[qemu.git] / slirp / tcp_timer.c

/*
 * Copyright (c) 1982, 1986, 1988, 1990, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *	@(#)tcp_timer.c	8.1 (Berkeley) 6/10/93
 * tcp_timer.c,v 1.2 1994/08/02 07:49:10 davidg Exp
 */

#include <slirp.h>

#ifdef LOG_ENABLED
struct   tcpstat tcpstat;        /* tcp statistics */
#endif

u_int32_t        tcp_now;                /* for RFC 1323 timestamps */

static struct tcpcb *tcp_timers(register struct tcpcb *tp, int timer);

/*
 * Fast timeout routine for processing delayed acks
 */
void
tcp_fasttimo(void)
{
	register struct socket *so;
	register struct tcpcb *tp;

	DEBUG_CALL("tcp_fasttimo");

	so = tcb.so_next;
	if (so)
	for (; so != &tcb; so = so->so_next)
		if ((tp = (struct tcpcb *)so->so_tcpcb) &&
		    (tp->t_flags & TF_DELACK)) {
			tp->t_flags &= ~TF_DELACK;
			tp->t_flags |= TF_ACKNOW;
			STAT(tcpstat.tcps_delack++);
			(void) tcp_output(tp);
		}
}

/*
 * Tcp protocol timeout routine called every 500 ms.
 * Updates the timers in all active tcb's and
 * causes finite state machine actions if timers expire.
 */
void
tcp_slowtimo(void)
{
	register struct socket *ip, *ipnxt;
	register struct tcpcb *tp;
	register int i;

	DEBUG_CALL("tcp_slowtimo");

	/*
	 * Search through tcb's and update active timers.
	 */
	ip = tcb.so_next;
	if (ip == 0)
	   return;
	for (; ip != &tcb; ip = ipnxt) {
		ipnxt = ip->so_next;
		tp = sototcpcb(ip);
		if (tp == 0)
			continue;
		for (i = 0; i < TCPT_NTIMERS; i++) {
			if (tp->t_timer[i] && --tp->t_timer[i] == 0) {
				tcp_timers(tp,i);
				if (ipnxt->so_prev != ip)
					goto tpgone;
			}
		}
		tp->t_idle++;
		if (tp->t_rtt)
		   tp->t_rtt++;
tpgone:
		;
	}
	tcp_iss += TCP_ISSINCR/PR_SLOWHZ;		/* increment iss */
#ifdef TCP_COMPAT_42
	if ((int)tcp_iss < 0)
		tcp_iss = 0;				/* XXX */
#endif
	tcp_now++;					/* for timestamps */
}

/*
 * Cancel all timers for TCP tp.
 */
void
tcp_canceltimers(struct tcpcb *tp)
{
	register int i;

	for (i = 0; i < TCPT_NTIMERS; i++)
		tp->t_timer[i] = 0;
}

const int tcp_backoff[TCP_MAXRXTSHIFT + 1] =
   { 1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64 };

/*
 * TCP timer processing.
 */
static struct tcpcb *
tcp_timers(register struct tcpcb *tp, int timer)
{
	register int rexmt;

	DEBUG_CALL("tcp_timers");

	switch (timer) {

	/*
	 * 2 MSL timeout in shutdown went off.  If we're closed but
	 * still waiting for peer to close and connection has been idle
	 * too long, or if 2MSL time is up from TIME_WAIT, delete connection
	 * control block.  Otherwise, check again in a bit.
	 */
	case TCPT_2MSL:
		if (tp->t_state != TCPS_TIME_WAIT &&
		    tp->t_idle <= TCP_MAXIDLE)
			tp->t_timer[TCPT_2MSL] = TCPTV_KEEPINTVL;
		else
			tp = tcp_close(tp);
		break;

	/*
	 * Retransmission timer went off.  Message has not
	 * been acked within retransmit interval.  Back off
	 * to a longer retransmit interval and retransmit one segment.
	 */
	case TCPT_REXMT:

		/*
		 * XXXXX If a packet has timed out, then remove all the queued
		 * packets for that session.
		 */

		if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
			/*
			 * This is a hack to suit our terminal server here at the uni of canberra
			 * since they have trouble with zeroes... It usually lets them through
			 * unharmed, but under some conditions, it'll eat the zeros.  If we
			 * keep retransmitting it, it'll keep eating the zeroes, so we keep
			 * retransmitting, and eventually the connection dies...
			 * (this only happens on incoming data)
			 *
			 * So, if we were gonna drop the connection from too many retransmits,
			 * don't... instead halve the t_maxseg, which might break up the NULLs and
			 * let them through
			 *
			 * *sigh*
			 */

			tp->t_maxseg >>= 1;
			if (tp->t_maxseg < 32) {
				/*
				 * We tried our best, now the connection must die!
				 */
				tp->t_rxtshift = TCP_MAXRXTSHIFT;
				STAT(tcpstat.tcps_timeoutdrop++);
				tp = tcp_drop(tp, tp->t_softerror);
				/* tp->t_softerror : ETIMEDOUT); */ /* XXX */
				return (tp); /* XXX */
			}

			/*
			 * Set rxtshift to 6, which is still at the maximum
			 * backoff time
			 */
			tp->t_rxtshift = 6;
		}
		STAT(tcpstat.tcps_rexmttimeo++);
		rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
		TCPT_RANGESET(tp->t_rxtcur, rexmt,
		    (short)tp->t_rttmin, TCPTV_REXMTMAX); /* XXX */
		tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;
		/*
		 * If losing, let the lower level know and try for
		 * a better route.  Also, if we backed off this far,
		 * our srtt estimate is probably bogus.  Clobber it
		 * so we'll take the next rtt measurement as our srtt;
		 * move the current srtt into rttvar to keep the current
		 * retransmit times until then.
		 */
		if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
/*			in_losing(tp->t_inpcb); */
			tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
			tp->t_srtt = 0;
		}
		tp->snd_nxt = tp->snd_una;
		/*
		 * If timing a segment in this window, stop the timer.
		 */
		tp->t_rtt = 0;
		/*
		 * Close the congestion window down to one segment
		 * (we'll open it by one segment for each ack we get).
		 * Since we probably have a window's worth of unacked
		 * data accumulated, this "slow start" keeps us from
		 * dumping all that data as back-to-back packets (which
		 * might overwhelm an intermediate gateway).
		 *
		 * There are two phases to the opening: Initially we
		 * open by one mss on each ack.  This makes the window
		 * size increase exponentially with time.  If the
		 * window is larger than the path can handle, this
		 * exponential growth results in dropped packet(s)
		 * almost immediately.  To get more time between
		 * drops but still "push" the network to take advantage
		 * of improving conditions, we switch from exponential
		 * to linear window opening at some threshold size.
		 * For a threshold, we use half the current window
		 * size, truncated to a multiple of the mss.
		 *
		 * (the minimum cwnd that will give us exponential
		 * growth is 2 mss.  We don't allow the threshold
		 * to go below this.)
		 */
		{
		u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
		if (win < 2)
			win = 2;
		tp->snd_cwnd = tp->t_maxseg;
		tp->snd_ssthresh = win * tp->t_maxseg;
		tp->t_dupacks = 0;
		}
		(void) tcp_output(tp);
		break;

	/*
	 * Persistence timer into zero window.
	 * Force a byte to be output, if possible.
	 */
	case TCPT_PERSIST:
		STAT(tcpstat.tcps_persisttimeo++);
		tcp_setpersist(tp);
		tp->t_force = 1;
		(void) tcp_output(tp);
		tp->t_force = 0;
		break;

	/*
	 * Keep-alive timer went off; send something
	 * or drop connection if idle for too long.
	 */
	case TCPT_KEEP:
		STAT(tcpstat.tcps_keeptimeo++);
		if (tp->t_state < TCPS_ESTABLISHED)
			goto dropit;

/*		if (tp->t_socket->so_options & SO_KEEPALIVE && */
		if ((SO_OPTIONS) && tp->t_state <= TCPS_CLOSE_WAIT) {
		    	if (tp->t_idle >= TCPTV_KEEP_IDLE + TCP_MAXIDLE)
				goto dropit;
			/*
			 * Send a packet designed to force a response
			 * if the peer is up and reachable:
			 * either an ACK if the connection is still alive,
			 * or an RST if the peer has closed the connection
			 * due to timeout or reboot.
			 * Using sequence number tp->snd_una-1
			 * causes the transmitted zero-length segment
			 * to lie outside the receive window;
			 * by the protocol spec, this requires the
			 * correspondent TCP to respond.
			 */
			STAT(tcpstat.tcps_keepprobe++);
#ifdef TCP_COMPAT_42
			/*
			 * The keepalive packet must have nonzero length
			 * to get a 4.2 host to respond.
			 */
			tcp_respond(tp, &tp->t_template, (struct mbuf *)NULL,
			    tp->rcv_nxt - 1, tp->snd_una - 1, 0);
#else
			tcp_respond(tp, &tp->t_template, (struct mbuf *)NULL,
			    tp->rcv_nxt, tp->snd_una - 1, 0);
#endif
			tp->t_timer[TCPT_KEEP] = TCPTV_KEEPINTVL;
		} else
			tp->t_timer[TCPT_KEEP] = TCPTV_KEEP_IDLE;
		break;

	dropit:
		STAT(tcpstat.tcps_keepdrops++);
		tp = tcp_drop(tp, 0); /* ETIMEDOUT); */
		break;
	}

	return (tp);
}
Commit	Line	Data
f0cbd3ec FB	1	/*
	2	* Copyright (c) 1982, 1986, 1988, 1990, 1993
	3	* The Regents of the University of California. All rights reserved.
	4	*
	5	* Redistribution and use in source and binary forms, with or without
	6	* modification, are permitted provided that the following conditions
	7	* are met:
	8	* 1. Redistributions of source code must retain the above copyright
	9	* notice, this list of conditions and the following disclaimer.
	10	* 2. Redistributions in binary form must reproduce the above copyright
	11	* notice, this list of conditions and the following disclaimer in the
	12	* documentation and/or other materials provided with the distribution.
2f5f8996	13	* 3. Neither the name of the University nor the names of its contributors
f0cbd3ec FB	14	* may be used to endorse or promote products derived from this software
	15	* without specific prior written permission.
	16	*
	17	* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
	18	* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	19	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	20	* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
	21	* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	22	* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	23	* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	24	* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	25	* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	26	* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	27	* SUCH DAMAGE.
	28	*
	29	* @(#)tcp_timer.c 8.1 (Berkeley) 6/10/93
	30	* tcp_timer.c,v 1.2 1994/08/02 07:49:10 davidg Exp
	31	*/
	32
	33	#include <slirp.h>
	34
31a60e22	35	#ifdef LOG_ENABLED
f0cbd3ec	36	struct tcpstat tcpstat; /* tcp statistics */
31a60e22 BS	37	#endif
31a60e22 BS	38
f0cbd3ec FB	39	u_int32_t tcp_now; /* for RFC 1323 timestamps */
f0cbd3ec FB	40
9634d903 BS	41	static struct tcpcb tcp_timers(register struct tcpcb tp, int timer);
9634d903 BS	42
f0cbd3ec FB	43	/*
	44	* Fast timeout routine for processing delayed acks
	45	*/
	46	void
aeed97c4	47	tcp_fasttimo(void)
f0cbd3ec FB	48	{
	49	register struct socket *so;
	50	register struct tcpcb *tp;
	51
	52	DEBUG_CALL("tcp_fasttimo");
5fafdf24	53
f0cbd3ec FB	54	so = tcb.so_next;
	55	if (so)
	56	for (; so != &tcb; so = so->so_next)
	57	if ((tp = (struct tcpcb *)so->so_tcpcb) &&
	58	(tp->t_flags & TF_DELACK)) {
	59	tp->t_flags &= ~TF_DELACK;
	60	tp->t_flags \|= TF_ACKNOW;
31a60e22	61	STAT(tcpstat.tcps_delack++);
f0cbd3ec FB	62	(void) tcp_output(tp);
	63	}
	64	}
	65
	66	/*
	67	* Tcp protocol timeout routine called every 500 ms.
	68	* Updates the timers in all active tcb's and
	69	* causes finite state machine actions if timers expire.
	70	*/
	71	void
aeed97c4	72	tcp_slowtimo(void)
f0cbd3ec FB	73	{
	74	register struct socket ip, ipnxt;
	75	register struct tcpcb *tp;
	76	register int i;
	77
	78	DEBUG_CALL("tcp_slowtimo");
5fafdf24	79
f0cbd3ec FB	80	/*
	81	* Search through tcb's and update active timers.
	82	*/
	83	ip = tcb.so_next;
	84	if (ip == 0)
	85	return;
	86	for (; ip != &tcb; ip = ipnxt) {
	87	ipnxt = ip->so_next;
	88	tp = sototcpcb(ip);
	89	if (tp == 0)
	90	continue;
	91	for (i = 0; i < TCPT_NTIMERS; i++) {
	92	if (tp->t_timer[i] && --tp->t_timer[i] == 0) {
	93	tcp_timers(tp,i);
	94	if (ipnxt->so_prev != ip)
	95	goto tpgone;
	96	}
	97	}
	98	tp->t_idle++;
	99	if (tp->t_rtt)
	100	tp->t_rtt++;
	101	tpgone:
	102	;
	103	}
	104	tcp_iss += TCP_ISSINCR/PR_SLOWHZ; /* increment iss */
	105	#ifdef TCP_COMPAT_42
	106	if ((int)tcp_iss < 0)
	107	tcp_iss = 0; /* XXX */
	108	#endif
	109	tcp_now++; /* for timestamps */
	110	}
	111
	112	/*
	113	* Cancel all timers for TCP tp.
	114	*/
	115	void
aeed97c4	116	tcp_canceltimers(struct tcpcb *tp)
f0cbd3ec FB	117	{
	118	register int i;
	119
	120	for (i = 0; i < TCPT_NTIMERS; i++)
	121	tp->t_timer[i] = 0;
	122	}
	123
9634d903	124	const int tcp_backoff[TCP_MAXRXTSHIFT + 1] =
f0cbd3ec FB	125	{ 1, 2, 4, 8, 16, 32, 64, 64, 64, 64, 64, 64, 64 };
	126
	127	/*
	128	* TCP timer processing.
	129	*/
9634d903 BS	130	static struct tcpcb *
9634d903 BS	131	tcp_timers(register struct tcpcb *tp, int timer)
f0cbd3ec FB	132	{
f0cbd3ec FB	133	register int rexmt;
5fafdf24	134
f0cbd3ec	135	DEBUG_CALL("tcp_timers");
5fafdf24	136
f0cbd3ec FB	137	switch (timer) {
	138
	139	/*
	140	* 2 MSL timeout in shutdown went off. If we're closed but
	141	* still waiting for peer to close and connection has been idle
	142	* too long, or if 2MSL time is up from TIME_WAIT, delete connection
	143	* control block. Otherwise, check again in a bit.
	144	*/
	145	case TCPT_2MSL:
	146	if (tp->t_state != TCPS_TIME_WAIT &&
9634d903 BS	147	tp->t_idle <= TCP_MAXIDLE)
9634d903 BS	148	tp->t_timer[TCPT_2MSL] = TCPTV_KEEPINTVL;
f0cbd3ec FB	149	else
	150	tp = tcp_close(tp);
	151	break;
	152
	153	/*
	154	* Retransmission timer went off. Message has not
	155	* been acked within retransmit interval. Back off
	156	* to a longer retransmit interval and retransmit one segment.
	157	*/
	158	case TCPT_REXMT:
3b46e624	159
f0cbd3ec FB	160	/*
	161	* XXXXX If a packet has timed out, then remove all the queued
	162	* packets for that session.
	163	*/
3b46e624	164
f0cbd3ec FB	165	if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
	166	/*
	167	* This is a hack to suit our terminal server here at the uni of canberra
	168	* since they have trouble with zeroes... It usually lets them through
	169	* unharmed, but under some conditions, it'll eat the zeros. If we
	170	* keep retransmitting it, it'll keep eating the zeroes, so we keep
	171	* retransmitting, and eventually the connection dies...
	172	* (this only happens on incoming data)
5fafdf24	173	*
f0cbd3ec FB	174	* So, if we were gonna drop the connection from too many retransmits,
	175	* don't... instead halve the t_maxseg, which might break up the NULLs and
	176	* let them through
5fafdf24	177	*
f0cbd3ec FB	178	* sigh
f0cbd3ec FB	179	*/
3b46e624	180
f0cbd3ec FB	181	tp->t_maxseg >>= 1;
	182	if (tp->t_maxseg < 32) {
	183	/*
	184	* We tried our best, now the connection must die!
	185	*/
	186	tp->t_rxtshift = TCP_MAXRXTSHIFT;
31a60e22	187	STAT(tcpstat.tcps_timeoutdrop++);
f0cbd3ec FB	188	tp = tcp_drop(tp, tp->t_softerror);
	189	/* tp->t_softerror : ETIMEDOUT); / / XXX */
	190	return (tp); /* XXX */
	191	}
3b46e624	192
f0cbd3ec FB	193	/*
	194	* Set rxtshift to 6, which is still at the maximum
	195	* backoff time
	196	*/
	197	tp->t_rxtshift = 6;
	198	}
31a60e22	199	STAT(tcpstat.tcps_rexmttimeo++);
f0cbd3ec FB	200	rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
	201	TCPT_RANGESET(tp->t_rxtcur, rexmt,
	202	(short)tp->t_rttmin, TCPTV_REXMTMAX); /* XXX */
	203	tp->t_timer[TCPT_REXMT] = tp->t_rxtcur;
	204	/*
	205	* If losing, let the lower level know and try for
	206	* a better route. Also, if we backed off this far,
	207	* our srtt estimate is probably bogus. Clobber it
	208	* so we'll take the next rtt measurement as our srtt;
	209	* move the current srtt into rttvar to keep the current
	210	* retransmit times until then.
	211	*/
	212	if (tp->t_rxtshift > TCP_MAXRXTSHIFT / 4) {
	213	/* in_losing(tp->t_inpcb); */
	214	tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
	215	tp->t_srtt = 0;
	216	}
	217	tp->snd_nxt = tp->snd_una;
	218	/*
	219	* If timing a segment in this window, stop the timer.
	220	*/
	221	tp->t_rtt = 0;
	222	/*
	223	* Close the congestion window down to one segment
	224	* (we'll open it by one segment for each ack we get).
	225	* Since we probably have a window's worth of unacked
	226	* data accumulated, this "slow start" keeps us from
	227	* dumping all that data as back-to-back packets (which
	228	* might overwhelm an intermediate gateway).
	229	*
	230	* There are two phases to the opening: Initially we
	231	* open by one mss on each ack. This makes the window
	232	* size increase exponentially with time. If the
	233	* window is larger than the path can handle, this
	234	* exponential growth results in dropped packet(s)
5fafdf24	235	* almost immediately. To get more time between
f0cbd3ec FB	236	* drops but still "push" the network to take advantage
	237	* of improving conditions, we switch from exponential
	238	* to linear window opening at some threshold size.
	239	* For a threshold, we use half the current window
	240	* size, truncated to a multiple of the mss.
	241	*
	242	* (the minimum cwnd that will give us exponential
	243	* growth is 2 mss. We don't allow the threshold
	244	* to go below this.)
	245	*/
	246	{
	247	u_int win = min(tp->snd_wnd, tp->snd_cwnd) / 2 / tp->t_maxseg;
	248	if (win < 2)
	249	win = 2;
	250	tp->snd_cwnd = tp->t_maxseg;
	251	tp->snd_ssthresh = win * tp->t_maxseg;
	252	tp->t_dupacks = 0;
	253	}
	254	(void) tcp_output(tp);
	255	break;
	256
	257	/*
	258	* Persistence timer into zero window.
	259	* Force a byte to be output, if possible.
	260	*/
	261	case TCPT_PERSIST:
31a60e22	262	STAT(tcpstat.tcps_persisttimeo++);
f0cbd3ec FB	263	tcp_setpersist(tp);
	264	tp->t_force = 1;
	265	(void) tcp_output(tp);
	266	tp->t_force = 0;
	267	break;
	268
	269	/*
	270	* Keep-alive timer went off; send something
	271	* or drop connection if idle for too long.
	272	*/
	273	case TCPT_KEEP:
31a60e22	274	STAT(tcpstat.tcps_keeptimeo++);
f0cbd3ec FB	275	if (tp->t_state < TCPS_ESTABLISHED)
	276	goto dropit;
	277
	278	/* if (tp->t_socket->so_options & SO_KEEPALIVE && */
9634d903 BS	279	if ((SO_OPTIONS) && tp->t_state <= TCPS_CLOSE_WAIT) {
9634d903 BS	280	if (tp->t_idle >= TCPTV_KEEP_IDLE + TCP_MAXIDLE)
f0cbd3ec FB	281	goto dropit;
	282	/*
	283	* Send a packet designed to force a response
	284	* if the peer is up and reachable:
	285	* either an ACK if the connection is still alive,
	286	* or an RST if the peer has closed the connection
	287	* due to timeout or reboot.
	288	* Using sequence number tp->snd_una-1
	289	* causes the transmitted zero-length segment
	290	* to lie outside the receive window;
	291	* by the protocol spec, this requires the
	292	* correspondent TCP to respond.
	293	*/
31a60e22	294	STAT(tcpstat.tcps_keepprobe++);
f0cbd3ec FB	295	#ifdef TCP_COMPAT_42
	296	/*
	297	* The keepalive packet must have nonzero length
	298	* to get a 4.2 host to respond.
	299	*/
	300	tcp_respond(tp, &tp->t_template, (struct mbuf *)NULL,
	301	tp->rcv_nxt - 1, tp->snd_una - 1, 0);
	302	#else
	303	tcp_respond(tp, &tp->t_template, (struct mbuf *)NULL,
	304	tp->rcv_nxt, tp->snd_una - 1, 0);
	305	#endif
9634d903	306	tp->t_timer[TCPT_KEEP] = TCPTV_KEEPINTVL;
f0cbd3ec	307	} else
9634d903	308	tp->t_timer[TCPT_KEEP] = TCPTV_KEEP_IDLE;
f0cbd3ec FB	309	break;
	310
	311	dropit:
31a60e22	312	STAT(tcpstat.tcps_keepdrops++);
f0cbd3ec FB	313	tp = tcp_drop(tp, 0); /* ETIMEDOUT); */
	314	break;
	315	}
	316
	317	return (tp);
	318	}