/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * 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.
 * 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.
 */

#include "opt_inet.h"
#include "opt_inet6.h"
#include "opt_rss.h"

#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/mbuf.h>
#include <sys/mutex.h>
#include <sys/protosw.h>
#include <sys/smp.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/sysctl.h>
#include <sys/systm.h>

#include <net/if.h>
#include <net/route.h>
#include <net/rss_config.h>
#include <net/vnet.h>
#include <net/netisr.h>

#include <netinet/in.h>
#include <netinet/in_kdtrace.h>
#include <netinet/in_pcb.h>
#include <netinet/in_rss.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/tcp_log_buf.h>
#include <netinet/tcp_seq.h>
#include <netinet/cc/cc.h>
#ifdef INET6
#include <netinet6/tcp6_var.h>
#endif
#include <netinet/tcpip.h>

int    tcp_persmin;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, persmin, CTLTYPE_INT | CTLFLAG_RW,
    &tcp_persmin, 0, sysctl_msec_to_ticks, "I",
    "minimum persistence interval");

int    tcp_persmax;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, persmax, CTLTYPE_INT | CTLFLAG_RW,
    &tcp_persmax, 0, sysctl_msec_to_ticks, "I",
    "maximum persistence interval");

int     tcp_keepinit;
SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINIT, keepinit, CTLTYPE_INT | CTLFLAG_RW,
    &tcp_keepinit, 0, sysctl_msec_to_ticks, "I",
    "time to establish connection");

int     tcp_keepidle;
SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPIDLE, keepidle, CTLTYPE_INT | CTLFLAG_RW,
    &tcp_keepidle, 0, sysctl_msec_to_ticks, "I",
    "time before keepalive probes begin");

int     tcp_keepintvl;
SYSCTL_PROC(_net_inet_tcp, TCPCTL_KEEPINTVL, keepintvl,
    CTLTYPE_INT | CTLFLAG_RW, &tcp_keepintvl, 0, sysctl_msec_to_ticks, "I",
    "time between keepalive probes");

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");

VNET_DEFINE(int, tcp_msl);
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl,
    CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_VNET,
    &VNET_NAME(tcp_msl), 0, sysctl_msec_to_ticks, "I",
    "Maximum segment lifetime");

VNET_DEFINE(int, tcp_msl_local);
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, msl_local,
    CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_VNET,
    &VNET_NAME(tcp_msl_local), 0, sysctl_msec_to_ticks, "I",
    "Maximum segment lifetime for local communication");

int     tcp_rexmit_initial;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_initial, CTLTYPE_INT | CTLFLAG_RW,
    &tcp_rexmit_initial, 0, sysctl_msec_to_ticks, "I",
    "Initial Retransmission Timeout");

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_max;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, rexmit_max, CTLTYPE_INT | CTLFLAG_RW,
    &tcp_rexmit_max, 0, sysctl_msec_to_ticks, "I",
    "Maximum 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");

VNET_DEFINE(int, tcp_always_keepalive) = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, always_keepalive, CTLFLAG_VNET|CTLFLAG_RW,
    &VNET_NAME(tcp_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");

int     tcp_keepcnt = TCPTV_KEEPCNT;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, keepcnt, CTLFLAG_RW, &tcp_keepcnt, 0,
    "Number of keepalive probes to send");

        /* max idle probes */
int     tcp_maxpersistidle;

int     tcp_rexmit_drop_options = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, rexmit_drop_options, CTLFLAG_RW,
    &tcp_rexmit_drop_options, 0,
    "Drop TCP options from 3rd and later retransmitted SYN");

int     tcp_maxunacktime = TCPTV_MAXUNACKTIME;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, maxunacktime, CTLTYPE_INT | CTLFLAG_RW,
    &tcp_maxunacktime, 0, sysctl_msec_to_ticks, "I",
    "Maximum time (in ms) that a session can linger without making progress");

VNET_DEFINE(int, tcp_pmtud_blackhole_detect);
SYSCTL_INT(_net_inet_tcp, OID_AUTO, pmtud_blackhole_detection,
    CTLFLAG_RW|CTLFLAG_VNET,
    &VNET_NAME(tcp_pmtud_blackhole_detect), 0,
    "Path MTU Discovery Black Hole Detection Enabled");

#ifdef INET
VNET_DEFINE(int, tcp_pmtud_blackhole_mss) = 1200;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, pmtud_blackhole_mss,
    CTLFLAG_RW|CTLFLAG_VNET,
    &VNET_NAME(tcp_pmtud_blackhole_mss), 0,
    "Path MTU Discovery Black Hole Detection lowered MSS");
#endif

#ifdef INET6
VNET_DEFINE(int, tcp_v6pmtud_blackhole_mss) = 1220;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, v6pmtud_blackhole_mss,
    CTLFLAG_RW|CTLFLAG_VNET,
    &VNET_NAME(tcp_v6pmtud_blackhole_mss), 0,
    "Path MTU Discovery IPv6 Black Hole Detection lowered MSS");
#endif

#ifdef  RSS
static int      per_cpu_timers = 1;
#else
static int      per_cpu_timers = 0;
#endif
SYSCTL_INT(_net_inet_tcp, OID_AUTO, per_cpu_timers, CTLFLAG_RW,
    &per_cpu_timers , 0, "run tcp timers on all cpus");

static int
sysctl_net_inet_tcp_retries(SYSCTL_HANDLER_ARGS)
{
        int error, new;

        new = V_tcp_retries;
        error = sysctl_handle_int(oidp, &new, 0, req);
        if (error == 0 && req->newptr) {
                if ((new < 1) || (new > TCP_MAXRXTSHIFT))
                        error = EINVAL;
                else
                        V_tcp_retries = new;
        }
        return (error);
}

VNET_DEFINE(int, tcp_retries) = TCP_MAXRXTSHIFT;
SYSCTL_PROC(_net_inet_tcp, OID_AUTO, retries,
    CTLTYPE_INT | CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(tcp_retries), 0, sysctl_net_inet_tcp_retries, "I",
    "maximum number of consecutive timer based retransmissions");

/*
 * Map the given inp to a CPU id.
 *
 * This queries RSS if it's compiled in, else it defaults to the current
 * CPU ID.
 */
inline int
inp_to_cpuid(struct inpcb *inp)
{
        u_int cpuid;

        if (per_cpu_timers) {
#ifdef  RSS
                cpuid = rss_hash2cpuid(inp->inp_flowid, inp->inp_flowtype);
                if (cpuid == NETISR_CPUID_NONE)
                        return (curcpu);        /* XXX */
                else
                        return (cpuid);
#endif
                /*
                 * We don't have a flowid -> cpuid mapping, so cheat and
                 * just map unknown cpuids to curcpu.  Not the best, but
                 * apparently better than defaulting to swi 0.
                 */
                cpuid = inp->inp_flowid % (mp_maxid + 1);
                if (! CPU_ABSENT(cpuid))
                        return (cpuid);
                return (curcpu);
        } else {
                return (0);
        }
}

int     tcp_backoff[TCP_MAXRXTSHIFT + 1] =
    { 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 512, 512, 512 };

int tcp_totbackoff = 2559;      /* sum of tcp_backoff[] */

/*
 * TCP timer processing.
 *
 * Each connection has 5 timers associated with it, which can be scheduled
 * simultaneously.  They all are serviced by one callout tcp_timer_enter().
 * This function executes the next timer via tcp_timersw[] vector.  Each
 * timer is supposed to return 'true' unless the connection was destroyed.
 * In the former case tcp_timer_enter() will schedule callout for next timer.
 */

typedef bool tcp_timer_t(struct tcpcb *);
static tcp_timer_t tcp_timer_delack;
static tcp_timer_t tcp_timer_2msl;
static tcp_timer_t tcp_timer_keep;
static tcp_timer_t tcp_timer_persist;
static tcp_timer_t tcp_timer_rexmt;

static tcp_timer_t * const tcp_timersw[TT_N] = {
        [TT_DELACK] = tcp_timer_delack,
        [TT_REXMT] = tcp_timer_rexmt,
        [TT_PERSIST] = tcp_timer_persist,
        [TT_KEEP] = tcp_timer_keep,
        [TT_2MSL] = tcp_timer_2msl,
};

/*
 * tcp_output_locked() s a timer specific variation of call to tcp_output(),
 * see tcp_var.h for the rest.  It handles drop request from advanced stacks,
 * but keeps tcpcb locked unless tcp_drop() destroyed it.
 * Returns true if tcpcb is valid and locked.
 */
static inline bool
tcp_output_locked(struct tcpcb *tp)
{
        int rv;

        INP_WLOCK_ASSERT(tptoinpcb(tp));

        if ((rv = tp->t_fb->tfb_tcp_output(tp)) < 0) {
                KASSERT(tp->t_fb->tfb_flags & TCP_FUNC_OUTPUT_CANDROP,
                    ("TCP stack %s requested tcp_drop(%p)",
                    tp->t_fb->tfb_tcp_block_name, tp));
                tp = tcp_drop(tp, -rv);
        }

        return (tp != NULL);
}

static bool
tcp_timer_delack(struct tcpcb *tp)
{
        struct epoch_tracker et;
#if defined(INVARIANTS) || defined(VIMAGE)
        struct inpcb *inp = tptoinpcb(tp);
#endif
        bool rv;

        INP_WLOCK_ASSERT(inp);

        CURVNET_SET(inp->inp_vnet);
        tp->t_flags |= TF_ACKNOW;
        TCPSTAT_INC(tcps_delack);
        NET_EPOCH_ENTER(et);
        rv = tcp_output_locked(tp);
        NET_EPOCH_EXIT(et);
        CURVNET_RESTORE();

        return (rv);
}

static bool
tcp_timer_2msl(struct tcpcb *tp)
{
        struct inpcb *inp = tptoinpcb(tp);
        bool close = false;

        INP_WLOCK_ASSERT(inp);

        TCP_PROBE2(debug__user, tp, PRU_SLOWTIMO);
        CURVNET_SET(inp->inp_vnet);
        tcp_log_end_status(tp, TCP_EI_STATUS_2MSL);
        tcp_free_sackholes(tp);
        /*
         * 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 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.
         *
         * XXXGL: check if inp_socket shall always be !NULL here?
         */
        if (tp->t_state == TCPS_TIME_WAIT) {
                close = true;
        } else if (tp->t_state == TCPS_FIN_WAIT_2 &&
            tcp_fast_finwait2_recycle && inp->inp_socket &&
            (inp->inp_socket->so_rcv.sb_state & SBS_CANTRCVMORE)) {
                TCPSTAT_INC(tcps_finwait2_drops);
                close = true;
        } else {
                if (ticks - tp->t_rcvtime <= TP_MAXIDLE(tp))
                        tcp_timer_activate(tp, TT_2MSL, TP_KEEPINTVL(tp));
                else
                        close = true;
        }
        if (close) {
                struct epoch_tracker et;

                NET_EPOCH_ENTER(et);
                tp = tcp_close(tp);
                NET_EPOCH_EXIT(et);
        }
        CURVNET_RESTORE();

        return (tp != NULL);
}

static bool
tcp_timer_keep(struct tcpcb *tp)
{
        struct epoch_tracker et;
        struct inpcb *inp = tptoinpcb(tp);
        struct tcptemp *t_template;

        INP_WLOCK_ASSERT(inp);

        TCP_PROBE2(debug__user, tp, PRU_SLOWTIMO);
        CURVNET_SET(inp->inp_vnet);
        /*
         * Because we don't regularly reset the keepalive callout in
         * the ESTABLISHED state, it may be that we don't actually need
         * to send a keepalive yet. If that occurs, schedule another
         * call for the next time the keepalive timer might expire.
         */
        if (TCPS_HAVEESTABLISHED(tp->t_state)) {
                u_int idletime;

                idletime = ticks - tp->t_rcvtime;
                if (idletime < TP_KEEPIDLE(tp)) {
                        tcp_timer_activate(tp, TT_KEEP,
                            TP_KEEPIDLE(tp) - idletime);
                        CURVNET_RESTORE();
                        return (true);
                }
        }

        /*
         * Keep-alive timer went off; send something
         * or drop connection if idle for too long.
         */
        TCPSTAT_INC(tcps_keeptimeo);
        if (tp->t_state < TCPS_ESTABLISHED)
                goto dropit;
        if ((V_tcp_always_keepalive ||
            inp->inp_socket->so_options & SO_KEEPALIVE) &&
            tp->t_state <= TCPS_CLOSING) {
                if (ticks - tp->t_rcvtime >= TP_KEEPIDLE(tp) + TP_MAXIDLE(tp))
                        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_INC(tcps_keepprobe);
                t_template = tcpip_maketemplate(inp);
                if (t_template) {
                        NET_EPOCH_ENTER(et);
                        tcp_respond(tp, t_template->tt_ipgen,
                                    &t_template->tt_t, (struct mbuf *)NULL,
                                    tp->rcv_nxt, tp->snd_una - 1, 0);
                        NET_EPOCH_EXIT(et);
                        free(t_template, M_TEMP);
                }
                tcp_timer_activate(tp, TT_KEEP, TP_KEEPINTVL(tp));
        } else
                tcp_timer_activate(tp, TT_KEEP, TP_KEEPIDLE(tp));

        CURVNET_RESTORE();
        return (true);

dropit:
        TCPSTAT_INC(tcps_keepdrops);
        NET_EPOCH_ENTER(et);
        tcp_log_end_status(tp, TCP_EI_STATUS_KEEP_MAX);
        tp = tcp_drop(tp, ETIMEDOUT);
        NET_EPOCH_EXIT(et);
        CURVNET_RESTORE();

        return (tp != NULL);
}

/*
 * Has this session exceeded the maximum time without seeing a substantive
 * acknowledgement? If so, return true; otherwise false.
 */
static bool
tcp_maxunacktime_check(struct tcpcb *tp)
{

        /* Are we tracking this timer for this session? */
        if (TP_MAXUNACKTIME(tp) == 0)
                return false;

        /* Do we have a current measurement. */
        if (tp->t_acktime == 0)
                return false;

        /* Are we within the acceptable range? */
        if (TSTMP_GT(TP_MAXUNACKTIME(tp) + tp->t_acktime, (u_int)ticks))
                return false;

        /* We exceeded the timer. */
        TCPSTAT_INC(tcps_progdrops);
        return true;
}

static bool
tcp_timer_persist(struct tcpcb *tp)
{
        struct epoch_tracker et;
#if defined(INVARIANTS) || defined(VIMAGE)
        struct inpcb *inp = tptoinpcb(tp);
#endif
        bool progdrop, rv;

        INP_WLOCK_ASSERT(inp);

        TCP_PROBE2(debug__user, tp, PRU_SLOWTIMO);
        CURVNET_SET(inp->inp_vnet);
        /*
         * Persistence timer into zero window.
         * Force a byte to be output, if possible.
         */
        TCPSTAT_INC(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.
         * Also, drop the connection if we haven't been making
         * progress.
         */
        progdrop = tcp_maxunacktime_check(tp);
        if (progdrop || (tp->t_rxtshift >= V_tcp_retries &&
            (ticks - tp->t_rcvtime >= tcp_maxpersistidle ||
             ticks - tp->t_rcvtime >= TCP_REXMTVAL(tp) * tcp_totbackoff))) {
                if (progdrop) {
                        tcp_log_end_status(tp, TCP_EI_STATUS_PROGRESS);
                } else {
                        TCPSTAT_INC(tcps_persistdrop);
                        tcp_log_end_status(tp, TCP_EI_STATUS_PERSIST_MAX);
                }
                goto dropit;
        }
        /*
         * If the user has closed the socket then drop a persisting
         * connection after a much reduced timeout.
         */
        if (tp->t_state > TCPS_CLOSE_WAIT &&
            (ticks - tp->t_rcvtime) >= TCPTV_PERSMAX) {
                TCPSTAT_INC(tcps_persistdrop);
                tcp_log_end_status(tp, TCP_EI_STATUS_PERSIST_MAX);
                goto dropit;
        }
        tcp_setpersist(tp);
        tp->t_flags |= TF_FORCEDATA;
        NET_EPOCH_ENTER(et);
        if ((rv = tcp_output_locked(tp)))
                tp->t_flags &= ~TF_FORCEDATA;
        NET_EPOCH_EXIT(et);
        CURVNET_RESTORE();

        return (rv);

dropit:
        NET_EPOCH_ENTER(et);
        tp = tcp_drop(tp, ETIMEDOUT);
        NET_EPOCH_EXIT(et);
        CURVNET_RESTORE();

        return (tp != NULL);
}

static bool
tcp_timer_rexmt(struct tcpcb *tp)
{
        struct epoch_tracker et;
        struct inpcb *inp = tptoinpcb(tp);
        int rexmt;
        bool isipv6, rv;

        INP_WLOCK_ASSERT(inp);

        TCP_PROBE2(debug__user, tp, PRU_SLOWTIMO);
        CURVNET_SET(inp->inp_vnet);
        if (tp->t_fb->tfb_tcp_rexmit_tmr) {
                /* The stack has a timer action too. */
                (*tp->t_fb->tfb_tcp_rexmit_tmr)(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 we've either exceeded the maximum number of retransmissions,
         * or we've gone long enough without making progress, then drop
         * the session.
         */
        if (++tp->t_rxtshift > V_tcp_retries || tcp_maxunacktime_check(tp)) {
                if (tp->t_rxtshift > V_tcp_retries)
                        TCPSTAT_INC(tcps_timeoutdrop);
                tp->t_rxtshift = V_tcp_retries;
                tcp_log_end_status(tp, TCP_EI_STATUS_RETRAN);
                NET_EPOCH_ENTER(et);
                tp = tcp_drop(tp, ETIMEDOUT);
                NET_EPOCH_EXIT(et);
                CURVNET_RESTORE();

                return (tp != NULL);
        }
        if (tp->t_state == TCPS_SYN_SENT) {
                /*
                 * If the SYN was retransmitted, indicate CWND to be
                 * limited to 1 segment in cc_conn_init().
                 */
                tp->snd_cwnd = 1;
        } else 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->t_flags))
                        tp->t_flags |= TF_WASFRECOVERY;
                else
                        tp->t_flags &= ~TF_WASFRECOVERY;
                if (IN_CONGRECOVERY(tp->t_flags))
                        tp->t_flags |= TF_WASCRECOVERY;
                else
                        tp->t_flags &= ~TF_WASCRECOVERY;
                if ((tp->t_flags & TF_RCVD_TSTMP) == 0)
                        tp->t_badrxtwin = ticks + (tp->t_srtt >> (TCP_RTT_SHIFT + 1));
                /* In the event that we've negotiated timestamps
                 * badrxtwin will be set to the value that we set
                 * the retransmitted packet's to_tsval to by tcp_output
                 */
                tp->t_flags |= TF_PREVVALID;
                tcp_resend_sackholes(tp);
        } else {
                tp->t_flags &= ~TF_PREVVALID;
                tcp_free_sackholes(tp);
        }
        TCPSTAT_INC(tcps_rexmttimeo);
        if ((tp->t_state == TCPS_SYN_SENT) ||
            (tp->t_state == TCPS_SYN_RECEIVED))
                rexmt = tcp_rexmit_initial * tcp_backoff[tp->t_rxtshift];
        else
                rexmt = TCP_REXMTVAL(tp) * tcp_backoff[tp->t_rxtshift];
        TCPT_RANGESET(tp->t_rxtcur, rexmt, tp->t_rttmin, tcp_rexmit_max);

        /*
         * We enter the path for PLMTUD if connection is established or, if
         * connection is FIN_WAIT_1 status, reason for the last is that if
         * amount of data we send is very small, we could send it in couple of
         * packets and process straight to FIN. In that case we won't catch
         * ESTABLISHED state.
         */
#ifdef INET6
        isipv6 = (inp->inp_vflag & INP_IPV6) ? true : false;
#else
        isipv6 = false;
#endif
        if (((V_tcp_pmtud_blackhole_detect == 1) ||
            (V_tcp_pmtud_blackhole_detect == 2 && !isipv6) ||
            (V_tcp_pmtud_blackhole_detect == 3 && isipv6)) &&
            ((tp->t_state == TCPS_ESTABLISHED) ||
            (tp->t_state == TCPS_FIN_WAIT_1))) {
                if (tp->t_rxtshift == 1) {
                        /*
                         * We enter blackhole detection after the first
                         * unsuccessful timer based retransmission.
                         * Then we reduce up to two times the MSS, each
                         * candidate giving two tries of retransmissions.
                         * But we give a candidate only two tries, if it
                         * actually reduces the MSS.
                         */
                        tp->t_blackhole_enter = 2;
                        tp->t_blackhole_exit = tp->t_blackhole_enter;
                        if (isipv6) {
#ifdef INET6
                                if (tp->t_maxseg > V_tcp_v6pmtud_blackhole_mss)
                                        tp->t_blackhole_exit += 2;
                                if (tp->t_maxseg > V_tcp_v6mssdflt &&
                                    V_tcp_v6pmtud_blackhole_mss > V_tcp_v6mssdflt)
                                        tp->t_blackhole_exit += 2;
#endif
                        } else {
#ifdef INET
                                if (tp->t_maxseg > V_tcp_pmtud_blackhole_mss)
                                        tp->t_blackhole_exit += 2;
                                if (tp->t_maxseg > V_tcp_mssdflt &&
                                    V_tcp_pmtud_blackhole_mss > V_tcp_mssdflt)
                                        tp->t_blackhole_exit += 2;
#endif
                        }
                }
                if (((tp->t_flags2 & (TF2_PLPMTU_PMTUD|TF2_PLPMTU_MAXSEGSNT)) ==
                    (TF2_PLPMTU_PMTUD|TF2_PLPMTU_MAXSEGSNT)) &&
                    (tp->t_rxtshift >= tp->t_blackhole_enter &&
                    tp->t_rxtshift < tp->t_blackhole_exit &&
                    (tp->t_rxtshift - tp->t_blackhole_enter) % 2 == 0)) {
                        /*
                         * Enter Path MTU Black-hole Detection mechanism:
                         * - Disable Path MTU Discovery (IP "DF" bit).
                         * - Reduce MTU to lower value than what we
                         *   negotiated with peer.
                         */
                        if ((tp->t_flags2 & TF2_PLPMTU_BLACKHOLE) == 0) {
                                /* Record that we may have found a black hole. */
                                tp->t_flags2 |= TF2_PLPMTU_BLACKHOLE;
                                /* Keep track of previous MSS. */
                                tp->t_pmtud_saved_maxseg = tp->t_maxseg;
                        }

                        /*
                         * Reduce the MSS to blackhole value or to the default
                         * in an attempt to retransmit.
                         */
#ifdef INET6
                        if (isipv6 &&
                            tp->t_maxseg > V_tcp_v6pmtud_blackhole_mss &&
                            V_tcp_v6pmtud_blackhole_mss > V_tcp_v6mssdflt) {
                                /* Use the sysctl tuneable blackhole MSS. */
                                tp->t_maxseg = V_tcp_v6pmtud_blackhole_mss;
                                TCPSTAT_INC(tcps_pmtud_blackhole_activated);
                        } else if (isipv6) {
                                /* Use the default MSS. */
                                tp->t_maxseg = V_tcp_v6mssdflt;
                                /*
                                 * Disable Path MTU Discovery when we switch to
                                 * minmss.
                                 */
                                tp->t_flags2 &= ~TF2_PLPMTU_PMTUD;
                                TCPSTAT_INC(tcps_pmtud_blackhole_activated_min_mss);
                        }
#endif
#if defined(INET6) && defined(INET)
                        else
#endif
#ifdef INET
                        if (tp->t_maxseg > V_tcp_pmtud_blackhole_mss &&
                            V_tcp_pmtud_blackhole_mss > V_tcp_mssdflt) {
                                /* Use the sysctl tuneable blackhole MSS. */
                                tp->t_maxseg = V_tcp_pmtud_blackhole_mss;
                                TCPSTAT_INC(tcps_pmtud_blackhole_activated);
                        } else {
                                /* Use the default MSS. */
                                tp->t_maxseg = V_tcp_mssdflt;
                                /*
                                 * Disable Path MTU Discovery when we switch to
                                 * minmss.
                                 */
                                tp->t_flags2 &= ~TF2_PLPMTU_PMTUD;
                                TCPSTAT_INC(tcps_pmtud_blackhole_activated_min_mss);
                        }
#endif
                        /*
                         * Reset the slow-start flight size
                         * as it may depend on the new MSS.
                         */
                        if (CC_ALGO(tp)->conn_init != NULL)
                                CC_ALGO(tp)->conn_init(&tp->t_ccv);
                } else {
                        /*
                         * If further retransmissions are still unsuccessful
                         * with a lowered MTU, maybe this isn't a blackhole and
                         * we restore the previous MSS and blackhole detection
                         * flags.
                         */
                        if ((tp->t_flags2 & TF2_PLPMTU_BLACKHOLE) &&
                            (tp->t_rxtshift >= tp->t_blackhole_exit)) {
                                tp->t_flags2 |= TF2_PLPMTU_PMTUD;
                                tp->t_flags2 &= ~TF2_PLPMTU_BLACKHOLE;
                                tp->t_maxseg = tp->t_pmtud_saved_maxseg;
                                if (tp->t_maxseg < V_tcp_mssdflt) {
                                        /*
                                         * The MSS is so small we should not
                                         * process incoming SACK's since we are
                                         * subject to attack in such a case.
                                         */
                                        tp->t_flags2 |= TF2_PROC_SACK_PROHIBIT;
                                } else {
                                        tp->t_flags2 &= ~TF2_PROC_SACK_PROHIBIT;
                                }
                                TCPSTAT_INC(tcps_pmtud_blackhole_failed);
                                /*
                                 * Reset the slow-start flight size as it
                                 * may depend on the new MSS.
                                 */
                                if (CC_ALGO(tp)->conn_init != NULL)
                                        CC_ALGO(tp)->conn_init(&tp->t_ccv);
                        }
                }
        }

        /*
         * Disable RFC1323 and SACK if we haven'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 (tcp_rexmit_drop_options && (tp->t_state == TCPS_SYN_SENT) &&
            (tp->t_rxtshift == 3))
                tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP|TF_SACK_PERMIT);
        /*
         * If we backed off this far, notify the L3 protocol that we're having
         * connection problems.
         */
        if (tp->t_rxtshift > TCP_RTT_INVALIDATE) {
#ifdef INET6
                if ((inp->inp_vflag & INP_IPV6) != 0)
                        in6_losing(inp);
                else
#endif
                        in_losing(inp);
        }
        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;

        /* Do not overwrite the snd_cwnd on SYN retransmissions. */
        if (tp->t_state != TCPS_SYN_SENT)
                cc_cong_signal(tp, NULL, CC_RTO);
        NET_EPOCH_ENTER(et);
        rv = tcp_output_locked(tp);
        NET_EPOCH_EXIT(et);
        CURVNET_RESTORE();

        return (rv);
}

static void
tcp_bblog_timer(struct tcpcb *tp, tt_which which, tt_what what, uint32_t ticks)
{
        struct tcp_log_buffer *lgb;
        uint64_t ms;

        INP_WLOCK_ASSERT(tptoinpcb(tp));
        if (tcp_bblogging_on(tp))
                lgb = tcp_log_event(tp, NULL, NULL, NULL, TCP_LOG_RTO, 0, 0,
                    NULL, false, NULL, NULL, 0, NULL);
        else
                lgb = NULL;
        if (lgb != NULL) {
                lgb->tlb_flex1 = (what << 8) | which;
                if (what == TT_STARTING) {
                        /* Convert ticks to ms and store it in tlb_flex2. */
                        if (hz == 1000)
                                lgb->tlb_flex2 = ticks;
                        else {
                                ms = (((uint64_t)ticks * 1000) + (hz - 1)) / hz;
                                if (ms > UINT32_MAX)
                                        lgb->tlb_flex2 = UINT32_MAX;
                                else
                                        lgb->tlb_flex2 = (uint32_t)ms;
                        }
                }
        }
}

static inline tt_which
tcp_timer_next(struct tcpcb *tp, sbintime_t *precision)
{
        tt_which i, rv;
        sbintime_t after, before;

        for (i = 0, rv = TT_N, after = before = SBT_MAX; i < TT_N; i++) {
                if (tp->t_timers[i] < after) {
                        after = tp->t_timers[i];
                        rv = i;
                }
                before = MIN(before, tp->t_timers[i] + tp->t_precisions[i]);
        }
        if (precision != NULL)
                *precision = before - after;

        return (rv);
}

static void
tcp_timer_enter(void *xtp)
{
        struct tcpcb *tp = xtp;
        struct inpcb *inp = tptoinpcb(tp);
        sbintime_t precision;
        tt_which which;

        INP_WLOCK_ASSERT(inp);

        which = tcp_timer_next(tp, NULL);
        MPASS(which < TT_N);
        tp->t_timers[which] = SBT_MAX;
        tp->t_precisions[which] = 0;

        tcp_bblog_timer(tp, which, TT_PROCESSING, 0);
        if (tcp_timersw[which](tp)) {
                tcp_bblog_timer(tp, which, TT_PROCESSED, 0);
                if ((which = tcp_timer_next(tp, &precision)) != TT_N) {
                        MPASS(tp->t_state > TCPS_CLOSED);
                        callout_reset_sbt_on(&tp->t_callout,
                            tp->t_timers[which], precision, tcp_timer_enter,
                            tp, inp_to_cpuid(inp), C_ABSOLUTE);
                }
                INP_WUNLOCK(inp);
        }
}

/*
 * Activate or stop (delta == 0) a TCP timer.
 */
void
tcp_timer_activate(struct tcpcb *tp, tt_which which, u_int delta)
{
        struct inpcb *inp = tptoinpcb(tp);
        sbintime_t precision;
        tt_what what;

#ifdef TCP_OFFLOAD
        if (tp->t_flags & TF_TOE)
                return;
#endif

        INP_WLOCK_ASSERT(inp);
        MPASS(tp->t_state > TCPS_CLOSED);

        if (delta > 0) {
                what = TT_STARTING;
                callout_when(tick_sbt * delta, 0, C_HARDCLOCK,
                    &tp->t_timers[which], &tp->t_precisions[which]);
        } else {
                what = TT_STOPPING;
                tp->t_timers[which] = SBT_MAX;
        }
        tcp_bblog_timer(tp, which, what, delta);

        if ((which = tcp_timer_next(tp, &precision)) != TT_N)
                callout_reset_sbt_on(&tp->t_callout, tp->t_timers[which],
                    precision, tcp_timer_enter, tp, inp_to_cpuid(inp),
                    C_ABSOLUTE);
        else
                callout_stop(&tp->t_callout);
}

bool
tcp_timer_active(struct tcpcb *tp, tt_which which)
{

        INP_WLOCK_ASSERT(tptoinpcb(tp));

        return (tp->t_timers[which] != SBT_MAX);
}

/*
 * Stop all timers associated with tcpcb.
 * Called when tcpcb moves to TCPS_CLOSED.
 */
void
tcp_timer_stop(struct tcpcb *tp)
{

        INP_WLOCK_ASSERT(tptoinpcb(tp));

        /*
         * We don't check return value from callout_stop().  There are two
         * reasons why it can return 0.  First, a legitimate one: we could have
         * been called from the callout itself.  Second, callout(9) has a bug.
         * It can race internally in softclock_call_cc(), when callout has
         * already completed, but cc_exec_curr still points at the callout.
         */
        (void )callout_stop(&tp->t_callout);
        /*
         * In case of being called from callout itself, we must make sure that
         * we don't reschedule.
         */
        for (tt_which i = 0; i < TT_N; i++)
                tp->t_timers[i] = SBT_MAX;
}