aboutgitcodebugslistschat
path: root/tcp.c
blob: 46b739ddaab889fc260d5189f88f448c00b8960a (plain) (tree)
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






















































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































                                                                                 
// SPDX-License-Identifier: AGPL-3.0-or-later

/* PASST - Plug A Simple Socket Transport
 *
 * tcp.c - TCP L2-L4 translation state machine
 *
 * Copyright (c) 2020-2021 Red Hat GmbH
 * Author: Stefano Brivio <sbrivio@redhat.com>
 *
 */

/**
 * DOC: Theory of Operation
 *
 *
 * Overview
 * --------
 *
 * This implementation maps TCP traffic between a single L2 interface (tap) and
 * native TCP (L4) sockets, mimicking and reproducing as closely as possible the
 * inferred behaviour of applications running on a guest, connected via said L2
 * interface. Four connection flows are supported:
 * - from the local host to the guest behind the tap interface:
 *   - this is the main use case for proxies in service meshes
 *   - we bind to all unbound local ports, and relay traffic between L4 sockets
 *     with local endpoints and the L2 interface
 * - from remote hosts to the guest behind the tap interface:
 *   - this might be needed for services that need to be addressed directly,
 *     and typically configured with special port forwarding rules (which are
 *     not needed here)
 *   - we also relay traffic between L4 sockets with remote endpoints and the L2
 *     interface
 * - from the guest to the local host:
 *   - this is not observed in practice, but implemented for completeness and
 *     transparency
 * - from the guest to external hosts:
 *   - this might be needed for applications running on the guest that need to
 *     directly access internet services (e.g. NTP)
 *
 * Relevant goals are:
 * - transparency: sockets need to behave as if guest applications were running
 *   directly on the host. This is achieved by:
 *   - avoiding port and address translations whenever possible
 *   - mirroring TCP dynamics by observation of socket parameters (TCP_INFO
 *     socket option) and TCP headers of packets coming from the tap interface,
 *     reapplying those parameters in both flow directions (including TCP_MSS,
 *     TCP_WINDOW_CLAMP socket options)
 * - simplicity: only a small subset of TCP logic is implemented here and
 *   delegated as much as possible to the TCP implementations of guest and host
 *   kernel. This is achieved by:
 *   - avoiding a complete TCP stack reimplementation, with a modified TCP state
 *     machine focused on the translation of observed states instead
 *   - mirroring TCP dynamics as described above and hence avoiding the need for
 *     segmentation, explicit queueing, and reassembly of segments
 * - security:
 *   - no dynamic memory allocation is performed
 *   - TODO: synflood protection
 *   - TODO: sequence collision attacks
 *
 * Portability is limited by usage of Linux-specific socket options.
 *
 *
 * Limits
 * ------
 *
 * To avoid the need for dynamic memory allocation, a maximum, reasonable amount
 * of connections is defined by TCP_MAX_CONNS below (currently 256k, close to
 * the maximum amount of file descriptors typically available to a process on
 * Linux).
 *
 * While fragmentation and reassembly are not implemented, tracking of missing
 * segments and retransmissions needs to be, thus data needs to linger on
 * sockets as long as it's not acknowledged by the guest, and read using
 * MSG_PEEK into a single, preallocated static buffer sized to the maximum
 * supported window, 64MiB. This imposes a practical limitation on window
 * scaling, that is, the maximum factor is 1024. If a bigger window scaling
 * factor is observed during connection establishment, connection is reset and
 * reestablished by omitting the scaling factor in the SYN segment. This
 * limitation only applies to the window scaling advertised by the guest, but
 * if exceeded, no window scaling will be allowed at all toward either endpoint.
 *
 *
 * Ports
 * -----
 *
 * To avoid the need for ad-hoc configuration of port forwarding or allowed
 * ports, listening sockets are opened and bound to all unbound ports on the
 * host, as far as process capabilities allow. This service needs to be started
 * after any application proxy that needs to bind to local ports.
 *
 * No port translation is needed for connections initiated remotely or by the
 * local host: source port from socket is reused while establishing connections
 * to the guest.
 * 
 * For connections initiated by the guest, it's not possible to force the same
 * source port as connections are established by the host kernel: that's the
 * only port translation needed.
 *
 *
 * Connection tracking and storage
 * -------------------------------
 *
 * Connection are tracked by the @tc array of struct tcp_conn, containing
 * addresses, ports, TCP states and parameters. This is statically allocated and
 * indices are the file descriptor numbers associated to inbound or outbound
 * sockets.
 *
 * IPv4 addresses are stored as IPv4-mapped IPv6 addresses to avoid the need for
 * separate data structures depending on the protocol version.
 *
 * - Inbound connection requests (to the guest) are mapped using the triple
 *   < source IP address, source port, destination port >
 * - Outbound connection requests (from the guest) are mapped using the triple
 *   < destination IP address, destination port, source port >
 *   where the source port is the one used by the guest, not the one used by the
 *   corresponding host socket
 *
 *
 * Initialisation
 * --------------
 *
 * Up to 2^15 + 2^14 listening sockets (excluding ephemeral ports, repeated for
 * IPv4 and IPv6) are opened and bound to wildcard addresses. Some will fail to
 * bind (for low ports, or ports already bound, e.g. by a proxy). These are
 * added to the epoll list, with no separate storage.
 *
 *
 * States and events
 * -----------------
 *
 * These states apply to connected sockets only, listening sockets are always
 * open after initialisation, in LISTEN state. A single state is maintained for
 * both sides of the connection, and most states are omitted as they are already
 * handled by host kernel and guest.
 *
 * - CLOSED			no connection
 *   No associated events: this is always a final state, new connections
 *   directly start from TAP_SYN_SENT or SOCK_SYN_SENT described below.
 *
 * - TAP_SYN_SENT		connect() in progress, triggered from tap
 *   - connect() completes	SYN,ACK to tap > TAP_SYN_RCVD
 *   - connect() aborts		RST to tap, close socket > CLOSED
 *   - RST from tap		close socket > CLOSED
 *
 * - SOCK_SYN_SENT		new connected socket, SYN sent to tap
 *   - SYN,ACK from tap		ACK to tap > ESTABLISHED
 *   - SYN,ACK timeout		RST to tap, close socket > CLOSED
 *   - socket error		RST to tap, close socket > CLOSED
 *   - RST from tap		close socket > CLOSED
 *
 * - TAP_SYN_RCVD		connect() completed, SYN,ACK sent to tap
 *   - ACK from tap		> ESTABLISHED
 *   - ACK timeout		RST to tap, close socket > CLOSED
 *   - socket error		RST to tap, close socket > CLOSED
 *   - RST from tap		close socket > CLOSED
 *
 * - ESTABLISHED		connection established, ready for data
 *   - zero-sized socket read	FIN to tap > ESTABLISHED_SOCK_FIN
 *   - data timeout		FIN to tap > ESTABLISHED_SOCK_FIN
 *   - socket error		RST to tap, close socket > CLOSED
 *   - FIN from tap		FIN,ACK to tap, close socket > FIN_WAIT_1
 *   - RST from tap		close socket > CLOSED
 *
 * - ESTABLISHED_SOCK_FIN	socket wants to close connection, data allowed
 *   - ACK from tap		> CLOSE_WAIT
 *   - ACK timeout		RST to tap, close socket > CLOSED
 *   - RST from tap		close socket > CLOSED
 *
 * - CLOSE_WAIT			socket wants to close connection, seen by tap
 *   - socket error		RST to tap, close socket > CLOSED
 *   - FIN from tap		ACK to tap, close socket > LAST_ACK
 *   - FIN timeout		RST to tap, close socket > CLOSED
 *   - RST from tap		close socket > CLOSED
 * 
 * - LAST_ACK			socket started close, tap completed it
 *   - anything from socket	close socket > CLOSED
 *   - socket error		RST to tap, close socket > CLOSED
 *   - ACK timeout		RST to tap, close socket > CLOSED
 *
 * - FIN_WAIT_1			tap wants to close connection, _FIN,ACK sent_
 *   - ACK from tap		close socket > CLOSED
 *   - socket error		RST to tap, close socket > CLOSED
 *   - ACK timeout		RST to tap, close socket > CLOSED
 *
 *
 * Connection setup
 * ----------------
 *
 * - inbound connection (from socket to guest): on accept() from listening
 *   socket, the new socket is mapped in connection tracking table, and
 *   three-way handshake initiated towards the guest, advertising MSS and window
 *   size and scaling from socket parameters
 * - outbound connection (from guest to socket): on SYN segment from guest, a
 *   new socket is created and mapped in connection tracking table, setting
 *   MSS and window clamping from header and option of the observed SYN segment
 *
 * 
 * Aging and timeout
 * -----------------
 *
 * Two bitmaps of TCP_MAX_CONNS bits indicate which connections need scheduled
 * actions:
 * - @tcp_act_fast is used to send ACK segments to the tap once TCP_INFO reports
 *   an increased number of acknowledged bytes sent on a socket, and examined
 *   every 20ms (one tenth of current TCP_DELACK_MAX on Linux): for each marked
 *   connection, a TCP_INFO query is performed and ACK segments are sent right
 *   away as needed
 * - @tcp_act_slow is used for state and retransmission timeouts, and examined
 *   every 2s: for each marked connection with an expired @timeout timestamp
 *   specific actions are taken depending on the connection state:
 *   - SOCK_SYN_SENT: after a 2MSL (240s) timeout waiting for a SYN,ACK segment
 *     from tap expires, connection is reset (RST to tap, socket closed)
 *   - TAP_SYN_RCVD: after a 2MSL (240s) timeout waiting for an ACK segment from
 *     tap expires, connection is reset (RST to tap, socket closed)
 *   - ESTABLISHED: after a timeout of 1s (TODO: implement requirements from
 *     RFC 6298) waiting for an ACK segment from tap expires, data from socket
 *     queue is retransmitted starting from the last ACK sequence
 *   - ESTABLISHED: after a two hours (current TCP_KEEPALIVE_TIME on Linux)
 *     timeout waiting for any activity expires, connection is reset (RST to
 *     tap, socket closed)
 *   - ESTABLISHED_SOCK_FIN: after a 2MSL (240s) timeout waiting for an ACK
 *     segment from tap expires, connection is reset (RST to tap, socket closed)
 *   - CLOSE_WAIT: after a 2MSL (240s) timeout waiting for a FIN segment from
 *     tap expires, connection is reset (RST to tap, socket closed)
 *   - LAST_ACK: after a 2MSL (240s) timeout waiting for an ACK segment from
 *     socket expires, connection is reset (RST to tap, socket closed)
 *   - FIN_WAIT_1: after a 2MSL (240s) timeout waiting for an ACK segment from
 *     tap expires, connection is reset (RST to tap, socket closed)
 *
 *
 * Data flows (from ESTABLISHED, ESTABLISHED_SOCK_FIN states)
 * ----------------------------------------------------------
 *
 * @seq_to_tap:		next sequence for packets to tap
 * @seq_ack_from_tap:	last ACK number received from tap
 * @seq_from_tap:	next sequence for packets from tap (not actually sent)
 * @seq_ack_to_tap:	last ACK number sent to tap
 *
 * @seq_init_from_tap:	initial sequence number from tap
 *
 * @tap_window:		last window size received from tap, scaled
 * @tcpi_acked_last:	most recent value of tcpi_bytes_acked (TCP_INFO)
 * 
 * - from socket to tap:
 *   - on new data from socket:
 *     - peek into buffer
 *     - send data to tap:
 *       - starting at offset (@seq_to_tap - @seq_ack_from_tap)
 *       - in MSS-sized segments
 *       - increasing @seq_to_tap at each segment
 *       - up to window (until @seq_to_tap - @seq_ack_from_tap <= @tap_window)
 *       - mark socket in bitmap for periodic ACK check, set @last_ts_to_tap
 *     - on read error, send RST to tap, close socket
 *     - on zero read, send FIN to tap, enter ESTABLISHED_SOCK_FIN
 *   - on ACK from tap:
 *     - check if it's the second duplicated ACK
 *     - consume buffer by difference between new ack_seq and @seq_ack_from_tap
 *     - update @seq_ack_from_tap from ack_seq in header
 *     - on two duplicated ACKs, reset @seq_to_tap to @seq_ack_from_tap, and
 *       resend with steps listed above
 *     - set TCP_WINDOW_CLAMP from TCP header from tap
 *     - on @seq_ack_from_tap == @seq_to_tap, mark in bitmap, umark otherwise
 *   - periodically:
 *     - if @seq_ack_from_tap < @seq_to_tap and the retransmission timer
 *       (TODO: implement requirements from RFC 6298, currently 3s fixed) from
 *       @last_ts_to_tap elapsed, reset @seq_to_tap to @seq_ack_from_tap, and
 *       resend data with the steps listed above
 *
 * - from tap to socket:
 *   - on packet from tap:
 *     - set TCP_WINDOW_CLAMP from TCP header from tap
 *     - check seq from header against @seq_from_tap, if data is missing, send
 *       two ACKs with number @seq_ack_to_tap, discard packet
 *     - otherwise queue data to socket, set @seq_from_tap to seq from header
 *       plus payload length
 *     - query socket for TCP_INFO, on tcpi_bytes_acked > @tcpi_acked_last,
 *       set @tcpi_acked_last to tcpi_bytes_acked, set @seq_ack_to_tap
 *       to (tcpi_bytes_acked + @seq_init_from_tap) % 2^32 and
 *       send ACK to tap
 *     - set @last_ts_sock
 *     - on @seq_ack_to_tap < @seq_from_tap, mark socket for later ACK in bitmap
 *   - periodically:
 *     - if socket is marked in bitmap, query socket for TCP_INFO, on
 *       tcpi_bytes_acked > @tcpi_acked_last, 
 *       set @tcpi_acked_last to tcpi_bytes_acked, set @seq_ack_to_tap
 *       to (tcpi_bytes_acked + @seq_init_from_tap) % 2^32 and
 *       send ACK to tap
 *     - on @seq_ack_to_tap == @seq_from_tap, unmark socket from bitmap
 */

#define _GNU_SOURCE
#include <stdio.h>
#include <errno.h>
#include <limits.h>
#include <net/ethernet.h>
#include <net/if.h>
#include <netinet/in.h>
#include <stdint.h>
#include <stddef.h>
#include <string.h>
#include <sys/epoll.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>
#include <linux/ip.h>
#include <linux/ipv6.h>
#include <linux/tcp.h>
#include <time.h>

#include "passt.h"
#include "tap.h"
#include "util.h"

/* Approximately maximum number of open descriptors per process */
#define MAX_CONNS			(256 * 1024)

#define MAX_WS				10
#define MAX_WINDOW			(1 << (16 + (MAX_WS)))
#define MSS_DEFAULT			536
#define WINDOW_DEFAULT			4380

#define SYN_TIMEOUT			240000		/* ms */
#define ACK_TIMEOUT			3000
#define ACT_TIMEOUT			7200000
#define FIN_TIMEOUT			240000
#define LAST_ACK_TIMEOUT		240000

#define SOCK_ACK_INTERVAL		20

/* We need to include <linux/tcp.h> for tcpi_bytes_acked, instead of
 * <netinet/tcp.h>, but that doesn't include a definition for SOL_TCP
 */
#define SOL_TCP				IPPROTO_TCP

static char tcp_in_buf[MAX_WINDOW];

static uint8_t tcp_act_fast[MAX_CONNS / 8] = { 0 };
static uint8_t tcp_act_slow[MAX_CONNS / 8] = { 0 };

enum tcp_state {
	CLOSED = 0,
	TAP_SYN_SENT,
	SOCK_SYN_SENT,
	TAP_SYN_RCVD,
	ESTABLISHED,
	ESTABLISHED_SOCK_FIN,
	CLOSE_WAIT,
	LAST_ACK,
	FIN_WAIT_1,
};

#define FIN		(1 << 0)
#define SYN		(1 << 1)
#define RST		(1 << 2)
#define ACK		(1 << 4)

#define OPT_EOL		0
#define OPT_NOP		1
#define OPT_MSS		2
#define OPT_WS		3
#define OPT_SACKP	4
#define OPT_SACK	5
#define OPT_TS		8

/**
 * struct tcp_conn - Descriptor for a TCP connection
 * @a.a6:		IPv6 remote address, can be IPv4-mapped
 * @a.a4.zero:		Zero prefix for IPv4-mapped, see RFC 6890, Table 20
 * @a.a4.one:		Ones prefix for IPv4-mapped
 * @a.a4.a:		IPv4 address
 * @tap_port:		Guest-facing tap port
 * @sock_port:		Remote, socket-facing port
 * @s:			TCP connection state
 * @seq_to_tap:		Next sequence for packets to tap
 * @seq_ack_from_tap:	Last ACK number received from tap
 * @seq_from_tap:	Next sequence for packets from tap (not actually sent)
 * @seq_ack_to_tap:	Last ACK number sent to tap
 * @seq_init_from_tap:	Initial sequence number from tap
 * @tcpi_acked_last:	Most recent value of tcpi_bytes_acked (TCP_INFO query)
 * @dup_acks:		Count of currently duplicated ACKs from tap
 * @ws_allowed:		Window scaling allowed
 * @ws:			Window scaling factor
 * @tap_window:		Last window size received from tap, scaled
 * @last_ts_sock:	Last activity timestamp from socket for timeout purposes
 * @last_ts_tap:	Last activity timestamp from tap for timeout purposes
 * @mss_guest:		Maximum segment size advertised by guest
 */
struct tcp_conn {
	union {
		struct in6_addr a6;
		struct {
			uint8_t zero[10];
			uint8_t one[2];
			struct in_addr a;
		} a4;
	} a;
	in_port_t tap_port;
	in_port_t sock_port;
	enum tcp_state s;

	uint32_t seq_to_tap;
	uint32_t seq_ack_from_tap;
	uint32_t seq_from_tap;
	uint32_t seq_ack_to_tap;
	uint32_t seq_init_from_tap;
	uint64_t tcpi_acked_last;
	int dup_acks;

	int ws_allowed;
	int ws;
	int tap_window;

	struct timespec last_ts_sock;
	struct timespec last_ts_tap;

	int mss_guest;
};

static struct tcp_conn tc[MAX_CONNS];

static int tcp_send_to_tap(struct ctx *c, int s, int flags, char *in, int len);

/**
 * tcp_act_fast_set() - Set socket in bitmap for "fast" timeout events
 * @s:		Socket file descriptor number
 */
static void tcp_act_fast_set(int s)
{
	tcp_act_fast[s / 8] |= 1 << (s % 8);
}

/**
 * tcp_act_fast_clear() - Clear socket from bitmap for "fast" timeout events
 * @s:		Socket file descriptor number
 */
static void tcp_act_fast_clear(int s)
{
	tcp_act_fast[s / 8] &= ~(1 << (s % 8));
}

/**
 * tcp_act_slow_set() - Set socket in bitmap for "slow" timeout events
 * @s:		Socket file descriptor number
 */
static void tcp_act_slow_set(int s)
{
	tcp_act_slow[s / 8] |= 1 << (s % 8);
}

/**
 * tcp_act_slow_clear() - Clear socket from bitmap for "slow" timeout events
 * @s:		Socket file descriptor number
 */
static void tcp_act_slow_clear(int s)
{
	tcp_act_slow[s / 8] &= ~(1 << (s % 8));
}

/**
 * tcp_opt_get() - Get option, and value if any, from TCP header
 * @th:		Pointer to TCP header
 * @len:	Length of buffer, including TCP header
 * @type:	Option type to look for
 * @optlen:	Optional, filled with option length if passed
 * @value:	Optional, set to start of option value if passed
 *
 * Return: Option value, meaningful for up to 4 bytes, -1 if not found
 */
static int tcp_opt_get(struct tcphdr *th, unsigned int len, uint8_t type,
		       uint8_t *optlen, void *value)
{
	uint8_t *p, __type, __optlen;

	len -= sizeof(*th);
	p = (uint8_t *)(th + 1);

	if (len > th->doff * 4 - sizeof(*th))
		len = th->doff * 4 - sizeof(*th);

	while (len >= 2) {
		switch (*p) {
		case OPT_EOL:
			return -1;
		case OPT_NOP:
			p++;
			len--;
			break;
		default:
			__type = *(p++);
			__optlen = *(p++);
			len -= 2;

			if (type == __type) {
				if (optlen)
					*optlen = __optlen;
				if (value)
					value = p;

				if (__optlen - 2 == 0)
					return 0;

				if (__optlen - 2 == 1)
					return *p;

				if (__optlen - 2 == 2)
					return ntohs(*(uint16_t *)p);

				return ntohl(*(uint32_t *)p);
			}

			p += __optlen - 2;
			len -= __optlen - 2;
		}
	}

	return -1;
}

/**
 * tcp_close_and_epoll_del() - Close socket and remove from epoll descriptor
 * @c:		Execution context
 * @s:		File descriptor number for socket
 */
static void tcp_close_and_epoll_del(struct ctx *c, int s)
{
	epoll_ctl(c->epollfd, EPOLL_CTL_DEL, s, NULL);
	close(s);
	tcp_act_fast_clear(s);
	tcp_act_slow_clear(s);
}

/**
 * tcp_rst() - Reset a connection: send RST segment to tap, close socket
 * @c:		Execution context
 * @s:		File descriptor number for socket
 */
static void tcp_rst(struct ctx *c, int s)
{
	if (s < 0)
		return;

	tcp_send_to_tap(c, s, RST, NULL, 0);
	tcp_close_and_epoll_del(c, s);
	tc[s].s = CLOSED;
}

/**
 * tcp_send_to_tap() - Send segment to tap, with options and values from socket
 * @c:		Execution context
 * @s:		File descriptor number for socket
 * @flags:	TCP flags to set
 * @in:		Input buffer, L4 header
 * @len:	Buffer length, at L4
 *
 * Return: -1 on error with connection reset, 0 otherwise
 */
static int tcp_send_to_tap(struct ctx *c, int s, int flags, char *in, int len)
{
	char buf[USHRT_MAX] = { 0 }, *data;
	struct tcp_info info = { 0 };
	socklen_t sl = sizeof(info);
	int ws = 0, have_info = 1;
	struct tcphdr *th;

	if (getsockopt(s, SOL_TCP, TCP_INFO, &info, &sl)) {
		if (!(flags & RST)) {
			tcp_rst(c, s);
			return -1;
		}

		have_info = 0;
	}

	th = (struct tcphdr *)buf;
	data = (char *)(th + 1);

	if (flags & SYN && have_info) {
		if (tc[s].ws_allowed)
			ws = info.tcpi_snd_wscale;

		/* Options: MSS, NOP and window scale if allowed (4-8 bytes) */
		*data++ = 2;
		*data++ = 4;
		*(uint16_t *)data = htons(info.tcpi_snd_mss);
		data += 2;

		if (ws) {
			*data++ = 1;

			*data++ = 3;
			*data++ = 3;
			*data++ = ws;

			th->doff = (20 + 8) / 4;
		} else {
			th->doff = (20 + 4) / 4;
		}

		th->seq = htonl(tc[s].seq_to_tap++);
	} else {
		th->doff = 20 / 4;

		th->seq = htonl(tc[s].seq_to_tap);
		tc[s].seq_to_tap += len;
	}

	if ((info.tcpi_bytes_acked > tc[s].tcpi_acked_last || (flags & ACK) ||
	     len) &&
	    have_info) {
		uint64_t ack_seq;

		th->ack = 1;
		/* info.tcpi_bytes_acked already includes one byte for SYN, but
		 * not for incoming connections.
		 */
		ack_seq = info.tcpi_bytes_acked + tc[s].seq_init_from_tap;
		if (!info.tcpi_bytes_acked)
			ack_seq++;
		ack_seq &= (uint32_t)~0U;

		tc[s].seq_ack_to_tap = ack_seq;
		th->ack_seq = htonl(tc[s].seq_ack_to_tap);

		tc[s].tcpi_acked_last = info.tcpi_bytes_acked;
	} else {
		if (!len && !flags)
			return 0;

		th->ack = th->ack_seq = 0;
	}

	th->rst = !!(flags & RST);
	th->syn = !!(flags & SYN);
	th->fin = !!(flags & FIN);

	th->source = tc[s].sock_port;
	th->dest = tc[s].tap_port;

	if (have_info)
		th->window = htons(info.tcpi_snd_wnd >> info.tcpi_snd_wscale);
	else
		th->window = WINDOW_DEFAULT;

	th->urg_ptr = 0;
	th->check = 0;

	memcpy(data, in, len);

	tap_ip_send(c, &tc[s].a.a6, IPPROTO_TCP, buf, th->doff * 4 + len);

	return 0;
}

/**
 * tcp_clamp_window() - Set window and scaling from option, clamp on socket
 * @s:		File descriptor number for socket
 * @th:		TCP header, from tap
 * @len:	Buffer length, at L4
 */
static void tcp_clamp_window(int s, struct tcphdr *th, int len)
{
	int ws;

	if (!tc[s].tap_window) {
		ws = tcp_opt_get(th, len, OPT_WS, NULL, NULL);
		if (ws >= 0 && ws <= MAX_WS) {
			tc[s].ws_allowed = 1;
			tc[s].ws = ws;
		} else {
			tc[s].ws_allowed = 0;
			tc[s].ws = 0;
		}

		/* First value is not scaled. Also, don't clamp yet, to avoid
		 * getting a zero scale just because we set a small window now.
		 */
		tc[s].tap_window = ntohs(th->window);
	} else {
		tc[s].tap_window = ntohs(th->window) << tc[s].ws;
		setsockopt(s, SOL_TCP, TCP_WINDOW_CLAMP,
			   &tc[s].tap_window, sizeof(tc[s].tap_window));
	}
}

/**
 * tcp_conn_from_tap() - Handle connection request (SYN segment) from tap
 * @c:		Execution context
 * @af:		Address family, AF_INET or AF_INET6
 * @addr:	Remote address, pointer to sin_addr or sin6_addr
 * @th:		TCP header from tap
 * @len:	Packet length at L4
 */
static void tcp_conn_from_tap(struct ctx *c, int af, void *addr,
			      struct tcphdr *th, size_t len)
{
	struct sockaddr_in addr4 = {
		.sin_family = AF_INET,
		.sin_port = th->dest,
		.sin_addr = *(struct in_addr *)addr,
	};
	struct sockaddr_in6 addr6 = {
		.sin6_family = AF_INET6,
		.sin6_port = th->dest,
		.sin6_addr = *(struct in6_addr *)addr,
	};
	struct epoll_event ev = { 0 };
	const struct sockaddr *sa;
	socklen_t sl;
	int s;

	s = socket(af, SOCK_STREAM | SOCK_NONBLOCK, IPPROTO_TCP);
	if (s < 0)
		return;

	tc[s].mss_guest = tcp_opt_get(th, len, OPT_MSS, NULL, NULL);
	if (tc[s].mss_guest < 0)
		tc[s].mss_guest = MSS_DEFAULT;
	sl = sizeof(tc[s].mss_guest);
	setsockopt(s, SOL_TCP, TCP_MAXSEG, &tc[s].mss_guest, sl);

	tcp_clamp_window(s, th, len);

	if (af == AF_INET) {
		sa = (const struct sockaddr *)&addr4;
		sl = sizeof(addr4);

		memset(&tc[s].a.a4.zero, 0, sizeof(tc[s].a.a4.zero));
		memset(&tc[s].a.a4.one, 0xff, sizeof(tc[s].a.a4.one));
		memcpy(&tc[s].a.a4.a, addr, sizeof(tc[s].a.a4.a));
	} else {
		sa = (const struct sockaddr *)&addr6;
		sl = sizeof(addr6);

		memcpy(&tc[s].a.a6, addr, sizeof(tc[s].a.a6));
	}

	tc[s].sock_port = th->dest;
	tc[s].tap_port = th->source;

	ev.events = EPOLLIN | EPOLLET | EPOLLRDHUP | EPOLLERR | EPOLLHUP;
	ev.data.fd = s;

	tc[s].seq_init_from_tap = ntohl(th->seq);
	tc[s].seq_from_tap = tc[s].seq_init_from_tap + 1;
	tc[s].seq_ack_to_tap = tc[s].seq_from_tap;

	/* TODO: RFC 6528 with SipHash, worth it? */
	tc[s].seq_ack_from_tap = tc[s].seq_to_tap = 0;

	if (connect(s, sa, sl)) {
		if (errno != EINPROGRESS) {
			tcp_rst(c, s);
			return;
		}

		ev.events |= EPOLLOUT;
		tc[s].s = TAP_SYN_SENT;
	} else {
		if (tcp_send_to_tap(c, s, SYN | ACK, NULL, 0))
			return;

		tc[s].s = TAP_SYN_RCVD;
	}

	epoll_ctl(c->epollfd, EPOLL_CTL_ADD, s, &ev);

	return;
}

/**
 * tcp_sock_lookup() - Look up socket given remote address and pair of ports
 * @af:		Address family, AF_INET or AF_INET6
 * @tap_port:	tap-facing port
 * @sock_port:	Socket-facing port
 *
 * Return: file descriptor number for socket, if found, -1 otherwise
 */
static int tcp_sock_lookup(int af, void *addr,
			   in_port_t tap_port, in_port_t sock_port)
{
	int i;

	/* TODO: hash table and lookup. This is just a dummy implementation. */
	for (i = 0; i < MAX_CONNS; i++) {
		if (af == AF_INET && IN6_IS_ADDR_V4MAPPED(&tc[i].a.a6)	&&
		    !memcmp(&tc[i].a.a4.a, addr, sizeof(tc[i].a.a4.a))	&&
		    tc[i].tap_port == tap_port				&&
		    tc[i].sock_port == sock_port			&&
		    tc[i].s)
			return i;

		if (af == AF_INET6					&&
		    !memcmp(&tc[i].a.a6, addr, sizeof(tc[i].a.a6))	&&
		    tc[i].tap_port == tap_port				&&
		    tc[i].sock_port == sock_port			&&
		    tc[i].s)
			return i;
	}

	return -1;
}

/**
 * tcp_conn_from_sock() - Handle new connection request from listening socket
 * @c:		Execution context
 * @fd:		File descriptor number for listening socket
 */
static void tcp_conn_from_sock(struct ctx *c, int fd)
{
	struct sockaddr_storage sa_r, sa_l;
	socklen_t sa_len = sizeof(sa_r);
	struct epoll_event ev = { 0 };
	struct sockaddr_in6 *sa6;
	struct sockaddr_in *sa4;
	int s;

	if (getsockname(fd, (struct sockaddr *)&sa_l, &sa_len))
		return;

	s = accept4(fd, (struct sockaddr *)&sa_r, &sa_len, SOCK_NONBLOCK);
	if (s == -1)
		return;

	if (sa_l.ss_family == AF_INET) {
		sa4 = (struct sockaddr_in *)&sa_r;

		memset(&tc[s].a.a4.zero, 0, sizeof(tc[s].a.a4.zero));
		memset(&tc[s].a.a4.one, 0xff, sizeof(tc[s].a.a4.one));
		memcpy(&tc[s].a.a4.a, &sa4->sin_addr, sizeof(tc[s].a.a4.a));

		tc[s].sock_port = sa4->sin_port;

		sa4 = (struct sockaddr_in *)&sa_l;
		tc[s].tap_port = sa4->sin_port;

	} else if (sa_l.ss_family == AF_INET6) {
		sa6 = (struct sockaddr_in6 *)&sa_r;

		memcpy(&tc[s].a.a6, &sa6->sin6_addr, sizeof(tc[s].a.a6));

		tc[s].sock_port = sa6->sin6_port;

		sa6 = (struct sockaddr_in6 *)&sa_l;
		tc[s].tap_port = sa6->sin6_port;
	}

	/* TODO: RFC 6528 with SipHash, worth it? */
	tc[s].seq_to_tap = 0;

	tc[s].ws_allowed = 1;

	clock_gettime(CLOCK_MONOTONIC, &tc[s].last_ts_sock);
	clock_gettime(CLOCK_MONOTONIC, &tc[s].last_ts_tap);

	ev.events = EPOLLIN | EPOLLET | EPOLLRDHUP | EPOLLERR | EPOLLHUP;
	ev.data.fd = s;
	epoll_ctl(c->epollfd, EPOLL_CTL_ADD, s, &ev);

	tc[s].s = SOCK_SYN_SENT;
	tcp_send_to_tap(c, s, SYN, NULL, 0);
}

/**
 * tcp_send_to_sock() - Send buffer to socket, update timestamp and sequence
 * @c:			Execution context
 * @s:			File descriptor number for socket
 * @seq:		Previous TCP sequence, host order
 * @data:		Data buffer
 * @len:		Length at L4
 * @extra_flags:	Additional flags for send(), if any
 *
 * Return: -1 on socket error with connection reset, 0 otherwise
 */
static int tcp_send_to_sock(struct ctx *c, int s, int seq, char *data, int len,
			    int extra_flags)
{
	int err = send(s, data, len, MSG_DONTWAIT | MSG_NOSIGNAL | extra_flags);

	if (err < 0) {
		if (errno == EAGAIN || errno == EWOULDBLOCK) {
			/* If we can't queue right now, do nothing, sender has
			 * to retransmit.
			 */
			return 0;
		}

		tcp_rst(c, s);
		return -1;
	}

	clock_gettime(CLOCK_MONOTONIC, &tc[s].last_ts_sock);
	tc[s].seq_from_tap = seq + len;

	return 0;
}

/**
 * tcp_check_dupack() - Check if given ACK number is duplicated, update counter
 * @s:		File descriptor number for socket
 * @ack_seq:	ACK sequence, host order
 *
 * Return: 1 on two duplicated ACKs observed, with counter reset, 0 otherwise
 */
static int tcp_check_dupack(int s, uint32_t ack_seq)
{
	if (ack_seq == tc[s].seq_ack_from_tap && ++tc[s].dup_acks == 2) {
		tc[s].dup_acks = 0;
		return 1;
	}

	return 0;
}

/**
 * tcp_sock_consume() - Consume (discard) data from socket buffer
 * @s:		File descriptor number for socket
 * @ack_seq:	ACK sequence, host order
 *
 * Return: -1 on invalid sequence, 0 otherwise
 */
static int tcp_sock_consume(int s, uint32_t ack_seq)
{
	int to_ack;

	/* Implicitly take care of wrap-arounds */
	to_ack = ack_seq - tc[s].seq_ack_from_tap;

	if (to_ack < 0)
		return -1;

	recv(s, NULL, to_ack, MSG_DONTWAIT | MSG_TRUNC);
	tc[s].seq_ack_from_tap = ack_seq;

	return 0;
}

/**
 * tcp_data_from_sock() - Handle new data from socket, queue to tap, in window
 * @c:		Execution context
 * @s:		File descriptor number for socket
 *
 * Return: non-zero on socket error or pending data, 0 otherwise
 */
static int tcp_data_from_sock(struct ctx *c, int s)
{
	int len, offset, left, send;

	/* Don't dequeue until acknowledged by guest */
	len = recv(s, tcp_in_buf, sizeof(tcp_in_buf), MSG_DONTWAIT | MSG_PEEK);
	if (len < 0) {
		if (errno != EAGAIN && errno != EWOULDBLOCK)
			tcp_rst(c, s);
		return 1;
	}

	if (len == 0) {
		if (tc[s].s >= ESTABLISHED_SOCK_FIN)
			return 0;

		tc[s].s = ESTABLISHED_SOCK_FIN;
		if (tcp_send_to_tap(c, s, FIN | ACK, NULL, 0))
			return 0;

		left = 0;
		goto out;
	}

	offset = tc[s].seq_to_tap - tc[s].seq_ack_from_tap;
	left = len - offset;
	while (left && offset + tc[s].mss_guest <= tc[s].tap_window) {
		if (left < tc[s].mss_guest)
			send = left;
		else
			send = tc[s].mss_guest;

		if (tcp_send_to_tap(c, s, 0, tcp_in_buf + offset, send))
			return 0;

		offset += send;
		left -= send;
	}

out:
	clock_gettime(CLOCK_MONOTONIC, &tc[s].last_ts_tap);
	tcp_act_slow_set(s);

	return !!left;
}

/**
 * tcp_tap_handler() - Handle packets from tap and state transitions
 * @c:		Execution context
 * @af:		Address family, AF_INET or AF_INET6
 * @in:		Input buffer
 * @len:	Length, including TCP header
 */
void tcp_tap_handler(struct ctx *c, int af, void *addr, char *in, size_t len)
{
	struct tcphdr *th = (struct tcphdr *)in;
	size_t off;
	int s, ws;

	if (len < sizeof(*th))
		return;

	off = th->doff * 4;
	if (off < sizeof(*th) || off > len)
		return;

	s = tcp_sock_lookup(af, addr, th->source, th->dest);

	if (s < 0) {
		if (th->syn)
			tcp_conn_from_tap(c, af, addr, th, len);
		return;
	}

	if (th->rst) {
		tcp_close_and_epoll_del(c, s);
		return;
	}

	tcp_clamp_window(s, th, len);

	if (th->ack)
		clock_gettime(CLOCK_MONOTONIC, &tc[s].last_ts_tap);

	switch (tc[s].s) {
	case SOCK_SYN_SENT:
		if (!th->syn || !th->ack)
			return;

		tc[s].mss_guest = tcp_opt_get(th, len, OPT_MSS, NULL, NULL);
		if (tc[s].mss_guest < 0)
			tc[s].mss_guest = MSS_DEFAULT;

		ws = tcp_opt_get(th, len, OPT_WS, NULL, NULL);
		if (ws > MAX_WS) {
			if (tcp_send_to_tap(c, s, RST, NULL, 0))
				return;

			tc[s].seq_to_tap = 0;
			tc[s].ws_allowed = 0;
			tcp_send_to_tap(c, s, SYN, NULL, 0);
			return;
		}

		tc[s].seq_from_tap = tc[s].seq_init_from_tap = ntohl(th->seq);
		tc[s].seq_ack_to_tap = tc[s].seq_from_tap;

		tc[s].s = ESTABLISHED;
		tcp_send_to_tap(c, s, ACK, NULL, 0);
		break;
	case TAP_SYN_SENT:
		break;
	case TAP_SYN_RCVD:
		if (th->fin) {
			shutdown(s, SHUT_WR);
			tc[s].s = FIN_WAIT_1;

			break;
		}

		if (!th->ack) {
			tcp_rst(c, s);
			return;
		}

		tc[s].seq_ack_from_tap = ntohl(th->ack_seq);

		tc[s].s = ESTABLISHED;
		break;
	case ESTABLISHED:
		if (th->ack) {
			int retrans = 0;

			if (len == th->doff)
				retrans = tcp_check_dupack(s, th->ack_seq);

			if (tcp_sock_consume(s, ntohl(th->ack_seq))) {
				tcp_rst(c, s);
				return;
			}

			if (retrans) {
				tc[s].seq_to_tap = tc[s].seq_ack_from_tap;
				tcp_data_from_sock(c, s);
			}
		}

		if (tcp_send_to_sock(c, s, ntohl(th->seq), in + off, len - off,
				     th->psh ? 0 : MSG_MORE))
			break;

		if (th->fin) {
			shutdown(s, SHUT_WR);
			tc[s].s = FIN_WAIT_1;
		}

		break;
	case ESTABLISHED_SOCK_FIN:
		if (tcp_send_to_sock(c, s, ntohl(th->seq), in + off, len - off,
				     th->psh ? 0 : MSG_MORE) < 0)
			break;

		if (th->ack) {
			shutdown(s, SHUT_RD);
			if (!tcp_data_from_sock(c, s))
				tc[s].s = CLOSE_WAIT;

			if (tcp_sock_consume(s, ntohl(th->ack_seq))) {
				tcp_rst(c, s);
				return;
			}
		}

		break;

	case CLOSE_WAIT:
		if (tcp_sock_consume(s, ntohl(th->ack_seq))) {
			tcp_rst(c, s);
			return;
		}

		if (th->fin) {
			shutdown(s, SHUT_WR);
			tc[s].s = LAST_ACK;
		}

		break;
	case FIN_WAIT_1:
	case LAST_ACK:
	case CLOSED:	/* ;) */
		break;
	}

	if (tc[s].seq_to_tap > tc[s].seq_ack_from_tap)
		tcp_act_slow_set(s);
	else
		tcp_act_slow_clear(s);

	if (tc[s].seq_from_tap > tc[s].seq_ack_to_tap)
		tcp_act_fast_set(s);
	else
		tcp_act_fast_clear(s);
}

/**
 * tcp_connect_finish() - Handle completion of connect() from EPOLLOUT event
 * @c:		Execution context
 * @s:		File descriptor number for socket
 */
static void tcp_connect_finish(struct ctx *c, int s)
{
	struct epoll_event ev = { 0 };
	socklen_t sl;
	int so;

	sl = sizeof(so);
	if (getsockopt(s, SOL_SOCKET, SO_ERROR, &so, &sl) || so) {
		tcp_rst(c, s);
		return;
	}

	if (tcp_send_to_tap(c, s, SYN | ACK, NULL, 0) < 0)
		return;

	ev.events = EPOLLIN | EPOLLET | EPOLLRDHUP | EPOLLERR | EPOLLHUP;
	ev.data.fd = s;
	epoll_ctl(c->epollfd, EPOLL_CTL_MOD, s, &ev);

	tc[s].s = TAP_SYN_RCVD;
}

/**
 * tcp_sock_handler() - Handle new data from socket
 * @c:		Execution context
 * @s:		File descriptor number for socket
 * @events:	epoll events bitmap
 */
void tcp_sock_handler(struct ctx *c, int s, uint32_t events)
{
	socklen_t sl;
	int so;

	if (tc[s].s == LAST_ACK) {
		tcp_close_and_epoll_del(c, s);
		return;
	}

	sl = sizeof(so);
	if ((events & EPOLLERR) ||
	    getsockopt(s, SOL_SOCKET, SO_ACCEPTCONN, &so, &sl)) {
		if (tc[s].s != CLOSED)
			tcp_rst(c, s);
		return;
	}

	if (so) {
		tcp_conn_from_sock(c, s);
		return;
	}

	if (events & EPOLLOUT) {	/* Implies TAP_SYN_SENT */
		tcp_connect_finish(c, s);
		return;
	}

	if (tc[s].s == ESTABLISHED)
		tcp_data_from_sock(c, s);

	if (events & EPOLLRDHUP || events & EPOLLHUP) {
		if (tc[s].s == ESTABLISHED)
			tc[s].s = ESTABLISHED_SOCK_FIN;

		tcp_send_to_tap(c, s, FIN | ACK, NULL, 0);

		if (tc[s].s == FIN_WAIT_1) {
			shutdown(s, SHUT_RD);

			if (tcp_sock_consume(s, ntohl(tc[s].seq_ack_from_tap))) {
				tcp_rst(c, s);
				return;
			}

			tcp_close_and_epoll_del(c, s);
			tc[s].s = CLOSED;
		}
	}
}

/**
 * tcp_sock_init() - Create and bind listening sockets for inbound connections
 * @c:		Execution context
 *
 * Return: 0 on success, -1 on failure
 */
int tcp_sock_init(struct ctx *c)
{
	in_port_t port;

	for (port = 0; port < (1 << 15) + (1 << 14); port++) {
		if (c->v4 && sock_l4_add(c, 4, IPPROTO_TCP, htons(port)) < 0)
			return -1;
		if (c->v6 && sock_l4_add(c, 6, IPPROTO_TCP, htons(port)) < 0)
			return -1;
	}

	return 0;
}

/**
 * tcp_periodic_fast_one() - Handler for "fast" timeout events on one socket
 * @c:		Execution context
 * @s:		File descriptor number for socket
 * @ts:		Timestamp from caller
 *
 * Return: 0 if socket needs to be monitored further, non-zero otherwise
 */
int tcp_periodic_fast_one(struct ctx *c, int s, struct timespec *ts)
{
	if (timespec_diff_ms(ts, &tc[s].last_ts_sock) < SOCK_ACK_INTERVAL)
		return 0;

	tc[s].last_ts_sock = *ts;

	tcp_send_to_tap(c, s, 0, NULL, 0);

	return tc[s].seq_from_tap == tc[s].seq_ack_to_tap;
}

/**
 * tcp_periodic_fast() - Handle sockets in "fast" event bitmap, clear as needed
 * @c:		Execution context
 */
void tcp_periodic_fast(struct ctx *c)
{
	long *word = (long *)tcp_act_fast, tmp;
	struct timespec now;
	unsigned int i;
	int n, s;

	clock_gettime(CLOCK_MONOTONIC, &now);

	for (i = 0; i < sizeof(tcp_act_fast) / sizeof(long); i++, word++) {
		tmp = *word;
		while ((n = ffsl(tmp))) {
			tmp &= ~(1UL << (n - 1));

			s = i * sizeof(long) * 8 + n - 1;

			if (tcp_periodic_fast_one(c, s, &now))
				*word &= ~(1UL << (n - 1));
		}
	}
}

/**
 * tcp_periodic_fast_one() - Handler for "slow" timeout events on one socket
 * @c:		Execution context
 * @s:		File descriptor number for socket
 * @ts:		Timestamp from caller
 */
void tcp_periodic_slow_one(struct ctx *c, int s, struct timespec *ts)
{
	switch (tc[s].s) {
	case SOCK_SYN_SENT:
	case TAP_SYN_SENT:
	case TAP_SYN_RCVD:
		if (timespec_diff_ms(ts, &tc[s].last_ts_tap) > SYN_TIMEOUT)
			tcp_rst(c, s);
		break;
	case ESTABLISHED_SOCK_FIN:
		if (timespec_diff_ms(ts, &tc[s].last_ts_tap) > FIN_TIMEOUT) {
			tcp_rst(c, s);
			break;
		}
		/* Falls through */
	case ESTABLISHED:
		if (tc[s].seq_ack_from_tap < tc[s].seq_to_tap &&
		    timespec_diff_ms(ts, &tc[s].last_ts_tap) > ACK_TIMEOUT) {
			tc[s].seq_to_tap = tc[s].seq_ack_from_tap;
			tcp_data_from_sock(c, s);
		}

		if (timespec_diff_ms(ts, &tc[s].last_ts_tap) > ACT_TIMEOUT &&
		    timespec_diff_ms(ts, &tc[s].last_ts_sock) > ACT_TIMEOUT)
			tcp_rst(c, s);

		break;
	case CLOSE_WAIT:
	case FIN_WAIT_1:
		if (timespec_diff_ms(ts, &tc[s].last_ts_tap) > FIN_TIMEOUT)
			tcp_rst(c, s);
		break;
	case LAST_ACK:
		if (timespec_diff_ms(ts, &tc[s].last_ts_sock) >
		    LAST_ACK_TIMEOUT)
			tcp_rst(c, s);
		break;
	case CLOSED:
		break;
	}
}

/**
 * tcp_periodic_slow() - Handle sockets in "slow" event bitmap
 * @c:		Execution context
 */
void tcp_periodic_slow(struct ctx *c)
{
	long *word = (long *)tcp_act_slow, tmp;
	struct timespec now;
	unsigned int i;
	int n;

	clock_gettime(CLOCK_MONOTONIC, &now);

	for (i = 0; i < sizeof(tcp_act_slow) / sizeof(long); i++, word++) {
		tmp = *word;
		while ((n = ffsl(tmp))) {
			tmp &= ~(1UL << (n - 1));
			tcp_periodic_slow_one(c, i * sizeof(long) * 8 + n - 1,
					      &now);
		}
	}
}