text: /*
   * INET		An implementation of the TCP/IP protocol suite for the LINUX
   *		operating system.  INET is implemented using the  BSD Socket
   *		interface as the means of communication with the user level.
   *
   *		Implementation of the Transmission Control Protocol(TCP).
   *
   * Version:	$Id: tcp_output.c,v 1.1.1.1 2001/02/22 14:59:02 serg Exp $
   *
   * Authors:	Ross Biro, <bir7@leland.Stanford.Edu>
   *		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	:	Retransmit queue handled by TCP.
   *				:	Fragmentation on mtu decrease
   *				:	Segment collapse on retransmit
   *				:	AF independence
   *
   *		Linus Torvalds	:	send_delayed_ack
   *		David S. Miller	:	Charge memory using the right skb
   *					during syn/ack processing.
   *		David S. Miller :	Output engine completely rewritten.
   *		Andrea Arcangeli:	SYNACK carry ts_recent in tsecr.
   *		Cacophonix Gaul :	draft-minshall-nagle-01
   *		J Hadi Salim	:	ECN support
   *
   */
  
  #include <net/tcp.h>
  
  #include <linux/smp_lock.h>
  
  /* People can turn this off for buggy TCP's found in printers etc. */
  int sysctl_tcp_retrans_collapse = 1;
  
  static __inline__
  void update_send_head(struct sock *sk, struct tcp_opt *tp, struct sk_buff *skb)
  {
  	tp->send_head = skb->next;
  	if (tp->send_head == (struct sk_buff *) &sk->write_queue)
  		tp->send_head = NULL;
  	tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
  	if (tp->packets_out++ == 0)
  		tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);
  }
  
  /* SND.NXT, if window was not shrunk.
   * If window has been shrunk, what should we make? It is not clear at all.
   * Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
   * Anything in between SND.UNA...SND.UNA+SND.WND also can be already
   * invalid. OK, let's make this for now:
   */
  static __inline__ __u32 tcp_acceptable_seq(struct sock *sk, struct tcp_opt *tp)
  {
  	if (!before(tp->snd_una+tp->snd_wnd, tp->snd_nxt))
  		return tp->snd_nxt;
  	else
  		return tp->snd_una+tp->snd_wnd;
  }
  
  /* Calculate mss to advertise in SYN segment.
   * RFC1122, RFC1063, draft-ietf-tcpimpl-pmtud-01 state that:
   *
   * 1. It is independent of path mtu.
   * 2. Ideally, it is maximal possible segment size i.e. 65535-40.
   * 3. For IPv4 it is reasonable to calculate it from maximal MTU of
   *    attached devices, because some buggy hosts are confused by
   *    large MSS.
   * 4. We do not make 3, we advertise MSS, calculated from first
   *    hop device mtu, but allow to raise it to ip_rt_min_advmss.
   *    This may be overriden via information stored in routing table.
   * 5. Value 65535 for MSS is valid in IPv6 and means "as large as possible,
   *    probably even Jumbo".
   */
  static __u16 tcp_advertise_mss(struct sock *sk)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  	struct dst_entry *dst = __sk_dst_get(sk);
  	int mss = tp->advmss;
  
  	if (dst && dst->advmss < mss) {
  		mss = dst->advmss;
  		tp->advmss = mss;
  	}
  
  	return (__u16)mss;
  }
  
  /* RFC2861. Reset CWND after idle period longer RTO to "restart window".
   * This is the first part of cwnd validation mechanism. */
  static void tcp_cwnd_restart(struct tcp_opt *tp)
  {
  	s32 delta = tcp_time_stamp - tp->lsndtime;
  	u32 restart_cwnd = tcp_init_cwnd(tp);
  	u32 cwnd = tp->snd_cwnd;
  
  	tp->snd_ssthresh = tcp_current_ssthresh(tp);
  	restart_cwnd = min(restart_cwnd, cwnd);
  
  	while ((delta -= tp->rto) > 0 && cwnd > restart_cwnd)
  		cwnd >>= 1;
  	tp->snd_cwnd = max(cwnd, restart_cwnd);
  	tp->snd_cwnd_stamp = tcp_time_stamp;
  	tp->snd_cwnd_used = 0;
  }
  
  static __inline__ void tcp_event_data_sent(struct tcp_opt *tp, struct sk_buff *skb)
  {
  	u32 now = tcp_time_stamp;
  
  	if (!tp->packets_out && (s32)(now - tp->lsndtime) > tp->rto)
  		tcp_cwnd_restart(tp);
  
  	tp->lsndtime = now;
  
  	/* If it is a reply for ato after last received
  	 * packet, enter pingpong mode.
  	 */
  	if ((u32)(now - tp->ack.lrcvtime) < tp->ack.ato)
  		tp->ack.pingpong = 1;
  }
  
  static __inline__ void tcp_event_ack_sent(struct sock *sk)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  
  	tcp_dec_quickack_mode(tp);
  	tcp_clear_xmit_timer(sk, TCP_TIME_DACK);
  }
  
  /* Chose a new window to advertise, update state in tcp_opt for the
   * socket, and return result with RFC1323 scaling applied.  The return
   * value can be stuffed directly into th->window for an outgoing
   * frame.
   */
  static __inline__ u16 tcp_select_window(struct sock *sk)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  	u32 cur_win = tcp_receive_window(tp);
  	u32 new_win = __tcp_select_window(sk);
  
  	/* Never shrink the offered window */
  	if(new_win < cur_win) {
  		/* Danger Will Robinson!
  		 * Don't update rcv_wup/rcv_wnd here or else
  		 * we will not be able to advertise a zero
  		 * window in time.  --DaveM
  		 *
  		 * Relax Will Robinson.
  		 */
  		new_win = cur_win;
  	}
  	tp->rcv_wnd = new_win;
  	tp->rcv_wup = tp->rcv_nxt;
  
  	/* RFC1323 scaling applied */
  	new_win >>= tp->rcv_wscale;
  
  #ifdef TCP_FORMAL_WINDOW
  	if (new_win == 0) {
  		/* If we advertise zero window, disable fast path. */
  		tp->pred_flags = 0;
  	} else if (cur_win == 0 && tp->pred_flags == 0 &&
  		   skb_queue_len(&tp->out_of_order_queue) == 0 &&
  		   !tp->urg_data) {
  		/* If we open zero window, enable fast path.
  		   Without this it will be open by the first data packet,
  		   it is too late to merge checksumming to copy.
  		 */
  		tcp_fast_path_on(tp);
  	}
  #endif
  
  	return new_win;
  }
  
  
  /* This routine actually transmits TCP packets queued in by
   * tcp_do_sendmsg().  This is used by both the initial
   * transmission and possible later retransmissions.
   * All SKB's seen here are completely headerless.  It is our
   * job to build the TCP header, and pass the packet down to
   * IP so it can do the same plus pass the packet off to the
   * device.
   *
   * We are working here with either a clone of the original
   * SKB, or a fresh unique copy made by the retransmit engine.
   */
  int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb)
  {
  	if(skb != NULL) {
  		struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  		struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);
  		int tcp_header_size = tp->tcp_header_len;
  		struct tcphdr *th;
  		int sysctl_flags;
  		int err;
  
  #define SYSCTL_FLAG_TSTAMPS	0x1
  #define SYSCTL_FLAG_WSCALE	0x2
  #define SYSCTL_FLAG_SACK	0x4
  
  		sysctl_flags = 0;
  		if (tcb->flags & TCPCB_FLAG_SYN) {
  			tcp_header_size = sizeof(struct tcphdr) + TCPOLEN_MSS;
  			if(sysctl_tcp_timestamps) {
  				tcp_header_size += TCPOLEN_TSTAMP_ALIGNED;
  				sysctl_flags |= SYSCTL_FLAG_TSTAMPS;
  			}
  			if(sysctl_tcp_window_scaling) {
  				tcp_header_size += TCPOLEN_WSCALE_ALIGNED;
  				sysctl_flags |= SYSCTL_FLAG_WSCALE;
  			}
  			if(sysctl_tcp_sack) {
  				sysctl_flags |= SYSCTL_FLAG_SACK;
  				if(!(sysctl_flags & SYSCTL_FLAG_TSTAMPS))
  					tcp_header_size += TCPOLEN_SACKPERM_ALIGNED;
  			}
  		} else if (tp->eff_sacks) {
  			/* A SACK is 2 pad bytes, a 2 byte header, plus
  			 * 2 32-bit sequence numbers for each SACK block.
  			 */
  			tcp_header_size += (TCPOLEN_SACK_BASE_ALIGNED +
  					    (tp->eff_sacks * TCPOLEN_SACK_PERBLOCK));
  		}
  		th = (struct tcphdr *) skb_push(skb, tcp_header_size);
  		skb->h.th = th;
  		skb_set_owner_w(skb, sk);
  
  		/* Build TCP header and checksum it. */
  		th->source		= sk->sport;
  		th->dest		= sk->dport;
  		th->seq			= htonl(tcb->seq);
  		th->ack_seq		= htonl(tp->rcv_nxt);
  		*(((__u16 *)th) + 6)	= htons(((tcp_header_size >> 2) << 12) | tcb->flags);
  		if (tcb->flags & TCPCB_FLAG_SYN) {
  			/* RFC1323: The window in SYN & SYN/ACK segments
  			 * is never scaled.
  			 */
  			th->window	= htons(tp->rcv_wnd);
  		} else {
  			th->window	= htons(tcp_select_window(sk));
  		}
  		th->check		= 0;
  		th->urg_ptr		= 0;
  
  		if (tp->urg_mode &&
  		    between(tp->snd_up, tcb->seq+1, tcb->seq+0xFFFF)) {
  			th->urg_ptr		= htons(tp->snd_up-tcb->seq);
  			th->urg			= 1;
  		}
  
  		if (tcb->flags & TCPCB_FLAG_SYN) {
  			tcp_syn_build_options((__u32 *)(th + 1),
  					      tcp_advertise_mss(sk),
  					      (sysctl_flags & SYSCTL_FLAG_TSTAMPS),
  					      (sysctl_flags & SYSCTL_FLAG_SACK),
  					      (sysctl_flags & SYSCTL_FLAG_WSCALE),
  					      tp->rcv_wscale,
  					      tcb->when,
  		      			      tp->ts_recent);
  		} else {
  			tcp_build_and_update_options((__u32 *)(th + 1),
  						     tp, tcb->when);
  
  			TCP_ECN_send(sk, tp, skb, tcp_header_size);
  		}
  		tp->af_specific->send_check(sk, th, skb->len, skb);
  
  		if (tcb->flags & TCPCB_FLAG_ACK)
  			tcp_event_ack_sent(sk);
  
  		if (skb->len != tcp_header_size)
  			tcp_event_data_sent(tp, skb);
  
  		TCP_INC_STATS(TcpOutSegs);
  
  		err = tp->af_specific->queue_xmit(skb);
  		if (err <= 0)
  			return err;
  
  		tcp_enter_cwr(tp);
  
  		/* NET_XMIT_CN is special. It does not guarantee,
  		 * that this packet is lost. It tells that device
  		 * is about to start to drop packets or already
  		 * drops some packets of the same priority and
  		 * invokes us to send less aggressively.
  		 */
  		return err == NET_XMIT_CN ? 0 : err;
  	}
  	return -ENOBUFS;
  #undef SYSCTL_FLAG_TSTAMPS
  #undef SYSCTL_FLAG_WSCALE
  #undef SYSCTL_FLAG_SACK
  }
  
  
  /* This is the main buffer sending routine. We queue the buffer
   * and decide whether to queue or transmit now.
   *
   * NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
   * otherwise socket can stall.
   */
  void tcp_send_skb(struct sock *sk, struct sk_buff *skb, int force_queue, unsigned cur_mss)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  
  	/* Advance write_seq and place onto the write_queue. */
  	tp->write_seq = TCP_SKB_CB(skb)->end_seq;
  	__skb_queue_tail(&sk->write_queue, skb);
  	tcp_charge_skb(sk, skb);
  
  	if (!force_queue && tp->send_head == NULL && tcp_snd_test(tp, skb, cur_mss, tp->nonagle)) {
  		/* Send it out now. */
  		TCP_SKB_CB(skb)->when = tcp_time_stamp;
  		if (tcp_transmit_skb(sk, skb_clone(skb, sk->allocation)) == 0) {
  			tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
  			tcp_minshall_update(tp, cur_mss, skb);
  			if (tp->packets_out++ == 0)
  				tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);
  			return;
  		}
  	}
  	/* Queue it, remembering where we must start sending. */
  	if (tp->send_head == NULL)
  		tp->send_head = skb;
  }
  
  /* Function to create two new TCP segments.  Shrinks the given segment
   * to the specified size and appends a new segment with the rest of the
   * packet to the list.  This won't be called frequently, I hope. 
   * Remember, these are still headerless SKBs at this point.
   */
  static int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len)
  {
  	struct tcp_opt *tp = &sk->tp_pinfo.af_tcp;
  	struct sk_buff *buff;
  	int nsize = skb->len - len;
  	u16 flags;
  
  	/* Get a new skb... force flag on. */
  	buff = tcp_alloc_skb(sk, nsize + MAX_TCP_HEADER, GFP_ATOMIC);
  	if (buff == NULL)
  		return -ENOMEM; /* We'll just try again later. */
  	tcp_charge_skb(sk, buff);
  
  	/* Reserve space for headers. */
  	skb_reserve(buff, MAX_TCP_HEADER);
  		
  	/* Correct the sequence numbers. */
  	TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
  	TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
  	
  	/* PSH and FIN should only be set in the second packet. */
  	flags = TCP_SKB_CB(skb)->flags;
  	TCP_SKB_CB(skb)->flags = flags & ~(TCPCB_FLAG_FIN|TCPCB_FLAG_PSH);
  	TCP_SKB_CB(buff)->flags = flags;
  	TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked&(TCPCB_LOST|TCPCB_EVER_RETRANS|TCPCB_AT_TAIL);
  	if (TCP_SKB_CB(buff)->sacked&TCPCB_LOST) {
  		tp->lost_out++;
  		tp->left_out++;
  	}
  	TCP_SKB_CB(buff)->sacked &= ~TCPCB_AT_TAIL;
  
  	/* Copy and checksum data tail into the new buffer. */
  	buff->csum = csum_partial_copy_nocheck(skb->data + len, skb_put(buff, nsize),
  					       nsize, 0);
  
  	/* This takes care of the FIN sequence number too. */
  	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
  	skb_trim(skb, len);
  
  	/* Rechecksum original buffer. */
  	skb->csum = csum_partial(skb->data, skb->len, 0);
  
  	/* Looks stupid, but our code really uses when of
  	 * skbs, which it never sent before. --ANK
  	 */
  	TCP_SKB_CB(buff)->when = TCP_SKB_CB(skb)->when;
  
  	/* Link BUFF into the send queue. */
  	__skb_append(skb, buff);
  
  	return 0;
  }
  
  /* This function synchronize snd mss to current pmtu/exthdr set.
  
     tp->user_mss is mss set by user by TCP_MAXSEG. It does NOT counts
     for TCP options, but includes only bare TCP header.
  
     tp->mss_clamp is mss negotiated at connection setup.
     It is minumum of user_mss and mss received with SYN.
     It also does not include TCP options.
  
     tp->pmtu_cookie is last pmtu, seen by this function.
  
     tp->mss_cache is current effective sending mss, including
     all tcp options except for SACKs. It is evaluated,
     taking into account current pmtu, but never exceeds
     tp->mss_clamp.
  
     NOTE1. rfc1122 clearly states that advertised MSS
     DOES NOT include either tcp or ip options.
  
     NOTE2. tp->pmtu_cookie and tp->mss_cache are READ ONLY outside
     this function.			--ANK (980731)
   */
  
  int tcp_sync_mss(struct sock *sk, u32 pmtu)
  {
  	struct tcp_opt *tp = &sk->tp_pinfo.af_tcp;
  	int mss_now;
  
  	/* Calculate base mss without TCP options:
  	   It is MMS_S - sizeof(tcphdr) of rfc1122
  	 */
  
  	mss_now = pmtu - tp->af_specific->net_header_len - sizeof(struct tcphdr);
  
  	/* Clamp it (mss_clamp does not include tcp options) */
  	if (mss_now > tp->mss_clamp)
  		mss_now = tp->mss_clamp;
  
  	/* Now subtract optional transport overhead */
  	mss_now -= tp->ext_header_len;
  
  	/* Then reserve room for full set of TCP options and 8 bytes of data */
  	if (mss_now < 48)
  		mss_now = 48;
  
  	/* Now subtract TCP options size, not including SACKs */
  	mss_now -= tp->tcp_header_len - sizeof(struct tcphdr);
  
  	/* Bound mss with half of window */
  	if (tp->max_window && mss_now > (tp->max_window>>1))
  		mss_now = max((tp->max_window>>1), 68 - tp->tcp_header_len);
  
  	/* And store cached results */
  	tp->pmtu_cookie = pmtu;
  	tp->mss_cache = mss_now;
  	return mss_now;
  }
  
  
  /* This routine writes packets to the network.  It advances the
   * send_head.  This happens as incoming acks open up the remote
   * window for us.
   *
   * Returns 1, if no segments are in flight and we have queued segments, but
   * cannot send anything now because of SWS or another problem.
   */
  int tcp_write_xmit(struct sock *sk)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  	unsigned int mss_now;
  
  	/* If we are closed, the bytes will have to remain here.
  	 * In time closedown will finish, we empty the write queue and all
  	 * will be happy.
  	 */
  	if(sk->state != TCP_CLOSE) {
  		struct sk_buff *skb;
  		int sent_pkts = 0;
  
  		/* Account for SACKS, we may need to fragment due to this.
  		 * It is just like the real MSS changing on us midstream.
  		 * We also handle things correctly when the user adds some
  		 * IP options mid-stream.  Silly to do, but cover it.
  		 */
  		mss_now = tcp_current_mss(sk); 
  
  		while((skb = tp->send_head) &&
  		      tcp_snd_test(tp, skb, mss_now, tcp_skb_is_last(sk, skb) ? tp->nonagle : 1)) {
  			if (skb->len > mss_now) {
  				if (tcp_fragment(sk, skb, mss_now))
  					break;
  			}
  
  			TCP_SKB_CB(skb)->when = tcp_time_stamp;
  			if (tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC)))
  				break;
  			/* Advance the send_head.  This one is sent out. */
  			update_send_head(sk, tp, skb);
  			tcp_minshall_update(tp, mss_now, skb);
  			sent_pkts = 1;
  		}
  
  		if (sent_pkts) {
  			tcp_cwnd_validate(sk, tp);
  			return 0;
  		}
  
  		return !tp->packets_out && tp->send_head;
  	}
  	return 0;
  }
  
  /* This function returns the amount that we can raise the
   * usable window based on the following constraints
   *  
   * 1. The window can never be shrunk once it is offered (RFC 793)
   * 2. We limit memory per socket
   *
   * RFC 1122:
   * "the suggested [SWS] avoidance algorithm for the receiver is to keep
   *  RECV.NEXT + RCV.WIN fixed until:
   *  RCV.BUFF - RCV.USER - RCV.WINDOW >= min(1/2 RCV.BUFF, MSS)"
   *
   * i.e. don't raise the right edge of the window until you can raise
   * it at least MSS bytes.
   *
   * Unfortunately, the recommended algorithm breaks header prediction,
   * since header prediction assumes th->window stays fixed.
   *
   * Strictly speaking, keeping th->window fixed violates the receiver
   * side SWS prevention criteria. The problem is that under this rule
   * a stream of single byte packets will cause the right side of the
   * window to always advance by a single byte.
   * 
   * Of course, if the sender implements sender side SWS prevention
   * then this will not be a problem.
   * 
   * BSD seems to make the following compromise:
   * 
   *	If the free space is less than the 1/4 of the maximum
   *	space available and the free space is less than 1/2 mss,
   *	then set the window to 0.
   *	[ Actually, bsd uses MSS and 1/4 of maximal _window_ ]
   *	Otherwise, just prevent the window from shrinking
   *	and from being larger than the largest representable value.
   *
   * This prevents incremental opening of the window in the regime
   * where TCP is limited by the speed of the reader side taking
   * data out of the TCP receive queue. It does nothing about
   * those cases where the window is constrained on the sender side
   * because the pipeline is full.
   *
   * BSD also seems to "accidentally" limit itself to windows that are a
   * multiple of MSS, at least until the free space gets quite small.
   * This would appear to be a side effect of the mbuf implementation.
   * Combining these two algorithms results in the observed behavior
   * of having a fixed window size at almost all times.
   *
   * Below we obtain similar behavior by forcing the offered window to
   * a multiple of the mss when it is feasible to do so.
   *
   * Note, we don't "adjust" for TIMESTAMP or SACK option bytes.
   * Regular options like TIMESTAMP are taken into account.
   */
  u32 __tcp_select_window(struct sock *sk)
  {
  	struct tcp_opt *tp = &sk->tp_pinfo.af_tcp;
  	/* MSS for the peer's data.  Previous verions used mss_clamp
  	 * here.  I don't know if the value based on our guesses
  	 * of peer's MSS is better for the performance.  It's more correct
  	 * but may be worse for the performance because of rcv_mss
  	 * fluctuations.  --SAW  1998/11/1
  	 */
  	unsigned int mss = tp->ack.rcv_mss;
  	int free_space;
  	u32 window;
  
  	/* Sometimes free_space can be < 0. */
  	free_space = tcp_space(sk); 
  	if (tp->window_clamp < mss)
  		mss = tp->window_clamp; 
  
  	if (free_space < (int)min(tp->window_clamp, tcp_full_space(sk)) / 2) {
  		tp->ack.quick = 0;
  
  		if (tcp_memory_pressure)
  			tp->rcv_ssthresh = min(tp->rcv_ssthresh, 4*tp->advmss);
  
  		if (free_space < ((int)mss))
  			return 0;
  	}
  
  	if (free_space > tp->rcv_ssthresh)
  		free_space = tp->rcv_ssthresh;
  
  	/* Get the largest window that is a nice multiple of mss.
  	 * Window clamp already applied above.
  	 * If our current window offering is within 1 mss of the
  	 * free space we just keep it. This prevents the divide
  	 * and multiply from happening most of the time.
  	 * We also don't do any window rounding when the free space
  	 * is too small.
  	 */
  	window = tp->rcv_wnd;
  	if ((((int) window) <= (free_space - ((int) mss))) ||
  	    (((int) window) > free_space))
  		window = (((unsigned int) free_space)/mss)*mss;
  
  	return window;
  }
  
  /* Attempt to collapse two adjacent SKB's during retransmission. */
  static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *skb, int mss_now)
  {
  	struct tcp_opt *tp = &sk->tp_pinfo.af_tcp;
  	struct sk_buff *next_skb = skb->next;
  
  	/* The first test we must make is that neither of these two
  	 * SKB's are still referenced by someone else.
  	 */
  	if(!skb_cloned(skb) && !skb_cloned(next_skb)) {
  		int skb_size = skb->len, next_skb_size = next_skb->len;
  		u16 flags = TCP_SKB_CB(skb)->flags;
  
  		/* Also punt if next skb has been SACK'd. */
  		if(TCP_SKB_CB(next_skb)->sacked & TCPCB_SACKED_ACKED)
  			return;
  
  		/* Next skb is out of window. */
  		if (after(TCP_SKB_CB(next_skb)->end_seq, tp->snd_una+tp->snd_wnd))
  			return;
  
  		/* Punt if not enough space exists in the first SKB for
  		 * the data in the second, or the total combined payload
  		 * would exceed the MSS.
  		 */
  		if ((next_skb_size > skb_tailroom(skb)) ||
  		    ((skb_size + next_skb_size) > mss_now))
  			return;
  
  		/* Ok.  We will be able to collapse the packet. */
  		__skb_unlink(next_skb, next_skb->list);
  
  		if(skb->len % 4) {
  			/* Must copy and rechecksum all data. */
  			memcpy(skb_put(skb, next_skb_size), next_skb->data, next_skb_size);
  			skb->csum = csum_partial(skb->data, skb->len, 0);
  		} else {
  			/* Optimize, actually we could also combine next_skb->csum
  			 * to skb->csum using a single add w/carry operation too.
  			 */
  			skb->csum = csum_partial_copy_nocheck(next_skb->data,
  							      skb_put(skb, next_skb_size),
  							      next_skb_size, skb->csum);
  		}
  	
  		/* Update sequence range on original skb. */
  		TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;
  
  		/* Merge over control information. */
  		flags |= TCP_SKB_CB(next_skb)->flags; /* This moves PSH/FIN etc. over */
  		TCP_SKB_CB(skb)->flags = flags;
  
  		/* All done, get rid of second SKB and account for it so
  		 * packet counting does not break.
  		 */
  		TCP_SKB_CB(skb)->sacked |= TCP_SKB_CB(next_skb)->sacked&(TCPCB_EVER_RETRANS|TCPCB_AT_TAIL);
  		if (TCP_SKB_CB(next_skb)->sacked&TCPCB_SACKED_RETRANS)
  			tp->retrans_out--;
  		if (TCP_SKB_CB(next_skb)->sacked&TCPCB_LOST) {
  			tp->lost_out--;
  			tp->left_out--;
  		}
  		if (!tp->sack_ok && tp->sacked_out) {
  			/* Reno case is special. Sigh... */
  			tp->sacked_out--;
  			tp->left_out--;
  		}
  		/* Not quite right: it can be > snd.fack, but
  		 * it is better to underestimate fackets.
  		 */
  		if (tp->fackets_out)
  			tp->fackets_out--;
  		tcp_free_skb(sk, next_skb);
  		tp->packets_out--;
  	}
  }
  
  /* 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)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  	struct sk_buff *skb;
  	unsigned int mss = tcp_current_mss(sk);
  	int lost = 0;
  
  	for_retrans_queue(skb, sk, tp) {
  		if (skb->len > 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--;
  			}
  			if (!(TCP_SKB_CB(skb)->sacked&TCPCB_LOST)) {
  				TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
  				tp->lost_out++;
  				lost = 1;
  			}
  		}
  	}
  
  	if (!lost)
  		return;
  
  	tp->left_out = tp->sacked_out + tp->lost_out;
  
   	/* 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 (tp->ca_state != TCP_CA_Loss) {
  		tp->high_seq = tp->snd_nxt;
  		tp->snd_ssthresh = tcp_current_ssthresh(tp);
  		tp->prior_ssthresh = 0;
  		tp->undo_marker = 0;
  		tp->ca_state = TCP_CA_Loss;
  	}
  	tcp_xmit_retransmit_queue(sk);
  }
  
  /* This retransmits one SKB.  Policy decisions and retransmit queue
   * state updates are done by the caller.  Returns non-zero if an
   * error occurred which prevented the send.
   */
  int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  	unsigned int cur_mss = tcp_current_mss(sk);
  	int err;
  
  	/* Do not sent more than we queued. 1/4 is reserved for possible
  	 * copying overhead: frgagmentation, tunneling, mangling etc.
  	 */
  	if (atomic_read(&sk->wmem_alloc) > min(sk->wmem_queued+(sk->wmem_queued>>2),sk->sndbuf))
  		return -EAGAIN;
  
  	if(skb->len > cur_mss) {
  		if(tcp_fragment(sk, skb, cur_mss))
  			return -ENOMEM; /* We'll try again later. */
  
  		/* New SKB created, account for it. */
  		tp->packets_out++;
  	}
  
  	/* Collapse two adjacent packets if worthwhile and we can. */
  	if(!(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN) &&
  	   (skb->len < (cur_mss >> 1)) &&
  	   (skb->next != tp->send_head) &&
  	   (skb->next != (struct sk_buff *)&sk->write_queue) &&
  	   (sysctl_tcp_retrans_collapse != 0))
  		tcp_retrans_try_collapse(sk, skb, cur_mss);
  
  	if(tp->af_specific->rebuild_header(sk))
  		return -EHOSTUNREACH; /* Routing failure or similar. */
  
  	/* Some Solaris stacks overoptimize and ignore the FIN on a
  	 * retransmit when old data is attached.  So strip it off
  	 * since it is cheap to do so and saves bytes on the network.
  	 */
  	if(skb->len > 0 &&
  	   (TCP_SKB_CB(skb)->flags & TCPCB_FLAG_FIN) &&
  	   tp->snd_una == (TCP_SKB_CB(skb)->end_seq - 1)) {
  		TCP_SKB_CB(skb)->seq = TCP_SKB_CB(skb)->end_seq - 1;
  		skb_trim(skb, 0);
  		skb->csum = 0;
  	}
  
  	/* Make a copy, if the first transmission SKB clone we made
  	 * is still in somebody's hands, else make a clone.
  	 */
  	TCP_SKB_CB(skb)->when = tcp_time_stamp;
  
  	err = tcp_transmit_skb(sk, (skb_cloned(skb) ?
  				    skb_copy(skb, GFP_ATOMIC):
  				    skb_clone(skb, GFP_ATOMIC)));
  
  	if (err == 0) {
  		/* Update global TCP statistics. */
  		TCP_INC_STATS(TcpRetransSegs);
  
  #if FASTRETRANS_DEBUG > 0
  		if (TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_RETRANS) {
  			if (net_ratelimit())
  				printk(KERN_DEBUG "retrans_out leaked.\n");
  		}
  #endif
  		TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
  		tp->retrans_out++;
  
  		/* Save stamp of the first retransmit. */
  		if (!tp->retrans_stamp)
  			tp->retrans_stamp = TCP_SKB_CB(skb)->when;
  
  		tp->undo_retrans++;
  
  		/* snd_nxt is stored to detect loss of retransmitted segment,
  		 * see tcp_input.c tcp_sacktag_write_queue().
  		 */
  		TCP_SKB_CB(skb)->ack_seq = tp->snd_nxt;
  	}
  	return err;
  }
  
  /* This gets called after a retransmit timeout, and the initially
   * retransmitted data is acknowledged.  It tries to continue
   * resending the rest of the retransmit queue, until either
   * we've sent it all or the congestion window limit is reached.
   * If doing SACK, the first ACK which comes back for a timeout
   * based retransmit packet might feed us FACK information again.
   * If so, we use it to avoid unnecessarily retransmissions.
   */
  void tcp_xmit_retransmit_queue(struct sock *sk)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  	struct sk_buff *skb;
  	int packet_cnt = tp->lost_out;
  
  	/* First pass: retransmit lost packets. */
  	if (packet_cnt) {
  		for_retrans_queue(skb, sk, tp) {
  			__u8 sacked = TCP_SKB_CB(skb)->sacked;
  
  			if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
  				return;
  
  			if (sacked&TCPCB_LOST) {
  				if (!(sacked&(TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))) {
  					if (tcp_retransmit_skb(sk, skb))
  						return;
  					if (tp->ca_state != TCP_CA_Loss)
  						NET_INC_STATS_BH(TCPFastRetrans);
  					else
  						NET_INC_STATS_BH(TCPSlowStartRetrans);
  
  					if (skb == skb_peek(&sk->write_queue))
  						tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);
  				}
  
  				if (--packet_cnt <= 0)
  					break;
  			}
  		}
  	}
  
  	/* OK, demanded retransmission is finished. */
  
  	/* Forward retransmissions are possible only during Recovery. */
  	if (tp->ca_state != TCP_CA_Recovery)
  		return;
  
  	/* No forward retransmissions in Reno are possible. */
  	if (!tp->sack_ok)
  		return;
  
  	/* Yeah, we have to make difficult choice between forward transmission
  	 * and retransmission... Both ways have their merits...
  	 *
  	 * For now we do not retrnamsit anything, while we have some new
  	 * segments to send.
  	 */
  
  	if (tcp_may_send_now(sk, tp))
  		return;
  
  	packet_cnt = 0;
  
  	for_retrans_queue(skb, sk, tp) {
  		if(++packet_cnt > tp->fackets_out)
  			break;
  
  		if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
  			break;
  
  		if(TCP_SKB_CB(skb)->sacked & TCPCB_TAGBITS)
  			continue;
  
  		/* Ok, retransmit it. */
  		if(tcp_retransmit_skb(sk, skb))
  			break;
  
  		if (skb == skb_peek(&sk->write_queue))
  			tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);
  
  		NET_INC_STATS_BH(TCPForwardRetrans);
  	}
  }
  
  
  /* Send a fin.  The caller locks the socket for us.  This cannot be
   * allowed to fail queueing a FIN frame under any circumstances.
   */
  void tcp_send_fin(struct sock *sk)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);	
  	struct sk_buff *skb = skb_peek_tail(&sk->write_queue);
  	unsigned int mss_now;
  	
  	/* Optimization, tack on the FIN if we have a queue of
  	 * unsent frames.  But be careful about outgoing SACKS
  	 * and IP options.
  	 */
  	mss_now = tcp_current_mss(sk); 
  
  	/* Please, find seven differences of 2.3.33 and loook
  	 * what I broke here. 8) --ANK
  	 */
  
  	if(tp->send_head != NULL) {
  		/* tcp_write_xmit() takes care of the rest. */
  		TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_FIN;
  		TCP_SKB_CB(skb)->end_seq++;
  		tp->write_seq++;
  
  		/* Special case to avoid Nagle bogosity.  If this
  		 * segment is the last segment, and it was queued
  		 * due to Nagle/SWS-avoidance, send it out now.
  		 */
  		if(tp->send_head == skb &&
  		   !after(tp->write_seq, tp->snd_una + tp->snd_wnd)) {
  			TCP_SKB_CB(skb)->when = tcp_time_stamp;
  			if (!tcp_transmit_skb(sk, skb_clone(skb, GFP_KERNEL)))
  				update_send_head(sk, tp, skb);
  			else
  				tcp_check_probe_timer(sk, tp);
  		}
  	} else {
  		/* Socket is locked, keep trying until memory is available. */
  		for (;;) {
  			skb = alloc_skb(MAX_TCP_HEADER, GFP_KERNEL);
  			if (skb)
  				break;
  			current->policy |= SCHED_YIELD;
  			schedule();
  		}
  
  		/* Reserve space for headers and prepare control bits. */
  		skb_reserve(skb, MAX_TCP_HEADER);
  		skb->csum = 0;
  		TCP_SKB_CB(skb)->flags = (TCPCB_FLAG_ACK | TCPCB_FLAG_FIN);
  		TCP_SKB_CB(skb)->sacked = 0;
  
  		/* FIN eats a sequence byte, write_seq advanced by tcp_send_skb(). */
  		TCP_SKB_CB(skb)->seq = tp->write_seq;
  		TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + 1;
  		tcp_send_skb(sk, skb, 0, mss_now);
  		__tcp_push_pending_frames(sk, tp, mss_now, 1);
  	}
  }
  
  /* We get here when a process closes a file descriptor (either due to
   * an explicit close() or as a byproduct of exit()'ing) and there
   * was unread data in the receive queue.  This behavior is recommended
   * by draft-ietf-tcpimpl-prob-03.txt section 3.10.  -DaveM
   */
  void tcp_send_active_reset(struct sock *sk, int priority)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  	struct sk_buff *skb;
  
  	/* NOTE: No TCP options attached and we never retransmit this. */
  	skb = alloc_skb(MAX_TCP_HEADER, priority);
  	if (!skb) {
  		NET_INC_STATS(TCPAbortFailed);
  		return;
  	}
  
  	/* Reserve space for headers and prepare control bits. */
  	skb_reserve(skb, MAX_TCP_HEADER);
  	skb->csum = 0;
  	TCP_SKB_CB(skb)->flags = (TCPCB_FLAG_ACK | TCPCB_FLAG_RST);
  	TCP_SKB_CB(skb)->sacked = 0;
  
  	/* Send it off. */
  	TCP_SKB_CB(skb)->seq = tcp_acceptable_seq(sk, tp);
  	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq;
  	TCP_SKB_CB(skb)->when = tcp_time_stamp;
  	if (tcp_transmit_skb(sk, skb))
  		NET_INC_STATS(TCPAbortFailed);
  }
  
  /* WARNING: This routine must only be called when we have already sent
   * a SYN packet that crossed the incoming SYN that caused this routine
   * to get called. If this assumption fails then the initial rcv_wnd
   * and rcv_wscale values will not be correct.
   */
  int tcp_send_synack(struct sock *sk)
  {
  	struct sk_buff* skb;
  
  	skb = skb_peek(&sk->write_queue);
  	if (skb == NULL || !(TCP_SKB_CB(skb)->flags&TCPCB_FLAG_SYN)) {
  		printk(KERN_DEBUG "tcp_send_synack: wrong queue state\n");
  		return -EFAULT;
  	}
  	if (!(TCP_SKB_CB(skb)->flags&TCPCB_FLAG_ACK)) {
  		if (skb_cloned(skb)) {
  			struct sk_buff *nskb = skb_copy(skb, GFP_ATOMIC);
  			if (nskb == NULL)
  				return -ENOMEM;
  			__skb_unlink(skb, &sk->write_queue);
  			__skb_queue_head(&sk->write_queue, nskb);
  			tcp_free_skb(sk, skb);
  			tcp_charge_skb(sk, nskb);
  			skb = nskb;
  		}
  
  		TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_ACK;
  		TCP_ECN_send_synack(&sk->tp_pinfo.af_tcp, skb);
  	}
  	TCP_SKB_CB(skb)->when = tcp_time_stamp;
  	return tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC));
  }
  
  /*
   * Prepare a SYN-ACK.
   */
  struct sk_buff * tcp_make_synack(struct sock *sk, struct dst_entry *dst,
  				 struct open_request *req)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  	struct tcphdr *th;
  	int tcp_header_size;
  	struct sk_buff *skb;
  
  	skb = sock_wmalloc(sk, MAX_TCP_HEADER + 15, 1, GFP_ATOMIC);
  	if (skb == NULL)
  		return NULL;
  
  	/* Reserve space for headers. */
  	skb_reserve(skb, MAX_TCP_HEADER);
  
  	skb->dst = dst_clone(dst);
  
  	tcp_header_size = (sizeof(struct tcphdr) + TCPOLEN_MSS +
  			   (req->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0) +
  			   (req->wscale_ok ? TCPOLEN_WSCALE_ALIGNED : 0) +
  			   /* SACK_PERM is in the place of NOP NOP of TS */
  			   ((req->sack_ok && !req->tstamp_ok) ? TCPOLEN_SACKPERM_ALIGNED : 0));
  	skb->h.th = th = (struct tcphdr *) skb_push(skb, tcp_header_size);
  
  	memset(th, 0, sizeof(struct tcphdr));
  	th->syn = 1;
  	th->ack = 1;
  	TCP_ECN_make_synack(req, th);
  	th->source = sk->sport;
  	th->dest = req->rmt_port;
  	TCP_SKB_CB(skb)->seq = req->snt_isn;
  	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + 1;
  	th->seq = htonl(TCP_SKB_CB(skb)->seq);
  	th->ack_seq = htonl(req->rcv_isn + 1);
  	if (req->rcv_wnd == 0) { /* ignored for retransmitted syns */
  		__u8 rcv_wscale; 
  		/* Set this up on the first call only */
  		req->window_clamp = tp->window_clamp ? : dst->window;
  		/* tcp_full_space because it is guaranteed to be the first packet */
  		tcp_select_initial_window(tcp_full_space(sk), 
  			dst->advmss - (req->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
  			&req->rcv_wnd,
  			&req->window_clamp,
  			req->wscale_ok,
  			&rcv_wscale);
  		req->rcv_wscale = rcv_wscale; 
  	}
  
  	/* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
  	th->window = htons(req->rcv_wnd);
  
  	TCP_SKB_CB(skb)->when = tcp_time_stamp;
  	tcp_syn_build_options((__u32 *)(th + 1), dst->advmss, req->tstamp_ok,
  			      req->sack_ok, req->wscale_ok, req->rcv_wscale,
  			      TCP_SKB_CB(skb)->when,
  			      req->ts_recent);
  
  	skb->csum = 0;
  	th->doff = (tcp_header_size >> 2);
  	TCP_INC_STATS(TcpOutSegs);
  	return skb;
  }
  
  int tcp_connect(struct sock *sk, struct sk_buff *buff)
  {
  	struct dst_entry *dst = __sk_dst_get(sk);
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  
  	/* Reserve space for headers. */
  	skb_reserve(buff, MAX_TCP_HEADER);
  
  	/* We'll fix this up when we get a response from the other end.
  	 * See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
  	 */
  	tp->tcp_header_len = sizeof(struct tcphdr) +
  		(sysctl_tcp_timestamps ? TCPOLEN_TSTAMP_ALIGNED : 0);
  
  	/* If user gave his TCP_MAXSEG, record it to clamp */
  	if (tp->user_mss)
  		tp->mss_clamp = tp->user_mss;
  	tp->max_window = 0;
  	tcp_sync_mss(sk, dst->pmtu);
  
  	if (!tp->window_clamp)
  		tp->window_clamp = dst->window;
  	tp->advmss = dst->advmss;
  	tcp_initialize_rcv_mss(sk);
  
  	tcp_select_initial_window(tcp_full_space(sk),
  				  tp->advmss - (tp->ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
  				  &tp->rcv_wnd,
  				  &tp->window_clamp,
  				  sysctl_tcp_window_scaling,
  				  &tp->rcv_wscale);
  
  	tp->rcv_ssthresh = tp->rcv_wnd;
  
  	/* Socket identity change complete, no longer
  	 * in TCP_CLOSE, so enter ourselves into the
  	 * hash tables.
  	 */
  	tcp_set_state(sk,TCP_SYN_SENT);
  	if (tp->af_specific->hash_connecting(sk))
  		goto err_out;
  
  	sk->err = 0;
  	sk->done = 0;
  	tp->snd_wnd = 0;
  	tcp_init_wl(tp, tp->write_seq, 0);
  	tp->snd_una = tp->write_seq;
  	tp->snd_sml = tp->write_seq;
  	tp->rcv_nxt = 0;
  	tp->rcv_wup = 0;
  	tp->copied_seq = 0;
  
  	tp->rto = TCP_TIMEOUT_INIT;
  	tp->retransmits = 0;
  	tcp_clear_retrans(tp);
  
  	TCP_SKB_CB(buff)->flags = TCPCB_FLAG_SYN;
  	TCP_ECN_send_syn(tp, buff);
  	TCP_SKB_CB(buff)->sacked = 0;
  	buff->csum = 0;
  	TCP_SKB_CB(buff)->seq = tp->write_seq++;
  	TCP_SKB_CB(buff)->end_seq = tp->write_seq;
  	tp->snd_nxt = tp->write_seq;
  	tp->pushed_seq = tp->write_seq;
  
  	/* Send it off. */
  	TCP_SKB_CB(buff)->when = tcp_time_stamp;
  	tp->retrans_stamp = TCP_SKB_CB(buff)->when;
  	__skb_queue_tail(&sk->write_queue, buff);
  	tcp_charge_skb(sk, buff);
  	tp->packets_out++;
  	tcp_transmit_skb(sk, skb_clone(buff, GFP_KERNEL));
  	TCP_INC_STATS(TcpActiveOpens);
  
  	/* Timer for repeating the SYN until an answer. */
  	tcp_reset_xmit_timer(sk, TCP_TIME_RETRANS, tp->rto);
  	return 0;
  
  err_out:
  	tcp_set_state(sk,TCP_CLOSE);
  	kfree_skb(buff);
  	return -EADDRNOTAVAIL;
  }
  
  /* Send out a delayed ack, the caller does the policy checking
   * to see if we should even be here.  See tcp_input.c:tcp_ack_snd_check()
   * for details.
   */
  void tcp_send_delayed_ack(struct sock *sk)
  {
  	struct tcp_opt *tp = &sk->tp_pinfo.af_tcp;
  	int ato = tp->ack.ato;
  	unsigned long timeout;
  
  	if (ato > TCP_DELACK_MIN) {
  		int max_ato = HZ/2;
  
  		if (tp->ack.pingpong || (tp->ack.pending&TCP_ACK_PUSHED))
  			max_ato = TCP_DELACK_MAX;
  
  		/* Slow path, intersegment interval is "high". */
  
  		/* If some rtt estimate is known, use it to bound delayed ack.
  		 * Do not use tp->rto here, use results of rtt measurements
  		 * directly.
  		 */
  		if (tp->srtt) {
  			int rtt = max(tp->srtt>>3, TCP_DELACK_MIN);
  
  			if (rtt < max_ato)
  				max_ato = rtt;
  		}
  
  		ato = min(ato, max_ato);
  	}
  
  	/* Stay within the limit we were given */
  	timeout = jiffies + ato;
  
  	/* Use new timeout only if there wasn't a older one earlier. */
  	if (tp->ack.pending&TCP_ACK_TIMER) {
  		/* If delack timer was blocked or is about to expire,
  		 * send ACK now.
  		 */
  		if (tp->ack.blocked || time_before_eq(tp->ack.timeout, jiffies+(ato>>2))) {
  			tcp_send_ack(sk);
  			return;
  		}
  
  		if (!time_before(timeout, tp->ack.timeout))
  			timeout = tp->ack.timeout;
  	}
  	tp->ack.pending |= TCP_ACK_SCHED|TCP_ACK_TIMER;
  	tp->ack.timeout = timeout;
  	if (!mod_timer(&tp->delack_timer, timeout))
  		sock_hold(sk);
  
  #ifdef TCP_FORMAL_WINDOW
  	/* Explanation. Header prediction path does not handle
  	 * case of zero window. If we send ACK immediately, pred_flags
  	 * are reset when sending ACK. If rcv_nxt is advanced and
  	 * ack is not sent, than delayed ack is scheduled.
  	 * Hence, it is the best place to check for zero window.
  	 */
  	if (tp->pred_flags) {
  		if (tcp_receive_window(tp) == 0)
  			tp->pred_flags = 0;
  	} else {
  		if (skb_queue_len(&tp->out_of_order_queue) == 0 &&
  		    !tp->urg_data)
  			tcp_fast_path_on(tp);
  	}
  #endif
  }
  
  /* This routine sends an ack and also updates the window. */
  void tcp_send_ack(struct sock *sk)
  {
  	/* If we have been reset, we may not send again. */
  	if(sk->state != TCP_CLOSE) {
  		struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  		struct sk_buff *buff;
  
  		/* We are not putting this on the write queue, so
  		 * tcp_transmit_skb() will set the ownership to this
  		 * sock.
  		 */
  		buff = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
  		if (buff == NULL) {
  			tcp_schedule_ack(tp);
  			tp->ack.ato = TCP_ATO_MIN;
  			tcp_reset_xmit_timer(sk, TCP_TIME_DACK, TCP_DELACK_MAX);
  			return;
  		}
  
  		/* Reserve space for headers and prepare control bits. */
  		skb_reserve(buff, MAX_TCP_HEADER);
  		buff->csum = 0;
  		TCP_SKB_CB(buff)->flags = TCPCB_FLAG_ACK;
  		TCP_SKB_CB(buff)->sacked = 0;
  
  		/* Send it off, this clears delayed acks for us. */
  		TCP_SKB_CB(buff)->seq = TCP_SKB_CB(buff)->end_seq = tcp_acceptable_seq(sk, tp);
  		TCP_SKB_CB(buff)->when = tcp_time_stamp;
  		tcp_transmit_skb(sk, buff);
  	}
  }
  
  /* This routine sends a packet with an out of date sequence
   * number. It assumes the other end will try to ack it.
   *
   * Question: what should we make while urgent mode?
   * 4.4BSD forces sending single byte of data. We cannot send
   * out of window data, because we have SND.NXT==SND.MAX...
   *
   * Current solution: to send TWO zero-length segments in urgent mode:
   * one is with SEG.SEQ=SND.UNA to deliver urgent pointer, another is
   * out-of-date with SND.UNA-1 to probe window.
   */
  static int tcp_xmit_probe_skb(struct sock *sk, int urgent)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  	struct sk_buff *skb;
  
  	/* We don't queue it, tcp_transmit_skb() sets ownership. */
  	skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
  	if (skb == NULL) 
  		return -1;
  
  	/* Reserve space for headers and set control bits. */
  	skb_reserve(skb, MAX_TCP_HEADER);
  	skb->csum = 0;
  	TCP_SKB_CB(skb)->flags = TCPCB_FLAG_ACK;
  	TCP_SKB_CB(skb)->sacked = urgent;
  
  	/* Use a previous sequence.  This should cause the other
  	 * end to send an ack.  Don't queue or clone SKB, just
  	 * send it.
  	 */
  	TCP_SKB_CB(skb)->seq = urgent ? tp->snd_una : tp->snd_una - 1;
  	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq;
  	TCP_SKB_CB(skb)->when = tcp_time_stamp;
  	return tcp_transmit_skb(sk, skb);
  }
  
  int tcp_write_wakeup(struct sock *sk)
  {
  	if (sk->state != TCP_CLOSE) {
  		struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  		struct sk_buff *skb;
  
  		if ((skb = tp->send_head) != NULL &&
  		    before(TCP_SKB_CB(skb)->seq, tp->snd_una+tp->snd_wnd)) {
  			int err;
  			int mss = tcp_current_mss(sk);
  			int seg_size = tp->snd_una+tp->snd_wnd-TCP_SKB_CB(skb)->seq;
  
  			if (before(tp->pushed_seq, TCP_SKB_CB(skb)->end_seq))
  				tp->pushed_seq = TCP_SKB_CB(skb)->end_seq;
  
  			/* We are probing the opening of a window
  			 * but the window size is != 0
  			 * must have been a result SWS avoidance ( sender )
  			 */
  			if (seg_size < TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq ||
  			    skb->len > mss) {
  				seg_size = min(seg_size, mss);
  				TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_PSH;
  				if (tcp_fragment(sk, skb, seg_size))
  					return -1;
  			}
  			TCP_SKB_CB(skb)->flags |= TCPCB_FLAG_PSH;
  			TCP_SKB_CB(skb)->when = tcp_time_stamp;
  			err = tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC));
  			if (!err) {
  				update_send_head(sk, tp, skb);
  			}
  			return err;
  		} else {
  			if (tp->urg_mode &&
  			    between(tp->snd_up, tp->snd_una+1, tp->snd_una+0xFFFF))
  				tcp_xmit_probe_skb(sk, TCPCB_URG);
  			return tcp_xmit_probe_skb(sk, 0);
  		}
  	}
  	return -1;
  }
  
  /* A window probe timeout has occurred.  If window is not closed send
   * a partial packet else a zero probe.
   */
  void tcp_send_probe0(struct sock *sk)
  {
  	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
  	int err;
  
  	err = tcp_write_wakeup(sk);
  
  	if (tp->packets_out || !tp->send_head) {
  		/* Cancel probe timer, if it is not required. */
  		tp->probes_out = 0;
  		tp->backoff = 0;
  		return;
  	}
  
  	if (err <= 0) {
  		tp->backoff++;
  		tp->probes_out++;
  		tcp_reset_xmit_timer (sk, TCP_TIME_PROBE0, 
  				      min(tp->rto << tp->backoff, TCP_RTO_MAX));
  	} else {
  		/* If packet was not sent due to local congestion,
  		 * do not backoff and do not remember probes_out.
  		 * Let local senders to fight for local resources.
  		 *
  		 * Use accumulated backoff yet.
  		 */
  		if (!tp->probes_out)
  			tp->probes_out=1;
  		tcp_reset_xmit_timer (sk, TCP_TIME_PROBE0, 
  				      min(tp->rto << tp->backoff, TCP_RESOURCE_PROBE_INTERVAL));
  	}
  }
  