/*
 * 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).
 *
 * 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 <linux/kernel.h>
#include <net/dst.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_SACK_RENEGING      0x2000 /* snd_una advanced to a sacked seq */

#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 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) >> 1;
        int window = tcp_win_from_space(sysctl_tcp_rmem[2]) >> 1;

        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_mem_reclaim(sk);
                }
        }
        icsk->icsk_ack.lrcvtime = now;

        TCP_ECN_check_ce(tp, skb);

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

/* 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.
         */
        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;
                unsigned long rtt;

                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;
                }

                rtt = dst_metric_rtt(dst, RTAX_RTT);
                m = 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)
                                set_dst_metric_rtt(dst, RTAX_RTT, tp->srtt);
                        else
                                set_dst_metric_rtt(dst, RTAX_RTT, rtt - (m >> 3));
                }

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

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

                        var = dst_metric_rtt(dst, RTAX_RTTVAR);
                        if (m >= var)
                                var = m;
                        else
                                var -= (var - m) >> 2;

                        set_dst_metric_rtt(dst, RTAX_RTTVAR, var);
                }

                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_metric(dst, RTAX_CWND) + 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_metric(dst, RTAX_CWND) + tp->snd_ssthresh) >> 1;
                        if (dst_metric(dst, RTAX_SSTHRESH) &&
                            !dst_metric_locked(dst, RTAX_SSTHRESH) &&
                            tp->snd_ssthresh > dst_metric(dst, RTAX_SSTHRESH))
                                dst->metrics[RTAX_SSTHRESH-1] = tp->snd_ssthresh;
                }

                if (!dst_metric_locked(dst, RTAX_REORDERING)) {
                        if (dst_metric(dst, RTAX_REORDERING) < 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)
{
        /* RFC3517 uses different metric in lost marker => reset on change */
        if (tcp_is_fack(tp))
                tp->lost_skb_hint = NULL;
        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_rtt(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_rtt(dst, RTAX_RTT) > tp->srtt) {
                tp->srtt = dst_metric_rtt(dst, RTAX_RTT);
                tp->rtt_seq = tp->snd_nxt;
        }
        if (dst_metric_rtt(dst, RTAX_RTTVAR) > tp->mdev) {
                tp->mdev = dst_metric_rtt(dst, RTAX_RTTVAR);
                tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
        }
        tcp_set_rto(sk);
        if (inet_csk(sk)->icsk_rto < TCP_TIMEOUT_INIT && !tp->rx_opt.saw_tstamp)
                goto reset;

cwnd:
        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;
        }
        goto cwnd;
}

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) {
                int mib_idx;

                tp->reordering = min(TCP_MAX_REORDERING, metric);

                /* This exciting event is worth to be remembered. 8) */
                if (ts)
                        mib_idx = LINUX_MIB_TCPTSREORDER;
                else if (tcp_is_reno(tp))
                        mib_idx = LINUX_MIB_TCPRENOREORDER;
                else if (tcp_is_fack(tp))
                        mib_idx = LINUX_MIB_TCPFACKREORDER;
                else
                        mib_idx = LINUX_MIB_TCPSACKREORDER;

                NET_INC_STATS_BH(sock_net(sk), mib_idx);
#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 must be called before lost_out is incremented */
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 = skb;

        if (!tp->lost_out ||
            after(TCP_SKB_CB(skb)->end_seq, tp->retransmit_high))
                tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
}

static void tcp_skb_mark_lost(struct tcp_sock *tp, struct sk_buff *skb)
{
        if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) {
                tcp_verify_retransmit_hint(tp, skb);

                tp->lost_out += tcp_skb_pcount(skb);
                TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
        }
}

static void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp,
                                            struct sk_buff *skb)
{
        tcp_verify_retransmit_hint(tp, skb);

        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;
        }
}

/* 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
 * highest SACK block). Also calculate the lowest snd_nxt among the remaining
 * retransmitted skbs to avoid some costly processing per ACKs.
 */
static void tcp_mark_lost_retrans(struct sock *sk)
{
        const struct inet_connection_sock *icsk = inet_csk(sk);
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb;
        int cnt = 0;
        u32 new_low_seq = tp->snd_nxt;
        u32 received_upto = tcp_highest_sack_seq(tp);

        if (!tcp_is_fack(tp) || !tp->retrans_out ||
            !after(received_upto, tp->lost_retrans_low) ||
            icsk->icsk_ca_state != TCP_CA_Recovery)
                return;

        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;

                /* TODO: We would like to get rid of tcp_is_fack(tp) only
                 * constraint here (see above) but figuring out that at
                 * least tp->reordering SACK blocks reside between ack_seq
                 * and received_upto is not easy task to do cheaply with
                 * the available datastructures.
                 *
                 * Whether FACK should check here for tp->reordering segs
                 * in-between one could argue for either way (it would be
                 * rather simple to implement as we could count fack_count
                 * during the walk and do tp->fackets_out - fack_count).
                 */
                if (after(received_upto, ack_seq)) {
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                        tp->retrans_out -= tcp_skb_pcount(skb);

                        tcp_skb_mark_lost_uncond_verify(tp, skb);
                        NET_INC_STATS_BH(sock_net(sk), 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;
}

static int tcp_check_dsack(struct sock *sk, struct sk_buff *ack_skb,
                           struct tcp_sack_block_wire *sp, int num_sacks,
                           u32 prior_snd_una)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 start_seq_0 = get_unaligned_be32(&sp[0].start_seq);
        u32 end_seq_0 = get_unaligned_be32(&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(sock_net(sk), LINUX_MIB_TCPDSACKRECV);
        } else if (num_sacks > 1) {
                u32 end_seq_1 = get_unaligned_be32(&sp[1].end_seq);
                u32 start_seq_1 = get_unaligned_be32(&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(sock_net(sk),
                                        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;
}

struct tcp_sacktag_state {
        int reord;
        int fack_count;
        int flag;
};

/* 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).
 *
 * FIXME: this could be merged to shift decision code
 */
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;
        unsigned int mss;

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

                if (!in_sack) {
                        pkt_len = start_seq - TCP_SKB_CB(skb)->seq;
                        if (pkt_len < mss)
                                pkt_len = mss;
                } else {
                        pkt_len = end_seq - TCP_SKB_CB(skb)->seq;
                        if (pkt_len < mss)
                                return -EINVAL;
                }

                /* Round if necessary so that SACKs cover only full MSSes
                 * and/or the remaining small portion (if present)
                 */
                if (pkt_len > mss) {
                        unsigned int new_len = (pkt_len / mss) * mss;
                        if (!in_sack && new_len < pkt_len) {
                                new_len += mss;
                                if (new_len > skb->len)
                                        return 0;
                        }
                        pkt_len = new_len;
                }
                err = tcp_fragment(sk, skb, pkt_len, mss);
                if (err < 0)
                        return err;
        }

        return in_sack;
}

static u8 tcp_sacktag_one(struct sk_buff *skb, struct sock *sk,
                          struct tcp_sacktag_state *state,
                          int dup_sack, int pcount)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u8 sacked = TCP_SKB_CB(skb)->sacked;
        int fack_count = state->fack_count;

        /* Account D-SACK for retransmitted packet. */
        if (dup_sack && (sacked & TCPCB_RETRANS)) {
                if (after(TCP_SKB_CB(skb)->end_seq, tp->undo_marker))
                        tp->undo_retrans--;
                if (sacked & TCPCB_SACKED_ACKED)
                        state->reord = min(fack_count, state->reord);
        }

        /* Nothing to do; acked frame is about to be dropped (was ACKed). */
        if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
                return sacked;

        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) {
                                sacked &= ~(TCPCB_LOST|TCPCB_SACKED_RETRANS);
                                tp->lost_out -= pcount;
                                tp->retrans_out -= pcount;
                        }
                } else {
                        if (!(sacked & TCPCB_RETRANS)) {
                                /* New sack for not retransmitted frame,
                                 * which was in hole. It is reordering.
                                 */
                                if (before(TCP_SKB_CB(skb)->seq,
                                           tcp_highest_sack_seq(tp)))
                                        state->reord = min(fack_count,
                                                           state->reord);

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

                        if (sacked & TCPCB_LOST) {
                                sacked &= ~TCPCB_LOST;
                                tp->lost_out -= pcount;
                        }
                }

                sacked |= TCPCB_SACKED_ACKED;
                state->flag |= FLAG_DATA_SACKED;
                tp->sacked_out += pcount;

                fack_count += pcount;

                /* Lost marker hint past SACKed? Tweak RFC3517 cnt */
                if (!tcp_is_fack(tp) && (tp->lost_skb_hint != NULL) &&
                    before(TCP_SKB_CB(skb)->seq,
                           TCP_SKB_CB(tp->lost_skb_hint)->seq))
                        tp->lost_cnt_hint += pcount;

                if (fack_count > tp->fackets_out)
                        tp->fackets_out = fack_count;
        }

        /* 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 && (sacked & TCPCB_SACKED_RETRANS)) {
                sacked &= ~TCPCB_SACKED_RETRANS;
                tp->retrans_out -= pcount;
        }

        return sacked;
}

static int tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
                           struct tcp_sacktag_state *state,
                           unsigned int pcount, int shifted, int mss,
                           int dup_sack)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *prev = tcp_write_queue_prev(sk, skb);

        BUG_ON(!pcount);

        /* Tweak before seqno plays */
        if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint &&
            !before(TCP_SKB_CB(tp->lost_skb_hint)->seq, TCP_SKB_CB(skb)->seq))
                tp->lost_cnt_hint += pcount;

        TCP_SKB_CB(prev)->end_seq += shifted;
        TCP_SKB_CB(skb)->seq += shifted;

        skb_shinfo(prev)->gso_segs += pcount;
        BUG_ON(skb_shinfo(skb)->gso_segs < pcount);
        skb_shinfo(skb)->gso_segs -= pcount;

        /* When we're adding to gso_segs == 1, gso_size will be zero,
         * in theory this shouldn't be necessary but as long as DSACK
         * code can come after this skb later on it's better to keep
         * setting gso_size to something.
         */
        if (!skb_shinfo(prev)->gso_size) {
                skb_shinfo(prev)->gso_size = mss;
                skb_shinfo(prev)->gso_type = sk->sk_gso_type;
        }

        /* CHECKME: To clear or not to clear? Mimics normal skb currently */
        if (skb_shinfo(skb)->gso_segs <= 1) {
                skb_shinfo(skb)->gso_size = 0;
                skb_shinfo(skb)->gso_type = 0;
        }

        /* We discard results */
        tcp_sacktag_one(skb, sk, state, dup_sack, pcount);

        /* Difference in this won't matter, both ACKed by the same cumul. ACK */
        TCP_SKB_CB(prev)->sacked |= (TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS);

        if (skb->len > 0) {
                BUG_ON(!tcp_skb_pcount(skb));
                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SACKSHIFTED);
                return 0;
        }

        /* Whole SKB was eaten :-) */

        if (skb == tp->retransmit_skb_hint)
                tp->retransmit_skb_hint = prev;
        if (skb == tp->scoreboard_skb_hint)
                tp->scoreboard_skb_hint = prev;
        if (skb == tp->lost_skb_hint) {
                tp->lost_skb_hint = prev;
                tp->lost_cnt_hint -= tcp_skb_pcount(prev);
        }

        TCP_SKB_CB(skb)->flags |= TCP_SKB_CB(prev)->flags;
        if (skb == tcp_highest_sack(sk))
                tcp_advance_highest_sack(sk, skb);

        tcp_unlink_write_queue(skb, sk);
        sk_wmem_free_skb(sk, skb);

        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SACKMERGED);

        return 1;
}

/* I wish gso_size would have a bit more sane initialization than
 * something-or-zero which complicates things
 */
static int tcp_skb_seglen(struct sk_buff *skb)
{
        return tcp_skb_pcount(skb) == 1 ? skb->len : tcp_skb_mss(skb);
}

/* Shifting pages past head area doesn't work */
static int skb_can_shift(struct sk_buff *skb)
{
        return !skb_headlen(skb) && skb_is_nonlinear(skb);
}

/* Try collapsing SACK blocks spanning across multiple skbs to a single
 * skb.
 */
static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
                                          struct tcp_sacktag_state *state,
                                          u32 start_seq, u32 end_seq,
                                          int dup_sack)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *prev;
        int mss;
        int pcount = 0;
        int len;
        int in_sack;

        if (!sk_can_gso(sk))
                goto fallback;

        /* Normally R but no L won't result in plain S */
        if (!dup_sack &&
            (TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_RETRANS)) == TCPCB_SACKED_RETRANS)
                goto fallback;
        if (!skb_can_shift(skb))
                goto fallback;
        /* This frame is about to be dropped (was ACKed). */
        if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
                goto fallback;

        /* Can only happen with delayed DSACK + discard craziness */
        if (unlikely(skb == tcp_write_queue_head(sk)))
                goto fallback;
        prev = tcp_write_queue_prev(sk, skb);

        if ((TCP_SKB_CB(prev)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED)
                goto fallback;

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

        if (in_sack) {
                len = skb->len;
                pcount = tcp_skb_pcount(skb);
                mss = tcp_skb_seglen(skb);

                /* TODO: Fix DSACKs to not fragment already SACKed and we can
                 * drop this restriction as unnecessary
                 */
                if (mss != tcp_skb_seglen(prev))
                        goto fallback;
        } else {
                if (!after(TCP_SKB_CB(skb)->end_seq, start_seq))
                        goto noop;
                /* CHECKME: This is non-MSS split case only?, this will
                 * cause skipped skbs due to advancing loop btw, original
                 * has that feature too
                 */
                if (tcp_skb_pcount(skb) <= 1)
                        goto noop;

                in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);
                if (!in_sack) {
                        /* TODO: head merge to next could be attempted here
                         * if (!after(TCP_SKB_CB(skb)->end_seq, end_seq)),
                         * though it might not be worth of the additional hassle
                         *
                         * ...we can probably just fallback to what was done
                         * previously. We could try merging non-SACKed ones
                         * as well but it probably isn't going to buy off
                         * because later SACKs might again split them, and
                         * it would make skb timestamp tracking considerably
                         * harder problem.
                         */
                        goto fallback;
                }

                len = end_seq - TCP_SKB_CB(skb)->seq;
                BUG_ON(len < 0);
                BUG_ON(len > skb->len);

                /* MSS boundaries should be honoured or else pcount will
                 * severely break even though it makes things bit trickier.
                 * Optimize common case to avoid most of the divides
                 */
                mss = tcp_skb_mss(skb);

                /* TODO: Fix DSACKs to not fragment already SACKed and we can
                 * drop this restriction as unnecessary
                 */
                if (mss != tcp_skb_seglen(prev))
                        goto fallback;

                if (len == mss) {
                        pcount = 1;
                } else if (len < mss) {
                        goto noop;
                } else {
                        pcount = len / mss;
                        len = pcount * mss;
                }
        }

        if (!skb_shift(prev, skb, len))
                goto fallback;
        if (!tcp_shifted_skb(sk, skb, state, pcount, len, mss, dup_sack))
                goto out;

        /* Hole filled allows collapsing with the next as well, this is very
         * useful when hole on every nth skb pattern happens
         */
        if (prev == tcp_write_queue_tail(sk))
                goto out;
        skb = tcp_write_queue_next(sk, prev);

        if (!skb_can_shift(skb) ||
            (skb == tcp_send_head(sk)) ||
            ((TCP_SKB_CB(skb)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED) ||
            (mss != tcp_skb_seglen(skb)))
                goto out;

        len = skb->len;
        if (skb_shift(prev, skb, len)) {
                pcount += tcp_skb_pcount(skb);
                tcp_shifted_skb(sk, skb, state, tcp_skb_pcount(skb), len, mss, 0);
        }

out:
        state->fack_count += pcount;
        return prev;

noop:
        return skb;

fallback:
        NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SACKSHIFTFALLBACK);
        return NULL;
}

static struct sk_buff *tcp_sacktag_walk(struct sk_buff *skb, struct sock *sk,
                                        struct tcp_sack_block *next_dup,
                                        struct tcp_sacktag_state *state,
                                        u32 start_seq, u32 end_seq,
                                        int dup_sack_in)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *tmp;

        tcp_for_write_queue_from(skb, sk) {
                int in_sack = 0;
                int dup_sack = dup_sack_in;

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

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

                if ((next_dup != NULL) &&
                    before(TCP_SKB_CB(skb)->seq, next_dup->end_seq)) {
                        in_sack = tcp_match_skb_to_sack(sk, skb,
                                                        next_dup->start_seq,
                                                        next_dup->end_seq);
                        if (in_sack > 0)
                                dup_sack = 1;
                }

                /* skb reference here is a bit tricky to get right, since
                 * shifting can eat and free both this skb and the next,
                 * so not even _safe variant of the loop is enough.
                 */
                if (in_sack <= 0) {
                        tmp = tcp_shift_skb_data(sk, skb, state,
                                                 start_seq, end_seq, dup_sack);
                        if (tmp != NULL) {
                                if (tmp != skb) {
                                        skb = tmp;
                                        continue;
                                }

                                in_sack = 0;
                        } else {
                                in_sack = tcp_match_skb_to_sack(sk, skb,
                                                                start_seq,
                                                                end_seq);
                        }
                }

                if (unlikely(in_sack < 0))
                        break;

                if (in_sack) {
                        TCP_SKB_CB(skb)->sacked = tcp_sacktag_one(skb, sk,
                                                                  state,
                                                                  dup_sack,
                                                                  tcp_skb_pcount(skb));

                        if (!before(TCP_SKB_CB(skb)->seq,
                                    tcp_highest_sack_seq(tp)))
                                tcp_advance_highest_sack(sk, skb);
                }

                state->fack_count += tcp_skb_pcount(skb);
        }
        return skb;
}

/* Avoid all extra work that is being done by sacktag while walking in
 * a normal way
 */
static struct sk_buff *tcp_sacktag_skip(struct sk_buff *skb, struct sock *sk,
                                        struct tcp_sacktag_state *state,
                                        u32 skip_to_seq)
{
        tcp_for_write_queue_from(skb, sk) {
                if (skb == tcp_send_head(sk))
                        break;

                if (after(TCP_SKB_CB(skb)->end_seq, skip_to_seq))
                        break;

                state->fack_count += tcp_skb_pcount(skb);
        }
        return skb;
}

static struct sk_buff *tcp_maybe_skipping_dsack(struct sk_buff *skb,
                                                struct sock *sk,
                                                struct tcp_sack_block *next_dup,
                                                struct tcp_sacktag_state *state,
                                                u32 skip_to_seq)
{
        if (next_dup == NULL)
                return skb;

        if (before(next_dup->start_seq, skip_to_seq)) {
                skb = tcp_sacktag_skip(skb, sk, state, next_dup->start_seq);
                skb = tcp_sacktag_walk(skb, sk, NULL, state,
                                       next_dup->start_seq, next_dup->end_seq,
                                       1);
        }

        return skb;
}

static int tcp_sack_cache_ok(struct tcp_sock *tp, struct tcp_sack_block *cache)
{
        return cache < tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
}

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_wire = (struct tcp_sack_block_wire *)(ptr+2);
        struct tcp_sack_block sp[TCP_NUM_SACKS];
        struct tcp_sack_block *cache;
        struct tcp_sacktag_state state;
        struct sk_buff *skb;
        int num_sacks = min(TCP_NUM_SACKS, (ptr[1] - TCPOLEN_SACK_BASE) >> 3);
        int used_sacks;
        int found_dup_sack = 0;
        int i, j;
        int first_sack_index;

        state.flag = 0;
        state.reord = tp->packets_out;

        if (!tp->sacked_out) {
                if (WARN_ON(tp->fackets_out))
                        tp->fackets_out = 0;
                tcp_highest_sack_reset(sk);
        }

        found_dup_sack = tcp_check_dsack(sk, ack_skb, sp_wire,
                                         num_sacks, prior_snd_una);
        if (found_dup_sack)
                state.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;

        used_sacks = 0;
        first_sack_index = 0;
        for (i = 0; i < num_sacks; i++) {
                int dup_sack = !i && found_dup_sack;

                sp[used_sacks].start_seq = get_unaligned_be32(&sp_wire[i].start_seq);
                sp[used_sacks].end_seq = get_unaligned_be32(&sp_wire[i].end_seq);

                if (!tcp_is_sackblock_valid(tp, dup_sack,
                                            sp[used_sacks].start_seq,
                                            sp[used_sacks].end_seq)) {
                        int mib_idx;

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

                        NET_INC_STATS_BH(sock_net(sk), mib_idx);
                        if (i == 0)
                                first_sack_index = -1;
                        continue;
                }

                /* Ignore very old stuff early */
                if (!after(sp[used_sacks].end_seq, prior_snd_una))
                        continue;

                used_sacks++;
        }

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

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

        skb = tcp_write_queue_head(sk);
        state.fack_count = 0;
        i = 0;

        if (!tp->sacked_out) {
                /* It's already past, so skip checking against it */
                cache = tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
        } else {
                cache = tp->recv_sack_cache;
                /* Skip empty blocks in at head of the cache */
                while (tcp_sack_cache_ok(tp, cache) && !cache->start_seq &&
                       !cache->end_seq)
                        cache++;
        }

        while (i < used_sacks) {
                u32 start_seq = sp[i].start_seq;
                u32 end_seq = sp[i].end_seq;
                int dup_sack = (found_dup_sack && (i == first_sack_index));
                struct tcp_sack_block *next_dup = NULL;

                if (found_dup_sack && ((i + 1) == first_sack_index))
                        next_dup = &sp[i + 1];

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

                /* Skip too early cached blocks */
                while (tcp_sack_cache_ok(tp, cache) &&
                       !before(start_seq, cache->end_seq))
                        cache++;

                /* Can skip some work by looking recv_sack_cache? */
                if (tcp_sack_cache_ok(tp, cache) && !dup_sack &&
                    after(end_seq, cache->start_seq)) {

                        /* Head todo? */
                        if (before(start_seq, cache->start_seq)) {
                                skb = tcp_sacktag_skip(skb, sk, &state,
                                                       start_seq);
                                skb = tcp_sacktag_walk(skb, sk, next_dup,
                                                       &state,
                                                       start_seq,
                                                       cache->start_seq,
                                                       dup_sack);
                        }

                        /* Rest of the block already fully processed? */
                        if (!after(end_seq, cache->end_seq))
                                goto advance_sp;

                        skb = tcp_maybe_skipping_dsack(skb, sk, next_dup,
                                                       &state,
                                                       cache->end_seq);

                        /* ...tail remains todo... */
                        if (tcp_highest_sack_seq(tp) == cache->end_seq) {
                                /* ...but better entrypoint exists! */
                                skb = tcp_highest_sack(sk);
                                if (skb == NULL)
                                        break;
                                state.fack_count = tp->fackets_out;
                                cache++;
                                goto walk;
                        }

                        skb = tcp_sacktag_skip(skb, sk, &state, cache->end_seq);
                        /* Check overlap against next cached too (past this one already) */
                        cache++;
                        continue;
                }

                if (!before(start_seq, tcp_highest_sack_seq(tp))) {
                        skb = tcp_highest_sack(sk);
                        if (skb == NULL)
                                break;
                        state.fack_count = tp->fackets_out;
                }
                skb = tcp_sacktag_skip(skb, sk, &state, start_seq);

walk:
                skb = tcp_sacktag_walk(skb, sk, next_dup, &state,
                                       start_seq, end_seq, dup_sack);

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

                i++;
        }

        /* Clear the head of the cache sack blocks so we can skip it next time */
        for (i = 0; i < ARRAY_SIZE(tp->recv_sack_cache) - used_sacks; i++) {
                tp->recv_sack_cache[i].start_seq = 0;
                tp->recv_sack_cache[i].end_seq = 0;
        }
        for (j = 0; j < used_sacks; j++)
                tp->recv_sack_cache[i++] = sp[j];

        tcp_mark_lost_retrans(sk);

        tcp_verify_left_out(tp);

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

out:

#if FASTRETRANS_DEBUG > 0
        WARN_ON((int)tp->sacked_out < 0);
        WARN_ON((int)tp->lost_out < 0);
        WARN_ON((int)tp->retrans_out < 0);
        WARN_ON((int)tcp_packets_in_flight(tp) < 0);
#endif
        return state.flag;
}

/* Limits sacked_out so that sum with lost_out isn't ever larger than
 * packets_out. Returns zero if sacked_out adjustement wasn't necessary.
 */
static int tcp_limit_reno_sacked(struct tcp_sock *tp)
{
        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;
                return 1;
        }
        return 0;
}

/* 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);
        if (tcp_limit_reno_sacked(tp))
                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;
}

static int tcp_is_sackfrto(const struct tcp_sock *tp)
{
        return (sysctl_tcp_frto == 0x2) && !tcp_is_reno(tp);
}

/* 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)
{