freebsd-dev/sys/netinet/tcp_timewait.c

/*-
 * 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.
 *
 *	@(#)tcp_subr.c	8.2 (Berkeley) 5/24/95
 */

#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");

#include "opt_inet.h"
#include "opt_inet6.h"
#include "opt_ipsec.h"
#include "opt_tcpdebug.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/callout.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/priv.h>
#include <sys/proc.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#ifndef INVARIANTS
#include <sys/syslog.h>
#endif
#include <sys/protosw.h>
#include <sys/random.h>

#include <vm/uma.h>

#include <net/route.h>
#include <net/if.h>
#include <net/if_var.h>
#include <net/vnet.h>

#include <netinet/in.h>
#include <netinet/in_kdtrace.h>
#include <netinet/in_pcb.h>
#include <netinet/in_systm.h>
#include <netinet/in_var.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <netinet/ip_var.h>
#ifdef INET6
#include <netinet/ip6.h>
#include <netinet6/in6_pcb.h>
#include <netinet6/ip6_var.h>
#include <netinet6/scope6_var.h>
#include <netinet6/nd6.h>
#endif
#include <netinet/tcp.h>
#include <netinet/tcp_fsm.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#include <netinet/tcp_hpts.h>
#ifdef INET6
#include <netinet6/tcp6_var.h>
#endif
#include <netinet/tcpip.h>
#ifdef TCPDEBUG
#include <netinet/tcp_debug.h>
#endif
#ifdef INET6
#include <netinet6/ip6protosw.h>
#endif

#include <netinet/udp.h>
#include <netinet/udp_var.h>

#include <netipsec/ipsec_support.h>

#include <machine/in_cksum.h>

#include <security/mac/mac_framework.h>

VNET_DEFINE_STATIC(uma_zone_t, tcptw_zone);
#define	V_tcptw_zone		VNET(tcptw_zone)
static int	maxtcptw;

/*
 * The timed wait queue contains references to each of the TCP sessions
 * currently in the TIME_WAIT state.  The queue pointers, including the
 * queue pointers in each tcptw structure, are protected using the global
 * timewait lock, which must be held over queue iteration and modification.
 *
 * Rules on tcptw usage:
 *  - a inpcb is always freed _after_ its tcptw
 *  - a tcptw relies on its inpcb reference counting for memory stability
 *  - a tcptw is dereferenceable only while its inpcb is locked
 */
VNET_DEFINE_STATIC(TAILQ_HEAD(, tcptw), twq_2msl);
#define	V_twq_2msl		VNET(twq_2msl)

/* Global timewait lock */
VNET_DEFINE_STATIC(struct rwlock, tw_lock);
#define	V_tw_lock		VNET(tw_lock)

#define	TW_LOCK_INIT(tw, d)	rw_init_flags(&(tw), (d), 0)
#define	TW_LOCK_DESTROY(tw)	rw_destroy(&(tw))
#define	TW_RLOCK(tw)		rw_rlock(&(tw))
#define	TW_WLOCK(tw)		rw_wlock(&(tw))
#define	TW_RUNLOCK(tw)		rw_runlock(&(tw))
#define	TW_WUNLOCK(tw)		rw_wunlock(&(tw))
#define	TW_LOCK_ASSERT(tw)	rw_assert(&(tw), RA_LOCKED)
#define	TW_RLOCK_ASSERT(tw)	rw_assert(&(tw), RA_RLOCKED)
#define	TW_WLOCK_ASSERT(tw)	rw_assert(&(tw), RA_WLOCKED)
#define	TW_UNLOCK_ASSERT(tw)	rw_assert(&(tw), RA_UNLOCKED)

static void	tcp_tw_2msl_reset(struct tcptw *, int);
static void	tcp_tw_2msl_stop(struct tcptw *, int);
static int	tcp_twrespond(struct tcptw *, int);

static int
tcptw_auto_size(void)
{
	int halfrange;

	/*
	 * Max out at half the ephemeral port range so that TIME_WAIT
	 * sockets don't tie up too many ephemeral ports.
	 */
	if (V_ipport_lastauto > V_ipport_firstauto)
		halfrange = (V_ipport_lastauto - V_ipport_firstauto) / 2;
	else
		halfrange = (V_ipport_firstauto - V_ipport_lastauto) / 2;
	/* Protect against goofy port ranges smaller than 32. */
	return (imin(imax(halfrange, 32), maxsockets / 5));
}

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

	if (maxtcptw == 0)
		new = tcptw_auto_size();
	else
		new = maxtcptw;
	error = sysctl_handle_int(oidp, &new, 0, req);
	if (error == 0 && req->newptr)
		if (new >= 32) {
			maxtcptw = new;
			uma_zone_set_max(V_tcptw_zone, maxtcptw);
		}
	return (error);
}

SYSCTL_PROC(_net_inet_tcp, OID_AUTO, maxtcptw,
    CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_NEEDGIANT,
    &maxtcptw, 0, sysctl_maxtcptw, "IU",
    "Maximum number of compressed TCP TIME_WAIT entries");

VNET_DEFINE_STATIC(bool, nolocaltimewait) = true;
#define	V_nolocaltimewait	VNET(nolocaltimewait)
SYSCTL_BOOL(_net_inet_tcp, OID_AUTO, nolocaltimewait, CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(nolocaltimewait), true,
    "Do not create compressed TCP TIME_WAIT entries for local connections");

void
tcp_tw_zone_change(void)
{

	if (maxtcptw == 0)
		uma_zone_set_max(V_tcptw_zone, tcptw_auto_size());
}

void
tcp_tw_init(void)
{

	V_tcptw_zone = uma_zcreate("tcptw", sizeof(struct tcptw),
	    NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
	TUNABLE_INT_FETCH("net.inet.tcp.maxtcptw", &maxtcptw);
	if (maxtcptw == 0)
		uma_zone_set_max(V_tcptw_zone, tcptw_auto_size());
	else
		uma_zone_set_max(V_tcptw_zone, maxtcptw);
	TAILQ_INIT(&V_twq_2msl);
	TW_LOCK_INIT(V_tw_lock, "tcptw");
}

#ifdef VIMAGE
void
tcp_tw_destroy(void)
{
	struct tcptw *tw;
	struct epoch_tracker et;

	NET_EPOCH_ENTER(et);
	while ((tw = TAILQ_FIRST(&V_twq_2msl)) != NULL)
		tcp_twclose(tw, 0);
	NET_EPOCH_EXIT(et);

	TW_LOCK_DESTROY(V_tw_lock);
	uma_zdestroy(V_tcptw_zone);
}
#endif

/*
 * Move a TCP connection into TIME_WAIT state.
 *    tcbinfo is locked.
 *    inp is locked, and is unlocked before returning.
 */
void
tcp_twstart(struct tcpcb *tp)
{
	struct tcptw twlocal, *tw;
	struct inpcb *inp = tp->t_inpcb;
	struct socket *so;
	uint32_t recwin;
	bool acknow, local;
#ifdef INET6
	bool isipv6 = inp->inp_inc.inc_flags & INC_ISIPV6;
#endif

	NET_EPOCH_ASSERT();
	INP_WLOCK_ASSERT(inp);

	/* A dropped inp should never transition to TIME_WAIT state. */
	KASSERT((inp->inp_flags & INP_DROPPED) == 0, ("tcp_twstart: "
	    "(inp->inp_flags & INP_DROPPED) != 0"));

	if (V_nolocaltimewait) {
#ifdef INET6
		if (isipv6)
			local = in6_localaddr(&inp->in6p_faddr);
		else
#endif
#ifdef INET
			local = in_localip(inp->inp_faddr);
#else
			local = false;
#endif
	} else
		local = false;

	/*
	 * For use only by DTrace.  We do not reference the state
	 * after this point so modifying it in place is not a problem.
	 */
	tcp_state_change(tp, TCPS_TIME_WAIT);

	if (local)
		tw = &twlocal;
	else
		tw = uma_zalloc(V_tcptw_zone, M_NOWAIT);
	if (tw == NULL) {
		/*
		 * Reached limit on total number of TIMEWAIT connections
		 * allowed. Remove a connection from TIMEWAIT queue in LRU
		 * fashion to make room for this connection.
		 * If that fails, use on stack tw at least to be able to
		 * run through tcp_twrespond() and standard tcpcb discard
		 * routine.
		 *
		 * XXX:  Check if it possible to always have enough room
		 * in advance based on guarantees provided by uma_zalloc().
		 */
		tw = tcp_tw_2msl_scan(1);
		if (tw == NULL) {
			tw = &twlocal;
			local = true;
		}
	}
	/*
	 * For !local case the tcptw will hold a reference on its inpcb
	 * until tcp_twclose is called.
	 */
	tw->tw_inpcb = inp;

	/*
	 * Recover last window size sent.
	 */
	so = inp->inp_socket;
	recwin = lmin(lmax(sbspace(&so->so_rcv), 0),
	    (long)TCP_MAXWIN << tp->rcv_scale);
	if (recwin < (so->so_rcv.sb_hiwat / 4) &&
	    recwin < tp->t_maxseg)
		recwin = 0;
	if (SEQ_GT(tp->rcv_adv, tp->rcv_nxt) &&
	    recwin < (tp->rcv_adv - tp->rcv_nxt))
		recwin = (tp->rcv_adv - tp->rcv_nxt);
	tw->last_win = (u_short)(recwin >> tp->rcv_scale);

	/*
	 * Set t_recent if timestamps are used on the connection.
	 */
	if ((tp->t_flags & (TF_REQ_TSTMP|TF_RCVD_TSTMP|TF_NOOPT)) ==
	    (TF_REQ_TSTMP|TF_RCVD_TSTMP)) {
		tw->t_recent = tp->ts_recent;
		tw->ts_offset = tp->ts_offset;
	} else {
		tw->t_recent = 0;
		tw->ts_offset = 0;
	}

	tw->snd_nxt = tp->snd_nxt;
	tw->t_port = tp->t_port;
	tw->rcv_nxt = tp->rcv_nxt;
	tw->tw_time = 0;
	tw->tw_flags = tp->t_flags;

/* XXX
 * If this code will
 * be used for fin-wait-2 state also, then we may need
 * a ts_recent from the last segment.
 */
	acknow = tp->t_flags & TF_ACKNOW;

	/*
	 * First, discard tcpcb state, which includes stopping its timers and
	 * freeing it.  tcp_discardcb() used to also release the inpcb, but
	 * that work is now done in the caller.
	 *
	 * Note: soisdisconnected() call used to be made in tcp_discardcb(),
	 * and might not be needed here any longer.
	 */
#ifdef TCPHPTS
	tcp_hpts_remove(inp);
#endif
	tcp_discardcb(tp);
	soisdisconnected(so);
	tw->tw_so_options = so->so_options;
	inp->inp_flags |= INP_TIMEWAIT;
	if (acknow)
		tcp_twrespond(tw, TH_ACK);
	if (local)
		in_pcbdrop(inp);
	else {
		in_pcbref(inp);	/* Reference from tw */
		tw->tw_cred = crhold(so->so_cred);
		inp->inp_ppcb = tw;
		TCPSTATES_INC(TCPS_TIME_WAIT);
		tcp_tw_2msl_reset(tw, 0);
	}

	/*
	 * If the inpcb owns the sole reference to the socket, then we can
	 * detach and free the socket as it is not needed in time wait.
	 */
	if (inp->inp_flags & INP_SOCKREF) {
		KASSERT(so->so_state & SS_PROTOREF,
		    ("tcp_twstart: !SS_PROTOREF"));
		inp->inp_flags &= ~INP_SOCKREF;
		INP_WUNLOCK(inp);
		SOCK_LOCK(so);
		so->so_state &= ~SS_PROTOREF;
		sofree(so);
	} else
		INP_WUNLOCK(inp);
}

/*
 * Returns 1 if the TIME_WAIT state was killed and we should start over,
 * looking for a pcb in the listen state.  Returns 0 otherwise.
 *
 * For pure SYN-segments the PCB shall be read-locked and the tcpopt pointer
 * may be NULL.  For the rest write-lock and valid tcpopt.
 */
int
tcp_twcheck(struct inpcb *inp, struct tcpopt *to, struct tcphdr *th,
    struct mbuf *m, int tlen)
{
	struct tcptw *tw;
	int thflags;
	tcp_seq seq;

	NET_EPOCH_ASSERT();
	INP_LOCK_ASSERT(inp);

	/*
	 * XXXRW: Time wait state for inpcb has been recycled, but inpcb is
	 * still present.  This is undesirable, but temporarily necessary
	 * until we work out how to handle inpcb's who's timewait state has
	 * been removed.
	 */
	tw = intotw(inp);
	if (tw == NULL)
		goto drop;

	thflags = tcp_get_flags(th);
#ifdef INVARIANTS
	if ((thflags & (TH_SYN | TH_ACK)) == TH_SYN)
		INP_RLOCK_ASSERT(inp);
	else {
		INP_WLOCK_ASSERT(inp);
		KASSERT(to != NULL,
		    ("%s: called without options on a non-SYN segment",
		    __func__));
	}
#endif

	/*
	 * NOTE: for FIN_WAIT_2 (to be added later),
	 * must validate sequence number before accepting RST
	 */

	/*
	 * If the segment contains RST:
	 *	Drop the segment - see Stevens, vol. 2, p. 964 and
	 *      RFC 1337.
	 */
	if (thflags & TH_RST)
		goto drop;

#if 0
/* PAWS not needed at the moment */
	/*
	 * RFC 1323 PAWS: If we have a timestamp reply on this segment
	 * and it's less than ts_recent, drop it.
	 */
	if ((to.to_flags & TOF_TS) != 0 && tp->ts_recent &&
	    TSTMP_LT(to.to_tsval, tp->ts_recent)) {
		if ((thflags & TH_ACK) == 0)
			goto drop;
		goto ack;
	}
	/*
	 * ts_recent is never updated because we never accept new segments.
	 */
#endif

	/*
	 * If a new connection request is received
	 * while in TIME_WAIT, drop the old connection
	 * and start over if the sequence numbers
	 * are above the previous ones.
	 * Allow UDP port number changes in this case.
	 */
	if ((thflags & TH_SYN) && SEQ_GT(th->th_seq, tw->rcv_nxt)) {
		/*
		 * In case we can't upgrade our lock just pretend we have
		 * lost this packet.
		 */
		if (((thflags & (TH_SYN | TH_ACK)) == TH_SYN) &&
		    INP_TRY_UPGRADE(inp) == 0)
			goto drop;
		tcp_twclose(tw, 0);
		TCPSTAT_INC(tcps_tw_recycles);
		return (1);
	}

	/*
	 * Send RST if UDP port numbers don't match
	 */
	if (tw->t_port != m->m_pkthdr.tcp_tun_port) {
		if (tcp_get_flags(th) & TH_ACK) {
			tcp_respond(NULL, mtod(m, void *), th, m,
			    (tcp_seq)0, th->th_ack, TH_RST);
		} else {
			if (tcp_get_flags(th) & TH_SYN)
				tlen++;
			if (tcp_get_flags(th) & TH_FIN)
				tlen++;
			tcp_respond(NULL, mtod(m, void *), th, m,
			    th->th_seq+tlen, (tcp_seq)0, TH_RST|TH_ACK);
		}
		INP_UNLOCK(inp);
		TCPSTAT_INC(tcps_tw_resets);
		return (0);
	}

	/*
	 * Drop the segment if it does not contain an ACK.
	 */
	if ((thflags & TH_ACK) == 0)
		goto drop;

	INP_WLOCK_ASSERT(inp);

	/*
	 * If timestamps were negotiated during SYN/ACK and a
	 * segment without a timestamp is received, silently drop
	 * the segment, unless the missing timestamps are tolerated.
	 * See section 3.2 of RFC 7323.
	 */
	if (((to->to_flags & TOF_TS) == 0) && (tw->t_recent != 0) &&
	    (V_tcp_tolerate_missing_ts == 0)) {
		goto drop;
	}

	/*
	 * Reset the 2MSL timer if this is a duplicate FIN.
	 */
	if (thflags & TH_FIN) {
		seq = th->th_seq + tlen + (thflags & TH_SYN ? 1 : 0);
		if (seq + 1 == tw->rcv_nxt)
			tcp_tw_2msl_reset(tw, 1);
	}

	/*
	 * Acknowledge the segment if it has data or is not a duplicate ACK.
	 */
	if (thflags != TH_ACK || tlen != 0 ||
	    th->th_seq != tw->rcv_nxt || th->th_ack != tw->snd_nxt) {
		TCP_PROBE5(receive, NULL, NULL, m, NULL, th);
		tcp_twrespond(tw, TH_ACK);
		TCPSTAT_INC(tcps_tw_responds);
		goto dropnoprobe;
	}
drop:
	TCP_PROBE5(receive, NULL, NULL, m, NULL, th);
dropnoprobe:
	INP_UNLOCK(inp);
	m_freem(m);
	return (0);
}

void
tcp_twclose(struct tcptw *tw, int reuse)
{
	struct socket *so;
	struct inpcb *inp;

	/*
	 * At this point, we are in one of two situations:
	 *
	 * (1) We have no socket, just an inpcb<->twtcp pair.  We can free
	 *     all state.
	 *
	 * (2) We have a socket -- if we own a reference, release it and
	 *     notify the socket layer.
	 */
	inp = tw->tw_inpcb;
	KASSERT((inp->inp_flags & INP_TIMEWAIT), ("tcp_twclose: !timewait"));
	KASSERT(intotw(inp) == tw, ("tcp_twclose: inp_ppcb != tw"));
	NET_EPOCH_ASSERT();
	INP_WLOCK_ASSERT(inp);

	tcp_tw_2msl_stop(tw, reuse);
	inp->inp_ppcb = NULL;
	in_pcbdrop(inp);

	so = inp->inp_socket;
	if (so != NULL) {
		/*
		 * If there's a socket, handle two cases: first, we own a
		 * strong reference, which we will now release, or we don't
		 * in which case another reference exists (XXXRW: think
		 * about this more), and we don't need to take action.
		 */
		if (inp->inp_flags & INP_SOCKREF) {
			inp->inp_flags &= ~INP_SOCKREF;
			INP_WUNLOCK(inp);
			SOCK_LOCK(so);
			KASSERT(so->so_state & SS_PROTOREF,
			    ("tcp_twclose: INP_SOCKREF && !SS_PROTOREF"));
			so->so_state &= ~SS_PROTOREF;
			sofree(so);
		} else {
			/*
			 * If we don't own the only reference, the socket and
			 * inpcb need to be left around to be handled by
			 * tcp_usr_detach() later.
			 */
			INP_WUNLOCK(inp);
		}
	} else {
		/*
		 * The socket has been already cleaned-up for us, only free the
		 * inpcb.
		 */
		in_pcbfree(inp);
	}
	TCPSTAT_INC(tcps_closed);
}

static int
tcp_twrespond(struct tcptw *tw, int flags)
{
	struct inpcb *inp = tw->tw_inpcb;
#if defined(INET6) || defined(INET)
	struct tcphdr *th = NULL;
#endif
	struct mbuf *m;
#ifdef INET
	struct ip *ip = NULL;
#endif
	u_int hdrlen, optlen, ulen;
	int error = 0;			/* Keep compiler happy */
	struct tcpopt to;
#ifdef INET6
	struct ip6_hdr *ip6 = NULL;
	int isipv6 = inp->inp_inc.inc_flags & INC_ISIPV6;
#endif
	struct udphdr *udp = NULL;
	hdrlen = 0;                     /* Keep compiler happy */

	INP_WLOCK_ASSERT(inp);

	m = m_gethdr(M_NOWAIT, MT_DATA);
	if (m == NULL)
		return (ENOBUFS);
	m->m_data += max_linkhdr;

#ifdef MAC
	mac_inpcb_create_mbuf(inp, m);
#endif

#ifdef INET6
	if (isipv6) {
		hdrlen = sizeof(struct ip6_hdr) + sizeof(struct tcphdr);
		ip6 = mtod(m, struct ip6_hdr *);
		if (tw->t_port) {
			udp = (struct udphdr *)(ip6 + 1);
			hdrlen += sizeof(struct udphdr);
			udp->uh_sport = htons(V_tcp_udp_tunneling_port);
			udp->uh_dport = tw->t_port;
			ulen = (hdrlen - sizeof(struct ip6_hdr));
			th = (struct tcphdr *)(udp + 1);
		} else
			th = (struct tcphdr *)(ip6 + 1);
		tcpip_fillheaders(inp, tw->t_port, ip6, th);
	}
#endif
#if defined(INET6) && defined(INET)
	else
#endif
#ifdef INET
	{
		hdrlen = sizeof(struct tcpiphdr);
		ip = mtod(m, struct ip *);
		if (tw->t_port) {
			udp = (struct udphdr *)(ip + 1);
			hdrlen += sizeof(struct udphdr);
			udp->uh_sport = htons(V_tcp_udp_tunneling_port);
			udp->uh_dport = tw->t_port;
			ulen = (hdrlen - sizeof(struct ip));
			th = (struct tcphdr *)(udp + 1);
		} else
			th = (struct tcphdr *)(ip + 1);
		tcpip_fillheaders(inp, tw->t_port, ip, th);
	}
#endif
	to.to_flags = 0;

	/*
	 * Send a timestamp and echo-reply if both our side and our peer
	 * have sent timestamps in our SYN's and this is not a RST.
	 */
	if (tw->t_recent && flags == TH_ACK) {
		to.to_flags |= TOF_TS;
		to.to_tsval = tcp_ts_getticks() + tw->ts_offset;
		to.to_tsecr = tw->t_recent;
	}
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
	if (tw->tw_flags & TF_SIGNATURE)
		to.to_flags |= TOF_SIGNATURE;
#endif
	optlen = tcp_addoptions(&to, (u_char *)(th + 1));

	if (udp) {
		ulen += optlen;
		udp->uh_ulen = htons(ulen);
	}
	m->m_len = hdrlen + optlen;
	m->m_pkthdr.len = m->m_len;

	KASSERT(max_linkhdr + m->m_len <= MHLEN, ("tcptw: mbuf too small"));

	th->th_seq = htonl(tw->snd_nxt);
	th->th_ack = htonl(tw->rcv_nxt);
	th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
	tcp_set_flags(th, flags);
	th->th_win = htons(tw->last_win);

#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
	if (tw->tw_flags & TF_SIGNATURE) {
		if (!TCPMD5_ENABLED() ||
		    TCPMD5_OUTPUT(m, th, to.to_signature) != 0)
			return (-1);
	}
#endif
#ifdef INET6
	if (isipv6) {
		if (tw->t_port) {
			m->m_pkthdr.csum_flags = CSUM_UDP_IPV6;
			m->m_pkthdr.csum_data = offsetof(struct udphdr, uh_sum);
			udp->uh_sum = in6_cksum_pseudo(ip6, ulen, IPPROTO_UDP, 0);
			th->th_sum = htons(0);
		} else {
			m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
			m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
			th->th_sum = in6_cksum_pseudo(ip6,
			    sizeof(struct tcphdr) + optlen, IPPROTO_TCP, 0);
		}
		ip6->ip6_hlim = in6_selecthlim(inp, NULL);
		TCP_PROBE5(send, NULL, NULL, ip6, NULL, th);
		error = ip6_output(m, inp->in6p_outputopts, NULL,
		    (tw->tw_so_options & SO_DONTROUTE), NULL, NULL, inp);
	}
#endif
#if defined(INET6) && defined(INET)
	else
#endif
#ifdef INET
	{
		if (tw->t_port) {
			m->m_pkthdr.csum_flags = CSUM_UDP;
			m->m_pkthdr.csum_data = offsetof(struct udphdr, uh_sum);
			udp->uh_sum = in_pseudo(ip->ip_src.s_addr,
			    ip->ip_dst.s_addr, htons(ulen + IPPROTO_UDP));
			th->th_sum = htons(0);
		} else {
			m->m_pkthdr.csum_flags = CSUM_TCP;
			m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
			th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
			    htons(sizeof(struct tcphdr) + optlen + IPPROTO_TCP));
		}
		ip->ip_len = htons(m->m_pkthdr.len);
		if (V_path_mtu_discovery)
			ip->ip_off |= htons(IP_DF);
		TCP_PROBE5(send, NULL, NULL, ip, NULL, th);
		error = ip_output(m, inp->inp_options, NULL,
		    ((tw->tw_so_options & SO_DONTROUTE) ? IP_ROUTETOIF : 0),
		    NULL, inp);
	}
#endif
	if (flags & TH_ACK)
		TCPSTAT_INC(tcps_sndacks);
	else
		TCPSTAT_INC(tcps_sndctrl);
	TCPSTAT_INC(tcps_sndtotal);
	return (error);
}

static void
tcp_tw_2msl_reset(struct tcptw *tw, int rearm)
{

	NET_EPOCH_ASSERT();
	INP_WLOCK_ASSERT(tw->tw_inpcb);

	TW_WLOCK(V_tw_lock);
	if (rearm)
		TAILQ_REMOVE(&V_twq_2msl, tw, tw_2msl);
	tw->tw_time = ticks + 2 * V_tcp_msl;
	TAILQ_INSERT_TAIL(&V_twq_2msl, tw, tw_2msl);
	TW_WUNLOCK(V_tw_lock);
}

static void
tcp_tw_2msl_stop(struct tcptw *tw, int reuse)
{
	struct ucred *cred;
	struct inpcb *inp;
	int released __unused;

	NET_EPOCH_ASSERT();

	TW_WLOCK(V_tw_lock);
	inp = tw->tw_inpcb;
	tw->tw_inpcb = NULL;

	TAILQ_REMOVE(&V_twq_2msl, tw, tw_2msl);
	cred = tw->tw_cred;
	tw->tw_cred = NULL;
	TW_WUNLOCK(V_tw_lock);

	if (cred != NULL)
		crfree(cred);

	released = in_pcbrele_wlocked(inp);
	KASSERT(!released, ("%s: inp should not be released here", __func__));

	if (!reuse)
		uma_zfree(V_tcptw_zone, tw);
	TCPSTATES_DEC(TCPS_TIME_WAIT);
}

struct tcptw *
tcp_tw_2msl_scan(int reuse)
{
	struct tcptw *tw;
	struct inpcb *inp;

	NET_EPOCH_ASSERT();

	for (;;) {
		TW_RLOCK(V_tw_lock);
		tw = TAILQ_FIRST(&V_twq_2msl);
		if (tw == NULL || (!reuse && (tw->tw_time - ticks) > 0)) {
			TW_RUNLOCK(V_tw_lock);
			break;
		}
		KASSERT(tw->tw_inpcb != NULL, ("%s: tw->tw_inpcb == NULL",
		    __func__));

		inp = tw->tw_inpcb;
		in_pcbref(inp);
		TW_RUNLOCK(V_tw_lock);

		INP_WLOCK(inp);
		tw = intotw(inp);
		if (in_pcbrele_wlocked(inp)) {
			if (__predict_true(tw == NULL)) {
				continue;
			} else {
				/* This should not happen as in TIMEWAIT
				 * state the inp should not be destroyed
				 * before its tcptw. If INVARIANTS is
				 * defined panic.
				 */
#ifdef INVARIANTS
				panic("%s: Panic before an infinite "
					  "loop: INP_TIMEWAIT && (INP_FREED "
					  "|| inp last reference) && tw != "
					  "NULL", __func__);
#else
				log(LOG_ERR, "%s: Avoid an infinite "
					"loop: INP_TIMEWAIT && (INP_FREED "
					"|| inp last reference) && tw != "
					"NULL", __func__);
#endif
				break;
			}
		}

		if (tw == NULL) {
			/* tcp_twclose() has already been called */
			INP_WUNLOCK(inp);
			continue;
		}

		tcp_twclose(tw, reuse);
		if (reuse)
			return tw;
	}

	return NULL;
}