/*
 * INET         An implementation of the TCP/IP protocol suite for the LINUX
 *              operating system.  INET is implemented using the  BSD Socket
 *              interface as the means of communication with the user level.
 *
 *              Implementation of the Transmission Control Protocol(TCP).
 *
 * Version:     $Id: tcp_input.c,v 1.243 2002/02/01 22:01:04 davem Exp $
 *
 * Authors:     Ross Biro
 *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
 *              Mark Evans, <evansmp@uhura.aston.ac.uk>
 *              Corey Minyard <wf-rch!minyard@relay.EU.net>
 *              Florian La Roche, <flla@stud.uni-sb.de>
 *              Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
 *              Linus Torvalds, <torvalds@cs.helsinki.fi>
 *              Alan Cox, <gw4pts@gw4pts.ampr.org>
 *              Matthew Dillon, <dillon@apollo.west.oic.com>
 *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 *              Jorge Cwik, <jorge@laser.satlink.net>
 */

/*
 * Changes:
 *              Pedro Roque     :       Fast Retransmit/Recovery.
 *                                      Two receive queues.
 *                                      Retransmit queue handled by TCP.
 *                                      Better retransmit timer handling.
 *                                      New congestion avoidance.
 *                                      Header prediction.
 *                                      Variable renaming.
 *
 *              Eric            :       Fast Retransmit.
 *              Randy Scott     :       MSS option defines.
 *              Eric Schenk     :       Fixes to slow start algorithm.
 *              Eric Schenk     :       Yet another double ACK bug.
 *              Eric Schenk     :       Delayed ACK bug fixes.
 *              Eric Schenk     :       Floyd style fast retrans war avoidance.
 *              David S. Miller :       Don't allow zero congestion window.
 *              Eric Schenk     :       Fix retransmitter so that it sends
 *                                      next packet on ack of previous packet.
 *              Andi Kleen      :       Moved open_request checking here
 *                                      and process RSTs for open_requests.
 *              Andi Kleen      :       Better prune_queue, and other fixes.
 *              Andrey Savochkin:       Fix RTT measurements in the presence of
 *                                      timestamps.
 *              Andrey Savochkin:       Check sequence numbers correctly when
 *                                      removing SACKs due to in sequence incoming
 *                                      data segments.
 *              Andi Kleen:             Make sure we never ack data there is not
 *                                      enough room for. Also make this condition
 *                                      a fatal error if it might still happen.
 *              Andi Kleen:             Add tcp_measure_rcv_mss to make
 *                                      connections with MSS<min(MTU,ann. MSS)
 *                                      work without delayed acks.
 *              Andi Kleen:             Process packets with PSH set in the
 *                                      fast path.
 *              J Hadi Salim:           ECN support
 *              Andrei Gurtov,
 *              Pasi Sarolahti,
 *              Panu Kuhlberg:          Experimental audit of TCP (re)transmission
 *                                      engine. Lots of bugs are found.
 *              Pasi Sarolahti:         F-RTO for dealing with spurious RTOs
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/sysctl.h>
#include <net/tcp.h>
#include <net/inet_common.h>
#include <linux/ipsec.h>
#include <asm/unaligned.h>
#include <net/netdma.h>

int sysctl_tcp_timestamps __read_mostly = 1;
int sysctl_tcp_window_scaling __read_mostly = 1;
int sysctl_tcp_sack __read_mostly = 1;
int sysctl_tcp_fack __read_mostly = 1;
int sysctl_tcp_reordering __read_mostly = TCP_FASTRETRANS_THRESH;
int sysctl_tcp_ecn __read_mostly;
int sysctl_tcp_dsack __read_mostly = 1;
int sysctl_tcp_app_win __read_mostly = 31;
int sysctl_tcp_adv_win_scale __read_mostly = 2;

int sysctl_tcp_stdurg __read_mostly;
int sysctl_tcp_rfc1337 __read_mostly;
int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
int sysctl_tcp_frto __read_mostly = 2;
int sysctl_tcp_frto_response __read_mostly;
int sysctl_tcp_nometrics_save __read_mostly;

int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
int sysctl_tcp_abc __read_mostly;

#define FLAG_DATA               0x01 /* Incoming frame contained data.          */
#define FLAG_WIN_UPDATE         0x02 /* Incoming ACK was a window update.       */
#define FLAG_DATA_ACKED         0x04 /* This ACK acknowledged new data.         */
#define FLAG_RETRANS_DATA_ACKED 0x08 /* "" "" some of which was retransmitted.  */
#define FLAG_SYN_ACKED          0x10 /* This ACK acknowledged SYN.              */
#define FLAG_DATA_SACKED        0x20 /* New SACK.                               */
#define FLAG_ECE                0x40 /* ECE in this ACK                         */
#define FLAG_DATA_LOST          0x80 /* SACK detected data lossage.             */
#define FLAG_SLOWPATH           0x100 /* Do not skip RFC checks for window update.*/
#define FLAG_ONLY_ORIG_SACKED   0x200 /* SACKs only non-rexmit sent before RTO */
#define FLAG_SND_UNA_ADVANCED   0x400 /* Snd_una was changed (!= FLAG_DATA_ACKED) */
#define FLAG_DSACKING_ACK       0x800 /* SACK blocks contained D-SACK info */
#define FLAG_NONHEAD_RETRANS_ACKED      0x1000 /* Non-head rexmitted data was ACKed */

#define FLAG_ACKED              (FLAG_DATA_ACKED|FLAG_SYN_ACKED)
#define FLAG_NOT_DUP            (FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
#define FLAG_CA_ALERT           (FLAG_DATA_SACKED|FLAG_ECE)
#define FLAG_FORWARD_PROGRESS   (FLAG_ACKED|FLAG_DATA_SACKED)
#define FLAG_ANY_PROGRESS       (FLAG_FORWARD_PROGRESS|FLAG_SND_UNA_ADVANCED)

#define IsSackFrto() (sysctl_tcp_frto == 0x2)

#define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH)
#define TCP_HP_BITS (~(TCP_RESERVED_BITS|TCP_FLAG_PSH))

/* Adapt the MSS value used to make delayed ack decision to the
 * real world.
 */
static void tcp_measure_rcv_mss(struct sock *sk,
                                const struct sk_buff *skb)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        const unsigned int lss = icsk->icsk_ack.last_seg_size;
        unsigned int len;

        icsk->icsk_ack.last_seg_size = 0;

        /* skb->len may jitter because of SACKs, even if peer
         * sends good full-sized frames.
         */
        len = skb_shinfo(skb)->gso_size ?: skb->len;
        if (len >= icsk->icsk_ack.rcv_mss) {
                icsk->icsk_ack.rcv_mss = len;
        } else {
                /* Otherwise, we make more careful check taking into account,
                 * that SACKs block is variable.
                 *
                 * "len" is invariant segment length, including TCP header.
                 */
                len += skb->data - skb_transport_header(skb);
                if (len >= TCP_MIN_RCVMSS + sizeof(struct tcphdr) ||
                    /* If PSH is not set, packet should be
                     * full sized, provided peer TCP is not badly broken.
                     * This observation (if it is correct 8)) allows
                     * to handle super-low mtu links fairly.
                     */
                    (len >= TCP_MIN_MSS + sizeof(struct tcphdr) &&
                     !(tcp_flag_word(tcp_hdr(skb)) & TCP_REMNANT))) {
                        /* Subtract also invariant (if peer is RFC compliant),
                         * tcp header plus fixed timestamp option length.
                         * Resulting "len" is MSS free of SACK jitter.
                         */
                        len -= tcp_sk(sk)->tcp_header_len;
                        icsk->icsk_ack.last_seg_size = len;
                        if (len == lss) {
                                icsk->icsk_ack.rcv_mss = len;
                                return;
                        }
                }
                if (icsk->icsk_ack.pending & ICSK_ACK_PUSHED)
                        icsk->icsk_ack.pending |= ICSK_ACK_PUSHED2;
                icsk->icsk_ack.pending |= ICSK_ACK_PUSHED;
        }
}

static void tcp_incr_quickack(struct sock *sk)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        unsigned quickacks = tcp_sk(sk)->rcv_wnd / (2 * icsk->icsk_ack.rcv_mss);

        if (quickacks==0)
                quickacks=2;
        if (quickacks > icsk->icsk_ack.quick)
                icsk->icsk_ack.quick = min(quickacks, TCP_MAX_QUICKACKS);
}

void tcp_enter_quickack_mode(struct sock *sk)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        tcp_incr_quickack(sk);
        icsk->icsk_ack.pingpong = 0;
        icsk->icsk_ack.ato = TCP_ATO_MIN;
}

/* Send ACKs quickly, if "quick" count is not exhausted
 * and the session is not interactive.
 */

static inline int tcp_in_quickack_mode(const struct sock *sk)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        return icsk->icsk_ack.quick && !icsk->icsk_ack.pingpong;
}

static inline void TCP_ECN_queue_cwr(struct tcp_sock *tp)
{
        if (tp->ecn_flags&TCP_ECN_OK)
                tp->ecn_flags |= TCP_ECN_QUEUE_CWR;
}

static inline void TCP_ECN_accept_cwr(struct tcp_sock *tp, struct sk_buff *skb)
{
        if (tcp_hdr(skb)->cwr)
                tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
}

static inline void TCP_ECN_withdraw_cwr(struct tcp_sock *tp)
{
        tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
}

static inline void TCP_ECN_check_ce(struct tcp_sock *tp, struct sk_buff *skb)
{
        if (tp->ecn_flags&TCP_ECN_OK) {
                if (INET_ECN_is_ce(TCP_SKB_CB(skb)->flags))
                        tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
                /* Funny extension: if ECT is not set on a segment,
                 * it is surely retransmit. It is not in ECN RFC,
                 * but Linux follows this rule. */
                else if (INET_ECN_is_not_ect((TCP_SKB_CB(skb)->flags)))
                        tcp_enter_quickack_mode((struct sock *)tp);
        }
}

static inline void TCP_ECN_rcv_synack(struct tcp_sock *tp, struct tcphdr *th)
{
        if ((tp->ecn_flags&TCP_ECN_OK) && (!th->ece || th->cwr))
                tp->ecn_flags &= ~TCP_ECN_OK;
}

static inline void TCP_ECN_rcv_syn(struct tcp_sock *tp, struct tcphdr *th)
{
        if ((tp->ecn_flags&TCP_ECN_OK) && (!th->ece || !th->cwr))
                tp->ecn_flags &= ~TCP_ECN_OK;
}

static inline int TCP_ECN_rcv_ecn_echo(struct tcp_sock *tp, struct tcphdr *th)
{
        if (th->ece && !th->syn && (tp->ecn_flags&TCP_ECN_OK))
                return 1;
        return 0;
}

/* Buffer size and advertised window tuning.
 *
 * 1. Tuning sk->sk_sndbuf, when connection enters established state.
 */

static void tcp_fixup_sndbuf(struct sock *sk)
{
        int sndmem = tcp_sk(sk)->rx_opt.mss_clamp + MAX_TCP_HEADER + 16 +
                     sizeof(struct sk_buff);

        if (sk->sk_sndbuf < 3 * sndmem)
                sk->sk_sndbuf = min(3 * sndmem, sysctl_tcp_wmem[2]);
}

/* 2. Tuning advertised window (window_clamp, rcv_ssthresh)
 *
 * All tcp_full_space() is split to two parts: "network" buffer, allocated
 * forward and advertised in receiver window (tp->rcv_wnd) and
 * "application buffer", required to isolate scheduling/application
 * latencies from network.
 * window_clamp is maximal advertised window. It can be less than
 * tcp_full_space(), in this case tcp_full_space() - window_clamp
 * is reserved for "application" buffer. The less window_clamp is
 * the smoother our behaviour from viewpoint of network, but the lower
 * throughput and the higher sensitivity of the connection to losses. 8)
 *
 * rcv_ssthresh is more strict window_clamp used at "slow start"
 * phase to predict further behaviour of this connection.
 * It is used for two goals:
 * - to enforce header prediction at sender, even when application
 *   requires some significant "application buffer". It is check #1.
 * - to prevent pruning of receive queue because of misprediction
 *   of receiver window. Check #2.
 *
 * The scheme does not work when sender sends good segments opening
 * window and then starts to feed us spaghetti. But it should work
 * in common situations. Otherwise, we have to rely on queue collapsing.
 */

/* Slow part of check#2. */
static int __tcp_grow_window(const struct sock *sk, const struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        /* Optimize this! */
        int truesize = tcp_win_from_space(skb->truesize)/2;
        int window = tcp_win_from_space(sysctl_tcp_rmem[2])/2;

        while (tp->rcv_ssthresh <= window) {
                if (truesize <= skb->len)
                        return 2 * inet_csk(sk)->icsk_ack.rcv_mss;

                truesize >>= 1;
                window >>= 1;
        }
        return 0;
}

static void tcp_grow_window(struct sock *sk,
                            struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);

        /* Check #1 */
        if (tp->rcv_ssthresh < tp->window_clamp &&
            (int)tp->rcv_ssthresh < tcp_space(sk) &&
            !tcp_memory_pressure) {
                int incr;

                /* Check #2. Increase window, if skb with such overhead
                 * will fit to rcvbuf in future.
                 */
                if (tcp_win_from_space(skb->truesize) <= skb->len)
                        incr = 2*tp->advmss;
                else
                        incr = __tcp_grow_window(sk, skb);

                if (incr) {
                        tp->rcv_ssthresh = min(tp->rcv_ssthresh + incr, tp->window_clamp);
                        inet_csk(sk)->icsk_ack.quick |= 1;
                }
        }
}

/* 3. Tuning rcvbuf, when connection enters established state. */

static void tcp_fixup_rcvbuf(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        int rcvmem = tp->advmss + MAX_TCP_HEADER + 16 + sizeof(struct sk_buff);

        /* Try to select rcvbuf so that 4 mss-sized segments
         * will fit to window and corresponding skbs will fit to our rcvbuf.
         * (was 3; 4 is minimum to allow fast retransmit to work.)
         */
        while (tcp_win_from_space(rcvmem) < tp->advmss)
                rcvmem += 128;
        if (sk->sk_rcvbuf < 4 * rcvmem)
                sk->sk_rcvbuf = min(4 * rcvmem, sysctl_tcp_rmem[2]);
}

/* 4. Try to fixup all. It is made immediately after connection enters
 *    established state.
 */
static void tcp_init_buffer_space(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        int maxwin;

        if (!(sk->sk_userlocks & SOCK_RCVBUF_LOCK))
                tcp_fixup_rcvbuf(sk);
        if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))
                tcp_fixup_sndbuf(sk);

        tp->rcvq_space.space = tp->rcv_wnd;

        maxwin = tcp_full_space(sk);

        if (tp->window_clamp >= maxwin) {
                tp->window_clamp = maxwin;

                if (sysctl_tcp_app_win && maxwin > 4 * tp->advmss)
                        tp->window_clamp = max(maxwin -
                                               (maxwin >> sysctl_tcp_app_win),
                                               4 * tp->advmss);
        }

        /* Force reservation of one segment. */
        if (sysctl_tcp_app_win &&
            tp->window_clamp > 2 * tp->advmss &&
            tp->window_clamp + tp->advmss > maxwin)
                tp->window_clamp = max(2 * tp->advmss, maxwin - tp->advmss);

        tp->rcv_ssthresh = min(tp->rcv_ssthresh, tp->window_clamp);
        tp->snd_cwnd_stamp = tcp_time_stamp;
}

/* 5. Recalculate window clamp after socket hit its memory bounds. */
static void tcp_clamp_window(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);

        icsk->icsk_ack.quick = 0;

        if (sk->sk_rcvbuf < sysctl_tcp_rmem[2] &&
            !(sk->sk_userlocks & SOCK_RCVBUF_LOCK) &&
            !tcp_memory_pressure &&
            atomic_read(&tcp_memory_allocated) < sysctl_tcp_mem[0]) {
                sk->sk_rcvbuf = min(atomic_read(&sk->sk_rmem_alloc),
                                    sysctl_tcp_rmem[2]);
        }
        if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf)
                tp->rcv_ssthresh = min(tp->window_clamp, 2U*tp->advmss);
}


/* Initialize RCV_MSS value.
 * RCV_MSS is an our guess about MSS used by the peer.
 * We haven't any direct information about the MSS.
 * It's better to underestimate the RCV_MSS rather than overestimate.
 * Overestimations make us ACKing less frequently than needed.
 * Underestimations are more easy to detect and fix by tcp_measure_rcv_mss().
 */
void tcp_initialize_rcv_mss(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        unsigned int hint = min_t(unsigned int, tp->advmss, tp->mss_cache);

        hint = min(hint, tp->rcv_wnd/2);
        hint = min(hint, TCP_MIN_RCVMSS);
        hint = max(hint, TCP_MIN_MSS);

        inet_csk(sk)->icsk_ack.rcv_mss = hint;
}

/* Receiver "autotuning" code.
 *
 * The algorithm for RTT estimation w/o timestamps is based on
 * Dynamic Right-Sizing (DRS) by Wu Feng and Mike Fisk of LANL.
 * <http://www.lanl.gov/radiant/website/pubs/drs/lacsi2001.ps>
 *
 * More detail on this code can be found at
 * <http://www.psc.edu/~jheffner/senior_thesis.ps>,
 * though this reference is out of date.  A new paper
 * is pending.
 */
static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
{
        u32 new_sample = tp->rcv_rtt_est.rtt;
        long m = sample;

        if (m == 0)
                m = 1;

        if (new_sample != 0) {
                /* If we sample in larger samples in the non-timestamp
                 * case, we could grossly overestimate the RTT especially
                 * with chatty applications or bulk transfer apps which
                 * are stalled on filesystem I/O.
                 *
                 * Also, since we are only going for a minimum in the
                 * non-timestamp case, we do not smooth things out
                 * else with timestamps disabled convergence takes too
                 * long.
                 */
                if (!win_dep) {
                        m -= (new_sample >> 3);
                        new_sample += m;
                } else if (m < new_sample)
                        new_sample = m << 3;
        } else {
                /* No previous measure. */
                new_sample = m << 3;
        }

        if (tp->rcv_rtt_est.rtt != new_sample)
                tp->rcv_rtt_est.rtt = new_sample;
}

static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
{
        if (tp->rcv_rtt_est.time == 0)
                goto new_measure;
        if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
                return;
        tcp_rcv_rtt_update(tp,
                           jiffies - tp->rcv_rtt_est.time,
                           1);

new_measure:
        tp->rcv_rtt_est.seq = tp->rcv_nxt + tp->rcv_wnd;
        tp->rcv_rtt_est.time = tcp_time_stamp;
}

static inline void tcp_rcv_rtt_measure_ts(struct sock *sk, const struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        if (tp->rx_opt.rcv_tsecr &&
            (TCP_SKB_CB(skb)->end_seq -
             TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss))
                tcp_rcv_rtt_update(tp, tcp_time_stamp - tp->rx_opt.rcv_tsecr, 0);
}

/*
 * This function should be called every time data is copied to user space.
 * It calculates the appropriate TCP receive buffer space.
 */
void tcp_rcv_space_adjust(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        int time;
        int space;

        if (tp->rcvq_space.time == 0)
                goto new_measure;

        time = tcp_time_stamp - tp->rcvq_space.time;
        if (time < (tp->rcv_rtt_est.rtt >> 3) ||
            tp->rcv_rtt_est.rtt == 0)
                return;

        space = 2 * (tp->copied_seq - tp->rcvq_space.seq);

        space = max(tp->rcvq_space.space, space);

        if (tp->rcvq_space.space != space) {
                int rcvmem;

                tp->rcvq_space.space = space;

                if (sysctl_tcp_moderate_rcvbuf &&
                    !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
                        int new_clamp = space;

                        /* Receive space grows, normalize in order to
                         * take into account packet headers and sk_buff
                         * structure overhead.
                         */
                        space /= tp->advmss;
                        if (!space)
                                space = 1;
                        rcvmem = (tp->advmss + MAX_TCP_HEADER +
                                  16 + sizeof(struct sk_buff));
                        while (tcp_win_from_space(rcvmem) < tp->advmss)
                                rcvmem += 128;
                        space *= rcvmem;
                        space = min(space, sysctl_tcp_rmem[2]);
                        if (space > sk->sk_rcvbuf) {
                                sk->sk_rcvbuf = space;

                                /* Make the window clamp follow along.  */
                                tp->window_clamp = new_clamp;
                        }
                }
        }

new_measure:
        tp->rcvq_space.seq = tp->copied_seq;
        tp->rcvq_space.time = tcp_time_stamp;
}

/* There is something which you must keep in mind when you analyze the
 * behavior of the tp->ato delayed ack timeout interval.  When a
 * connection starts up, we want to ack as quickly as possible.  The
 * problem is that "good" TCP's do slow start at the beginning of data
 * transmission.  The means that until we send the first few ACK's the
 * sender will sit on his end and only queue most of his data, because
 * he can only send snd_cwnd unacked packets at any given time.  For
 * each ACK we send, he increments snd_cwnd and transmits more of his
 * queue.  -DaveM
 */
static void tcp_event_data_recv(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);
        u32 now;

        inet_csk_schedule_ack(sk);

        tcp_measure_rcv_mss(sk, skb);

        tcp_rcv_rtt_measure(tp);

        now = tcp_time_stamp;

        if (!icsk->icsk_ack.ato) {
                /* The _first_ data packet received, initialize
                 * delayed ACK engine.
                 */
                tcp_incr_quickack(sk);
                icsk->icsk_ack.ato = TCP_ATO_MIN;
        } else {
                int m = now - icsk->icsk_ack.lrcvtime;

                if (m <= TCP_ATO_MIN/2) {
                        /* The fastest case is the first. */
                        icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + TCP_ATO_MIN / 2;
                } else if (m < icsk->icsk_ack.ato) {
                        icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + m;
                        if (icsk->icsk_ack.ato > icsk->icsk_rto)
                                icsk->icsk_ack.ato = icsk->icsk_rto;
                } else if (m > icsk->icsk_rto) {
                        /* Too long gap. Apparently sender failed to
                         * restart window, so that we send ACKs quickly.
                         */
                        tcp_incr_quickack(sk);
                        sk_stream_mem_reclaim(sk);
                }
        }
        icsk->icsk_ack.lrcvtime = now;

        TCP_ECN_check_ce(tp, skb);

        if (skb->len >= 128)
                tcp_grow_window(sk, skb);
}

static u32 tcp_rto_min(struct sock *sk)
{
        struct dst_entry *dst = __sk_dst_get(sk);
        u32 rto_min = TCP_RTO_MIN;

        if (dst && dst_metric_locked(dst, RTAX_RTO_MIN))
                rto_min = dst->metrics[RTAX_RTO_MIN-1];
        return rto_min;
}

/* Called to compute a smoothed rtt estimate. The data fed to this
 * routine either comes from timestamps, or from segments that were
 * known _not_ to have been retransmitted [see Karn/Partridge
 * Proceedings SIGCOMM 87]. The algorithm is from the SIGCOMM 88
 * piece by Van Jacobson.
 * NOTE: the next three routines used to be one big routine.
 * To save cycles in the RFC 1323 implementation it was better to break
 * it up into three procedures. -- erics
 */
static void tcp_rtt_estimator(struct sock *sk, const __u32 mrtt)
{
        struct tcp_sock *tp = tcp_sk(sk);
        long m = mrtt; /* RTT */

        /*      The following amusing code comes from Jacobson's
         *      article in SIGCOMM '88.  Note that rtt and mdev
         *      are scaled versions of rtt and mean deviation.
         *      This is designed to be as fast as possible
         *      m stands for "measurement".
         *
         *      On a 1990 paper the rto value is changed to:
         *      RTO = rtt + 4 * mdev
         *
         * Funny. This algorithm seems to be very broken.
         * These formulae increase RTO, when it should be decreased, increase
         * too slowly, when it should be increased quickly, decrease too quickly
         * etc. I guess in BSD RTO takes ONE value, so that it is absolutely
         * does not matter how to _calculate_ it. Seems, it was trap
         * that VJ failed to avoid. 8)
         */
        if (m == 0)
                m = 1;
        if (tp->srtt != 0) {
                m -= (tp->srtt >> 3);   /* m is now error in rtt est */
                tp->srtt += m;          /* rtt = 7/8 rtt + 1/8 new */
                if (m < 0) {
                        m = -m;         /* m is now abs(error) */
                        m -= (tp->mdev >> 2);   /* similar update on mdev */
                        /* This is similar to one of Eifel findings.
                         * Eifel blocks mdev updates when rtt decreases.
                         * This solution is a bit different: we use finer gain
                         * for mdev in this case (alpha*beta).
                         * Like Eifel it also prevents growth of rto,
                         * but also it limits too fast rto decreases,
                         * happening in pure Eifel.
                         */
                        if (m > 0)
                                m >>= 3;
                } else {
                        m -= (tp->mdev >> 2);   /* similar update on mdev */
                }
                tp->mdev += m;          /* mdev = 3/4 mdev + 1/4 new */
                if (tp->mdev > tp->mdev_max) {
                        tp->mdev_max = tp->mdev;
                        if (tp->mdev_max > tp->rttvar)
                                tp->rttvar = tp->mdev_max;
                }
                if (after(tp->snd_una, tp->rtt_seq)) {
                        if (tp->mdev_max < tp->rttvar)
                                tp->rttvar -= (tp->rttvar-tp->mdev_max)>>2;
                        tp->rtt_seq = tp->snd_nxt;
                        tp->mdev_max = tcp_rto_min(sk);
                }
        } else {
                /* no previous measure. */
                tp->srtt = m<<3;        /* take the measured time to be rtt */
                tp->mdev = m<<1;        /* make sure rto = 3*rtt */
                tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
                tp->rtt_seq = tp->snd_nxt;
        }
}

/* Calculate rto without backoff.  This is the second half of Van Jacobson's
 * routine referred to above.
 */
static inline void tcp_set_rto(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        /* Old crap is replaced with new one. 8)
         *
         * More seriously:
         * 1. If rtt variance happened to be less 50msec, it is hallucination.
         *    It cannot be less due to utterly erratic ACK generation made
         *    at least by solaris and freebsd. "Erratic ACKs" has _nothing_
         *    to do with delayed acks, because at cwnd>2 true delack timeout
         *    is invisible. Actually, Linux-2.4 also generates erratic
         *    ACKs in some circumstances.
         */
        inet_csk(sk)->icsk_rto = (tp->srtt >> 3) + tp->rttvar;

        /* 2. Fixups made earlier cannot be right.
         *    If we do not estimate RTO correctly without them,
         *    all the algo is pure shit and should be replaced
         *    with correct one. It is exactly, which we pretend to do.
         */
}

/* NOTE: clamping at TCP_RTO_MIN is not required, current algo
 * guarantees that rto is higher.
 */
static inline void tcp_bound_rto(struct sock *sk)
{
        if (inet_csk(sk)->icsk_rto > TCP_RTO_MAX)
                inet_csk(sk)->icsk_rto = TCP_RTO_MAX;
}

/* Save metrics learned by this TCP session.
   This function is called only, when TCP finishes successfully
   i.e. when it enters TIME-WAIT or goes from LAST-ACK to CLOSE.
 */
void tcp_update_metrics(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct dst_entry *dst = __sk_dst_get(sk);

        if (sysctl_tcp_nometrics_save)
                return;

        dst_confirm(dst);

        if (dst && (dst->flags&DST_HOST)) {
                const struct inet_connection_sock *icsk = inet_csk(sk);
                int m;

                if (icsk->icsk_backoff || !tp->srtt) {
                        /* This session failed to estimate rtt. Why?
                         * Probably, no packets returned in time.
                         * Reset our results.
                         */
                        if (!(dst_metric_locked(dst, RTAX_RTT)))
                                dst->metrics[RTAX_RTT-1] = 0;
                        return;
                }

                m = dst_metric(dst, RTAX_RTT) - tp->srtt;

                /* If newly calculated rtt larger than stored one,
                 * store new one. Otherwise, use EWMA. Remember,
                 * rtt overestimation is always better than underestimation.
                 */
                if (!(dst_metric_locked(dst, RTAX_RTT))) {
                        if (m <= 0)
                                dst->metrics[RTAX_RTT-1] = tp->srtt;
                        else
                                dst->metrics[RTAX_RTT-1] -= (m>>3);
                }

                if (!(dst_metric_locked(dst, RTAX_RTTVAR))) {
                        if (m < 0)
                                m = -m;

                        /* Scale deviation to rttvar fixed point */
                        m >>= 1;
                        if (m < tp->mdev)
                                m = tp->mdev;

                        if (m >= dst_metric(dst, RTAX_RTTVAR))
                                dst->metrics[RTAX_RTTVAR-1] = m;
                        else
                                dst->metrics[RTAX_RTTVAR-1] -=
                                        (dst->metrics[RTAX_RTTVAR-1] - m)>>2;
                }

                if (tp->snd_ssthresh >= 0xFFFF) {
                        /* Slow start still did not finish. */
                        if (dst_metric(dst, RTAX_SSTHRESH) &&
                            !dst_metric_locked(dst, RTAX_SSTHRESH) &&
                            (tp->snd_cwnd >> 1) > dst_metric(dst, RTAX_SSTHRESH))
                                dst->metrics[RTAX_SSTHRESH-1] = tp->snd_cwnd >> 1;
                        if (!dst_metric_locked(dst, RTAX_CWND) &&
                            tp->snd_cwnd > dst_metric(dst, RTAX_CWND))
                                dst->metrics[RTAX_CWND-1] = tp->snd_cwnd;
                } else if (tp->snd_cwnd > tp->snd_ssthresh &&
                           icsk->icsk_ca_state == TCP_CA_Open) {
                        /* Cong. avoidance phase, cwnd is reliable. */
                        if (!dst_metric_locked(dst, RTAX_SSTHRESH))
                                dst->metrics[RTAX_SSTHRESH-1] =
                                        max(tp->snd_cwnd >> 1, tp->snd_ssthresh);
                        if (!dst_metric_locked(dst, RTAX_CWND))
                                dst->metrics[RTAX_CWND-1] = (dst->metrics[RTAX_CWND-1] + tp->snd_cwnd) >> 1;
                } else {
                        /* Else slow start did not finish, cwnd is non-sense,
                           ssthresh may be also invalid.
                         */
                        if (!dst_metric_locked(dst, RTAX_CWND))
                                dst->metrics[RTAX_CWND-1] = (dst->metrics[RTAX_CWND-1] + tp->snd_ssthresh) >> 1;
                        if (dst->metrics[RTAX_SSTHRESH-1] &&
                            !dst_metric_locked(dst, RTAX_SSTHRESH) &&
                            tp->snd_ssthresh > dst->metrics[RTAX_SSTHRESH-1])
                                dst->metrics[RTAX_SSTHRESH-1] = tp->snd_ssthresh;
                }

                if (!dst_metric_locked(dst, RTAX_REORDERING)) {
                        if (dst->metrics[RTAX_REORDERING-1] < tp->reordering &&
                            tp->reordering != sysctl_tcp_reordering)
                                dst->metrics[RTAX_REORDERING-1] = tp->reordering;
                }
        }
}

/* Numbers are taken from RFC3390.
 *
 * John Heffner states:
 *
 *      The RFC specifies a window of no more than 4380 bytes
 *      unless 2*MSS > 4380.  Reading the pseudocode in the RFC
 *      is a bit misleading because they use a clamp at 4380 bytes
 *      rather than use a multiplier in the relevant range.
 */
__u32 tcp_init_cwnd(struct tcp_sock *tp, struct dst_entry *dst)
{
        __u32 cwnd = (dst ? dst_metric(dst, RTAX_INITCWND) : 0);

        if (!cwnd) {
                if (tp->mss_cache > 1460)
                        cwnd = 2;
                else
                        cwnd = (tp->mss_cache > 1095) ? 3 : 4;
        }
        return min_t(__u32, cwnd, tp->snd_cwnd_clamp);
}

/* Set slow start threshold and cwnd not falling to slow start */
void tcp_enter_cwr(struct sock *sk, const int set_ssthresh)
{
        struct tcp_sock *tp = tcp_sk(sk);
        const struct inet_connection_sock *icsk = inet_csk(sk);

        tp->prior_ssthresh = 0;
        tp->bytes_acked = 0;
        if (icsk->icsk_ca_state < TCP_CA_CWR) {
                tp->undo_marker = 0;
                if (set_ssthresh)
                        tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
                tp->snd_cwnd = min(tp->snd_cwnd,
                                   tcp_packets_in_flight(tp) + 1U);
                tp->snd_cwnd_cnt = 0;
                tp->high_seq = tp->snd_nxt;
                tp->snd_cwnd_stamp = tcp_time_stamp;
                TCP_ECN_queue_cwr(tp);

                tcp_set_ca_state(sk, TCP_CA_CWR);
        }
}

/*
 * Packet counting of FACK is based on in-order assumptions, therefore TCP
 * disables it when reordering is detected
 */
static void tcp_disable_fack(struct tcp_sock *tp)
{
        tp->rx_opt.sack_ok &= ~2;
}

/* Take a notice that peer is sending D-SACKs */
static void tcp_dsack_seen(struct tcp_sock *tp)
{
        tp->rx_opt.sack_ok |= 4;
}

/* Initialize metrics on socket. */

static void tcp_init_metrics(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct dst_entry *dst = __sk_dst_get(sk);

        if (dst == NULL)
                goto reset;

        dst_confirm(dst);

        if (dst_metric_locked(dst, RTAX_CWND))
                tp->snd_cwnd_clamp = dst_metric(dst, RTAX_CWND);
        if (dst_metric(dst, RTAX_SSTHRESH)) {
                tp->snd_ssthresh = dst_metric(dst, RTAX_SSTHRESH);
                if (tp->snd_ssthresh > tp->snd_cwnd_clamp)
                        tp->snd_ssthresh = tp->snd_cwnd_clamp;
        }
        if (dst_metric(dst, RTAX_REORDERING) &&
            tp->reordering != dst_metric(dst, RTAX_REORDERING)) {
                tcp_disable_fack(tp);
                tp->reordering = dst_metric(dst, RTAX_REORDERING);
        }

        if (dst_metric(dst, RTAX_RTT) == 0)
                goto reset;

        if (!tp->srtt && dst_metric(dst, RTAX_RTT) < (TCP_TIMEOUT_INIT << 3))
                goto reset;

        /* Initial rtt is determined from SYN,SYN-ACK.
         * The segment is small and rtt may appear much
         * less than real one. Use per-dst memory
         * to make it more realistic.
         *
         * A bit of theory. RTT is time passed after "normal" sized packet
         * is sent until it is ACKed. In normal circumstances sending small
         * packets force peer to delay ACKs and calculation is correct too.
         * The algorithm is adaptive and, provided we follow specs, it
         * NEVER underestimate RTT. BUT! If peer tries to make some clever
         * tricks sort of "quick acks" for time long enough to decrease RTT
         * to low value, and then abruptly stops to do it and starts to delay
         * ACKs, wait for troubles.
         */
        if (dst_metric(dst, RTAX_RTT) > tp->srtt) {
                tp->srtt = dst_metric(dst, RTAX_RTT);
                tp->rtt_seq = tp->snd_nxt;
        }
        if (dst_metric(dst, RTAX_RTTVAR) > tp->mdev) {
                tp->mdev = dst_metric(dst, RTAX_RTTVAR);
                tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
        }
        tcp_set_rto(sk);
        tcp_bound_rto(sk);
        if (inet_csk(sk)->icsk_rto < TCP_TIMEOUT_INIT && !tp->rx_opt.saw_tstamp)
                goto reset;
        tp->snd_cwnd = tcp_init_cwnd(tp, dst);
        tp->snd_cwnd_stamp = tcp_time_stamp;
        return;

reset:
        /* Play conservative. If timestamps are not
         * supported, TCP will fail to recalculate correct
         * rtt, if initial rto is too small. FORGET ALL AND RESET!
         */
        if (!tp->rx_opt.saw_tstamp && tp->srtt) {
                tp->srtt = 0;
                tp->mdev = tp->mdev_max = tp->rttvar = TCP_TIMEOUT_INIT;
                inet_csk(sk)->icsk_rto = TCP_TIMEOUT_INIT;
        }
}

static void tcp_update_reordering(struct sock *sk, const int metric,
                                  const int ts)
{
        struct tcp_sock *tp = tcp_sk(sk);
        if (metric > tp->reordering) {
                tp->reordering = min(TCP_MAX_REORDERING, metric);

                /* This exciting event is worth to be remembered. 8) */
                if (ts)
                        NET_INC_STATS_BH(LINUX_MIB_TCPTSREORDER);
                else if (tcp_is_reno(tp))
                        NET_INC_STATS_BH(LINUX_MIB_TCPRENOREORDER);
                else if (tcp_is_fack(tp))
                        NET_INC_STATS_BH(LINUX_MIB_TCPFACKREORDER);
                else
                        NET_INC_STATS_BH(LINUX_MIB_TCPSACKREORDER);
#if FASTRETRANS_DEBUG > 1
                printk(KERN_DEBUG "Disorder%d %d %u f%u s%u rr%d\n",
                       tp->rx_opt.sack_ok, inet_csk(sk)->icsk_ca_state,
                       tp->reordering,
                       tp->fackets_out,
                       tp->sacked_out,
                       tp->undo_marker ? tp->undo_retrans : 0);
#endif
                tcp_disable_fack(tp);
        }
}

/* This procedure tags the retransmission queue when SACKs arrive.
 *
 * We have three tag bits: SACKED(S), RETRANS(R) and LOST(L).
 * Packets in queue with these bits set are counted in variables
 * sacked_out, retrans_out and lost_out, correspondingly.
 *
 * Valid combinations are:
 * Tag  InFlight        Description
 * 0    1               - orig segment is in flight.
 * S    0               - nothing flies, orig reached receiver.
 * L    0               - nothing flies, orig lost by net.
 * R    2               - both orig and retransmit are in flight.
 * L|R  1               - orig is lost, retransmit is in flight.
 * S|R  1               - orig reached receiver, retrans is still in flight.
 * (L|S|R is logically valid, it could occur when L|R is sacked,
 *  but it is equivalent to plain S and code short-curcuits it to S.
 *  L|S is logically invalid, it would mean -1 packet in flight 8))
 *
 * These 6 states form finite state machine, controlled by the following events:
 * 1. New ACK (+SACK) arrives. (tcp_sacktag_write_queue())
 * 2. Retransmission. (tcp_retransmit_skb(), tcp_xmit_retransmit_queue())
 * 3. Loss detection event of one of three flavors:
 *      A. Scoreboard estimator decided the packet is lost.
 *         A'. Reno "three dupacks" marks head of queue lost.
 *         A''. Its FACK modfication, head until snd.fack is lost.

 *      B. SACK arrives sacking data transmitted after never retransmitted
 *         hole was sent out.
 *      C. SACK arrives sacking SND.NXT at the moment, when the
 *         segment was retransmitted.
 * 4. D-SACK added new rule: D-SACK changes any tag to S.
 *
 * It is pleasant to note, that state diagram turns out to be commutative,
 * so that we are allowed not to be bothered by order of our actions,
 * when multiple events arrive simultaneously. (see the function below).
 *
 * Reordering detection.
 * --------------------
 * Reordering metric is maximal distance, which a packet can be displaced
 * in packet stream. With SACKs we can estimate it:
 *
 * 1. SACK fills old hole and the corresponding segment was not
 *    ever retransmitted -> reordering. Alas, we cannot use it
 *    when segment was retransmitted.
 * 2. The last flaw is solved with D-SACK. D-SACK arrives
 *    for retransmitted and already SACKed segment -> reordering..
 * Both of these heuristics are not used in Loss state, when we cannot
 * account for retransmits accurately.
 *
 * SACK block validation.
 * ----------------------
 *
 * SACK block range validation checks that the received SACK block fits to
 * the expected sequence limits, i.e., it is between SND.UNA and SND.NXT.
 * Note that SND.UNA is not included to the range though being valid because
 * it means that the receiver is rather inconsistent with itself reporting
 * SACK reneging when it should advance SND.UNA. Such SACK block this is
 * perfectly valid, however, in light of RFC2018 which explicitly states
 * that "SACK block MUST reflect the newest segment.  Even if the newest
 * segment is going to be discarded ...", not that it looks very clever
 * in case of head skb. Due to potentional receiver driven attacks, we
 * choose to avoid immediate execution of a walk in write queue due to
 * reneging and defer head skb's loss recovery to standard loss recovery
 * procedure that will eventually trigger (nothing forbids us doing this).
 *
 * Implements also blockage to start_seq wrap-around. Problem lies in the
 * fact that though start_seq (s) is before end_seq (i.e., not reversed),
 * there's no guarantee that it will be before snd_nxt (n). The problem
 * happens when start_seq resides between end_seq wrap (e_w) and snd_nxt
 * wrap (s_w):
 *
 *         <- outs wnd ->                          <- wrapzone ->
 *         u     e      n                         u_w   e_w  s n_w
 *         |     |      |                          |     |   |  |
 * |<------------+------+----- TCP seqno space --------------+---------->|
 * ...-- <2^31 ->|                                           |<--------...
 * ...---- >2^31 ------>|                                    |<--------...
 *
 * Current code wouldn't be vulnerable but it's better still to discard such
 * crazy SACK blocks. Doing this check for start_seq alone closes somewhat
 * similar case (end_seq after snd_nxt wrap) as earlier reversed check in
 * snd_nxt wrap -> snd_una region will then become "well defined", i.e.,
 * equal to the ideal case (infinite seqno space without wrap caused issues).
 *
 * With D-SACK the lower bound is extended to cover sequence space below
 * SND.UNA down to undo_marker, which is the last point of interest. Yet
 * again, D-SACK block must not to go across snd_una (for the same reason as
 * for the normal SACK blocks, explained above). But there all simplicity
 * ends, TCP might receive valid D-SACKs below that. As long as they reside
 * fully below undo_marker they do not affect behavior in anyway and can
 * therefore be safely ignored. In rare cases (which are more or less
 * theoretical ones), the D-SACK will nicely cross that boundary due to skb
 * fragmentation and packet reordering past skb's retransmission. To consider
 * them correctly, the acceptable range must be extended even more though
 * the exact amount is rather hard to quantify. However, tp->max_window can
 * be used as an exaggerated estimate.
 */
static int tcp_is_sackblock_valid(struct tcp_sock *tp, int is_dsack,
                                  u32 start_seq, u32 end_seq)
{
        /* Too far in future, or reversed (interpretation is ambiguous) */
        if (after(end_seq, tp->snd_nxt) || !before(start_seq, end_seq))
                return 0;

        /* Nasty start_seq wrap-around check (see comments above) */
        if (!before(start_seq, tp->snd_nxt))
                return 0;

        /* In outstanding window? ...This is valid exit for D-SACKs too.
         * start_seq == snd_una is non-sensical (see comments above)
         */
        if (after(start_seq, tp->snd_una))
                return 1;

        if (!is_dsack || !tp->undo_marker)
                return 0;

        /* ...Then it's D-SACK, and must reside below snd_una completely */
        if (!after(end_seq, tp->snd_una))
                return 0;

        if (!before(start_seq, tp->undo_marker))
                return 1;

        /* Too old */
        if (!after(end_seq, tp->undo_marker))
                return 0;

        /* Undo_marker boundary crossing (overestimates a lot). Known already:
         *   start_seq < undo_marker and end_seq >= undo_marker.
         */
        return !before(start_seq, end_seq - tp->max_window);
}

/* Check for lost retransmit. This superb idea is borrowed from "ratehalving".
 * Event "C". Later note: FACK people cheated me again 8), we have to account
 * for reordering! Ugly, but should help.
 *
 * Search retransmitted skbs from write_queue that were sent when snd_nxt was
 * less than what is now known to be received by the other end (derived from
 * SACK blocks by the caller). Also calculate the lowest snd_nxt among the
 * remaining retransmitted skbs to avoid some costly processing per ACKs.
 */
static int tcp_mark_lost_retrans(struct sock *sk, u32 received_upto)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
        int flag = 0;
        int cnt = 0;
        u32 new_low_seq = tp->snd_nxt;

        tcp_for_write_queue(skb, sk) {
                u32 ack_seq = TCP_SKB_CB(skb)->ack_seq;

                if (skb == tcp_send_head(sk))
                        break;
                if (cnt == tp->retrans_out)
                        break;
                if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
                        continue;

                if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS))
                        continue;

                if (after(received_upto, ack_seq) &&
                    (tcp_is_fack(tp) ||
                     !before(received_upto,
                             ack_seq + tp->reordering * tp->mss_cache))) {
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                        tp->retrans_out -= tcp_skb_pcount(skb);

                        /* clear lost hint */
                        tp->retransmit_skb_hint = NULL;

                        if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) {
                                tp->lost_out += tcp_skb_pcount(skb);
                                TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                                flag |= FLAG_DATA_SACKED;
                                NET_INC_STATS_BH(LINUX_MIB_TCPLOSTRETRANSMIT);
                        }
                } else {
                        if (before(ack_seq, new_low_seq))
                                new_low_seq = ack_seq;
                        cnt += tcp_skb_pcount(skb);
                }
        }

        if (tp->retrans_out)
                tp->lost_retrans_low = new_low_seq;

        return flag;
}

static int tcp_check_dsack(struct tcp_sock *tp, struct sk_buff *ack_skb,
                           struct tcp_sack_block_wire *sp, int num_sacks,
                           u32 prior_snd_una)
{
        u32 start_seq_0 = ntohl(get_unaligned(&sp[0].start_seq));
        u32 end_seq_0 = ntohl(get_unaligned(&sp[0].end_seq));
        int dup_sack = 0;

        if (before(start_seq_0, TCP_SKB_CB(ack_skb)->ack_seq)) {
                dup_sack = 1;
                tcp_dsack_seen(tp);
                NET_INC_STATS_BH(LINUX_MIB_TCPDSACKRECV);
        } else if (num_sacks > 1) {
                u32 end_seq_1 = ntohl(get_unaligned(&sp[1].end_seq));
                u32 start_seq_1 = ntohl(get_unaligned(&sp[1].start_seq));

                if (!after(end_seq_0, end_seq_1) &&
                    !before(start_seq_0, start_seq_1)) {
                        dup_sack = 1;
                        tcp_dsack_seen(tp);
                        NET_INC_STATS_BH(LINUX_MIB_TCPDSACKOFORECV);
                }
        }

        /* D-SACK for already forgotten data... Do dumb counting. */
        if (dup_sack &&
            !after(end_seq_0, prior_snd_una) &&
            after(end_seq_0, tp->undo_marker))
                tp->undo_retrans--;

        return dup_sack;
}

/* Check if skb is fully within the SACK block. In presence of GSO skbs,
 * the incoming SACK may not exactly match but we can find smaller MSS
 * aligned portion of it that matches. Therefore we might need to fragment
 * which may fail and creates some hassle (caller must handle error case
 * returns).
 */
static int tcp_match_skb_to_sack(struct sock *sk, struct sk_buff *skb,
                                 u32 start_seq, u32 end_seq)
{
        int in_sack, err;
        unsigned int pkt_len;

        in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
                  !before(end_seq, TCP_SKB_CB(skb)->end_seq);

        if (tcp_skb_pcount(skb) > 1 && !in_sack &&
            after(TCP_SKB_CB(skb)->end_seq, start_seq)) {

                in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);

                if (!in_sack)
                        pkt_len = start_seq - TCP_SKB_CB(skb)->seq;
                else
                        pkt_len = end_seq - TCP_SKB_CB(skb)->seq;
                err = tcp_fragment(sk, skb, pkt_len, skb_shinfo(skb)->gso_size);
                if (err < 0)
                        return err;
        }

        return in_sack;
}

static int
tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_una)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        unsigned char *ptr = (skb_transport_header(ack_skb) +
                              TCP_SKB_CB(ack_skb)->sacked);
        struct tcp_sack_block_wire *sp = (struct tcp_sack_block_wire *)(ptr+2);
        struct sk_buff *cached_skb;
        int num_sacks = (ptr[1] - TCPOLEN_SACK_BASE)>>3;
        int reord = tp->packets_out;
        int prior_fackets;
        u32 highest_sack_end_seq = tp->lost_retrans_low;
        int flag = 0;
        int found_dup_sack = 0;
        int cached_fack_count;
        int i;
        int first_sack_index;
        int force_one_sack;

        if (!tp->sacked_out) {
                if (WARN_ON(tp->fackets_out))
                        tp->fackets_out = 0;
                tp->highest_sack = tp->snd_una;
        }
        prior_fackets = tp->fackets_out;

        found_dup_sack = tcp_check_dsack(tp, ack_skb, sp,
                                         num_sacks, prior_snd_una);
        if (found_dup_sack)
                flag |= FLAG_DSACKING_ACK;

        /* Eliminate too old ACKs, but take into
         * account more or less fresh ones, they can
         * contain valid SACK info.
         */
        if (before(TCP_SKB_CB(ack_skb)->ack_seq, prior_snd_una - tp->max_window))
                return 0;

        if (!tp->packets_out)
                goto out;

        /* SACK fastpath:
         * if the only SACK change is the increase of the end_seq of
         * the first block then only apply that SACK block
         * and use retrans queue hinting otherwise slowpath */
        force_one_sack = 1;
        for (i = 0; i < num_sacks; i++) {
                __be32 start_seq = sp[i].start_seq;
                __be32 end_seq = sp[i].end_seq;

                if (i == 0) {
                        if (tp->recv_sack_cache[i].start_seq != start_seq)
                                force_one_sack = 0;
                } else {
                        if ((tp->recv_sack_cache[i].start_seq != start_seq) ||
                            (tp->recv_sack_cache[i].end_seq != end_seq))
                                force_one_sack = 0;
                }
                tp->recv_sack_cache[i].start_seq = start_seq;
                tp->recv_sack_cache[i].end_seq = end_seq;
        }
        /* Clear the rest of the cache sack blocks so they won't match mistakenly. */
        for (; i < ARRAY_SIZE(tp->recv_sack_cache); i++) {
                tp->recv_sack_cache[i].start_seq = 0;
                tp->recv_sack_cache[i].end_seq = 0;
        }

        first_sack_index = 0;
        if (force_one_sack)
                num_sacks = 1;
        else {
                int j;
                tp->fastpath_skb_hint = NULL;

                /* order SACK blocks to allow in order walk of the retrans queue */
                for (i = num_sacks-1; i > 0; i--) {
                        for (j = 0; j < i; j++){
                                if (after(ntohl(sp[j].start_seq),
                                          ntohl(sp[j+1].start_seq))){
                                        struct tcp_sack_block_wire tmp;

                                        tmp = sp[j];
                                        sp[j] = sp[j+1];
                                        sp[j+1] = tmp;

                                        /* Track where the first SACK block goes to */
                                        if (j == first_sack_index)
                                                first_sack_index = j+1;
                                }

                        }
                }
        }

        /* Use SACK fastpath hint if valid */
        cached_skb = tp->fastpath_skb_hint;
        cached_fack_count = tp->fastpath_cnt_hint;
        if (!cached_skb) {
                cached_skb = tcp_write_queue_head(sk);
                cached_fack_count = 0;
        }

        for (i = 0; i < num_sacks; i++) {
                struct sk_buff *skb;
                __u32 start_seq = ntohl(sp->start_seq);
                __u32 end_seq = ntohl(sp->end_seq);
                int fack_count;
                int dup_sack = (found_dup_sack && (i == first_sack_index));
                int next_dup = (found_dup_sack && (i+1 == first_sack_index));

                sp++;

                if (!tcp_is_sackblock_valid(tp, dup_sack, start_seq, end_seq)) {
                        if (dup_sack) {
                                if (!tp->undo_marker)
                                        NET_INC_STATS_BH(LINUX_MIB_TCPDSACKIGNOREDNOUNDO);
                                else
                                        NET_INC_STATS_BH(LINUX_MIB_TCPDSACKIGNOREDOLD);
                        } else {
                                /* Don't count olds caused by ACK reordering */
                                if ((TCP_SKB_CB(ack_skb)->ack_seq != tp->snd_una) &&
                                    !after(end_seq, tp->snd_una))
                                        continue;
                                NET_INC_STATS_BH(LINUX_MIB_TCPSACKDISCARD);
                        }
                        continue;
                }

                skb = cached_skb;
                fack_count = cached_fack_count;

                /* Event "B" in the comment above. */
                if (after(end_seq, tp->high_seq))
                        flag |= FLAG_DATA_LOST;

                tcp_for_write_queue_from(skb, sk) {
                        int in_sack = 0;
                        u8 sacked;

                        if (skb == tcp_send_head(sk))
                                break;

                        cached_skb = skb;
                        cached_fack_count = fack_count;
                        if (i == first_sack_index) {
                                tp->fastpath_skb_hint = skb;
                                tp->fastpath_cnt_hint = fack_count;
                        }

                        /* The retransmission queue is always in order, so
                         * we can short-circuit the walk early.
                         */
                        if (!before(TCP_SKB_CB(skb)->seq, end_seq))
                                break;

                        dup_sack = (found_dup_sack && (i == first_sack_index));

                        /* Due to sorting DSACK may reside within this SACK block! */
                        if (next_dup) {
                                u32 dup_start = ntohl(sp->start_seq);
                                u32 dup_end = ntohl(sp->end_seq);

                                if (before(TCP_SKB_CB(skb)->seq, dup_end)) {
                                        in_sack = tcp_match_skb_to_sack(sk, skb, dup_start, dup_end);
                                        if (in_sack > 0)
                                                dup_sack = 1;
                                }
                        }

                        /* DSACK info lost if out-of-mem, try SACK still */
                        if (in_sack <= 0)
                                in_sack = tcp_match_skb_to_sack(sk, skb, start_seq, end_seq);
                        if (unlikely(in_sack < 0))
                                break;

                        sacked = TCP_SKB_CB(skb)->sacked;

                        /* Account D-SACK for retransmitted packet. */
                        if ((dup_sack && in_sack) &&
                            (sacked & TCPCB_RETRANS) &&
                            after(TCP_SKB_CB(skb)->end_seq, tp->undo_marker))
                                tp->undo_retrans--;

                        /* The frame is ACKed. */
                        if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una)) {
                                if (sacked&TCPCB_RETRANS) {
                                        if ((dup_sack && in_sack) &&
                                            (sacked&TCPCB_SACKED_ACKED))
                                                reord = min(fack_count, reord);
                                }

                                /* Nothing to do; acked frame is about to be dropped. */
                                fack_count += tcp_skb_pcount(skb);
                                continue;
                        }

                        if (!in_sack) {
                                fack_count += tcp_skb_pcount(skb);
                                continue;
                        }

                        if (!(sacked&TCPCB_SACKED_ACKED)) {
                                if (sacked & TCPCB_SACKED_RETRANS) {
                                        /* If the segment is not tagged as lost,
                                         * we do not clear RETRANS, believing
                                         * that retransmission is still in flight.
                                         */
                                        if (sacked & TCPCB_LOST) {
                                                TCP_SKB_CB(skb)->sacked &= ~(TCPCB_LOST|TCPCB_SACKED_RETRANS);
                                                tp->lost_out -= tcp_skb_pcount(skb);
                                                tp->retrans_out -= tcp_skb_pcount(skb);

                                                /* clear lost hint */
                                                tp->retransmit_skb_hint = NULL;
                                        }
                                } else {
                                        if (!(sacked & TCPCB_RETRANS)) {
                                                /* New sack for not retransmitted frame,
                                                 * which was in hole. It is reordering.
                                                 */
                                                if (fack_count < prior_fackets)
                                                        reord = min(fack_count, reord);

                                                /* SACK enhanced F-RTO (RFC4138; Appendix B) */
                                                if (!after(TCP_SKB_CB(skb)->end_seq, tp->frto_highmark))
                                                        flag |= FLAG_ONLY_ORIG_SACKED;
                                        }

                                        if (sacked & TCPCB_LOST) {
                                                TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
                                                tp->lost_out -= tcp_skb_pcount(skb);

                                                /* clear lost hint */
                                                tp->retransmit_skb_hint = NULL;
                                        }
                                }

                                TCP_SKB_CB(skb)->sacked |= TCPCB_SACKED_ACKED;
                                flag |= FLAG_DATA_SACKED;
                                tp->sacked_out += tcp_skb_pcount(skb);

                                fack_count += tcp_skb_pcount(skb);
                                if (fack_count > tp->fackets_out)
                                        tp->fackets_out = fack_count;

                                if (after(TCP_SKB_CB(skb)->seq, tp->highest_sack)) {
                                        tp->highest_sack = TCP_SKB_CB(skb)->seq;
                                        highest_sack_end_seq = TCP_SKB_CB(skb)->end_seq;
                                }
                        } else {
                                if (dup_sack && (sacked&TCPCB_RETRANS))
                                        reord = min(fack_count, reord);

                                fack_count += tcp_skb_pcount(skb);
                        }

                        /* D-SACK. We can detect redundant retransmission
                         * in S|R and plain R frames and clear it.
                         * undo_retrans is decreased above, L|R frames
                         * are accounted above as well.
                         */
                        if (dup_sack &&
                            (TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_RETRANS)) {
                                TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                                tp->retrans_out -= tcp_skb_pcount(skb);
                                tp->retransmit_skb_hint = NULL;
                        }
                }

                /* SACK enhanced FRTO (RFC4138, Appendix B): Clearing correct
                 * due to in-order walk
                 */
                if (after(end_seq, tp->frto_highmark))
                        flag &= ~FLAG_ONLY_ORIG_SACKED;
        }

        if (tp->retrans_out &&
            after(highest_sack_end_seq, tp->lost_retrans_low) &&
            icsk->icsk_ca_state == TCP_CA_Recovery)
                flag |= tcp_mark_lost_retrans(sk, highest_sack_end_seq);

        tcp_verify_left_out(tp);

        if ((reord < tp->fackets_out) && icsk->icsk_ca_state != TCP_CA_Loss &&
            (!tp->frto_highmark || after(tp->snd_una, tp->frto_highmark)))
                tcp_update_reordering(sk, tp->fackets_out - reord, 0);

out:

#if FASTRETRANS_DEBUG > 0
        BUG_TRAP((int)tp->sacked_out >= 0);
        BUG_TRAP((int)tp->lost_out >= 0);
        BUG_TRAP((int)tp->retrans_out >= 0);
        BUG_TRAP((int)tcp_packets_in_flight(tp) >= 0);
#endif
        return flag;
}

/* If we receive more dupacks than we expected counting segments
 * in assumption of absent reordering, interpret this as reordering.
 * The only another reason could be bug in receiver TCP.
 */
static void tcp_check_reno_reordering(struct sock *sk, const int addend)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 holes;

        holes = max(tp->lost_out, 1U);
        holes = min(holes, tp->packets_out);

        if ((tp->sacked_out + holes) > tp->packets_out) {
                tp->sacked_out = tp->packets_out - holes;
                tcp_update_reordering(sk, tp->packets_out + addend, 0);
        }
}

/* Emulate SACKs for SACKless connection: account for a new dupack. */

static void tcp_add_reno_sack(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        tp->sacked_out++;
        tcp_check_reno_reordering(sk, 0);
        tcp_verify_left_out(tp);
}

/* Account for ACK, ACKing some data in Reno Recovery phase. */

static void tcp_remove_reno_sacks(struct sock *sk, int acked)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (acked > 0) {
                /* One ACK acked hole. The rest eat duplicate ACKs. */
                if (acked-1 >= tp->sacked_out)
                        tp->sacked_out = 0;
                else
                        tp->sacked_out -= acked-1;
        }
        tcp_check_reno_reordering(sk, acked);
        tcp_verify_left_out(tp);
}

static inline void tcp_reset_reno_sack(struct tcp_sock *tp)
{
        tp->sacked_out = 0;
}

/* F-RTO can only be used if TCP has never retransmitted anything other than
 * head (SACK enhanced variant from Appendix B of RFC4138 is more robust here)
 */
int tcp_use_frto(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;

        if (!sysctl_tcp_frto)
                return 0;

        if (IsSackFrto())
                return 1;

        /* Avoid expensive walking of rexmit queue if possible */
        if (tp->retrans_out > 1)
                return 0;

        skb = tcp_write_queue_head(sk);
        skb = tcp_write_queue_next(sk, skb);    /* Skips head */
        tcp_for_write_queue_from(skb, sk) {
                if (skb == tcp_send_head(sk))
                        break;
                if (TCP_SKB_CB(skb)->sacked&TCPCB_RETRANS)
                        return 0;
                /* Short-circuit when first non-SACKed skb has been checked */
                if (!(TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_ACKED))
                        break;
        }
        return 1;
}

/* RTO occurred, but do not yet enter Loss state. Instead, defer RTO
 * recovery a bit and use heuristics in tcp_process_frto() to detect if
 * the RTO was spurious. Only clear SACKED_RETRANS of the head here to
 * keep retrans_out counting accurate (with SACK F-RTO, other than head
 * may still have that bit set); TCPCB_LOST and remaining SACKED_RETRANS
 * bits are handled if the Loss state is really to be entered (in
 * tcp_enter_frto_loss).
 *
 * Do like tcp_enter_loss() would; when RTO expires the second time it
 * does:
 *  "Reduce ssthresh if it has not yet been made inside this window."
 */
void tcp_enter_frto(struct sock *sk)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;

        if ((!tp->frto_counter && icsk->icsk_ca_state <= TCP_CA_Disorder) ||
            tp->snd_una == tp->high_seq ||
            ((icsk->icsk_ca_state == TCP_CA_Loss || tp->frto_counter) &&
             !icsk->icsk_retransmits)) {
                tp->prior_ssthresh = tcp_current_ssthresh(sk);
                /* Our state is too optimistic in ssthresh() call because cwnd
                 * is not reduced until tcp_enter_frto_loss() when previous F-RTO
                 * recovery has not yet completed. Pattern would be this: RTO,
                 * Cumulative ACK, RTO (2xRTO for the same segment does not end
                 * up here twice).
                 * RFC4138 should be more specific on what to do, even though
                 * RTO is quite unlikely to occur after the first Cumulative ACK
                 * due to back-off and complexity of triggering events ...
                 */
                if (tp->frto_counter) {
                        u32 stored_cwnd;
                        stored_cwnd = tp->snd_cwnd;
                        tp->snd_cwnd = 2;
                        tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
                        tp->snd_cwnd = stored_cwnd;
                } else {
                        tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
                }
                /* ... in theory, cong.control module could do "any tricks" in
                 * ssthresh(), which means that ca_state, lost bits and lost_out
                 * counter would have to be faked before the call occurs. We
                 * consider that too expensive, unlikely and hacky, so modules
                 * using these in ssthresh() must deal these incompatibility
                 * issues if they receives CA_EVENT_FRTO and frto_counter != 0
                 */
                tcp_ca_event(sk, CA_EVENT_FRTO);
        }

        tp->undo_marker = tp->snd_una;
        tp->undo_retrans = 0;

        skb = tcp_write_queue_head(sk);
        if (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS)
                tp->undo_marker = 0;
        if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
                TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                tp->retrans_out -= tcp_skb_pcount(skb);
        }
        tcp_verify_left_out(tp);

        /* Too bad if TCP was application limited */
        tp->snd_cwnd = min(tp->snd_cwnd, tcp_packets_in_flight(tp) + 1);

        /* Earlier loss recovery underway (see RFC4138; Appendix B).
         * The last condition is necessary at least in tp->frto_counter case.
         */
        if (IsSackFrto() && (tp->frto_counter ||
            ((1 << icsk->icsk_ca_state) & (TCPF_CA_Recovery|TCPF_CA_Loss))) &&
            after(tp->high_seq, tp->snd_una)) {
                tp->frto_highmark = tp->high_seq;
        } else {
                tp->frto_highmark = tp->snd_nxt;
        }
        tcp_set_ca_state(sk, TCP_CA_Disorder);
        tp->high_seq = tp->snd_nxt;
        tp->frto_counter = 1;
}

/* Enter Loss state after F-RTO was applied. Dupack arrived after RTO,
 * which indicates that we should follow the traditional RTO recovery,
 * i.e. mark everything lost and do go-back-N retransmission.
 */
static void tcp_enter_frto_loss(struct sock *sk, int allowed_segments, int flag)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;

        tp->lost_out = 0;
        tp->retrans_out = 0;
        if (tcp_is_reno(tp))
                tcp_reset_reno_sack(tp);

        tcp_for_write_queue(skb, sk) {
                if (skb == tcp_send_head(sk))
                        break;

                TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
                /*
                 * Count the retransmission made on RTO correctly (only when
                 * waiting for the first ACK and did not get it)...
                 */
                if ((tp->frto_counter == 1) && !(flag&FLAG_DATA_ACKED)) {
                        /* For some reason this R-bit might get cleared? */
                        if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
                                tp->retrans_out += tcp_skb_pcount(skb);
                        /* ...enter this if branch just for the first segment */
                        flag |= FLAG_DATA_ACKED;
                } else {
                        if (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS)
                                tp->undo_marker = 0;
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                }

                /* Don't lost mark skbs that were fwd transmitted after RTO */
                if (!(TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_ACKED) &&
                    !after(TCP_SKB_CB(skb)->end_seq, tp->frto_highmark)) {
                        TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                        tp->lost_out += tcp_skb_pcount(skb);
                }
        }
        tcp_verify_left_out(tp);

        tp->snd_cwnd = tcp_packets_in_flight(tp) + allowed_segments;
        tp->snd_cwnd_cnt = 0;
        tp->snd_cwnd_stamp = tcp_time_stamp;
        tp->frto_counter = 0;
        tp->bytes_acked = 0;

        tp->reordering = min_t(unsigned int, tp->reordering,
                                             sysctl_tcp_reordering);
        tcp_set_ca_state(sk, TCP_CA_Loss);
        tp->high_seq = tp->frto_highmark;
        TCP_ECN_queue_cwr(tp);

        tcp_clear_retrans_hints_partial(tp);
}

static void tcp_clear_retrans_partial(struct tcp_sock *tp)
{
        tp->retrans_out = 0;
        tp->lost_out = 0;

        tp->undo_marker = 0;
        tp->undo_retrans = 0;
}

void tcp_clear_retrans(struct tcp_sock *tp)
{
        tcp_clear_retrans_partial(tp);

        tp->fackets_out = 0;
        tp->sacked_out = 0;
}

/* Enter Loss state. If "how" is not zero, forget all SACK information
 * and reset tags completely, otherwise preserve SACKs. If receiver
 * dropped its ofo queue, we will know this due to reneging detection.
 */
void tcp_enter_loss(struct sock *sk, int how)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;

        /* Reduce ssthresh if it has not yet been made inside this window. */
        if (icsk->icsk_ca_state <= TCP_CA_Disorder || tp->snd_una == tp->high_seq ||
            (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
                tp->prior_ssthresh = tcp_current_ssthresh(sk);
                tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
                tcp_ca_event(sk, CA_EVENT_LOSS);
        }
        tp->snd_cwnd       = 1;
        tp->snd_cwnd_cnt   = 0;
        tp->snd_cwnd_stamp = tcp_time_stamp;

        tp->bytes_acked = 0;
        tcp_clear_retrans_partial(tp);

        if (tcp_is_reno(tp))
                tcp_reset_reno_sack(tp);

        if (!how) {
                /* Push undo marker, if it was plain RTO and nothing
                 * was retransmitted. */
                tp->undo_marker = tp->snd_una;
                tcp_clear_retrans_hints_partial(tp);
        } else {
                tp->sacked_out = 0;
                tp->fackets_out = 0;
                tcp_clear_all_retrans_hints(tp);
        }

        tcp_for_write_queue(skb, sk) {
                if (skb == tcp_send_head(sk))
                        break;

                if (TCP_SKB_CB(skb)->sacked&TCPCB_RETRANS)
                        tp->undo_marker = 0;
                TCP_SKB_CB(skb)->sacked &= (~TCPCB_TAGBITS)|TCPCB_SACKED_ACKED;
                if (!(TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_ACKED) || how) {
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
                        TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                        tp->lost_out += tcp_skb_pcount(skb);
                }
        }
        tcp_verify_left_out(tp);

        tp->reordering = min_t(unsigned int, tp->reordering,
                                             sysctl_tcp_reordering);
        tcp_set_ca_state(sk, TCP_CA_Loss);
        tp->high_seq = tp->snd_nxt;
        TCP_ECN_queue_cwr(tp);
        /* Abort F-RTO algorithm if one is in progress */
        tp->frto_counter = 0;
}

static int tcp_check_sack_reneging(struct sock *sk)
{
        struct sk_buff *skb;

        /* If ACK arrived pointing to a remembered SACK,
         * it means that our remembered SACKs do not reflect
         * real state of receiver i.e.
         * receiver _host_ is heavily congested (or buggy).
         * Do processing similar to RTO timeout.
         */
        if ((skb = tcp_write_queue_head(sk)) != NULL &&
            (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) {
                struct inet_connection_sock *icsk = inet_csk(sk);
                NET_INC_STATS_BH(LINUX_MIB_TCPSACKRENEGING);

                tcp_enter_loss(sk, 1);
                icsk->icsk_retransmits++;
                tcp_retransmit_skb(sk, tcp_write_queue_head(sk));
                inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                          icsk->icsk_rto, TCP_RTO_MAX);
                return 1;
        }
        return 0;
}

static inline int tcp_fackets_out(struct tcp_sock *tp)
{
        return tcp_is_reno(tp) ? tp->sacked_out+1 : tp->fackets_out;
}

static inline int tcp_skb_timedout(struct sock *sk, struct sk_buff *skb)
{
        return (tcp_time_stamp - TCP_SKB_CB(skb)->when > inet_csk(sk)->icsk_rto);
}

static inline int tcp_head_timedout(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        return tp->packets_out &&
               tcp_skb_timedout(sk, tcp_write_queue_head(sk));
}

/* Linux NewReno/SACK/FACK/ECN state machine.
 * --------------------------------------
 *
 * "Open"       Normal state, no dubious events, fast path.
 * "Disorder"   In all the respects it is "Open",
 *              but requires a bit more attention. It is entered when
 *              we see some SACKs or dupacks. It is split of "Open"
 *              mainly to move some processing from fast path to slow one.
 * "CWR"        CWND was reduced due to some Congestion Notification event.
 *              It can be ECN, ICMP source quench, local device congestion.
 * "Recovery"   CWND was reduced, we are fast-retransmitting.
 * "Loss"       CWND was reduced due to RTO timeout or SACK reneging.
 *
 * tcp_fastretrans_alert() is entered:
 * - each incoming ACK, if state is not "Open"
 * - when arrived ACK is unusual, namely:
 *      * SACK
 *      * Duplicate ACK.
 *      * ECN ECE.
 *
 * Counting packets in flight is pretty simple.
 *
 *      in_flight = packets_out - left_out + retrans_out
 *
 *      packets_out is SND.NXT-SND.UNA counted in packets.
 *
 *      retrans_out is number of retransmitted segments.
 *
 *      left_out is number of segments left network, but not ACKed yet.
 *
 *              left_out = sacked_out + lost_out
 *
 *     sacked_out: Packets, which arrived to receiver out of order
 *                 and hence not ACKed. With SACKs this number is simply
 *                 amount of SACKed data. Even without SACKs
 *                 it is easy to give pretty reliable estimate of this number,
 *                 counting duplicate ACKs.
 *
 *       lost_out: Packets lost by network. TCP has no explicit
 *                 "loss notification" feedback from network (for now).
 *                 It means that this number can be only _guessed_.
 *                 Actually, it is the heuristics to predict lossage that
 *                 distinguishes different algorithms.
 *
 *      F.e. after RTO, when all the queue is considered as lost,
 *      lost_out = packets_out and in_flight = retrans_out.
 *
 *              Essentially, we have now two algorithms counting
 *              lost packets.
 *
 *              FACK: It is the simplest heuristics. As soon as we decided
 *              that something is lost, we decide that _all_ not SACKed
 *              packets until the most forward SACK are lost. I.e.
 *              lost_out = fackets_out - sacked_out and left_out = fackets_out.
 *              It is absolutely correct estimate, if network does not reorder
 *              packets. And it loses any connection to reality when reordering
 *              takes place. We use FACK by default until reordering
 *              is suspected on the path to this destination.
 *
 *              NewReno: when Recovery is entered, we assume that one segment
 *              is lost (classic Reno). While we are in Recovery and
 *              a partial ACK arrives, we assume that one more packet
 *              is lost (NewReno). This heuristics are the same in NewReno
 *              and SACK.
 *
 *  Imagine, that's all! Forget about all this shamanism about CWND inflation
 *  deflation etc. CWND is real congestion window, never inflated, changes
 *  only according to classic VJ rules.
 *
 * Really tricky (and requiring careful tuning) part of algorithm
 * is hidden in functions tcp_time_to_recover() and tcp_xmit_retransmit_queue().
 * The first determines the moment _when_ we should reduce CWND and,
 * hence, slow down forward transmission. In fact, it determines the moment
 * when we decide that hole is caused by loss, rather than by a reorder.
 *
 * tcp_xmit_retransmit_queue() decides, _what_ we should retransmit to fill
 * holes, caused by lost packets.
 *
 * And the most logically complicated part of algorithm is undo
 * heuristics. We detect false retransmits due to both too early
 * fast retransmit (reordering) and underestimated RTO, analyzing
 * timestamps and D-SACKs. When we detect that some segments were
 * retransmitted by mistake and CWND reduction was wrong, we undo
 * window reduction and abort recovery phase. This logic is hidden
 * inside several functions named tcp_try_undo_<something>.
 */

/* This function decides, when we should leave Disordered state
 * and enter Recovery phase, reducing congestion window.
 *
 * Main question: may we further continue forward transmission
 * with the same cwnd?
 */
static int tcp_time_to_recover(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        __u32 packets_out;

        /* Do not perform any recovery during F-RTO algorithm */
        if (tp->frto_counter)
                return 0;

        /* Trick#1: The loss is proven. */
        if (tp->lost_out)
                return 1;

        /* Not-A-Trick#2 : Classic rule... */
        if (tcp_fackets_out(tp) > tp->reordering)
                return 1;

        /* Trick#3 : when we use RFC2988 timer restart, fast
         * retransmit can be triggered by timeout of queue head.
         */
        if (tcp_head_timedout(sk))
                return 1;

        /* Trick#4: It is still not OK... But will it be useful to delay
         * recovery more?
         */
        packets_out = tp->packets_out;
        if (packets_out <= tp->reordering &&
            tp->sacked_out >= max_t(__u32, packets_out/2, sysctl_tcp_reordering) &&
            !tcp_may_send_now(sk)) {
                /* We have nothing to send. This connection is limited
                 * either by receiver window or by application.

                 */
                return 1;
        }

        return 0;
}

/* RFC: This is from the original, I doubt that this is necessary at all:
 * clear xmit_retrans hint if seq of this skb is beyond hint. How could we
 * retransmitted past LOST markings in the first place? I'm not fully sure
 * about undo and end of connection cases, which can cause R without L?
 */
static void tcp_verify_retransmit_hint(struct tcp_sock *tp,
                                       struct sk_buff *skb)
{
        if ((tp->retransmit_skb_hint != NULL) &&
            before(TCP_SKB_CB(skb)->seq,
            TCP_SKB_CB(tp->retransmit_skb_hint)->seq))
                tp->retransmit_skb_hint = NULL;
}

/* Mark head of queue up as lost. */
static void tcp_mark_head_lost(struct sock *sk, int packets)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
        int cnt;

        BUG_TRAP(packets <= tp->packets_out);
        if (tp->lost_skb_hint) {
                skb = tp->lost_skb_hint;
                cnt = tp->lost_cnt_hint;
        } else {
                skb = tcp_write_queue_head(sk);
                cnt = 0;
        }

        tcp_for_write_queue_from(skb, sk) {
                if (skb == tcp_send_head(sk))
                        break;
                /* TODO: do this better */
                /* this is not the most efficient way to do this... */
                tp->lost_skb_hint = skb;
                tp->lost_cnt_hint = cnt;
                cnt += tcp_skb_pcount(skb);
                if (cnt > packets || after(TCP_SKB_CB(skb)->end_seq, tp->high_seq))
                        break;
                if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_SACKED_ACKED|TCPCB_LOST))) {
                        TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                        tp->lost_out += tcp_skb_pcount(skb);
                        tcp_verify_retransmit_hint(tp, skb);
                }
        }
        tcp_verify_left_out(tp);
}

/* Account newly detected lost packet(s) */

static void tcp_update_scoreboard(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tcp_is_fack(tp)) {
                int lost = tp->fackets_out - tp->reordering;
                if (lost <= 0)
                        lost = 1;
                tcp_mark_head_lost(sk, lost);
        } else {
                tcp_mark_head_lost(sk, 1);
        }

        /* New heuristics: it is possible only after we switched
         * to restart timer each time when something is ACKed.
         * Hence, we can detect timed out packets during fast
         * retransmit without falling to slow start.
         */
        if (!tcp_is_reno(tp) && tcp_head_timedout(sk)) {
                struct sk_buff *skb;

                skb = tp->scoreboard_skb_hint ? tp->scoreboard_skb_hint
                        : tcp_write_queue_head(sk);

                tcp_for_write_queue_from(skb, sk) {
                        if (skb == tcp_send_head(sk))
                                break;
                        if (!tcp_skb_timedout(sk, skb))
                                break;

                        if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_SACKED_ACKED|TCPCB_LOST))) {
                                TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
                                tp->lost_out += tcp_skb_pcount(skb);
                                tcp_verify_retransmit_hint(tp, skb);
                        }
                }

                tp->scoreboard_skb_hint = skb;

                tcp_verify_left_out(tp);
        }
}

/* CWND moderation, preventing bursts due to too big ACKs
 * in dubious situations.
 */
static inline void tcp_moderate_cwnd(struct tcp_sock *tp)
{
        tp->snd_cwnd = min(tp->snd_cwnd,
                           tcp_packets_in_flight(tp)+tcp_max_burst(tp));
        tp->snd_cwnd_stamp = tcp_time_stamp;
}

/* Lower bound on congestion window is slow start threshold
 * unless congestion avoidance choice decides to overide it.
 */
static inline u32 tcp_cwnd_min(const struct sock *sk)
{
        const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;

        return ca_ops->min_cwnd ? ca_ops->min_cwnd(sk) : tcp_sk(sk)->snd_ssthresh;
}

/* Decrease cwnd each second ack. */
static void tcp_cwnd_down(struct sock *sk, int flag)
{
        struct tcp_sock *tp = tcp_sk(sk);
        int decr = tp->snd_cwnd_cnt + 1;

        if ((flag&(FLAG_ANY_PROGRESS|FLAG_DSACKING_ACK)) ||
            (tcp_is_reno(tp) && !(flag&FLAG_NOT_DUP))) {
                tp->snd_cwnd_cnt = decr&1;
                decr >>= 1;

                if (decr && tp->snd_cwnd > tcp_cwnd_min(sk))
                        tp->snd_cwnd -= decr;

                tp->snd_cwnd = min(tp->snd_cwnd, tcp_packets_in_flight(tp)+1);
                tp->snd_cwnd_stamp = tcp_time_stamp;
        }
}

/* Nothing was retransmitted or returned timestamp is less
 * than timestamp of the first retransmission.
 */
static inline int tcp_packet_delayed(struct tcp_sock *tp)
{
        return !tp->retrans_stamp ||
                (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
                 (__s32)(tp->rx_opt.rcv_tsecr - tp->retrans_stamp) < 0);
}

/* Undo procedures. */

#if FASTRETRANS_DEBUG > 1
static void DBGUNDO(struct sock *sk, const char *msg)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_sock *inet = inet_sk(sk);

        printk(KERN_DEBUG "Undo %s %u.%u.%u.%u/%u c%u l%u ss%u/%u p%u\n",
               msg,
               NIPQUAD(inet->daddr), ntohs(inet->dport),
               tp->snd_cwnd, tcp_left_out(tp),
               tp->snd_ssthresh, tp->prior_ssthresh,
               tp->packets_out);
}
#else
#define DBGUNDO(x...) do { } while (0)
#endif

static void tcp_undo_cwr(struct sock *sk, const int undo)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tp->prior_ssthresh) {
                const struct inet_connection_sock *icsk = inet_csk(sk);

                if (icsk->icsk_ca_ops->undo_cwnd)
                        tp->snd_cwnd = icsk->icsk_ca_ops->undo_cwnd(sk);
                else
                        tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh<<1);

                if (undo && tp->prior_ssthresh > tp->snd_ssthresh) {
                        tp->snd_ssthresh = tp->prior_ssthresh;
                        TCP_ECN_withdraw_cwr(tp);
                }
        } else {
                tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh);
        }
        tcp_moderate_cwnd(tp);
        tp->snd_cwnd_stamp = tcp_time_stamp;

        /* There is something screwy going on with the retrans hints after
           an undo */
        tcp_clear_all_retrans_hints(tp);
}

static inline int tcp_may_undo(struct tcp_sock *tp)
{
        return tp->undo_marker &&
                (!tp->undo_retrans || tcp_packet_delayed(tp));
}

/* People celebrate: "We love our President!" */
static int tcp_try_undo_recovery(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tcp_may_undo(tp)) {
                /* Happy end! We did not retransmit anything
                 * or our original transmission succeeded.
                 */
                DBGUNDO(sk, inet_csk(sk)->icsk_ca_state == TCP_CA_Loss ? "loss" : "retrans");
                tcp_undo_cwr(sk, 1);
                if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss)
                        NET_INC_STATS_BH(LINUX_MIB_TCPLOSSUNDO);
                else
                        NET_INC_STATS_BH(LINUX_MIB_TCPFULLUNDO);
                tp->undo_marker = 0;
        }
        if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) {
                /* Hold old state until something *above* high_seq
                 * is ACKed. For Reno it is MUST to prevent false
                 * fast retransmits (RFC2582). SACK TCP is safe. */
                tcp_moderate_cwnd(tp);
                return 1;
        }
        tcp_set_ca_state(sk, TCP_CA_Open);
        return 0;
}

/* Try to undo cwnd reduction, because D-SACKs acked all retransmitted data */
static void tcp_try_undo_dsack(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tp->undo_marker && !tp->undo_retrans) {
                DBGUNDO(sk, "D-SACK");
                tcp_undo_cwr(sk, 1);
                tp->undo_marker = 0;
                NET_INC_STATS_BH(LINUX_MIB_TCPDSACKUNDO);
        }
}

/* Undo during fast recovery after partial ACK. */

static int tcp_try_undo_partial(struct sock *sk, int acked)
{
        struct tcp_sock *tp = tcp_sk(sk);
        /* Partial ACK arrived. Force Hoe's retransmit. */
        int failed = tcp_is_reno(tp) || tp->fackets_out>tp->reordering;

        if (tcp_may_undo(tp)) {
                /* Plain luck! Hole if filled with delayed
                 * packet, rather than with a retransmit.
                 */
                if (tp->retrans_out == 0)
                        tp->retrans_stamp = 0;

                tcp_update_reordering(sk, tcp_fackets_out(tp) + acked, 1);

                DBGUNDO(sk, "Hoe");
                tcp_undo_cwr(sk, 0);
                NET_INC_STATS_BH(LINUX_MIB_TCPPARTIALUNDO);

                /* So... Do not make Hoe's retransmit yet.
                 * If the first packet was delayed, the rest
                 * ones are most probably delayed as well.
                 */
                failed = 0;
        }
        return failed;
}

/* Undo during loss recovery after partial ACK. */
static int tcp_try_undo_loss(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tcp_may_undo(tp)) {
                struct sk_buff *skb;
                tcp_for_write_queue(skb, sk) {
                        if (skb == tcp_send_head(sk))
                                break;
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
                }

                tcp_clear_all_retrans_hints(tp);

                DBGUNDO(sk, "partial loss");
                tp->lost_out = 0;
                tcp_undo_cwr(sk, 1);
                NET_INC_STATS_BH(LINUX_MIB_TCPLOSSUNDO);
                inet_csk(sk)->icsk_retransmits = 0;
                tp->undo_marker = 0;
                if (tcp_is_sack(tp))
                        tcp_set_ca_state(sk, TCP_CA_Open);
                return 1;
        }
        return 0;
}

static inline void tcp_complete_cwr(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
        tp->snd_cwnd_stamp = tcp_time_stamp;
        tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
}

static void tcp_try_to_open(struct sock *sk, int flag)
{
        struct tcp_sock *tp = tcp_sk(sk);

        tcp_verify_left_out(tp);

        if (tp->retrans_out == 0)
                tp->retrans_stamp = 0;

        if (flag&FLAG_ECE)
                tcp_enter_cwr(sk, 1);

        if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
                int state = TCP_CA_Open;

                if (tcp_left_out(tp) || tp->retrans_out || tp->undo_marker)
                        state = TCP_CA_Disorder;

                if (inet_csk(sk)->icsk_ca_state != state) {
                        tcp_set_ca_state(sk, state);
                        tp->high_seq = tp->snd_nxt;
                }
                tcp_moderate_cwnd(tp);
        } else {
                tcp_cwnd_down(sk, flag);
        }
}

static void tcp_mtup_probe_failed(struct sock *sk)
{
        struct inet_connection_sock *icsk = inet_csk(sk);

        icsk->icsk_mtup.search_high = icsk->icsk_mtup.probe_size - 1;
        icsk->icsk_mtup.probe_size = 0;
}

static void tcp_mtup_probe_success(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);

        /* FIXME: breaks with very large cwnd */
        tp->prior_ssthresh = tcp_current_ssthresh(sk);
        tp->snd_cwnd = tp->snd_cwnd *
                       tcp_mss_to_mtu(sk, tp->mss_cache) /
                       icsk->icsk_mtup.probe_size;
        tp->snd_cwnd_cnt = 0;
        tp->snd_cwnd_stamp = tcp_time_stamp;
        tp->rcv_ssthresh = tcp_current_ssthresh(sk);

        icsk->icsk_mtup.search_low = icsk->icsk_mtup.probe_size;
        icsk->icsk_mtup.probe_size = 0;
        tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
}


/* Process an event, which can update packets-in-flight not trivially.
 * Main goal of this function is to calculate new estimate for left_out,
 * taking into account both packets sitting in receiver's buffer and
 * packets lost by network.
 *
 * Besides that it does CWND reduction, when packet loss is detected
 * and changes state of machine.
 *
 * It does _not_ decide what to send, it is made in function
 * tcp_xmit_retransmit_queue().
 */
static void
tcp_fastretrans_alert(struct sock *sk, int pkts_acked, int flag)
{
        struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        int is_dupack = !(flag&(FLAG_SND_UNA_ADVANCED|FLAG_NOT_DUP));
        int do_lost = is_dupack || ((flag&FLAG_DATA_SACKED) &&
                                    (tp->fackets_out > tp->reordering));

        /* Some technical things:
         * 1. Reno does not count dupacks (sacked_out) automatically. */
        if (!tp->packets_out)
                tp->sacked_out = 0;

        if (WARN_ON(!tp->sacked_out && tp->fackets_out))
                tp->fackets_out = 0;

        /* Now state machine starts.
         * A. ECE, hence prohibit cwnd undoing, the reduction is required. */
        if (flag&FLAG_ECE)
                tp->prior_ssthresh = 0;

        /* B. In all the states check for reneging SACKs. */
        if (tp->sacked_out && tcp_check_sack_reneging(sk))
                return;

        /* C. Process data loss notification, provided it is valid. */
        if ((flag&FLAG_DATA_LOST) &&
            before(tp->snd_una, tp->high_seq) &&
            icsk->icsk_ca_state != TCP_CA_Open &&
            tp->fackets_out > tp->reordering) {
                tcp_mark_head_lost(sk, tp->fackets_out - tp->reordering);
                NET_INC_STATS_BH(LINUX_MIB_TCPLOSS);
        }

        /* D. Check consistency of the current state. */
        tcp_verify_left_out(tp);

        /* E. Check state exit conditions. State can be terminated
         *    when high_seq is ACKed. */
        if (icsk->icsk_ca_state == TCP_CA_Open) {
                BUG_TRAP(tp->retrans_out == 0);
                tp->retrans_stamp = 0;
        } else if (!before(tp->snd_una, tp->high_seq)) {
                switch (icsk->icsk_ca_state) {
                case TCP_CA_Loss:
                        icsk->icsk_retransmits = 0;
                        if (tcp_try_undo_recovery(sk))
                                return;
                        break;

                case TCP_CA_CWR:
                        /* CWR is to be held something *above* high_seq
                         * is ACKed for CWR bit to reach receiver. */
                        if (tp->snd_una != tp->high_seq) {
                                tcp_complete_cwr(sk);
                                tcp_set_ca_state(sk, TCP_CA_Open);
                        }
                        break;

                case TCP_CA_Disorder:
                        tcp_try_undo_dsack(sk);
                        if (!tp->undo_marker ||
                            /* For SACK case do not Open to allow to undo
                             * catching for all duplicate ACKs. */
                            tcp_is_reno(tp) || tp->snd_una != tp->high_seq) {
                                tp->undo_marker = 0;
                                tcp_set_ca_state(sk, TCP_CA_Open);
                        }
                        break;

                case TCP_CA_Recovery:
                        if (tcp_is_reno(tp))
                                tcp_reset_reno_sack(tp);
                        if (tcp_try_undo_recovery(sk))
                                return;
                        tcp_complete_cwr(sk);
                        break;
                }
        }

        /* F. Process state. */
        switch (icsk->icsk_ca_state) {
        case TCP_CA_Recovery:
                if (!(flag & FLAG_SND_UNA_ADVANCED)) {
                        if (tcp_is_reno(tp) && is_dupack)
                                tcp_add_reno_sack(sk);
                } else
                        do_lost = tcp_try_undo_partial(sk, pkts_acked);
                break;
        case TCP_CA_Loss:
                if (flag&FLAG_DATA_ACKED)
                        icsk->icsk_retransmits = 0;
                if (!tcp_try_undo_loss(sk)) {
                        tcp_moderate_cwnd(tp);
                        tcp_xmit_retransmit_queue(sk);
                        return;
                }
                if (icsk->icsk_ca_state != TCP_CA_Open)
                        return;
                /* Loss is undone; fall through to processing in Open state. */
        default:
                if (tcp_is_reno(tp)) {
                        if (flag & FLAG_SND_UNA_ADVANCED)
                                tcp_reset_reno_sack(tp);
                        if (is_dupack)
                                tcp_add_reno_sack(sk);
                }

                if (icsk->icsk_ca_state == TCP_CA_Disorder)
                        tcp_try_undo_dsack(sk);

                if (!tcp_time_to_recover(sk)) {
                        tcp_try_to_open(sk, flag);
                        return;
                }

                /* MTU probe failure: don't reduce cwnd */
                if (icsk->icsk_ca_state < TCP_CA_CWR &&
                    icsk->icsk_mtup.probe_size &&
                    tp->snd_una == tp->mtu_probe.probe_seq_start) {
                        tcp_mtup_probe_failed(sk);
                        /* Restores the reduction we did in tcp_mtup_probe() */
                        tp->snd_cwnd++;
                        tcp_simple_retransmit(sk);
                        return;
                }

                /* Otherwise enter Recovery state */

                if (tcp_is_reno(tp))
                        NET_INC_STATS_BH(LINUX_MIB_TCPRENORECOVERY);
                else
                        NET_INC_STATS_BH(LINUX_MIB_TCPSACKRECOVERY);

                tp->high_seq = tp->snd_nxt;
                tp->prior_ssthresh = 0;
                tp->undo_marker = tp->snd_una;
                tp->undo_retrans = tp->retrans_out;

                if (icsk->icsk_ca_state < TCP_CA_CWR) {
                        if (!(flag&FLAG_ECE))
                                tp->prior_ssthresh = tcp_current_ssthresh(sk);
                        tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
                        TCP_ECN_queue_cwr(tp);
                }

                tp->bytes_acked = 0;
                tp->snd_cwnd_cnt = 0;
                tcp_set_ca_state(sk, TCP_CA_Recovery);
        }

        if (do_lost || tcp_head_timedout(sk))
                tcp_update_scoreboard(sk);
        tcp_cwnd_down(sk, flag);
        tcp_xmit_retransmit_queue(sk);
}

/* Read draft-ietf-tcplw-high-performance before mucking
 * with this code. (Supersedes RFC1323)
 */
static void tcp_ack_saw_tstamp(struct sock *sk, int flag)
{
        /* RTTM Rule: A TSecr value received in a segment is used to
         * update the averaged RTT measurement only if the segment
         * acknowledges some new data, i.e., only if it advances the
         * left edge of the send window.
         *
         * See draft-ietf-tcplw-high-performance-00, section 3.3.
         * 1998/04/10 Andrey V. Savochkin <saw@msu.ru>
         *
         * Changed: reset backoff as soon as we see the first valid sample.
         * If we do not, we get strongly overestimated rto. With timestamps
         * samples are accepted even from very old segments: f.e., when rtt=1
         * increases to 8, we retransmit 5 times and after 8 seconds delayed
         * answer arrives rto becomes 120 seconds! If at least one of segments
         * in window is lost... Voila.                          --ANK (010210)
         */
        struct tcp_sock *tp = tcp_sk(sk);
        const __u32 seq_rtt = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
        tcp_rtt_estimator(sk, seq_rtt);
        tcp_set_rto(sk);
        inet_csk(sk)->icsk_backoff = 0;
        tcp_bound_rto(sk);
}

static void tcp_ack_no_tstamp(struct sock *sk, u32 seq_rtt, int flag)
{
        /* We don't have a timestamp. Can only use
         * packets that are not retransmitted to determine
         * rtt estimates. Also, we must not reset the
         * backoff for rto until we get a non-retransmitted
         * packet. This allows us to deal with a situation
         * where the network delay has increased suddenly.
         * I.e. Karn's algorithm. (SIGCOMM '87, p5.)
         */

        if (flag & FLAG_RETRANS_DATA_ACKED)
                return;

        tcp_rtt_estimator(sk, seq_rtt);
        tcp_set_rto(sk);
        inet_csk(sk)->icsk_backoff = 0;
        tcp_bound_rto(sk);
}

static inline void tcp_ack_update_rtt(struct sock *sk, const int flag,
                                      const s32 seq_rtt)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        /* Note that peer MAY send zero echo. In this case it is ignored. (rfc1323) */
        if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
                tcp_ack_saw_tstamp(sk, flag);
        else if (seq_rtt >= 0)
                tcp_ack_no_tstamp(sk, seq_rtt, flag);
}

static void tcp_cong_avoid(struct sock *sk, u32 ack,
                           u32 in_flight, int good)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        icsk->icsk_ca_ops->cong_avoid(sk, ack, in_flight, good);
        tcp_sk(sk)->snd_cwnd_stamp = tcp_time_stamp;
}

/* Restart timer after forward progress on connection.
 * RFC2988 recommends to restart timer to now+rto.
 */
static void tcp_rearm_rto(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (!tp->packets_out) {
                inet_csk_clear_xmit_timer(sk, ICSK_TIME_RETRANS);
        } else {
                inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
        }
}

/* If we get here, the whole TSO packet has not been acked. */
static u32 tcp_tso_acked(struct sock *sk, struct sk_buff *skb)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 packets_acked;

        BUG_ON(!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una));

        packets_acked = tcp_skb_pcount(skb);
        if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
                return 0;
        packets_acked -= tcp_skb_pcount(skb);

        if (packets_acked) {
                BUG_ON(tcp_skb_pcount(skb) == 0);
                BUG_ON(!before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq));
        }

        return packets_acked;
}

/* Remove acknowledged frames from the retransmission queue. If our packet
 * is before the ack sequence we can discard it as it's confirmed to have
 * arrived at the other end.
 */
static int tcp_clean_rtx_queue(struct sock *sk, s32 *seq_rtt_p,
                               int prior_fackets)
{
        struct tcp_sock *tp = tcp_sk(sk);
        const struct inet_connection_sock *icsk = inet_csk(sk);
        struct sk_buff *skb;
        u32 now = tcp_time_stamp;
        int fully_acked = 1;
        int flag = 0;
        int prior_packets = tp->packets_out;
        u32 cnt = 0;
        u32 reord = tp->packets_out;
        s32 seq_rtt = -1;
        s32 ca_seq_rtt = -1;
        ktime_t last_ackt = net_invalid_timestamp();

        while ((skb = tcp_write_queue_head(sk)) && skb != tcp_send_head(sk)) {
                struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
                u32 end_seq;
                u32 packets_acked;
                u8 sacked = scb->sacked;

                /* Determine how many packets and what bytes were acked, tso and else */
                if (after(scb->end_seq, tp->snd_una)) {
                        if (tcp_skb_pcount(skb) == 1 ||
                            !after(tp->snd_una, scb->seq))
                                break;

                        packets_acked = tcp_tso_acked(sk, skb);
                        if (!packets_acked)
                                break;

                        fully_acked = 0;
                        end_seq = tp->snd_una;
                } else {
                        packets_acked = tcp_skb_pcount(skb);
                        end_seq = scb->end_seq;
                }

                /* MTU probing checks */
                if (fully_acked && icsk->icsk_mtup.probe_size &&
                    !after(tp->mtu_probe.probe_seq_end, scb->end_seq)) {
                        tcp_mtup_probe_success(sk, skb);
                }

                if (sacked) {
                        if (sacked & TCPCB_RETRANS) {
                                if (sacked & TCPCB_SACKED_RETRANS)
                                        tp->retrans_out -= packets_acked;
                                flag |= FLAG_RETRANS_DATA_ACKED;
                                ca_seq_rtt = -1;
                                seq_rtt = -1;
                                if ((flag & FLAG_DATA_ACKED) ||
                                    (packets_acked > 1))
                                        flag |= FLAG_NONHEAD_RETRANS_ACKED;
                        } else {
                                ca_seq_rtt = now - scb->when;
                                last_ackt = skb->tstamp;
                                if (seq_rtt < 0) {
                                        seq_rtt = ca_seq_rtt;
                                }
                                if (!(sacked & TCPCB_SACKED_ACKED))
                                        reord = min(cnt, reord);
                        }

                        if (sacked & TCPCB_SACKED_ACKED)
                                tp->sacked_out -= packets_acked;
                        if (sacked & TCPCB_LOST)
                                tp->lost_out -= packets_acked;

                        if ((sacked & TCPCB_URG) && tp->urg_mode &&
                            !before(end_seq, tp->snd_up))
                                tp->urg_mode = 0;
                } else {
                        ca_seq_rtt = now - scb->when;
                        last_ackt = skb->tstamp;
                        if (seq_rtt < 0) {
                                seq_rtt = ca_seq_rtt;
                        }
                        reord = min(cnt, reord);
                }
                tp->packets_out -= packets_acked;
                cnt += packets_acked;

                /* Initial outgoing SYN's get put onto the write_queue
                 * just like anything else we transmit.  It is not
                 * true data, and if we misinform our callers that
                 * this ACK acks real data, we will erroneously exit
                 * connection startup slow start one packet too
                 * quickly.  This is severely frowned upon behavior.
                 */
                if (!(scb->flags & TCPCB_FLAG_SYN)) {
                        flag |= FLAG_DATA_ACKED;
                } else {
                        flag |= FLAG_SYN_ACKED;
                        tp->retrans_stamp = 0;
                }

                if (!fully_acked)
                        break;

                tcp_unlink_write_queue(skb, sk);
                sk_stream_free_skb(sk, skb);
                tcp_clear_all_retrans_hints(tp);
        }

        if (flag & FLAG_ACKED) {
                u32 pkts_acked = prior_packets - tp->packets_out;
                const struct tcp_congestion_ops *ca_ops
                        = inet_csk(sk)->icsk_ca_ops;

                tcp_ack_update_rtt(sk, flag, seq_rtt);
                tcp_rearm_rto(sk);

                if (tcp_is_reno(tp)) {
                        tcp_remove_reno_sacks(sk, pkts_acked);
                } else {
                        /* Non-retransmitted hole got filled? That's reordering */
                        if (reord < prior_fackets)
                                tcp_update_reordering(sk, tp->fackets_out - reord, 0);
                }

                tp->fackets_out -= min(pkts_acked, tp->fackets_out);
                /* hint's skb might be NULL but we don't need to care */
                tp->fastpath_cnt_hint -= min_t(u32, pkts_acked,
                                               tp->fastpath_cnt_hint);
                if (ca_ops->pkts_acked) {
                        s32 rtt_us = -1;

                        /* Is the ACK triggering packet unambiguous? */
                        if (!(flag & FLAG_RETRANS_DATA_ACKED)) {
                                /* High resolution needed and available? */
                                if (ca_ops->flags & TCP_CONG_RTT_STAMP &&
                                    !ktime_equal(last_ackt,
                                                 net_invalid_timestamp()))
                                        rtt_us = ktime_us_delta(ktime_get_real(),
                                                                last_ackt);
                                else if (ca_seq_rtt > 0)
                                        rtt_us = jiffies_to_usecs(ca_seq_rtt);
                        }

                        ca_ops->pkts_acked(sk, pkts_acked, rtt_us);
                }
        }

#if FASTRETRANS_DEBUG > 0
        BUG_TRAP((int)tp->sacked_out >= 0);
        BUG_TRAP((int)tp->lost_out >= 0);
        BUG_TRAP((int)tp->retrans_out >= 0);
        if (!tp->packets_out && tcp_is_sack(tp)) {
                icsk = inet_csk(sk);
                if (tp->lost_out) {
                        printk(KERN_DEBUG "Leak l=%u %d\n",
                               tp->lost_out, icsk->icsk_ca_state);
                        tp->lost_out = 0;
                }
                if (tp->sacked_out) {
                        printk(KERN_DEBUG "Leak s=%u %d\n",
                               tp->sacked_out, icsk->icsk_ca_state);
                        tp->sacked_out = 0;
                }
                if (tp->retrans_out) {
                        printk(KERN_DEBUG "Leak r=%u %d\n",
                               tp->retrans_out, icsk->icsk_ca_state);
                        tp->retrans_out = 0;
                }
        }
#endif
        *seq_rtt_p = seq_rtt;
        return flag;
}

static void tcp_ack_probe(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        struct inet_connection_sock *icsk = inet_csk(sk);

        /* Was it a usable window open? */

        if (!after(TCP_SKB_CB(tcp_send_head(sk))->end_seq,
                   tp->snd_una + tp->snd_wnd)) {
                icsk->icsk_backoff = 0;
                inet_csk_clear_xmit_timer(sk, ICSK_TIME_PROBE0);
                /* Socket must be waked up by subsequent tcp_data_snd_check().
                 * This function is not for random using!
                 */
        } else {
                inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
                                          min(icsk->icsk_rto << icsk->icsk_backoff, TCP_RTO_MAX),
                                          TCP_RTO_MAX);
        }
}

static inline int tcp_ack_is_dubious(const struct sock *sk, const int flag)
{
        return (!(flag & FLAG_NOT_DUP) || (flag & FLAG_CA_ALERT) ||
                inet_csk(sk)->icsk_ca_state != TCP_CA_Open);
}

static inline int tcp_may_raise_cwnd(const struct sock *sk, const int flag)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        return (!(flag & FLAG_ECE) || tp->snd_cwnd < tp->snd_ssthresh) &&
                !((1 << inet_csk(sk)->icsk_ca_state) & (TCPF_CA_Recovery | TCPF_CA_CWR));
}

/* Check that window update is acceptable.
 * The function assumes that snd_una<=ack<=snd_next.
 */
static inline int tcp_may_update_window(const struct tcp_sock *tp, const u32 ack,
                                        const u32 ack_seq, const u32 nwin)
{
        return (after(ack, tp->snd_una) ||
                after(ack_seq, tp->snd_wl1) ||
                (ack_seq == tp->snd_wl1 && nwin > tp->snd_wnd));
}

/* Update our send window.
 *
 * Window update algorithm, described in RFC793/RFC1122 (used in linux-2.2
 * and in FreeBSD. NetBSD's one is even worse.) is wrong.
 */
static int tcp_ack_update_window(struct sock *sk, struct sk_buff *skb, u32 ack,
                                 u32 ack_seq)
{
        struct tcp_sock *tp = tcp_sk(sk);
        int flag = 0;
        u32 nwin = ntohs(tcp_hdr(skb)->window);

        if (likely(!tcp_hdr(skb)->syn))
                nwin <<= tp->rx_opt.snd_wscale;

        if (tcp_may_update_window(tp, ack, ack_seq, nwin)) {
                flag |= FLAG_WIN_UPDATE;
                tcp_update_wl(tp, ack, ack_seq);

                if (tp->snd_wnd != nwin) {
                        tp->snd_wnd = nwin;

                        /* Note, it is the only place, where
                         * fast path is recovered for sending TCP.
                         */
                        tp->pred_flags = 0;
                        tcp_fast_path_check(sk);

                        if (nwin > tp->max_window) {
                                tp->max_window = nwin;
                                tcp_sync_mss(sk, inet_csk(sk)->icsk_pmtu_cookie);
                        }
                }
        }

        tp->snd_una = ack;

        return flag;
}

/* A very conservative spurious RTO response algorithm: reduce cwnd and
 * continue in congestion avoidance.
 */
static void tcp_conservative_spur_to_response(struct tcp_sock *tp)
{
        tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
        tp->snd_cwnd_cnt = 0;
        tp->bytes_acked = 0;
        TCP_ECN_queue_cwr(tp);
        tcp_moderate_cwnd(tp);
}

/* A conservative spurious RTO response algorithm: reduce cwnd using
 * rate halving and continue in congestion avoidance.
 */
static void tcp_ratehalving_spur_to_response(struct sock *sk)
{
        tcp_enter_cwr(sk, 0);
}

static void tcp_undo_spur_to_response(struct sock *sk, int flag)
{
        if (flag&FLAG_ECE)
                tcp_ratehalving_spur_to_response(sk);
        else
                tcp_undo_cwr(sk, 1);
}

/* F-RTO spurious RTO detection algorithm (RFC4138)
 *
 * F-RTO affects during two new ACKs following RTO (well, almost, see inline
 * comments). State (ACK number) is kept in frto_counter. When ACK advances
 * window (but not to or beyond highest sequence sent before RTO):
 *   On First ACK,  send two new segments out.
 *   On Second ACK, RTO was likely spurious. Do spurious response (response
 *                  algorithm is not part of the F-RTO detection algorithm
 *                  given in RFC4138 but can be selected separately).
 * Otherwise (basically on duplicate ACK), RTO was (likely) caused by a loss
 * and TCP falls back to conventional RTO recovery. F-RTO allows overriding
 * of Nagle, this is done using frto_counter states 2 and 3, when a new data
 * segment of any size sent during F-RTO, state 2 is upgraded to 3.
 *
 * Rationale: if the RTO was spurious, new ACKs should arrive from the
 * original window even after we transmit two new data segments.
 *
 * SACK version:
 *   on first step, wait until first cumulative ACK arrives, then move to
 *   the second step. In second step, the next ACK decides.
 *
 * F-RTO is implemented (mainly) in four functions:
 *   - tcp_use_frto() is used to determine if TCP is can use F-RTO
 *   - tcp_enter_frto() prepares TCP state on RTO if F-RTO is used, it is
 *     called when tcp_use_frto() showed green light
 *   - tcp_process_frto() handles incoming ACKs during F-RTO algorithm
 *   - tcp_enter_frto_loss() is called if there is not enough evidence
 *     to prove that the RTO is indeed spurious. It transfers the control
 *     from F-RTO to the conventional RTO recovery
 */
static int tcp_process_frto(struct sock *sk, int flag)
{
        struct tcp_sock *tp = tcp_sk(sk);

        tcp_verify_left_out(tp);

        /* Duplicate the behavior from Loss state (fastretrans_alert) */
        if (flag&FLAG_DATA_ACKED)
                inet_csk(sk)->icsk_retransmits = 0;

        if ((flag & FLAG_NONHEAD_RETRANS_ACKED) ||
            ((tp->frto_counter >= 2) && (flag & FLAG_RETRANS_DATA_ACKED)))
                tp->undo_marker = 0;

        if (!before(tp->snd_una, tp->frto_highmark)) {
                tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3), flag);
                return 1;
        }

        if (!IsSackFrto() || tcp_is_reno(tp)) {
                /* RFC4138 shortcoming in step 2; should also have case c):
                 * ACK isn't duplicate nor advances window, e.g., opposite dir
                 * data, winupdate
                 */
                if (!(flag&FLAG_ANY_PROGRESS) && (flag&FLAG_NOT_DUP))
                        return 1;

                if (!(flag&FLAG_DATA_ACKED)) {
                        tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 0 : 3),
                                            flag);
                        return 1;
                }
        } else {
                if (!(flag&FLAG_DATA_ACKED) && (tp->frto_counter == 1)) {
                        /* Prevent sending of new data. */
                        tp->snd_cwnd = min(tp->snd_cwnd,
                                           tcp_packets_in_flight(tp));
                        return 1;
                }

                if ((tp->frto_counter >= 2) &&
                    (!(flag&FLAG_FORWARD_PROGRESS) ||
                     ((flag&FLAG_DATA_SACKED) && !(flag&FLAG_ONLY_ORIG_SACKED)))) {
                        /* RFC4138 shortcoming (see comment above) */
                        if (!(flag&FLAG_FORWARD_PROGRESS) && (flag&FLAG_NOT_DUP))