summaryrefslogtreecommitdiffhomepage
path: root/pkg/tcpip/transport/tcp/connect.go
blob: 61a173fbb0d7d973cb6f9bd91849b084a5c296da (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
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
// Copyright 2018 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package tcp

import (
	"encoding/binary"
	"math"
	"time"

	"gvisor.dev/gvisor/pkg/rand"
	"gvisor.dev/gvisor/pkg/sleep"
	"gvisor.dev/gvisor/pkg/sync"
	"gvisor.dev/gvisor/pkg/tcpip"
	"gvisor.dev/gvisor/pkg/tcpip/buffer"
	"gvisor.dev/gvisor/pkg/tcpip/hash/jenkins"
	"gvisor.dev/gvisor/pkg/tcpip/header"
	"gvisor.dev/gvisor/pkg/tcpip/seqnum"
	"gvisor.dev/gvisor/pkg/tcpip/stack"
	"gvisor.dev/gvisor/pkg/waiter"
)

// maxSegmentsPerWake is the maximum number of segments to process in the main
// protocol goroutine per wake-up. Yielding [after this number of segments are
// processed] allows other events to be processed as well (e.g., timeouts,
// resets, etc.).
const maxSegmentsPerWake = 100

type handshakeState int

// The following are the possible states of the TCP connection during a 3-way
// handshake. A depiction of the states and transitions can be found in RFC 793,
// page 23.
const (
	handshakeSynSent handshakeState = iota
	handshakeSynRcvd
	handshakeCompleted
)

// The following are used to set up sleepers.
const (
	wakerForNotification = iota
	wakerForNewSegment
	wakerForResend
)

const (
	// Maximum space available for options.
	maxOptionSize = 40
)

// handshake holds the state used during a TCP 3-way handshake.
//
// NOTE: handshake.ep.mu is held during handshake processing. It is released if
// we are going to block and reacquired when we start processing an event.
type handshake struct {
	ep     *endpoint
	state  handshakeState
	active bool
	flags  uint8
	ackNum seqnum.Value

	// iss is the initial send sequence number, as defined in RFC 793.
	iss seqnum.Value

	// rcvWnd is the receive window, as defined in RFC 793.
	rcvWnd seqnum.Size

	// sndWnd is the send window, as defined in RFC 793.
	sndWnd seqnum.Size

	// mss is the maximum segment size received from the peer.
	mss uint16

	// sndWndScale is the send window scale, as defined in RFC 1323. A
	// negative value means no scaling is supported by the peer.
	sndWndScale int

	// rcvWndScale is the receive window scale, as defined in RFC 1323.
	rcvWndScale int

	// startTime is the time at which the first SYN/SYN-ACK was sent.
	startTime time.Time

	// deferAccept if non-zero will drop the final ACK for a passive
	// handshake till an ACK segment with data is received or the timeout is
	// hit.
	deferAccept time.Duration

	// acked is true if the the final ACK for a 3-way handshake has
	// been received. This is required to stop retransmitting the
	// original SYN-ACK when deferAccept is enabled.
	acked bool

	// sendSYNOpts is the cached values for the SYN options to be sent.
	sendSYNOpts header.TCPSynOptions
}

func (e *endpoint) newHandshake() *handshake {
	h := &handshake{
		ep:          e,
		active:      true,
		rcvWnd:      seqnum.Size(e.initialReceiveWindow()),
		rcvWndScale: e.rcvWndScaleForHandshake(),
	}
	h.resetState()
	// Store reference to handshake state in endpoint.
	e.h = h
	return h
}

func (e *endpoint) newPassiveHandshake(isn, irs seqnum.Value, opts *header.TCPSynOptions, deferAccept time.Duration) *handshake {
	h := e.newHandshake()
	h.resetToSynRcvd(isn, irs, opts, deferAccept)
	return h
}

// FindWndScale determines the window scale to use for the given maximum window
// size.
func FindWndScale(wnd seqnum.Size) int {
	if wnd < 0x10000 {
		return 0
	}

	max := seqnum.Size(math.MaxUint16)
	s := 0
	for wnd > max && s < header.MaxWndScale {
		s++
		max <<= 1
	}

	return s
}

// resetState resets the state of the handshake object such that it becomes
// ready for a new 3-way handshake.
func (h *handshake) resetState() {
	b := make([]byte, 4)
	if _, err := rand.Read(b); err != nil {
		panic(err)
	}

	h.state = handshakeSynSent
	h.flags = header.TCPFlagSyn
	h.ackNum = 0
	h.mss = 0
	h.iss = generateSecureISN(h.ep.ID, h.ep.stack.Seed())
}

// generateSecureISN generates a secure Initial Sequence number based on the
// recommendation here https://tools.ietf.org/html/rfc6528#page-3.
func generateSecureISN(id stack.TransportEndpointID, seed uint32) seqnum.Value {
	isnHasher := jenkins.Sum32(seed)
	isnHasher.Write([]byte(id.LocalAddress))
	isnHasher.Write([]byte(id.RemoteAddress))
	portBuf := make([]byte, 2)
	binary.LittleEndian.PutUint16(portBuf, id.LocalPort)
	isnHasher.Write(portBuf)
	binary.LittleEndian.PutUint16(portBuf, id.RemotePort)
	isnHasher.Write(portBuf)
	// The time period here is 64ns. This is similar to what linux uses
	// generate a sequence number that overlaps less than one
	// time per MSL (2 minutes).
	//
	// A 64ns clock ticks 10^9/64 = 15625000) times in a second.
	// To wrap the whole 32 bit space would require
	// 2^32/1562500 ~ 274 seconds.
	//
	// Which sort of guarantees that we won't reuse the ISN for a new
	// connection for the same tuple for at least 274s.
	isn := isnHasher.Sum32() + uint32(time.Now().UnixNano()>>6)
	return seqnum.Value(isn)
}

// effectiveRcvWndScale returns the effective receive window scale to be used.
// If the peer doesn't support window scaling, the effective rcv wnd scale is
// zero; otherwise it's the value calculated based on the initial rcv wnd.
func (h *handshake) effectiveRcvWndScale() uint8 {
	if h.sndWndScale < 0 {
		return 0
	}
	return uint8(h.rcvWndScale)
}

// resetToSynRcvd resets the state of the handshake object to the SYN-RCVD
// state.
func (h *handshake) resetToSynRcvd(iss seqnum.Value, irs seqnum.Value, opts *header.TCPSynOptions, deferAccept time.Duration) {
	h.active = false
	h.state = handshakeSynRcvd
	h.flags = header.TCPFlagSyn | header.TCPFlagAck
	h.iss = iss
	h.ackNum = irs + 1
	h.mss = opts.MSS
	h.sndWndScale = opts.WS
	h.deferAccept = deferAccept
	h.ep.setEndpointState(StateSynRecv)
}

// checkAck checks if the ACK number, if present, of a segment received during
// a TCP 3-way handshake is valid. If it's not, a RST segment is sent back in
// response.
func (h *handshake) checkAck(s *segment) bool {
	if s.flagIsSet(header.TCPFlagAck) && s.ackNumber != h.iss+1 {
		// RFC 793, page 36, states that a reset must be generated when
		// the connection is in any non-synchronized state and an
		// incoming segment acknowledges something not yet sent. The
		// connection remains in the same state.
		ack := s.sequenceNumber.Add(s.logicalLen())
		h.ep.sendRaw(buffer.VectorisedView{}, header.TCPFlagRst|header.TCPFlagAck, s.ackNumber, ack, 0)
		return false
	}

	return true
}

// synSentState handles a segment received when the TCP 3-way handshake is in
// the SYN-SENT state.
func (h *handshake) synSentState(s *segment) tcpip.Error {
	// RFC 793, page 37, states that in the SYN-SENT state, a reset is
	// acceptable if the ack field acknowledges the SYN.
	if s.flagIsSet(header.TCPFlagRst) {
		if s.flagIsSet(header.TCPFlagAck) && s.ackNumber == h.iss+1 {
			// RFC 793, page 67, states that "If the RST bit is set [and] If the ACK
			// was acceptable then signal the user "error: connection reset", drop
			// the segment, enter CLOSED state, delete TCB, and return."
			h.ep.workerCleanup = true
			// Although the RFC above calls out ECONNRESET, Linux actually returns
			// ECONNREFUSED here so we do as well.
			return &tcpip.ErrConnectionRefused{}
		}
		return nil
	}

	if !h.checkAck(s) {
		return nil
	}

	// We are in the SYN-SENT state. We only care about segments that have
	// the SYN flag.
	if !s.flagIsSet(header.TCPFlagSyn) {
		return nil
	}

	// Parse the SYN options.
	rcvSynOpts := parseSynSegmentOptions(s)

	// Remember if the Timestamp option was negotiated.
	h.ep.maybeEnableTimestamp(&rcvSynOpts)

	// Remember if the SACKPermitted option was negotiated.
	h.ep.maybeEnableSACKPermitted(&rcvSynOpts)

	// Remember the sequence we'll ack from now on.
	h.ackNum = s.sequenceNumber + 1
	h.flags |= header.TCPFlagAck
	h.mss = rcvSynOpts.MSS
	h.sndWndScale = rcvSynOpts.WS

	// If this is a SYN ACK response, we only need to acknowledge the SYN
	// and the handshake is completed.
	if s.flagIsSet(header.TCPFlagAck) {
		h.state = handshakeCompleted

		h.ep.transitionToStateEstablishedLocked(h)

		h.ep.sendRaw(buffer.VectorisedView{}, header.TCPFlagAck, h.iss+1, h.ackNum, h.rcvWnd>>h.effectiveRcvWndScale())
		return nil
	}

	// A SYN segment was received, but no ACK in it. We acknowledge the SYN
	// but resend our own SYN and wait for it to be acknowledged in the
	// SYN-RCVD state.
	h.state = handshakeSynRcvd
	ttl := h.ep.ttl
	amss := h.ep.amss
	h.ep.setEndpointState(StateSynRecv)
	synOpts := header.TCPSynOptions{
		WS:    int(h.effectiveRcvWndScale()),
		TS:    rcvSynOpts.TS,
		TSVal: h.ep.timestamp(),
		TSEcr: h.ep.recentTimestamp(),

		// We only send SACKPermitted if the other side indicated it
		// permits SACK. This is not explicitly defined in the RFC but
		// this is the behaviour implemented by Linux.
		SACKPermitted: rcvSynOpts.SACKPermitted,
		MSS:           amss,
	}
	if ttl == 0 {
		ttl = h.ep.route.DefaultTTL()
	}
	h.ep.sendSynTCP(h.ep.route, tcpFields{
		id:     h.ep.ID,
		ttl:    ttl,
		tos:    h.ep.sendTOS,
		flags:  h.flags,
		seq:    h.iss,
		ack:    h.ackNum,
		rcvWnd: h.rcvWnd,
	}, synOpts)
	return nil
}

// synRcvdState handles a segment received when the TCP 3-way handshake is in
// the SYN-RCVD state.
func (h *handshake) synRcvdState(s *segment) tcpip.Error {
	if s.flagIsSet(header.TCPFlagRst) {
		// RFC 793, page 37, states that in the SYN-RCVD state, a reset
		// is acceptable if the sequence number is in the window.
		if s.sequenceNumber.InWindow(h.ackNum, h.rcvWnd) {
			return &tcpip.ErrConnectionRefused{}
		}
		return nil
	}

	if !h.checkAck(s) {
		return nil
	}

	// RFC 793, Section 3.9, page 69, states that in the SYN-RCVD state, a
	// sequence number outside of the window causes an ACK with the proper seq
	// number and "After sending the acknowledgment, drop the unacceptable
	// segment and return."
	if !s.sequenceNumber.InWindow(h.ackNum, h.rcvWnd) {
		if h.ep.allowOutOfWindowAck() {
			h.ep.sendRaw(buffer.VectorisedView{}, header.TCPFlagAck, h.iss+1, h.ackNum, h.rcvWnd)
		}
		return nil
	}

	if s.flagIsSet(header.TCPFlagSyn) && s.sequenceNumber != h.ackNum-1 {
		// We received two SYN segments with different sequence
		// numbers, so we reset this and restart the whole
		// process, except that we don't reset the timer.
		ack := s.sequenceNumber.Add(s.logicalLen())
		seq := seqnum.Value(0)
		if s.flagIsSet(header.TCPFlagAck) {
			seq = s.ackNumber
		}
		h.ep.sendRaw(buffer.VectorisedView{}, header.TCPFlagRst|header.TCPFlagAck, seq, ack, 0)

		if !h.active {
			return &tcpip.ErrInvalidEndpointState{}
		}

		h.resetState()
		synOpts := header.TCPSynOptions{
			WS:            h.rcvWndScale,
			TS:            h.ep.sendTSOk,
			TSVal:         h.ep.timestamp(),
			TSEcr:         h.ep.recentTimestamp(),
			SACKPermitted: h.ep.sackPermitted,
			MSS:           h.ep.amss,
		}
		h.ep.sendSynTCP(h.ep.route, tcpFields{
			id:     h.ep.ID,
			ttl:    h.ep.ttl,
			tos:    h.ep.sendTOS,
			flags:  h.flags,
			seq:    h.iss,
			ack:    h.ackNum,
			rcvWnd: h.rcvWnd,
		}, synOpts)
		return nil
	}

	// We have previously received (and acknowledged) the peer's SYN. If the
	// peer acknowledges our SYN, the handshake is completed.
	if s.flagIsSet(header.TCPFlagAck) {
		// If deferAccept is not zero and this is a bare ACK and the
		// timeout is not hit then drop the ACK.
		if h.deferAccept != 0 && s.data.Size() == 0 && time.Since(h.startTime) < h.deferAccept {
			h.acked = true
			h.ep.stack.Stats().DroppedPackets.Increment()
			return nil
		}

		// If the timestamp option is negotiated and the segment does
		// not carry a timestamp option then the segment must be dropped
		// as per https://tools.ietf.org/html/rfc7323#section-3.2.
		if h.ep.sendTSOk && !s.parsedOptions.TS {
			h.ep.stack.Stats().DroppedPackets.Increment()
			return nil
		}

		// Update timestamp if required. See RFC7323, section-4.3.
		if h.ep.sendTSOk && s.parsedOptions.TS {
			h.ep.updateRecentTimestamp(s.parsedOptions.TSVal, h.ackNum, s.sequenceNumber)
		}
		h.state = handshakeCompleted

		h.ep.transitionToStateEstablishedLocked(h)

		// If the segment has data then requeue it for the receiver
		// to process it again once main loop is started.
		if s.data.Size() > 0 {
			s.incRef()
			h.ep.enqueueSegment(s)
		}
		return nil
	}

	return nil
}

func (h *handshake) handleSegment(s *segment) tcpip.Error {
	h.sndWnd = s.window
	if !s.flagIsSet(header.TCPFlagSyn) && h.sndWndScale > 0 {
		h.sndWnd <<= uint8(h.sndWndScale)
	}

	switch h.state {
	case handshakeSynRcvd:
		return h.synRcvdState(s)
	case handshakeSynSent:
		return h.synSentState(s)
	}
	return nil
}

// processSegments goes through the segment queue and processes up to
// maxSegmentsPerWake (if they're available).
func (h *handshake) processSegments() tcpip.Error {
	for i := 0; i < maxSegmentsPerWake; i++ {
		s := h.ep.segmentQueue.dequeue()
		if s == nil {
			return nil
		}

		err := h.handleSegment(s)
		s.decRef()
		if err != nil {
			return err
		}

		// We stop processing packets once the handshake is completed,
		// otherwise we may process packets meant to be processed by
		// the main protocol goroutine.
		if h.state == handshakeCompleted {
			break
		}
	}

	// If the queue is not empty, make sure we'll wake up in the next
	// iteration.
	if !h.ep.segmentQueue.empty() {
		h.ep.newSegmentWaker.Assert()
	}

	return nil
}

// start sends the first SYN/SYN-ACK. It does not block, even if link address
// resolution is required.
func (h *handshake) start() {
	h.startTime = time.Now()
	h.ep.amss = calculateAdvertisedMSS(h.ep.userMSS, h.ep.route)
	var sackEnabled tcpip.TCPSACKEnabled
	if err := h.ep.stack.TransportProtocolOption(ProtocolNumber, &sackEnabled); err != nil {
		// If stack returned an error when checking for SACKEnabled
		// status then just default to switching off SACK negotiation.
		sackEnabled = false
	}

	synOpts := header.TCPSynOptions{
		WS:            h.rcvWndScale,
		TS:            true,
		TSVal:         h.ep.timestamp(),
		TSEcr:         h.ep.recentTimestamp(),
		SACKPermitted: bool(sackEnabled),
		MSS:           h.ep.amss,
	}

	// start() is also called in a listen context so we want to make sure we only
	// send the TS/SACK option when we received the TS/SACK in the initial SYN.
	if h.state == handshakeSynRcvd {
		synOpts.TS = h.ep.sendTSOk
		synOpts.SACKPermitted = h.ep.sackPermitted && bool(sackEnabled)
		if h.sndWndScale < 0 {
			// Disable window scaling if the peer did not send us
			// the window scaling option.
			synOpts.WS = -1
		}
	}

	h.sendSYNOpts = synOpts
	h.ep.sendSynTCP(h.ep.route, tcpFields{
		id:     h.ep.ID,
		ttl:    h.ep.ttl,
		tos:    h.ep.sendTOS,
		flags:  h.flags,
		seq:    h.iss,
		ack:    h.ackNum,
		rcvWnd: h.rcvWnd,
	}, synOpts)
}

// complete completes the TCP 3-way handshake initiated by h.start().
func (h *handshake) complete() tcpip.Error {
	// Set up the wakers.
	var s sleep.Sleeper
	resendWaker := sleep.Waker{}
	s.AddWaker(&resendWaker, wakerForResend)
	s.AddWaker(&h.ep.notificationWaker, wakerForNotification)
	s.AddWaker(&h.ep.newSegmentWaker, wakerForNewSegment)
	defer s.Done()

	// Initialize the resend timer.
	timer, err := newBackoffTimer(time.Second, MaxRTO, resendWaker.Assert)
	if err != nil {
		return err
	}
	defer timer.stop()
	for h.state != handshakeCompleted {
		// Unlock before blocking, and reacquire again afterwards (h.ep.mu is held
		// throughout handshake processing).
		h.ep.mu.Unlock()
		index, _ := s.Fetch(true /* block */)
		h.ep.mu.Lock()
		switch index {

		case wakerForResend:
			if err := timer.reset(); err != nil {
				return err
			}
			// Resend the SYN/SYN-ACK only if the following conditions hold.
			//  - It's an active handshake (deferAccept does not apply)
			//  - It's a passive handshake and we have not yet got the final-ACK.
			//  - It's a passive handshake and we got an ACK but deferAccept is
			//    enabled and we are now past the deferAccept duration.
			// The last is required to provide a way for the peer to complete
			// the connection with another ACK or data (as ACKs are never
			// retransmitted on their own).
			if h.active || !h.acked || h.deferAccept != 0 && time.Since(h.startTime) > h.deferAccept {
				h.ep.sendSynTCP(h.ep.route, tcpFields{
					id:     h.ep.ID,
					ttl:    h.ep.ttl,
					tos:    h.ep.sendTOS,
					flags:  h.flags,
					seq:    h.iss,
					ack:    h.ackNum,
					rcvWnd: h.rcvWnd,
				}, h.sendSYNOpts)
			}

		case wakerForNotification:
			n := h.ep.fetchNotifications()
			if (n&notifyClose)|(n&notifyAbort) != 0 {
				return &tcpip.ErrAborted{}
			}
			if n&notifyDrain != 0 {
				for !h.ep.segmentQueue.empty() {
					s := h.ep.segmentQueue.dequeue()
					err := h.handleSegment(s)
					s.decRef()
					if err != nil {
						return err
					}
					if h.state == handshakeCompleted {
						return nil
					}
				}
				close(h.ep.drainDone)
				h.ep.mu.Unlock()
				<-h.ep.undrain
				h.ep.mu.Lock()
			}
			if n&notifyError != 0 {
				return h.ep.lastErrorLocked()
			}
		case wakerForNewSegment:
			if err := h.processSegments(); err != nil {
				return err
			}
		}
	}

	return nil
}

type backoffTimer struct {
	timeout    time.Duration
	maxTimeout time.Duration
	t          *time.Timer
}

func newBackoffTimer(timeout, maxTimeout time.Duration, f func()) (*backoffTimer, tcpip.Error) {
	if timeout > maxTimeout {
		return nil, &tcpip.ErrTimeout{}
	}
	bt := &backoffTimer{timeout: timeout, maxTimeout: maxTimeout}
	bt.t = time.AfterFunc(timeout, f)
	return bt, nil
}

func (bt *backoffTimer) reset() tcpip.Error {
	bt.timeout *= 2
	if bt.timeout > bt.maxTimeout {
		return &tcpip.ErrTimeout{}
	}
	bt.t.Reset(bt.timeout)
	return nil
}

func (bt *backoffTimer) stop() {
	bt.t.Stop()
}

func parseSynSegmentOptions(s *segment) header.TCPSynOptions {
	synOpts := header.ParseSynOptions(s.options, s.flagIsSet(header.TCPFlagAck))
	if synOpts.TS {
		s.parsedOptions.TSVal = synOpts.TSVal
		s.parsedOptions.TSEcr = synOpts.TSEcr
	}
	return synOpts
}

var optionPool = sync.Pool{
	New: func() interface{} {
		return &[maxOptionSize]byte{}
	},
}

func getOptions() []byte {
	return (*optionPool.Get().(*[maxOptionSize]byte))[:]
}

func putOptions(options []byte) {
	// Reslice to full capacity.
	optionPool.Put(optionsToArray(options))
}

func makeSynOptions(opts header.TCPSynOptions) []byte {
	// Emulate linux option order. This is as follows:
	//
	// if md5: NOP NOP MD5SIG 18 md5sig(16)
	// if mss: MSS 4 mss(2)
	// if ts and sack_advertise:
	//	SACK 2 TIMESTAMP 2 timestamp(8)
	// elif ts: NOP NOP TIMESTAMP 10 timestamp(8)
	// elif sack: NOP NOP SACK 2
	// if wscale: NOP WINDOW 3 ws(1)
	// if sack_blocks: NOP NOP SACK ((2 + (#blocks * 8))
	//	[for each block] start_seq(4) end_seq(4)
	// if fastopen_cookie:
	//	if exp: EXP (4 + len(cookie)) FASTOPEN_MAGIC(2)
	// 	else: FASTOPEN (2 + len(cookie))
	//	cookie(variable) [padding to four bytes]
	//
	options := getOptions()

	// Always encode the mss.
	offset := header.EncodeMSSOption(uint32(opts.MSS), options)

	// Special ordering is required here. If both TS and SACK are enabled,
	// then the SACK option precedes TS, with no padding. If they are
	// enabled individually, then we see padding before the option.
	if opts.TS && opts.SACKPermitted {
		offset += header.EncodeSACKPermittedOption(options[offset:])
		offset += header.EncodeTSOption(opts.TSVal, opts.TSEcr, options[offset:])
	} else if opts.TS {
		offset += header.EncodeNOP(options[offset:])
		offset += header.EncodeNOP(options[offset:])
		offset += header.EncodeTSOption(opts.TSVal, opts.TSEcr, options[offset:])
	} else if opts.SACKPermitted {
		offset += header.EncodeNOP(options[offset:])
		offset += header.EncodeNOP(options[offset:])
		offset += header.EncodeSACKPermittedOption(options[offset:])
	}

	// Initialize the WS option.
	if opts.WS >= 0 {
		offset += header.EncodeNOP(options[offset:])
		offset += header.EncodeWSOption(opts.WS, options[offset:])
	}

	// Padding to the end; note that this never apply unless we add a
	// fastopen option, we always expect the offset to remain the same.
	if delta := header.AddTCPOptionPadding(options, offset); delta != 0 {
		panic("unexpected option encoding")
	}

	return options[:offset]
}

// tcpFields is a struct to carry different parameters required by the
// send*TCP variant functions below.
type tcpFields struct {
	id     stack.TransportEndpointID
	ttl    uint8
	tos    uint8
	flags  byte
	seq    seqnum.Value
	ack    seqnum.Value
	rcvWnd seqnum.Size
	opts   []byte
	txHash uint32
}

func (e *endpoint) sendSynTCP(r *stack.Route, tf tcpFields, opts header.TCPSynOptions) tcpip.Error {
	tf.opts = makeSynOptions(opts)
	// We ignore SYN send errors and let the callers re-attempt send.
	if err := e.sendTCP(r, tf, buffer.VectorisedView{}, nil); err != nil {
		e.stats.SendErrors.SynSendToNetworkFailed.Increment()
	}
	putOptions(tf.opts)
	return nil
}

func (e *endpoint) sendTCP(r *stack.Route, tf tcpFields, data buffer.VectorisedView, gso *stack.GSO) tcpip.Error {
	tf.txHash = e.txHash
	if err := sendTCP(r, tf, data, gso, e.owner); err != nil {
		e.stats.SendErrors.SegmentSendToNetworkFailed.Increment()
		return err
	}
	e.stats.SegmentsSent.Increment()
	return nil
}

func buildTCPHdr(r *stack.Route, tf tcpFields, pkt *stack.PacketBuffer, gso *stack.GSO) {
	optLen := len(tf.opts)
	tcp := header.TCP(pkt.TransportHeader().Push(header.TCPMinimumSize + optLen))
	pkt.TransportProtocolNumber = header.TCPProtocolNumber
	tcp.Encode(&header.TCPFields{
		SrcPort:    tf.id.LocalPort,
		DstPort:    tf.id.RemotePort,
		SeqNum:     uint32(tf.seq),
		AckNum:     uint32(tf.ack),
		DataOffset: uint8(header.TCPMinimumSize + optLen),
		Flags:      tf.flags,
		WindowSize: uint16(tf.rcvWnd),
	})
	copy(tcp[header.TCPMinimumSize:], tf.opts)

	xsum := r.PseudoHeaderChecksum(ProtocolNumber, uint16(pkt.Size()))
	// Only calculate the checksum if offloading isn't supported.
	if gso != nil && gso.NeedsCsum {
		// This is called CHECKSUM_PARTIAL in the Linux kernel. We
		// calculate a checksum of the pseudo-header and save it in the
		// TCP header, then the kernel calculate a checksum of the
		// header and data and get the right sum of the TCP packet.
		tcp.SetChecksum(xsum)
	} else if r.RequiresTXTransportChecksum() {
		xsum = header.ChecksumCombine(xsum, pkt.Data().AsRange().Checksum())
		tcp.SetChecksum(^tcp.CalculateChecksum(xsum))
	}
}

func sendTCPBatch(r *stack.Route, tf tcpFields, data buffer.VectorisedView, gso *stack.GSO, owner tcpip.PacketOwner) tcpip.Error {
	// We need to shallow clone the VectorisedView here as ReadToView will
	// split the VectorisedView and Trim underlying views as it splits. Not
	// doing the clone here will cause the underlying views of data itself
	// to be altered.
	data = data.Clone(nil)

	optLen := len(tf.opts)
	if tf.rcvWnd > math.MaxUint16 {
		tf.rcvWnd = math.MaxUint16
	}

	mss := int(gso.MSS)
	n := (data.Size() + mss - 1) / mss

	size := data.Size()
	hdrSize := header.TCPMinimumSize + int(r.MaxHeaderLength()) + optLen
	var pkts stack.PacketBufferList
	for i := 0; i < n; i++ {
		packetSize := mss
		if packetSize > size {
			packetSize = size
		}
		size -= packetSize
		pkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
			ReserveHeaderBytes: hdrSize,
		})
		pkt.Hash = tf.txHash
		pkt.Owner = owner
		pkt.Data().ReadFromVV(&data, packetSize)
		buildTCPHdr(r, tf, pkt, gso)
		tf.seq = tf.seq.Add(seqnum.Size(packetSize))
		pkts.PushBack(pkt)
	}

	if tf.ttl == 0 {
		tf.ttl = r.DefaultTTL()
	}
	sent, err := r.WritePackets(gso, pkts, stack.NetworkHeaderParams{Protocol: ProtocolNumber, TTL: tf.ttl, TOS: tf.tos})
	if err != nil {
		r.Stats().TCP.SegmentSendErrors.IncrementBy(uint64(n - sent))
	}
	r.Stats().TCP.SegmentsSent.IncrementBy(uint64(sent))
	return err
}

// sendTCP sends a TCP segment with the provided options via the provided
// network endpoint and under the provided identity.
func sendTCP(r *stack.Route, tf tcpFields, data buffer.VectorisedView, gso *stack.GSO, owner tcpip.PacketOwner) tcpip.Error {
	optLen := len(tf.opts)
	if tf.rcvWnd > math.MaxUint16 {
		tf.rcvWnd = math.MaxUint16
	}

	if r.Loop&stack.PacketLoop == 0 && gso != nil && gso.Type == stack.GSOSW && int(gso.MSS) < data.Size() {
		return sendTCPBatch(r, tf, data, gso, owner)
	}

	pkt := stack.NewPacketBuffer(stack.PacketBufferOptions{
		ReserveHeaderBytes: header.TCPMinimumSize + int(r.MaxHeaderLength()) + optLen,
		Data:               data,
	})
	pkt.Hash = tf.txHash
	pkt.Owner = owner
	buildTCPHdr(r, tf, pkt, gso)

	if tf.ttl == 0 {
		tf.ttl = r.DefaultTTL()
	}
	if err := r.WritePacket(gso, stack.NetworkHeaderParams{Protocol: ProtocolNumber, TTL: tf.ttl, TOS: tf.tos}, pkt); err != nil {
		r.Stats().TCP.SegmentSendErrors.Increment()
		return err
	}
	r.Stats().TCP.SegmentsSent.Increment()
	if (tf.flags & header.TCPFlagRst) != 0 {
		r.Stats().TCP.ResetsSent.Increment()
	}
	return nil
}

// makeOptions makes an options slice.
func (e *endpoint) makeOptions(sackBlocks []header.SACKBlock) []byte {
	options := getOptions()
	offset := 0

	// N.B. the ordering here matches the ordering used by Linux internally
	// and described in the raw makeOptions function. We don't include
	// unnecessary cases here (post connection.)
	if e.sendTSOk {
		// Embed the timestamp if timestamp has been enabled.
		//
		// We only use the lower 32 bits of the unix time in
		// milliseconds. This is similar to what Linux does where it
		// uses the lower 32 bits of the jiffies value in the tsVal
		// field of the timestamp option.
		//
		// Further, RFC7323 section-5.4 recommends millisecond
		// resolution as the lowest recommended resolution for the
		// timestamp clock.
		//
		// Ref: https://tools.ietf.org/html/rfc7323#section-5.4.
		offset += header.EncodeNOP(options[offset:])
		offset += header.EncodeNOP(options[offset:])
		offset += header.EncodeTSOption(e.timestamp(), e.recentTimestamp(), options[offset:])
	}
	if e.sackPermitted && len(sackBlocks) > 0 {
		offset += header.EncodeNOP(options[offset:])
		offset += header.EncodeNOP(options[offset:])
		offset += header.EncodeSACKBlocks(sackBlocks, options[offset:])
	}

	// We expect the above to produce an aligned offset.
	if delta := header.AddTCPOptionPadding(options, offset); delta != 0 {
		panic("unexpected option encoding")
	}

	return options[:offset]
}

// sendRaw sends a TCP segment to the endpoint's peer.
func (e *endpoint) sendRaw(data buffer.VectorisedView, flags byte, seq, ack seqnum.Value, rcvWnd seqnum.Size) tcpip.Error {
	var sackBlocks []header.SACKBlock
	if e.EndpointState() == StateEstablished && e.rcv.pendingRcvdSegments.Len() > 0 && (flags&header.TCPFlagAck != 0) {
		sackBlocks = e.sack.Blocks[:e.sack.NumBlocks]
	}
	options := e.makeOptions(sackBlocks)
	err := e.sendTCP(e.route, tcpFields{
		id:     e.ID,
		ttl:    e.ttl,
		tos:    e.sendTOS,
		flags:  flags,
		seq:    seq,
		ack:    ack,
		rcvWnd: rcvWnd,
		opts:   options,
	}, data, e.gso)
	putOptions(options)
	return err
}

func (e *endpoint) handleWrite() {
	e.sndBufMu.Lock()
	next := e.drainSendQueueLocked()
	e.sndBufMu.Unlock()

	e.sendData(next)
}

// Move packets from send queue to send list.
//
// Precondition: e.sndBufMu must be locked.
func (e *endpoint) drainSendQueueLocked() *segment {
	first := e.sndQueue.Front()
	if first != nil {
		e.snd.writeList.PushBackList(&e.sndQueue)
		e.sndBufInQueue = 0
	}
	return first
}

// Precondition: e.mu must be locked.
func (e *endpoint) sendData(next *segment) {
	// Initialize the next segment to write if it's currently nil.
	if e.snd.writeNext == nil {
		e.snd.writeNext = next
	}

	// Push out any new packets.
	e.snd.sendData()
}

func (e *endpoint) handleClose() {
	if !e.EndpointState().connected() {
		return
	}
	// Drain the send queue.
	e.handleWrite()

	// Mark send side as closed.
	e.snd.closed = true
}

// resetConnectionLocked puts the endpoint in an error state with the given
// error code and sends a RST if and only if the error is not ErrConnectionReset
// indicating that the connection is being reset due to receiving a RST. This
// method must only be called from the protocol goroutine.
func (e *endpoint) resetConnectionLocked(err tcpip.Error) {
	// Only send a reset if the connection is being aborted for a reason
	// other than receiving a reset.
	e.setEndpointState(StateError)
	e.hardError = err
	switch err.(type) {
	case *tcpip.ErrConnectionReset, *tcpip.ErrTimeout:
	default:
		// The exact sequence number to be used for the RST is the same as the
		// one used by Linux. We need to handle the case of window being shrunk
		// which can cause sndNxt to be outside the acceptable window on the
		// receiver.
		//
		// See: https://www.snellman.net/blog/archive/2016-02-01-tcp-rst/ for more
		// information.
		sndWndEnd := e.snd.sndUna.Add(e.snd.sndWnd)
		resetSeqNum := sndWndEnd
		if !sndWndEnd.LessThan(e.snd.sndNxt) || e.snd.sndNxt.Size(sndWndEnd) < (1<<e.snd.sndWndScale) {
			resetSeqNum = e.snd.sndNxt
		}
		e.sendRaw(buffer.VectorisedView{}, header.TCPFlagAck|header.TCPFlagRst, resetSeqNum, e.rcv.rcvNxt, 0)
	}
}

// completeWorkerLocked is called by the worker goroutine when it's about to
// exit.
func (e *endpoint) completeWorkerLocked() {
	// Worker is terminating(either due to moving to
	// CLOSED or ERROR state, ensure we release all
	// registrations port reservations even if the socket
	// itself is not yet closed by the application.
	e.workerRunning = false
	if e.workerCleanup {
		e.cleanupLocked()
	}
}

// transitionToStateEstablisedLocked transitions a given endpoint
// to an established state using the handshake parameters provided.
// It also initializes sender/receiver.
func (e *endpoint) transitionToStateEstablishedLocked(h *handshake) {
	// Transfer handshake state to TCP connection. We disable
	// receive window scaling if the peer doesn't support it
	// (indicated by a negative send window scale).
	e.snd = newSender(e, h.iss, h.ackNum-1, h.sndWnd, h.mss, h.sndWndScale)

	e.rcvListMu.Lock()
	e.rcv = newReceiver(e, h.ackNum-1, h.rcvWnd, h.effectiveRcvWndScale())
	// Bootstrap the auto tuning algorithm. Starting at zero will
	// result in a really large receive window after the first auto
	// tuning adjustment.
	e.rcvAutoParams.prevCopied = int(h.rcvWnd)
	e.rcvListMu.Unlock()

	e.setEndpointState(StateEstablished)
}

// transitionToStateCloseLocked ensures that the endpoint is
// cleaned up from the transport demuxer, "before" moving to
// StateClose. This will ensure that no packet will be
// delivered to this endpoint from the demuxer when the endpoint
// is transitioned to StateClose.
func (e *endpoint) transitionToStateCloseLocked() {
	s := e.EndpointState()
	if s == StateClose {
		return
	}

	if s.connected() {
		e.stack.Stats().TCP.CurrentConnected.Decrement()
		e.stack.Stats().TCP.EstablishedClosed.Increment()
	}

	// Mark the endpoint as fully closed for reads/writes.
	e.cleanupLocked()
	e.setEndpointState(StateClose)
}

// tryDeliverSegmentFromClosedEndpoint attempts to deliver the parsed
// segment to any other endpoint other than the current one. This is called
// only when the endpoint is in StateClose and we want to deliver the segment
// to any other listening endpoint. We reply with RST if we cannot find one.
func (e *endpoint) tryDeliverSegmentFromClosedEndpoint(s *segment) {
	ep := e.stack.FindTransportEndpoint(e.NetProto, e.TransProto, e.ID, s.nicID)
	if ep == nil && e.NetProto == header.IPv6ProtocolNumber && e.TransportEndpointInfo.ID.LocalAddress.To4() != "" {
		// Dual-stack socket, try IPv4.
		ep = e.stack.FindTransportEndpoint(header.IPv4ProtocolNumber, e.TransProto, e.ID, s.nicID)
	}
	if ep == nil {
		replyWithReset(e.stack, s, stack.DefaultTOS, 0 /* ttl */)
		s.decRef()
		return
	}

	if e == ep {
		panic("current endpoint not removed from demuxer, enqueing segments to itself")
	}

	if ep := ep.(*endpoint); ep.enqueueSegment(s) {
		ep.newSegmentWaker.Assert()
	}
}

// Drain segment queue from the endpoint and try to re-match the segment to a
// different endpoint. This is used when the current endpoint is transitioned to
// StateClose and has been unregistered from the transport demuxer.
func (e *endpoint) drainClosingSegmentQueue() {
	for {
		s := e.segmentQueue.dequeue()
		if s == nil {
			break
		}

		e.tryDeliverSegmentFromClosedEndpoint(s)
	}
}

func (e *endpoint) handleReset(s *segment) (ok bool, err tcpip.Error) {
	if e.rcv.acceptable(s.sequenceNumber, 0) {
		// RFC 793, page 37 states that "in all states
		// except SYN-SENT, all reset (RST) segments are
		// validated by checking their SEQ-fields." So
		// we only process it if it's acceptable.
		switch e.EndpointState() {
		// In case of a RST in CLOSE-WAIT linux moves
		// the socket to closed state with an error set
		// to indicate EPIPE.
		//
		// Technically this seems to be at odds w/ RFC.
		// As per https://tools.ietf.org/html/rfc793#section-2.7
		// page 69 the behavior for a segment arriving
		// w/ RST bit set in CLOSE-WAIT is inlined below.
		//
		//  ESTABLISHED
		//  FIN-WAIT-1
		//  FIN-WAIT-2
		//  CLOSE-WAIT

		//  If the RST bit is set then, any outstanding RECEIVEs and
		//  SEND should receive "reset" responses. All segment queues
		//  should be flushed.  Users should also receive an unsolicited
		//  general "connection reset" signal. Enter the CLOSED state,
		//  delete the TCB, and return.
		case StateCloseWait:
			e.transitionToStateCloseLocked()
			e.hardError = &tcpip.ErrAborted{}
			e.notifyProtocolGoroutine(notifyTickleWorker)
			return false, nil
		default:
			// RFC 793, page 37 states that "in all states
			// except SYN-SENT, all reset (RST) segments are
			// validated by checking their SEQ-fields." So
			// we only process it if it's acceptable.

			// Notify protocol goroutine. This is required when
			// handleSegment is invoked from the processor goroutine
			// rather than the worker goroutine.
			e.notifyProtocolGoroutine(notifyResetByPeer)
			return false, &tcpip.ErrConnectionReset{}
		}
	}
	return true, nil
}

// handleSegments processes all inbound segments.
func (e *endpoint) handleSegments(fastPath bool) tcpip.Error {
	checkRequeue := true
	for i := 0; i < maxSegmentsPerWake; i++ {
		if e.EndpointState().closed() {
			return nil
		}
		s := e.segmentQueue.dequeue()
		if s == nil {
			checkRequeue = false
			break
		}

		cont, err := e.handleSegment(s)
		s.decRef()
		if err != nil {
			return err
		}
		if !cont {
			return nil
		}
	}

	// When fastPath is true we don't want to wake up the worker
	// goroutine. If the endpoint has more segments to process the
	// dispatcher will call handleSegments again anyway.
	if !fastPath && checkRequeue && !e.segmentQueue.empty() {
		e.newSegmentWaker.Assert()
	}

	// Send an ACK for all processed packets if needed.
	if e.rcv.rcvNxt != e.snd.maxSentAck {
		e.snd.sendAck()
	}

	e.resetKeepaliveTimer(true /* receivedData */)

	return nil
}

func (e *endpoint) probeSegment() {
	if e.probe != nil {
		e.probe(e.completeState())
	}
}

// handleSegment handles a given segment and notifies the worker goroutine if
// if the connection should be terminated.
func (e *endpoint) handleSegment(s *segment) (cont bool, err tcpip.Error) {
	// Invoke the tcp probe if installed. The tcp probe function will update
	// the TCPEndpointState after the segment is processed.
	defer e.probeSegment()

	if s.flagIsSet(header.TCPFlagRst) {
		if ok, err := e.handleReset(s); !ok {
			return false, err
		}
	} else if s.flagIsSet(header.TCPFlagSyn) {
		// See: https://tools.ietf.org/html/rfc5961#section-4.1
		//   1) If the SYN bit is set, irrespective of the sequence number, TCP
		//    MUST send an ACK (also referred to as challenge ACK) to the remote
		//    peer:
		//
		//    <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
		//
		//    After sending the acknowledgment, TCP MUST drop the unacceptable
		//    segment and stop processing further.
		//
		// By sending an ACK, the remote peer is challenged to confirm the loss
		// of the previous connection and the request to start a new connection.
		// A legitimate peer, after restart, would not have a TCB in the
		// synchronized state.  Thus, when the ACK arrives, the peer should send
		// a RST segment back with the sequence number derived from the ACK
		// field that caused the RST.

		// This RST will confirm that the remote peer has indeed closed the
		// previous connection.  Upon receipt of a valid RST, the local TCP
		// endpoint MUST terminate its connection.  The local TCP endpoint
		// should then rely on SYN retransmission from the remote end to
		// re-establish the connection.
		e.snd.maybeSendOutOfWindowAck(s)
	} else if s.flagIsSet(header.TCPFlagAck) {
		// Patch the window size in the segment according to the
		// send window scale.
		s.window <<= e.snd.sndWndScale

		// RFC 793, page 41 states that "once in the ESTABLISHED
		// state all segments must carry current acknowledgment
		// information."
		drop, err := e.rcv.handleRcvdSegment(s)
		if err != nil {
			return false, err
		}
		if drop {
			return true, nil
		}

		// Now check if the received segment has caused us to transition
		// to a CLOSED state, if yes then terminate processing and do
		// not invoke the sender.
		state := e.state
		if state == StateClose {
			// When we get into StateClose while processing from the queue,
			// return immediately and let the protocolMainloop handle it.
			//
			// We can reach StateClose only while processing a previous segment
			// or a notification from the protocolMainLoop (caller goroutine).
			// This means that with this return, the segment dequeue below can
			// never occur on a closed endpoint.
			return false, nil
		}

		e.snd.handleRcvdSegment(s)
	}

	return true, nil
}

// keepaliveTimerExpired is called when the keepaliveTimer fires. We send TCP
// keepalive packets periodically when the connection is idle. If we don't hear
// from the other side after a number of tries, we terminate the connection.
func (e *endpoint) keepaliveTimerExpired() tcpip.Error {
	userTimeout := e.userTimeout

	e.keepalive.Lock()
	if !e.SocketOptions().GetKeepAlive() || !e.keepalive.timer.checkExpiration() {
		e.keepalive.Unlock()
		return nil
	}

	// If a userTimeout is set then abort the connection if it is
	// exceeded.
	if userTimeout != 0 && time.Since(e.rcv.lastRcvdAckTime) >= userTimeout && e.keepalive.unacked > 0 {
		e.keepalive.Unlock()
		e.stack.Stats().TCP.EstablishedTimedout.Increment()
		return &tcpip.ErrTimeout{}
	}

	if e.keepalive.unacked >= e.keepalive.count {
		e.keepalive.Unlock()
		e.stack.Stats().TCP.EstablishedTimedout.Increment()
		return &tcpip.ErrTimeout{}
	}

	// RFC1122 4.2.3.6: TCP keepalive is a dataless ACK with
	// seg.seq = snd.nxt-1.
	e.keepalive.unacked++
	e.keepalive.Unlock()
	e.snd.sendSegmentFromView(buffer.VectorisedView{}, header.TCPFlagAck, e.snd.sndNxt-1)
	e.resetKeepaliveTimer(false)
	return nil
}

// resetKeepaliveTimer restarts or stops the keepalive timer, depending on
// whether it is enabled for this endpoint.
func (e *endpoint) resetKeepaliveTimer(receivedData bool) {
	e.keepalive.Lock()
	if receivedData {
		e.keepalive.unacked = 0
	}
	// Start the keepalive timer IFF it's enabled and there is no pending
	// data to send.
	if !e.SocketOptions().GetKeepAlive() || e.snd == nil || e.snd.sndUna != e.snd.sndNxt {
		e.keepalive.timer.disable()
		e.keepalive.Unlock()
		return
	}
	if e.keepalive.unacked > 0 {
		e.keepalive.timer.enable(e.keepalive.interval)
	} else {
		e.keepalive.timer.enable(e.keepalive.idle)
	}
	e.keepalive.Unlock()
}

// disableKeepaliveTimer stops the keepalive timer.
func (e *endpoint) disableKeepaliveTimer() {
	e.keepalive.Lock()
	e.keepalive.timer.disable()
	e.keepalive.Unlock()
}

// protocolMainLoop is the main loop of the TCP protocol. It runs in its own
// goroutine and is responsible for sending segments and handling received
// segments.
func (e *endpoint) protocolMainLoop(handshake bool, wakerInitDone chan<- struct{}) tcpip.Error {
	e.mu.Lock()
	var closeTimer *time.Timer
	var closeWaker sleep.Waker

	epilogue := func() {
		// e.mu is expected to be hold upon entering this section.
		if e.snd != nil {
			e.snd.resendTimer.cleanup()
			e.snd.probeTimer.cleanup()
			e.snd.reorderTimer.cleanup()
		}

		if closeTimer != nil {
			closeTimer.Stop()
		}

		e.completeWorkerLocked()

		if e.drainDone != nil {
			close(e.drainDone)
		}

		e.mu.Unlock()

		e.drainClosingSegmentQueue()

		// When the protocol loop exits we should wake up our waiters.
		e.waiterQueue.Notify(waiter.EventHUp | waiter.EventErr | waiter.EventIn | waiter.EventOut)
	}

	if handshake {
		if err := e.h.complete(); err != nil {
			e.lastErrorMu.Lock()
			e.lastError = err
			e.lastErrorMu.Unlock()

			e.setEndpointState(StateError)
			e.hardError = err

			e.workerCleanup = true
			// Lock released below.
			epilogue()
			return err
		}
	}

	// Reaching this point means that we successfully completed the 3-way
	// handshake with our peer.
	//
	// Completing the 3-way handshake is an indication that the route is valid
	// and the remote is reachable as the only way we can complete a handshake
	// is if our SYN reached the remote and their ACK reached us.
	e.route.ConfirmReachable()

	drained := e.drainDone != nil
	if drained {
		close(e.drainDone)
		e.mu.Unlock()
		<-e.undrain
		e.mu.Lock()
	}

	// Set up the functions that will be called when the main protocol loop
	// wakes up.
	funcs := []struct {
		w *sleep.Waker
		f func() tcpip.Error
	}{
		{
			w: &e.sndWaker,
			f: func() tcpip.Error {
				e.handleWrite()
				return nil
			},
		},
		{
			w: &e.sndCloseWaker,
			f: func() tcpip.Error {
				e.handleClose()
				return nil
			},
		},
		{
			w: &closeWaker,
			f: func() tcpip.Error {
				// This means the socket is being closed due
				// to the TCP-FIN-WAIT2 timeout was hit. Just
				// mark the socket as closed.
				e.transitionToStateCloseLocked()
				e.workerCleanup = true
				return nil
			},
		},
		{
			w: &e.snd.resendWaker,
			f: func() tcpip.Error {
				if !e.snd.retransmitTimerExpired() {
					e.stack.Stats().TCP.EstablishedTimedout.Increment()
					return &tcpip.ErrTimeout{}
				}
				return nil
			},
		},
		{
			w: &e.snd.probeWaker,
			f: e.snd.probeTimerExpired,
		},
		{
			w: &e.newSegmentWaker,
			f: func() tcpip.Error {
				return e.handleSegments(false /* fastPath */)
			},
		},
		{
			w: &e.keepalive.waker,
			f: e.keepaliveTimerExpired,
		},
		{
			w: &e.notificationWaker,
			f: func() tcpip.Error {
				n := e.fetchNotifications()
				if n&notifyNonZeroReceiveWindow != 0 {
					e.rcv.nonZeroWindow()
				}

				if n&notifyMTUChanged != 0 {
					e.sndBufMu.Lock()
					count := e.packetTooBigCount
					e.packetTooBigCount = 0
					mtu := e.sndMTU
					e.sndBufMu.Unlock()

					e.snd.updateMaxPayloadSize(mtu, count)
				}

				if n&notifyReset != 0 || n&notifyAbort != 0 {
					return &tcpip.ErrConnectionAborted{}
				}

				if n&notifyResetByPeer != 0 {
					return &tcpip.ErrConnectionReset{}
				}

				if n&notifyClose != 0 && closeTimer == nil {
					if e.EndpointState() == StateFinWait2 && e.closed {
						// The socket has been closed and we are in FIN_WAIT2
						// so start the FIN_WAIT2 timer.
						closeTimer = time.AfterFunc(e.tcpLingerTimeout, closeWaker.Assert)
					}
				}

				if n&notifyKeepaliveChanged != 0 {
					// The timer could fire in background
					// when the endpoint is drained. That's
					// OK. See above.
					e.resetKeepaliveTimer(true)
				}

				if n&notifyDrain != 0 {
					for !e.segmentQueue.empty() {
						if err := e.handleSegments(false /* fastPath */); err != nil {
							return err
						}
					}
					if !e.EndpointState().closed() {
						// Only block the worker if the endpoint
						// is not in closed state or error state.
						close(e.drainDone)
						e.mu.Unlock()
						<-e.undrain
						e.mu.Lock()
					}
				}

				if n&notifyTickleWorker != 0 {
					// Just a tickle notification. No need to do
					// anything.
					return nil
				}

				return nil
			},
		},
		{
			w: &e.snd.reorderWaker,
			f: e.snd.rc.reorderTimerExpired,
		},
	}

	// Initialize the sleeper based on the wakers in funcs.
	var s sleep.Sleeper
	for i := range funcs {
		s.AddWaker(funcs[i].w, i)
	}

	// Notify the caller that the waker initialization is complete and the
	// endpoint is ready.
	if wakerInitDone != nil {
		close(wakerInitDone)
	}

	// Tell waiters that the endpoint is connected and writable.
	e.waiterQueue.Notify(waiter.EventOut)

	// The following assertions and notifications are needed for restored
	// endpoints. Fresh newly created endpoints have empty states and should
	// not invoke any.
	if !e.segmentQueue.empty() {
		e.newSegmentWaker.Assert()
	}

	e.rcvListMu.Lock()
	if !e.rcvList.Empty() {
		e.waiterQueue.Notify(waiter.EventIn)
	}
	e.rcvListMu.Unlock()

	if e.workerCleanup {
		e.notifyProtocolGoroutine(notifyClose)
	}

	// Main loop. Handle segments until both send and receive ends of the
	// connection have completed.
	cleanupOnError := func(err tcpip.Error) {
		e.stack.Stats().TCP.CurrentConnected.Decrement()
		e.workerCleanup = true
		if err != nil {
			e.resetConnectionLocked(err)
		}
		// Lock released below.
		epilogue()
	}

loop:
	for {
		switch e.EndpointState() {
		case StateTimeWait, StateClose, StateError:
			break loop
		}

		e.mu.Unlock()
		v, _ := s.Fetch(true /* block */)
		e.mu.Lock()

		// We need to double check here because the notification may be
		// stale by the time we got around to processing it.
		switch e.EndpointState() {
		case StateError:
			// If the endpoint has already transitioned to an ERROR
			// state just pass nil here as any reset that may need
			// to be sent etc should already have been done and we
			// just want to terminate the loop and cleanup the
			// endpoint.
			cleanupOnError(nil)
			return nil
		case StateTimeWait:
			fallthrough
		case StateClose:
			break loop
		default:
			if err := funcs[v].f(); err != nil {
				cleanupOnError(err)
				return nil
			}
		}
	}

	var reuseTW func()
	if e.EndpointState() == StateTimeWait {
		// Disable close timer as we now entering real TIME_WAIT.
		if closeTimer != nil {
			closeTimer.Stop()
		}
		// Mark the current sleeper done so as to free all associated
		// wakers.
		s.Done()
		// Wake up any waiters before we enter TIME_WAIT.
		e.waiterQueue.Notify(waiter.EventHUp | waiter.EventErr | waiter.EventIn | waiter.EventOut)
		e.workerCleanup = true
		reuseTW = e.doTimeWait()
	}

	// Handle any StateError transition from StateTimeWait.
	if e.EndpointState() == StateError {
		cleanupOnError(nil)
		return nil
	}

	e.transitionToStateCloseLocked()

	// Lock released below.
	epilogue()

	// A new SYN was received during TIME_WAIT and we need to abort
	// the timewait and redirect the segment to the listener queue
	if reuseTW != nil {
		reuseTW()
	}

	return nil
}

// handleTimeWaitSegments processes segments received during TIME_WAIT
// state.
func (e *endpoint) handleTimeWaitSegments() (extendTimeWait bool, reuseTW func()) {
	checkRequeue := true
	for i := 0; i < maxSegmentsPerWake; i++ {
		s := e.segmentQueue.dequeue()
		if s == nil {
			checkRequeue = false
			break
		}
		extTW, newSyn := e.rcv.handleTimeWaitSegment(s)
		if newSyn {
			info := e.TransportEndpointInfo
			newID := info.ID
			newID.RemoteAddress = ""
			newID.RemotePort = 0
			netProtos := []tcpip.NetworkProtocolNumber{info.NetProto}
			// If the local address is an IPv4 address then also
			// look for IPv6 dual stack endpoints that might be
			// listening on the local address.
			if newID.LocalAddress.To4() != "" {
				netProtos = []tcpip.NetworkProtocolNumber{header.IPv4ProtocolNumber, header.IPv6ProtocolNumber}
			}
			for _, netProto := range netProtos {
				if listenEP := e.stack.FindTransportEndpoint(netProto, info.TransProto, newID, s.nicID); listenEP != nil {
					tcpEP := listenEP.(*endpoint)
					if EndpointState(tcpEP.State()) == StateListen {
						reuseTW = func() {
							if !tcpEP.enqueueSegment(s) {
								s.decRef()
								return
							}
							tcpEP.newSegmentWaker.Assert()
						}
						// We explicitly do not decRef
						// the segment as it's still
						// valid and being reflected to
						// a listening endpoint.
						return false, reuseTW
					}
				}
			}
		}
		if extTW {
			extendTimeWait = true
		}
		s.decRef()
	}
	if checkRequeue && !e.segmentQueue.empty() {
		e.newSegmentWaker.Assert()
	}
	return extendTimeWait, nil
}

// doTimeWait is responsible for handling the TCP behaviour once a socket
// enters the TIME_WAIT state. Optionally it can return a closure that
// should be executed after releasing the endpoint registrations. This is
// done in cases where a new SYN is received during TIME_WAIT that carries
// a sequence number larger than one see on the connection.
func (e *endpoint) doTimeWait() (twReuse func()) {
	// Trigger a 2 * MSL time wait state. During this period
	// we will drop all incoming segments.
	// NOTE: On Linux this is not configurable and is fixed at 60 seconds.
	timeWaitDuration := DefaultTCPTimeWaitTimeout

	// Get the stack wide configuration.
	var tcpTW tcpip.TCPTimeWaitTimeoutOption
	if err := e.stack.TransportProtocolOption(ProtocolNumber, &tcpTW); err == nil {
		timeWaitDuration = time.Duration(tcpTW)
	}

	const newSegment = 1
	const notification = 2
	const timeWaitDone = 3

	var s sleep.Sleeper
	defer s.Done()
	s.AddWaker(&e.newSegmentWaker, newSegment)
	s.AddWaker(&e.notificationWaker, notification)

	var timeWaitWaker sleep.Waker
	s.AddWaker(&timeWaitWaker, timeWaitDone)
	timeWaitTimer := time.AfterFunc(timeWaitDuration, timeWaitWaker.Assert)
	defer timeWaitTimer.Stop()

	for {
		e.mu.Unlock()
		v, _ := s.Fetch(true /* block */)
		e.mu.Lock()
		switch v {
		case newSegment:
			extendTimeWait, reuseTW := e.handleTimeWaitSegments()
			if reuseTW != nil {
				return reuseTW
			}
			if extendTimeWait {
				timeWaitTimer.Reset(timeWaitDuration)
			}
		case notification:
			n := e.fetchNotifications()
			if n&notifyAbort != 0 {
				return nil
			}
			if n&notifyDrain != 0 {
				for !e.segmentQueue.empty() {
					// Ignore extending TIME_WAIT during a
					// save. For sockets in TIME_WAIT we just
					// terminate the TIME_WAIT early.
					e.handleTimeWaitSegments()
				}
				close(e.drainDone)
				e.mu.Unlock()
				<-e.undrain
				e.mu.Lock()
				return nil
			}
		case timeWaitDone:
			return nil
		}
	}
}