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33 results

stacktrace.h

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  • tcp_input.c 171.40 KiB
    /*
     * 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
     */
    
    #define pr_fmt(fmt) "TCP: " fmt
    
    #include <linux/mm.h>
    #include <linux/slab.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;
    EXPORT_SYMBOL(sysctl_tcp_reordering);
    int sysctl_tcp_ecn __read_mostly = 2;
    EXPORT_SYMBOL(sysctl_tcp_ecn);
    int sysctl_tcp_dsack __read_mostly = 1;
    int sysctl_tcp_app_win __read_mostly = 31;
    int sysctl_tcp_adv_win_scale __read_mostly = 1;
    EXPORT_SYMBOL(sysctl_tcp_adv_win_scale);
    
    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_thin_dupack __read_mostly;
    
    int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
    int sysctl_tcp_abc __read_mostly;
    int sysctl_tcp_early_retrans __read_mostly = 2;
    
    #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_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_MSS_DEFAULT + 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 int 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);
    }
    
    static 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 bool 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, const 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, const struct sk_buff *skb)
    {
    	if (!(tp->ecn_flags & TCP_ECN_OK))
    		return;
    
    	switch (TCP_SKB_CB(skb)->ip_dsfield & INET_ECN_MASK) {
    	case INET_ECN_NOT_ECT:
    		/* Funny extension: if ECT is not set on a segment,
    		 * and we already seen ECT on a previous segment,
    		 * it is probably a retransmit.
    		 */
    		if (tp->ecn_flags & TCP_ECN_SEEN)
    			tcp_enter_quickack_mode((struct sock *)tp);
    		break;
    	case INET_ECN_CE:
    		tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
    		/* fallinto */
    	default:
    		tp->ecn_flags |= TCP_ECN_SEEN;
    	}
    }
    
    static inline void TCP_ECN_rcv_synack(struct tcp_sock *tp, const 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, const struct tcphdr *th)
    {
    	if ((tp->ecn_flags & TCP_ECN_OK) && (!th->ece || !th->cwr))
    		tp->ecn_flags &= ~TCP_ECN_OK;
    }
    
    static bool TCP_ECN_rcv_ecn_echo(const struct tcp_sock *tp, const struct tcphdr *th)
    {
    	if (th->ece && !th->syn && (tp->ecn_flags & TCP_ECN_OK))
    		return true;
    	return false;
    }
    
    /* 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 = SKB_TRUESIZE(tcp_sk(sk)->rx_opt.mss_clamp + MAX_TCP_HEADER);
    
    	sndmem *= TCP_INIT_CWND;
    	if (sk->sk_sndbuf < sndmem)
    		sk->sk_sndbuf = min(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, const 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) &&
    	    !sk_under_memory_pressure(sk)) {
    		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) {
    			incr = max_t(int, incr, 2 * skb->len);
    			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)
    {
    	u32 mss = tcp_sk(sk)->advmss;
    	u32 icwnd = TCP_DEFAULT_INIT_RCVWND;
    	int rcvmem;
    
    	/* Limit to 10 segments if mss <= 1460,
    	 * or 14600/mss segments, with a minimum of two segments.
    	 */
    	if (mss > 1460)
    		icwnd = max_t(u32, (1460 * TCP_DEFAULT_INIT_RCVWND) / mss, 2);
    
    	rcvmem = SKB_TRUESIZE(mss + MAX_TCP_HEADER);
    	while (tcp_win_from_space(rcvmem) < mss)
    		rcvmem += 128;
    
    	rcvmem *= icwnd;
    
    	if (sk->sk_rcvbuf < rcvmem)
    		sk->sk_rcvbuf = min(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) &&
    	    !sk_under_memory_pressure(sk) &&
    	    sk_memory_allocated(sk) < sk_prot_mem_limits(sk, 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)
    {
    	const 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_MSS_DEFAULT);
    	hint = max(hint, TCP_MIN_MSS);
    
    	inet_csk(sk)->icsk_ack.rcv_mss = hint;
    }
    EXPORT_SYMBOL(tcp_initialize_rcv_mss);
    
    /* 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://public.lanl.gov/radiant/pubs.html#DRS>
     *
     * More detail on this code can be found at
     * <http://staff.psc.edu/jheffner/>,
     * 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 {
    			m <<= 3;
    			if (m < new_sample)
    				new_sample = m;
    		}
    	} 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, tcp_time_stamp - 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 = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER);
    			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.
     */
    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 = __tcp_set_rto(tp);
    
    	/* 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.
    	 */
    	tcp_bound_rto(sk);
    }
    
    __u32 tcp_init_cwnd(const struct tcp_sock *tp, const struct dst_entry *dst)
    {
    	__u32 cwnd = (dst ? dst_metric(dst, RTAX_INITCWND) : 0);
    
    	if (!cwnd)
    		cwnd = TCP_INIT_CWND;
    	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
     */
    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 &= ~TCP_FACK_ENABLED;
    }
    
    /* Take a notice that peer is sending D-SACKs */
    static void tcp_dsack_seen(struct tcp_sock *tp)
    {
    	tp->rx_opt.sack_ok |= TCP_DSACK_SEEN;
    }
    
    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
    		pr_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);
    	}
    
    	if (metric > 0)
    		tcp_disable_early_retrans(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 two flavors:
     *	A. Scoreboard estimator decided the packet is lost.
     *	   A'. Reno "three dupacks" marks head of queue lost.
     *	   A''. Its FACK modification, head until snd.fack is lost.
     *	B. 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 bool tcp_is_sackblock_valid(struct tcp_sock *tp, bool 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 false;
    
    	/* Nasty start_seq wrap-around check (see comments above) */
    	if (!before(start_seq, tp->snd_nxt))
    		return false;
    
    	/* 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 true;
    
    	if (!is_dsack || !tp->undo_marker)
    		return false;
    
    	/* ...Then it's D-SACK, and must reside below snd_una completely */
    	if (after(end_seq, tp->snd_una))
    		return false;
    
    	if (!before(start_seq, tp->undo_marker))
    		return true;
    
    	/* Too old */
    	if (!after(end_seq, tp->undo_marker))
    		return false;
    
    	/* 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 "B". 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 bool tcp_check_dsack(struct sock *sk, const 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);
    	bool dup_sack = false;
    
    	if (before(start_seq_0, TCP_SKB_CB(ack_skb)->ack_seq)) {
    		dup_sack = true;
    		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 = true;
    			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 && tp->undo_marker && tp->undo_retrans &&
    	    !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 err;
    	bool in_sack;
    	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;
    }
    
    /* Mark the given newly-SACKed range as such, adjusting counters and hints. */
    static u8 tcp_sacktag_one(struct sock *sk,
    			  struct tcp_sacktag_state *state, u8 sacked,
    			  u32 start_seq, u32 end_seq,
    			  bool dup_sack, int pcount)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	int fack_count = state->fack_count;
    
    	/* Account D-SACK for retransmitted packet. */
    	if (dup_sack && (sacked & TCPCB_RETRANS)) {
    		if (tp->undo_marker && tp->undo_retrans &&
    		    after(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(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(start_seq,
    					   tcp_highest_sack_seq(tp)))
    					state->reord = min(fack_count,
    							   state->reord);
    
    				/* SACK enhanced F-RTO (RFC4138; Appendix B) */
    				if (!after(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(start_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;
    }
    
    /* Shift newly-SACKed bytes from this skb to the immediately previous
     * already-SACKed sk_buff. Mark the newly-SACKed bytes as such.
     */
    static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
    			    struct tcp_sacktag_state *state,
    			    unsigned int pcount, int shifted, int mss,
    			    bool dup_sack)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	struct sk_buff *prev = tcp_write_queue_prev(sk, skb);
    	u32 start_seq = TCP_SKB_CB(skb)->seq;	/* start of newly-SACKed */
    	u32 end_seq = start_seq + shifted;	/* end of newly-SACKed */
    
    	BUG_ON(!pcount);
    
    	/* Adjust counters and hints for the newly sacked sequence
    	 * range but discard the return value since prev is already
    	 * marked. We must tag the range first because the seq
    	 * advancement below implicitly advances
    	 * tcp_highest_sack_seq() when skb is highest_sack.
    	 */
    	tcp_sacktag_one(sk, state, TCP_SKB_CB(skb)->sacked,
    			start_seq, end_seq, dup_sack, pcount);
    
    	if (skb == tp->lost_skb_hint)
    		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;
    	}
    
    	/* 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 false;
    	}
    
    	/* 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)->tcp_flags |= TCP_SKB_CB(prev)->tcp_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 true;
    }
    
    /* I wish gso_size would have a bit more sane initialization than
     * something-or-zero which complicates things
     */
    static int tcp_skb_seglen(const 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(const 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,
    					  bool 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;
    		}
    	}
    
    	/* tcp_sacktag_one() won't SACK-tag ranges below snd_una */
    	if (!after(TCP_SKB_CB(skb)->seq + len, tp->snd_una))
    		goto fallback;
    
    	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,
    					bool 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;
    		bool 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 = true;
    		}
    
    		/* 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(sk,
    						state,
    						TCP_SKB_CB(skb)->sacked,
    						TCP_SKB_CB(skb)->seq,
    						TCP_SKB_CB(skb)->end_seq,
    						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(const struct tcp_sock *tp, const 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, const 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);
    	const 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;
    	bool found_dup_sack = false;
    	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++) {
    		bool 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;
    		bool 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];
    
    		/* 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 false if sacked_out adjustement wasn't necessary.
     */
    static bool 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 true;
    	}
    	return false;
    }
    
    /* 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)
     */
    bool tcp_use_frto(struct sock *sk)
    {
    	const struct tcp_sock *tp = tcp_sk(sk);
    	const struct inet_connection_sock *icsk = inet_csk(sk);
    	struct sk_buff *skb;
    
    	if (!sysctl_tcp_frto)
    		return false;
    
    	/* MTU probe and F-RTO won't really play nicely along currently */
    	if (icsk->icsk_mtup.probe_size)
    		return false;
    
    	if (tcp_is_sackfrto(tp))
    		return true;
    
    	/* Avoid expensive walking of rexmit queue if possible */
    	if (tp->retrans_out > 1)
    		return false;
    
    	skb = tcp_write_queue_head(sk);
    	if (tcp_skb_is_last(sk, skb))
    		return true;
    	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 false;
    		/* Short-circuit when first non-SACKed skb has been checked */
    		if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
    			break;
    	}
    	return true;
    }
    
    /* 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 (tcp_is_sackfrto(tp) && (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;
    		}
    
    		/* Marking forward transmissions that were made after RTO lost
    		 * can cause unnecessary retransmissions in some scenarios,
    		 * SACK blocks will mitigate that in some but not in all cases.
    		 * We used to not mark them but it was causing break-ups with
    		 * receivers that do only in-order receival.
    		 *
    		 * TODO: we could detect presence of such receiver and select
    		 * different behavior per flow.
    		 */
    		if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) {
    			TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
    			tp->lost_out += tcp_skb_pcount(skb);
    			tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
    		}
    	}
    	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->snd_nxt;
    	TCP_ECN_queue_cwr(tp);
    
    	tcp_clear_all_retrans_hints(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;
    	} 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);
    			tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
    		}
    	}
    	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;
    }
    
    /* 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.
     */
    static bool tcp_check_sack_reneging(struct sock *sk, int flag)
    {
    	if (flag & FLAG_SACK_RENEGING) {
    		struct inet_connection_sock *icsk = inet_csk(sk);
    		NET_INC_STATS_BH(sock_net(sk), 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 true;
    	}
    	return false;
    }
    
    static inline int tcp_fackets_out(const struct tcp_sock *tp)
    {
    	return tcp_is_reno(tp) ? tp->sacked_out + 1 : tp->fackets_out;
    }
    
    /* Heurestics to calculate number of duplicate ACKs. There's no dupACKs
     * counter when SACK is enabled (without SACK, sacked_out is used for
     * that purpose).
     *
     * Instead, with FACK TCP uses fackets_out that includes both SACKed
     * segments up to the highest received SACK block so far and holes in
     * between them.
     *
     * With reordering, holes may still be in flight, so RFC3517 recovery
     * uses pure sacked_out (total number of SACKed segments) even though
     * it violates the RFC that uses duplicate ACKs, often these are equal
     * but when e.g. out-of-window ACKs or packet duplication occurs,
     * they differ. Since neither occurs due to loss, TCP should really
     * ignore them.
     */
    static inline int tcp_dupack_heuristics(const struct tcp_sock *tp)
    {
    	return tcp_is_fack(tp) ? tp->fackets_out : tp->sacked_out + 1;
    }
    
    static bool tcp_pause_early_retransmit(struct sock *sk, int flag)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	unsigned long delay;
    
    	/* Delay early retransmit and entering fast recovery for
    	 * max(RTT/4, 2msec) unless ack has ECE mark, no RTT samples
    	 * available, or RTO is scheduled to fire first.
    	 */
    	if (sysctl_tcp_early_retrans < 2 || (flag & FLAG_ECE) || !tp->srtt)
    		return false;
    
    	delay = max_t(unsigned long, (tp->srtt >> 5), msecs_to_jiffies(2));
    	if (!time_after(inet_csk(sk)->icsk_timeout, (jiffies + delay)))
    		return false;
    
    	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, delay, TCP_RTO_MAX);
    	tp->early_retrans_delayed = 1;
    	return true;
    }
    
    static inline int tcp_skb_timedout(const struct sock *sk,
    				   const struct sk_buff *skb)
    {
    	return tcp_time_stamp - TCP_SKB_CB(skb)->when > inet_csk(sk)->icsk_rto;
    }
    
    static inline int tcp_head_timedout(const struct sock *sk)
    {
    	const 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 bool tcp_time_to_recover(struct sock *sk, int flag)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	__u32 packets_out;
    
    	/* Do not perform any recovery during F-RTO algorithm */
    	if (tp->frto_counter)
    		return false;
    
    	/* Trick#1: The loss is proven. */
    	if (tp->lost_out)
    		return true;
    
    	/* Not-A-Trick#2 : Classic rule... */
    	if (tcp_dupack_heuristics(tp) > tp->reordering)
    		return true;
    
    	/* Trick#3 : when we use RFC2988 timer restart, fast
    	 * retransmit can be triggered by timeout of queue head.
    	 */
    	if (tcp_is_fack(tp) && tcp_head_timedout(sk))
    		return true;
    
    	/* 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 true;
    	}
    
    	/* If a thin stream is detected, retransmit after first
    	 * received dupack. Employ only if SACK is supported in order
    	 * to avoid possible corner-case series of spurious retransmissions
    	 * Use only if there are no unsent data.
    	 */
    	if ((tp->thin_dupack || sysctl_tcp_thin_dupack) &&
    	    tcp_stream_is_thin(tp) && tcp_dupack_heuristics(tp) > 1 &&
    	    tcp_is_sack(tp) && !tcp_send_head(sk))
    		return true;
    
    	/* Trick#6: TCP early retransmit, per RFC5827.  To avoid spurious
    	 * retransmissions due to small network reorderings, we implement
    	 * Mitigation A.3 in the RFC and delay the retransmission for a short
    	 * interval if appropriate.
    	 */
    	if (tp->do_early_retrans && !tp->retrans_out && tp->sacked_out &&
    	    (tp->packets_out == (tp->sacked_out + 1) && tp->packets_out < 4) &&
    	    !tcp_may_send_now(sk))
    		return !tcp_pause_early_retransmit(sk, flag);
    
    	return false;
    }
    
    /* 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.
     *
     * Usefulness of this as is very questionable, since we should know which of
     * the segments is the next to timeout which is relatively expensive to find
     * in general case unless we add some data structure just for that. The
     * current approach certainly won't find the right one too often and when it
     * finally does find _something_ it usually marks large part of the window
     * right away (because a retransmission with a larger timestamp blocks the
     * loop from advancing). -ij
     */
    static void tcp_timeout_skbs(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	struct sk_buff *skb;
    
    	if (!tcp_is_fack(tp) || !tcp_head_timedout(sk))
    		return;
    
    	skb = tp->scoreboard_skb_hint;
    	if (tp->scoreboard_skb_hint == NULL)
    		skb = 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;
    
    		tcp_skb_mark_lost(tp, skb);
    	}
    
    	tp->scoreboard_skb_hint = skb;
    
    	tcp_verify_left_out(tp);
    }
    
    /* Detect loss in event "A" above by marking head of queue up as lost.
     * For FACK or non-SACK(Reno) senders, the first "packets" number of segments
     * are considered lost. For RFC3517 SACK, a segment is considered lost if it
     * has at least tp->reordering SACKed seqments above it; "packets" refers to
     * the maximum SACKed segments to pass before reaching this limit.
     */
    static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	struct sk_buff *skb;
    	int cnt, oldcnt;
    	int err;
    	unsigned int mss;
    	/* Use SACK to deduce losses of new sequences sent during recovery */
    	const u32 loss_high = tcp_is_sack(tp) ?  tp->snd_nxt : tp->high_seq;
    
    	WARN_ON(packets > tp->packets_out);
    	if (tp->lost_skb_hint) {
    		skb = tp->lost_skb_hint;
    		cnt = tp->lost_cnt_hint;
    		/* Head already handled? */
    		if (mark_head && skb != tcp_write_queue_head(sk))
    			return;
    	} 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;
    
    		if (after(TCP_SKB_CB(skb)->end_seq, loss_high))
    			break;
    
    		oldcnt = cnt;
    		if (tcp_is_fack(tp) || tcp_is_reno(tp) ||
    		    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
    			cnt += tcp_skb_pcount(skb);
    
    		if (cnt > packets) {
    			if ((tcp_is_sack(tp) && !tcp_is_fack(tp)) ||
    			    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) ||
    			    (oldcnt >= packets))
    				break;
    
    			mss = skb_shinfo(skb)->gso_size;
    			err = tcp_fragment(sk, skb, (packets - oldcnt) * mss, mss);
    			if (err < 0)
    				break;
    			cnt = packets;
    		}
    
    		tcp_skb_mark_lost(tp, skb);
    
    		if (mark_head)
    			break;
    	}
    	tcp_verify_left_out(tp);
    }
    
    /* Account newly detected lost packet(s) */
    
    static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	if (tcp_is_reno(tp)) {
    		tcp_mark_head_lost(sk, 1, 1);
    	} else if (tcp_is_fack(tp)) {
    		int lost = tp->fackets_out - tp->reordering;
    		if (lost <= 0)
    			lost = 1;
    		tcp_mark_head_lost(sk, lost, 0);
    	} else {
    		int sacked_upto = tp->sacked_out - tp->reordering;
    		if (sacked_upto >= 0)
    			tcp_mark_head_lost(sk, sacked_upto, 0);
    		else if (fast_rexmit)
    			tcp_mark_head_lost(sk, 1, 1);
    	}
    
    	tcp_timeout_skbs(sk);
    }
    
    /* 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(const struct tcp_sock *tp)
    {
    	return !tp->retrans_stamp ||
    		(tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
    		 before(tp->rx_opt.rcv_tsecr, tp->retrans_stamp));
    }
    
    /* 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);
    
    	if (sk->sk_family == AF_INET) {
    		pr_debug("Undo %s %pI4/%u c%u l%u ss%u/%u p%u\n",
    			 msg,
    			 &inet->inet_daddr, ntohs(inet->inet_dport),
    			 tp->snd_cwnd, tcp_left_out(tp),
    			 tp->snd_ssthresh, tp->prior_ssthresh,
    			 tp->packets_out);
    	}
    #if IS_ENABLED(CONFIG_IPV6)
    	else if (sk->sk_family == AF_INET6) {
    		struct ipv6_pinfo *np = inet6_sk(sk);
    		pr_debug("Undo %s %pI6/%u c%u l%u ss%u/%u p%u\n",
    			 msg,
    			 &np->daddr, ntohs(inet->inet_dport),
    			 tp->snd_cwnd, tcp_left_out(tp),
    			 tp->snd_ssthresh, tp->prior_ssthresh,
    			 tp->packets_out);
    	}
    #endif
    }
    #else
    #define DBGUNDO(x...) do { } while (0)
    #endif
    
    static void tcp_undo_cwr(struct sock *sk, const bool undo_ssthresh)
    {
    	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_ssthresh && 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);
    	}
    	tp->snd_cwnd_stamp = tcp_time_stamp;
    }
    
    static inline int tcp_may_undo(const struct tcp_sock *tp)
    {
    	return tp->undo_marker && (!tp->undo_retrans || tcp_packet_delayed(tp));
    }
    
    /* People celebrate: "We love our President!" */
    static bool tcp_try_undo_recovery(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	if (tcp_may_undo(tp)) {
    		int mib_idx;
    
    		/* 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, true);
    		if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss)
    			mib_idx = LINUX_MIB_TCPLOSSUNDO;
    		else
    			mib_idx = LINUX_MIB_TCPFULLUNDO;
    
    		NET_INC_STATS_BH(sock_net(sk), mib_idx);
    		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 true;
    	}
    	tcp_set_ca_state(sk, TCP_CA_Open);
    	return false;
    }
    
    /* 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, true);
    		tp->undo_marker = 0;
    		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPDSACKUNDO);
    	}
    }
    
    /* We can clear retrans_stamp when there are no retransmissions in the
     * window. It would seem that it is trivially available for us in
     * tp->retrans_out, however, that kind of assumptions doesn't consider
     * what will happen if errors occur when sending retransmission for the
     * second time. ...It could the that such segment has only
     * TCPCB_EVER_RETRANS set at the present time. It seems that checking
     * the head skb is enough except for some reneging corner cases that
     * are not worth the effort.
     *
     * Main reason for all this complexity is the fact that connection dying
     * time now depends on the validity of the retrans_stamp, in particular,
     * that successive retransmissions of a segment must not advance
     * retrans_stamp under any conditions.
     */
    static bool tcp_any_retrans_done(const struct sock *sk)
    {
    	const struct tcp_sock *tp = tcp_sk(sk);
    	struct sk_buff *skb;
    
    	if (tp->retrans_out)
    		return true;
    
    	skb = tcp_write_queue_head(sk);
    	if (unlikely(skb && TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS))
    		return true;
    
    	return false;
    }
    
    /* 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) || (tcp_fackets_out(tp) > tp->reordering);
    
    	if (tcp_may_undo(tp)) {
    		/* Plain luck! Hole if filled with delayed
    		 * packet, rather than with a retransmit.
    		 */
    		if (!tcp_any_retrans_done(sk))
    			tp->retrans_stamp = 0;
    
    		tcp_update_reordering(sk, tcp_fackets_out(tp) + acked, 1);
    
    		DBGUNDO(sk, "Hoe");
    		tcp_undo_cwr(sk, false);
    		NET_INC_STATS_BH(sock_net(sk), 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 bool 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, true);
    		NET_INC_STATS_BH(sock_net(sk), 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 true;
    	}
    	return false;
    }
    
    static inline void tcp_complete_cwr(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	/* Do not moderate cwnd if it's already undone in cwr or recovery. */
    	if (tp->undo_marker) {
    		if (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR) {
    			tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
    			tp->snd_cwnd_stamp = tcp_time_stamp;
    		} else if (tp->snd_ssthresh < TCP_INFINITE_SSTHRESH) {
    			/* PRR algorithm. */
    			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_keep_open(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	int state = TCP_CA_Open;
    
    	if (tcp_left_out(tp) || tcp_any_retrans_done(sk))
    		state = TCP_CA_Disorder;
    
    	if (inet_csk(sk)->icsk_ca_state != state) {
    		tcp_set_ca_state(sk, state);
    		tp->high_seq = tp->snd_nxt;
    	}
    }
    
    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->frto_counter && !tcp_any_retrans_done(sk))
    		tp->retrans_stamp = 0;
    
    	if (flag & FLAG_ECE)
    		tcp_enter_cwr(sk, 1);
    
    	if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
    		tcp_try_keep_open(sk);
    		if (inet_csk(sk)->icsk_ca_state != TCP_CA_Open)
    			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 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->snd_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);
    }
    
    /* Do a simple retransmit without using the backoff mechanisms in
     * tcp_timer. This is used for path mtu discovery.
     * The socket is already locked here.
     */
    void tcp_simple_retransmit(struct sock *sk)
    {
    	const struct inet_connection_sock *icsk = inet_csk(sk);
    	struct tcp_sock *tp = tcp_sk(sk);
    	struct sk_buff *skb;
    	unsigned int mss = tcp_current_mss(sk);
    	u32 prior_lost = tp->lost_out;
    
    	tcp_for_write_queue(skb, sk) {
    		if (skb == tcp_send_head(sk))
    			break;
    		if (tcp_skb_seglen(skb) > mss &&
    		    !(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) {
    			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_skb_mark_lost_uncond_verify(tp, skb);
    		}
    	}
    
    	tcp_clear_retrans_hints_partial(tp);
    
    	if (prior_lost == tp->lost_out)
    		return;
    
    	if (tcp_is_reno(tp))
    		tcp_limit_reno_sacked(tp);
    
    	tcp_verify_left_out(tp);
    
    	/* Don't muck with the congestion window here.
    	 * Reason is that we do not increase amount of _data_
    	 * in network, but units changed and effective
    	 * cwnd/ssthresh really reduced now.
    	 */
    	if (icsk->icsk_ca_state != TCP_CA_Loss) {
    		tp->high_seq = tp->snd_nxt;
    		tp->snd_ssthresh = tcp_current_ssthresh(sk);
    		tp->prior_ssthresh = 0;
    		tp->undo_marker = 0;
    		tcp_set_ca_state(sk, TCP_CA_Loss);
    	}
    	tcp_xmit_retransmit_queue(sk);
    }
    EXPORT_SYMBOL(tcp_simple_retransmit);
    
    /* This function implements the PRR algorithm, specifcally the PRR-SSRB
     * (proportional rate reduction with slow start reduction bound) as described in
     * http://www.ietf.org/id/draft-mathis-tcpm-proportional-rate-reduction-01.txt.
     * It computes the number of packets to send (sndcnt) based on packets newly
     * delivered:
     *   1) If the packets in flight is larger than ssthresh, PRR spreads the
     *	cwnd reductions across a full RTT.
     *   2) If packets in flight is lower than ssthresh (such as due to excess
     *	losses and/or application stalls), do not perform any further cwnd
     *	reductions, but instead slow start up to ssthresh.
     */
    static void tcp_update_cwnd_in_recovery(struct sock *sk, int newly_acked_sacked,
    					int fast_rexmit, int flag)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	int sndcnt = 0;
    	int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);
    
    	if (tcp_packets_in_flight(tp) > tp->snd_ssthresh) {
    		u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +
    			       tp->prior_cwnd - 1;
    		sndcnt = div_u64(dividend, tp->prior_cwnd) - tp->prr_out;
    	} else {
    		sndcnt = min_t(int, delta,
    			       max_t(int, tp->prr_delivered - tp->prr_out,
    				     newly_acked_sacked) + 1);
    	}
    
    	sndcnt = max(sndcnt, (fast_rexmit ? 1 : 0));
    	tp->snd_cwnd = tcp_packets_in_flight(tp) + sndcnt;
    }
    
    static void tcp_enter_recovery(struct sock *sk, bool ece_ack)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	int mib_idx;
    
    	if (tcp_is_reno(tp))
    		mib_idx = LINUX_MIB_TCPRENORECOVERY;
    	else
    		mib_idx = LINUX_MIB_TCPSACKRECOVERY;
    
    	NET_INC_STATS_BH(sock_net(sk), mib_idx);
    
    	tp->high_seq = tp->snd_nxt;
    	tp->prior_ssthresh = 0;
    	tp->undo_marker = tp->snd_una;
    	tp->undo_retrans = tp->retrans_out;
    
    	if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
    		if (!ece_ack)
    			tp->prior_ssthresh = tcp_current_ssthresh(sk);
    		tp->snd_ssthresh = inet_csk(sk)->icsk_ca_ops->ssthresh(sk);
    		TCP_ECN_queue_cwr(tp);
    	}
    
    	tp->bytes_acked = 0;
    	tp->snd_cwnd_cnt = 0;
    	tp->prior_cwnd = tp->snd_cwnd;
    	tp->prr_delivered = 0;
    	tp->prr_out = 0;
    	tcp_set_ca_state(sk, TCP_CA_Recovery);
    }
    
    /* 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 newly_acked_sacked, bool is_dupack,
    				  int flag)
    {
    	struct inet_connection_sock *icsk = inet_csk(sk);
    	struct tcp_sock *tp = tcp_sk(sk);
    	int do_lost = is_dupack || ((flag & FLAG_DATA_SACKED) &&
    				    (tcp_fackets_out(tp) > tp->reordering));
    	int fast_rexmit = 0;
    
    	if (WARN_ON(!tp->packets_out && tp->sacked_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 (tcp_check_sack_reneging(sk, flag))
    		return;
    
    	/* C. Check consistency of the current state. */
    	tcp_verify_left_out(tp);
    
    	/* D. Check state exit conditions. State can be terminated
    	 *    when high_seq is ACKed. */
    	if (icsk->icsk_ca_state == TCP_CA_Open) {
    		WARN_ON(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_Recovery:
    			if (tcp_is_reno(tp))
    				tcp_reset_reno_sack(tp);
    			if (tcp_try_undo_recovery(sk))
    				return;
    			tcp_complete_cwr(sk);
    			break;
    		}
    	}
    
    	/* E. 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_is_reno(tp) && flag & FLAG_SND_UNA_ADVANCED)
    			tcp_reset_reno_sack(tp);
    		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, flag)) {
    			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 */
    		tcp_enter_recovery(sk, (flag & FLAG_ECE));
    		fast_rexmit = 1;
    	}
    
    	if (do_lost || (tcp_is_fack(tp) && tcp_head_timedout(sk)))
    		tcp_update_scoreboard(sk, fast_rexmit);
    	tp->prr_delivered += newly_acked_sacked;
    	tcp_update_cwnd_in_recovery(sk, newly_acked_sacked, fast_rexmit, flag);
    	tcp_xmit_retransmit_queue(sk);
    }
    
    void tcp_valid_rtt_meas(struct sock *sk, u32 seq_rtt)
    {
    	tcp_rtt_estimator(sk, seq_rtt);
    	tcp_set_rto(sk);
    	inet_csk(sk)->icsk_backoff = 0;
    }
    EXPORT_SYMBOL(tcp_valid_rtt_meas);
    
    /* 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);
    
    	tcp_valid_rtt_meas(sk, tcp_time_stamp - tp->rx_opt.rcv_tsecr);
    }
    
    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_valid_rtt_meas(sk, seq_rtt);
    }
    
    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)
    {
    	const struct inet_connection_sock *icsk = inet_csk(sk);
    	icsk->icsk_ca_ops->cong_avoid(sk, ack, in_flight);
    	tcp_sk(sk)->snd_cwnd_stamp = tcp_time_stamp;
    }
    
    /* Restart timer after forward progress on connection.
     * RFC2988 recommends to restart timer to now+rto.
     */
    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 {
    		u32 rto = inet_csk(sk)->icsk_rto;
    		/* Offset the time elapsed after installing regular RTO */
    		if (tp->early_retrans_delayed) {
    			struct sk_buff *skb = tcp_write_queue_head(sk);
    			const u32 rto_time_stamp = TCP_SKB_CB(skb)->when + rto;
    			s32 delta = (s32)(rto_time_stamp - tcp_time_stamp);
    			/* delta may not be positive if the socket is locked
    			 * when the delayed ER timer fires and is rescheduled.
    			 */
    			if (delta > 0)
    				rto = delta;
    		}
    		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
    					  TCP_RTO_MAX);
    	}
    	tp->early_retrans_delayed = 0;
    }
    
    /* This function is called when the delayed ER timer fires. TCP enters
     * fast recovery and performs fast-retransmit.
     */
    void tcp_resume_early_retransmit(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	tcp_rearm_rto(sk);
    
    	/* Stop if ER is disabled after the delayed ER timer is scheduled */
    	if (!tp->do_early_retrans)
    		return;
    
    	tcp_enter_recovery(sk, false);
    	tcp_update_scoreboard(sk, 1);
    	tcp_xmit_retransmit_queue(sk);
    }
    
    /* 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, int prior_fackets,
    			       u32 prior_snd_una)
    {
    	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 = true;
    	int flag = 0;
    	u32 pkts_acked = 0;
    	u32 reord = tp->packets_out;
    	u32 prior_sacked = tp->sacked_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 acked_pcount;
    		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;
    
    			acked_pcount = tcp_tso_acked(sk, skb);
    			if (!acked_pcount)
    				break;
    
    			fully_acked = false;
    		} else {
    			acked_pcount = tcp_skb_pcount(skb);
    		}
    
    		if (sacked & TCPCB_RETRANS) {
    			if (sacked & TCPCB_SACKED_RETRANS)
    				tp->retrans_out -= acked_pcount;
    			flag |= FLAG_RETRANS_DATA_ACKED;
    			ca_seq_rtt = -1;
    			seq_rtt = -1;
    			if ((flag & FLAG_DATA_ACKED) || (acked_pcount > 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(pkts_acked, reord);
    		}
    
    		if (sacked & TCPCB_SACKED_ACKED)
    			tp->sacked_out -= acked_pcount;
    		if (sacked & TCPCB_LOST)
    			tp->lost_out -= acked_pcount;
    
    		tp->packets_out -= acked_pcount;
    		pkts_acked += acked_pcount;
    
    		/* 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->tcp_flags & TCPHDR_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_wmem_free_skb(sk, skb);
    		tp->scoreboard_skb_hint = NULL;
    		if (skb == tp->retransmit_skb_hint)
    			tp->retransmit_skb_hint = NULL;
    		if (skb == tp->lost_skb_hint)
    			tp->lost_skb_hint = NULL;
    	}
    
    	if (likely(between(tp->snd_up, prior_snd_una, tp->snd_una)))
    		tp->snd_up = tp->snd_una;
    
    	if (skb && (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
    		flag |= FLAG_SACK_RENEGING;
    
    	if (flag & FLAG_ACKED) {
    		const struct tcp_congestion_ops *ca_ops
    			= inet_csk(sk)->icsk_ca_ops;
    
    		if (unlikely(icsk->icsk_mtup.probe_size &&
    			     !after(tp->mtu_probe.probe_seq_end, tp->snd_una))) {
    			tcp_mtup_probe_success(sk);
    		}
    
    		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 {
    			int delta;
    
    			/* Non-retransmitted hole got filled? That's reordering */
    			if (reord < prior_fackets)
    				tcp_update_reordering(sk, tp->fackets_out - reord, 0);
    
    			delta = tcp_is_fack(tp) ? pkts_acked :
    						  prior_sacked - tp->sacked_out;
    			tp->lost_cnt_hint -= min(tp->lost_cnt_hint, delta);
    		}
    
    		tp->fackets_out -= min(pkts_acked, tp->fackets_out);
    
    		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
    	WARN_ON((int)tp->sacked_out < 0);
    	WARN_ON((int)tp->lost_out < 0);
    	WARN_ON((int)tp->retrans_out < 0);
    	if (!tp->packets_out && tcp_is_sack(tp)) {
    		icsk = inet_csk(sk);
    		if (tp->lost_out) {
    			pr_debug("Leak l=%u %d\n",
    				 tp->lost_out, icsk->icsk_ca_state);
    			tp->lost_out = 0;
    		}
    		if (tp->sacked_out) {
    			pr_debug("Leak s=%u %d\n",
    				 tp->sacked_out, icsk->icsk_ca_state);
    			tp->sacked_out = 0;
    		}
    		if (tp->retrans_out) {
    			pr_debug("Leak r=%u %d\n",
    				 tp->retrans_out, icsk->icsk_ca_state);
    			tp->retrans_out = 0;
    		}
    	}
    #endif
    	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, tcp_wnd_end(tp))) {
    		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, const 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_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, true);
    }
    
    /* 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 bool 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 true;
    	}
    
    	if (!tcp_is_sackfrto(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 true;
    
    		if (!(flag & FLAG_DATA_ACKED)) {
    			tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 0 : 3),
    					    flag);
    			return true;
    		}
    	} 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 true;
    		}
    
    		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))
    				return true;
    
    			tcp_enter_frto_loss(sk, 3, flag);
    			return true;
    		}
    	}
    
    	if (tp->frto_counter == 1) {
    		/* tcp_may_send_now needs to see updated state */
    		tp->snd_cwnd = tcp_packets_in_flight(tp) + 2;
    		tp->frto_counter = 2;
    
    		if (!tcp_may_send_now(sk))
    			tcp_enter_frto_loss(sk, 2, flag);
    
    		return true;
    	} else {
    		switch (sysctl_tcp_frto_response) {
    		case 2:
    			tcp_undo_spur_to_response(sk, flag);
    			break;
    		case 1:
    			tcp_conservative_spur_to_response(tp);
    			break;
    		default:
    			tcp_ratehalving_spur_to_response(sk);
    			break;
    		}
    		tp->frto_counter = 0;
    		tp->undo_marker = 0;
    		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSPURIOUSRTOS);
    	}
    	return false;
    }
    
    /* This routine deals with incoming acks, but not outgoing ones. */
    static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
    {
    	struct inet_connection_sock *icsk = inet_csk(sk);
    	struct tcp_sock *tp = tcp_sk(sk);
    	u32 prior_snd_una = tp->snd_una;
    	u32 ack_seq = TCP_SKB_CB(skb)->seq;
    	u32 ack = TCP_SKB_CB(skb)->ack_seq;
    	bool is_dupack = false;
    	u32 prior_in_flight;
    	u32 prior_fackets;
    	int prior_packets;
    	int prior_sacked = tp->sacked_out;
    	int pkts_acked = 0;
    	int newly_acked_sacked = 0;
    	bool frto_cwnd = false;
    
    	/* If the ack is older than previous acks
    	 * then we can probably ignore it.
    	 */
    	if (before(ack, prior_snd_una))
    		goto old_ack;
    
    	/* If the ack includes data we haven't sent yet, discard
    	 * this segment (RFC793 Section 3.9).
    	 */
    	if (after(ack, tp->snd_nxt))
    		goto invalid_ack;
    
    	if (tp->early_retrans_delayed)
    		tcp_rearm_rto(sk);
    
    	if (after(ack, prior_snd_una))
    		flag |= FLAG_SND_UNA_ADVANCED;
    
    	if (sysctl_tcp_abc) {
    		if (icsk->icsk_ca_state < TCP_CA_CWR)
    			tp->bytes_acked += ack - prior_snd_una;
    		else if (icsk->icsk_ca_state == TCP_CA_Loss)
    			/* we assume just one segment left network */
    			tp->bytes_acked += min(ack - prior_snd_una,
    					       tp->mss_cache);
    	}
    
    	prior_fackets = tp->fackets_out;
    	prior_in_flight = tcp_packets_in_flight(tp);
    
    	if (!(flag & FLAG_SLOWPATH) && after(ack, prior_snd_una)) {
    		/* Window is constant, pure forward advance.
    		 * No more checks are required.
    		 * Note, we use the fact that SND.UNA>=SND.WL2.
    		 */
    		tcp_update_wl(tp, ack_seq);
    		tp->snd_una = ack;
    		flag |= FLAG_WIN_UPDATE;
    
    		tcp_ca_event(sk, CA_EVENT_FAST_ACK);
    
    		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPHPACKS);
    	} else {
    		if (ack_seq != TCP_SKB_CB(skb)->end_seq)
    			flag |= FLAG_DATA;
    		else
    			NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPPUREACKS);
    
    		flag |= tcp_ack_update_window(sk, skb, ack, ack_seq);
    
    		if (TCP_SKB_CB(skb)->sacked)
    			flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una);
    
    		if (TCP_ECN_rcv_ecn_echo(tp, tcp_hdr(skb)))
    			flag |= FLAG_ECE;
    
    		tcp_ca_event(sk, CA_EVENT_SLOW_ACK);
    	}
    
    	/* We passed data and got it acked, remove any soft error
    	 * log. Something worked...
    	 */
    	sk->sk_err_soft = 0;
    	icsk->icsk_probes_out = 0;
    	tp->rcv_tstamp = tcp_time_stamp;
    	prior_packets = tp->packets_out;
    	if (!prior_packets)
    		goto no_queue;
    
    	/* See if we can take anything off of the retransmit queue. */
    	flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una);
    
    	pkts_acked = prior_packets - tp->packets_out;
    	newly_acked_sacked = (prior_packets - prior_sacked) -
    			     (tp->packets_out - tp->sacked_out);
    
    	if (tp->frto_counter)
    		frto_cwnd = tcp_process_frto(sk, flag);
    	/* Guarantee sacktag reordering detection against wrap-arounds */
    	if (before(tp->frto_highmark, tp->snd_una))
    		tp->frto_highmark = 0;
    
    	if (tcp_ack_is_dubious(sk, flag)) {
    		/* Advance CWND, if state allows this. */
    		if ((flag & FLAG_DATA_ACKED) && !frto_cwnd &&
    		    tcp_may_raise_cwnd(sk, flag))
    			tcp_cong_avoid(sk, ack, prior_in_flight);
    		is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));
    		tcp_fastretrans_alert(sk, pkts_acked, newly_acked_sacked,
    				      is_dupack, flag);
    	} else {
    		if ((flag & FLAG_DATA_ACKED) && !frto_cwnd)
    			tcp_cong_avoid(sk, ack, prior_in_flight);
    	}
    
    	if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP)) {
    		struct dst_entry *dst = __sk_dst_get(sk);
    		if (dst)
    			dst_confirm(dst);
    	}
    	return 1;
    
    no_queue:
    	/* If data was DSACKed, see if we can undo a cwnd reduction. */
    	if (flag & FLAG_DSACKING_ACK)
    		tcp_fastretrans_alert(sk, pkts_acked, newly_acked_sacked,
    				      is_dupack, flag);
    	/* If this ack opens up a zero window, clear backoff.  It was
    	 * being used to time the probes, and is probably far higher than
    	 * it needs to be for normal retransmission.
    	 */
    	if (tcp_send_head(sk))
    		tcp_ack_probe(sk);
    	return 1;
    
    invalid_ack:
    	SOCK_DEBUG(sk, "Ack %u after %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
    	return -1;
    
    old_ack:
    	/* If data was SACKed, tag it and see if we should send more data.
    	 * If data was DSACKed, see if we can undo a cwnd reduction.
    	 */
    	if (TCP_SKB_CB(skb)->sacked) {
    		flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una);
    		newly_acked_sacked = tp->sacked_out - prior_sacked;
    		tcp_fastretrans_alert(sk, pkts_acked, newly_acked_sacked,
    				      is_dupack, flag);
    	}
    
    	SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
    	return 0;
    }
    
    /* Look for tcp options. Normally only called on SYN and SYNACK packets.
     * But, this can also be called on packets in the established flow when
     * the fast version below fails.
     */
    void tcp_parse_options(const struct sk_buff *skb, struct tcp_options_received *opt_rx,
    		       const u8 **hvpp, int estab)
    {
    	const unsigned char *ptr;
    	const struct tcphdr *th = tcp_hdr(skb);
    	int length = (th->doff * 4) - sizeof(struct tcphdr);
    
    	ptr = (const unsigned char *)(th + 1);
    	opt_rx->saw_tstamp = 0;
    
    	while (length > 0) {
    		int opcode = *ptr++;
    		int opsize;
    
    		switch (opcode) {
    		case TCPOPT_EOL:
    			return;
    		case TCPOPT_NOP:	/* Ref: RFC 793 section 3.1 */
    			length--;
    			continue;
    		default:
    			opsize = *ptr++;
    			if (opsize < 2) /* "silly options" */
    				return;
    			if (opsize > length)
    				return;	/* don't parse partial options */
    			switch (opcode) {
    			case TCPOPT_MSS:
    				if (opsize == TCPOLEN_MSS && th->syn && !estab) {
    					u16 in_mss = get_unaligned_be16(ptr);
    					if (in_mss) {
    						if (opt_rx->user_mss &&
    						    opt_rx->user_mss < in_mss)
    							in_mss = opt_rx->user_mss;
    						opt_rx->mss_clamp = in_mss;
    					}
    				}
    				break;
    			case TCPOPT_WINDOW:
    				if (opsize == TCPOLEN_WINDOW && th->syn &&
    				    !estab && sysctl_tcp_window_scaling) {
    					__u8 snd_wscale = *(__u8 *)ptr;
    					opt_rx->wscale_ok = 1;
    					if (snd_wscale > 14) {
    						net_info_ratelimited("%s: Illegal window scaling value %d >14 received\n",
    								     __func__,
    								     snd_wscale);
    						snd_wscale = 14;
    					}
    					opt_rx->snd_wscale = snd_wscale;
    				}
    				break;
    			case TCPOPT_TIMESTAMP:
    				if ((opsize == TCPOLEN_TIMESTAMP) &&
    				    ((estab && opt_rx->tstamp_ok) ||
    				     (!estab && sysctl_tcp_timestamps))) {
    					opt_rx->saw_tstamp = 1;
    					opt_rx->rcv_tsval = get_unaligned_be32(ptr);
    					opt_rx->rcv_tsecr = get_unaligned_be32(ptr + 4);
    				}
    				break;
    			case TCPOPT_SACK_PERM:
    				if (opsize == TCPOLEN_SACK_PERM && th->syn &&
    				    !estab && sysctl_tcp_sack) {
    					opt_rx->sack_ok = TCP_SACK_SEEN;
    					tcp_sack_reset(opt_rx);
    				}
    				break;
    
    			case TCPOPT_SACK:
    				if ((opsize >= (TCPOLEN_SACK_BASE + TCPOLEN_SACK_PERBLOCK)) &&
    				   !((opsize - TCPOLEN_SACK_BASE) % TCPOLEN_SACK_PERBLOCK) &&
    				   opt_rx->sack_ok) {
    					TCP_SKB_CB(skb)->sacked = (ptr - 2) - (unsigned char *)th;
    				}
    				break;
    #ifdef CONFIG_TCP_MD5SIG
    			case TCPOPT_MD5SIG:
    				/*
    				 * The MD5 Hash has already been
    				 * checked (see tcp_v{4,6}_do_rcv()).
    				 */
    				break;
    #endif
    			case TCPOPT_COOKIE:
    				/* This option is variable length.
    				 */
    				switch (opsize) {
    				case TCPOLEN_COOKIE_BASE:
    					/* not yet implemented */
    					break;
    				case TCPOLEN_COOKIE_PAIR:
    					/* not yet implemented */
    					break;
    				case TCPOLEN_COOKIE_MIN+0:
    				case TCPOLEN_COOKIE_MIN+2:
    				case TCPOLEN_COOKIE_MIN+4:
    				case TCPOLEN_COOKIE_MIN+6:
    				case TCPOLEN_COOKIE_MAX:
    					/* 16-bit multiple */
    					opt_rx->cookie_plus = opsize;
    					*hvpp = ptr;
    					break;
    				default:
    					/* ignore option */
    					break;
    				}
    				break;
    			}
    
    			ptr += opsize-2;
    			length -= opsize;
    		}
    	}
    }
    EXPORT_SYMBOL(tcp_parse_options);
    
    static bool tcp_parse_aligned_timestamp(struct tcp_sock *tp, const struct tcphdr *th)
    {
    	const __be32 *ptr = (const __be32 *)(th + 1);
    
    	if (*ptr == htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16)
    			  | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP)) {
    		tp->rx_opt.saw_tstamp = 1;
    		++ptr;
    		tp->rx_opt.rcv_tsval = ntohl(*ptr);
    		++ptr;
    		tp->rx_opt.rcv_tsecr = ntohl(*ptr);
    		return true;
    	}
    	return false;
    }
    
    /* Fast parse options. This hopes to only see timestamps.
     * If it is wrong it falls back on tcp_parse_options().
     */
    static bool tcp_fast_parse_options(const struct sk_buff *skb,
    				   const struct tcphdr *th,
    				   struct tcp_sock *tp, const u8 **hvpp)
    {
    	/* In the spirit of fast parsing, compare doff directly to constant
    	 * values.  Because equality is used, short doff can be ignored here.
    	 */
    	if (th->doff == (sizeof(*th) / 4)) {
    		tp->rx_opt.saw_tstamp = 0;
    		return false;
    	} else if (tp->rx_opt.tstamp_ok &&
    		   th->doff == ((sizeof(*th) + TCPOLEN_TSTAMP_ALIGNED) / 4)) {
    		if (tcp_parse_aligned_timestamp(tp, th))
    			return true;
    	}
    	tcp_parse_options(skb, &tp->rx_opt, hvpp, 1);
    	return true;
    }
    
    #ifdef CONFIG_TCP_MD5SIG
    /*
     * Parse MD5 Signature option
     */
    const u8 *tcp_parse_md5sig_option(const struct tcphdr *th)
    {
    	int length = (th->doff << 2) - sizeof(*th);
    	const u8 *ptr = (const u8 *)(th + 1);
    
    	/* If the TCP option is too short, we can short cut */
    	if (length < TCPOLEN_MD5SIG)
    		return NULL;
    
    	while (length > 0) {
    		int opcode = *ptr++;
    		int opsize;
    
    		switch(opcode) {
    		case TCPOPT_EOL:
    			return NULL;
    		case TCPOPT_NOP:
    			length--;
    			continue;
    		default:
    			opsize = *ptr++;
    			if (opsize < 2 || opsize > length)
    				return NULL;
    			if (opcode == TCPOPT_MD5SIG)
    				return opsize == TCPOLEN_MD5SIG ? ptr : NULL;
    		}
    		ptr += opsize - 2;
    		length -= opsize;
    	}
    	return NULL;
    }
    EXPORT_SYMBOL(tcp_parse_md5sig_option);
    #endif
    
    static inline void tcp_store_ts_recent(struct tcp_sock *tp)
    {
    	tp->rx_opt.ts_recent = tp->rx_opt.rcv_tsval;
    	tp->rx_opt.ts_recent_stamp = get_seconds();
    }
    
    static inline void tcp_replace_ts_recent(struct tcp_sock *tp, u32 seq)
    {
    	if (tp->rx_opt.saw_tstamp && !after(seq, tp->rcv_wup)) {
    		/* PAWS bug workaround wrt. ACK frames, the PAWS discard
    		 * extra check below makes sure this can only happen
    		 * for pure ACK frames.  -DaveM
    		 *
    		 * Not only, also it occurs for expired timestamps.
    		 */
    
    		if (tcp_paws_check(&tp->rx_opt, 0))
    			tcp_store_ts_recent(tp);
    	}
    }
    
    /* Sorry, PAWS as specified is broken wrt. pure-ACKs -DaveM
     *
     * It is not fatal. If this ACK does _not_ change critical state (seqs, window)
     * it can pass through stack. So, the following predicate verifies that
     * this segment is not used for anything but congestion avoidance or
     * fast retransmit. Moreover, we even are able to eliminate most of such
     * second order effects, if we apply some small "replay" window (~RTO)
     * to timestamp space.
     *
     * All these measures still do not guarantee that we reject wrapped ACKs
     * on networks with high bandwidth, when sequence space is recycled fastly,
     * but it guarantees that such events will be very rare and do not affect
     * connection seriously. This doesn't look nice, but alas, PAWS is really
     * buggy extension.
     *
     * [ Later note. Even worse! It is buggy for segments _with_ data. RFC
     * states that events when retransmit arrives after original data are rare.
     * It is a blatant lie. VJ forgot about fast retransmit! 8)8) It is
     * the biggest problem on large power networks even with minor reordering.
     * OK, let's give it small replay window. If peer clock is even 1hz, it is safe
     * up to bandwidth of 18Gigabit/sec. 8) ]
     */
    
    static int tcp_disordered_ack(const struct sock *sk, const struct sk_buff *skb)
    {
    	const struct tcp_sock *tp = tcp_sk(sk);
    	const struct tcphdr *th = tcp_hdr(skb);
    	u32 seq = TCP_SKB_CB(skb)->seq;
    	u32 ack = TCP_SKB_CB(skb)->ack_seq;
    
    	return (/* 1. Pure ACK with correct sequence number. */
    		(th->ack && seq == TCP_SKB_CB(skb)->end_seq && seq == tp->rcv_nxt) &&
    
    		/* 2. ... and duplicate ACK. */
    		ack == tp->snd_una &&
    
    		/* 3. ... and does not update window. */
    		!tcp_may_update_window(tp, ack, seq, ntohs(th->window) << tp->rx_opt.snd_wscale) &&
    
    		/* 4. ... and sits in replay window. */
    		(s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) <= (inet_csk(sk)->icsk_rto * 1024) / HZ);
    }
    
    static inline int tcp_paws_discard(const struct sock *sk,
    				   const struct sk_buff *skb)
    {
    	const struct tcp_sock *tp = tcp_sk(sk);
    
    	return !tcp_paws_check(&tp->rx_opt, TCP_PAWS_WINDOW) &&
    	       !tcp_disordered_ack(sk, skb);
    }
    
    /* Check segment sequence number for validity.
     *
     * Segment controls are considered valid, if the segment
     * fits to the window after truncation to the window. Acceptability
     * of data (and SYN, FIN, of course) is checked separately.
     * See tcp_data_queue(), for example.
     *
     * Also, controls (RST is main one) are accepted using RCV.WUP instead
     * of RCV.NXT. Peer still did not advance his SND.UNA when we
     * delayed ACK, so that hisSND.UNA<=ourRCV.WUP.
     * (borrowed from freebsd)
     */
    
    static inline int tcp_sequence(const struct tcp_sock *tp, u32 seq, u32 end_seq)
    {
    	return	!before(end_seq, tp->rcv_wup) &&
    		!after(seq, tp->rcv_nxt + tcp_receive_window(tp));
    }
    
    /* When we get a reset we do this. */
    static void tcp_reset(struct sock *sk)
    {
    	/* We want the right error as BSD sees it (and indeed as we do). */
    	switch (sk->sk_state) {
    	case TCP_SYN_SENT:
    		sk->sk_err = ECONNREFUSED;
    		break;
    	case TCP_CLOSE_WAIT:
    		sk->sk_err = EPIPE;
    		break;
    	case TCP_CLOSE:
    		return;
    	default:
    		sk->sk_err = ECONNRESET;
    	}
    	/* This barrier is coupled with smp_rmb() in tcp_poll() */
    	smp_wmb();
    
    	if (!sock_flag(sk, SOCK_DEAD))
    		sk->sk_error_report(sk);
    
    	tcp_done(sk);
    }
    
    /*
     * 	Process the FIN bit. This now behaves as it is supposed to work
     *	and the FIN takes effect when it is validly part of sequence
     *	space. Not before when we get holes.
     *
     *	If we are ESTABLISHED, a received fin moves us to CLOSE-WAIT
     *	(and thence onto LAST-ACK and finally, CLOSE, we never enter
     *	TIME-WAIT)
     *
     *	If we are in FINWAIT-1, a received FIN indicates simultaneous
     *	close and we go into CLOSING (and later onto TIME-WAIT)
     *
     *	If we are in FINWAIT-2, a received FIN moves us to TIME-WAIT.
     */
    static void tcp_fin(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	inet_csk_schedule_ack(sk);
    
    	sk->sk_shutdown |= RCV_SHUTDOWN;
    	sock_set_flag(sk, SOCK_DONE);
    
    	switch (sk->sk_state) {
    	case TCP_SYN_RECV:
    	case TCP_ESTABLISHED:
    		/* Move to CLOSE_WAIT */
    		tcp_set_state(sk, TCP_CLOSE_WAIT);
    		inet_csk(sk)->icsk_ack.pingpong = 1;
    		break;
    
    	case TCP_CLOSE_WAIT:
    	case TCP_CLOSING:
    		/* Received a retransmission of the FIN, do
    		 * nothing.
    		 */
    		break;
    	case TCP_LAST_ACK:
    		/* RFC793: Remain in the LAST-ACK state. */
    		break;
    
    	case TCP_FIN_WAIT1:
    		/* This case occurs when a simultaneous close
    		 * happens, we must ack the received FIN and
    		 * enter the CLOSING state.
    		 */
    		tcp_send_ack(sk);
    		tcp_set_state(sk, TCP_CLOSING);
    		break;
    	case TCP_FIN_WAIT2:
    		/* Received a FIN -- send ACK and enter TIME_WAIT. */
    		tcp_send_ack(sk);
    		tcp_time_wait(sk, TCP_TIME_WAIT, 0);
    		break;
    	default:
    		/* Only TCP_LISTEN and TCP_CLOSE are left, in these
    		 * cases we should never reach this piece of code.
    		 */
    		pr_err("%s: Impossible, sk->sk_state=%d\n",
    		       __func__, sk->sk_state);
    		break;
    	}
    
    	/* It _is_ possible, that we have something out-of-order _after_ FIN.
    	 * Probably, we should reset in this case. For now drop them.
    	 */
    	__skb_queue_purge(&tp->out_of_order_queue);
    	if (tcp_is_sack(tp))
    		tcp_sack_reset(&tp->rx_opt);
    	sk_mem_reclaim(sk);
    
    	if (!sock_flag(sk, SOCK_DEAD)) {
    		sk->sk_state_change(sk);
    
    		/* Do not send POLL_HUP for half duplex close. */
    		if (sk->sk_shutdown == SHUTDOWN_MASK ||
    		    sk->sk_state == TCP_CLOSE)
    			sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_HUP);
    		else
    			sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
    	}
    }
    
    static inline bool tcp_sack_extend(struct tcp_sack_block *sp, u32 seq,
    				  u32 end_seq)
    {
    	if (!after(seq, sp->end_seq) && !after(sp->start_seq, end_seq)) {
    		if (before(seq, sp->start_seq))
    			sp->start_seq = seq;
    		if (after(end_seq, sp->end_seq))
    			sp->end_seq = end_seq;
    		return true;
    	}
    	return false;
    }
    
    static void tcp_dsack_set(struct sock *sk, u32 seq, u32 end_seq)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	if (tcp_is_sack(tp) && sysctl_tcp_dsack) {
    		int mib_idx;
    
    		if (before(seq, tp->rcv_nxt))
    			mib_idx = LINUX_MIB_TCPDSACKOLDSENT;
    		else
    			mib_idx = LINUX_MIB_TCPDSACKOFOSENT;
    
    		NET_INC_STATS_BH(sock_net(sk), mib_idx);
    
    		tp->rx_opt.dsack = 1;
    		tp->duplicate_sack[0].start_seq = seq;
    		tp->duplicate_sack[0].end_seq = end_seq;
    	}
    }
    
    static void tcp_dsack_extend(struct sock *sk, u32 seq, u32 end_seq)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	if (!tp->rx_opt.dsack)
    		tcp_dsack_set(sk, seq, end_seq);
    	else
    		tcp_sack_extend(tp->duplicate_sack, seq, end_seq);
    }
    
    static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
    	    before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
    		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
    		tcp_enter_quickack_mode(sk);
    
    		if (tcp_is_sack(tp) && sysctl_tcp_dsack) {
    			u32 end_seq = TCP_SKB_CB(skb)->end_seq;
    
    			if (after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))
    				end_seq = tp->rcv_nxt;
    			tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, end_seq);
    		}
    	}
    
    	tcp_send_ack(sk);
    }
    
    /* These routines update the SACK block as out-of-order packets arrive or
     * in-order packets close up the sequence space.
     */
    static void tcp_sack_maybe_coalesce(struct tcp_sock *tp)
    {
    	int this_sack;
    	struct tcp_sack_block *sp = &tp->selective_acks[0];
    	struct tcp_sack_block *swalk = sp + 1;
    
    	/* See if the recent change to the first SACK eats into
    	 * or hits the sequence space of other SACK blocks, if so coalesce.
    	 */
    	for (this_sack = 1; this_sack < tp->rx_opt.num_sacks;) {
    		if (tcp_sack_extend(sp, swalk->start_seq, swalk->end_seq)) {
    			int i;
    
    			/* Zap SWALK, by moving every further SACK up by one slot.
    			 * Decrease num_sacks.
    			 */
    			tp->rx_opt.num_sacks--;
    			for (i = this_sack; i < tp->rx_opt.num_sacks; i++)
    				sp[i] = sp[i + 1];
    			continue;
    		}
    		this_sack++, swalk++;
    	}
    }
    
    static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	struct tcp_sack_block *sp = &tp->selective_acks[0];
    	int cur_sacks = tp->rx_opt.num_sacks;
    	int this_sack;
    
    	if (!cur_sacks)
    		goto new_sack;
    
    	for (this_sack = 0; this_sack < cur_sacks; this_sack++, sp++) {
    		if (tcp_sack_extend(sp, seq, end_seq)) {
    			/* Rotate this_sack to the first one. */
    			for (; this_sack > 0; this_sack--, sp--)
    				swap(*sp, *(sp - 1));
    			if (cur_sacks > 1)
    				tcp_sack_maybe_coalesce(tp);
    			return;
    		}
    	}
    
    	/* Could not find an adjacent existing SACK, build a new one,
    	 * put it at the front, and shift everyone else down.  We
    	 * always know there is at least one SACK present already here.
    	 *
    	 * If the sack array is full, forget about the last one.
    	 */
    	if (this_sack >= TCP_NUM_SACKS) {
    		this_sack--;
    		tp->rx_opt.num_sacks--;
    		sp--;
    	}
    	for (; this_sack > 0; this_sack--, sp--)
    		*sp = *(sp - 1);
    
    new_sack:
    	/* Build the new head SACK, and we're done. */
    	sp->start_seq = seq;
    	sp->end_seq = end_seq;
    	tp->rx_opt.num_sacks++;
    }
    
    /* RCV.NXT advances, some SACKs should be eaten. */
    
    static void tcp_sack_remove(struct tcp_sock *tp)
    {
    	struct tcp_sack_block *sp = &tp->selective_acks[0];
    	int num_sacks = tp->rx_opt.num_sacks;
    	int this_sack;
    
    	/* Empty ofo queue, hence, all the SACKs are eaten. Clear. */
    	if (skb_queue_empty(&tp->out_of_order_queue)) {
    		tp->rx_opt.num_sacks = 0;
    		return;
    	}
    
    	for (this_sack = 0; this_sack < num_sacks;) {
    		/* Check if the start of the sack is covered by RCV.NXT. */
    		if (!before(tp->rcv_nxt, sp->start_seq)) {
    			int i;
    
    			/* RCV.NXT must cover all the block! */
    			WARN_ON(before(tp->rcv_nxt, sp->end_seq));
    
    			/* Zap this SACK, by moving forward any other SACKS. */
    			for (i=this_sack+1; i < num_sacks; i++)
    				tp->selective_acks[i-1] = tp->selective_acks[i];
    			num_sacks--;
    			continue;
    		}
    		this_sack++;
    		sp++;
    	}
    	tp->rx_opt.num_sacks = num_sacks;
    }
    
    /* This one checks to see if we can put data from the
     * out_of_order queue into the receive_queue.
     */
    static void tcp_ofo_queue(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	__u32 dsack_high = tp->rcv_nxt;
    	struct sk_buff *skb;
    
    	while ((skb = skb_peek(&tp->out_of_order_queue)) != NULL) {
    		if (after(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
    			break;
    
    		if (before(TCP_SKB_CB(skb)->seq, dsack_high)) {
    			__u32 dsack = dsack_high;
    			if (before(TCP_SKB_CB(skb)->end_seq, dsack_high))
    				dsack_high = TCP_SKB_CB(skb)->end_seq;
    			tcp_dsack_extend(sk, TCP_SKB_CB(skb)->seq, dsack);
    		}
    
    		if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
    			SOCK_DEBUG(sk, "ofo packet was already received\n");
    			__skb_unlink(skb, &tp->out_of_order_queue);
    			__kfree_skb(skb);
    			continue;
    		}
    		SOCK_DEBUG(sk, "ofo requeuing : rcv_next %X seq %X - %X\n",
    			   tp->rcv_nxt, TCP_SKB_CB(skb)->seq,
    			   TCP_SKB_CB(skb)->end_seq);
    
    		__skb_unlink(skb, &tp->out_of_order_queue);
    		__skb_queue_tail(&sk->sk_receive_queue, skb);
    		tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
    		if (tcp_hdr(skb)->fin)
    			tcp_fin(sk);
    	}
    }
    
    static bool tcp_prune_ofo_queue(struct sock *sk);
    static int tcp_prune_queue(struct sock *sk);
    
    static int tcp_try_rmem_schedule(struct sock *sk, unsigned int size)
    {
    	if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf ||
    	    !sk_rmem_schedule(sk, size)) {
    
    		if (tcp_prune_queue(sk) < 0)
    			return -1;
    
    		if (!sk_rmem_schedule(sk, size)) {
    			if (!tcp_prune_ofo_queue(sk))
    				return -1;
    
    			if (!sk_rmem_schedule(sk, size))
    				return -1;
    		}
    	}
    	return 0;
    }
    
    /**
     * tcp_try_coalesce - try to merge skb to prior one
     * @sk: socket
     * @to: prior buffer
     * @from: buffer to add in queue
     * @fragstolen: pointer to boolean
     *
     * Before queueing skb @from after @to, try to merge them
     * to reduce overall memory use and queue lengths, if cost is small.
     * Packets in ofo or receive queues can stay a long time.
     * Better try to coalesce them right now to avoid future collapses.
     * Returns true if caller should free @from instead of queueing it
     */
    static bool tcp_try_coalesce(struct sock *sk,
    			     struct sk_buff *to,
    			     struct sk_buff *from,
    			     bool *fragstolen)
    {
    	int delta;
    
    	*fragstolen = false;
    
    	if (tcp_hdr(from)->fin)
    		return false;
    
    	/* Its possible this segment overlaps with prior segment in queue */
    	if (TCP_SKB_CB(from)->seq != TCP_SKB_CB(to)->end_seq)
    		return false;
    
    	if (!skb_try_coalesce(to, from, fragstolen, &delta))
    		return false;
    
    	atomic_add(delta, &sk->sk_rmem_alloc);
    	sk_mem_charge(sk, delta);
    	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
    	TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
    	TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
    	return true;
    }
    
    static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	struct sk_buff *skb1;
    	u32 seq, end_seq;
    
    	TCP_ECN_check_ce(tp, skb);
    
    	if (unlikely(tcp_try_rmem_schedule(sk, skb->truesize))) {
    		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFODROP);
    		__kfree_skb(skb);
    		return;
    	}
    
    	/* Disable header prediction. */
    	tp->pred_flags = 0;
    	inet_csk_schedule_ack(sk);
    
    	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFOQUEUE);
    	SOCK_DEBUG(sk, "out of order segment: rcv_next %X seq %X - %X\n",
    		   tp->rcv_nxt, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);
    
    	skb1 = skb_peek_tail(&tp->out_of_order_queue);
    	if (!skb1) {
    		/* Initial out of order segment, build 1 SACK. */
    		if (tcp_is_sack(tp)) {
    			tp->rx_opt.num_sacks = 1;
    			tp->selective_acks[0].start_seq = TCP_SKB_CB(skb)->seq;
    			tp->selective_acks[0].end_seq =
    						TCP_SKB_CB(skb)->end_seq;
    		}
    		__skb_queue_head(&tp->out_of_order_queue, skb);
    		goto end;
    	}
    
    	seq = TCP_SKB_CB(skb)->seq;
    	end_seq = TCP_SKB_CB(skb)->end_seq;
    
    	if (seq == TCP_SKB_CB(skb1)->end_seq) {
    		bool fragstolen;
    
    		if (!tcp_try_coalesce(sk, skb1, skb, &fragstolen)) {
    			__skb_queue_after(&tp->out_of_order_queue, skb1, skb);
    		} else {
    			kfree_skb_partial(skb, fragstolen);
    			skb = NULL;
    		}
    
    		if (!tp->rx_opt.num_sacks ||
    		    tp->selective_acks[0].end_seq != seq)
    			goto add_sack;
    
    		/* Common case: data arrive in order after hole. */
    		tp->selective_acks[0].end_seq = end_seq;
    		goto end;
    	}
    
    	/* Find place to insert this segment. */
    	while (1) {
    		if (!after(TCP_SKB_CB(skb1)->seq, seq))
    			break;
    		if (skb_queue_is_first(&tp->out_of_order_queue, skb1)) {
    			skb1 = NULL;
    			break;
    		}
    		skb1 = skb_queue_prev(&tp->out_of_order_queue, skb1);
    	}
    
    	/* Do skb overlap to previous one? */
    	if (skb1 && before(seq, TCP_SKB_CB(skb1)->end_seq)) {
    		if (!after(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
    			/* All the bits are present. Drop. */
    			NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFOMERGE);
    			__kfree_skb(skb);
    			skb = NULL;
    			tcp_dsack_set(sk, seq, end_seq);
    			goto add_sack;
    		}
    		if (after(seq, TCP_SKB_CB(skb1)->seq)) {
    			/* Partial overlap. */
    			tcp_dsack_set(sk, seq,
    				      TCP_SKB_CB(skb1)->end_seq);
    		} else {
    			if (skb_queue_is_first(&tp->out_of_order_queue,
    					       skb1))
    				skb1 = NULL;
    			else
    				skb1 = skb_queue_prev(
    					&tp->out_of_order_queue,
    					skb1);
    		}
    	}
    	if (!skb1)
    		__skb_queue_head(&tp->out_of_order_queue, skb);
    	else
    		__skb_queue_after(&tp->out_of_order_queue, skb1, skb);
    
    	/* And clean segments covered by new one as whole. */
    	while (!skb_queue_is_last(&tp->out_of_order_queue, skb)) {
    		skb1 = skb_queue_next(&tp->out_of_order_queue, skb);
    
    		if (!after(end_seq, TCP_SKB_CB(skb1)->seq))
    			break;
    		if (before(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
    			tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
    					 end_seq);
    			break;
    		}
    		__skb_unlink(skb1, &tp->out_of_order_queue);
    		tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
    				 TCP_SKB_CB(skb1)->end_seq);
    		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFOMERGE);
    		__kfree_skb(skb1);
    	}
    
    add_sack:
    	if (tcp_is_sack(tp))
    		tcp_sack_new_ofo_skb(sk, seq, end_seq);
    end:
    	if (skb)
    		skb_set_owner_r(skb, sk);
    }
    
    static int __must_check tcp_queue_rcv(struct sock *sk, struct sk_buff *skb, int hdrlen,
    		  bool *fragstolen)
    {
    	int eaten;
    	struct sk_buff *tail = skb_peek_tail(&sk->sk_receive_queue);
    
    	__skb_pull(skb, hdrlen);
    	eaten = (tail &&
    		 tcp_try_coalesce(sk, tail, skb, fragstolen)) ? 1 : 0;
    	tcp_sk(sk)->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
    	if (!eaten) {
    		__skb_queue_tail(&sk->sk_receive_queue, skb);
    		skb_set_owner_r(skb, sk);
    	}
    	return eaten;
    }
    
    int tcp_send_rcvq(struct sock *sk, struct msghdr *msg, size_t size)
    {
    	struct sk_buff *skb;
    	struct tcphdr *th;
    	bool fragstolen;
    
    	if (tcp_try_rmem_schedule(sk, size + sizeof(*th)))
    		goto err;
    
    	skb = alloc_skb(size + sizeof(*th), sk->sk_allocation);
    	if (!skb)
    		goto err;
    
    	th = (struct tcphdr *)skb_put(skb, sizeof(*th));
    	skb_reset_transport_header(skb);
    	memset(th, 0, sizeof(*th));
    
    	if (memcpy_fromiovec(skb_put(skb, size), msg->msg_iov, size))
    		goto err_free;
    
    	TCP_SKB_CB(skb)->seq = tcp_sk(sk)->rcv_nxt;
    	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + size;
    	TCP_SKB_CB(skb)->ack_seq = tcp_sk(sk)->snd_una - 1;
    
    	if (tcp_queue_rcv(sk, skb, sizeof(*th), &fragstolen)) {
    		WARN_ON_ONCE(fragstolen); /* should not happen */
    		__kfree_skb(skb);
    	}
    	return size;
    
    err_free:
    	kfree_skb(skb);
    err:
    	return -ENOMEM;
    }
    
    static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
    {
    	const struct tcphdr *th = tcp_hdr(skb);
    	struct tcp_sock *tp = tcp_sk(sk);
    	int eaten = -1;
    	bool fragstolen = false;
    
    	if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq)
    		goto drop;
    
    	skb_dst_drop(skb);
    	__skb_pull(skb, th->doff * 4);
    
    	TCP_ECN_accept_cwr(tp, skb);
    
    	tp->rx_opt.dsack = 0;
    
    	/*  Queue data for delivery to the user.
    	 *  Packets in sequence go to the receive queue.
    	 *  Out of sequence packets to the out_of_order_queue.
    	 */
    	if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
    		if (tcp_receive_window(tp) == 0)
    			goto out_of_window;
    
    		/* Ok. In sequence. In window. */
    		if (tp->ucopy.task == current &&
    		    tp->copied_seq == tp->rcv_nxt && tp->ucopy.len &&
    		    sock_owned_by_user(sk) && !tp->urg_data) {
    			int chunk = min_t(unsigned int, skb->len,
    					  tp->ucopy.len);
    
    			__set_current_state(TASK_RUNNING);
    
    			local_bh_enable();
    			if (!skb_copy_datagram_iovec(skb, 0, tp->ucopy.iov, chunk)) {
    				tp->ucopy.len -= chunk;
    				tp->copied_seq += chunk;
    				eaten = (chunk == skb->len);
    				tcp_rcv_space_adjust(sk);
    			}
    			local_bh_disable();
    		}
    
    		if (eaten <= 0) {
    queue_and_out:
    			if (eaten < 0 &&
    			    tcp_try_rmem_schedule(sk, skb->truesize))
    				goto drop;
    
    			eaten = tcp_queue_rcv(sk, skb, 0, &fragstolen);
    		}
    		tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
    		if (skb->len)
    			tcp_event_data_recv(sk, skb);
    		if (th->fin)
    			tcp_fin(sk);
    
    		if (!skb_queue_empty(&tp->out_of_order_queue)) {
    			tcp_ofo_queue(sk);
    
    			/* RFC2581. 4.2. SHOULD send immediate ACK, when
    			 * gap in queue is filled.
    			 */
    			if (skb_queue_empty(&tp->out_of_order_queue))
    				inet_csk(sk)->icsk_ack.pingpong = 0;
    		}
    
    		if (tp->rx_opt.num_sacks)
    			tcp_sack_remove(tp);
    
    		tcp_fast_path_check(sk);
    
    		if (eaten > 0)
    			kfree_skb_partial(skb, fragstolen);
    		else if (!sock_flag(sk, SOCK_DEAD))
    			sk->sk_data_ready(sk, 0);
    		return;
    	}
    
    	if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
    		/* A retransmit, 2nd most common case.  Force an immediate ack. */
    		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
    		tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);
    
    out_of_window:
    		tcp_enter_quickack_mode(sk);
    		inet_csk_schedule_ack(sk);
    drop:
    		__kfree_skb(skb);
    		return;
    	}
    
    	/* Out of window. F.e. zero window probe. */
    	if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt + tcp_receive_window(tp)))
    		goto out_of_window;
    
    	tcp_enter_quickack_mode(sk);
    
    	if (before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
    		/* Partial packet, seq < rcv_next < end_seq */
    		SOCK_DEBUG(sk, "partial packet: rcv_next %X seq %X - %X\n",
    			   tp->rcv_nxt, TCP_SKB_CB(skb)->seq,
    			   TCP_SKB_CB(skb)->end_seq);
    
    		tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, tp->rcv_nxt);
    
    		/* If window is closed, drop tail of packet. But after
    		 * remembering D-SACK for its head made in previous line.
    		 */
    		if (!tcp_receive_window(tp))
    			goto out_of_window;
    		goto queue_and_out;
    	}
    
    	tcp_data_queue_ofo(sk, skb);
    }
    
    static struct sk_buff *tcp_collapse_one(struct sock *sk, struct sk_buff *skb,
    					struct sk_buff_head *list)
    {
    	struct sk_buff *next = NULL;
    
    	if (!skb_queue_is_last(list, skb))
    		next = skb_queue_next(list, skb);
    
    	__skb_unlink(skb, list);
    	__kfree_skb(skb);
    	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED);
    
    	return next;
    }
    
    /* Collapse contiguous sequence of skbs head..tail with
     * sequence numbers start..end.
     *
     * If tail is NULL, this means until the end of the list.
     *
     * Segments with FIN/SYN are not collapsed (only because this
     * simplifies code)
     */
    static void
    tcp_collapse(struct sock *sk, struct sk_buff_head *list,
    	     struct sk_buff *head, struct sk_buff *tail,
    	     u32 start, u32 end)
    {
    	struct sk_buff *skb, *n;
    	bool end_of_skbs;
    
    	/* First, check that queue is collapsible and find
    	 * the point where collapsing can be useful. */
    	skb = head;
    restart:
    	end_of_skbs = true;
    	skb_queue_walk_from_safe(list, skb, n) {
    		if (skb == tail)
    			break;
    		/* No new bits? It is possible on ofo queue. */
    		if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
    			skb = tcp_collapse_one(sk, skb, list);
    			if (!skb)
    				break;
    			goto restart;
    		}
    
    		/* The first skb to collapse is:
    		 * - not SYN/FIN and
    		 * - bloated or contains data before "start" or
    		 *   overlaps to the next one.
    		 */
    		if (!tcp_hdr(skb)->syn && !tcp_hdr(skb)->fin &&
    		    (tcp_win_from_space(skb->truesize) > skb->len ||
    		     before(TCP_SKB_CB(skb)->seq, start))) {
    			end_of_skbs = false;
    			break;
    		}
    
    		if (!skb_queue_is_last(list, skb)) {
    			struct sk_buff *next = skb_queue_next(list, skb);
    			if (next != tail &&
    			    TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(next)->seq) {
    				end_of_skbs = false;
    				break;
    			}
    		}
    
    		/* Decided to skip this, advance start seq. */
    		start = TCP_SKB_CB(skb)->end_seq;
    	}
    	if (end_of_skbs || tcp_hdr(skb)->syn || tcp_hdr(skb)->fin)
    		return;
    
    	while (before(start, end)) {
    		struct sk_buff *nskb;
    		unsigned int header = skb_headroom(skb);
    		int copy = SKB_MAX_ORDER(header, 0);
    
    		/* Too big header? This can happen with IPv6. */
    		if (copy < 0)
    			return;
    		if (end - start < copy)
    			copy = end - start;
    		nskb = alloc_skb(copy + header, GFP_ATOMIC);
    		if (!nskb)
    			return;
    
    		skb_set_mac_header(nskb, skb_mac_header(skb) - skb->head);
    		skb_set_network_header(nskb, (skb_network_header(skb) -
    					      skb->head));
    		skb_set_transport_header(nskb, (skb_transport_header(skb) -
    						skb->head));
    		skb_reserve(nskb, header);
    		memcpy(nskb->head, skb->head, header);
    		memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
    		TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
    		__skb_queue_before(list, skb, nskb);
    		skb_set_owner_r(nskb, sk);
    
    		/* Copy data, releasing collapsed skbs. */
    		while (copy > 0) {
    			int offset = start - TCP_SKB_CB(skb)->seq;
    			int size = TCP_SKB_CB(skb)->end_seq - start;
    
    			BUG_ON(offset < 0);
    			if (size > 0) {
    				size = min(copy, size);
    				if (skb_copy_bits(skb, offset, skb_put(nskb, size), size))
    					BUG();
    				TCP_SKB_CB(nskb)->end_seq += size;
    				copy -= size;
    				start += size;
    			}
    			if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
    				skb = tcp_collapse_one(sk, skb, list);
    				if (!skb ||
    				    skb == tail ||
    				    tcp_hdr(skb)->syn ||
    				    tcp_hdr(skb)->fin)
    					return;
    			}
    		}
    	}
    }
    
    /* Collapse ofo queue. Algorithm: select contiguous sequence of skbs
     * and tcp_collapse() them until all the queue is collapsed.
     */
    static void tcp_collapse_ofo_queue(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	struct sk_buff *skb = skb_peek(&tp->out_of_order_queue);
    	struct sk_buff *head;
    	u32 start, end;
    
    	if (skb == NULL)
    		return;
    
    	start = TCP_SKB_CB(skb)->seq;
    	end = TCP_SKB_CB(skb)->end_seq;
    	head = skb;
    
    	for (;;) {
    		struct sk_buff *next = NULL;
    
    		if (!skb_queue_is_last(&tp->out_of_order_queue, skb))
    			next = skb_queue_next(&tp->out_of_order_queue, skb);
    		skb = next;
    
    		/* Segment is terminated when we see gap or when
    		 * we are at the end of all the queue. */
    		if (!skb ||
    		    after(TCP_SKB_CB(skb)->seq, end) ||
    		    before(TCP_SKB_CB(skb)->end_seq, start)) {
    			tcp_collapse(sk, &tp->out_of_order_queue,
    				     head, skb, start, end);
    			head = skb;
    			if (!skb)
    				break;
    			/* Start new segment */
    			start = TCP_SKB_CB(skb)->seq;
    			end = TCP_SKB_CB(skb)->end_seq;
    		} else {
    			if (before(TCP_SKB_CB(skb)->seq, start))
    				start = TCP_SKB_CB(skb)->seq;
    			if (after(TCP_SKB_CB(skb)->end_seq, end))
    				end = TCP_SKB_CB(skb)->end_seq;
    		}
    	}
    }
    
    /*
     * Purge the out-of-order queue.
     * Return true if queue was pruned.
     */
    static bool tcp_prune_ofo_queue(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	bool res = false;
    
    	if (!skb_queue_empty(&tp->out_of_order_queue)) {
    		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_OFOPRUNED);
    		__skb_queue_purge(&tp->out_of_order_queue);
    
    		/* Reset SACK state.  A conforming SACK implementation will
    		 * do the same at a timeout based retransmit.  When a connection
    		 * is in a sad state like this, we care only about integrity
    		 * of the connection not performance.
    		 */
    		if (tp->rx_opt.sack_ok)
    			tcp_sack_reset(&tp->rx_opt);
    		sk_mem_reclaim(sk);
    		res = true;
    	}
    	return res;
    }
    
    /* Reduce allocated memory if we can, trying to get
     * the socket within its memory limits again.
     *
     * Return less than zero if we should start dropping frames
     * until the socket owning process reads some of the data
     * to stabilize the situation.
     */
    static int tcp_prune_queue(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	SOCK_DEBUG(sk, "prune_queue: c=%x\n", tp->copied_seq);
    
    	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PRUNECALLED);
    
    	if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf)
    		tcp_clamp_window(sk);
    	else if (sk_under_memory_pressure(sk))
    		tp->rcv_ssthresh = min(tp->rcv_ssthresh, 4U * tp->advmss);
    
    	tcp_collapse_ofo_queue(sk);
    	if (!skb_queue_empty(&sk->sk_receive_queue))
    		tcp_collapse(sk, &sk->sk_receive_queue,
    			     skb_peek(&sk->sk_receive_queue),
    			     NULL,
    			     tp->copied_seq, tp->rcv_nxt);
    	sk_mem_reclaim(sk);
    
    	if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
    		return 0;
    
    	/* Collapsing did not help, destructive actions follow.
    	 * This must not ever occur. */
    
    	tcp_prune_ofo_queue(sk);
    
    	if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
    		return 0;
    
    	/* If we are really being abused, tell the caller to silently
    	 * drop receive data on the floor.  It will get retransmitted
    	 * and hopefully then we'll have sufficient space.
    	 */
    	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_RCVPRUNED);
    
    	/* Massive buffer overcommit. */
    	tp->pred_flags = 0;
    	return -1;
    }
    
    /* RFC2861, slow part. Adjust cwnd, after it was not full during one rto.
     * As additional protections, we do not touch cwnd in retransmission phases,
     * and if application hit its sndbuf limit recently.
     */
    void tcp_cwnd_application_limited(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	if (inet_csk(sk)->icsk_ca_state == TCP_CA_Open &&
    	    sk->sk_socket && !test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
    		/* Limited by application or receiver window. */
    		u32 init_win = tcp_init_cwnd(tp, __sk_dst_get(sk));
    		u32 win_used = max(tp->snd_cwnd_used, init_win);
    		if (win_used < tp->snd_cwnd) {
    			tp->snd_ssthresh = tcp_current_ssthresh(sk);
    			tp->snd_cwnd = (tp->snd_cwnd + win_used) >> 1;
    		}
    		tp->snd_cwnd_used = 0;
    	}
    	tp->snd_cwnd_stamp = tcp_time_stamp;
    }
    
    static bool tcp_should_expand_sndbuf(const struct sock *sk)
    {
    	const struct tcp_sock *tp = tcp_sk(sk);
    
    	/* If the user specified a specific send buffer setting, do
    	 * not modify it.
    	 */
    	if (sk->sk_userlocks & SOCK_SNDBUF_LOCK)
    		return false;
    
    	/* If we are under global TCP memory pressure, do not expand.  */
    	if (sk_under_memory_pressure(sk))
    		return false;
    
    	/* If we are under soft global TCP memory pressure, do not expand.  */
    	if (sk_memory_allocated(sk) >= sk_prot_mem_limits(sk, 0))
    		return false;
    
    	/* If we filled the congestion window, do not expand.  */
    	if (tp->packets_out >= tp->snd_cwnd)
    		return false;
    
    	return true;
    }
    
    /* When incoming ACK allowed to free some skb from write_queue,
     * we remember this event in flag SOCK_QUEUE_SHRUNK and wake up socket
     * on the exit from tcp input handler.
     *
     * PROBLEM: sndbuf expansion does not work well with largesend.
     */
    static void tcp_new_space(struct sock *sk)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	if (tcp_should_expand_sndbuf(sk)) {
    		int sndmem = SKB_TRUESIZE(max_t(u32,
    						tp->rx_opt.mss_clamp,
    						tp->mss_cache) +
    					  MAX_TCP_HEADER);
    		int demanded = max_t(unsigned int, tp->snd_cwnd,
    				     tp->reordering + 1);
    		sndmem *= 2 * demanded;
    		if (sndmem > sk->sk_sndbuf)
    			sk->sk_sndbuf = min(sndmem, sysctl_tcp_wmem[2]);
    		tp->snd_cwnd_stamp = tcp_time_stamp;
    	}
    
    	sk->sk_write_space(sk);
    }
    
    static void tcp_check_space(struct sock *sk)
    {
    	if (sock_flag(sk, SOCK_QUEUE_SHRUNK)) {
    		sock_reset_flag(sk, SOCK_QUEUE_SHRUNK);
    		if (sk->sk_socket &&
    		    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
    			tcp_new_space(sk);
    	}
    }
    
    static inline void tcp_data_snd_check(struct sock *sk)
    {
    	tcp_push_pending_frames(sk);
    	tcp_check_space(sk);
    }
    
    /*
     * Check if sending an ack is needed.
     */
    static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	    /* More than one full frame received... */
    	if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss &&
    	     /* ... and right edge of window advances far enough.
    	      * (tcp_recvmsg() will send ACK otherwise). Or...
    	      */
    	     __tcp_select_window(sk) >= tp->rcv_wnd) ||
    	    /* We ACK each frame or... */
    	    tcp_in_quickack_mode(sk) ||
    	    /* We have out of order data. */
    	    (ofo_possible && skb_peek(&tp->out_of_order_queue))) {
    		/* Then ack it now */
    		tcp_send_ack(sk);
    	} else {
    		/* Else, send delayed ack. */
    		tcp_send_delayed_ack(sk);
    	}
    }
    
    static inline void tcp_ack_snd_check(struct sock *sk)
    {
    	if (!inet_csk_ack_scheduled(sk)) {
    		/* We sent a data segment already. */
    		return;
    	}
    	__tcp_ack_snd_check(sk, 1);
    }
    
    /*
     *	This routine is only called when we have urgent data
     *	signaled. Its the 'slow' part of tcp_urg. It could be
     *	moved inline now as tcp_urg is only called from one
     *	place. We handle URGent data wrong. We have to - as
     *	BSD still doesn't use the correction from RFC961.
     *	For 1003.1g we should support a new option TCP_STDURG to permit
     *	either form (or just set the sysctl tcp_stdurg).
     */
    
    static void tcp_check_urg(struct sock *sk, const struct tcphdr *th)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	u32 ptr = ntohs(th->urg_ptr);
    
    	if (ptr && !sysctl_tcp_stdurg)
    		ptr--;
    	ptr += ntohl(th->seq);
    
    	/* Ignore urgent data that we've already seen and read. */
    	if (after(tp->copied_seq, ptr))
    		return;
    
    	/* Do not replay urg ptr.
    	 *
    	 * NOTE: interesting situation not covered by specs.
    	 * Misbehaving sender may send urg ptr, pointing to segment,
    	 * which we already have in ofo queue. We are not able to fetch
    	 * such data and will stay in TCP_URG_NOTYET until will be eaten
    	 * by recvmsg(). Seems, we are not obliged to handle such wicked
    	 * situations. But it is worth to think about possibility of some
    	 * DoSes using some hypothetical application level deadlock.
    	 */
    	if (before(ptr, tp->rcv_nxt))
    		return;
    
    	/* Do we already have a newer (or duplicate) urgent pointer? */
    	if (tp->urg_data && !after(ptr, tp->urg_seq))
    		return;
    
    	/* Tell the world about our new urgent pointer. */
    	sk_send_sigurg(sk);
    
    	/* We may be adding urgent data when the last byte read was
    	 * urgent. To do this requires some care. We cannot just ignore
    	 * tp->copied_seq since we would read the last urgent byte again
    	 * as data, nor can we alter copied_seq until this data arrives
    	 * or we break the semantics of SIOCATMARK (and thus sockatmark())
    	 *
    	 * NOTE. Double Dutch. Rendering to plain English: author of comment
    	 * above did something sort of 	send("A", MSG_OOB); send("B", MSG_OOB);
    	 * and expect that both A and B disappear from stream. This is _wrong_.
    	 * Though this happens in BSD with high probability, this is occasional.
    	 * Any application relying on this is buggy. Note also, that fix "works"
    	 * only in this artificial test. Insert some normal data between A and B and we will
    	 * decline of BSD again. Verdict: it is better to remove to trap
    	 * buggy users.
    	 */
    	if (tp->urg_seq == tp->copied_seq && tp->urg_data &&
    	    !sock_flag(sk, SOCK_URGINLINE) && tp->copied_seq != tp->rcv_nxt) {
    		struct sk_buff *skb = skb_peek(&sk->sk_receive_queue);
    		tp->copied_seq++;
    		if (skb && !before(tp->copied_seq, TCP_SKB_CB(skb)->end_seq)) {
    			__skb_unlink(skb, &sk->sk_receive_queue);
    			__kfree_skb(skb);
    		}
    	}
    
    	tp->urg_data = TCP_URG_NOTYET;
    	tp->urg_seq = ptr;
    
    	/* Disable header prediction. */
    	tp->pred_flags = 0;
    }
    
    /* This is the 'fast' part of urgent handling. */
    static void tcp_urg(struct sock *sk, struct sk_buff *skb, const struct tcphdr *th)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	/* Check if we get a new urgent pointer - normally not. */
    	if (th->urg)
    		tcp_check_urg(sk, th);
    
    	/* Do we wait for any urgent data? - normally not... */
    	if (tp->urg_data == TCP_URG_NOTYET) {
    		u32 ptr = tp->urg_seq - ntohl(th->seq) + (th->doff * 4) -
    			  th->syn;
    
    		/* Is the urgent pointer pointing into this packet? */
    		if (ptr < skb->len) {
    			u8 tmp;
    			if (skb_copy_bits(skb, ptr, &tmp, 1))
    				BUG();
    			tp->urg_data = TCP_URG_VALID | tmp;
    			if (!sock_flag(sk, SOCK_DEAD))
    				sk->sk_data_ready(sk, 0);
    		}
    	}
    }
    
    static int tcp_copy_to_iovec(struct sock *sk, struct sk_buff *skb, int hlen)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	int chunk = skb->len - hlen;
    	int err;
    
    	local_bh_enable();
    	if (skb_csum_unnecessary(skb))
    		err = skb_copy_datagram_iovec(skb, hlen, tp->ucopy.iov, chunk);
    	else
    		err = skb_copy_and_csum_datagram_iovec(skb, hlen,
    						       tp->ucopy.iov);
    
    	if (!err) {
    		tp->ucopy.len -= chunk;
    		tp->copied_seq += chunk;
    		tcp_rcv_space_adjust(sk);
    	}
    
    	local_bh_disable();
    	return err;
    }
    
    static __sum16 __tcp_checksum_complete_user(struct sock *sk,
    					    struct sk_buff *skb)
    {
    	__sum16 result;
    
    	if (sock_owned_by_user(sk)) {
    		local_bh_enable();
    		result = __tcp_checksum_complete(skb);
    		local_bh_disable();
    	} else {
    		result = __tcp_checksum_complete(skb);
    	}
    	return result;
    }
    
    static inline int tcp_checksum_complete_user(struct sock *sk,
    					     struct sk_buff *skb)
    {
    	return !skb_csum_unnecessary(skb) &&
    	       __tcp_checksum_complete_user(sk, skb);
    }
    
    #ifdef CONFIG_NET_DMA
    static bool tcp_dma_try_early_copy(struct sock *sk, struct sk_buff *skb,
    				  int hlen)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	int chunk = skb->len - hlen;
    	int dma_cookie;
    	bool copied_early = false;
    
    	if (tp->ucopy.wakeup)
    		return false;
    
    	if (!tp->ucopy.dma_chan && tp->ucopy.pinned_list)
    		tp->ucopy.dma_chan = net_dma_find_channel();
    
    	if (tp->ucopy.dma_chan && skb_csum_unnecessary(skb)) {
    
    		dma_cookie = dma_skb_copy_datagram_iovec(tp->ucopy.dma_chan,
    							 skb, hlen,
    							 tp->ucopy.iov, chunk,
    							 tp->ucopy.pinned_list);
    
    		if (dma_cookie < 0)
    			goto out;
    
    		tp->ucopy.dma_cookie = dma_cookie;
    		copied_early = true;
    
    		tp->ucopy.len -= chunk;
    		tp->copied_seq += chunk;
    		tcp_rcv_space_adjust(sk);
    
    		if ((tp->ucopy.len == 0) ||
    		    (tcp_flag_word(tcp_hdr(skb)) & TCP_FLAG_PSH) ||
    		    (atomic_read(&sk->sk_rmem_alloc) > (sk->sk_rcvbuf >> 1))) {
    			tp->ucopy.wakeup = 1;
    			sk->sk_data_ready(sk, 0);
    		}
    	} else if (chunk > 0) {
    		tp->ucopy.wakeup = 1;
    		sk->sk_data_ready(sk, 0);
    	}
    out:
    	return copied_early;
    }
    #endif /* CONFIG_NET_DMA */
    
    /* Does PAWS and seqno based validation of an incoming segment, flags will
     * play significant role here.
     */
    static int tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
    			      const struct tcphdr *th, int syn_inerr)
    {
    	const u8 *hash_location;
    	struct tcp_sock *tp = tcp_sk(sk);
    
    	/* RFC1323: H1. Apply PAWS check first. */
    	if (tcp_fast_parse_options(skb, th, tp, &hash_location) &&
    	    tp->rx_opt.saw_tstamp &&
    	    tcp_paws_discard(sk, skb)) {
    		if (!th->rst) {
    			NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
    			tcp_send_dupack(sk, skb);
    			goto discard;
    		}
    		/* Reset is accepted even if it did not pass PAWS. */
    	}
    
    	/* Step 1: check sequence number */
    	if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) {
    		/* RFC793, page 37: "In all states except SYN-SENT, all reset
    		 * (RST) segments are validated by checking their SEQ-fields."
    		 * And page 69: "If an incoming segment is not acceptable,
    		 * an acknowledgment should be sent in reply (unless the RST
    		 * bit is set, if so drop the segment and return)".
    		 */
    		if (!th->rst)
    			tcp_send_dupack(sk, skb);
    		goto discard;
    	}
    
    	/* Step 2: check RST bit */
    	if (th->rst) {
    		tcp_reset(sk);
    		goto discard;
    	}
    
    	/* ts_recent update must be made after we are sure that the packet
    	 * is in window.
    	 */
    	tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);
    
    	/* step 3: check security and precedence [ignored] */
    
    	/* step 4: Check for a SYN in window. */
    	if (th->syn && !before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
    		if (syn_inerr)
    			TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
    		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONSYN);
    		tcp_reset(sk);
    		return -1;
    	}
    
    	return 1;
    
    discard:
    	__kfree_skb(skb);
    	return 0;
    }
    
    /*
     *	TCP receive function for the ESTABLISHED state.
     *
     *	It is split into a fast path and a slow path. The fast path is
     * 	disabled when:
     *	- A zero window was announced from us - zero window probing
     *        is only handled properly in the slow path.
     *	- Out of order segments arrived.
     *	- Urgent data is expected.
     *	- There is no buffer space left
     *	- Unexpected TCP flags/window values/header lengths are received
     *	  (detected by checking the TCP header against pred_flags)
     *	- Data is sent in both directions. Fast path only supports pure senders
     *	  or pure receivers (this means either the sequence number or the ack
     *	  value must stay constant)
     *	- Unexpected TCP option.
     *
     *	When these conditions are not satisfied it drops into a standard
     *	receive procedure patterned after RFC793 to handle all cases.
     *	The first three cases are guaranteed by proper pred_flags setting,
     *	the rest is checked inline. Fast processing is turned on in
     *	tcp_data_queue when everything is OK.
     */
    int tcp_rcv_established(struct sock *sk, struct sk_buff *skb,
    			const struct tcphdr *th, unsigned int len)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	int res;
    
    	if (sk->sk_rx_dst) {
    		struct dst_entry *dst = sk->sk_rx_dst;
    		if (unlikely(dst->obsolete)) {
    			if (dst->ops->check(dst, 0) == NULL) {
    				dst_release(dst);
    				sk->sk_rx_dst = NULL;
    			}
    		}
    	}
    	if (unlikely(sk->sk_rx_dst == NULL))
    		sk->sk_rx_dst = dst_clone(skb_dst(skb));
    
    	/*
    	 *	Header prediction.
    	 *	The code loosely follows the one in the famous
    	 *	"30 instruction TCP receive" Van Jacobson mail.
    	 *
    	 *	Van's trick is to deposit buffers into socket queue
    	 *	on a device interrupt, to call tcp_recv function
    	 *	on the receive process context and checksum and copy
    	 *	the buffer to user space. smart...
    	 *
    	 *	Our current scheme is not silly either but we take the
    	 *	extra cost of the net_bh soft interrupt processing...
    	 *	We do checksum and copy also but from device to kernel.
    	 */
    
    	tp->rx_opt.saw_tstamp = 0;
    
    	/*	pred_flags is 0xS?10 << 16 + snd_wnd
    	 *	if header_prediction is to be made
    	 *	'S' will always be tp->tcp_header_len >> 2
    	 *	'?' will be 0 for the fast path, otherwise pred_flags is 0 to
    	 *  turn it off	(when there are holes in the receive
    	 *	 space for instance)
    	 *	PSH flag is ignored.
    	 */
    
    	if ((tcp_flag_word(th) & TCP_HP_BITS) == tp->pred_flags &&
    	    TCP_SKB_CB(skb)->seq == tp->rcv_nxt &&
    	    !after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
    		int tcp_header_len = tp->tcp_header_len;
    
    		/* Timestamp header prediction: tcp_header_len
    		 * is automatically equal to th->doff*4 due to pred_flags
    		 * match.
    		 */
    
    		/* Check timestamp */
    		if (tcp_header_len == sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) {
    			/* No? Slow path! */
    			if (!tcp_parse_aligned_timestamp(tp, th))
    				goto slow_path;
    
    			/* If PAWS failed, check it more carefully in slow path */
    			if ((s32)(tp->rx_opt.rcv_tsval - tp->rx_opt.ts_recent) < 0)
    				goto slow_path;
    
    			/* DO NOT update ts_recent here, if checksum fails
    			 * and timestamp was corrupted part, it will result
    			 * in a hung connection since we will drop all
    			 * future packets due to the PAWS test.
    			 */
    		}
    
    		if (len <= tcp_header_len) {
    			/* Bulk data transfer: sender */
    			if (len == tcp_header_len) {
    				/* Predicted packet is in window by definition.
    				 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
    				 * Hence, check seq<=rcv_wup reduces to:
    				 */
    				if (tcp_header_len ==
    				    (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
    				    tp->rcv_nxt == tp->rcv_wup)
    					tcp_store_ts_recent(tp);
    
    				/* We know that such packets are checksummed
    				 * on entry.
    				 */
    				tcp_ack(sk, skb, 0);
    				__kfree_skb(skb);
    				tcp_data_snd_check(sk);
    				return 0;
    			} else { /* Header too small */
    				TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
    				goto discard;
    			}
    		} else {
    			int eaten = 0;
    			int copied_early = 0;
    			bool fragstolen = false;
    
    			if (tp->copied_seq == tp->rcv_nxt &&
    			    len - tcp_header_len <= tp->ucopy.len) {
    #ifdef CONFIG_NET_DMA
    				if (tcp_dma_try_early_copy(sk, skb, tcp_header_len)) {
    					copied_early = 1;
    					eaten = 1;
    				}
    #endif
    				if (tp->ucopy.task == current &&
    				    sock_owned_by_user(sk) && !copied_early) {
    					__set_current_state(TASK_RUNNING);
    
    					if (!tcp_copy_to_iovec(sk, skb, tcp_header_len))
    						eaten = 1;
    				}
    				if (eaten) {
    					/* Predicted packet is in window by definition.
    					 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
    					 * Hence, check seq<=rcv_wup reduces to:
    					 */
    					if (tcp_header_len ==
    					    (sizeof(struct tcphdr) +
    					     TCPOLEN_TSTAMP_ALIGNED) &&
    					    tp->rcv_nxt == tp->rcv_wup)
    						tcp_store_ts_recent(tp);
    
    					tcp_rcv_rtt_measure_ts(sk, skb);
    
    					__skb_pull(skb, tcp_header_len);
    					tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
    					NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPHPHITSTOUSER);
    				}
    				if (copied_early)
    					tcp_cleanup_rbuf(sk, skb->len);
    			}
    			if (!eaten) {
    				if (tcp_checksum_complete_user(sk, skb))
    					goto csum_error;
    
    				/* Predicted packet is in window by definition.
    				 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
    				 * Hence, check seq<=rcv_wup reduces to:
    				 */
    				if (tcp_header_len ==
    				    (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
    				    tp->rcv_nxt == tp->rcv_wup)
    					tcp_store_ts_recent(tp);
    
    				tcp_rcv_rtt_measure_ts(sk, skb);
    
    				if ((int)skb->truesize > sk->sk_forward_alloc)
    					goto step5;
    
    				NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPHPHITS);
    
    				/* Bulk data transfer: receiver */
    				eaten = tcp_queue_rcv(sk, skb, tcp_header_len,
    						      &fragstolen);
    			}
    
    			tcp_event_data_recv(sk, skb);
    
    			if (TCP_SKB_CB(skb)->ack_seq != tp->snd_una) {
    				/* Well, only one small jumplet in fast path... */
    				tcp_ack(sk, skb, FLAG_DATA);
    				tcp_data_snd_check(sk);
    				if (!inet_csk_ack_scheduled(sk))
    					goto no_ack;
    			}
    
    			if (!copied_early || tp->rcv_nxt != tp->rcv_wup)
    				__tcp_ack_snd_check(sk, 0);
    no_ack:
    #ifdef CONFIG_NET_DMA
    			if (copied_early)
    				__skb_queue_tail(&sk->sk_async_wait_queue, skb);
    			else
    #endif
    			if (eaten)
    				kfree_skb_partial(skb, fragstolen);
    			else
    				sk->sk_data_ready(sk, 0);
    			return 0;
    		}
    	}
    
    slow_path:
    	if (len < (th->doff << 2) || tcp_checksum_complete_user(sk, skb))
    		goto csum_error;
    
    	/*
    	 *	Standard slow path.
    	 */
    
    	res = tcp_validate_incoming(sk, skb, th, 1);
    	if (res <= 0)
    		return -res;
    
    step5:
    	if (th->ack && tcp_ack(sk, skb, FLAG_SLOWPATH) < 0)
    		goto discard;
    
    	tcp_rcv_rtt_measure_ts(sk, skb);
    
    	/* Process urgent data. */
    	tcp_urg(sk, skb, th);
    
    	/* step 7: process the segment text */
    	tcp_data_queue(sk, skb);
    
    	tcp_data_snd_check(sk);
    	tcp_ack_snd_check(sk);
    	return 0;
    
    csum_error:
    	TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
    
    discard:
    	__kfree_skb(skb);
    	return 0;
    }
    EXPORT_SYMBOL(tcp_rcv_established);
    
    void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	struct inet_connection_sock *icsk = inet_csk(sk);
    
    	tcp_set_state(sk, TCP_ESTABLISHED);
    
    	if (skb != NULL) {
    		sk->sk_rx_dst = dst_clone(skb_dst(skb));
    		security_inet_conn_established(sk, skb);
    	}
    
    	/* Make sure socket is routed, for correct metrics.  */
    	icsk->icsk_af_ops->rebuild_header(sk);
    
    	tcp_init_metrics(sk);
    
    	tcp_init_congestion_control(sk);
    
    	/* Prevent spurious tcp_cwnd_restart() on first data
    	 * packet.
    	 */
    	tp->lsndtime = tcp_time_stamp;
    
    	tcp_init_buffer_space(sk);
    
    	if (sock_flag(sk, SOCK_KEEPOPEN))
    		inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tp));
    
    	if (!tp->rx_opt.snd_wscale)
    		__tcp_fast_path_on(tp, tp->snd_wnd);
    	else
    		tp->pred_flags = 0;
    
    	if (!sock_flag(sk, SOCK_DEAD)) {
    		sk->sk_state_change(sk);
    		sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
    	}
    }
    
    static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
    					 const struct tcphdr *th, unsigned int len)
    {
    	const u8 *hash_location;
    	struct inet_connection_sock *icsk = inet_csk(sk);
    	struct tcp_sock *tp = tcp_sk(sk);
    	struct tcp_cookie_values *cvp = tp->cookie_values;
    	int saved_clamp = tp->rx_opt.mss_clamp;
    
    	tcp_parse_options(skb, &tp->rx_opt, &hash_location, 0);
    
    	if (th->ack) {
    		/* rfc793:
    		 * "If the state is SYN-SENT then
    		 *    first check the ACK bit
    		 *      If the ACK bit is set
    		 *	  If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send
    		 *        a reset (unless the RST bit is set, if so drop
    		 *        the segment and return)"
    		 *
    		 *  We do not send data with SYN, so that RFC-correct
    		 *  test reduces to:
    		 */
    		if (TCP_SKB_CB(skb)->ack_seq != tp->snd_nxt)
    			goto reset_and_undo;
    
    		if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
    		    !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,
    			     tcp_time_stamp)) {
    			NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSACTIVEREJECTED);
    			goto reset_and_undo;
    		}
    
    		/* Now ACK is acceptable.
    		 *
    		 * "If the RST bit is set
    		 *    If the ACK was acceptable then signal the user "error:
    		 *    connection reset", drop the segment, enter CLOSED state,
    		 *    delete TCB, and return."
    		 */
    
    		if (th->rst) {
    			tcp_reset(sk);
    			goto discard;
    		}
    
    		/* rfc793:
    		 *   "fifth, if neither of the SYN or RST bits is set then
    		 *    drop the segment and return."
    		 *
    		 *    See note below!
    		 *                                        --ANK(990513)
    		 */
    		if (!th->syn)
    			goto discard_and_undo;
    
    		/* rfc793:
    		 *   "If the SYN bit is on ...
    		 *    are acceptable then ...
    		 *    (our SYN has been ACKed), change the connection
    		 *    state to ESTABLISHED..."
    		 */
    
    		TCP_ECN_rcv_synack(tp, th);
    
    		tp->snd_wl1 = TCP_SKB_CB(skb)->seq;
    		tcp_ack(sk, skb, FLAG_SLOWPATH);
    
    		/* Ok.. it's good. Set up sequence numbers and
    		 * move to established.
    		 */
    		tp->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
    		tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;
    
    		/* RFC1323: The window in SYN & SYN/ACK segments is
    		 * never scaled.
    		 */
    		tp->snd_wnd = ntohs(th->window);
    		tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
    
    		if (!tp->rx_opt.wscale_ok) {
    			tp->rx_opt.snd_wscale = tp->rx_opt.rcv_wscale = 0;
    			tp->window_clamp = min(tp->window_clamp, 65535U);
    		}
    
    		if (tp->rx_opt.saw_tstamp) {
    			tp->rx_opt.tstamp_ok	   = 1;
    			tp->tcp_header_len =
    				sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
    			tp->advmss	    -= TCPOLEN_TSTAMP_ALIGNED;
    			tcp_store_ts_recent(tp);
    		} else {
    			tp->tcp_header_len = sizeof(struct tcphdr);
    		}
    
    		if (tcp_is_sack(tp) && sysctl_tcp_fack)
    			tcp_enable_fack(tp);
    
    		tcp_mtup_init(sk);
    		tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
    		tcp_initialize_rcv_mss(sk);
    
    		/* Remember, tcp_poll() does not lock socket!
    		 * Change state from SYN-SENT only after copied_seq
    		 * is initialized. */
    		tp->copied_seq = tp->rcv_nxt;
    
    		if (cvp != NULL &&
    		    cvp->cookie_pair_size > 0 &&
    		    tp->rx_opt.cookie_plus > 0) {
    			int cookie_size = tp->rx_opt.cookie_plus
    					- TCPOLEN_COOKIE_BASE;
    			int cookie_pair_size = cookie_size
    					     + cvp->cookie_desired;
    
    			/* A cookie extension option was sent and returned.
    			 * Note that each incoming SYNACK replaces the
    			 * Responder cookie.  The initial exchange is most
    			 * fragile, as protection against spoofing relies
    			 * entirely upon the sequence and timestamp (above).
    			 * This replacement strategy allows the correct pair to
    			 * pass through, while any others will be filtered via
    			 * Responder verification later.
    			 */
    			if (sizeof(cvp->cookie_pair) >= cookie_pair_size) {
    				memcpy(&cvp->cookie_pair[cvp->cookie_desired],
    				       hash_location, cookie_size);
    				cvp->cookie_pair_size = cookie_pair_size;
    			}
    		}
    
    		smp_mb();
    
    		tcp_finish_connect(sk, skb);
    
    		if (sk->sk_write_pending ||
    		    icsk->icsk_accept_queue.rskq_defer_accept ||
    		    icsk->icsk_ack.pingpong) {
    			/* Save one ACK. Data will be ready after
    			 * several ticks, if write_pending is set.
    			 *
    			 * It may be deleted, but with this feature tcpdumps
    			 * look so _wonderfully_ clever, that I was not able
    			 * to stand against the temptation 8)     --ANK
    			 */
    			inet_csk_schedule_ack(sk);
    			icsk->icsk_ack.lrcvtime = tcp_time_stamp;
    			tcp_enter_quickack_mode(sk);
    			inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
    						  TCP_DELACK_MAX, TCP_RTO_MAX);
    
    discard:
    			__kfree_skb(skb);
    			return 0;
    		} else {
    			tcp_send_ack(sk);
    		}
    		return -1;
    	}
    
    	/* No ACK in the segment */
    
    	if (th->rst) {
    		/* rfc793:
    		 * "If the RST bit is set
    		 *
    		 *      Otherwise (no ACK) drop the segment and return."
    		 */
    
    		goto discard_and_undo;
    	}
    
    	/* PAWS check. */
    	if (tp->rx_opt.ts_recent_stamp && tp->rx_opt.saw_tstamp &&
    	    tcp_paws_reject(&tp->rx_opt, 0))
    		goto discard_and_undo;
    
    	if (th->syn) {
    		/* We see SYN without ACK. It is attempt of
    		 * simultaneous connect with crossed SYNs.
    		 * Particularly, it can be connect to self.
    		 */
    		tcp_set_state(sk, TCP_SYN_RECV);
    
    		if (tp->rx_opt.saw_tstamp) {
    			tp->rx_opt.tstamp_ok = 1;
    			tcp_store_ts_recent(tp);
    			tp->tcp_header_len =
    				sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
    		} else {
    			tp->tcp_header_len = sizeof(struct tcphdr);
    		}
    
    		tp->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
    		tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;
    
    		/* RFC1323: The window in SYN & SYN/ACK segments is
    		 * never scaled.
    		 */
    		tp->snd_wnd    = ntohs(th->window);
    		tp->snd_wl1    = TCP_SKB_CB(skb)->seq;
    		tp->max_window = tp->snd_wnd;
    
    		TCP_ECN_rcv_syn(tp, th);
    
    		tcp_mtup_init(sk);
    		tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
    		tcp_initialize_rcv_mss(sk);
    
    		tcp_send_synack(sk);
    #if 0
    		/* Note, we could accept data and URG from this segment.
    		 * There are no obstacles to make this.
    		 *
    		 * However, if we ignore data in ACKless segments sometimes,
    		 * we have no reasons to accept it sometimes.
    		 * Also, seems the code doing it in step6 of tcp_rcv_state_process
    		 * is not flawless. So, discard packet for sanity.
    		 * Uncomment this return to process the data.
    		 */
    		return -1;
    #else
    		goto discard;
    #endif
    	}
    	/* "fifth, if neither of the SYN or RST bits is set then
    	 * drop the segment and return."
    	 */
    
    discard_and_undo:
    	tcp_clear_options(&tp->rx_opt);
    	tp->rx_opt.mss_clamp = saved_clamp;
    	goto discard;
    
    reset_and_undo:
    	tcp_clear_options(&tp->rx_opt);
    	tp->rx_opt.mss_clamp = saved_clamp;
    	return 1;
    }
    
    /*
     *	This function implements the receiving procedure of RFC 793 for
     *	all states except ESTABLISHED and TIME_WAIT.
     *	It's called from both tcp_v4_rcv and tcp_v6_rcv and should be
     *	address independent.
     */
    
    int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
    			  const struct tcphdr *th, unsigned int len)
    {
    	struct tcp_sock *tp = tcp_sk(sk);
    	struct inet_connection_sock *icsk = inet_csk(sk);
    	int queued = 0;
    	int res;
    
    	tp->rx_opt.saw_tstamp = 0;
    
    	switch (sk->sk_state) {
    	case TCP_CLOSE:
    		goto discard;
    
    	case TCP_LISTEN:
    		if (th->ack)
    			return 1;
    
    		if (th->rst)
    			goto discard;
    
    		if (th->syn) {
    			if (th->fin)
    				goto discard;
    			if (icsk->icsk_af_ops->conn_request(sk, skb) < 0)
    				return 1;
    
    			/* Now we have several options: In theory there is
    			 * nothing else in the frame. KA9Q has an option to
    			 * send data with the syn, BSD accepts data with the
    			 * syn up to the [to be] advertised window and
    			 * Solaris 2.1 gives you a protocol error. For now
    			 * we just ignore it, that fits the spec precisely
    			 * and avoids incompatibilities. It would be nice in
    			 * future to drop through and process the data.
    			 *
    			 * Now that TTCP is starting to be used we ought to
    			 * queue this data.
    			 * But, this leaves one open to an easy denial of
    			 * service attack, and SYN cookies can't defend
    			 * against this problem. So, we drop the data
    			 * in the interest of security over speed unless
    			 * it's still in use.
    			 */
    			kfree_skb(skb);
    			return 0;
    		}
    		goto discard;
    
    	case TCP_SYN_SENT:
    		queued = tcp_rcv_synsent_state_process(sk, skb, th, len);
    		if (queued >= 0)
    			return queued;
    
    		/* Do step6 onward by hand. */
    		tcp_urg(sk, skb, th);
    		__kfree_skb(skb);
    		tcp_data_snd_check(sk);
    		return 0;
    	}
    
    	res = tcp_validate_incoming(sk, skb, th, 0);
    	if (res <= 0)
    		return -res;
    
    	/* step 5: check the ACK field */
    	if (th->ack) {
    		int acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH) > 0;
    
    		switch (sk->sk_state) {
    		case TCP_SYN_RECV:
    			if (acceptable) {
    				tp->copied_seq = tp->rcv_nxt;
    				smp_mb();
    				tcp_set_state(sk, TCP_ESTABLISHED);
    				sk->sk_state_change(sk);
    
    				/* Note, that this wakeup is only for marginal
    				 * crossed SYN case. Passively open sockets
    				 * are not waked up, because sk->sk_sleep ==
    				 * NULL and sk->sk_socket == NULL.
    				 */
    				if (sk->sk_socket)
    					sk_wake_async(sk,
    						      SOCK_WAKE_IO, POLL_OUT);
    
    				tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
    				tp->snd_wnd = ntohs(th->window) <<
    					      tp->rx_opt.snd_wscale;
    				tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
    
    				if (tp->rx_opt.tstamp_ok)
    					tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;
    
    				/* Make sure socket is routed, for
    				 * correct metrics.
    				 */
    				icsk->icsk_af_ops->rebuild_header(sk);
    
    				tcp_init_metrics(sk);
    
    				tcp_init_congestion_control(sk);
    
    				/* Prevent spurious tcp_cwnd_restart() on
    				 * first data packet.
    				 */
    				tp->lsndtime = tcp_time_stamp;
    
    				tcp_mtup_init(sk);
    				tcp_initialize_rcv_mss(sk);
    				tcp_init_buffer_space(sk);
    				tcp_fast_path_on(tp);
    			} else {
    				return 1;
    			}
    			break;
    
    		case TCP_FIN_WAIT1:
    			if (tp->snd_una == tp->write_seq) {
    				struct dst_entry *dst;
    
    				tcp_set_state(sk, TCP_FIN_WAIT2);
    				sk->sk_shutdown |= SEND_SHUTDOWN;
    
    				dst = __sk_dst_get(sk);
    				if (dst)
    					dst_confirm(dst);
    
    				if (!sock_flag(sk, SOCK_DEAD))
    					/* Wake up lingering close() */
    					sk->sk_state_change(sk);
    				else {
    					int tmo;
    
    					if (tp->linger2 < 0 ||
    					    (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
    					     after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt))) {
    						tcp_done(sk);
    						NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
    						return 1;
    					}
    
    					tmo = tcp_fin_time(sk);
    					if (tmo > TCP_TIMEWAIT_LEN) {
    						inet_csk_reset_keepalive_timer(sk, tmo - TCP_TIMEWAIT_LEN);
    					} else if (th->fin || sock_owned_by_user(sk)) {
    						/* Bad case. We could lose such FIN otherwise.
    						 * It is not a big problem, but it looks confusing
    						 * and not so rare event. We still can lose it now,
    						 * if it spins in bh_lock_sock(), but it is really
    						 * marginal case.
    						 */
    						inet_csk_reset_keepalive_timer(sk, tmo);
    					} else {
    						tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
    						goto discard;
    					}
    				}
    			}
    			break;
    
    		case TCP_CLOSING:
    			if (tp->snd_una == tp->write_seq) {
    				tcp_time_wait(sk, TCP_TIME_WAIT, 0);
    				goto discard;
    			}
    			break;
    
    		case TCP_LAST_ACK:
    			if (tp->snd_una == tp->write_seq) {
    				tcp_update_metrics(sk);
    				tcp_done(sk);
    				goto discard;
    			}
    			break;
    		}
    	} else
    		goto discard;
    
    	/* step 6: check the URG bit */
    	tcp_urg(sk, skb, th);
    
    	/* step 7: process the segment text */
    	switch (sk->sk_state) {
    	case TCP_CLOSE_WAIT:
    	case TCP_CLOSING:
    	case TCP_LAST_ACK:
    		if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
    			break;
    	case TCP_FIN_WAIT1:
    	case TCP_FIN_WAIT2:
    		/* RFC 793 says to queue data in these states,
    		 * RFC 1122 says we MUST send a reset.
    		 * BSD 4.4 also does reset.
    		 */
    		if (sk->sk_shutdown & RCV_SHUTDOWN) {
    			if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
    			    after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
    				NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
    				tcp_reset(sk);
    				return 1;
    			}
    		}
    		/* Fall through */
    	case TCP_ESTABLISHED:
    		tcp_data_queue(sk, skb);
    		queued = 1;
    		break;
    	}
    
    	/* tcp_data could move socket to TIME-WAIT */
    	if (sk->sk_state != TCP_CLOSE) {
    		tcp_data_snd_check(sk);
    		tcp_ack_snd_check(sk);
    	}
    
    	if (!queued) {
    discard:
    		__kfree_skb(skb);
    	}
    	return 0;
    }
    EXPORT_SYMBOL(tcp_rcv_state_process);