summaryrefslogtreecommitdiff
path: root/pfinet/linux-src/net/ipv4/tcp_output.c
blob: 2ac5e8a2b7a0173043471b8609480fbd9756b6eb (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
/*
 * 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 1.108.2.1 1999/05/14 23:07:36 davem 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.
 *
 */

#include <net/tcp.h>

extern int sysctl_tcp_timestamps;
extern int sysctl_tcp_window_scaling;
extern int sysctl_tcp_sack;

/* People can turn this off for buggy TCP's found in printers etc. */
int sysctl_tcp_retrans_collapse = 1;

/* Get rid of any delayed acks, we sent one already.. */
static __inline__ void clear_delayed_acks(struct sock * sk)
{
	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

	tp->delayed_acks = 0;
	if(tcp_in_quickack_mode(tp))
		tcp_exit_quickack_mode(tp);
	tcp_clear_xmit_timer(sk, TIME_DACK);
}

static __inline__ void update_send_head(struct sock *sk)
{
	struct tcp_opt *tp = &sk->tp_pinfo.af_tcp;
	
	tp->send_head = tp->send_head->next;
	if (tp->send_head == (struct sk_buff *) &sk->write_queue)
		tp->send_head = NULL;
}

/* 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.
 */
void 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;

#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->sack_ok && tp->num_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->num_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(TCP_SKB_CB(skb)->seq);
		th->ack_seq		= htonl(tp->rcv_nxt);
		th->doff		= (tcp_header_size >> 2);
		th->res1		= 0;
		*(((__u8 *)th) + 13)	= tcb->flags;
		if(!(tcb->flags & TCPCB_FLAG_SYN))
			th->window	= htons(tcp_select_window(sk));
		th->check		= 0;
		th->urg_ptr		= ntohs(tcb->urg_ptr);
		if(tcb->flags & TCPCB_FLAG_SYN) {
			/* RFC1323: The window in SYN & SYN/ACK segments
			 * is never scaled.
			 */
			th->window	= htons(tp->rcv_wnd);
			tcp_syn_build_options((__u32 *)(th + 1), tp->mss_clamp,
					      (sysctl_flags & SYSCTL_FLAG_TSTAMPS),
					      (sysctl_flags & SYSCTL_FLAG_SACK),
					      (sysctl_flags & SYSCTL_FLAG_WSCALE),
					      tp->rcv_wscale,
					      TCP_SKB_CB(skb)->when,
		      			      tp->ts_recent);
		} else {
			tcp_build_and_update_options((__u32 *)(th + 1),
						     tp, TCP_SKB_CB(skb)->when);
		}
		tp->af_specific->send_check(sk, th, skb->len, skb);

		clear_delayed_acks(sk);
		tp->last_ack_sent = tp->rcv_nxt;
		tcp_statistics.TcpOutSegs++;
		tp->af_specific->queue_xmit(skb);
	}
#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.
 */
void tcp_send_skb(struct sock *sk, struct sk_buff *skb, int force_queue)
{
	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 - TCP_SKB_CB(skb)->seq);
	__skb_queue_tail(&sk->write_queue, skb);

	if (!force_queue && tp->send_head == NULL && tcp_snd_test(sk, skb)) {
		/* Send it out now. */
		TCP_SKB_CB(skb)->when = tcp_time_stamp;
		tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
		tp->packets_out++;
		tcp_transmit_skb(sk, skb_clone(skb, GFP_KERNEL));
		if(!tcp_timer_is_set(sk, TIME_RETRANS))
			tcp_reset_xmit_timer(sk, TIME_RETRANS, tp->rto);
	} else {
		/* Queue it, remembering where we must start sending. */
		if (tp->send_head == NULL)
			tp->send_head = skb;
		if (!force_queue && tp->packets_out == 0 && !tp->pending) {
			tp->pending = TIME_PROBE0;
			tcp_reset_xmit_timer(sk, TIME_PROBE0, tp->rto);
		}
	}
}

/* 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 sk_buff *buff;
	int nsize = skb->len - len;
	u16 flags;

	/* Get a new skb... force flag on. */
	buff = sock_wmalloc(sk,
			    (nsize + MAX_HEADER + sk->prot->max_header),
			    1, GFP_ATOMIC);
	if (buff == NULL)
		return -1; /* We'll just try again later. */

	/* Reserve space for headers. */
	skb_reserve(buff, MAX_HEADER + sk->prot->max_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);
	if(flags & TCPCB_FLAG_URG) {
		u16 old_urg_ptr = TCP_SKB_CB(skb)->urg_ptr;

		/* Urgent data is always a pain in the ass. */
		if(old_urg_ptr > len) {
			TCP_SKB_CB(skb)->flags &= ~(TCPCB_FLAG_URG);
			TCP_SKB_CB(skb)->urg_ptr = 0;
			TCP_SKB_CB(buff)->urg_ptr = old_urg_ptr - len;
		} else {
			flags &= ~(TCPCB_FLAG_URG);
		}
	}
	if(!(flags & TCPCB_FLAG_URG))
		TCP_SKB_CB(buff)->urg_ptr = 0;
	TCP_SKB_CB(buff)->flags = flags;
	TCP_SKB_CB(buff)->sacked = 0;

	/* Copy and checksum data tail into the new buffer. */
	buff->csum = csum_partial_copy(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 TCP options size, not including SACKs */
	mss_now -= tp->tcp_header_len - sizeof(struct tcphdr);

	/* Now subtract optional transport overhead */
	mss_now -= tp->ext_header_len;

	/* It we got too small (or even negative) value,
	   clamp it by 8 from below. Why 8 ?
	   Well, it could be 1 with the same success,
	   but if IP accepted segment of length 1,
	   it would love 8 even more 8)		--ANK (980731)
	 */
	if (mss_now < 8)
		mss_now = 8;

	/* 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.
 */
void tcp_write_xmit(struct sock *sk)
{
	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
	unsigned int mss_now;

	/* 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); 

	/* If we are zapped, the bytes will have to remain here.
	 * In time closedown will empty the write queue and all
	 * will be happy.
	 */
	if(!sk->zapped) {
		struct sk_buff *skb;
		int sent_pkts = 0;

		/* Anything on the transmit queue that fits the window can
		 * be added providing we are:
		 *
		 * a) following SWS avoidance [and Nagle algorithm]
		 * b) not exceeding our congestion window.
		 * c) not retransmitting [Nagle]
		 */
		while((skb = tp->send_head) && tcp_snd_test(sk, skb)) {
			if (skb->len > mss_now) {
				if (tcp_fragment(sk, skb, mss_now))
					break;
			}

			/* Advance the send_head.  This one is going out. */
			update_send_head(sk);
			TCP_SKB_CB(skb)->when = tcp_time_stamp;
			tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
			tp->packets_out++;
			tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC));
			sent_pkts = 1;
		}

		/* If we sent anything, make sure the retransmit
		 * timer is active.
		 */
		if (sent_pkts && !tcp_timer_is_set(sk, TIME_RETRANS))
			tcp_reset_xmit_timer(sk, TIME_RETRANS, tp->rto);
	}
}

/* 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.
 *	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.
 */
u32 __tcp_select_window(struct sock *sk)
{
	struct tcp_opt *tp = &sk->tp_pinfo.af_tcp;
	unsigned int mss = tp->mss_cache;
	int free_space;
	u32 window;

	/* Sometimes free_space can be < 0. */
	free_space = (sk->rcvbuf - atomic_read(&sk->rmem_alloc)) / 2;
	if (tp->window_clamp) {
		if (free_space > ((int) tp->window_clamp))
			free_space = tp->window_clamp;
		mss = min(tp->window_clamp, mss);
	} else {
		printk("tcp_select_window: tp->window_clamp == 0.\n");
	}

	if (mss < 1) {
		mss = 1;
		printk("tcp_select_window: sk->mss fell to 0.\n");
	}
	
	if ((free_space < (sk->rcvbuf/4)) && (free_space < ((int) (mss/2)))) {
		window = 0;
		tp->pred_flags = 0; 
	} else {
		/* 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 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;

		/* Punt if the first SKB has URG set. */
		if(flags & TCPCB_FLAG_URG)
			return;
	
		/* Also punt if next skb has been SACK'd. */
		if(TCP_SKB_CB(next_skb)->sacked & TCPCB_SACKED_ACKED)
			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(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 */
		if(flags & TCPCB_FLAG_URG) {
			u16 urgptr = TCP_SKB_CB(next_skb)->urg_ptr;
			TCP_SKB_CB(skb)->urg_ptr = urgptr + skb_size;
		}
		TCP_SKB_CB(skb)->flags = flags;

		/* All done, get rid of second SKB and account for it so
		 * packet counting does not break.
		 */
		kfree_skb(next_skb);
		sk->tp_pinfo.af_tcp.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, *old_next_skb;
	unsigned int mss = tcp_current_mss(sk);

 	/* Don't muck with the congestion window here. */
 	tp->dup_acks = 0;
 	tp->high_seq = tp->snd_nxt;
 	tp->retrans_head = NULL;

 	/* Input control flow will see that this was retransmitted
	 * and not use it for RTT calculation in the absence of
	 * the timestamp option.
	 */
	for (old_next_skb = skb = skb_peek(&sk->write_queue);
	     ((skb != tp->send_head) &&
	      (skb != (struct sk_buff *)&sk->write_queue));
	     skb = skb->next) {
		int resend_skb = 0;

		/* Our goal is to push out the packets which we
		 * sent already, but are being chopped up now to
		 * account for the PMTU information we have.
		 *
		 * As we resend the queue, packets are fragmented
		 * into two pieces, and when we try to send the
		 * second piece it may be collapsed together with
		 * a subsequent packet, and so on.  -DaveM
		 */
		if (old_next_skb != skb || skb->len > mss)
			resend_skb = 1;
		old_next_skb = skb->next;
		if (resend_skb != 0)
			tcp_retransmit_skb(sk, skb);
	}
}

static __inline__ void update_retrans_head(struct sock *sk)
{
	struct tcp_opt *tp = &sk->tp_pinfo.af_tcp;
	
	tp->retrans_head = tp->retrans_head->next;
	if((tp->retrans_head == tp->send_head) ||
	   (tp->retrans_head == (struct sk_buff *) &sk->write_queue)) {
		tp->retrans_head = NULL;
		tp->rexmt_done = 1;
	}
}

/* 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);

	if(skb->len > cur_mss) {
		if(tcp_fragment(sk, skb, cur_mss))
			return 1; /* 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 1; /* 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;
	}

	/* Ok, we're gonna send it out, update state. */
	TCP_SKB_CB(skb)->sacked |= TCPCB_SACKED_RETRANS;
	tp->retrans_out++;

	/* 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;
	if(skb_cloned(skb))
		skb = skb_copy(skb, GFP_ATOMIC);
	else
		skb = skb_clone(skb, GFP_ATOMIC);

	tcp_transmit_skb(sk, skb);

	/* Update global TCP statistics and return success. */
	sk->prot->retransmits++;
	tcp_statistics.TcpRetransSegs++;

	return 0;
}

/* 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;

	if (tp->retrans_head == NULL &&
	    tp->rexmt_done == 0)
		tp->retrans_head = skb_peek(&sk->write_queue);
	if (tp->retrans_head == tp->send_head)
		tp->retrans_head = NULL;

	/* Each time, advance the retrans_head if we got
	 * a packet out or we skipped one because it was
	 * SACK'd.  -DaveM
	 */
	while ((skb = tp->retrans_head) != NULL) {
		/* If it has been ack'd by a SACK block, we don't
		 * retransmit it.
		 */
		if(!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) {
			/* Send it out, punt if error occurred. */
			if(tcp_retransmit_skb(sk, skb))
				break;

			update_retrans_head(sk);
		
			/* Stop retransmitting if we've hit the congestion
			 * window limit.
			 */
			if (tp->retrans_out >= tp->snd_cwnd)
				break;
		} else {
			update_retrans_head(sk);
		}
	}
}

/* Using FACK information, retransmit all missing frames at the receiver
 * up to the forward most SACK'd packet (tp->fackets_out) if the packet
 * has not been retransmitted already.
 */
void tcp_fack_retransmit(struct sock *sk)
{
	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
	struct sk_buff *skb = skb_peek(&sk->write_queue);
	int packet_cnt = 0;

	while((skb != NULL) &&
	      (skb != tp->send_head) &&
	      (skb != (struct sk_buff *)&sk->write_queue)) {
		__u8 sacked = TCP_SKB_CB(skb)->sacked;

		if(sacked & (TCPCB_SACKED_ACKED | TCPCB_SACKED_RETRANS))
			goto next_packet;

		/* Ok, retransmit it. */
		if(tcp_retransmit_skb(sk, skb))
			break;

		if(tcp_packets_in_flight(tp) >= tp->snd_cwnd)
			break;
next_packet:
		packet_cnt++;
		if(packet_cnt >= tp->fackets_out)
			break;
		skb = skb->next;
	}
}

/* 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); 

	if((tp->send_head != NULL) && (skb->len < mss_now)) {
		/* 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 &&
		   !sk->nonagle &&
		   skb->len < (tp->mss_cache >> 1) &&
		   tp->packets_out &&
		   !(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_URG)) {
			update_send_head(sk);
			TCP_SKB_CB(skb)->when = tcp_time_stamp;
			tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
			tp->packets_out++;
			tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC));
			if(!tcp_timer_is_set(sk, TIME_RETRANS))
				tcp_reset_xmit_timer(sk, TIME_RETRANS, tp->rto);
		}
	} else {
		/* Socket is locked, keep trying until memory is available. */
		do {
			skb = sock_wmalloc(sk,
					   (MAX_HEADER +
					    sk->prot->max_header),
					   1, GFP_KERNEL);
		} while (skb == NULL);

		/* Reserve space for headers and prepare control bits. */
		skb_reserve(skb, MAX_HEADER + sk->prot->max_header);
		skb->csum = 0;
		TCP_SKB_CB(skb)->flags = (TCPCB_FLAG_ACK | TCPCB_FLAG_FIN);
		TCP_SKB_CB(skb)->sacked = 0;
		TCP_SKB_CB(skb)->urg_ptr = 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);
	}
}

/* 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)
{
	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_HEADER + sk->prot->max_header, GFP_KERNEL);
	if (!skb)
		return;

	/* Reserve space for headers and prepare control bits. */
	skb_reserve(skb, MAX_HEADER + sk->prot->max_header);
	skb->csum = 0;
	TCP_SKB_CB(skb)->flags = (TCPCB_FLAG_ACK | TCPCB_FLAG_RST);
	TCP_SKB_CB(skb)->sacked = 0;
	TCP_SKB_CB(skb)->urg_ptr = 0;

	/* Send it off. */
	TCP_SKB_CB(skb)->seq = tp->write_seq;
	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq;
	TCP_SKB_CB(skb)->when = tcp_time_stamp;
	tcp_transmit_skb(sk, skb);
}

/* 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 tcp_opt* tp = &(sk->tp_pinfo.af_tcp);
	struct sk_buff* skb;	
	
	skb = sock_wmalloc(sk, (MAX_HEADER + sk->prot->max_header),
			   1, GFP_ATOMIC);
	if (skb == NULL) 
		return -ENOMEM;

	/* Reserve space for headers and prepare control bits. */
	skb_reserve(skb, MAX_HEADER + sk->prot->max_header);
	skb->csum = 0;
	TCP_SKB_CB(skb)->flags = (TCPCB_FLAG_ACK | TCPCB_FLAG_SYN);
	TCP_SKB_CB(skb)->sacked = 0;
	TCP_SKB_CB(skb)->urg_ptr = 0;

	/* SYN eats a sequence byte. */
	TCP_SKB_CB(skb)->seq = tp->snd_una;
	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + 1;
	__skb_queue_tail(&sk->write_queue, skb);
	TCP_SKB_CB(skb)->when = tcp_time_stamp;
	tp->packets_out++;
	tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC));
	return 0;
}

/*
 * Prepare a SYN-ACK.
 */
struct sk_buff * tcp_make_synack(struct sock *sk, struct dst_entry *dst,
				 struct open_request *req, int mss)
{
	struct tcphdr *th;
	int tcp_header_size;
	struct sk_buff *skb;

	skb = sock_wmalloc(sk, MAX_HEADER + sk->prot->max_header, 1, GFP_ATOMIC);
	if (skb == NULL)
		return NULL;

	/* Reserve space for headers. */
	skb_reserve(skb, MAX_HEADER + sk->prot->max_header);

	skb->dst = dst_clone(dst);

	/* Don't offer more than they did.
	 * This way we don't have to memorize who said what.
	 * FIXME: maybe this should be changed for better performance
	 * with syncookies.
	 */
	req->mss = min(mss, req->mss);
	if (req->mss < 8) {
		printk(KERN_DEBUG "initial req->mss below 8\n");
		req->mss = 8;
	}

	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;
	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 = skb->dst->window;
		tcp_select_initial_window(sock_rspace(sk)/2,req->mss,
			&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), req->mss, 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_statistics.TcpOutSegs++; 
	return skb;
}

void tcp_connect(struct sock *sk, struct sk_buff *buff, int mtu)
{
	struct dst_entry *dst = sk->dst_cache;
	struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);

	/* Reserve space for headers. */
	skb_reserve(buff, MAX_HEADER + sk->prot->max_header);

	tp->snd_wnd = 0;
	tp->snd_wl1 = 0;
	tp->snd_wl2 = tp->write_seq;
	tp->snd_una = tp->write_seq;
	tp->rcv_nxt = 0;

	sk->err = 0;
	
	/* 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;
	tcp_sync_mss(sk, mtu);

	/* Now unpleasant action: if initial pmtu is too low
	   set lower clamp. I am not sure that it is good.
	   To be more exact, I do not think that clamping at value, which
	   is apparently transient and may improve in future is good idea.
	   It would be better to wait until peer will returns its MSS
	   (probably 65535 too) and now advertise something sort of 65535
	   or at least first hop device mtu. Is it clear, what I mean?
	   We should tell peer what maximal mss we expect to RECEIVE,
	   it has nothing to do with pmtu.
	   I am afraid someone will be confused by such huge value.
	                                                   --ANK (980731)
	 */
	if (tp->mss_cache + tp->tcp_header_len - sizeof(struct tcphdr) < tp->mss_clamp )
		tp->mss_clamp = tp->mss_cache + tp->tcp_header_len - sizeof(struct tcphdr);

	TCP_SKB_CB(buff)->flags = TCPCB_FLAG_SYN;
	TCP_SKB_CB(buff)->sacked = 0;
	TCP_SKB_CB(buff)->urg_ptr = 0;
	buff->csum = 0;
	TCP_SKB_CB(buff)->seq = tp->write_seq++;
	TCP_SKB_CB(buff)->end_seq = tp->write_seq;
	tp->snd_nxt = TCP_SKB_CB(buff)->end_seq;

	tp->window_clamp = dst->window;
	tcp_select_initial_window(sock_rspace(sk)/2,tp->mss_clamp,
		&tp->rcv_wnd,
		&tp->window_clamp,
		sysctl_tcp_window_scaling,
		&tp->rcv_wscale);
	/* Ok, now lock the socket before we make it visible to
	 * the incoming packet engine.
	 */
	lock_sock(sk);

	/* Socket identity change complete, no longer
	 * in TCP_CLOSE, so enter ourselves into the
	 * hash tables.
	 */
	tcp_set_state(sk,TCP_SYN_SENT);
	sk->prot->hash(sk);

	tp->rto = dst->rtt;
	tcp_init_xmit_timers(sk);
	tp->retransmits = 0;
	tp->fackets_out = 0;
	tp->retrans_out = 0;

	/* Send it off. */
	__skb_queue_tail(&sk->write_queue, buff);
	TCP_SKB_CB(buff)->when = tcp_time_stamp;
	tp->packets_out++;
	tcp_transmit_skb(sk, skb_clone(buff, GFP_KERNEL));
	tcp_statistics.TcpActiveOpens++;

	/* Timer for repeating the SYN until an answer. */
	tcp_reset_xmit_timer(sk, TIME_RETRANS, tp->rto);

	/* Now, it is safe to release the socket. */
	release_sock(sk);
}

/* 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 tcp_opt *tp, int max_timeout)
{
	unsigned long timeout;

	/* Stay within the limit we were given */
	timeout = tp->ato;
	if (timeout > max_timeout)
		timeout = max_timeout;
	timeout += jiffies;

	/* Use new timeout only if there wasn't a older one earlier. */
	if (!tp->delack_timer.prev) {
		tp->delack_timer.expires = timeout;
		add_timer(&tp->delack_timer);
        } else {
		if (time_before(timeout, tp->delack_timer.expires))
			mod_timer(&tp->delack_timer, timeout);
	}
}

/* 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->zapped) {
		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_HEADER + sk->prot->max_header, GFP_ATOMIC);
		if (buff == NULL) {
			/* Force it to send an ack. We don't have to do this
			 * (ACK is unreliable) but it's much better use of
			 * bandwidth on slow links to send a spare ack than
			 * resend packets.
			 *
			 * This is the one possible way that we can delay an
			 * ACK and have tp->ato indicate that we are in
			 * quick ack mode, so clear it.
			 */
			if(tcp_in_quickack_mode(tp))
				tcp_exit_quickack_mode(tp);
			tcp_send_delayed_ack(tp, HZ/2);
			return;
		}

		/* Reserve space for headers and prepare control bits. */
		skb_reserve(buff, MAX_HEADER + sk->prot->max_header);
		buff->csum = 0;
		TCP_SKB_CB(buff)->flags = TCPCB_FLAG_ACK;
		TCP_SKB_CB(buff)->sacked = 0;
		TCP_SKB_CB(buff)->urg_ptr = 0;

		/* Send it off, this clears delayed acks for us. */
		TCP_SKB_CB(buff)->seq = TCP_SKB_CB(buff)->end_seq = tp->snd_nxt;
		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.
 */
void tcp_write_wakeup(struct sock *sk)
{
	/* After a valid reset we can send no more. */
	if (!sk->zapped) {
		struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
		struct sk_buff *skb;

		/* Write data can still be transmitted/retransmitted in the
		 * following states.  If any other state is encountered, return.
		 * [listen/close will never occur here anyway]
		 */
		if ((1 << sk->state) &
		    ~(TCPF_ESTABLISHED|TCPF_CLOSE_WAIT|TCPF_FIN_WAIT1|
		      TCPF_LAST_ACK|TCPF_CLOSING))
			return;

		if (before(tp->snd_nxt, tp->snd_una + tp->snd_wnd) &&
		    ((skb = tp->send_head) != NULL)) {
			unsigned long win_size;

			/* We are probing the opening of a window
			 * but the window size is != 0
			 * must have been a result SWS avoidance ( sender )
			 */
			win_size = tp->snd_wnd - (tp->snd_nxt - tp->snd_una);
			if (win_size < TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq) {
				if (tcp_fragment(sk, skb, win_size))
					return; /* Let a retransmit get it. */
			}
			update_send_head(sk);
			TCP_SKB_CB(skb)->when = tcp_time_stamp;
			tp->snd_nxt = TCP_SKB_CB(skb)->end_seq;
			tp->packets_out++;
			tcp_transmit_skb(sk, skb_clone(skb, GFP_ATOMIC));
			if (!tcp_timer_is_set(sk, TIME_RETRANS))
				tcp_reset_xmit_timer(sk, TIME_RETRANS, tp->rto);
		} else {
			/* We don't queue it, tcp_transmit_skb() sets ownership. */
			skb = alloc_skb(MAX_HEADER + sk->prot->max_header,
					GFP_ATOMIC);
			if (skb == NULL) 
				return;

			/* Reserve space for headers and set control bits. */
			skb_reserve(skb, MAX_HEADER + sk->prot->max_header);
			skb->csum = 0;
			TCP_SKB_CB(skb)->flags = TCPCB_FLAG_ACK;
			TCP_SKB_CB(skb)->sacked = 0;
			TCP_SKB_CB(skb)->urg_ptr = 0;

			/* 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 = tp->snd_nxt - 1;
			TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq;
			TCP_SKB_CB(skb)->when = tcp_time_stamp;
			tcp_transmit_skb(sk, skb);
		}
	}
}

/* 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);

	tcp_write_wakeup(sk);
	tp->pending = TIME_PROBE0;
	tp->backoff++;
	tp->probes_out++;
	tcp_reset_xmit_timer (sk, TIME_PROBE0, 
			      min(tp->rto << tp->backoff, 120*HZ));
}