7209 tcp_time_wait_collector() collecting all the CPU time

[unleashed.git] / usr / src / uts / common / inet / tcp / tcp_time_wait.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */

/*
 * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
 * Copyright 2016 Joyent, Inc.
 */

/*
 * This file contains functions related to TCP time wait processing.  Also
 * refer to the time wait handling comments in tcp_impl.h.
 */

#include <sys/types.h>
#include <sys/strsun.h>
#include <sys/squeue_impl.h>
#include <sys/squeue.h>
#include <sys/callo.h>

#include <inet/common.h>
#include <inet/ip.h>
#include <inet/tcp.h>
#include <inet/tcp_impl.h>
#include <inet/tcp_cluster.h>

static void tcp_time_wait_purge(tcp_t *, tcp_squeue_priv_t *);

#define TW_BUCKET(t)                                    \
        (((t) / MSEC_TO_TICK(TCP_TIME_WAIT_DELAY)) % TCP_TIME_WAIT_BUCKETS)

#define TW_BUCKET_NEXT(b)       (((b) + 1) % TCP_TIME_WAIT_BUCKETS)


/*
 * Remove a connection from the list of detached TIME_WAIT connections.
 * It returns B_FALSE if it can't remove the connection from the list
 * as the connection has already been removed from the list due to an
 * earlier call to tcp_time_wait_remove(); otherwise it returns B_TRUE.
 */
boolean_t
tcp_time_wait_remove(tcp_t *tcp, tcp_squeue_priv_t *tsp)
{
        boolean_t       locked = B_FALSE;

        if (tsp == NULL) {
                tsp = *((tcp_squeue_priv_t **)
                    squeue_getprivate(tcp->tcp_connp->conn_sqp, SQPRIVATE_TCP));
                mutex_enter(&tsp->tcp_time_wait_lock);
                locked = B_TRUE;
        } else {
                ASSERT(MUTEX_HELD(&tsp->tcp_time_wait_lock));
        }

        /* 0 means that the tcp_t has not been added to the time wait list. */
        if (tcp->tcp_time_wait_expire == 0) {
                ASSERT(tcp->tcp_time_wait_next == NULL);
                ASSERT(tcp->tcp_time_wait_prev == NULL);
                if (locked)
                        mutex_exit(&tsp->tcp_time_wait_lock);
                return (B_FALSE);
        }
        ASSERT(TCP_IS_DETACHED(tcp));
        ASSERT(tcp->tcp_state == TCPS_TIME_WAIT);
        ASSERT(tsp->tcp_time_wait_cnt > 0);

        if (tcp->tcp_time_wait_next != NULL) {
                tcp->tcp_time_wait_next->tcp_time_wait_prev =
                    tcp->tcp_time_wait_prev;
        }
        if (tcp->tcp_time_wait_prev != NULL) {
                tcp->tcp_time_wait_prev->tcp_time_wait_next =
                    tcp->tcp_time_wait_next;
        } else {
                unsigned int bucket;

                bucket = TW_BUCKET(tcp->tcp_time_wait_expire);
                ASSERT(tsp->tcp_time_wait_bucket[bucket] == tcp);
                tsp->tcp_time_wait_bucket[bucket] = tcp->tcp_time_wait_next;
        }
        tcp->tcp_time_wait_next = NULL;
        tcp->tcp_time_wait_prev = NULL;
        tcp->tcp_time_wait_expire = 0;
        tsp->tcp_time_wait_cnt--;

        if (locked)
                mutex_exit(&tsp->tcp_time_wait_lock);
        return (B_TRUE);
}

/* Constants used for fast checking of a localhost address */
#if defined(_BIG_ENDIAN)
#define IPv4_LOCALHOST  0x7f000000U
#define IPv4_LH_MASK    0xffffff00U
#else
#define IPv4_LOCALHOST  0x0000007fU
#define IPv4_LH_MASK    0x00ffffffU
#endif

#define IS_LOCAL_HOST(x)        ( \
        ((x)->tcp_connp->conn_ipversion == IPV4_VERSION && \
        ((x)->tcp_connp->conn_laddr_v4 & IPv4_LH_MASK) == IPv4_LOCALHOST) || \
        ((x)->tcp_connp->conn_ipversion == IPV6_VERSION && \
        IN6_IS_ADDR_LOOPBACK(&(x)->tcp_connp->conn_laddr_v6)))


/*
 * Add a connection to the list of detached TIME_WAIT connections
 * and set its time to expire.
 */
void
tcp_time_wait_append(tcp_t *tcp)
{
        tcp_stack_t     *tcps = tcp->tcp_tcps;
        squeue_t        *sqp = tcp->tcp_connp->conn_sqp;
        tcp_squeue_priv_t *tsp =
            *((tcp_squeue_priv_t **)squeue_getprivate(sqp, SQPRIVATE_TCP));
        int64_t         now, schedule;
        unsigned int    bucket;

        tcp_timers_stop(tcp);

        /* Freed above */
        ASSERT(tcp->tcp_timer_tid == 0);
        ASSERT(tcp->tcp_ack_tid == 0);

        /* must have happened at the time of detaching the tcp */
        ASSERT(TCP_IS_DETACHED(tcp));
        ASSERT(tcp->tcp_state == TCPS_TIME_WAIT);
        ASSERT(tcp->tcp_ptpahn == NULL);
        ASSERT(tcp->tcp_flow_stopped == 0);
        ASSERT(tcp->tcp_time_wait_next == NULL);
        ASSERT(tcp->tcp_time_wait_prev == NULL);
        ASSERT(tcp->tcp_time_wait_expire == 0);
        ASSERT(tcp->tcp_listener == NULL);

        TCP_DBGSTAT(tcps, tcp_time_wait);
        mutex_enter(&tsp->tcp_time_wait_lock);

        /*
         * Immediately expire loopback connections.  Since there is no worry
         * about packets on the local host showing up after a long network
         * delay, this is safe and allows much higher rates of connection churn
         * for applications operating locally.
         *
         * This typically bypasses the tcp_free_list fast path due to squeue
         * re-entry for the loopback close operation.
         */
        if (tcp->tcp_loopback) {
                tcp_time_wait_purge(tcp, tsp);
                mutex_exit(&tsp->tcp_time_wait_lock);
                return;
        }

        /*
         * In order to reap TIME_WAITs reliably, we should use a source of time
         * that is not adjustable by the user.  While it would be more accurate
         * to grab this timestamp before (potentially) sleeping on the
         * tcp_time_wait_lock, doing so complicates bucket addressing later.
         */
        now = ddi_get_lbolt64();

        /*
         * Each squeue uses an arbitrary time offset when scheduling
         * expiration timers.  This prevents the bucketing from forcing
         * tcp_time_wait_collector to run in locksetup across squeues.
         *
         * This offset is (re)initialized when a new TIME_WAIT connection is
         * added to an squeue which has no connections waiting to expire.
         */
        if (tsp->tcp_time_wait_tid == 0) {
                ASSERT(tsp->tcp_time_wait_cnt == 0);
                tsp->tcp_time_wait_offset =
                    now % MSEC_TO_TICK(TCP_TIME_WAIT_DELAY);
        }
        now -= tsp->tcp_time_wait_offset;

        /*
         * Use the netstack-defined timeout, rounded up to the minimum
         * time_wait_collector interval.
         */
        schedule = now + MSEC_TO_TICK(tcps->tcps_time_wait_interval);
        tcp->tcp_time_wait_expire = schedule;

        /*
         * Append the connection into the appropriate bucket.
         */
        bucket = TW_BUCKET(tcp->tcp_time_wait_expire);
        tcp->tcp_time_wait_next = tsp->tcp_time_wait_bucket[bucket];
        tsp->tcp_time_wait_bucket[bucket] = tcp;
        if (tcp->tcp_time_wait_next != NULL) {
                ASSERT(tcp->tcp_time_wait_next->tcp_time_wait_prev == NULL);
                tcp->tcp_time_wait_next->tcp_time_wait_prev = tcp;
        }
        tsp->tcp_time_wait_cnt++;

        /*
         * Round delay up to the nearest bucket boundary.
         */
        schedule += MSEC_TO_TICK(TCP_TIME_WAIT_DELAY);
        schedule -= schedule % MSEC_TO_TICK(TCP_TIME_WAIT_DELAY);

        /*
         * The newly inserted entry may require a tighter schedule for the
         * expiration timer.
         */
        if (schedule < tsp->tcp_time_wait_schedule) {
                callout_id_t old_tid = tsp->tcp_time_wait_tid;

                tsp->tcp_time_wait_schedule = schedule;
                tsp->tcp_time_wait_tid =
                    timeout_generic(CALLOUT_NORMAL,
                    tcp_time_wait_collector, sqp,
                    TICK_TO_NSEC(schedule - now),
                    CALLOUT_TCP_RESOLUTION, CALLOUT_FLAG_ROUNDUP);

                /*
                 * It is possible for the timer to fire before the untimeout
                 * action is able to complete.  In that case, the exclusion
                 * offered by the tcp_time_wait_collector_active flag will
                 * prevent multiple collector threads from processing records
                 * simultaneously from the same squeue.
                 */
                mutex_exit(&tsp->tcp_time_wait_lock);
                (void) untimeout_default(old_tid, 0);
                return;
        }

        /*
         * Start a fresh timer if none exists.
         */
        if (tsp->tcp_time_wait_schedule == 0) {
                ASSERT(tsp->tcp_time_wait_tid == 0);

                tsp->tcp_time_wait_schedule = schedule;
                tsp->tcp_time_wait_tid =
                    timeout_generic(CALLOUT_NORMAL,
                    tcp_time_wait_collector, sqp,
                    TICK_TO_NSEC(schedule - now),
                    CALLOUT_TCP_RESOLUTION, CALLOUT_FLAG_ROUNDUP);
        }
        mutex_exit(&tsp->tcp_time_wait_lock);
}

/*
 * Wrapper to call tcp_close_detached() via squeue to clean up TIME-WAIT
 * tcp_t.  Used in tcp_time_wait_collector().
 */
/* ARGSUSED */
static void
tcp_timewait_close(void *arg, mblk_t *mp, void *arg2, ip_recv_attr_t *dummy)
{
        conn_t  *connp = (conn_t *)arg;
        tcp_t   *tcp = connp->conn_tcp;

        ASSERT(tcp != NULL);
        if (tcp->tcp_state == TCPS_CLOSED) {
                return;
        }

        ASSERT((connp->conn_family == AF_INET &&
            connp->conn_ipversion == IPV4_VERSION) ||
            (connp->conn_family == AF_INET6 &&
            (connp->conn_ipversion == IPV4_VERSION ||
            connp->conn_ipversion == IPV6_VERSION)));
        ASSERT(!tcp->tcp_listener);

        ASSERT(TCP_IS_DETACHED(tcp));

        /*
         * Because they have no upstream client to rebind or tcp_close()
         * them later, we axe the connection here and now.
         */
        tcp_close_detached(tcp);
}


static void
tcp_time_wait_purge(tcp_t *tcp, tcp_squeue_priv_t *tsp)
{
        mblk_t *mp;
        conn_t *connp = tcp->tcp_connp;
        kmutex_t *lock;

        ASSERT(MUTEX_HELD(&tsp->tcp_time_wait_lock));
        ASSERT(connp->conn_fanout != NULL);

        lock = &connp->conn_fanout->connf_lock;

        /*
         * This is essentially a TIME_WAIT reclaim fast path optimization for
         * performance where the connection is checked under the fanout lock
         * (so that no one else can get access to the conn_t) that the refcnt
         * is 2 (one each for TCP and the classifier hash list).  That is the
         * case and clustering callbacks are not enabled, the conn can be
         * removed under the fanout lock and avoid clean-up under the squeue.
         *
         * This optimization is forgone when clustering is enabled since the
         * clustering callback must be made before setting the CONDEMNED flag
         * and after dropping all locks
         *
         * See the comments in tcp_closei_local for additional information
         * regarding the refcnt logic.
         */
        if (mutex_tryenter(lock)) {
                mutex_enter(&connp->conn_lock);
                if (connp->conn_ref == 2 && cl_inet_disconnect == NULL) {
                        ipcl_hash_remove_locked(connp, connp->conn_fanout);
                        /*
                         * Set the CONDEMNED flag now itself so that the refcnt
                         * cannot increase due to any walker.
                         */
                        connp->conn_state_flags |= CONN_CONDEMNED;
                        mutex_exit(&connp->conn_lock);
                        mutex_exit(lock);
                        if (tsp->tcp_free_list_cnt < tcp_free_list_max_cnt) {
                                /*
                                 * Add to head of tcp_free_list
                                 */
                                tcp_cleanup(tcp);
                                ASSERT(connp->conn_latch == NULL);
                                ASSERT(connp->conn_policy == NULL);
                                ASSERT(tcp->tcp_tcps == NULL);
                                ASSERT(connp->conn_netstack == NULL);

                                tcp->tcp_time_wait_next = tsp->tcp_free_list;
                                tcp->tcp_in_free_list = B_TRUE;
                                tsp->tcp_free_list = tcp;
                                tsp->tcp_free_list_cnt++;
                        } else {
                                /*
                                 * Do not add to tcp_free_list
                                 */
                                tcp_bind_hash_remove(tcp);
                                ixa_cleanup(tcp->tcp_connp->conn_ixa);
                                tcp_ipsec_cleanup(tcp);
                                CONN_DEC_REF(tcp->tcp_connp);
                        }

                        /*
                         * With the fast-path complete, we can bail.
                         */
                        return;
                } else {
                        /*
                         * Fall back to slow path.
                         */
                        CONN_INC_REF_LOCKED(connp);
                        mutex_exit(&connp->conn_lock);
                        mutex_exit(lock);
                }
        } else {
                CONN_INC_REF(connp);
        }

        /*
         * We can reuse the closemp here since conn has detached (otherwise we
         * wouldn't even be in time_wait list). It is safe to change
         * tcp_closemp_used without taking a lock as no other thread can
         * concurrently access it at this point in the connection lifecycle.
         */
        if (tcp->tcp_closemp.b_prev == NULL) {
                tcp->tcp_closemp_used = B_TRUE;
        } else {
                cmn_err(CE_PANIC,
                    "tcp_timewait_collector: concurrent use of tcp_closemp: "
                    "connp %p tcp %p\n", (void *)connp, (void *)tcp);
        }

        TCP_DEBUG_GETPCSTACK(tcp->tcmp_stk, 15);
        mp = &tcp->tcp_closemp;
        mutex_exit(&tsp->tcp_time_wait_lock);
        SQUEUE_ENTER_ONE(connp->conn_sqp, mp, tcp_timewait_close, connp, NULL,
            SQ_FILL, SQTAG_TCP_TIMEWAIT);
        mutex_enter(&tsp->tcp_time_wait_lock);
}

/*
 * Purge any tcp_t instances associated with this squeue which have expired
 * from the TIME_WAIT state.
 */
void
tcp_time_wait_collector(void *arg)
{
        tcp_t *tcp;
        int64_t now, sched_active, sched_cur, sched_new;
        unsigned int idx;

        squeue_t *sqp = (squeue_t *)arg;
        tcp_squeue_priv_t *tsp =
            *((tcp_squeue_priv_t **)squeue_getprivate(sqp, SQPRIVATE_TCP));

        mutex_enter(&tsp->tcp_time_wait_lock);

        /*
         * Because of timer scheduling complexity and the fact that the
         * tcp_time_wait_lock is dropped during tcp_time_wait_purge, it is
         * possible for multiple tcp_time_wait_collector threads to run against
         * the same squeue.  This flag is used to exclude other collectors from
         * the squeue during execution.
         */
        if (tsp->tcp_time_wait_collector_active) {
                mutex_exit(&tsp->tcp_time_wait_lock);
                return;
        }
        tsp->tcp_time_wait_collector_active = B_TRUE;

        /*
         * After its assignment here, the value of sched_active must not be
         * altered as it is used to validate the state of the
         * tcp_time_wait_collector callout schedule for this squeue.
         *
         * The same does not hold true of sched_cur, which holds the timestamp
         * of the bucket undergoing processing.  While it is initially equal to
         * sched_active, certain conditions below can walk it forward,
         * triggering the retry loop.
         */
        sched_cur = sched_active = tsp->tcp_time_wait_schedule;

        /*
         * Purge the free list if necessary
         */
        if (tsp->tcp_free_list != NULL) {
                TCP_G_STAT(tcp_freelist_cleanup);
                while ((tcp = tsp->tcp_free_list) != NULL) {
                        tsp->tcp_free_list = tcp->tcp_time_wait_next;
                        tcp->tcp_time_wait_next = NULL;
                        tsp->tcp_free_list_cnt--;
                        ASSERT(tcp->tcp_tcps == NULL);
                        CONN_DEC_REF(tcp->tcp_connp);
                }
                ASSERT(tsp->tcp_free_list_cnt == 0);
        }

        /*
         * If there are no connections pending, clear timer-related state to be
         * reinitialized by the next caller.
         */
        if (tsp->tcp_time_wait_cnt == 0) {
                tsp->tcp_time_wait_offset = 0;
                tsp->tcp_time_wait_schedule = 0;
                tsp->tcp_time_wait_tid = 0;
                tsp->tcp_time_wait_collector_active = B_FALSE;
                mutex_exit(&tsp->tcp_time_wait_lock);
                return;
        }

retry:
        /*
         * Grab the bucket which we were scheduled to cleanse.
         */
        idx = TW_BUCKET(sched_cur - 1);
        now = ddi_get_lbolt64() - tsp->tcp_time_wait_offset;
        tcp = tsp->tcp_time_wait_bucket[idx];

        while (tcp != NULL) {
                /*
                 * Since the bucket count is sized to prevent wrap-around
                 * during typical operation and timers are schedule to process
                 * buckets with only expired connections, there is only one
                 * reason to encounter a connection expiring in the future:
                 * The tcp_time_wait_collector thread has been so delayed in
                 * its processing that connections have wrapped around the
                 * timing wheel into this bucket.
                 *
                 * In that case, the remaining entires in the bucket can be
                 * ignored since, being appended sequentially, they should all
                 * expire in the future.
                 */
                if (now < tcp->tcp_time_wait_expire) {
                        break;
                }

                /*
                 * Pull the connection out of the bucket.
                 */
                VERIFY(tcp_time_wait_remove(tcp, tsp));

                /*
                 * Purge the connection.
                 *
                 * While tcp_time_wait_lock will be temporarily dropped as part
                 * of the process, there is no risk of the timer being
                 * (re)scheduled while the collector is running since a value
                 * corresponding to the past is left in tcp_time_wait_schedule.
                 */
                tcp_time_wait_purge(tcp, tsp);

                /*
                 * Because tcp_time_wait_remove clears the tcp_time_wait_next
                 * field, the next item must be grabbed directly from the
                 * bucket itself.
                 */
                tcp = tsp->tcp_time_wait_bucket[idx];
        }

        if (tsp->tcp_time_wait_cnt == 0) {
                /*
                 * There is not a need for the collector to schedule a new
                 * timer if no pending items remain.  The timer state can be
                 * cleared only if it was untouched while the collector dropped
                 * its locks during tcp_time_wait_purge.
                 */
                if (tsp->tcp_time_wait_schedule == sched_active) {
                        tsp->tcp_time_wait_offset = 0;
                        tsp->tcp_time_wait_schedule = 0;
                        tsp->tcp_time_wait_tid = 0;
                }
                tsp->tcp_time_wait_collector_active = B_FALSE;
                mutex_exit(&tsp->tcp_time_wait_lock);
                return;
        } else {
                unsigned int nidx;

                /*
                 * Locate the next bucket containing entries.
                 */
                sched_new = sched_cur + MSEC_TO_TICK(TCP_TIME_WAIT_DELAY);
                nidx = TW_BUCKET_NEXT(idx);
                while (tsp->tcp_time_wait_bucket[nidx] == NULL) {
                        if (nidx == idx) {
                                break;
                        }
                        nidx = TW_BUCKET_NEXT(nidx);
                        sched_new += MSEC_TO_TICK(TCP_TIME_WAIT_DELAY);
                }
                ASSERT(tsp->tcp_time_wait_bucket[nidx] != NULL);
        }

        /*
         * It is possible that the system is under such dire load that between
         * the timer scheduling and TIME_WAIT processing delay, execution
         * overran the interval allocated to this bucket.
         */
        now = ddi_get_lbolt64() - tsp->tcp_time_wait_offset;
        if (sched_new <= now) {
                /*
                 * Attempt to right the situation by immediately performing a
                 * purge on the next bucket.  This loop will continue as needed
                 * until the schedule can be pushed out ahead of the clock.
                 */
                sched_cur = sched_new;
                DTRACE_PROBE3(tcp__time__wait__overrun,
                    tcp_squeue_priv_t *, tsp, int64_t, sched_new, int64_t, now);
                goto retry;
        }

        /*
         * Another thread may have snuck in to reschedule the timer while locks
         * were dropped during tcp_time_wait_purge.  Defer to the running timer
         * if that is the case.
         */
        if (tsp->tcp_time_wait_schedule != sched_active) {
                tsp->tcp_time_wait_collector_active = B_FALSE;
                mutex_exit(&tsp->tcp_time_wait_lock);
                return;
        }

        /*
         * Schedule the next timer.
         */
        tsp->tcp_time_wait_schedule = sched_new;
        tsp->tcp_time_wait_tid =
            timeout_generic(CALLOUT_NORMAL,
            tcp_time_wait_collector, sqp,
            TICK_TO_NSEC(sched_new - now),
            CALLOUT_TCP_RESOLUTION, CALLOUT_FLAG_ROUNDUP);
        tsp->tcp_time_wait_collector_active = B_FALSE;
        mutex_exit(&tsp->tcp_time_wait_lock);
}

/*
 * tcp_time_wait_processing() handles processing of incoming packets when
 * the tcp_t is in the TIME_WAIT state.
 *
 * A TIME_WAIT tcp_t that has an associated open TCP end point (not in
 * detached state) is never put on the time wait list.
 */
void
tcp_time_wait_processing(tcp_t *tcp, mblk_t *mp, uint32_t seg_seq,
    uint32_t seg_ack, int seg_len, tcpha_t *tcpha, ip_recv_attr_t *ira)
{
        int32_t         bytes_acked;
        int32_t         gap;
        int32_t         rgap;
        tcp_opt_t       tcpopt;
        uint_t          flags;
        uint32_t        new_swnd = 0;
        conn_t          *nconnp;
        conn_t          *connp = tcp->tcp_connp;
        tcp_stack_t     *tcps = tcp->tcp_tcps;

        BUMP_LOCAL(tcp->tcp_ibsegs);
        DTRACE_PROBE2(tcp__trace__recv, mblk_t *, mp, tcp_t *, tcp);

        flags = (unsigned int)tcpha->tha_flags & 0xFF;
        new_swnd = ntohs(tcpha->tha_win) <<
            ((tcpha->tha_flags & TH_SYN) ? 0 : tcp->tcp_snd_ws);

        if (tcp->tcp_snd_ts_ok && !(tcpha->tha_flags & TH_RST)) {
                int options;
                if (tcp->tcp_snd_sack_ok)
                        tcpopt.tcp = tcp;
                else
                        tcpopt.tcp = NULL;
                options = tcp_parse_options(tcpha, &tcpopt);
                if (!(options & TCP_OPT_TSTAMP_PRESENT)) {
                        DTRACE_TCP1(droppedtimestamp, tcp_t *, tcp);
                        goto done;
                } else if (!tcp_paws_check(tcp, &tcpopt)) {
                        tcp_xmit_ctl(NULL, tcp, tcp->tcp_snxt, tcp->tcp_rnxt,
                            TH_ACK);
                        goto done;
                }
        }
        gap = seg_seq - tcp->tcp_rnxt;
        rgap = tcp->tcp_rwnd - (gap + seg_len);
        if (gap < 0) {
                TCPS_BUMP_MIB(tcps, tcpInDataDupSegs);
                TCPS_UPDATE_MIB(tcps, tcpInDataDupBytes,
                    (seg_len > -gap ? -gap : seg_len));
                seg_len += gap;
                if (seg_len < 0 || (seg_len == 0 && !(flags & TH_FIN))) {
                        if (flags & TH_RST) {
                                goto done;
                        }
                        if ((flags & TH_FIN) && seg_len == -1) {
                                /*
                                 * When TCP receives a duplicate FIN in
                                 * TIME_WAIT state, restart the 2 MSL timer.
                                 * See page 73 in RFC 793. Make sure this TCP
                                 * is already on the TIME_WAIT list. If not,
                                 * just restart the timer.
                                 */
                                if (TCP_IS_DETACHED(tcp)) {
                                        if (tcp_time_wait_remove(tcp, NULL) ==
                                            B_TRUE) {
                                                tcp_time_wait_append(tcp);
                                                TCP_DBGSTAT(tcps,
                                                    tcp_rput_time_wait);
                                        }
                                } else {
                                        ASSERT(tcp != NULL);
                                        TCP_TIMER_RESTART(tcp,
                                            tcps->tcps_time_wait_interval);
                                }
                                tcp_xmit_ctl(NULL, tcp, tcp->tcp_snxt,
                                    tcp->tcp_rnxt, TH_ACK);
                                goto done;
                        }
                        flags |=  TH_ACK_NEEDED;
                        seg_len = 0;
                        goto process_ack;
                }

                /* Fix seg_seq, and chew the gap off the front. */
                seg_seq = tcp->tcp_rnxt;
        }

        if ((flags & TH_SYN) && gap > 0 && rgap < 0) {
                /*
                 * Make sure that when we accept the connection, pick
                 * an ISS greater than (tcp_snxt + tcp_iss_incr/2) for the
                 * old connection.
                 *
                 * The next ISS generated is equal to tcp_iss_incr_extra
                 * + tcp_iss_incr/2 + other components depending on the
                 * value of tcp_strong_iss.  We pre-calculate the new
                 * ISS here and compare with tcp_snxt to determine if
                 * we need to make adjustment to tcp_iss_incr_extra.
                 *
                 * The above calculation is ugly and is a
                 * waste of CPU cycles...
                 */
                uint32_t new_iss = tcps->tcps_iss_incr_extra;
                int32_t adj;
                ip_stack_t *ipst = tcps->tcps_netstack->netstack_ip;

                switch (tcps->tcps_strong_iss) {
                case 2: {
                        /* Add time and MD5 components. */
                        uint32_t answer[4];
                        struct {
                                uint32_t ports;
                                in6_addr_t src;
                                in6_addr_t dst;
                        } arg;
                        MD5_CTX context;

                        mutex_enter(&tcps->tcps_iss_key_lock);
                        context = tcps->tcps_iss_key;
                        mutex_exit(&tcps->tcps_iss_key_lock);
                        arg.ports = connp->conn_ports;
                        /* We use MAPPED addresses in tcp_iss_init */
                        arg.src = connp->conn_laddr_v6;
                        arg.dst = connp->conn_faddr_v6;
                        MD5Update(&context, (uchar_t *)&arg,
                            sizeof (arg));
                        MD5Final((uchar_t *)answer, &context);
                        answer[0] ^= answer[1] ^ answer[2] ^ answer[3];
                        new_iss += (gethrtime() >> ISS_NSEC_SHT) + answer[0];
                        break;
                }
                case 1:
                        /* Add time component and min random (i.e. 1). */
                        new_iss += (gethrtime() >> ISS_NSEC_SHT) + 1;
                        break;
                default:
                        /* Add only time component. */
                        new_iss += (uint32_t)gethrestime_sec() *
                            tcps->tcps_iss_incr;
                        break;
                }
                if ((adj = (int32_t)(tcp->tcp_snxt - new_iss)) > 0) {
                        /*
                         * New ISS not guaranteed to be tcp_iss_incr/2
                         * ahead of the current tcp_snxt, so add the
                         * difference to tcp_iss_incr_extra.
                         */
                        tcps->tcps_iss_incr_extra += adj;
                }
                /*
                 * If tcp_clean_death() can not perform the task now,
                 * drop the SYN packet and let the other side re-xmit.
                 * Otherwise pass the SYN packet back in, since the
                 * old tcp state has been cleaned up or freed.
                 */
                if (tcp_clean_death(tcp, 0) == -1)
                        goto done;
                nconnp = ipcl_classify(mp, ira, ipst);
                if (nconnp != NULL) {
                        TCP_STAT(tcps, tcp_time_wait_syn_success);
                        /* Drops ref on nconnp */
                        tcp_reinput(nconnp, mp, ira, ipst);
                        return;
                }
                goto done;
        }

        /*
         * rgap is the amount of stuff received out of window.  A negative
         * value is the amount out of window.
         */
        if (rgap < 0) {
                TCPS_BUMP_MIB(tcps, tcpInDataPastWinSegs);
                TCPS_UPDATE_MIB(tcps, tcpInDataPastWinBytes, -rgap);
                /* Fix seg_len and make sure there is something left. */
                seg_len += rgap;
                if (seg_len <= 0) {
                        if (flags & TH_RST) {
                                goto done;
                        }
                        flags |=  TH_ACK_NEEDED;
                        seg_len = 0;
                        goto process_ack;
                }
        }
        /*
         * Check whether we can update tcp_ts_recent. This test is from RFC
         * 7323, section 5.3.
         */
        if (tcp->tcp_snd_ts_ok && !(flags & TH_RST) &&
            TSTMP_GEQ(tcpopt.tcp_opt_ts_val, tcp->tcp_ts_recent) &&
            SEQ_LEQ(seg_seq, tcp->tcp_rack)) {
                tcp->tcp_ts_recent = tcpopt.tcp_opt_ts_val;
                tcp->tcp_last_rcv_lbolt = ddi_get_lbolt64();
        }

        if (seg_seq != tcp->tcp_rnxt && seg_len > 0) {
                /* Always ack out of order packets */
                flags |= TH_ACK_NEEDED;
                seg_len = 0;
        } else if (seg_len > 0) {
                TCPS_BUMP_MIB(tcps, tcpInClosed);
                TCPS_BUMP_MIB(tcps, tcpInDataInorderSegs);
                TCPS_UPDATE_MIB(tcps, tcpInDataInorderBytes, seg_len);
        }
        if (flags & TH_RST) {
                (void) tcp_clean_death(tcp, 0);
                goto done;
        }
        if (flags & TH_SYN) {
                tcp_xmit_ctl("TH_SYN", tcp, seg_ack, seg_seq + 1,
                    TH_RST|TH_ACK);
                /*
                 * Do not delete the TCP structure if it is in
                 * TIME_WAIT state.  Refer to RFC 1122, 4.2.2.13.
                 */
                goto done;
        }
process_ack:
        if (flags & TH_ACK) {
                bytes_acked = (int)(seg_ack - tcp->tcp_suna);
                if (bytes_acked <= 0) {
                        if (bytes_acked == 0 && seg_len == 0 &&
                            new_swnd == tcp->tcp_swnd)
                                TCPS_BUMP_MIB(tcps, tcpInDupAck);
                } else {
                        /* Acks something not sent */
                        flags |= TH_ACK_NEEDED;
                }
        }
        if (flags & TH_ACK_NEEDED) {
                /*
                 * Time to send an ack for some reason.
                 */
                tcp_xmit_ctl(NULL, tcp, tcp->tcp_snxt,
                    tcp->tcp_rnxt, TH_ACK);
        }
done:
        freemsg(mp);
}