef TCP_SIGNATURE
	int sigoff = 0;
#endif
	struct mbuf *m;
	struct ip *ip = NULL;
	struct ipovly *ipov = NULL;
	struct tcphdr *th;
	u_char opt[TCP_MAXOLEN];
	unsigned int ipoptlen, optlen, hdrlen;
	int idle;
	boolean_t sendalot;
	struct ip6_hdr *ip6 = NULL;
#ifdef INET6
	const boolean_t isipv6 = (inp->inp_vflag & INP_IPV6) != 0;
#else
	const boolean_t isipv6 = FALSE;
#endif

	KKASSERT(so->so_port == &curthread->td_msgport);

	/*
	 * Determine length of data that should be transmitted,
	 * and flags that will be used.
	 * If there is some data or critical controls (SYN, RST)
	 * to send, then transmit; otherwise, investigate further.
	 */

	/*
	 * If we have been idle for a while, the send congestion window
	 * could be no longer representative of the current state of the link.
	 * So unless we are expecting more acks to come in, slow-start from
	 * scratch to re-determine the send congestion window.
	 */
	if (tp->snd_max == tp->snd_una &&
	    (ticks - tp->t_rcvtime) >= tp->t_rxtcur) {
		if (tcp_do_rfc3390) {
			int initial_cwnd =
			    min(4 * tp->t_maxseg, max(2 * tp->t_maxseg, 4380));

			tp->snd_cwnd = min(tp->snd_cwnd, initial_cwnd);
		} else {
			tp->snd_cwnd = tp->t_maxseg;
		}
		tp->snd_wacked = 0;
	}

	/*
	 * Calculate whether the transmit stream was previously idle 
	 * and adjust TF_LASTIDLE for the next time.
	 */
	idle = (tp->t_flags & TF_LASTIDLE) || (tp->snd_max == tp->snd_una);
	if (idle && (tp->t_flags & TF_MORETOCOME))
		tp->t_flags |= TF_LASTIDLE;
	else
		tp->t_flags &= ~TF_LASTIDLE;

	if (TCP_DO_SACK(tp) && tp->snd_nxt != tp->snd_max &&
	    !IN_FASTRECOVERY(tp))
		nsacked = tcp_sack_bytes_below(&tp->scb, tp->snd_nxt);

again:
	/* Make use of SACK information when slow-starting after a RTO. */
	if (TCP_DO_SACK(tp) && tp->snd_nxt != tp->snd_max &&
	    !IN_FASTRECOVERY(tp)) {
		tcp_seq old_snd_nxt = tp->snd_nxt;

		tcp_sack_skip_sacked(&tp->scb, &tp->snd_nxt);
		nsacked += tp->snd_nxt - old_snd_nxt;
	}

	sendalot = FALSE;
	off = tp->snd_nxt - tp->snd_una;
	sendwin = min(tp->snd_wnd, tp->snd_cwnd + nsacked);
	sendwin = min(sendwin, tp->snd_bwnd);

	flags = tcp_outflags[tp->t_state];
	/*
	 * Get standard flags, and add SYN or FIN if requested by 'hidden'
	 * state flags.
	 */
	if (tp->t_flags & TF_NEEDFIN)
		flags |= TH_FIN;
	if (tp->t_flags & TF_NEEDSYN)
		flags |= TH_SYN;

	/*
	 * If in persist timeout with window of 0, send 1 byte.
	 * Otherwise, if window is small but nonzero
	 * and timer expired, we will send what we can
	 * and go to transmit state.
	 */
	if (tp->t_flags & TF_FORCE) {
		if (sendwin == 0) {
			/*
			 * If we still have some data to send, then
			 * clear the FIN bit.  Usually this would
			 * happen below when it realizes that we
			 * aren't sending all the data.  However,
			 * if we have exactly 1 byte of unsent data,
			 * then it won't clear the FIN bit below,
			 * and if we are in persist state, we wind
			 * up sending the packet without recording
			 * that we sent the FIN bit.
			 *
			 * We can't just blindly clear the FIN bit,
			 * because if we don't have any more data
			 * to send then the probe will be the FIN
			 * itself.
			 */
			if (off < so->so_snd.ssb_cc)
				flags &= ~TH_FIN;
			sendwin = 1;
		} else {
			tcp_callout_stop(tp, tp->tt_persist);
			tp->t_rxtshift = 0;
		}
	}

	/*
	 * If snd_nxt == snd_max and we have transmitted a FIN, the
	 * offset will be > 0 even if so_snd.ssb_cc is 0, resulting in
	 * a negative length.  This can also occur when TCP opens up
	 * its congestion window while receiving additional duplicate
	 * acks after fast-retransmit because TCP will reset snd_nxt
	 * to snd_max after the fast-retransmit.
	 *
	 * In the normal retransmit-FIN-only case, however, snd_nxt will
	 * be set to snd_una, the offset will be 0, and the length may
	 * wind up 0.
	 */
	len = (long)ulmin(so->so_snd.ssb_cc, sendwin) - off;

	/*
	 * Lop off SYN bit if it has already been sent.  However, if this
	 * is SYN-SENT state and if segment contains data, suppress sending
	 * segment (sending the segment would be an option if we still
	 * did TAO and the remote host supported it).
	 */
	if ((flags & TH_SYN) && SEQ_GT(tp->snd_nxt, tp->snd_una)) {
		flags &= ~TH_SYN;
		off--, len++;
		if (len > 0 && tp->t_state == TCPS_SYN_SENT)
			return 0;
	}

	/*
	 * Be careful not to send data and/or FIN on SYN segments.
	 * This measure is needed to prevent interoperability problems
	 * with not fully conformant TCP implementations.
	 */
	if (flags & TH_SYN) {
		len = 0;
		flags &= ~TH_FIN;
	}

	if (len < 0) {
		/*
		 * If FIN has been sent but not acked,
		 * but we haven't been called to retransmit,
		 * len will be < 0.  Otherwise, window shrank
		 * after we sent into it.  If window shrank to 0,
		 * cancel pending retransmit, pull snd_nxt back
		 * to (closed) window, and set the persist timer
		 * if it isn't already going.  If the window didn't
		 * close completely, just wait for an ACK.
		 */
		len = 0;
		if (sendwin == 0) {
			tcp_callout_stop(tp, tp->tt_rexmt);
			tp->t_rxtshift = 0;
			tp->snd_nxt = tp->snd_una;
			if (!tcp_callout_active(tp, tp->tt_persist))
				tcp_setpersist(tp);
		}
	}

	KASSERT(len >= 0, ("%s: len < 0", __func__));
	/*
	 * Automatic sizing of send socket buffer.  Often the send buffer
	 * size is not optimally adjusted to the actual network conditions
	 * at hand (delay bandwidth product).  Setting the buffer size too
	 * small limits throughput on links with high bandwidth and high
	 * delay (eg. trans-continental/oceanic links).  Setting the
	 * buffer size too big consumes too much real kernel memory,
	 * especially with many connections on busy servers.
	 *
	 * The criteria to step up the send buffer one notch are:
	 *  1. receive window of remote host is larger than send buffer
	 *     (with a fudge factor of 5/4th);
	 *  2. send buffer is filled to 7/8th with data (so we actually
	 *     have data to make use of it);
	 *  3. send buffer fill has not hit maximal automatic size;
	 *  4. our send window (slow start and cogestion controlled) is
	 *     larger than sent but unacknowledged data in send buffer.
	 *
	 * The remote host receive window scaling factor may limit the
	 * growing of the send buffer before it reaches its allowed
	 * maximum.
	 *
	 * It scales directly with slow start or congestion window
	 * and does at most one step per received ACK.  This fast
	 * scaling has the drawback of growing the send buffer beyond
	 * what is strictly necessary to make full use of a given
	 * delay*bandwith product.  However testing has shown this not
	 * to be much of an problem.  At worst we are trading wasting
	 * of available bandwith (the non-use of it) for wasting some
	 * socket buffer memory.
	 *
	 * TODO: Shrink send buffer during idle periods together
	 * with congestion window.  Requires another timer.  Has to
	 * wait for upcoming tcp timer rewrite.
	 */
	if (tcp_do_autosndbuf && so->so_snd.ssb_flags & SSB_AUTOSIZE) {
		if ((tp->snd_wnd / 4 * 5) >= so->so_snd.ssb_hiwat &&
		    so->so_snd.ssb_cc >= (so->so_snd.ssb_hiwat / 8 * 7) &&
		    so->so_snd.ssb_cc < tcp_autosndbuf_max &&
		    sendwin >= (so->so_snd.ssb_cc - (tp->snd_nxt - tp->snd_una))) {
			u_long newsize;

			newsize = ulmin(so->so_snd.ssb_hiwat +
					 tcp_autosndbuf_inc,
					tcp_autosndbuf_max);
			if (!ssb_reserve(&so->so_snd, newsize, so, NULL))
				atomic_clear_int(&so->so_snd.ssb_flags, SSB_AUTOSIZE);
			if (newsize >= (TCP_MAXWIN << tp->snd_scale))
				atomic_clear_int(&so->so_snd.ssb_flags, SSB_AUTOSIZE);
		}
	}

	/*
	 * Truncate to the maximum segment length and ensure that FIN is
	 * removed if the length no longer contains the last data byte.
	 */
	if (len > tp->t_maxseg) {
		len = tp->t_maxseg;
		sendalot = TRUE;
	}
	if (SEQ_LT(tp->snd_nxt + len, tp->snd_una + so->so_snd.ssb_cc))
		flags &= ~TH_FIN;

	recvwin = ssb_space(&so->so_rcv);

	/*
	 * Sender silly window avoidance.   We transmit under the following
	 * conditions when len is non-zero:
	 *
	 *	- We have a full segment
	 *	- This is the last buffer in a write()/send() and we are
	 *	  either idle or running NODELAY
	 *	- we've timed out (e.g. persist timer)
	 *	- we have more then 1/2 the maximum send window's worth of
	 *	  data (receiver may be limiting the window size)
	 *	- we need to retransmit
	 */
	if (len) {
		if (len == tp->t_maxseg)
			goto send;
		/*
		 * NOTE! on localhost connections an 'ack' from the remote
		 * end may occur synchronously with the output and cause
		 * us to flush a buffer queued with moretocome.  XXX
		 *
		 * note: the len + off check is almost certainly unnecessary.
		 */
		if (!(tp->t_flags & TF_MORETOCOME) &&	/* normal case */
		    (idle || (tp->t_flags & TF_NODELAY)) &&
		    len + off >= so->so_snd.ssb_cc &&
		    !(tp->t_flags & TF_NOPUSH)) {
			goto send;
		}
		if (tp->t_flags & TF_FORCE)		/* typ. timeout case */
			goto send;
		if (len >= tp->max_sndwnd / 2 && tp->max_sndwnd > 0)
			goto send;
		if (SEQ_LT(tp->snd_nxt, tp->snd_max))	/* retransmit case */
			goto send;
	}

	/*
	 * Compare available window to amount of window
	 * known to peer (as advertised window less
	 * next expected input).  If the difference is at least two
	 * max size segments, or at least 50% of the maximum possible
	 * window, then want to send a window update to peer.
	 */
	if (recvwin > 0) {
		/*
		 * "adv" is the amount we can increase the window,
		 * taking into account that we are limited by
		 * TCP_MAXWIN << tp->rcv_scale.
		 */
		long adv = min(recvwin, (long)TCP_MAXWIN << tp->rcv_scale) -
			(tp->rcv_adv - tp->rcv_nxt);
		long hiwat;

		/*
		 * This ack case typically occurs when the user has drained
		 * the TCP socket buffer sufficiently to warrent an ack
		 * containing a 'pure window update'... that is, an ack that
		 * ONLY updates the tcp window.
		 *
		 * It is unclear why we would need to do a pure window update
		 * past 2 segments if we are going to do one at 1/2 the high
		 * water mark anyway, especially since under normal conditions
		 * the user program will drain the socket buffer quickly.
		 * The 2-segment pure window update will often add a large
		 * number of extra, unnecessary acks to the stream.
		 *
		 * avoid_pure_win_update now defaults to 1.
		 */
		if (avoid_pure_win_update == 0 ||
		    (tp->t_flags & TF_RXRESIZED)) {
			if (adv >= (long) (2 * tp->t_maxseg)) {
				goto send;
			}
		}
		hiwat = (long)(TCP_MAXWIN << tp->rcv_scale);
		if (hiwat > (long)so->so_rcv.ssb_hiwat)
			hiwat = (long)so->so_rcv.ssb_hiwat;
		if (adv >= hiwat / 2)
			goto send;
	}

	/*
	 * Send if we owe the peer an ACK, RST, SYN, or urgent data.  ACKNOW
	 * is also a catch-all for the retransmit timer timeout case.
	 */
	if (tp->t_flags & TF_ACKNOW)
		goto send;
	if ((flags & TH_RST) ||
	    ((flags & TH_SYN) && !(tp->t_flags & TF_NEEDSYN)))
		goto send;
	if (SEQ_GT(tp->snd_up, tp->snd_una))
		goto send;
	/*
	 * If our state indicates that FIN should be sent
	 * and we have not yet done so, then we need to send.
	 */
	if (flags & TH_FIN &&
	    (!(tp->t_flags & TF_SENTFIN) || tp->snd_nxt == tp->snd_una))
		goto send;

	/*
	 * TCP window updates are not reliable, rather a polling protocol
	 * using ``persist'' packets is used to insure receipt of window
	 * updates.  The three ``states'' for the output side are:
	 *	idle			not doing retransmits or persists
	 *	persisting		to move a small or zero window
	 *	(re)transmitting	and thereby not persisting
	 *
	 * tcp_callout_active(tp, tp->tt_persist)
	 *	is true when we are in persist state.
	 * The TF_FORCE flag in tp->t_flags
	 *	is set when we are called to send a persist packet.
	 * tcp_callout_active(tp, tp->tt_rexmt)
	 *	is set when we are retransmitting
	 * The output side is idle when both timers are zero.
	 *
	 * If send window is too small, there is data to transmit, and no
	 * retransmit or persist is pending, then go to persist state.
	 * If nothing happens soon, send when timer expires:
	 * if window is nonzero, transmit what we can,
	 * otherwise force out a byte.
	 */
	if (so->so_snd.ssb_cc > 0 &&
	    !tcp_callout_active(tp, tp->tt_rexmt) &&
	    !tcp_callout_active(tp, tp->tt_persist)) {
		tp->t_rxtshift = 0;
		tcp_setpersist(tp);
	}

	/*
	 * No reason to send a segment, just return.
	 */
	return (0);

send:
	/*
	 * Before ESTABLISHED, force sending of initial options
	 * unless TCP set not to do any options.
	 * NOTE: we assume that the IP/TCP header plus TCP options
	 * always fit in a single mbuf, leaving room for a maximum
	 * link header, i.e.
	 *	max_linkhdr + sizeof(struct tcpiphdr) + optlen <= MCLBYTES
	 */
	optlen = 0;
	if (isipv6)
		hdrlen = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
	else
		hdrlen = sizeof(struct tcpiphdr);
	if (flags & TH_SYN) {
		tp->snd_nxt = tp->iss;
		if (!(tp->t_flags & TF_NOOPT)) {
			u_short mss;

			opt[0] = TCPOPT_MAXSEG;
			opt[1] = TCPOLEN_MAXSEG;
			mss = htons((u_short) tcp_mssopt(tp));
			memcpy(opt + 2, &mss, sizeof mss);
			optlen = TCPOLEN_MAXSEG;

			if ((tp->t_flags & TF_REQ_SCALE) &&
			    (!(flags & TH_ACK) ||
			     (tp->t_flags & TF_RCVD_SCALE))) {
				*((u_int32_t *)(opt + optlen)) = htonl(
					TCPOPT_NOP << 24 |
					TCPOPT_WINDOW << 16 |
					TCPOLEN_WINDOW << 8 |
					tp->request_r_scale);
				optlen += 4;
			}

			if ((tcp_do_sack && !(flags & TH_ACK)) ||
			    tp->t_flags & TF_SACK_PERMITTED) {
				uint32_t *lp = (uint32_t *)(opt + optlen);

				*lp = htonl(TCPOPT_SACK_PERMITTED_ALIGNED);
				optlen += TCPOLEN_SACK_PERMITTED_ALIGNED;
			}
		}
	}

	/*
	 * Send a timestamp and echo-reply if this is a SYN and our side
	 * wants to use timestamps (TF_REQ_TSTMP is set) or both our side
	 * and our peer have sent timestamps in our SYN's.
	 */
	if ((tp->t_flags & (TF_REQ_TSTMP | TF_NOOPT)) == TF_REQ_TSTMP &&
	    !(flags & TH_RST) &&
	    (!(flags & TH_ACK) || (tp->t_flags & TF_RCVD_TSTMP))) {
		u_int32_t *lp = (u_int32_t *)(opt + optlen);

		/* Form timestamp option as shown in appendix A of RFC 1323. */
		*lp++ = htonl(TCPOPT_TSTAMP_HDR);
		*lp++ = htonl(ticks);
		*lp   = htonl(tp->ts_recent);
		optlen += TCPOLEN_TSTAMP_APPA;
	}

	/* Set receive buffer autosizing timestamp. */
	if (tp->rfbuf_ts == 0 && (so->so_rcv.ssb_flags & SSB_AUTOSIZE))
		tp->rfbuf_ts = ticks;

	/*
	 * If this is a SACK connection and we have a block to report,
	 * fill in the SACK blocks in the TCP options.
	 */
	if ((tp->t_flags & (TF_SACK_PERMITTED | TF_NOOPT)) ==
		TF_SACK_PERMITTED &&
	    (!LIST_EMPTY(&tp->t_segq) ||
	     tp->reportblk.rblk_start != tp->reportblk.rblk_end))
		tcp_sack_fill_report(tp, opt, &optlen);

#ifdef TCP_SIGNATURE
	if (tp->t_flags & TF_SIGNATURE) {
		int i;
		u_char *bp;
		/*
		 * Initialize TCP-MD5 option (RFC2385)
		 */
		bp = (u_char *)opt + optlen;
		*bp++ = TCPOPT_SIGNATURE;
		*bp++ = TCPOLEN_SIGNATURE;
		sigoff = optlen + 2;
		for (i = 0; i < TCP_SIGLEN; i++)
			*bp++ = 0;
		optlen += TCPOLEN_SIGNATURE;
		/*
		 * Terminate options list and maintain 32-bit alignment.
		 */
		*bp++ = TCPOPT_NOP;
		*bp++ = TCPOPT_EOL;
		optlen += 2;
	}
#endif /* TCP_SIGNATURE */
	KASSERT(optlen <= TCP_MAXOLEN, ("too many TCP options"));
	hdrlen += optlen;

	if (isipv6) {
		ipoptlen = ip6_optlen(inp);
	} else {
		if (inp->inp_options) {
			ipoptlen = inp->inp_options->m_len -
			    offsetof(struct ipoption, ipopt_list);
		} else {
Example #2
0
/*
 * Tcp output routine: figure out what should be sent and send it.
 */
int
tcp_output(struct tcpcb *tp)
{
	struct inpcb * const inp = tp->t_inpcb;
	struct socket *so = inp->inp_socket;
	long len, recvwin, sendwin;
	int nsacked = 0;
	int off, flags, error = 0;
#ifdef TCP_SIGNATURE
	int sigoff = 0;
#endif
	struct mbuf *m;
	struct ip *ip;
	struct tcphdr *th;
	u_char opt[TCP_MAXOLEN];
	unsigned int ipoptlen, optlen, hdrlen;
	int idle;
	boolean_t sendalot;
	struct ip6_hdr *ip6;
#ifdef INET6
	const boolean_t isipv6 = INP_ISIPV6(inp);
#else
	const boolean_t isipv6 = FALSE;
#endif
	boolean_t can_tso = FALSE, use_tso;
	boolean_t report_sack, idle_cwv = FALSE;
	u_int segsz, tso_hlen, tso_lenmax = 0;
	int segcnt = 0;
	boolean_t need_sched = FALSE;

	KKASSERT(so->so_port == &curthread->td_msgport);

	/*
	 * Determine length of data that should be transmitted,
	 * and flags that will be used.
	 * If there is some data or critical controls (SYN, RST)
	 * to send, then transmit; otherwise, investigate further.
	 */

	/*
	 * If we have been idle for a while, the send congestion window
	 * could be no longer representative of the current state of the
	 * link; need to validate congestion window.  However, we should
	 * not perform congestion window validation here, since we could
	 * be asked to send pure ACK.
	 */
	if (tp->snd_max == tp->snd_una &&
	    (ticks - tp->snd_last) >= tp->t_rxtcur && tcp_idle_restart)
		idle_cwv = TRUE;

	/*
	 * Calculate whether the transmit stream was previously idle 
	 * and adjust TF_LASTIDLE for the next time.
	 */
	idle = (tp->t_flags & TF_LASTIDLE) || (tp->snd_max == tp->snd_una);
	if (idle && (tp->t_flags & TF_MORETOCOME))
		tp->t_flags |= TF_LASTIDLE;
	else
		tp->t_flags &= ~TF_LASTIDLE;

	if (TCP_DO_SACK(tp) && tp->snd_nxt != tp->snd_max &&
	    !IN_FASTRECOVERY(tp))
		nsacked = tcp_sack_bytes_below(&tp->scb, tp->snd_nxt);

	/*
	 * Find out whether TSO could be used or not
	 *
	 * For TSO capable devices, the following assumptions apply to
	 * the processing of TCP flags:
	 * - If FIN is set on the large TCP segment, the device must set
	 *   FIN on the last segment that it creates from the large TCP
	 *   segment.
	 * - If PUSH is set on the large TCP segment, the device must set
	 *   PUSH on the last segment that it creates from the large TCP
	 *   segment.
	 */
#if !defined(IPSEC) && !defined(FAST_IPSEC)
	if (tcp_do_tso
#ifdef TCP_SIGNATURE
	    && (tp->t_flags & TF_SIGNATURE) == 0
#endif
	) {
		if (!isipv6) {
			struct rtentry *rt = inp->inp_route.ro_rt;

			if (rt != NULL && (rt->rt_flags & RTF_UP) &&
			    (rt->rt_ifp->if_hwassist & CSUM_TSO)) {
				can_tso = TRUE;
				tso_lenmax = rt->rt_ifp->if_tsolen;
			}
		}
	}
#endif	/* !IPSEC && !FAST_IPSEC */

again:
	m = NULL;
	ip = NULL;
	th = NULL;
	ip6 = NULL;

	if ((tp->t_flags & (TF_SACK_PERMITTED | TF_NOOPT)) ==
		TF_SACK_PERMITTED &&
	    (!TAILQ_EMPTY(&tp->t_segq) ||
	     tp->reportblk.rblk_start != tp->reportblk.rblk_end))
		report_sack = TRUE;
	else
		report_sack = FALSE;

	/* Make use of SACK information when slow-starting after a RTO. */
	if (TCP_DO_SACK(tp) && tp->snd_nxt != tp->snd_max &&
	    !IN_FASTRECOVERY(tp)) {
		tcp_seq old_snd_nxt = tp->snd_nxt;

		tcp_sack_skip_sacked(&tp->scb, &tp->snd_nxt);
		nsacked += tp->snd_nxt - old_snd_nxt;
	}

	sendalot = FALSE;
	off = tp->snd_nxt - tp->snd_una;
	sendwin = min(tp->snd_wnd, tp->snd_cwnd + nsacked);
	sendwin = min(sendwin, tp->snd_bwnd);

	flags = tcp_outflags[tp->t_state];
	/*
	 * Get standard flags, and add SYN or FIN if requested by 'hidden'
	 * state flags.
	 */
	if (tp->t_flags & TF_NEEDFIN)
		flags |= TH_FIN;
	if (tp->t_flags & TF_NEEDSYN)
		flags |= TH_SYN;

	/*
	 * If in persist timeout with window of 0, send 1 byte.
	 * Otherwise, if window is small but nonzero
	 * and timer expired, we will send what we can
	 * and go to transmit state.
	 */
	if (tp->t_flags & TF_FORCE) {
		if (sendwin == 0) {
			/*
			 * If we still have some data to send, then
			 * clear the FIN bit.  Usually this would
			 * happen below when it realizes that we
			 * aren't sending all the data.  However,
			 * if we have exactly 1 byte of unsent data,
			 * then it won't clear the FIN bit below,
			 * and if we are in persist state, we wind
			 * up sending the packet without recording
			 * that we sent the FIN bit.
			 *
			 * We can't just blindly clear the FIN bit,
			 * because if we don't have any more data
			 * to send then the probe will be the FIN
			 * itself.
			 */
			if (off < so->so_snd.ssb_cc)
				flags &= ~TH_FIN;
			sendwin = 1;
		} else {
			tcp_callout_stop(tp, tp->tt_persist);
			tp->t_rxtshift = 0;
		}
	}

	/*
	 * If snd_nxt == snd_max and we have transmitted a FIN, the
	 * offset will be > 0 even if so_snd.ssb_cc is 0, resulting in
	 * a negative length.  This can also occur when TCP opens up
	 * its congestion window while receiving additional duplicate
	 * acks after fast-retransmit because TCP will reset snd_nxt
	 * to snd_max after the fast-retransmit.
	 *
	 * A negative length can also occur when we are in the
	 * TCPS_SYN_RECEIVED state due to a simultanious connect where
	 * our SYN has not been acked yet.
	 *
	 * In the normal retransmit-FIN-only case, however, snd_nxt will
	 * be set to snd_una, the offset will be 0, and the length may
	 * wind up 0.
	 */
	len = (long)ulmin(so->so_snd.ssb_cc, sendwin) - off;

	/*
	 * Lop off SYN bit if it has already been sent.  However, if this
	 * is SYN-SENT state and if segment contains data, suppress sending
	 * segment (sending the segment would be an option if we still
	 * did TAO and the remote host supported it).
	 */
	if ((flags & TH_SYN) && SEQ_GT(tp->snd_nxt, tp->snd_una)) {
		flags &= ~TH_SYN;
		off--, len++;
		if (len > 0 && tp->t_state == TCPS_SYN_SENT) {
			tp->t_flags &= ~(TF_ACKNOW | TF_XMITNOW);
			return 0;
		}
	}

	/*
	 * Be careful not to send data and/or FIN on SYN segments.
	 * This measure is needed to prevent interoperability problems
	 * with not fully conformant TCP implementations.
	 */
	if (flags & TH_SYN) {
		len = 0;
		flags &= ~TH_FIN;
	}

	if (len < 0) {
		/*
		 * A negative len can occur if our FIN has been sent but not
		 * acked, or if we are in a simultanious connect in the
		 * TCPS_SYN_RECEIVED state with our SYN sent but not yet
		 * acked.
		 *
		 * If our window has contracted to 0 in the FIN case
		 * (which can only occur if we have NOT been called to
		 * retransmit as per code a few paragraphs up) then we
		 * want to shift the retransmit timer over to the
		 * persist timer.
		 *
		 * However, if we are in the TCPS_SYN_RECEIVED state
		 * (the SYN case) we will be in a simultanious connect and
		 * the window may be zero degeneratively.  In this case we
		 * do not want to shift to the persist timer after the SYN
		 * or the SYN+ACK transmission.
		 */
		len = 0;
		if (sendwin == 0 && tp->t_state != TCPS_SYN_RECEIVED) {
			tcp_callout_stop(tp, tp->tt_rexmt);
			tp->t_rxtshift = 0;
			tp->snd_nxt = tp->snd_una;
			if (!tcp_callout_active(tp, tp->tt_persist))
				tcp_setpersist(tp);
		}
	}

	KASSERT(len >= 0, ("%s: len < 0", __func__));
	/*
	 * Automatic sizing of send socket buffer.  Often the send buffer
	 * size is not optimally adjusted to the actual network conditions
	 * at hand (delay bandwidth product).  Setting the buffer size too
	 * small limits throughput on links with high bandwidth and high
	 * delay (eg. trans-continental/oceanic links).  Setting the
	 * buffer size too big consumes too much real kernel memory,
	 * especially with many connections on busy servers.
	 *
	 * The criteria to step up the send buffer one notch are:
	 *  1. receive window of remote host is larger than send buffer
	 *     (with a fudge factor of 5/4th);
	 *  2. hiwat has not significantly exceeded bwnd (inflight)
	 *     (bwnd is a maximal value if inflight is disabled).
	 *  3. send buffer is filled to 7/8th with data (so we actually
	 *     have data to make use of it);
	 *  4. hiwat has not hit maximal automatic size;
	 *  5. our send window (slow start and cogestion controlled) is
	 *     larger than sent but unacknowledged data in send buffer.
	 *
	 * The remote host receive window scaling factor may limit the
	 * growing of the send buffer before it reaches its allowed
	 * maximum.
	 *
	 * It scales directly with slow start or congestion window
	 * and does at most one step per received ACK.  This fast
	 * scaling has the drawback of growing the send buffer beyond
	 * what is strictly necessary to make full use of a given
	 * delay*bandwith product.  However testing has shown this not
	 * to be much of an problem.  At worst we are trading wasting
	 * of available bandwith (the non-use of it) for wasting some
	 * socket buffer memory.
	 *
	 * The criteria for shrinking the buffer is based solely on
	 * the inflight code (snd_bwnd).  If inflight is disabled,
	 * the buffer will not be shrinked.  Note that snd_bwnd already
	 * has a fudge factor.  Our test adds a little hysteresis.
	 */
	if (tcp_do_autosndbuf && (so->so_snd.ssb_flags & SSB_AUTOSIZE)) {
		const int asbinc = tcp_autosndbuf_inc;
		const int hiwat = so->so_snd.ssb_hiwat;
		const int lowat = so->so_snd.ssb_lowat;
		u_long newsize;

		if ((tp->snd_wnd / 4 * 5) >= hiwat &&
		    so->so_snd.ssb_cc >= (hiwat / 8 * 7) &&
		    hiwat < tp->snd_bwnd + hiwat / 10 &&
		    hiwat + asbinc < tcp_autosndbuf_max &&
		    hiwat < (TCP_MAXWIN << tp->snd_scale) &&
		    sendwin >= (so->so_snd.ssb_cc -
				(tp->snd_nxt - tp->snd_una))) {
			newsize = ulmin(hiwat + asbinc, tcp_autosndbuf_max);
			if (!ssb_reserve(&so->so_snd, newsize, so, NULL))
				atomic_clear_int(&so->so_snd.ssb_flags, SSB_AUTOSIZE);
#if 0
			if (newsize >= (TCP_MAXWIN << tp->snd_scale))
				atomic_clear_int(&so->so_snd.ssb_flags, SSB_AUTOSIZE);
#endif
		} else if ((long)tp->snd_bwnd <
			   (long)(hiwat * 3 / 4 - lowat - asbinc) &&
			   hiwat > tp->t_maxseg * 2 + asbinc &&
			   hiwat + asbinc >= tcp_autosndbuf_min &&
			   tcp_do_autosndbuf == 1) {
			newsize = ulmax(hiwat - asbinc, tp->t_maxseg * 2);
			ssb_reserve(&so->so_snd, newsize, so, NULL);
		}
	}

	/*
	 * Don't use TSO, if:
	 * - Congestion window needs validation
	 * - There are SACK blocks to report
	 * - RST or SYN flags is set
	 * - URG will be set
	 *
	 * XXX
	 * Checking for SYN|RST looks overkill, just to be safe than sorry
	 */
	use_tso = can_tso;
	if (report_sack || idle_cwv || (flags & (TH_RST | TH_SYN)))
		use_tso = FALSE;
	if (use_tso) {
		tcp_seq ugr_nxt = tp->snd_nxt;

		if ((flags & TH_FIN) && (tp->t_flags & TF_SENTFIN) &&
		    tp->snd_nxt == tp->snd_max)
			--ugr_nxt;

		if (SEQ_GT(tp->snd_up, ugr_nxt))
			use_tso = FALSE;
	}

	if (use_tso) {
		/*
		 * Find out segment size and header length for TSO
		 */
		error = tcp_tso_getsize(tp, &segsz, &tso_hlen);
		if (error)
			use_tso = FALSE;
	}
	if (!use_tso) {
		segsz = tp->t_maxseg;
		tso_hlen = 0; /* not used */
	}

	/*
	 * Truncate to the maximum segment length if not TSO, and ensure that
	 * FIN is removed if the length no longer contains the last data byte.
	 */
	if (len > segsz) {
		if (!use_tso) {
			len = segsz;
			++segcnt;
		} else {
			int nsegs;

			if (__predict_false(tso_lenmax < segsz))
				tso_lenmax = segsz << 1;

			/*
			 * Truncate TSO transfers to (IP_MAXPACKET - iphlen -
			 * thoff), and make sure that we send equal size
			 * transfers down the stack (rather than big-small-
			 * big-small-...).
			 */
			len = min(len, tso_lenmax);
			nsegs = min(len, (IP_MAXPACKET - tso_hlen)) / segsz;
			KKASSERT(nsegs > 0);

			len = nsegs * segsz;

			if (len <= segsz) {
				use_tso = FALSE;
				++segcnt;
			} else {
				segcnt += nsegs;
			}
		}
		sendalot = TRUE;
	} else {
		use_tso = FALSE;
		if (len > 0)
			++segcnt;
	}
	if (SEQ_LT(tp->snd_nxt + len, tp->snd_una + so->so_snd.ssb_cc))
		flags &= ~TH_FIN;

	recvwin = ssb_space(&so->so_rcv);

	/*
	 * Sender silly window avoidance.   We transmit under the following
	 * conditions when len is non-zero:
	 *
	 *	- We have a full segment
	 *	- This is the last buffer in a write()/send() and we are
	 *	  either idle or running NODELAY
	 *	- we've timed out (e.g. persist timer)
	 *	- we have more then 1/2 the maximum send window's worth of
	 *	  data (receiver may be limiting the window size)
	 *	- we need to retransmit
	 */
	if (len) {
		if (len >= segsz)
			goto send;
		/*
		 * NOTE! on localhost connections an 'ack' from the remote
		 * end may occur synchronously with the output and cause
		 * us to flush a buffer queued with moretocome.  XXX
		 *
		 * note: the len + off check is almost certainly unnecessary.
		 */
		if (!(tp->t_flags & TF_MORETOCOME) &&	/* normal case */
		    (idle || (tp->t_flags & TF_NODELAY)) &&
		    len + off >= so->so_snd.ssb_cc &&
		    !(tp->t_flags & TF_NOPUSH)) {
			goto send;
		}
		if (tp->t_flags & TF_FORCE)		/* typ. timeout case */
			goto send;
		if (len >= tp->max_sndwnd / 2 && tp->max_sndwnd > 0)
			goto send;
		if (SEQ_LT(tp->snd_nxt, tp->snd_max))	/* retransmit case */
			goto send;
		if (tp->t_flags & TF_XMITNOW)
			goto send;
	}

	/*
	 * Compare available window to amount of window
	 * known to peer (as advertised window less
	 * next expected input).  If the difference is at least two
	 * max size segments, or at least 50% of the maximum possible
	 * window, then want to send a window update to peer.
	 */
	if (recvwin > 0) {
		/*
		 * "adv" is the amount we can increase the window,
		 * taking into account that we are limited by
		 * TCP_MAXWIN << tp->rcv_scale.
		 */
		long adv = min(recvwin, (long)TCP_MAXWIN << tp->rcv_scale) -
			(tp->rcv_adv - tp->rcv_nxt);
		long hiwat;

		/*
		 * This ack case typically occurs when the user has drained
		 * the TCP socket buffer sufficiently to warrent an ack
		 * containing a 'pure window update'... that is, an ack that
		 * ONLY updates the tcp window.
		 *
		 * It is unclear why we would need to do a pure window update
		 * past 2 segments if we are going to do one at 1/2 the high
		 * water mark anyway, especially since under normal conditions
		 * the user program will drain the socket buffer quickly.
		 * The 2-segment pure window update will often add a large
		 * number of extra, unnecessary acks to the stream.
		 *
		 * avoid_pure_win_update now defaults to 1.
		 */
		if (avoid_pure_win_update == 0 ||
		    (tp->t_flags & TF_RXRESIZED)) {
			if (adv >= (long) (2 * segsz)) {
				goto send;
			}
		}
		hiwat = (long)(TCP_MAXWIN << tp->rcv_scale);
		if (hiwat > (long)so->so_rcv.ssb_hiwat)
			hiwat = (long)so->so_rcv.ssb_hiwat;
		if (adv >= hiwat / 2)
			goto send;
	}

	/*
	 * Send if we owe the peer an ACK, RST, SYN, or urgent data.  ACKNOW
	 * is also a catch-all for the retransmit timer timeout case.
	 */
	if (tp->t_flags & TF_ACKNOW)
		goto send;
	if ((flags & TH_RST) ||
	    ((flags & TH_SYN) && !(tp->t_flags & TF_NEEDSYN)))
		goto send;
	if (SEQ_GT(tp->snd_up, tp->snd_una))
		goto send;
	/*
	 * If our state indicates that FIN should be sent
	 * and we have not yet done so, then we need to send.
	 */
	if ((flags & TH_FIN) &&
	    (!(tp->t_flags & TF_SENTFIN) || tp->snd_nxt == tp->snd_una))
		goto send;

	/*
	 * TCP window updates are not reliable, rather a polling protocol
	 * using ``persist'' packets is used to insure receipt of window
	 * updates.  The three ``states'' for the output side are:
	 *	idle			not doing retransmits or persists
	 *	persisting		to move a small or zero window
	 *	(re)transmitting	and thereby not persisting
	 *
	 * tcp_callout_active(tp, tp->tt_persist)
	 *	is true when we are in persist state.
	 * The TF_FORCE flag in tp->t_flags
	 *	is set when we are called to send a persist packet.
	 * tcp_callout_active(tp, tp->tt_rexmt)
	 *	is set when we are retransmitting
	 * The output side is idle when both timers are zero.
	 *
	 * If send window is too small, there is data to transmit, and no
	 * retransmit or persist is pending, then go to persist state.
	 *
	 * If nothing happens soon, send when timer expires:
	 * if window is nonzero, transmit what we can, otherwise force out
	 * a byte.
	 *
	 * Don't try to set the persist state if we are in TCPS_SYN_RECEIVED
	 * with data pending.  This situation can occur during a
	 * simultanious connect.
	 */
	if (so->so_snd.ssb_cc > 0 &&
	    tp->t_state != TCPS_SYN_RECEIVED &&
	    !tcp_callout_active(tp, tp->tt_rexmt) &&
	    !tcp_callout_active(tp, tp->tt_persist)) {
		tp->t_rxtshift = 0;
		tcp_setpersist(tp);
	}

	/*
	 * No reason to send a segment, just return.
	 */
	tp->t_flags &= ~TF_XMITNOW;
	return (0);

send:
	if (need_sched && len > 0) {
		tcp_output_sched(tp);
		return 0;
	}

	/*
	 * Before ESTABLISHED, force sending of initial options
	 * unless TCP set not to do any options.
	 * NOTE: we assume that the IP/TCP header plus TCP options
	 * always fit in a single mbuf, leaving room for a maximum
	 * link header, i.e.
	 *	max_linkhdr + sizeof(struct tcpiphdr) + optlen <= MCLBYTES
	 */
	optlen = 0;
	if (isipv6)
		hdrlen = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
	else
		hdrlen = sizeof(struct tcpiphdr);
	if (flags & TH_SYN) {
		tp->snd_nxt = tp->iss;
		if (!(tp->t_flags & TF_NOOPT)) {
			u_short mss;

			opt[0] = TCPOPT_MAXSEG;
			opt[1] = TCPOLEN_MAXSEG;
			mss = htons((u_short) tcp_mssopt(tp));
			memcpy(opt + 2, &mss, sizeof mss);
			optlen = TCPOLEN_MAXSEG;

			if ((tp->t_flags & TF_REQ_SCALE) &&
			    (!(flags & TH_ACK) ||
			     (tp->t_flags & TF_RCVD_SCALE))) {
				*((u_int32_t *)(opt + optlen)) = htonl(
					TCPOPT_NOP << 24 |
					TCPOPT_WINDOW << 16 |
					TCPOLEN_WINDOW << 8 |
					tp->request_r_scale);
				optlen += 4;
			}

			if ((tcp_do_sack && !(flags & TH_ACK)) ||
			    tp->t_flags & TF_SACK_PERMITTED) {
				uint32_t *lp = (uint32_t *)(opt + optlen);

				*lp = htonl(TCPOPT_SACK_PERMITTED_ALIGNED);
				optlen += TCPOLEN_SACK_PERMITTED_ALIGNED;
			}
		}
	}

	/*
	 * Send a timestamp and echo-reply if this is a SYN and our side
	 * wants to use timestamps (TF_REQ_TSTMP is set) or both our side
	 * and our peer have sent timestamps in our SYN's.
	 */
	if ((tp->t_flags & (TF_REQ_TSTMP | TF_NOOPT)) == TF_REQ_TSTMP &&
	    !(flags & TH_RST) &&
	    (!(flags & TH_ACK) || (tp->t_flags & TF_RCVD_TSTMP))) {
		u_int32_t *lp = (u_int32_t *)(opt + optlen);

		/* Form timestamp option as shown in appendix A of RFC 1323. */
		*lp++ = htonl(TCPOPT_TSTAMP_HDR);
		*lp++ = htonl(ticks);
		*lp   = htonl(tp->ts_recent);
		optlen += TCPOLEN_TSTAMP_APPA;
	}

	/* Set receive buffer autosizing timestamp. */
	if (tp->rfbuf_ts == 0 && (so->so_rcv.ssb_flags & SSB_AUTOSIZE))
		tp->rfbuf_ts = ticks;

	/*
	 * If this is a SACK connection and we have a block to report,
	 * fill in the SACK blocks in the TCP options.
	 */
	if (report_sack)
		tcp_sack_fill_report(tp, opt, &optlen);

#ifdef TCP_SIGNATURE
	if (tp->t_flags & TF_SIGNATURE) {
		int i;
		u_char *bp;
		/*
		 * Initialize TCP-MD5 option (RFC2385)
		 */
		bp = (u_char *)opt + optlen;
		*bp++ = TCPOPT_SIGNATURE;
		*bp++ = TCPOLEN_SIGNATURE;
		sigoff = optlen + 2;
		for (i = 0; i < TCP_SIGLEN; i++)
			*bp++ = 0;
		optlen += TCPOLEN_SIGNATURE;
		/*
		 * Terminate options list and maintain 32-bit alignment.
		 */
		*bp++ = TCPOPT_NOP;
		*bp++ = TCPOPT_EOL;
		optlen += 2;
	}
#endif /* TCP_SIGNATURE */
	KASSERT(optlen <= TCP_MAXOLEN, ("too many TCP options"));
	hdrlen += optlen;

	if (isipv6) {
		ipoptlen = ip6_optlen(inp);
	} else {
		if (inp->inp_options) {
			ipoptlen = inp->inp_options->m_len -
			    offsetof(struct ipoption, ipopt_list);
		} else {