freebsd-dev/sys/netinet/tcp_timer.c
Andre Oppermann f58747375d Handle a race condition on >2 core machines in tcp_timer() when
a timer issues a shutdown and a simultaneous close on the socket
happens.  This race condition is inherent in the current socket/
inpcb life cycle system but can be handled well.

Reported by:	kris
Tested by:	kris (on 8-core machine)
2007-06-09 17:49:39 +00:00

693 lines
20 KiB
C

/*-
* Copyright (c) 1982, 1986, 1988, 1990, 1993, 1995
* 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.
* 4. 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 8.2 (Berkeley) 5/24/95
* $FreeBSD$
*/
#include "opt_inet6.h"
#include "opt_tcpdebug.h"
#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/ktr.h>
#include <sys/lock.h>
#include <sys/limits.h>
#include <sys/mbuf.h>
#include <sys/mutex.h>
#include <sys/protosw.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/sysctl.h>
#include <sys/systm.h>
#include <net/route.h>
#include <netinet/in.h>
#include <netinet/in_pcb.h>
#include <netinet/in_systm.h>
#ifdef INET6
#include <netinet6/in6_pcb.h>
#endif
#include <netinet/ip_var.h>
#include <netinet/tcp.h>
#include <netinet/tcp_fsm.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#include <netinet/tcpip.h>
#ifdef TCPDEBUG
#include <netinet/tcp_debug.h>
#endif
int tcp_keepinit;
SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINIT, keepinit, CTLTYPE_INT|CTLFLAG_RW,
&tcp_keepinit, 0, sysctl_msec_to_ticks, "I", "");
int tcp_keepidle;
SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT|CTLFLAG_RW,
&tcp_keepidle, 0, sysctl_msec_to_ticks, "I", "");
int tcp_keepintvl;
SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl, CTLTYPE_INT|CTLFLAG_RW,
&tcp_keepintvl, 0, sysctl_msec_to_ticks, "I", "");
int tcp_delacktime;
SYSCTL_PROC(_net_inet_tcp, TCPCTL_DELACKTIME, delacktime, CTLTYPE_INT|CTLFLAG_RW,
&tcp_delacktime, 0, sysctl_msec_to_ticks, "I",
"Time before a delayed ACK is sent");
int tcp_msl;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl, CTLTYPE_INT|CTLFLAG_RW,
&tcp_msl, 0, sysctl_msec_to_ticks, "I", "Maximum segment lifetime");
int tcp_rexmit_min;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_min, CTLTYPE_INT|CTLFLAG_RW,
&tcp_rexmit_min, 0, sysctl_msec_to_ticks, "I",
"Minimum Retransmission Timeout");
int tcp_rexmit_slop;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_slop, CTLTYPE_INT|CTLFLAG_RW,
&tcp_rexmit_slop, 0, sysctl_msec_to_ticks, "I",
"Retransmission Timer Slop");
static int always_keepalive = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_RW,
&always_keepalive , 0, "Assume SO_KEEPALIVE on all TCP connections");
int tcp_fast_finwait2_recycle = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, fast_finwait2_recycle, CTLFLAG_RW,
&tcp_fast_finwait2_recycle, 0,
"Recycle closed FIN_WAIT_2 connections faster");
int tcp_finwait2_timeout;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, finwait2_timeout, CTLTYPE_INT|CTLFLAG_RW,
&tcp_finwait2_timeout, 0, sysctl_msec_to_ticks, "I", "FIN-WAIT2 timeout");
static int tcp_keepcnt = TCPTV_KEEPCNT;
/* max idle probes */
int tcp_maxpersistidle;
/* max idle time in persist */
int tcp_maxidle;
static void tcp_timer(void *);
static int tcp_timer_delack(struct tcpcb *, struct inpcb *);
static int tcp_timer_2msl(struct tcpcb *, struct inpcb *);
static int tcp_timer_keep(struct tcpcb *, struct inpcb *);
static int tcp_timer_persist(struct tcpcb *, struct inpcb *);
static int tcp_timer_rexmt(struct tcpcb *, struct inpcb *);
/*
* Tcp protocol timeout routine called every 500 ms.
* Updates timestamps used for TCP
* causes finite state machine actions if timers expire.
*/
void
tcp_slowtimo(void)
{
tcp_maxidle = tcp_keepcnt * tcp_keepintvl;
INP_INFO_WLOCK(&tcbinfo);
(void) tcp_tw_2msl_scan(0);
INP_INFO_WUNLOCK(&tcbinfo);
}
int tcp_syn_backoff[TCP_MAXRXTSHIFT + 1] =
{ 1, 1, 1, 1, 1, 2, 4, 8, 16, 32, 64, 64, 64 };
int tcp_backoff[TCP_MAXRXTSHIFT + 1] =
{ 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 512, 512, 512 };
static int tcp_totbackoff = 2559; /* sum of tcp_backoff[] */
static int tcp_timer_race;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, timer_race, CTLFLAG_RD, &tcp_timer_race,
0, "Count of t_inpcb races on tcp_discardcb");
void
tcp_timer_activate(struct tcpcb *tp, int timer_type, u_int delta)
{
struct inpcb *inp = tp->t_inpcb;
struct tcp_timer *tt = tp->t_timers;
int tick = ticks; /* Stable time base. */
int next = delta ? tick + delta : 0;
INP_LOCK_ASSERT(inp);
CTR6(KTR_NET, "%p %s inp %p active %x delta %i nextc %i",
tp, __func__, inp, tt->tt_active, delta, tt->tt_nextc);
/* Set new value for timer. */
switch(timer_type) {
case TT_DELACK:
CTR4(KTR_NET, "%p %s TT_DELACK old %i new %i",
tp, __func__, tt->tt_delack, next);
tt->tt_delack = next;
break;
case TT_REXMT:
CTR4(KTR_NET, "%p %s TT_REXMT old %i new %i",
tp, __func__, tt->tt_rexmt, next);
tt->tt_rexmt = next;
break;
case TT_PERSIST:
CTR4(KTR_NET, "%p %s TT_PERSIST old %i new %i",
tp, __func__, tt->tt_persist, next);
tt->tt_persist = next;
break;
case TT_KEEP:
CTR4(KTR_NET, "%p %s TT_KEEP old %i new %i",
tp, __func__, tt->tt_keep, next);
tt->tt_keep = next;
break;
case TT_2MSL:
CTR4(KTR_NET, "%p %s TT_2MSL old %i new %i",
tp, __func__, tt->tt_2msl, next);
tt->tt_2msl = next;
break;
case 0: /* Dummy for timer rescan. */
CTR3(KTR_NET, "%p %s timer rescan new %i", tp, __func__, next);
break;
}
/* If some other timer is active and is schedules sooner just return. */
if (tt->tt_active != timer_type && tt->tt_nextc < next &&
callout_active(&tt->tt_timer))
return;
/* Select next timer to schedule. */
tt->tt_nextc = INT_MAX;
tt->tt_active = 0;
if (tt->tt_delack && tt->tt_delack < tt->tt_nextc) {
tt->tt_nextc = tt->tt_delack;
tt->tt_active = TT_DELACK;
}
if (tt->tt_rexmt && tt->tt_rexmt < tt->tt_nextc) {
tt->tt_nextc = tt->tt_rexmt;
tt->tt_active = TT_REXMT;
}
if (tt->tt_persist && tt->tt_persist < tt->tt_nextc) {
tt->tt_nextc = tt->tt_persist;
tt->tt_active = TT_PERSIST;
}
if (tt->tt_keep && tt->tt_keep < tt->tt_nextc) {
tt->tt_nextc = tt->tt_keep;
tt->tt_active = TT_KEEP;
}
if (tt->tt_2msl && tt->tt_2msl < tt->tt_nextc) {
tt->tt_nextc = tt->tt_2msl;
tt->tt_active = TT_2MSL;
}
/* Rearm callout with new timer if we found one. */
if (tt->tt_active) {
CTR4(KTR_NET, "%p %s callout_reset active %x nextc in %i",
tp, __func__, tt->tt_active, tt->tt_nextc - tick);
callout_reset(&tt->tt_timer,
tt->tt_nextc - tick, tcp_timer, (void *)inp);
} else {
CTR2(KTR_NET, "%p %s callout_stop", tp, __func__);
callout_stop(&tt->tt_timer);
tt->tt_nextc = 0;
}
return;
}
int
tcp_timer_active(struct tcpcb *tp, int timer_type)
{
switch (timer_type) {
case TT_DELACK:
CTR3(KTR_NET, "%p %s TT_DELACK %i",
tp, __func__, tp->t_timers->tt_delack);
return (tp->t_timers->tt_delack ? 1 : 0);
break;
case TT_REXMT:
CTR3(KTR_NET, "%p %s TT_REXMT %i",
tp, __func__, tp->t_timers->tt_rexmt);
return (tp->t_timers->tt_rexmt ? 1 : 0);
break;
case TT_PERSIST:
CTR3(KTR_NET, "%p %s TT_PERSIST %i",
tp, __func__, tp->t_timers->tt_persist);
return (tp->t_timers->tt_persist ? 1 : 0);
break;
case TT_KEEP:
CTR3(KTR_NET, "%p %s TT_KEEP %i",
tp, __func__, tp->t_timers->tt_keep);
return (tp->t_timers->tt_keep ? 1 : 0);
break;
case TT_2MSL:
CTR3(KTR_NET, "%p %s TT_2MSL %i",
tp, __func__, tp->t_timers->tt_2msl);
return (tp->t_timers->tt_2msl ? 1 : 0);
break;
}
return (0);
}
static void
tcp_timer(void *xinp)
{
struct inpcb *inp = (struct inpcb *)xinp;
struct tcpcb *tp = intotcpcb(inp);
struct tcp_timer *tt;
int tick = ticks;
int down, timer;
/* INP lock was obtained by callout. */
INP_LOCK_ASSERT(inp);
/*
* We've got a couple of race conditions here:
* - The tcpcb was converted into a compressed TW pcb. All our
* timers have been stopped while this callout already tried
* to obtain the inpcb lock. TW pcbs have their own timers
* and we just return.
*/
if (inp->inp_vflag & INP_TIMEWAIT)
return;
/*
* - The tcpcb was discarded. All our timers have been stopped
* while this callout already tried to obtain the inpcb lock
* and we just return.
*/
if (tp == NULL)
return;
tt = tp->t_timers; /* Initialize. */
CTR6(KTR_NET, "%p %s inp %p active %x tick %i nextc %i",
tp, __func__, inp, tt->tt_active, tick, tt->tt_nextc);
/*
* - We may have been waiting on the lock while the tcpcb has
* been scheduled for destruction. In this case no active
* timers remain and we just return.
*/
if (tt->tt_active == 0)
goto done;
/*
* - The timer was rescheduled while this callout was already
* waiting on the lock. This may happen when a packet just
* came in. Rescan and reschedule the the timer in case we
* just turned it off.
*/
if (tick < tt->tt_nextc)
goto rescan;
/*
* Mark as done. The active bit in struct callout is not
* automatically cleared. See callout(9) for more info.
* In tcp_discardcb() we depend on the correctly cleared
* active bit for faster processing.
*/
callout_deactivate(&tt->tt_timer);
/* Check which timer has fired and remove this timer activation. */
timer = tt->tt_active;
tt->tt_active = 0;
tt->tt_nextc = 0;
switch (timer) {
case TT_DELACK:
CTR2(KTR_NET, "%p %s running TT_DELACK", tp, __func__);
tt->tt_delack = 0;
down = tcp_timer_delack(tp, inp); /* down == 0 */
break;
case TT_REXMT:
CTR2(KTR_NET, "%p %s running TT_REXMT", tp, __func__);
tt->tt_rexmt = 0;
down = tcp_timer_rexmt(tp, inp);
break;
case TT_PERSIST:
CTR2(KTR_NET, "%p %s running TT_PERSIST", tp, __func__);
tt->tt_persist = 0;
down = tcp_timer_persist(tp, inp);
break;
case TT_KEEP:
CTR2(KTR_NET, "%p %s running TT_KEEP", tp, __func__);
tt->tt_keep = 0;
down = tcp_timer_keep(tp, inp);
break;
case TT_2MSL:
CTR2(KTR_NET, "%p %s running TT_2MSL", tp, __func__);
tt->tt_2msl = 0;
down = tcp_timer_2msl(tp, inp);
break;
default:
CTR2(KTR_NET, "%p %s running nothing", tp, __func__);
down = 0;
}
CTR4(KTR_NET, "%p %s down %i active %x",
tp, __func__, down, tt->tt_active);
/* Do we still exist? */
if (down)
goto shutdown;
rescan:
/* Rescan if no timer was reactivated above. */
if (tt->tt_active == 0)
tcp_timer_activate(tp, 0, 0);
done:
INP_UNLOCK(inp); /* CALLOUT_RETURNUNLOCKED */
return;
shutdown:
INP_UNLOCK(inp); /* Prevent LOR at expense of race. */
INP_INFO_WLOCK(&tcbinfo);
INP_LOCK(inp);
/*
* XXX: When our tcpcb went into TIMEWAIT, is gone or no
* longer the one we used to work with we've lost the race.
* This race is inherent in the current socket/inpcb life
* cycle system.
*/
if ((inp->inp_vflag & INP_TIMEWAIT) || inp->inp_ppcb == NULL ||
inp->inp_ppcb != tp) {
CTR3(KTR_NET, "%p %s inp %p lost shutdown race",
tp, __func__, inp);
tcp_timer_race++;
INP_UNLOCK(inp); /* CALLOUT_RETURNUNLOCKED */
INP_INFO_WUNLOCK(&tcbinfo);
return;
}
KASSERT(tp == inp->inp_ppcb, ("%s: tp changed", __func__));
/* Shutdown the connection. */
switch (down) {
case 1:
tp = tcp_close(tp);
break;
case 2:
tp = tcp_drop(tp,
tp->t_softerror ? tp->t_softerror : ETIMEDOUT);
break;
}
CTR3(KTR_NET, "%p %s inp %p after shutdown", tp, __func__, inp);
if (tp)
INP_UNLOCK(inp); /* CALLOUT_RETURNUNLOCKED */
INP_INFO_WUNLOCK(&tcbinfo);
return;
}
/*
* TCP timer processing.
*/
static int
tcp_timer_delack(struct tcpcb *tp, struct inpcb *inp)
{
tp->t_flags |= TF_ACKNOW;
tcpstat.tcps_delack++;
(void) tcp_output(tp);
return (0);
}
static int
tcp_timer_2msl(struct tcpcb *tp, struct inpcb *inp)
{
#ifdef TCPDEBUG
int ostate;
ostate = tp->t_state;
#endif
/*
* 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.
*
* If fastrecycle of FIN_WAIT_2, in FIN_WAIT_2 and receiver has closed,
* there's no point in hanging onto FIN_WAIT_2 socket. Just close it.
* Ignore fact that there were recent incoming segments.
*/
if (tcp_fast_finwait2_recycle && tp->t_state == TCPS_FIN_WAIT_2 &&
tp->t_inpcb->inp_socket &&
(tp->t_inpcb->inp_socket->so_rcv.sb_state & SBS_CANTRCVMORE)) {
tcpstat.tcps_finwait2_drops++;
return (1); /* tcp_close */
} else {
if (tp->t_state != TCPS_TIME_WAIT &&
(ticks - tp->t_rcvtime) <= tcp_maxidle)
tcp_timer_activate(tp, TT_2MSL, tcp_keepintvl);
else
return (1); /* tcp_close */
}
#ifdef TCPDEBUG
if (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
PRU_SLOWTIMO);
#endif
return (0);
}
static int
tcp_timer_keep(struct tcpcb *tp, struct inpcb *inp)
{
struct tcptemp *t_template;
#ifdef TCPDEBUG
int ostate;
ostate = tp->t_state;
#endif
/*
* Keep-alive timer went off; send something
* or drop connection if idle for too long.
*/
tcpstat.tcps_keeptimeo++;
if (tp->t_state < TCPS_ESTABLISHED)
goto dropit;
if ((always_keepalive || inp->inp_socket->so_options & SO_KEEPALIVE) &&
tp->t_state <= TCPS_CLOSING) {
if ((ticks - tp->t_rcvtime) >= tcp_keepidle + 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.
*/
tcpstat.tcps_keepprobe++;
t_template = tcpip_maketemplate(inp);
if (t_template) {
tcp_respond(tp, t_template->tt_ipgen,
&t_template->tt_t, (struct mbuf *)NULL,
tp->rcv_nxt, tp->snd_una - 1, 0);
(void) m_free(dtom(t_template));
}
tcp_timer_activate(tp, TT_KEEP, tcp_keepintvl);
} else
tcp_timer_activate(tp, TT_KEEP, tcp_keepidle);
#ifdef TCPDEBUG
if (inp->inp_socket->so_options & SO_DEBUG)
tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
PRU_SLOWTIMO);
#endif
return (0);
dropit:
tcpstat.tcps_keepdrops++;
return (2); /* tcp_drop() */
}
static int
tcp_timer_persist(struct tcpcb *tp, struct inpcb *inp)
{
#ifdef TCPDEBUG
int ostate;
ostate = tp->t_state;
#endif
/*
* Persistance timer into zero window.
* Force a byte to be output, if possible.
*/
tcpstat.tcps_persisttimeo++;
/*
* Hack: if the peer is dead/unreachable, we do not
* time out if the window is closed. After a full
* backoff, drop the connection if the idle time
* (no responses to probes) reaches the maximum
* backoff that we would use if retransmitting.
*/
if (tp->t_rxtshift == TCP_MAXRXTSHIFT &&
((ticks - tp->t_rcvtime) >= tcp_maxpersistidle ||
(ticks - tp->t_rcvtime) >= TCP_REXMTVAL(tp) * tcp_totbackoff)) {
tcpstat.tcps_persistdrop++;
return (2); /* tcp_drop() */
}
tcp_setpersist(tp);
tp->t_flags |= TF_FORCEDATA;
(void) tcp_output(tp);
tp->t_flags &= ~TF_FORCEDATA;
#ifdef TCPDEBUG
if (tp != NULL && tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
tcp_trace(TA_USER, ostate, tp, NULL, NULL, PRU_SLOWTIMO);
#endif
return (0);
}
static int
tcp_timer_rexmt(struct tcpcb *tp, struct inpcb *inp)
{
int rexmt;
#ifdef TCPDEBUG
int ostate;
ostate = tp->t_state;
#endif
tcp_free_sackholes(tp);
/*
* Retransmission timer went off. Message has not
* been acked within retransmit interval. Back off
* to a longer retransmit interval and retransmit one segment.
*/
if (++tp->t_rxtshift > TCP_MAXRXTSHIFT) {
tp->t_rxtshift = TCP_MAXRXTSHIFT;
tcpstat.tcps_timeoutdrop++;
return (2); /* tcp_drop() */
}
if (tp->t_rxtshift == 1) {
/*
* first retransmit; record ssthresh and cwnd so they can
* be recovered if this turns out to be a "bad" retransmit.
* A retransmit is considered "bad" if an ACK for this
* segment is received within RTT/2 interval; the assumption
* here is that the ACK was already in flight. See
* "On Estimating End-to-End Network Path Properties" by
* Allman and Paxson for more details.
*/
tp->snd_cwnd_prev = tp->snd_cwnd;
tp->snd_ssthresh_prev = tp->snd_ssthresh;
tp->snd_recover_prev = tp->snd_recover;
if (IN_FASTRECOVERY(tp))
tp->t_flags |= TF_WASFRECOVERY;
else
tp->t_flags &= ~TF_WASFRECOVERY;
tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
}
tcpstat.tcps_rexmttimeo++;
if (tp->t_state == TCPS_SYN_SENT)
rexmt = TCP_REXMTVAL(tp) * tcp_syn_backoff[tp->t_rxtshift];
else
rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
TCPT_RANGESET(tp->t_rxtcur, rexmt,
tp->t_rttmin, TCPTV_REXMTMAX);
/*
* Disable rfc1323 if we havn't got any response to
* our third SYN to work-around some broken terminal servers
* (most of which have hopefully been retired) that have bad VJ
* header compression code which trashes TCP segments containing
* unknown-to-them TCP options.
*/
if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP);
/*
* 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) {
#ifdef INET6
if ((tp->t_inpcb->inp_vflag & INP_IPV6) != 0)
in6_losing(tp->t_inpcb);
else
#endif
tp->t_rttvar += (tp->t_srtt >> TCP_RTT_SHIFT);
tp->t_srtt = 0;
}
tp->snd_nxt = tp->snd_una;
tp->snd_recover = tp->snd_max;
/*
* Force a segment to be sent.
*/
tp->t_flags |= TF_ACKNOW;
/*
* If timing a segment in this window, stop the timer.
*/
tp->t_rtttime = 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 threshhold size.
* For a threshhold, 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 threshhold
* 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;
}
EXIT_FASTRECOVERY(tp);
(void) tcp_output(tp);
#ifdef TCPDEBUG
if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
PRU_SLOWTIMO);
#endif
return (0);
}