xnu-7195.101.1.tar.gz

[apple/xnu.git] / bsd / netinet / mptcp.c

/*
 * Copyright (c) 2012-2018 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 *
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */

/*
 * A note on the MPTCP/NECP-interactions:
 *
 * MPTCP uses NECP-callbacks to get notified of interface/policy events.
 * MPTCP registers to these events at the MPTCP-layer for interface-events
 * through a call to necp_client_register_multipath_cb.
 * To get per-flow events (aka per TCP-subflow), we register to it with
 * necp_client_register_socket_flow. Both registrations happen by using the
 * necp-client-uuid that comes from the app.
 *
 * The locking is rather tricky. In general, we expect the lock-ordering to
 * happen from necp-fd -> necp->client -> mpp_lock.
 *
 * There are however some subtleties.
 *
 * 1. When registering the multipath_cb, we are holding the mpp_lock. This is
 * safe, because it is the very first time this MPTCP-connection goes into NECP.
 * As we go into NECP we take the NECP-locks and thus are guaranteed that no
 * NECP-locks will deadlock us. Because these NECP-events will also first take
 * the NECP-locks. Either they win the race and thus won't find our
 * MPTCP-connection. Or, MPTCP wins the race and thus it will safely install
 * the callbacks while holding the NECP lock.
 *
 * 2. When registering the subflow-callbacks we must unlock the mpp_lock. This,
 * because we have already registered callbacks and we might race against an
 * NECP-event that will match on our socket. So, we have to unlock to be safe.
 *
 * 3. When removing the multipath_cb, we do it in mp_pcbdispose(). The
 * so_usecount has reached 0. We must be careful to not remove the mpp_socket
 * pointers before we unregistered the callback. Because, again we might be
 * racing against an NECP-event. Unregistering must happen with an unlocked
 * mpp_lock, because of the lock-ordering constraint. It could be that
 * before we had a chance to unregister an NECP-event triggers. That's why
 * we need to check for the so_usecount in mptcp_session_necp_cb. If we get
 * there while the socket is being garbage-collected, the use-count will go
 * down to 0 and we exit. Removal of the multipath_cb again happens by taking
 * the NECP-locks so any running NECP-events will finish first and exit cleanly.
 *
 * 4. When removing the subflow-callback, we do it in in_pcbdispose(). Again,
 * the socket-lock must be unlocked for lock-ordering constraints. This gets a
 * bit tricky here, as in tcp_garbage_collect we hold the mp_so and so lock.
 * So, we drop the mp_so-lock as soon as the subflow is unlinked with
 * mptcp_subflow_del. Then, in in_pcbdispose we drop the subflow-lock.
 * If an NECP-event was waiting on the lock in mptcp_subflow_necp_cb, when it
 * gets it, it will realize that the subflow became non-MPTCP and retry (see
 * tcp_lock). Then it waits again on the subflow-lock. When we drop this lock
 * in in_pcbdispose, and enter necp_inpcb_dispose, this one will have to wait
 * for the NECP-lock (held by the other thread that is taking care of the NECP-
 * event). So, the event now finally gets the subflow-lock and then hits an
 * so_usecount that is 0 and exits. Eventually, we can remove the subflow from
 * the NECP callback.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/mbuf.h>
#include <sys/mcache.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/syslog.h>
#include <sys/protosw.h>

#include <kern/zalloc.h>
#include <kern/locks.h>

#include <mach/sdt.h>

#include <net/if.h>
#include <netinet/in.h>
#include <netinet/in_var.h>
#include <netinet/tcp.h>
#include <netinet/tcp_fsm.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_var.h>
#include <netinet/mptcp_var.h>
#include <netinet/mptcp.h>
#include <netinet/mptcp_seq.h>
#include <netinet/mptcp_opt.h>
#include <netinet/mptcp_timer.h>

int mptcp_enable = 1;
SYSCTL_INT(_net_inet_mptcp, OID_AUTO, enable, CTLFLAG_RW | CTLFLAG_LOCKED,
    &mptcp_enable, 0, "Enable Multipath TCP Support");

/*
 * Number of times to try negotiating MPTCP on SYN retransmissions.
 * We haven't seen any reports of a middlebox that is dropping all SYN-segments
 * that have an MPTCP-option. Thus, let's be generous and retransmit it 4 times.
 */
int mptcp_mpcap_retries = 4;
SYSCTL_INT(_net_inet_mptcp, OID_AUTO, mptcp_cap_retr,
    CTLFLAG_RW | CTLFLAG_LOCKED,
    &mptcp_mpcap_retries, 0, "Number of MP Capable SYN Retries");

/*
 * By default, DSS checksum is turned off, revisit if we ever do
 * MPTCP for non SSL Traffic.
 */
int mptcp_dss_csum = 0;
SYSCTL_INT(_net_inet_mptcp, OID_AUTO, dss_csum, CTLFLAG_RW | CTLFLAG_LOCKED,
    &mptcp_dss_csum, 0, "Enable DSS checksum");

/*
 * When mptcp_fail_thresh number of retransmissions are sent, subflow failover
 * is attempted on a different path.
 */
int mptcp_fail_thresh = 1;
SYSCTL_INT(_net_inet_mptcp, OID_AUTO, fail, CTLFLAG_RW | CTLFLAG_LOCKED,
    &mptcp_fail_thresh, 0, "Failover threshold");

/*
 * MPTCP subflows have TCP keepalives set to ON. Set a conservative keeptime
 * as carrier networks mostly have a 30 minute to 60 minute NAT Timeout.
 * Some carrier networks have a timeout of 10 or 15 minutes.
 */
int mptcp_subflow_keeptime = 60 * 14;
SYSCTL_INT(_net_inet_mptcp, OID_AUTO, keepalive, CTLFLAG_RW | CTLFLAG_LOCKED,
    &mptcp_subflow_keeptime, 0, "Keepalive in seconds");

int mptcp_rtthist_rtthresh = 600;
SYSCTL_INT(_net_inet_mptcp, OID_AUTO, rtthist_thresh, CTLFLAG_RW | CTLFLAG_LOCKED,
    &mptcp_rtthist_rtthresh, 0, "Rtt threshold");

int mptcp_rtothresh = 1500;
SYSCTL_INT(_net_inet_mptcp, OID_AUTO, rto_thresh, CTLFLAG_RW | CTLFLAG_LOCKED,
    &mptcp_rtothresh, 0, "RTO threshold");

/*
 * Probe the preferred path, when it is not in use
 */
uint32_t mptcp_probeto = 1000;
SYSCTL_UINT(_net_inet_mptcp, OID_AUTO, probeto, CTLFLAG_RW | CTLFLAG_LOCKED,
    &mptcp_probeto, 0, "Disable probing by setting to 0");

uint32_t mptcp_probecnt = 5;
SYSCTL_UINT(_net_inet_mptcp, OID_AUTO, probecnt, CTLFLAG_RW | CTLFLAG_LOCKED,
    &mptcp_probecnt, 0, "Number of probe writes");

static int
mptcp_reass_present(struct socket *mp_so)
{
        struct mptses *mpte = mpsotompte(mp_so);
        struct mptcb *mp_tp = mpte->mpte_mptcb;
        struct tseg_qent *q;
        int dowakeup = 0;
        int flags = 0;

        /*
         * Present data to user, advancing rcv_nxt through
         * completed sequence space.
         */
        if (mp_tp->mpt_state < MPTCPS_ESTABLISHED) {
                return flags;
        }
        q = LIST_FIRST(&mp_tp->mpt_segq);
        if (!q || q->tqe_m->m_pkthdr.mp_dsn != mp_tp->mpt_rcvnxt) {
                return flags;
        }

        /*
         * If there is already another thread doing reassembly for this
         * connection, it is better to let it finish the job --
         * (radar 16316196)
         */
        if (mp_tp->mpt_flags & MPTCPF_REASS_INPROG) {
                return flags;
        }

        mp_tp->mpt_flags |= MPTCPF_REASS_INPROG;

        do {
                mp_tp->mpt_rcvnxt += q->tqe_len;
                LIST_REMOVE(q, tqe_q);
                if (mp_so->so_state & SS_CANTRCVMORE) {
                        m_freem(q->tqe_m);
                } else {
                        flags = !!(q->tqe_m->m_pkthdr.pkt_flags & PKTF_MPTCP_DFIN);
                        if (sbappendstream_rcvdemux(mp_so, q->tqe_m)) {
                                dowakeup = 1;
                        }
                }
                zfree(tcp_reass_zone, q);
                mp_tp->mpt_reassqlen--;
                q = LIST_FIRST(&mp_tp->mpt_segq);
        } while (q && q->tqe_m->m_pkthdr.mp_dsn == mp_tp->mpt_rcvnxt);
        mp_tp->mpt_flags &= ~MPTCPF_REASS_INPROG;

        if (dowakeup) {
                sorwakeup(mp_so); /* done with socket lock held */
        }
        return flags;
}

static int
mptcp_reass(struct socket *mp_so, struct pkthdr *phdr, int *tlenp, struct mbuf *m)
{
        struct mptcb *mp_tp = mpsotomppcb(mp_so)->mpp_pcbe->mpte_mptcb;
        u_int64_t mb_dsn = phdr->mp_dsn;
        struct tseg_qent *q;
        struct tseg_qent *p = NULL;
        struct tseg_qent *nq;
        struct tseg_qent *te = NULL;
        uint32_t qlimit;

        /*
         * Limit the number of segments in the reassembly queue to prevent
         * holding on to too many segments (and thus running out of mbufs).
         * Make sure to let the missing segment through which caused this
         * queue.  Always keep one global queue entry spare to be able to
         * process the missing segment.
         */
        qlimit = MIN(MAX(100, mp_so->so_rcv.sb_hiwat >> 10),
            (tcp_autorcvbuf_max >> 10));
        if (mb_dsn != mp_tp->mpt_rcvnxt &&
            (mp_tp->mpt_reassqlen + 1) >= qlimit) {
                tcpstat.tcps_mptcp_rcvmemdrop++;
                m_freem(m);
                *tlenp = 0;
                return 0;
        }

        /* Allocate a new queue entry. If we can't, just drop the pkt. XXX */
        te = (struct tseg_qent *) zalloc(tcp_reass_zone);
        if (te == NULL) {
                tcpstat.tcps_mptcp_rcvmemdrop++;
                m_freem(m);
                return 0;
        }

        mp_tp->mpt_reassqlen++;

        /*
         * Find a segment which begins after this one does.
         */
        LIST_FOREACH(q, &mp_tp->mpt_segq, tqe_q) {
                if (MPTCP_SEQ_GT(q->tqe_m->m_pkthdr.mp_dsn, mb_dsn)) {
                        break;
                }
                p = q;
        }

        /*
         * If there is a preceding segment, it may provide some of
         * our data already.  If so, drop the data from the incoming
         * segment.  If it provides all of our data, drop us.
         */
        if (p != NULL) {
                int64_t i;
                /* conversion to int (in i) handles seq wraparound */
                i = p->tqe_m->m_pkthdr.mp_dsn + p->tqe_len - mb_dsn;
                if (i > 0) {
                        if (i >= *tlenp) {
                                tcpstat.tcps_mptcp_rcvduppack++;
                                m_freem(m);
                                zfree(tcp_reass_zone, te);
                                te = NULL;
                                mp_tp->mpt_reassqlen--;
                                /*
                                 * Try to present any queued data
                                 * at the left window edge to the user.
                                 * This is needed after the 3-WHS
                                 * completes.
                                 */
                                goto out;
                        }
                        VERIFY(i <= INT_MAX);
                        m_adj(m, (int)i);
                        *tlenp -= i;
                        phdr->mp_dsn += i;
                }
        }

        tcpstat.tcps_mp_oodata++;

        /*
         * While we overlap succeeding segments trim them or,
         * if they are completely covered, dequeue them.
         */
        while (q) {
                int64_t i = (mb_dsn + *tlenp) - q->tqe_m->m_pkthdr.mp_dsn;
                if (i <= 0) {
                        break;
                }

                if (i < q->tqe_len) {
                        q->tqe_m->m_pkthdr.mp_dsn += i;
                        q->tqe_len -= i;

                        VERIFY(i <= INT_MAX);
                        m_adj(q->tqe_m, (int)i);
                        break;
                }

                nq = LIST_NEXT(q, tqe_q);
                LIST_REMOVE(q, tqe_q);
                m_freem(q->tqe_m);
                zfree(tcp_reass_zone, q);
                mp_tp->mpt_reassqlen--;
                q = nq;
        }

        /* Insert the new segment queue entry into place. */
        te->tqe_m = m;
        te->tqe_th = NULL;
        te->tqe_len = *tlenp;

        if (p == NULL) {
                LIST_INSERT_HEAD(&mp_tp->mpt_segq, te, tqe_q);
        } else {
                LIST_INSERT_AFTER(p, te, tqe_q);
        }

out:
        return mptcp_reass_present(mp_so);
}

/*
 * MPTCP input, called when data has been read from a subflow socket.
 */
void
mptcp_input(struct mptses *mpte, struct mbuf *m)
{
        struct socket *mp_so;
        struct mptcb *mp_tp = NULL;
        int count = 0, wakeup = 0;
        struct mbuf *save = NULL, *prev = NULL;
        struct mbuf *freelist = NULL, *tail = NULL;

        VERIFY(m->m_flags & M_PKTHDR);

        mp_so = mptetoso(mpte);
        mp_tp = mpte->mpte_mptcb;

        socket_lock_assert_owned(mp_so);

        DTRACE_MPTCP(input);

        mp_tp->mpt_rcvwnd = mptcp_sbspace(mp_tp);

        /*
         * Each mbuf contains MPTCP Data Sequence Map
         * Process the data for reassembly, delivery to MPTCP socket
         * client, etc.
         *
         */
        count = mp_so->so_rcv.sb_cc;

        /*
         * In the degraded fallback case, data is accepted without DSS map
         */
        if (mp_tp->mpt_flags & MPTCPF_FALLBACK_TO_TCP) {
                struct mbuf *iter;
                int mb_dfin = 0;
fallback:
                mptcp_sbrcv_grow(mp_tp);

                iter = m;
                while (iter) {
                        if ((iter->m_flags & M_PKTHDR) &&
                            (iter->m_pkthdr.pkt_flags & PKTF_MPTCP_DFIN)) {
                                mb_dfin = 1;
                        }

                        if ((iter->m_flags & M_PKTHDR) && m_pktlen(iter) == 0) {
                                /* Don't add zero-length packets, so jump it! */
                                if (prev == NULL) {
                                        m = iter->m_next;
                                        m_free(iter);
                                        iter = m;
                                } else {
                                        prev->m_next = iter->m_next;
                                        m_free(iter);
                                        iter = prev->m_next;
                                }

                                /* It was a zero-length packet so next one must be a pkthdr */
                                VERIFY(iter == NULL || iter->m_flags & M_PKTHDR);
                        } else {
                                prev = iter;
                                iter = iter->m_next;
                        }
                }

                /*
                 * assume degraded flow as this may be the first packet
                 * without DSS, and the subflow state is not updated yet.
                 */
                if (sbappendstream_rcvdemux(mp_so, m)) {
                        sorwakeup(mp_so);
                }

                DTRACE_MPTCP5(receive__degraded, struct mbuf *, m,
                    struct socket *, mp_so,
                    struct sockbuf *, &mp_so->so_rcv,
                    struct sockbuf *, &mp_so->so_snd,
                    struct mptses *, mpte);
                count = mp_so->so_rcv.sb_cc - count;

                mp_tp->mpt_rcvnxt += count;

                if (mb_dfin) {
                        mptcp_close_fsm(mp_tp, MPCE_RECV_DATA_FIN);
                        socantrcvmore(mp_so);
                }
                return;
        }

        do {
                u_int64_t mb_dsn;
                int32_t mb_datalen;
                int64_t todrop;
                int mb_dfin = 0;

                VERIFY(m->m_flags & M_PKTHDR);

                /* If fallback occurs, mbufs will not have PKTF_MPTCP set */
                if (!(m->m_pkthdr.pkt_flags & PKTF_MPTCP)) {
                        goto fallback;
                }

                save = m->m_next;
                /*
                 * A single TCP packet formed of multiple mbufs
                 * holds DSS mapping in the first mbuf of the chain.
                 * Other mbufs in the chain may have M_PKTHDR set
                 * even though they belong to the same TCP packet
                 * and therefore use the DSS mapping stored in the
                 * first mbuf of the mbuf chain. mptcp_input() can
                 * get an mbuf chain with multiple TCP packets.
                 */
                while (save && (!(save->m_flags & M_PKTHDR) ||
                    !(save->m_pkthdr.pkt_flags & PKTF_MPTCP))) {
                        prev = save;
                        save = save->m_next;
                }
                if (prev) {
                        prev->m_next = NULL;
                } else {
                        m->m_next = NULL;
                }

                mb_dsn = m->m_pkthdr.mp_dsn;
                mb_datalen = m->m_pkthdr.mp_rlen;

                todrop = (mb_dsn + mb_datalen) - (mp_tp->mpt_rcvnxt + mp_tp->mpt_rcvwnd);
                if (todrop > 0) {
                        tcpstat.tcps_mptcp_rcvpackafterwin++;

                        os_log_info(mptcp_log_handle, "%s - %lx: dropping dsn %u dlen %u rcvnxt %u rcvwnd %u todrop %lld\n",
                            __func__, (unsigned long)VM_KERNEL_ADDRPERM(mpte),
                            (uint32_t)mb_dsn, mb_datalen, (uint32_t)mp_tp->mpt_rcvnxt,
                            mp_tp->mpt_rcvwnd, todrop);

                        if (todrop >= mb_datalen) {
                                if (freelist == NULL) {
                                        freelist = m;
                                } else {
                                        tail->m_next = m;
                                }

                                if (prev != NULL) {
                                        tail = prev;
                                } else {
                                        tail = m;
                                }

                                m = save;
                                prev = save = NULL;
                                continue;
                        } else {
                                VERIFY(todrop <= INT_MAX);
                                m_adj(m, (int)-todrop);
                                mb_datalen -= todrop;
                                m->m_pkthdr.mp_rlen -= todrop;
                        }

                        /*
                         * We drop from the right edge of the mbuf, thus the
                         * DATA_FIN is dropped as well
                         */
                        m->m_pkthdr.pkt_flags &= ~PKTF_MPTCP_DFIN;
                }

                if (MPTCP_SEQ_LT(mb_dsn, mp_tp->mpt_rcvnxt)) {
                        if (MPTCP_SEQ_LEQ((mb_dsn + mb_datalen),
                            mp_tp->mpt_rcvnxt)) {
                                if (freelist == NULL) {
                                        freelist = m;
                                } else {
                                        tail->m_next = m;
                                }

                                if (prev != NULL) {
                                        tail = prev;
                                } else {
                                        tail = m;
                                }

                                m = save;
                                prev = save = NULL;
                                continue;
                        } else {
                                VERIFY((mp_tp->mpt_rcvnxt - mb_dsn) <= INT_MAX);
                                m_adj(m, (int)(mp_tp->mpt_rcvnxt - mb_dsn));
                                mb_datalen -= (mp_tp->mpt_rcvnxt - mb_dsn);
                                mb_dsn = mp_tp->mpt_rcvnxt;
                                VERIFY(mb_datalen >= 0 && mb_datalen <= USHRT_MAX);
                                m->m_pkthdr.mp_rlen = (uint16_t)mb_datalen;
                                m->m_pkthdr.mp_dsn = mb_dsn;
                        }
                }

                if (MPTCP_SEQ_GT(mb_dsn, mp_tp->mpt_rcvnxt) ||
                    !LIST_EMPTY(&mp_tp->mpt_segq)) {
                        mb_dfin = mptcp_reass(mp_so, &m->m_pkthdr, &mb_datalen, m);

                        goto next;
                }
                mb_dfin = !!(m->m_pkthdr.pkt_flags & PKTF_MPTCP_DFIN);

                mptcp_sbrcv_grow(mp_tp);

                if (sbappendstream_rcvdemux(mp_so, m)) {
                        wakeup = 1;
                }

                DTRACE_MPTCP6(receive, struct mbuf *, m, struct socket *, mp_so,
                    struct sockbuf *, &mp_so->so_rcv,
                    struct sockbuf *, &mp_so->so_snd,
                    struct mptses *, mpte,
                    struct mptcb *, mp_tp);
                count = mp_so->so_rcv.sb_cc - count;
                tcpstat.tcps_mp_rcvtotal++;
                tcpstat.tcps_mp_rcvbytes += count;

                mp_tp->mpt_rcvnxt += count;

next:
                if (mb_dfin) {
                        mptcp_close_fsm(mp_tp, MPCE_RECV_DATA_FIN);
                        socantrcvmore(mp_so);
                }
                m = save;
                prev = save = NULL;
                count = mp_so->so_rcv.sb_cc;
        } while (m);

        if (freelist) {
                m_freem(freelist);
        }

        if (wakeup) {
                sorwakeup(mp_so);
        }
}

boolean_t
mptcp_can_send_more(struct mptcb *mp_tp, boolean_t ignore_reinject)
{
        struct socket *mp_so = mptetoso(mp_tp->mpt_mpte);

        /*
         * Always send if there is data in the reinject-queue.
         */
        if (!ignore_reinject && mp_tp->mpt_mpte->mpte_reinjectq) {
                return TRUE;
        }

        /*
         * Don't send, if:
         *
         * 1. snd_nxt >= snd_max : Means, basically everything has been sent.
         *    Except when using TFO, we might be doing a 0-byte write.
         * 2. snd_una + snd_wnd <= snd_nxt: No space in the receiver's window
         * 3. snd_nxt + 1 == snd_max and we are closing: A DATA_FIN is scheduled.
         */

        if (!(mp_so->so_flags1 & SOF1_PRECONNECT_DATA) && MPTCP_SEQ_GEQ(mp_tp->mpt_sndnxt, mp_tp->mpt_sndmax)) {
                return FALSE;
        }

        if (MPTCP_SEQ_LEQ(mp_tp->mpt_snduna + mp_tp->mpt_sndwnd, mp_tp->mpt_sndnxt)) {
                return FALSE;
        }

        if (mp_tp->mpt_sndnxt + 1 == mp_tp->mpt_sndmax && mp_tp->mpt_state > MPTCPS_CLOSE_WAIT) {
                return FALSE;
        }

        if (mp_tp->mpt_state >= MPTCPS_FIN_WAIT_2) {
                return FALSE;
        }

        return TRUE;
}

/*
 * MPTCP output.
 */
int
mptcp_output(struct mptses *mpte)
{
        struct mptcb *mp_tp;
        struct mptsub *mpts;
        struct mptsub *mpts_tried = NULL;
        struct socket *mp_so;
        struct mptsub *preferred_mpts = NULL;
        uint64_t old_snd_nxt;
        int error = 0;

        mp_so = mptetoso(mpte);
        mp_tp = mpte->mpte_mptcb;

        socket_lock_assert_owned(mp_so);

        if (mp_so->so_flags & SOF_DEFUNCT) {
                return 0;
        }

        VERIFY(!(mpte->mpte_mppcb->mpp_flags & MPP_WUPCALL));
        mpte->mpte_mppcb->mpp_flags |= MPP_WUPCALL;

        old_snd_nxt = mp_tp->mpt_sndnxt;
        while (mptcp_can_send_more(mp_tp, FALSE)) {
                /* get the "best" subflow to be used for transmission */
                mpts = mptcp_get_subflow(mpte, &preferred_mpts);
                if (mpts == NULL) {
                        mptcplog((LOG_INFO, "%s: no subflow\n", __func__),
                            MPTCP_SENDER_DBG, MPTCP_LOGLVL_LOG);
                        break;
                }

                /* In case there's just one flow, we reattempt later */
                if (mpts_tried != NULL &&
                    (mpts == mpts_tried || (mpts->mpts_flags & MPTSF_FAILINGOVER))) {
                        mpts_tried->mpts_flags &= ~MPTSF_FAILINGOVER;
                        mpts_tried->mpts_flags |= MPTSF_ACTIVE;
                        mptcp_start_timer(mpte, MPTT_REXMT);
                        break;
                }

                /*
                 * Automatic sizing of send socket buffer. Increase the send
                 * socket buffer size if all of the following criteria are met
                 *      1. the receiver has enough buffer space for this data
                 *      2. send buffer is filled to 7/8th with data (so we actually
                 *         have data to make use of it);
                 */
                if ((mp_so->so_snd.sb_flags & (SB_AUTOSIZE | SB_TRIM)) == SB_AUTOSIZE &&
                    tcp_cansbgrow(&mp_so->so_snd)) {
                        if ((mp_tp->mpt_sndwnd / 4 * 5) >= mp_so->so_snd.sb_hiwat &&
                            mp_so->so_snd.sb_cc >= (mp_so->so_snd.sb_hiwat / 8 * 7)) {
                                if (sbreserve(&mp_so->so_snd,
                                    min(mp_so->so_snd.sb_hiwat + tcp_autosndbuf_inc,
                                    tcp_autosndbuf_max)) == 1) {
                                        mp_so->so_snd.sb_idealsize = mp_so->so_snd.sb_hiwat;
                                }
                        }
                }

                DTRACE_MPTCP3(output, struct mptses *, mpte, struct mptsub *, mpts,
                    struct socket *, mp_so);
                error = mptcp_subflow_output(mpte, mpts, 0);
                if (error) {
                        /* can be a temporary loss of source address or other error */
                        mpts->mpts_flags |= MPTSF_FAILINGOVER;
                        mpts->mpts_flags &= ~MPTSF_ACTIVE;
                        mpts_tried = mpts;
                        if (error != ECANCELED) {
                                os_log_error(mptcp_log_handle, "%s - %lx: Error = %d mpts_flags %#x\n",
                                    __func__, (unsigned long)VM_KERNEL_ADDRPERM(mpte),
                                    error, mpts->mpts_flags);
                        }
                        break;
                }
                /* The model is to have only one active flow at a time */
                mpts->mpts_flags |= MPTSF_ACTIVE;
                mpts->mpts_probesoon = mpts->mpts_probecnt = 0;

                /* Allows us to update the smoothed rtt */
                if (mptcp_probeto && mpts != preferred_mpts && preferred_mpts != NULL) {
                        if (preferred_mpts->mpts_probesoon) {
                                if ((tcp_now - preferred_mpts->mpts_probesoon) > mptcp_probeto) {
                                        mptcp_subflow_output(mpte, preferred_mpts, MPTCP_SUBOUT_PROBING);
                                        if (preferred_mpts->mpts_probecnt >= mptcp_probecnt) {
                                                preferred_mpts->mpts_probesoon = 0;
                                                preferred_mpts->mpts_probecnt = 0;
                                        }
                                }
                        } else {
                                preferred_mpts->mpts_probesoon = tcp_now;
                                preferred_mpts->mpts_probecnt = 0;
                        }
                }

                if (mpte->mpte_active_sub == NULL) {
                        mpte->mpte_active_sub = mpts;
                } else if (mpte->mpte_active_sub != mpts) {
                        mpte->mpte_active_sub->mpts_flags &= ~MPTSF_ACTIVE;
                        mpte->mpte_active_sub = mpts;

                        mptcpstats_inc_switch(mpte, mpts);
                }
        }

        if (mp_tp->mpt_state > MPTCPS_CLOSE_WAIT) {
                if (mp_tp->mpt_sndnxt + 1 == mp_tp->mpt_sndmax &&
                    mp_tp->mpt_snduna == mp_tp->mpt_sndnxt) {
                        mptcp_finish_usrclosed(mpte);
                }
        }

        mptcp_handle_deferred_upcalls(mpte->mpte_mppcb, MPP_WUPCALL);

        /* subflow errors should not be percolated back up */
        return 0;
}


static struct mptsub *
mptcp_choose_subflow(struct mptsub *mpts, struct mptsub *curbest, int *currtt)
{
        struct tcpcb *tp = sototcpcb(mpts->mpts_socket);

        /*
         * Lower RTT? Take it, if it's our first one, or
         * it doesn't has any loss, or the current one has
         * loss as well.
         */
        if (tp->t_srtt && *currtt > tp->t_srtt &&
            (curbest == NULL || tp->t_rxtshift == 0 ||
            sototcpcb(curbest->mpts_socket)->t_rxtshift)) {
                *currtt = tp->t_srtt;
                return mpts;
        }

        /*
         * If we find a subflow without loss, take it always!
         */
        if (curbest &&
            sototcpcb(curbest->mpts_socket)->t_rxtshift &&
            tp->t_rxtshift == 0) {
                *currtt = tp->t_srtt;
                return mpts;
        }

        return curbest != NULL ? curbest : mpts;
}

static struct mptsub *
mptcp_return_subflow(struct mptsub *mpts)
{
        if (mpts && mptcp_subflow_cwnd_space(mpts->mpts_socket) <= 0) {
                return NULL;
        }

        return mpts;
}

static boolean_t
mptcp_subflow_is_slow(struct mptses *mpte, struct mptsub *mpts)
{
        struct tcpcb *tp = sototcpcb(mpts->mpts_socket);
        int fail_thresh = mptcp_fail_thresh;

        if (mpte->mpte_svctype == MPTCP_SVCTYPE_HANDOVER || mpte->mpte_svctype == MPTCP_SVCTYPE_PURE_HANDOVER) {
                fail_thresh *= 2;
        }

        return tp->t_rxtshift >= fail_thresh &&
               (mptetoso(mpte)->so_snd.sb_cc || mpte->mpte_reinjectq);
}

/*
 * Return the most eligible subflow to be used for sending data.
 */
struct mptsub *
mptcp_get_subflow(struct mptses *mpte, struct mptsub **preferred)
{
        struct tcpcb *besttp, *secondtp;
        struct inpcb *bestinp, *secondinp;
        struct mptsub *mpts;
        struct mptsub *best = NULL;
        struct mptsub *second_best = NULL;
        int exp_rtt = INT_MAX, cheap_rtt = INT_MAX;

        /*
         * First Step:
         * Choose the best subflow for cellular and non-cellular interfaces.
         */

        TAILQ_FOREACH(mpts, &mpte->mpte_subflows, mpts_entry) {
                struct socket *so = mpts->mpts_socket;
                struct tcpcb *tp = sototcpcb(so);
                struct inpcb *inp = sotoinpcb(so);

                mptcplog((LOG_DEBUG, "%s mpts %u mpts_flags %#x, suspended %u sostate %#x tpstate %u cellular %d rtt %u rxtshift %u cheap %u exp %u cwnd %d\n",
                    __func__, mpts->mpts_connid, mpts->mpts_flags,
                    INP_WAIT_FOR_IF_FEEDBACK(inp), so->so_state, tp->t_state,
                    inp->inp_last_outifp ? IFNET_IS_CELLULAR(inp->inp_last_outifp) : -1,
                    tp->t_srtt, tp->t_rxtshift, cheap_rtt, exp_rtt,
                    mptcp_subflow_cwnd_space(so)),
                    MPTCP_SOCKET_DBG, MPTCP_LOGLVL_VERBOSE);

                /*
                 * First, the hard conditions to reject subflows
                 * (e.g., not connected,...)
                 */
                if (inp->inp_last_outifp == NULL) {
                        continue;
                }

                if (INP_WAIT_FOR_IF_FEEDBACK(inp)) {
                        continue;
                }

                /* There can only be one subflow in degraded state */
                if (mpts->mpts_flags & MPTSF_MP_DEGRADED) {
                        best = mpts;
                        break;
                }

                /*
                 * If this subflow is waiting to finally send, do it!
                 */
                if (so->so_flags1 & SOF1_PRECONNECT_DATA) {
                        return mptcp_return_subflow(mpts);
                }

                /*
                 * Only send if the subflow is MP_CAPABLE. The exceptions to
                 * this rule (degraded or TFO) have been taken care of above.
                 */
                if (!(mpts->mpts_flags & MPTSF_MP_CAPABLE)) {
                        continue;
                }

                if ((so->so_state & SS_ISDISCONNECTED) ||
                    !(so->so_state & SS_ISCONNECTED) ||
                    !TCPS_HAVEESTABLISHED(tp->t_state) ||
                    tp->t_state > TCPS_CLOSE_WAIT) {
                        continue;
                }

                /*
                 * Second, the soft conditions to find the subflow with best
                 * conditions for each set (aka cellular vs non-cellular)
                 */
                if (IFNET_IS_CELLULAR(inp->inp_last_outifp)) {
                        second_best = mptcp_choose_subflow(mpts, second_best,
                            &exp_rtt);
                } else {
                        best = mptcp_choose_subflow(mpts, best, &cheap_rtt);
                }
        }

        /*
         * If there is no preferred or backup subflow, and there is no active
         * subflow use the last usable subflow.
         */
        if (best == NULL) {
                return mptcp_return_subflow(second_best);
        }

        if (second_best == NULL) {
                return mptcp_return_subflow(best);
        }

        besttp = sototcpcb(best->mpts_socket);
        bestinp = sotoinpcb(best->mpts_socket);
        secondtp = sototcpcb(second_best->mpts_socket);
        secondinp = sotoinpcb(second_best->mpts_socket);

        if (preferred != NULL) {
                *preferred = mptcp_return_subflow(best);
        }

        /*
         * Second Step: Among best and second_best. Choose the one that is
         * most appropriate for this particular service-type.
         */
        if (mpte->mpte_svctype == MPTCP_SVCTYPE_PURE_HANDOVER) {
                return mptcp_return_subflow(best);
        } else if (mpte->mpte_svctype == MPTCP_SVCTYPE_HANDOVER) {
                /*
                 * Only handover if Symptoms tells us to do so.
                 */
                if (!IFNET_IS_CELLULAR(bestinp->inp_last_outifp) &&
                    mptcp_is_wifi_unusable_for_session(mpte) != 0 && mptcp_subflow_is_slow(mpte, best)) {
                        return mptcp_return_subflow(second_best);
                }

                return mptcp_return_subflow(best);
        } else if (mpte->mpte_svctype == MPTCP_SVCTYPE_INTERACTIVE) {
                int rtt_thresh = mptcp_rtthist_rtthresh << TCP_RTT_SHIFT;
                int rto_thresh = mptcp_rtothresh;

                /* Adjust with symptoms information */
                if (!IFNET_IS_CELLULAR(bestinp->inp_last_outifp) &&
                    mptcp_is_wifi_unusable_for_session(mpte) != 0) {
                        rtt_thresh /= 2;
                        rto_thresh /= 2;
                }

                if (besttp->t_srtt && secondtp->t_srtt &&
                    besttp->t_srtt >= rtt_thresh &&
                    secondtp->t_srtt < rtt_thresh) {
                        tcpstat.tcps_mp_sel_rtt++;
                        mptcplog((LOG_DEBUG, "%s: best cid %d at rtt %d,  second cid %d at rtt %d\n", __func__,
                            best->mpts_connid, besttp->t_srtt >> TCP_RTT_SHIFT,
                            second_best->mpts_connid,
                            secondtp->t_srtt >> TCP_RTT_SHIFT),
                            MPTCP_SENDER_DBG, MPTCP_LOGLVL_LOG);
                        return mptcp_return_subflow(second_best);
                }

                if (mptcp_subflow_is_slow(mpte, best) &&
                    secondtp->t_rxtshift == 0) {
                        return mptcp_return_subflow(second_best);
                }

                /* Compare RTOs, select second_best if best's rto exceeds rtothresh */
                if (besttp->t_rxtcur && secondtp->t_rxtcur &&
                    besttp->t_rxtcur >= rto_thresh &&
                    secondtp->t_rxtcur < rto_thresh) {
                        tcpstat.tcps_mp_sel_rto++;
                        mptcplog((LOG_DEBUG, "%s: best cid %d at rto %d, second cid %d at rto %d\n", __func__,
                            best->mpts_connid, besttp->t_rxtcur,
                            second_best->mpts_connid, secondtp->t_rxtcur),
                            MPTCP_SENDER_DBG, MPTCP_LOGLVL_LOG);

                        return mptcp_return_subflow(second_best);
                }

                /*
                 * None of the above conditions for sending on the secondary
                 * were true. So, let's schedule on the best one, if he still
                 * has some space in the congestion-window.
                 */
                return mptcp_return_subflow(best);
        } else if (mpte->mpte_svctype >= MPTCP_SVCTYPE_AGGREGATE) {
                struct mptsub *tmp;

                /*
                 * We only care about RTT when aggregating
                 */
                if (besttp->t_srtt > secondtp->t_srtt) {
                        tmp = best;
                        best = second_best;
                        besttp = secondtp;
                        bestinp = secondinp;

                        second_best = tmp;
                        secondtp = sototcpcb(second_best->mpts_socket);
                        secondinp = sotoinpcb(second_best->mpts_socket);
                }

                /* Is there still space in the congestion window? */
                if (mptcp_subflow_cwnd_space(bestinp->inp_socket) <= 0) {
                        return mptcp_return_subflow(second_best);
                }

                return mptcp_return_subflow(best);
        } else {
                panic("Unknown service-type configured for MPTCP");
        }

        return NULL;
}

static const char *
mptcp_event_to_str(uint32_t event)
{
        const char *c = "UNDEFINED";
        switch (event) {
        case MPCE_CLOSE:
                c = "MPCE_CLOSE";
                break;
        case MPCE_RECV_DATA_ACK:
                c = "MPCE_RECV_DATA_ACK";
                break;
        case MPCE_RECV_DATA_FIN:
                c = "MPCE_RECV_DATA_FIN";
                break;
        }
        return c;
}

static const char *
mptcp_state_to_str(mptcp_state_t state)
{
        const char *c = "UNDEFINED";
        switch (state) {
        case MPTCPS_CLOSED:
                c = "MPTCPS_CLOSED";
                break;
        case MPTCPS_LISTEN:
                c = "MPTCPS_LISTEN";
                break;
        case MPTCPS_ESTABLISHED:
                c = "MPTCPS_ESTABLISHED";
                break;
        case MPTCPS_CLOSE_WAIT:
                c = "MPTCPS_CLOSE_WAIT";
                break;
        case MPTCPS_FIN_WAIT_1:
                c = "MPTCPS_FIN_WAIT_1";
                break;
        case MPTCPS_CLOSING:
                c = "MPTCPS_CLOSING";
                break;
        case MPTCPS_LAST_ACK:
                c = "MPTCPS_LAST_ACK";
                break;
        case MPTCPS_FIN_WAIT_2:
                c = "MPTCPS_FIN_WAIT_2";
                break;
        case MPTCPS_TIME_WAIT:
                c = "MPTCPS_TIME_WAIT";
                break;
        case MPTCPS_TERMINATE:
                c = "MPTCPS_TERMINATE";
                break;
        }
        return c;
}

void
mptcp_close_fsm(struct mptcb *mp_tp, uint32_t event)
{
        struct socket *mp_so = mptetoso(mp_tp->mpt_mpte);

        socket_lock_assert_owned(mp_so);

        mptcp_state_t old_state = mp_tp->mpt_state;

        DTRACE_MPTCP2(state__change, struct mptcb *, mp_tp,
            uint32_t, event);

        switch (mp_tp->mpt_state) {
        case MPTCPS_CLOSED:
        case MPTCPS_LISTEN:
                mp_tp->mpt_state = MPTCPS_TERMINATE;
                break;

        case MPTCPS_ESTABLISHED:
                if (event == MPCE_CLOSE) {
                        mp_tp->mpt_state = MPTCPS_FIN_WAIT_1;
                        mp_tp->mpt_sndmax += 1; /* adjust for Data FIN */
                } else if (event == MPCE_RECV_DATA_FIN) {
                        mp_tp->mpt_rcvnxt += 1; /* adj remote data FIN */
                        mp_tp->mpt_state = MPTCPS_CLOSE_WAIT;
                }
                break;

        case MPTCPS_CLOSE_WAIT:
                if (event == MPCE_CLOSE) {
                        mp_tp->mpt_state = MPTCPS_LAST_ACK;
                        mp_tp->mpt_sndmax += 1; /* adjust for Data FIN */
                }
                break;

        case MPTCPS_FIN_WAIT_1:
                if (event == MPCE_RECV_DATA_ACK) {
                        mp_tp->mpt_state = MPTCPS_FIN_WAIT_2;
                } else if (event == MPCE_RECV_DATA_FIN) {
                        mp_tp->mpt_rcvnxt += 1; /* adj remote data FIN */
                        mp_tp->mpt_state = MPTCPS_CLOSING;
                }
                break;

        case MPTCPS_CLOSING:
                if (event == MPCE_RECV_DATA_ACK) {
                        mp_tp->mpt_state = MPTCPS_TIME_WAIT;
                }
                break;

        case MPTCPS_LAST_ACK:
                if (event == MPCE_RECV_DATA_ACK) {
                        mptcp_close(mp_tp->mpt_mpte, mp_tp);
                }
                break;

        case MPTCPS_FIN_WAIT_2:
                if (event == MPCE_RECV_DATA_FIN) {
                        mp_tp->mpt_rcvnxt += 1; /* adj remote data FIN */
                        mp_tp->mpt_state = MPTCPS_TIME_WAIT;
                }
                break;

        case MPTCPS_TIME_WAIT:
        case MPTCPS_TERMINATE:
                break;

        default:
                VERIFY(0);
                /* NOTREACHED */
        }
        DTRACE_MPTCP2(state__change, struct mptcb *, mp_tp,
            uint32_t, event);
        mptcplog((LOG_INFO, "%s: %s to %s on event %s\n", __func__,
            mptcp_state_to_str(old_state),
            mptcp_state_to_str(mp_tp->mpt_state),
            mptcp_event_to_str(event)),
            MPTCP_STATE_DBG, MPTCP_LOGLVL_LOG);
}

/* If you change this function, match up mptcp_update_rcv_state_f */
void
mptcp_update_dss_rcv_state(struct mptcp_dsn_opt *dss_info, struct tcpcb *tp,
    uint16_t csum)
{
        struct mptcb *mp_tp = tptomptp(tp);
        u_int64_t full_dsn = 0;

        NTOHL(dss_info->mdss_dsn);
        NTOHL(dss_info->mdss_subflow_seqn);
        NTOHS(dss_info->mdss_data_len);

        /* XXX for autosndbuf grow sb here */
        MPTCP_EXTEND_DSN(mp_tp->mpt_rcvnxt, dss_info->mdss_dsn, full_dsn);
        mptcp_update_rcv_state_meat(mp_tp, tp,
            full_dsn, dss_info->mdss_subflow_seqn, dss_info->mdss_data_len,
            csum);
}

void
mptcp_update_rcv_state_meat(struct mptcb *mp_tp, struct tcpcb *tp,
    u_int64_t full_dsn, u_int32_t seqn, u_int16_t mdss_data_len,
    uint16_t csum)
{
        if (mdss_data_len == 0) {
                os_log_error(mptcp_log_handle, "%s - %lx: Infinite Mapping.\n",
                    __func__, (unsigned long)VM_KERNEL_ADDRPERM(mp_tp->mpt_mpte));

                if ((mp_tp->mpt_flags & MPTCPF_CHECKSUM) && (csum != 0)) {
                        os_log_error(mptcp_log_handle, "%s - %lx: Bad checksum %x \n",
                            __func__, (unsigned long)VM_KERNEL_ADDRPERM(mp_tp->mpt_mpte), csum);
                }
                mptcp_notify_mpfail(tp->t_inpcb->inp_socket);
                return;
        }

        mptcp_notify_mpready(tp->t_inpcb->inp_socket);

        tp->t_rcv_map.mpt_dsn = full_dsn;
        tp->t_rcv_map.mpt_sseq = seqn;
        tp->t_rcv_map.mpt_len = mdss_data_len;
        tp->t_rcv_map.mpt_csum = csum;
        tp->t_mpflags |= TMPF_EMBED_DSN;
}


static int
mptcp_validate_dss_map(struct socket *so, struct tcpcb *tp, struct mbuf *m,
    int hdrlen)
{
        u_int32_t datalen;

        if (!(m->m_pkthdr.pkt_flags & PKTF_MPTCP)) {
                return 0;
        }

        datalen = m->m_pkthdr.mp_rlen;

        /* unacceptable DSS option, fallback to TCP */
        if (m->m_pkthdr.len > ((int) datalen + hdrlen)) {
                os_log_error(mptcp_log_handle, "%s - %lx: mbuf len %d, MPTCP expected %d",
                    __func__, (unsigned long)VM_KERNEL_ADDRPERM(tptomptp(tp)->mpt_mpte), m->m_pkthdr.len, datalen);
        } else {
                return 0;
        }
        tp->t_mpflags |= TMPF_SND_MPFAIL;
        mptcp_notify_mpfail(so);
        m_freem(m);
        return -1;
}

int
mptcp_input_preproc(struct tcpcb *tp, struct mbuf *m, struct tcphdr *th,
    int drop_hdrlen)
{
        mptcp_insert_rmap(tp, m, th);
        if (mptcp_validate_dss_map(tp->t_inpcb->inp_socket, tp, m,
            drop_hdrlen) != 0) {
                return -1;
        }
        return 0;
}

static uint16_t
mptcp_input_csum(struct tcpcb *tp, struct mbuf *m, uint64_t dsn, uint32_t sseq,
    uint16_t dlen, uint16_t csum, int dfin)
{
        struct mptcb *mp_tp = tptomptp(tp);
        int real_len = dlen - dfin;
        uint32_t sum = 0;

        VERIFY(real_len >= 0);

        if (mp_tp == NULL) {
                return 0;
        }

        if (!(mp_tp->mpt_flags & MPTCPF_CHECKSUM)) {
                return 0;
        }

        if (tp->t_mpflags & TMPF_TCP_FALLBACK) {
                return 0;
        }

        /*
         * The remote side may send a packet with fewer bytes than the
         * claimed DSS checksum length.
         */
        if ((int)m_length2(m, NULL) < real_len) {
                return 0xffff;
        }

        if (real_len != 0) {
                sum = m_sum16(m, 0, real_len);
        }

        sum += in_pseudo64(htonll(dsn), htonl(sseq), htons(dlen) + csum);
        ADDCARRY(sum);

        DTRACE_MPTCP3(checksum__result, struct tcpcb *, tp, struct mbuf *, m,
            uint32_t, sum);

        return ~sum & 0xffff;
}

/*
 * MPTCP Checksum support
 * The checksum is calculated whenever the MPTCP DSS option is included
 * in the TCP packet. The checksum includes the sum of the MPTCP psuedo
 * header and the actual data indicated by the length specified in the
 * DSS option.
 */

int
mptcp_validate_csum(struct tcpcb *tp, struct mbuf *m, uint64_t dsn,
    uint32_t sseq, uint16_t dlen, uint16_t csum, int dfin)
{
        uint16_t mptcp_csum;

        mptcp_csum = mptcp_input_csum(tp, m, dsn, sseq, dlen, csum, dfin);
        if (mptcp_csum) {
                tp->t_mpflags |= TMPF_SND_MPFAIL;
                mptcp_notify_mpfail(tp->t_inpcb->inp_socket);
                m_freem(m);
                tcpstat.tcps_mp_badcsum++;
                return -1;
        }
        return 0;
}

uint16_t
mptcp_output_csum(struct mbuf *m, uint64_t dss_val, uint32_t sseq, uint16_t dlen)
{
        uint32_t sum = 0;

        if (dlen) {
                sum = m_sum16(m, 0, dlen);
        }

        dss_val = mptcp_hton64(dss_val);
        sseq = htonl(sseq);
        dlen = htons(dlen);
        sum += in_pseudo64(dss_val, sseq, dlen);

        ADDCARRY(sum);
        sum = ~sum & 0xffff;
        DTRACE_MPTCP2(checksum__result, struct mbuf *, m, uint32_t, sum);
        mptcplog((LOG_DEBUG, "%s: sum = %x \n", __func__, sum),
            MPTCP_SENDER_DBG, MPTCP_LOGLVL_VERBOSE);

        return (uint16_t)sum;
}

/*
 * When WiFi signal starts fading, there's more loss and RTT spikes.
 * Check if there has been a large spike by comparing against
 * a tolerable RTT spike threshold.
 */
boolean_t
mptcp_no_rto_spike(struct socket *so)
{
        struct tcpcb *tp = intotcpcb(sotoinpcb(so));
        int32_t spike = 0;

        if (tp->t_rxtcur > mptcp_rtothresh) {
                spike = tp->t_rxtcur - mptcp_rtothresh;

                mptcplog((LOG_DEBUG, "%s: spike = %d rto = %d best = %d cur = %d\n",
                    __func__, spike,
                    tp->t_rxtcur, tp->t_rttbest >> TCP_RTT_SHIFT,
                    tp->t_rttcur),
                    (MPTCP_SOCKET_DBG | MPTCP_SENDER_DBG), MPTCP_LOGLVL_LOG);
        }

        if (spike > 0) {
                return FALSE;
        } else {
                return TRUE;
        }
}

void
mptcp_handle_deferred_upcalls(struct mppcb *mpp, uint32_t flag)
{
        VERIFY(mpp->mpp_flags & flag);
        mpp->mpp_flags &= ~flag;

        if (mptcp_should_defer_upcall(mpp)) {
                return;
        }

        if (mpp->mpp_flags & MPP_SHOULD_WORKLOOP) {
                mpp->mpp_flags &= ~MPP_SHOULD_WORKLOOP;

                mptcp_subflow_workloop(mpp->mpp_pcbe);
        }

        if (mpp->mpp_flags & MPP_SHOULD_RWAKEUP) {
                mpp->mpp_flags &= ~MPP_SHOULD_RWAKEUP;

                sorwakeup(mpp->mpp_socket);
        }

        if (mpp->mpp_flags & MPP_SHOULD_WWAKEUP) {
                mpp->mpp_flags &= ~MPP_SHOULD_WWAKEUP;

                sowwakeup(mpp->mpp_socket);
        }
}

static void
mptcp_reset_itfinfo(struct mpt_itf_info *info)
{
        memset(info, 0, sizeof(*info));
}

void
mptcp_session_necp_cb(void *handle, int action, uint32_t interface_index,
    uint32_t necp_flags, __unused bool *viable)
{
        boolean_t has_v4 = !!(necp_flags & NECP_CLIENT_RESULT_FLAG_HAS_IPV4);
        boolean_t has_v6 = !!(necp_flags & NECP_CLIENT_RESULT_FLAG_HAS_IPV6);
        boolean_t has_nat64 = !!(necp_flags & NECP_CLIENT_RESULT_FLAG_HAS_NAT64);
        boolean_t low_power = !!(necp_flags & NECP_CLIENT_RESULT_FLAG_INTERFACE_LOW_POWER);
        struct mppcb *mp = (struct mppcb *)handle;
        struct mptses *mpte = mptompte(mp);
        struct socket *mp_so;
        struct mptcb *mp_tp;
        uint32_t i, ifindex;
        struct ifnet *ifp;
        int locked = 0;

        ifindex = interface_index;
        VERIFY(ifindex != IFSCOPE_NONE);

        /* About to be garbage-collected (see note about MPTCP/NECP interactions) */
        if (mp->mpp_socket->so_usecount == 0) {
                return;
        }

        mp_so = mptetoso(mpte);

        if (action != NECP_CLIENT_CBACTION_INITIAL) {
                socket_lock(mp_so, 1);
                locked = 1;

                /* Check again, because it might have changed while waiting */
                if (mp->mpp_socket->so_usecount == 0) {
                        goto out;
                }
        }

        socket_lock_assert_owned(mp_so);

        mp_tp = mpte->mpte_mptcb;

        ifnet_head_lock_shared();
        ifp = ifindex2ifnet[ifindex];
        ifnet_head_done();

        os_log(mptcp_log_handle, "%s - %lx: action: %u ifindex %u delegated to %u usecount %u mpt_flags %#x state %u v4 %u v6 %u nat64 %u power %u\n",
            __func__, (unsigned long)VM_KERNEL_ADDRPERM(mpte), action, ifindex,
            ifp && ifp->if_delegated.ifp ? ifp->if_delegated.ifp->if_index : IFSCOPE_NONE,
            mp->mpp_socket->so_usecount, mp_tp->mpt_flags, mp_tp->mpt_state,
            has_v4, has_v6, has_nat64, low_power);

        /* No need on fallen back sockets */
        if (mp_tp->mpt_flags & MPTCPF_FALLBACK_TO_TCP) {
                goto out;
        }

        /*
         * When the interface goes in low-power mode we don't want to establish
         * new subflows on it. Thus, mark it internally as non-viable.
         */
        if (low_power) {
                action = NECP_CLIENT_CBACTION_NONVIABLE;
        }

        if (action == NECP_CLIENT_CBACTION_NONVIABLE) {
                for (i = 0; i < mpte->mpte_itfinfo_size; i++) {
                        if (mpte->mpte_itfinfo[i].ifindex == IFSCOPE_NONE) {
                                continue;
                        }

                        if (mpte->mpte_itfinfo[i].ifindex == ifindex) {
                                mptcp_reset_itfinfo(&mpte->mpte_itfinfo[i]);
                        }
                }

                mptcp_sched_create_subflows(mpte);
        } else if (action == NECP_CLIENT_CBACTION_VIABLE ||
            action == NECP_CLIENT_CBACTION_INITIAL) {
                int found_slot = 0, slot_index = -1;
                struct sockaddr *dst;

                if (ifp == NULL) {
                        goto out;
                }

                if (IFNET_IS_COMPANION_LINK(ifp)) {
                        goto out;
                }

                if (IFNET_IS_EXPENSIVE(ifp) &&
                    (mp_so->so_restrictions & SO_RESTRICT_DENY_EXPENSIVE)) {
                        goto out;
                }

                if (IFNET_IS_CONSTRAINED(ifp) &&
                    (mp_so->so_restrictions & SO_RESTRICT_DENY_CONSTRAINED)) {
                        goto out;
                }

                if (IFNET_IS_CELLULAR(ifp) &&
                    (mp_so->so_restrictions & SO_RESTRICT_DENY_CELLULAR)) {
                        goto out;
                }

                if (IS_INTF_CLAT46(ifp)) {
                        has_v4 = FALSE;
                }

                /* Look for the slot on where to store/update the interface-info. */
                for (i = 0; i < mpte->mpte_itfinfo_size; i++) {
                        /* Found a potential empty slot where we can put it */
                        if (mpte->mpte_itfinfo[i].ifindex == 0) {
                                found_slot = 1;
                                slot_index = i;
                        }

                        /*
                         * The interface is already in our array. Check if we
                         * need to update it.
                         */
                        if (mpte->mpte_itfinfo[i].ifindex == ifindex &&
                            (mpte->mpte_itfinfo[i].has_v4_conn != has_v4 ||
                            mpte->mpte_itfinfo[i].has_v6_conn != has_v6 ||
                            mpte->mpte_itfinfo[i].has_nat64_conn != has_nat64)) {
                                found_slot = 1;
                                slot_index = i;
                                break;
                        }

                        if (mpte->mpte_itfinfo[i].ifindex == ifindex) {
                                /*
                                 * Ok, it's already there and we don't need
                                 * to update it
                                 */
                                goto out;
                        }
                }

                dst = mptcp_get_session_dst(mpte, has_v6, has_v4);
                if (dst && dst->sa_family == AF_INET &&
                    has_v6 && !has_nat64 && !has_v4) {
                        if (found_slot) {
                                mpte->mpte_itfinfo[slot_index].ifindex = ifindex;
                                mpte->mpte_itfinfo[slot_index].has_v4_conn = has_v4;
                                mpte->mpte_itfinfo[slot_index].has_v6_conn = has_v6;
                                mpte->mpte_itfinfo[slot_index].has_nat64_conn = has_nat64;
                        }
                        goto out;
                }

                if (found_slot == 0) {
                        int new_size = mpte->mpte_itfinfo_size * 2;
                        struct mpt_itf_info *info = _MALLOC(sizeof(*info) * new_size, M_TEMP, M_ZERO);

                        if (info == NULL) {
                                os_log_error(mptcp_log_handle, "%s - %lx: malloc failed for %u\n",
                                    __func__, (unsigned long)VM_KERNEL_ADDRPERM(mpte), new_size);
                                goto out;
                        }

                        memcpy(info, mpte->mpte_itfinfo, mpte->mpte_itfinfo_size * sizeof(*info));

                        if (mpte->mpte_itfinfo_size > MPTE_ITFINFO_SIZE) {
                                _FREE(mpte->mpte_itfinfo, M_TEMP);
                        }

                        /* We allocated a new one, thus the first must be empty */
                        slot_index = mpte->mpte_itfinfo_size;

                        mpte->mpte_itfinfo = info;
                        mpte->mpte_itfinfo_size = new_size;
                }

                VERIFY(slot_index >= 0 && slot_index < (int)mpte->mpte_itfinfo_size);
                mpte->mpte_itfinfo[slot_index].ifindex = ifindex;
                mpte->mpte_itfinfo[slot_index].has_v4_conn = has_v4;
                mpte->mpte_itfinfo[slot_index].has_v6_conn = has_v6;
                mpte->mpte_itfinfo[slot_index].has_nat64_conn = has_nat64;

                mptcp_sched_create_subflows(mpte);
        }

out:
        if (locked) {
                socket_unlock(mp_so, 1);
        }
}

void
mptcp_set_restrictions(struct socket *mp_so)
{
        struct mptses *mpte = mpsotompte(mp_so);
        uint32_t i;

        socket_lock_assert_owned(mp_so);

        ifnet_head_lock_shared();

        for (i = 0; i < mpte->mpte_itfinfo_size; i++) {
                struct mpt_itf_info *info = &mpte->mpte_itfinfo[i];
                uint32_t ifindex = info->ifindex;
                struct ifnet *ifp;

                if (ifindex == IFSCOPE_NONE) {
                        continue;
                }

                ifp = ifindex2ifnet[ifindex];
                if (ifp == NULL) {
                        continue;
                }

                if (IFNET_IS_EXPENSIVE(ifp) &&
                    (mp_so->so_restrictions & SO_RESTRICT_DENY_EXPENSIVE)) {
                        info->ifindex = IFSCOPE_NONE;
                }

                if (IFNET_IS_CONSTRAINED(ifp) &&
                    (mp_so->so_restrictions & SO_RESTRICT_DENY_CONSTRAINED)) {
                        info->ifindex = IFSCOPE_NONE;
                }

                if (IFNET_IS_CELLULAR(ifp) &&
                    (mp_so->so_restrictions & SO_RESTRICT_DENY_CELLULAR)) {
                        info->ifindex = IFSCOPE_NONE;
                }
        }

        ifnet_head_done();
}