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[dragonfly/vkernel-mp.git] / sys / netinet / tcp_syncache.c

/*
 * Copyright (c) 2003, 2004 Jeffrey M. Hsu.  All rights reserved.
 * Copyright (c) 2003, 2004 The DragonFly Project.  All rights reserved.
 *
 * This code is derived from software contributed to The DragonFly Project
 * by Jeffrey M. Hsu.
 *
 * 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 DragonFly Project 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 COPYRIGHT HOLDERS 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
 * COPYRIGHT HOLDERS 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.
 */

/*
 * Copyright (c) 2003, 2004 Jeffrey M. Hsu.  All rights reserved.
 *
 * License terms: all terms for the DragonFly license above plus the following:
 *
 * 4. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *
 *      This product includes software developed by Jeffrey M. Hsu
 *      for the DragonFly Project.
 *
 *    This requirement may be waived with permission from Jeffrey Hsu.
 *    This requirement will sunset and may be removed on July 8 2005,
 *    after which the standard DragonFly license (as shown above) will
 *    apply.
 */

/*
 * All advertising materials mentioning features or use of this software
 * must display the following acknowledgement:
 *   This product includes software developed by Jeffrey M. Hsu.
 *
 * Copyright (c) 2001 Networks Associates Technologies, Inc.
 * All rights reserved.
 *
 * This software was developed for the FreeBSD Project by Jonathan Lemon
 * and NAI Labs, the Security Research Division of Network Associates, Inc.
 * under DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
 * DARPA CHATS research program.
 *
 * 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. The name of the author may not be used to endorse or promote
 *    products derived from this software without specific prior written
 *    permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR 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 AUTHOR 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.
 *
 * $FreeBSD: src/sys/netinet/tcp_syncache.c,v 1.5.2.14 2003/02/24 04:02:27 silby Exp $
 * $DragonFly: src/sys/netinet/tcp_syncache.c,v 1.27 2006/12/23 00:57:52 swildner Exp $
 */

#include "opt_inet6.h"
#include "opt_ipsec.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/md5.h>
#include <sys/proc.h>           /* for proc0 declaration */
#include <sys/random.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/in_cksum.h>

#include <sys/msgport2.h>

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

#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/in_var.h>
#include <netinet/in_pcb.h>
#include <netinet/ip_var.h>
#include <netinet/ip6.h>
#ifdef INET6
#include <netinet/icmp6.h>
#include <netinet6/nd6.h>
#endif
#include <netinet6/ip6_var.h>
#include <netinet6/in6_pcb.h>
#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 <netinet6/tcp6_var.h>

#ifdef IPSEC
#include <netinet6/ipsec.h>
#ifdef INET6
#include <netinet6/ipsec6.h>
#endif
#include <netproto/key/key.h>
#endif /*IPSEC*/

#ifdef FAST_IPSEC
#include <netproto/ipsec/ipsec.h>
#ifdef INET6
#include <netproto/ipsec/ipsec6.h>
#endif
#include <netproto/ipsec/key.h>
#define IPSEC
#endif /*FAST_IPSEC*/

#include <vm/vm_zone.h>

static int tcp_syncookies = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_RW,
    &tcp_syncookies, 0,
    "Use TCP SYN cookies if the syncache overflows");

static void      syncache_drop(struct syncache *, struct syncache_head *);
static void      syncache_free(struct syncache *);
static void      syncache_insert(struct syncache *, struct syncache_head *);
struct syncache *syncache_lookup(struct in_conninfo *, struct syncache_head **);
static int       syncache_respond(struct syncache *, struct mbuf *);
static struct    socket *syncache_socket(struct syncache *, struct socket *);
static void      syncache_timer(void *);
static u_int32_t syncookie_generate(struct syncache *);
static struct syncache *syncookie_lookup(struct in_conninfo *,
                    struct tcphdr *, struct socket *);

/*
 * Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
 * 3 retransmits corresponds to a timeout of (1 + 2 + 4 + 8 == 15) seconds,
 * the odds are that the user has given up attempting to connect by then.
 */
#define SYNCACHE_MAXREXMTS              3

/* Arbitrary values */
#define TCP_SYNCACHE_HASHSIZE           512
#define TCP_SYNCACHE_BUCKETLIMIT        30

struct netmsg_sc_timer {
        struct lwkt_msg nm_lmsg;
        struct msgrec *nm_mrec;         /* back pointer to containing msgrec */
};

struct msgrec {
        struct netmsg_sc_timer msg;
        lwkt_port_t port;               /* constant after init */
        int slot;                       /* constant after init */
};

static int syncache_timer_handler(lwkt_msg_t);

struct tcp_syncache {
        struct  vm_zone *zone;
        u_int   hashsize;
        u_int   hashmask;
        u_int   bucket_limit;
        u_int   cache_limit;
        u_int   rexmt_limit;
        u_int   hash_secret;
};
static struct tcp_syncache tcp_syncache;

struct tcp_syncache_percpu {
        struct syncache_head    *hashbase;
        u_int                   cache_count;
        TAILQ_HEAD(, syncache)  timerq[SYNCACHE_MAXREXMTS + 1];
        struct callout          tt_timerq[SYNCACHE_MAXREXMTS + 1];
        struct msgrec           mrec[SYNCACHE_MAXREXMTS + 1];
};
static struct tcp_syncache_percpu tcp_syncache_percpu[MAXCPU];

static struct lwkt_port syncache_null_rport;

SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache, CTLFLAG_RW, 0, "TCP SYN cache");

SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_RD,
     &tcp_syncache.bucket_limit, 0, "Per-bucket hash limit for syncache");

SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_RD,
     &tcp_syncache.cache_limit, 0, "Overall entry limit for syncache");

/* XXX JH */
#if 0
SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, count, CTLFLAG_RD,
     &tcp_syncache.cache_count, 0, "Current number of entries in syncache");
#endif

SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_RD,
     &tcp_syncache.hashsize, 0, "Size of TCP syncache hashtable");

SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit, CTLFLAG_RW,
     &tcp_syncache.rexmt_limit, 0, "Limit on SYN/ACK retransmissions");

static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache");

#define SYNCACHE_HASH(inc, mask)                                        \
        ((tcp_syncache.hash_secret ^                                    \
          (inc)->inc_faddr.s_addr ^                                     \
          ((inc)->inc_faddr.s_addr >> 16) ^                             \
          (inc)->inc_fport ^ (inc)->inc_lport) & mask)

#define SYNCACHE_HASH6(inc, mask)                                       \
        ((tcp_syncache.hash_secret ^                                    \
          (inc)->inc6_faddr.s6_addr32[0] ^                              \
          (inc)->inc6_faddr.s6_addr32[3] ^                              \
          (inc)->inc_fport ^ (inc)->inc_lport) & mask)

#define ENDPTS_EQ(a, b) (                                               \
        (a)->ie_fport == (b)->ie_fport &&                               \
        (a)->ie_lport == (b)->ie_lport &&                               \
        (a)->ie_faddr.s_addr == (b)->ie_faddr.s_addr &&                 \
        (a)->ie_laddr.s_addr == (b)->ie_laddr.s_addr                    \
)

#define ENDPTS6_EQ(a, b) (memcmp(a, b, sizeof(*a)) == 0)

static __inline void
syncache_timeout(struct tcp_syncache_percpu *syncache_percpu,
                 struct syncache *sc, int slot)
{
        sc->sc_rxtslot = slot;
        sc->sc_rxttime = ticks + TCPTV_RTOBASE * tcp_backoff[slot];
        TAILQ_INSERT_TAIL(&syncache_percpu->timerq[slot], sc, sc_timerq);
        if (!callout_active(&syncache_percpu->tt_timerq[slot])) {
                callout_reset(&syncache_percpu->tt_timerq[slot],
                              TCPTV_RTOBASE * tcp_backoff[slot],
                              syncache_timer,
                              &syncache_percpu->mrec[slot]);
        }
}

static void
syncache_free(struct syncache *sc)
{
        struct rtentry *rt;
#ifdef INET6
        const boolean_t isipv6 = sc->sc_inc.inc_isipv6;
#else
        const boolean_t isipv6 = FALSE;
#endif

        if (sc->sc_ipopts)
                m_free(sc->sc_ipopts);

        rt = isipv6 ? sc->sc_route6.ro_rt : sc->sc_route.ro_rt;
        if (rt != NULL) {
                /*
                 * If this is the only reference to a protocol-cloned
                 * route, remove it immediately.
                 */
                if ((rt->rt_flags & RTF_WASCLONED) && rt->rt_refcnt == 1)
                        rtrequest(RTM_DELETE, rt_key(rt), rt->rt_gateway,
                                  rt_mask(rt), rt->rt_flags, NULL);
                RTFREE(rt);
        }

        zfree(tcp_syncache.zone, sc);
}

void
syncache_init(void)
{
        int i, cpu;

        tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
        tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT;
        tcp_syncache.cache_limit =
            tcp_syncache.hashsize * tcp_syncache.bucket_limit;
        tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS;
        tcp_syncache.hash_secret = karc4random();

        TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize",
            &tcp_syncache.hashsize);
        TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit",
            &tcp_syncache.cache_limit);
        TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit",
            &tcp_syncache.bucket_limit);
        if (!powerof2(tcp_syncache.hashsize)) {
                kprintf("WARNING: syncache hash size is not a power of 2.\n");
                tcp_syncache.hashsize = 512;    /* safe default */
        }
        tcp_syncache.hashmask = tcp_syncache.hashsize - 1;

        lwkt_initport_null_rport(&syncache_null_rport, NULL);

        for (cpu = 0; cpu < ncpus2; cpu++) {
                struct tcp_syncache_percpu *syncache_percpu;

                syncache_percpu = &tcp_syncache_percpu[cpu];
                /* Allocate the hash table. */
                MALLOC(syncache_percpu->hashbase, struct syncache_head *,
                    tcp_syncache.hashsize * sizeof(struct syncache_head),
                    M_SYNCACHE, M_WAITOK);

                /* Initialize the hash buckets. */
                for (i = 0; i < tcp_syncache.hashsize; i++) {
                        struct syncache_head *bucket;

                        bucket = &syncache_percpu->hashbase[i];
                        TAILQ_INIT(&bucket->sch_bucket);
                        bucket->sch_length = 0;
                }

                for (i = 0; i <= SYNCACHE_MAXREXMTS; i++) {
                        /* Initialize the timer queues. */
                        TAILQ_INIT(&syncache_percpu->timerq[i]);
                        callout_init(&syncache_percpu->tt_timerq[i]);

                        syncache_percpu->mrec[i].slot = i;
                        syncache_percpu->mrec[i].port = tcp_cport(cpu);
                        syncache_percpu->mrec[i].msg.nm_mrec =
                            &syncache_percpu->mrec[i];
                        lwkt_initmsg(&syncache_percpu->mrec[i].msg.nm_lmsg,
                            &syncache_null_rport, 0,
                            lwkt_cmd_func(syncache_timer_handler),
                            lwkt_cmd_op_none);
                }
        }

        /*
         * Allocate the syncache entries.  Allow the zone to allocate one
         * more entry than cache limit, so a new entry can bump out an
         * older one.
         */
        tcp_syncache.zone = zinit("syncache", sizeof(struct syncache),
            tcp_syncache.cache_limit * ncpus2, ZONE_INTERRUPT, 0);
        tcp_syncache.cache_limit -= 1;
}

static void
syncache_insert(struct syncache *sc, struct syncache_head *sch)
{
        struct tcp_syncache_percpu *syncache_percpu;
        struct syncache *sc2;
        int i;

        syncache_percpu = &tcp_syncache_percpu[mycpu->gd_cpuid];

        /*
         * Make sure that we don't overflow the per-bucket
         * limit or the total cache size limit.
         */
        if (sch->sch_length >= tcp_syncache.bucket_limit) {
                /*
                 * The bucket is full, toss the oldest element.
                 */
                sc2 = TAILQ_FIRST(&sch->sch_bucket);
                sc2->sc_tp->ts_recent = ticks;
                syncache_drop(sc2, sch);
                tcpstat.tcps_sc_bucketoverflow++;
        } else if (syncache_percpu->cache_count >= tcp_syncache.cache_limit) {
                /*
                 * The cache is full.  Toss the oldest entry in the
                 * entire cache.  This is the front entry in the
                 * first non-empty timer queue with the largest
                 * timeout value.
                 */
                for (i = SYNCACHE_MAXREXMTS; i >= 0; i--) {
                        sc2 = TAILQ_FIRST(&syncache_percpu->timerq[i]);
                        if (sc2 != NULL)
                                break;
                }
                sc2->sc_tp->ts_recent = ticks;
                syncache_drop(sc2, NULL);
                tcpstat.tcps_sc_cacheoverflow++;
        }

        /* Initialize the entry's timer. */
        syncache_timeout(syncache_percpu, sc, 0);

        /* Put it into the bucket. */
        TAILQ_INSERT_TAIL(&sch->sch_bucket, sc, sc_hash);
        sch->sch_length++;
        syncache_percpu->cache_count++;
        tcpstat.tcps_sc_added++;
}

static void
syncache_drop(struct syncache *sc, struct syncache_head *sch)
{
        struct tcp_syncache_percpu *syncache_percpu;
#ifdef INET6
        const boolean_t isipv6 = sc->sc_inc.inc_isipv6;
#else
        const boolean_t isipv6 = FALSE;
#endif

        syncache_percpu = &tcp_syncache_percpu[mycpu->gd_cpuid];

        if (sch == NULL) {
                if (isipv6) {
                        sch = &syncache_percpu->hashbase[
                            SYNCACHE_HASH6(&sc->sc_inc, tcp_syncache.hashmask)];
                } else {
                        sch = &syncache_percpu->hashbase[
                            SYNCACHE_HASH(&sc->sc_inc, tcp_syncache.hashmask)];
                }
        }

        TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
        sch->sch_length--;
        syncache_percpu->cache_count--;

        /*
         * Remove the entry from the syncache timer/timeout queue.  Note
         * that we do not try to stop any running timer since we do not know
         * whether the timer's message is in-transit or not.  Since timeouts
         * are fairly long, taking an unneeded callout does not detrimentally
         * effect performance.
         */
        TAILQ_REMOVE(&syncache_percpu->timerq[sc->sc_rxtslot], sc, sc_timerq);

        syncache_free(sc);
}

/*
 * Place a timeout message on the TCP thread's message queue.
 * This routine runs in soft interrupt context.
 *
 * An invariant is for this routine to be called, the callout must
 * have been active.  Note that the callout is not deactivated until
 * after the message has been processed in syncache_timer_handler() below.
 */
static void
syncache_timer(void *p)
{
        struct netmsg_sc_timer *msg = p;

        lwkt_sendmsg(msg->nm_mrec->port, &msg->nm_lmsg);
}

/*
 * Service a timer message queued by timer expiration.
 * This routine runs in the TCP protocol thread.
 *
 * Walk the timer queues, looking for SYN,ACKs that need to be retransmitted.
 * If we have retransmitted an entry the maximum number of times, expire it.
 *
 * When we finish processing timed-out entries, we restart the timer if there
 * are any entries still on the queue and deactivate it otherwise.  Only after
 * a timer has been deactivated here can it be restarted by syncache_timeout().
 */
static int
syncache_timer_handler(lwkt_msg_t msg)
{
        struct tcp_syncache_percpu *syncache_percpu;
        struct syncache *sc, *nsc;
        struct inpcb *inp;
        int slot;

        slot = ((struct netmsg_sc_timer *)msg)->nm_mrec->slot;
        syncache_percpu = &tcp_syncache_percpu[mycpu->gd_cpuid];

        nsc = TAILQ_FIRST(&syncache_percpu->timerq[slot]);
        while (nsc != NULL) {
                if (ticks < nsc->sc_rxttime)
                        break;  /* finished because timerq sorted by time */
                sc = nsc;
                inp = sc->sc_tp->t_inpcb;
                if (slot == SYNCACHE_MAXREXMTS ||
                    slot >= tcp_syncache.rexmt_limit ||
                    inp->inp_gencnt != sc->sc_inp_gencnt) {
                        nsc = TAILQ_NEXT(sc, sc_timerq);
                        syncache_drop(sc, NULL);
                        tcpstat.tcps_sc_stale++;
                        continue;
                }
                /*
                 * syncache_respond() may call back into the syncache to
                 * to modify another entry, so do not obtain the next
                 * entry on the timer chain until it has completed.
                 */
                syncache_respond(sc, NULL);
                nsc = TAILQ_NEXT(sc, sc_timerq);
                tcpstat.tcps_sc_retransmitted++;
                TAILQ_REMOVE(&syncache_percpu->timerq[slot], sc, sc_timerq);
                syncache_timeout(syncache_percpu, sc, slot + 1);
        }
        if (nsc != NULL)
                callout_reset(&syncache_percpu->tt_timerq[slot],
                    nsc->sc_rxttime - ticks, syncache_timer,
                    &syncache_percpu->mrec[slot]);
        else
                callout_deactivate(&syncache_percpu->tt_timerq[slot]);

        lwkt_replymsg(msg, 0);
        return (EASYNC);
}

/*
 * Find an entry in the syncache.
 */
struct syncache *
syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
{
        struct tcp_syncache_percpu *syncache_percpu;
        struct syncache *sc;
        struct syncache_head *sch;

        syncache_percpu = &tcp_syncache_percpu[mycpu->gd_cpuid];
#ifdef INET6
        if (inc->inc_isipv6) {
                sch = &syncache_percpu->hashbase[
                    SYNCACHE_HASH6(inc, tcp_syncache.hashmask)];
                *schp = sch;
                TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash)
                        if (ENDPTS6_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
                                return (sc);
        } else
#endif
        {
                sch = &syncache_percpu->hashbase[
                    SYNCACHE_HASH(inc, tcp_syncache.hashmask)];
                *schp = sch;
                TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
#ifdef INET6
                        if (sc->sc_inc.inc_isipv6)
                                continue;
#endif
                        if (ENDPTS_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
                                return (sc);
                }
        }
        return (NULL);
}

/*
 * This function is called when we get a RST for a
 * non-existent connection, so that we can see if the
 * connection is in the syn cache.  If it is, zap it.
 */
void
syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th)
{
        struct syncache *sc;
        struct syncache_head *sch;

        sc = syncache_lookup(inc, &sch);
        if (sc == NULL)
                return;
        /*
         * If the RST bit is set, check the sequence number to see
         * if this is a valid reset segment.
         * RFC 793 page 37:
         *   In all states except SYN-SENT, all reset (RST) segments
         *   are validated by checking their SEQ-fields.  A reset is
         *   valid if its sequence number is in the window.
         *
         *   The sequence number in the reset segment is normally an
         *   echo of our outgoing acknowlegement numbers, but some hosts
         *   send a reset with the sequence number at the rightmost edge
         *   of our receive window, and we have to handle this case.
         */
        if (SEQ_GEQ(th->th_seq, sc->sc_irs) &&
            SEQ_LEQ(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
                syncache_drop(sc, sch);
                tcpstat.tcps_sc_reset++;
        }
}

void
syncache_badack(struct in_conninfo *inc)
{
        struct syncache *sc;
        struct syncache_head *sch;

        sc = syncache_lookup(inc, &sch);
        if (sc != NULL) {
                syncache_drop(sc, sch);
                tcpstat.tcps_sc_badack++;
        }
}

void
syncache_unreach(struct in_conninfo *inc, struct tcphdr *th)
{
        struct syncache *sc;
        struct syncache_head *sch;

        /* we are called at splnet() here */
        sc = syncache_lookup(inc, &sch);
        if (sc == NULL)
                return;

        /* If the sequence number != sc_iss, then it's a bogus ICMP msg */
        if (ntohl(th->th_seq) != sc->sc_iss)
                return;

        /*
         * If we've rertransmitted 3 times and this is our second error,
         * we remove the entry.  Otherwise, we allow it to continue on.
         * This prevents us from incorrectly nuking an entry during a
         * spurious network outage.
         *
         * See tcp_notify().
         */
        if ((sc->sc_flags & SCF_UNREACH) == 0 || sc->sc_rxtslot < 3) {
                sc->sc_flags |= SCF_UNREACH;
                return;
        }
        syncache_drop(sc, sch);
        tcpstat.tcps_sc_unreach++;
}

/*
 * Build a new TCP socket structure from a syncache entry.
 */
static struct socket *
syncache_socket(struct syncache *sc, struct socket *lso)
{
        struct inpcb *inp = NULL, *linp;
        struct socket *so;
        struct tcpcb *tp;
#ifdef INET6
        const boolean_t isipv6 = sc->sc_inc.inc_isipv6;
#else
        const boolean_t isipv6 = FALSE;
#endif

        /*
         * Ok, create the full blown connection, and set things up
         * as they would have been set up if we had created the
         * connection when the SYN arrived.  If we can't create
         * the connection, abort it.
         */
        so = sonewconn(lso, SS_ISCONNECTED);
        if (so == NULL) {
                /*
                 * Drop the connection; we will send a RST if the peer
                 * retransmits the ACK,
                 */
                tcpstat.tcps_listendrop++;
                goto abort;
        }

        inp = so->so_pcb;

        /*
         * Insert new socket into hash list.
         */
        inp->inp_inc.inc_isipv6 = sc->sc_inc.inc_isipv6;
        if (isipv6) {
                inp->in6p_laddr = sc->sc_inc.inc6_laddr;
        } else {
#ifdef INET6
                inp->inp_vflag &= ~INP_IPV6;
                inp->inp_vflag |= INP_IPV4;
#endif
                inp->inp_laddr = sc->sc_inc.inc_laddr;
        }
        inp->inp_lport = sc->sc_inc.inc_lport;
        if (in_pcbinsporthash(inp) != 0) {
                /*
                 * Undo the assignments above if we failed to
                 * put the PCB on the hash lists.
                 */
                if (isipv6)
                        inp->in6p_laddr = kin6addr_any;
                else
                        inp->inp_laddr.s_addr = INADDR_ANY;
                inp->inp_lport = 0;
                goto abort;
        }
        linp = so->so_pcb;
#ifdef IPSEC
        /* copy old policy into new socket's */
        if (ipsec_copy_policy(linp->inp_sp, inp->inp_sp))
                kprintf("syncache_expand: could not copy policy\n");
#endif
        if (isipv6) {
                struct in6_addr laddr6;
                struct sockaddr_in6 sin6;
                /*
                 * Inherit socket options from the listening socket.
                 * Note that in6p_inputopts are not (and should not be)
                 * copied, since it stores previously received options and is
                 * used to detect if each new option is different than the
                 * previous one and hence should be passed to a user.
                 * If we copied in6p_inputopts, a user would not be able to
                 * receive options just after calling the accept system call.
                 */
                inp->inp_flags |= linp->inp_flags & INP_CONTROLOPTS;
                if (linp->in6p_outputopts)
                        inp->in6p_outputopts =
                            ip6_copypktopts(linp->in6p_outputopts, M_INTWAIT);
                inp->in6p_route = sc->sc_route6;
                sc->sc_route6.ro_rt = NULL;

                sin6.sin6_family = AF_INET6;
                sin6.sin6_len = sizeof sin6;
                sin6.sin6_addr = sc->sc_inc.inc6_faddr;
                sin6.sin6_port = sc->sc_inc.inc_fport;
                sin6.sin6_flowinfo = sin6.sin6_scope_id = 0;
                laddr6 = inp->in6p_laddr;
                if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr))
                        inp->in6p_laddr = sc->sc_inc.inc6_laddr;
                if (in6_pcbconnect(inp, (struct sockaddr *)&sin6, &thread0)) {
                        inp->in6p_laddr = laddr6;
                        goto abort;
                }
        } else {
                struct in_addr laddr;
                struct sockaddr_in sin;

                inp->inp_options = ip_srcroute();
                if (inp->inp_options == NULL) {
                        inp->inp_options = sc->sc_ipopts;
                        sc->sc_ipopts = NULL;
                }
                inp->inp_route = sc->sc_route;
                sc->sc_route.ro_rt = NULL;

                sin.sin_family = AF_INET;
                sin.sin_len = sizeof sin;
                sin.sin_addr = sc->sc_inc.inc_faddr;
                sin.sin_port = sc->sc_inc.inc_fport;
                bzero(sin.sin_zero, sizeof sin.sin_zero);
                laddr = inp->inp_laddr;
                if (inp->inp_laddr.s_addr == INADDR_ANY)
                        inp->inp_laddr = sc->sc_inc.inc_laddr;
                if (in_pcbconnect(inp, (struct sockaddr *)&sin, &thread0)) {
                        inp->inp_laddr = laddr;
                        goto abort;
                }
        }

        tp = intotcpcb(inp);
        tp->t_state = TCPS_SYN_RECEIVED;
        tp->iss = sc->sc_iss;
        tp->irs = sc->sc_irs;
        tcp_rcvseqinit(tp);
        tcp_sendseqinit(tp);
        tp->snd_wl1 = sc->sc_irs;
        tp->rcv_up = sc->sc_irs + 1;
        tp->rcv_wnd = sc->sc_wnd;
        tp->rcv_adv += tp->rcv_wnd;

        tp->t_flags = sototcpcb(lso)->t_flags & (TF_NOPUSH | TF_NODELAY);
        if (sc->sc_flags & SCF_NOOPT)
                tp->t_flags |= TF_NOOPT;
        if (sc->sc_flags & SCF_WINSCALE) {
                tp->t_flags |= TF_REQ_SCALE | TF_RCVD_SCALE;
                tp->requested_s_scale = sc->sc_requested_s_scale;
                tp->request_r_scale = sc->sc_request_r_scale;
        }
        if (sc->sc_flags & SCF_TIMESTAMP) {
                tp->t_flags |= TF_REQ_TSTMP | TF_RCVD_TSTMP;
                tp->ts_recent = sc->sc_tsrecent;
                tp->ts_recent_age = ticks;
        }
        if (sc->sc_flags & SCF_CC) {
                /*
                 * Initialization of the tcpcb for transaction;
                 *   set SND.WND = SEG.WND,
                 *   initialize CCsend and CCrecv.
                 */
                tp->t_flags |= TF_REQ_CC | TF_RCVD_CC;
                tp->cc_send = sc->sc_cc_send;
                tp->cc_recv = sc->sc_cc_recv;
        }
        if (sc->sc_flags & SCF_SACK_PERMITTED)
                tp->t_flags |= TF_SACK_PERMITTED;

        tcp_mss(tp, sc->sc_peer_mss);

        /*
         * If the SYN,ACK was retransmitted, reset cwnd to 1 segment.
         */
        if (sc->sc_rxtslot != 0)
                tp->snd_cwnd = tp->t_maxseg;
        callout_reset(tp->tt_keep, tcp_keepinit, tcp_timer_keep, tp);

        tcpstat.tcps_accepts++;
        return (so);

abort:
        if (so != NULL)
                soabort(so);
        return (NULL);
}

/*
 * This function gets called when we receive an ACK for a
 * socket in the LISTEN state.  We look up the connection
 * in the syncache, and if its there, we pull it out of
 * the cache and turn it into a full-blown connection in
 * the SYN-RECEIVED state.
 */
int
syncache_expand(struct in_conninfo *inc, struct tcphdr *th, struct socket **sop,
                struct mbuf *m)
{
        struct syncache *sc;
        struct syncache_head *sch;
        struct socket *so;

        sc = syncache_lookup(inc, &sch);
        if (sc == NULL) {
                /*
                 * There is no syncache entry, so see if this ACK is
                 * a returning syncookie.  To do this, first:
                 *  A. See if this socket has had a syncache entry dropped in
                 *     the past.  We don't want to accept a bogus syncookie
                 *     if we've never received a SYN.
                 *  B. check that the syncookie is valid.  If it is, then
                 *     cobble up a fake syncache entry, and return.
                 */
                if (!tcp_syncookies)
                        return (0);
                sc = syncookie_lookup(inc, th, *sop);
                if (sc == NULL)
                        return (0);
                sch = NULL;
                tcpstat.tcps_sc_recvcookie++;
        }

        /*
         * If seg contains an ACK, but not for our SYN/ACK, send a RST.
         */
        if (th->th_ack != sc->sc_iss + 1)
                return (0);

        so = syncache_socket(sc, *sop);
        if (so == NULL) {
#if 0
resetandabort:
                /* XXXjlemon check this - is this correct? */
                tcp_respond(NULL, m, m, th,
                    th->th_seq + tlen, (tcp_seq)0, TH_RST | TH_ACK);
#endif
                m_freem(m);                     /* XXX only needed for above */
                tcpstat.tcps_sc_aborted++;
        } else {
                tcpstat.tcps_sc_completed++;
        }
        if (sch == NULL)
                syncache_free(sc);
        else
                syncache_drop(sc, sch);
        *sop = so;
        return (1);
}

/*
 * Given a LISTEN socket and an inbound SYN request, add
 * this to the syn cache, and send back a segment:
 *      <SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
 * to the source.
 *
 * IMPORTANT NOTE: We do _NOT_ ACK data that might accompany the SYN.
 * Doing so would require that we hold onto the data and deliver it
 * to the application.  However, if we are the target of a SYN-flood
 * DoS attack, an attacker could send data which would eventually
 * consume all available buffer space if it were ACKed.  By not ACKing
 * the data, we avoid this DoS scenario.
 */
int
syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
             struct socket **sop, struct mbuf *m)
{
        struct tcp_syncache_percpu *syncache_percpu;
        struct tcpcb *tp;
        struct socket *so;
        struct syncache *sc = NULL;
        struct syncache_head *sch;
        struct mbuf *ipopts = NULL;
        struct rmxp_tao *taop;
        int win;

        syncache_percpu = &tcp_syncache_percpu[mycpu->gd_cpuid];
        so = *sop;
        tp = sototcpcb(so);

        /*
         * Remember the IP options, if any.
         */
#ifdef INET6
        if (!inc->inc_isipv6)
#endif
                ipopts = ip_srcroute();

        /*
         * See if we already have an entry for this connection.
         * If we do, resend the SYN,ACK, and reset the retransmit timer.
         *
         * XXX
         * The syncache should be re-initialized with the contents
         * of the new SYN which may have different options.
         */
        sc = syncache_lookup(inc, &sch);
        if (sc != NULL) {
                tcpstat.tcps_sc_dupsyn++;
                if (ipopts) {
                        /*
                         * If we were remembering a previous source route,
                         * forget it and use the new one we've been given.
                         */
                        if (sc->sc_ipopts)
                                m_free(sc->sc_ipopts);
                        sc->sc_ipopts = ipopts;
                }
                /*
                 * Update timestamp if present.
                 */
                if (sc->sc_flags & SCF_TIMESTAMP)
                        sc->sc_tsrecent = to->to_tsval;

                /* Just update the TOF_SACK_PERMITTED for now. */
                if (tcp_do_sack && (to->to_flags & TOF_SACK_PERMITTED))
                        sc->sc_flags |= SCF_SACK_PERMITTED;
                else
                        sc->sc_flags &= ~SCF_SACK_PERMITTED;

                /*
                 * PCB may have changed, pick up new values.
                 */
                sc->sc_tp = tp;
                sc->sc_inp_gencnt = tp->t_inpcb->inp_gencnt;
                if (syncache_respond(sc, m) == 0) {
                        TAILQ_REMOVE(&syncache_percpu->timerq[sc->sc_rxtslot],
                            sc, sc_timerq);
                        syncache_timeout(syncache_percpu, sc, sc->sc_rxtslot);
                        tcpstat.tcps_sndacks++;
                        tcpstat.tcps_sndtotal++;
                }
                *sop = NULL;
                return (1);
        }

        /*
         * This allocation is guaranteed to succeed because we
         * preallocate one more syncache entry than cache_limit.
         */
        sc = zalloc(tcp_syncache.zone);

        /*
         * Fill in the syncache values.
         */
        sc->sc_tp = tp;
        sc->sc_inp_gencnt = tp->t_inpcb->inp_gencnt;
        sc->sc_ipopts = ipopts;
        sc->sc_inc.inc_fport = inc->inc_fport;
        sc->sc_inc.inc_lport = inc->inc_lport;
#ifdef INET6
        sc->sc_inc.inc_isipv6 = inc->inc_isipv6;
        if (inc->inc_isipv6) {
                sc->sc_inc.inc6_faddr = inc->inc6_faddr;
                sc->sc_inc.inc6_laddr = inc->inc6_laddr;
                sc->sc_route6.ro_rt = NULL;
        } else
#endif
        {
                sc->sc_inc.inc_faddr = inc->inc_faddr;
                sc->sc_inc.inc_laddr = inc->inc_laddr;
                sc->sc_route.ro_rt = NULL;
        }
        sc->sc_irs = th->th_seq;
        sc->sc_flags = 0;
        sc->sc_peer_mss = to->to_flags & TOF_MSS ? to->to_mss : 0;
        if (tcp_syncookies)
                sc->sc_iss = syncookie_generate(sc);
        else
                sc->sc_iss = karc4random();

        /* Initial receive window: clip sbspace to [0 .. TCP_MAXWIN] */
        win = sbspace(&so->so_rcv);
        win = imax(win, 0);
        win = imin(win, TCP_MAXWIN);
        sc->sc_wnd = win;

        if (tcp_do_rfc1323) {
                /*
                 * A timestamp received in a SYN makes
                 * it ok to send timestamp requests and replies.
                 */
                if (to->to_flags & TOF_TS) {
                        sc->sc_tsrecent = to->to_tsval;
                        sc->sc_flags |= SCF_TIMESTAMP;
                }
                if (to->to_flags & TOF_SCALE) {
                        int wscale = 0;

                        /* Compute proper scaling value from buffer space */
                        while (wscale < TCP_MAX_WINSHIFT &&
                            (TCP_MAXWIN << wscale) < so->so_rcv.sb_hiwat)
                                wscale++;
                        sc->sc_request_r_scale = wscale;
                        sc->sc_requested_s_scale = to->to_requested_s_scale;
                        sc->sc_flags |= SCF_WINSCALE;
                }
        }
        if (tcp_do_rfc1644) {
                /*
                 * A CC or CC.new option received in a SYN makes
                 * it ok to send CC in subsequent segments.
                 */
                if (to->to_flags & (TOF_CC | TOF_CCNEW)) {
                        sc->sc_cc_recv = to->to_cc;
                        sc->sc_cc_send = CC_INC(tcp_ccgen);
                        sc->sc_flags |= SCF_CC;
                }
        }
        if (tcp_do_sack && (to->to_flags & TOF_SACK_PERMITTED))
                sc->sc_flags |= SCF_SACK_PERMITTED;
        if (tp->t_flags & TF_NOOPT)
                sc->sc_flags = SCF_NOOPT;

        /*
         * XXX
         * We have the option here of not doing TAO (even if the segment
         * qualifies) and instead fall back to a normal 3WHS via the syncache.
         * This allows us to apply synflood protection to TAO-qualifying SYNs
         * also. However, there should be a hueristic to determine when to
         * do this, and is not present at the moment.
         */

        /*
         * Perform TAO test on incoming CC (SEG.CC) option, if any.
         * - compare SEG.CC against cached CC from the same host, if any.
         * - if SEG.CC > chached value, SYN must be new and is accepted
         *      immediately: save new CC in the cache, mark the socket
         *      connected, enter ESTABLISHED state, turn on flag to
         *      send a SYN in the next segment.
         *      A virtual advertised window is set in rcv_adv to
         *      initialize SWS prevention.  Then enter normal segment
         *      processing: drop SYN, process data and FIN.
         * - otherwise do a normal 3-way handshake.
         */
        taop = tcp_gettaocache(&sc->sc_inc);
        if (to->to_flags & TOF_CC) {
                if ((tp->t_flags & TF_NOPUSH) &&
                    sc->sc_flags & SCF_CC &&
                    taop != NULL && taop->tao_cc != 0 &&
                    CC_GT(to->to_cc, taop->tao_cc)) {
                        sc->sc_rxtslot = 0;
                        so = syncache_socket(sc, *sop);
                        if (so != NULL) {
                                taop->tao_cc = to->to_cc;
                                *sop = so;
                        }
                        syncache_free(sc);
                        return (so != NULL);
                }
        } else {
                /*
                 * No CC option, but maybe CC.NEW: invalidate cached value.
                 */
                if (taop != NULL)
                        taop->tao_cc = 0;
        }
        /*
         * TAO test failed or there was no CC option,
         *    do a standard 3-way handshake.
         */
        if (syncache_respond(sc, m) == 0) {
                syncache_insert(sc, sch);
                tcpstat.tcps_sndacks++;
                tcpstat.tcps_sndtotal++;
        } else {
                syncache_free(sc);
                tcpstat.tcps_sc_dropped++;
        }
        *sop = NULL;
        return (1);
}

static int
syncache_respond(struct syncache *sc, struct mbuf *m)
{
        u_int8_t *optp;
        int optlen, error;
        u_int16_t tlen, hlen, mssopt;
        struct ip *ip = NULL;
        struct rtentry *rt;
        struct tcphdr *th;
        struct ip6_hdr *ip6 = NULL;
#ifdef INET6
        const boolean_t isipv6 = sc->sc_inc.inc_isipv6;
#else
        const boolean_t isipv6 = FALSE;
#endif

        if (isipv6) {
                rt = tcp_rtlookup6(&sc->sc_inc);
                if (rt != NULL)
                        mssopt = rt->rt_ifp->if_mtu -
                             (sizeof(struct ip6_hdr) + sizeof(struct tcphdr));
                else
                        mssopt = tcp_v6mssdflt;
                hlen = sizeof(struct ip6_hdr);
        } else {
                rt = tcp_rtlookup(&sc->sc_inc);
                if (rt != NULL)
                        mssopt = rt->rt_ifp->if_mtu -
                             (sizeof(struct ip) + sizeof(struct tcphdr));
                else
                        mssopt = tcp_mssdflt;
                hlen = sizeof(struct ip);
        }

        /* Compute the size of the TCP options. */
        if (sc->sc_flags & SCF_NOOPT) {
                optlen = 0;
        } else {
                optlen = TCPOLEN_MAXSEG +
                    ((sc->sc_flags & SCF_WINSCALE) ? 4 : 0) +
                    ((sc->sc_flags & SCF_TIMESTAMP) ? TCPOLEN_TSTAMP_APPA : 0) +
                    ((sc->sc_flags & SCF_CC) ? TCPOLEN_CC_APPA * 2 : 0) +
                    ((sc->sc_flags & SCF_SACK_PERMITTED) ?
                        TCPOLEN_SACK_PERMITTED_ALIGNED : 0);
        }
        tlen = hlen + sizeof(struct tcphdr) + optlen;

        /*
         * XXX
         * assume that the entire packet will fit in a header mbuf
         */
        KASSERT(max_linkhdr + tlen <= MHLEN, ("syncache: mbuf too small"));

        /*
         * XXX shouldn't this reuse the mbuf if possible ?
         * Create the IP+TCP header from scratch.
         */
        if (m)
                m_freem(m);

        m = m_gethdr(MB_DONTWAIT, MT_HEADER);
        if (m == NULL)
                return (ENOBUFS);
        m->m_data += max_linkhdr;
        m->m_len = tlen;
        m->m_pkthdr.len = tlen;
        m->m_pkthdr.rcvif = NULL;

        if (isipv6) {
                ip6 = mtod(m, struct ip6_hdr *);
                ip6->ip6_vfc = IPV6_VERSION;
                ip6->ip6_nxt = IPPROTO_TCP;
                ip6->ip6_src = sc->sc_inc.inc6_laddr;
                ip6->ip6_dst = sc->sc_inc.inc6_faddr;
                ip6->ip6_plen = htons(tlen - hlen);
                /* ip6_hlim is set after checksum */
                /* ip6_flow = ??? */

                th = (struct tcphdr *)(ip6 + 1);
        } else {
                ip = mtod(m, struct ip *);
                ip->ip_v = IPVERSION;
                ip->ip_hl = sizeof(struct ip) >> 2;
                ip->ip_len = tlen;
                ip->ip_id = 0;
                ip->ip_off = 0;
                ip->ip_sum = 0;
                ip->ip_p = IPPROTO_TCP;
                ip->ip_src = sc->sc_inc.inc_laddr;
                ip->ip_dst = sc->sc_inc.inc_faddr;
                ip->ip_ttl = sc->sc_tp->t_inpcb->inp_ip_ttl;   /* XXX */
                ip->ip_tos = sc->sc_tp->t_inpcb->inp_ip_tos;   /* XXX */

                /*
                 * See if we should do MTU discovery.  Route lookups are
                 * expensive, so we will only unset the DF bit if:
                 *
                 *      1) path_mtu_discovery is disabled
                 *      2) the SCF_UNREACH flag has been set
                 */
                if (path_mtu_discovery
                    && ((sc->sc_flags & SCF_UNREACH) == 0)) {
                       ip->ip_off |= IP_DF;
                }

                th = (struct tcphdr *)(ip + 1);
        }
        th->th_sport = sc->sc_inc.inc_lport;
        th->th_dport = sc->sc_inc.inc_fport;

        th->th_seq = htonl(sc->sc_iss);
        th->th_ack = htonl(sc->sc_irs + 1);
        th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
        th->th_x2 = 0;
        th->th_flags = TH_SYN | TH_ACK;
        th->th_win = htons(sc->sc_wnd);
        th->th_urp = 0;

        /* Tack on the TCP options. */
        if (optlen == 0)
                goto no_options;
        optp = (u_int8_t *)(th + 1);
        *optp++ = TCPOPT_MAXSEG;
        *optp++ = TCPOLEN_MAXSEG;
        *optp++ = (mssopt >> 8) & 0xff;
        *optp++ = mssopt & 0xff;

        if (sc->sc_flags & SCF_WINSCALE) {
                *((u_int32_t *)optp) = htonl(TCPOPT_NOP << 24 |
                    TCPOPT_WINDOW << 16 | TCPOLEN_WINDOW << 8 |
                    sc->sc_request_r_scale);
                optp += 4;
        }

        if (sc->sc_flags & SCF_TIMESTAMP) {
                u_int32_t *lp = (u_int32_t *)(optp);

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

        /*
         * Send CC and CC.echo if we received CC from our peer.
         */
        if (sc->sc_flags & SCF_CC) {
                u_int32_t *lp = (u_int32_t *)(optp);

                *lp++ = htonl(TCPOPT_CC_HDR(TCPOPT_CC));
                *lp++ = htonl(sc->sc_cc_send);
                *lp++ = htonl(TCPOPT_CC_HDR(TCPOPT_CCECHO));
                *lp   = htonl(sc->sc_cc_recv);
                optp += TCPOLEN_CC_APPA * 2;
        }

        if (sc->sc_flags & SCF_SACK_PERMITTED) {
                *((u_int32_t *)optp) = htonl(TCPOPT_SACK_PERMITTED_ALIGNED);
                optp += TCPOLEN_SACK_PERMITTED_ALIGNED;
        }

no_options:
        if (isipv6) {
                struct route_in6 *ro6 = &sc->sc_route6;

                th->th_sum = 0;
                th->th_sum = in6_cksum(m, IPPROTO_TCP, hlen, tlen - hlen);
                ip6->ip6_hlim = in6_selecthlim(NULL,
                    ro6->ro_rt ? ro6->ro_rt->rt_ifp : NULL);
                error = ip6_output(m, NULL, ro6, 0, NULL, NULL,
                                sc->sc_tp->t_inpcb);
        } else {
                th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
                                       htons(tlen - hlen + IPPROTO_TCP));
                m->m_pkthdr.csum_flags = CSUM_TCP;
                m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
                error = ip_output(m, sc->sc_ipopts, &sc->sc_route, 0, NULL,
                                sc->sc_tp->t_inpcb);
        }
        return (error);
}

/*
 * cookie layers:
 *
 *      |. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .|
 *      | peer iss                                                      |
 *      | MD5(laddr,faddr,secret,lport,fport)             |. . . . . . .|
 *      |                     0                       |(A)|             |
 * (A): peer mss index
 */

/*
 * The values below are chosen to minimize the size of the tcp_secret
 * table, as well as providing roughly a 16 second lifetime for the cookie.
 */

#define SYNCOOKIE_WNDBITS       5       /* exposed bits for window indexing */
#define SYNCOOKIE_TIMESHIFT     1       /* scale ticks to window time units */

#define SYNCOOKIE_WNDMASK       ((1 << SYNCOOKIE_WNDBITS) - 1)
#define SYNCOOKIE_NSECRETS      (1 << SYNCOOKIE_WNDBITS)
#define SYNCOOKIE_TIMEOUT \
    (hz * (1 << SYNCOOKIE_WNDBITS) / (1 << SYNCOOKIE_TIMESHIFT))
#define SYNCOOKIE_DATAMASK      ((3 << SYNCOOKIE_WNDBITS) | SYNCOOKIE_WNDMASK)

static struct {
        u_int32_t       ts_secbits[4];
        u_int           ts_expire;
} tcp_secret[SYNCOOKIE_NSECRETS];

static int tcp_msstab[] = { 0, 536, 1460, 8960 };

static MD5_CTX syn_ctx;

#define MD5Add(v)       MD5Update(&syn_ctx, (u_char *)&v, sizeof(v))

struct md5_add {
        u_int32_t laddr, faddr;
        u_int32_t secbits[4];
        u_int16_t lport, fport;
};

#ifdef CTASSERT
CTASSERT(sizeof(struct md5_add) == 28);
#endif

/*
 * Consider the problem of a recreated (and retransmitted) cookie.  If the
 * original SYN was accepted, the connection is established.  The second
 * SYN is inflight, and if it arrives with an ISN that falls within the
 * receive window, the connection is killed.
 *
 * However, since cookies have other problems, this may not be worth
 * worrying about.
 */

static u_int32_t
syncookie_generate(struct syncache *sc)
{
        u_int32_t md5_buffer[4];
        u_int32_t data;
        int idx, i;
        struct md5_add add;
#ifdef INET6
        const boolean_t isipv6 = sc->sc_inc.inc_isipv6;
#else
        const boolean_t isipv6 = FALSE;
#endif

        idx = ((ticks << SYNCOOKIE_TIMESHIFT) / hz) & SYNCOOKIE_WNDMASK;
        if (tcp_secret[idx].ts_expire < ticks) {
                for (i = 0; i < 4; i++)
                        tcp_secret[idx].ts_secbits[i] = karc4random();
                tcp_secret[idx].ts_expire = ticks + SYNCOOKIE_TIMEOUT;
        }
        for (data = sizeof(tcp_msstab) / sizeof(int) - 1; data > 0; data--)
                if (tcp_msstab[data] <= sc->sc_peer_mss)
                        break;
        data = (data << SYNCOOKIE_WNDBITS) | idx;
        data ^= sc->sc_irs;                             /* peer's iss */
        MD5Init(&syn_ctx);
        if (isipv6) {
                MD5Add(sc->sc_inc.inc6_laddr);
                MD5Add(sc->sc_inc.inc6_faddr);
                add.laddr = 0;
                add.faddr = 0;
        } else {
                add.laddr = sc->sc_inc.inc_laddr.s_addr;
                add.faddr = sc->sc_inc.inc_faddr.s_addr;
        }
        add.lport = sc->sc_inc.inc_lport;
        add.fport = sc->sc_inc.inc_fport;
        add.secbits[0] = tcp_secret[idx].ts_secbits[0];
        add.secbits[1] = tcp_secret[idx].ts_secbits[1];
        add.secbits[2] = tcp_secret[idx].ts_secbits[2];
        add.secbits[3] = tcp_secret[idx].ts_secbits[3];
        MD5Add(add);
        MD5Final((u_char *)&md5_buffer, &syn_ctx);
        data ^= (md5_buffer[0] & ~SYNCOOKIE_WNDMASK);
        return (data);
}

static struct syncache *
syncookie_lookup(struct in_conninfo *inc, struct tcphdr *th, struct socket *so)
{
        u_int32_t md5_buffer[4];
        struct syncache *sc;
        u_int32_t data;
        int wnd, idx;
        struct md5_add add;

        data = (th->th_ack - 1) ^ (th->th_seq - 1);     /* remove ISS */
        idx = data & SYNCOOKIE_WNDMASK;
        if (tcp_secret[idx].ts_expire < ticks ||
            sototcpcb(so)->ts_recent + SYNCOOKIE_TIMEOUT < ticks)
                return (NULL);
        MD5Init(&syn_ctx);
#ifdef INET6
        if (inc->inc_isipv6) {
                MD5Add(inc->inc6_laddr);
                MD5Add(inc->inc6_faddr);
                add.laddr = 0;
                add.faddr = 0;
        } else
#endif
        {
                add.laddr = inc->inc_laddr.s_addr;
                add.faddr = inc->inc_faddr.s_addr;
        }
        add.lport = inc->inc_lport;
        add.fport = inc->inc_fport;
        add.secbits[0] = tcp_secret[idx].ts_secbits[0];
        add.secbits[1] = tcp_secret[idx].ts_secbits[1];
        add.secbits[2] = tcp_secret[idx].ts_secbits[2];
        add.secbits[3] = tcp_secret[idx].ts_secbits[3];
        MD5Add(add);
        MD5Final((u_char *)&md5_buffer, &syn_ctx);
        data ^= md5_buffer[0];
        if (data & ~SYNCOOKIE_DATAMASK)
                return (NULL);
        data = data >> SYNCOOKIE_WNDBITS;

        /*
         * This allocation is guaranteed to succeed because we
         * preallocate one more syncache entry than cache_limit.
         */
        sc = zalloc(tcp_syncache.zone);

        /*
         * Fill in the syncache values.
         * XXX duplicate code from syncache_add
         */
        sc->sc_ipopts = NULL;
        sc->sc_inc.inc_fport = inc->inc_fport;
        sc->sc_inc.inc_lport = inc->inc_lport;
#ifdef INET6
        sc->sc_inc.inc_isipv6 = inc->inc_isipv6;
        if (inc->inc_isipv6) {
                sc->sc_inc.inc6_faddr = inc->inc6_faddr;
                sc->sc_inc.inc6_laddr = inc->inc6_laddr;
                sc->sc_route6.ro_rt = NULL;
        } else
#endif
        {
                sc->sc_inc.inc_faddr = inc->inc_faddr;
                sc->sc_inc.inc_laddr = inc->inc_laddr;
                sc->sc_route.ro_rt = NULL;
        }
        sc->sc_irs = th->th_seq - 1;
        sc->sc_iss = th->th_ack - 1;
        wnd = sbspace(&so->so_rcv);
        wnd = imax(wnd, 0);
        wnd = imin(wnd, TCP_MAXWIN);
        sc->sc_wnd = wnd;
        sc->sc_flags = 0;
        sc->sc_rxtslot = 0;
        sc->sc_peer_mss = tcp_msstab[data];
        return (sc);
}