summaryrefslogtreecommitdiffstats
path: root/net/ipv4/tcp_bbr.c
blob: dbcc9352a48f07a12484e45f3baf0a733e244f75 (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
/* Bottleneck Bandwidth and RTT (BBR) congestion control
 *
 * BBR congestion control computes the sending rate based on the delivery
 * rate (throughput) estimated from ACKs. In a nutshell:
 *
 *   On each ACK, update our model of the network path:
 *      bottleneck_bandwidth = windowed_max(delivered / elapsed, 10 round trips)
 *      min_rtt = windowed_min(rtt, 10 seconds)
 *   pacing_rate = pacing_gain * bottleneck_bandwidth
 *   cwnd = max(cwnd_gain * bottleneck_bandwidth * min_rtt, 4)
 *
 * The core algorithm does not react directly to packet losses or delays,
 * although BBR may adjust the size of next send per ACK when loss is
 * observed, or adjust the sending rate if it estimates there is a
 * traffic policer, in order to keep the drop rate reasonable.
 *
 * Here is a state transition diagram for BBR:
 *
 *             |
 *             V
 *    +---> STARTUP  ----+
 *    |        |         |
 *    |        V         |
 *    |      DRAIN   ----+
 *    |        |         |
 *    |        V         |
 *    +---> PROBE_BW ----+
 *    |      ^    |      |
 *    |      |    |      |
 *    |      +----+      |
 *    |                  |
 *    +---- PROBE_RTT <--+
 *
 * A BBR flow starts in STARTUP, and ramps up its sending rate quickly.
 * When it estimates the pipe is full, it enters DRAIN to drain the queue.
 * In steady state a BBR flow only uses PROBE_BW and PROBE_RTT.
 * A long-lived BBR flow spends the vast majority of its time remaining
 * (repeatedly) in PROBE_BW, fully probing and utilizing the pipe's bandwidth
 * in a fair manner, with a small, bounded queue. *If* a flow has been
 * continuously sending for the entire min_rtt window, and hasn't seen an RTT
 * sample that matches or decreases its min_rtt estimate for 10 seconds, then
 * it briefly enters PROBE_RTT to cut inflight to a minimum value to re-probe
 * the path's two-way propagation delay (min_rtt). When exiting PROBE_RTT, if
 * we estimated that we reached the full bw of the pipe then we enter PROBE_BW;
 * otherwise we enter STARTUP to try to fill the pipe.
 *
 * BBR is described in detail in:
 *   "BBR: Congestion-Based Congestion Control",
 *   Neal Cardwell, Yuchung Cheng, C. Stephen Gunn, Soheil Hassas Yeganeh,
 *   Van Jacobson. ACM Queue, Vol. 14 No. 5, September-October 2016.
 *
 * There is a public e-mail list for discussing BBR development and testing:
 *   https://groups.google.com/forum/#!forum/bbr-dev
 *
 * NOTE: BBR might be used with the fq qdisc ("man tc-fq") with pacing enabled,
 * otherwise TCP stack falls back to an internal pacing using one high
 * resolution timer per TCP socket and may use more resources.
 */
#include <linux/module.h>
#include <net/tcp.h>
#include <linux/inet_diag.h>
#include <linux/inet.h>
#include <linux/random.h>
#include <linux/win_minmax.h>

/* Scale factor for rate in pkt/uSec unit to avoid truncation in bandwidth
 * estimation. The rate unit ~= (1500 bytes / 1 usec / 2^24) ~= 715 bps.
 * This handles bandwidths from 0.06pps (715bps) to 256Mpps (3Tbps) in a u32.
 * Since the minimum window is >=4 packets, the lower bound isn't
 * an issue. The upper bound isn't an issue with existing technologies.
 */
#define BW_SCALE 24
#define BW_UNIT (1 << BW_SCALE)

#define BBR_SCALE 8	/* scaling factor for fractions in BBR (e.g. gains) */
#define BBR_UNIT (1 << BBR_SCALE)

/* BBR has the following modes for deciding how fast to send: */
enum bbr_mode {
	BBR_STARTUP,	/* ramp up sending rate rapidly to fill pipe */
	BBR_DRAIN,	/* drain any queue created during startup */
	BBR_PROBE_BW,	/* discover, share bw: pace around estimated bw */
	BBR_PROBE_RTT,	/* cut inflight to min to probe min_rtt */
};

/* BBR congestion control block */
struct bbr {
	u32	min_rtt_us;	        /* min RTT in min_rtt_win_sec window */
	u32	min_rtt_stamp;	        /* timestamp of min_rtt_us */
	u32	probe_rtt_done_stamp;   /* end time for BBR_PROBE_RTT mode */
	struct minmax bw;	/* Max recent delivery rate in pkts/uS << 24 */
	u32	rtt_cnt;	    /* count of packet-timed rounds elapsed */
	u32     next_rtt_delivered; /* scb->tx.delivered at end of round */
	u64	cycle_mstamp;	     /* time of this cycle phase start */
	u32     mode:3,		     /* current bbr_mode in state machine */
		prev_ca_state:3,     /* CA state on previous ACK */
		packet_conservation:1,  /* use packet conservation? */
		restore_cwnd:1,	     /* decided to revert cwnd to old value */
		round_start:1,	     /* start of packet-timed tx->ack round? */
		tso_segs_goal:7,     /* segments we want in each skb we send */
		idle_restart:1,	     /* restarting after idle? */
		probe_rtt_round_done:1,  /* a BBR_PROBE_RTT round at 4 pkts? */
		unused:5,
		lt_is_sampling:1,    /* taking long-term ("LT") samples now? */
		lt_rtt_cnt:7,	     /* round trips in long-term interval */
		lt_use_bw:1;	     /* use lt_bw as our bw estimate? */
	u32	lt_bw;		     /* LT est delivery rate in pkts/uS << 24 */
	u32	lt_last_delivered;   /* LT intvl start: tp->delivered */
	u32	lt_last_stamp;	     /* LT intvl start: tp->delivered_mstamp */
	u32	lt_last_lost;	     /* LT intvl start: tp->lost */
	u32	pacing_gain:10,	/* current gain for setting pacing rate */
		cwnd_gain:10,	/* current gain for setting cwnd */
		full_bw_cnt:3,	/* number of rounds without large bw gains */
		cycle_idx:3,	/* current index in pacing_gain cycle array */
		unused_b:6;
	u32	prior_cwnd;	/* prior cwnd upon entering loss recovery */
	u32	full_bw;	/* recent bw, to estimate if pipe is full */
};

#define CYCLE_LEN	8	/* number of phases in a pacing gain cycle */

/* Window length of bw filter (in rounds): */
static const int bbr_bw_rtts = CYCLE_LEN + 2;
/* Window length of min_rtt filter (in sec): */
static const u32 bbr_min_rtt_win_sec = 10;
/* Minimum time (in ms) spent at bbr_cwnd_min_target in BBR_PROBE_RTT mode: */
static const u32 bbr_probe_rtt_mode_ms = 200;
/* Skip TSO below the following bandwidth (bits/sec): */
static const int bbr_min_tso_rate = 1200000;

/* We use a high_gain value of 2/ln(2) because it's the smallest pacing gain
 * that will allow a smoothly increasing pacing rate that will double each RTT
 * and send the same number of packets per RTT that an un-paced, slow-starting
 * Reno or CUBIC flow would:
 */
static const int bbr_high_gain  = BBR_UNIT * 2885 / 1000 + 1;
/* The pacing gain of 1/high_gain in BBR_DRAIN is calculated to typically drain
 * the queue created in BBR_STARTUP in a single round:
 */
static const int bbr_drain_gain = BBR_UNIT * 1000 / 2885;
/* The gain for deriving steady-state cwnd tolerates delayed/stretched ACKs: */
static const int bbr_cwnd_gain  = BBR_UNIT * 2;
/* The pacing_gain values for the PROBE_BW gain cycle, to discover/share bw: */
static const int bbr_pacing_gain[] = {
	BBR_UNIT * 5 / 4,	/* probe for more available bw */
	BBR_UNIT * 3 / 4,	/* drain queue and/or yield bw to other flows */
	BBR_UNIT, BBR_UNIT, BBR_UNIT,	/* cruise at 1.0*bw to utilize pipe, */
	BBR_UNIT, BBR_UNIT, BBR_UNIT	/* without creating excess queue... */
};
/* Randomize the starting gain cycling phase over N phases: */
static const u32 bbr_cycle_rand = 7;

/* Try to keep at least this many packets in flight, if things go smoothly. For
 * smooth functioning, a sliding window protocol ACKing every other packet
 * needs at least 4 packets in flight:
 */
static const u32 bbr_cwnd_min_target = 4;

/* To estimate if BBR_STARTUP mode (i.e. high_gain) has filled pipe... */
/* If bw has increased significantly (1.25x), there may be more bw available: */
static const u32 bbr_full_bw_thresh = BBR_UNIT * 5 / 4;
/* But after 3 rounds w/o significant bw growth, estimate pipe is full: */
static const u32 bbr_full_bw_cnt = 3;

/* "long-term" ("LT") bandwidth estimator parameters... */
/* The minimum number of rounds in an LT bw sampling interval: */
static const u32 bbr_lt_intvl_min_rtts = 4;
/* If lost/delivered ratio > 20%, interval is "lossy" and we may be policed: */
static const u32 bbr_lt_loss_thresh = 50;
/* If 2 intervals have a bw ratio <= 1/8, their bw is "consistent": */
static const u32 bbr_lt_bw_ratio = BBR_UNIT / 8;
/* If 2 intervals have a bw diff <= 4 Kbit/sec their bw is "consistent": */
static const u32 bbr_lt_bw_diff = 4000 / 8;
/* If we estimate we're policed, use lt_bw for this many round trips: */
static const u32 bbr_lt_bw_max_rtts = 48;

/* Do we estimate that STARTUP filled the pipe? */
static bool bbr_full_bw_reached(const struct sock *sk)
{
	const struct bbr *bbr = inet_csk_ca(sk);

	return bbr->full_bw_cnt >= bbr_full_bw_cnt;
}

/* Return the windowed max recent bandwidth sample, in pkts/uS << BW_SCALE. */
static u32 bbr_max_bw(const struct sock *sk)
{
	struct bbr *bbr = inet_csk_ca(sk);

	return minmax_get(&bbr->bw);
}

/* Return the estimated bandwidth of the path, in pkts/uS << BW_SCALE. */
static u32 bbr_bw(const struct sock *sk)
{
	struct bbr *bbr = inet_csk_ca(sk);

	return bbr->lt_use_bw ? bbr->lt_bw : bbr_max_bw(sk);
}

/* Return rate in bytes per second, optionally with a gain.
 * The order here is chosen carefully to avoid overflow of u64. This should
 * work for input rates of up to 2.9Tbit/sec and gain of 2.89x.
 */
static u64 bbr_rate_bytes_per_sec(struct sock *sk, u64 rate, int gain)
{
	rate *= tcp_mss_to_mtu(sk, tcp_sk(sk)->mss_cache);
	rate *= gain;
	rate >>= BBR_SCALE;
	rate *= USEC_PER_SEC;
	return rate >> BW_SCALE;
}

/* Pace using current bw estimate and a gain factor. In order to help drive the
 * network toward lower queues while maintaining high utilization and low
 * latency, the average pacing rate aims to be slightly (~1%) lower than the
 * estimated bandwidth. This is an important aspect of the design. In this
 * implementation this slightly lower pacing rate is achieved implicitly by not
 * including link-layer headers in the packet size used for the pacing rate.
 */
static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain)
{
	struct bbr *bbr = inet_csk_ca(sk);
	u64 rate = bw;

	rate = bbr_rate_bytes_per_sec(sk, rate, gain);
	rate = min_t(u64, rate, sk->sk_max_pacing_rate);
	if (bbr->mode != BBR_STARTUP || rate > sk->sk_pacing_rate)
		sk->sk_pacing_rate = rate;
}

/* Return count of segments we want in the skbs we send, or 0 for default. */
static u32 bbr_tso_segs_goal(struct sock *sk)
{
	struct bbr *bbr = inet_csk_ca(sk);

	return bbr->tso_segs_goal;
}

static void bbr_set_tso_segs_goal(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);
	u32 min_segs;

	min_segs = sk->sk_pacing_rate < (bbr_min_tso_rate >> 3) ? 1 : 2;
	bbr->tso_segs_goal = min(tcp_tso_autosize(sk, tp->mss_cache, min_segs),
				 0x7FU);
}

/* Save "last known good" cwnd so we can restore it after losses or PROBE_RTT */
static void bbr_save_cwnd(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);

	if (bbr->prev_ca_state < TCP_CA_Recovery && bbr->mode != BBR_PROBE_RTT)
		bbr->prior_cwnd = tp->snd_cwnd;  /* this cwnd is good enough */
	else  /* loss recovery or BBR_PROBE_RTT have temporarily cut cwnd */
		bbr->prior_cwnd = max(bbr->prior_cwnd, tp->snd_cwnd);
}

static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);

	if (event == CA_EVENT_TX_START && tp->app_limited) {
		bbr->idle_restart = 1;
		/* Avoid pointless buffer overflows: pace at est. bw if we don't
		 * need more speed (we're restarting from idle and app-limited).
		 */
		if (bbr->mode == BBR_PROBE_BW)
			bbr_set_pacing_rate(sk, bbr_bw(sk), BBR_UNIT);
	}
}

/* Find target cwnd. Right-size the cwnd based on min RTT and the
 * estimated bottleneck bandwidth:
 *
 * cwnd = bw * min_rtt * gain = BDP * gain
 *
 * The key factor, gain, controls the amount of queue. While a small gain
 * builds a smaller queue, it becomes more vulnerable to noise in RTT
 * measurements (e.g., delayed ACKs or other ACK compression effects). This
 * noise may cause BBR to under-estimate the rate.
 *
 * To achieve full performance in high-speed paths, we budget enough cwnd to
 * fit full-sized skbs in-flight on both end hosts to fully utilize the path:
 *   - one skb in sending host Qdisc,
 *   - one skb in sending host TSO/GSO engine
 *   - one skb being received by receiver host LRO/GRO/delayed-ACK engine
 * Don't worry, at low rates (bbr_min_tso_rate) this won't bloat cwnd because
 * in such cases tso_segs_goal is 1. The minimum cwnd is 4 packets,
 * which allows 2 outstanding 2-packet sequences, to try to keep pipe
 * full even with ACK-every-other-packet delayed ACKs.
 */
static u32 bbr_target_cwnd(struct sock *sk, u32 bw, int gain)
{
	struct bbr *bbr = inet_csk_ca(sk);
	u32 cwnd;
	u64 w;

	/* If we've never had a valid RTT sample, cap cwnd at the initial
	 * default. This should only happen when the connection is not using TCP
	 * timestamps and has retransmitted all of the SYN/SYNACK/data packets
	 * ACKed so far. In this case, an RTO can cut cwnd to 1, in which
	 * case we need to slow-start up toward something safe: TCP_INIT_CWND.
	 */
	if (unlikely(bbr->min_rtt_us == ~0U))	 /* no valid RTT samples yet? */
		return TCP_INIT_CWND;  /* be safe: cap at default initial cwnd*/

	w = (u64)bw * bbr->min_rtt_us;

	/* Apply a gain to the given value, then remove the BW_SCALE shift. */
	cwnd = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT;

	/* Allow enough full-sized skbs in flight to utilize end systems. */
	cwnd += 3 * bbr->tso_segs_goal;

	/* Reduce delayed ACKs by rounding up cwnd to the next even number. */
	cwnd = (cwnd + 1) & ~1U;

	return cwnd;
}

/* An optimization in BBR to reduce losses: On the first round of recovery, we
 * follow the packet conservation principle: send P packets per P packets acked.
 * After that, we slow-start and send at most 2*P packets per P packets acked.
 * After recovery finishes, or upon undo, we restore the cwnd we had when
 * recovery started (capped by the target cwnd based on estimated BDP).
 *
 * TODO(ycheng/ncardwell): implement a rate-based approach.
 */
static bool bbr_set_cwnd_to_recover_or_restore(
	struct sock *sk, const struct rate_sample *rs, u32 acked, u32 *new_cwnd)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);
	u8 prev_state = bbr->prev_ca_state, state = inet_csk(sk)->icsk_ca_state;
	u32 cwnd = tp->snd_cwnd;

	/* An ACK for P pkts should release at most 2*P packets. We do this
	 * in two steps. First, here we deduct the number of lost packets.
	 * Then, in bbr_set_cwnd() we slow start up toward the target cwnd.
	 */
	if (rs->losses > 0)
		cwnd = max_t(s32, cwnd - rs->losses, 1);

	if (state == TCP_CA_Recovery && prev_state != TCP_CA_Recovery) {
		/* Starting 1st round of Recovery, so do packet conservation. */
		bbr->packet_conservation = 1;
		bbr->next_rtt_delivered = tp->delivered;  /* start round now */
		/* Cut unused cwnd from app behavior, TSQ, or TSO deferral: */
		cwnd = tcp_packets_in_flight(tp) + acked;
	} else if (prev_state >= TCP_CA_Recovery && state < TCP_CA_Recovery) {
		/* Exiting loss recovery; restore cwnd saved before recovery. */
		bbr->restore_cwnd = 1;
		bbr->packet_conservation = 0;
	}
	bbr->prev_ca_state = state;

	if (bbr->restore_cwnd) {
		/* Restore cwnd after exiting loss recovery or PROBE_RTT. */
		cwnd = max(cwnd, bbr->prior_cwnd);
		bbr->restore_cwnd = 0;
	}

	if (bbr->packet_conservation) {
		*new_cwnd = max(cwnd, tcp_packets_in_flight(tp) + acked);
		return true;	/* yes, using packet conservation */
	}
	*new_cwnd = cwnd;
	return false;
}

/* Slow-start up toward target cwnd (if bw estimate is growing, or packet loss
 * has drawn us down below target), or snap down to target if we're above it.
 */
static void bbr_set_cwnd(struct sock *sk, const struct rate_sample *rs,
			 u32 acked, u32 bw, int gain)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);
	u32 cwnd = 0, target_cwnd = 0;

	if (!acked)
		return;

	if (bbr_set_cwnd_to_recover_or_restore(sk, rs, acked, &cwnd))
		goto done;

	/* If we're below target cwnd, slow start cwnd toward target cwnd. */
	target_cwnd = bbr_target_cwnd(sk, bw, gain);
	if (bbr_full_bw_reached(sk))  /* only cut cwnd if we filled the pipe */
		cwnd = min(cwnd + acked, target_cwnd);
	else if (cwnd < target_cwnd || tp->delivered < TCP_INIT_CWND)
		cwnd = cwnd + acked;
	cwnd = max(cwnd, bbr_cwnd_min_target);

done:
	tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);	/* apply global cap */
	if (bbr->mode == BBR_PROBE_RTT)  /* drain queue, refresh min_rtt */
		tp->snd_cwnd = min(tp->snd_cwnd, bbr_cwnd_min_target);
}

/* End cycle phase if it's time and/or we hit the phase's in-flight target. */
static bool bbr_is_next_cycle_phase(struct sock *sk,
				    const struct rate_sample *rs)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);
	bool is_full_length =
		tcp_stamp_us_delta(tp->delivered_mstamp, bbr->cycle_mstamp) >
		bbr->min_rtt_us;
	u32 inflight, bw;

	/* The pacing_gain of 1.0 paces at the estimated bw to try to fully
	 * use the pipe without increasing the queue.
	 */
	if (bbr->pacing_gain == BBR_UNIT)
		return is_full_length;		/* just use wall clock time */

	inflight = rs->prior_in_flight;  /* what was in-flight before ACK? */
	bw = bbr_max_bw(sk);

	/* A pacing_gain > 1.0 probes for bw by trying to raise inflight to at
	 * least pacing_gain*BDP; this may take more than min_rtt if min_rtt is
	 * small (e.g. on a LAN). We do not persist if packets are lost, since
	 * a path with small buffers may not hold that much.
	 */
	if (bbr->pacing_gain > BBR_UNIT)
		return is_full_length &&
			(rs->losses ||  /* perhaps pacing_gain*BDP won't fit */
			 inflight >= bbr_target_cwnd(sk, bw, bbr->pacing_gain));

	/* A pacing_gain < 1.0 tries to drain extra queue we added if bw
	 * probing didn't find more bw. If inflight falls to match BDP then we
	 * estimate queue is drained; persisting would underutilize the pipe.
	 */
	return is_full_length ||
		inflight <= bbr_target_cwnd(sk, bw, BBR_UNIT);
}

static void bbr_advance_cycle_phase(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);

	bbr->cycle_idx = (bbr->cycle_idx + 1) & (CYCLE_LEN - 1);
	bbr->cycle_mstamp = tp->delivered_mstamp;
	bbr->pacing_gain = bbr_pacing_gain[bbr->cycle_idx];
}

/* Gain cycling: cycle pacing gain to converge to fair share of available bw. */
static void bbr_update_cycle_phase(struct sock *sk,
				   const struct rate_sample *rs)
{
	struct bbr *bbr = inet_csk_ca(sk);

	if ((bbr->mode == BBR_PROBE_BW) && !bbr->lt_use_bw &&
	    bbr_is_next_cycle_phase(sk, rs))
		bbr_advance_cycle_phase(sk);
}

static void bbr_reset_startup_mode(struct sock *sk)
{
	struct bbr *bbr = inet_csk_ca(sk);

	bbr->mode = BBR_STARTUP;
	bbr->pacing_gain = bbr_high_gain;
	bbr->cwnd_gain	 = bbr_high_gain;
}

static void bbr_reset_probe_bw_mode(struct sock *sk)
{
	struct bbr *bbr = inet_csk_ca(sk);

	bbr->mode = BBR_PROBE_BW;
	bbr->pacing_gain = BBR_UNIT;
	bbr->cwnd_gain = bbr_cwnd_gain;
	bbr->cycle_idx = CYCLE_LEN - 1 - prandom_u32_max(bbr_cycle_rand);
	bbr_advance_cycle_phase(sk);	/* flip to next phase of gain cycle */
}

static void bbr_reset_mode(struct sock *sk)
{
	if (!bbr_full_bw_reached(sk))
		bbr_reset_startup_mode(sk);
	else
		bbr_reset_probe_bw_mode(sk);
}

/* Start a new long-term sampling interval. */
static void bbr_reset_lt_bw_sampling_interval(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);

	bbr->lt_last_stamp = div_u64(tp->delivered_mstamp, USEC_PER_MSEC);
	bbr->lt_last_delivered = tp->delivered;
	bbr->lt_last_lost = tp->lost;
	bbr->lt_rtt_cnt = 0;
}

/* Completely reset long-term bandwidth sampling. */
static void bbr_reset_lt_bw_sampling(struct sock *sk)
{
	struct bbr *bbr = inet_csk_ca(sk);

	bbr->lt_bw = 0;
	bbr->lt_use_bw = 0;
	bbr->lt_is_sampling = false;
	bbr_reset_lt_bw_sampling_interval(sk);
}

/* Long-term bw sampling interval is done. Estimate whether we're policed. */
static void bbr_lt_bw_interval_done(struct sock *sk, u32 bw)
{
	struct bbr *bbr = inet_csk_ca(sk);
	u32 diff;

	if (bbr->lt_bw) {  /* do we have bw from a previous interval? */
		/* Is new bw close to the lt_bw from the previous interval? */
		diff = abs(bw - bbr->lt_bw);
		if ((diff * BBR_UNIT <= bbr_lt_bw_ratio * bbr->lt_bw) ||
		    (bbr_rate_bytes_per_sec(sk, diff, BBR_UNIT) <=
		     bbr_lt_bw_diff)) {
			/* All criteria are met; estimate we're policed. */
			bbr->lt_bw = (bw + bbr->lt_bw) >> 1;  /* avg 2 intvls */
			bbr->lt_use_bw = 1;
			bbr->pacing_gain = BBR_UNIT;  /* try to avoid drops */
			bbr->lt_rtt_cnt = 0;
			return;
		}
	}
	bbr->lt_bw = bw;
	bbr_reset_lt_bw_sampling_interval(sk);
}

/* Token-bucket traffic policers are common (see "An Internet-Wide Analysis of
 * Traffic Policing", SIGCOMM 2016). BBR detects token-bucket policers and
 * explicitly models their policed rate, to reduce unnecessary losses. We
 * estimate that we're policed if we see 2 consecutive sampling intervals with
 * consistent throughput and high packet loss. If we think we're being policed,
 * set lt_bw to the "long-term" average delivery rate from those 2 intervals.
 */
static void bbr_lt_bw_sampling(struct sock *sk, const struct rate_sample *rs)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);
	u32 lost, delivered;
	u64 bw;
	u32 t;

	if (bbr->lt_use_bw) {	/* already using long-term rate, lt_bw? */
		if (bbr->mode == BBR_PROBE_BW && bbr->round_start &&
		    ++bbr->lt_rtt_cnt >= bbr_lt_bw_max_rtts) {
			bbr_reset_lt_bw_sampling(sk);    /* stop using lt_bw */
			bbr_reset_probe_bw_mode(sk);  /* restart gain cycling */
		}
		return;
	}

	/* Wait for the first loss before sampling, to let the policer exhaust
	 * its tokens and estimate the steady-state rate allowed by the policer.
	 * Starting samples earlier includes bursts that over-estimate the bw.
	 */
	if (!bbr->lt_is_sampling) {
		if (!rs->losses)
			return;
		bbr_reset_lt_bw_sampling_interval(sk);
		bbr->lt_is_sampling = true;
	}

	/* To avoid underestimates, reset sampling if we run out of data. */
	if (rs->is_app_limited) {
		bbr_reset_lt_bw_sampling(sk);
		return;
	}

	if (bbr->round_start)
		bbr->lt_rtt_cnt++;	/* count round trips in this interval */
	if (bbr->lt_rtt_cnt < bbr_lt_intvl_min_rtts)
		return;		/* sampling interval needs to be longer */
	if (bbr->lt_rtt_cnt > 4 * bbr_lt_intvl_min_rtts) {
		bbr_reset_lt_bw_sampling(sk);  /* interval is too long */
		return;
	}

	/* End sampling interval when a packet is lost, so we estimate the
	 * policer tokens were exhausted. Stopping the sampling before the
	 * tokens are exhausted under-estimates the policed rate.
	 */
	if (!rs->losses)
		return;

	/* Calculate packets lost and delivered in sampling interval. */
	lost = tp->lost - bbr->lt_last_lost;
	delivered = tp->delivered - bbr->lt_last_delivered;
	/* Is loss rate (lost/delivered) >= lt_loss_thresh? If not, wait. */
	if (!delivered || (lost << BBR_SCALE) < bbr_lt_loss_thresh * delivered)
		return;

	/* Find average delivery rate in this sampling interval. */
	t = div_u64(tp->delivered_mstamp, USEC_PER_MSEC) - bbr->lt_last_stamp;
	if ((s32)t < 1)
		return;		/* interval is less than one ms, so wait */
	/* Check if can multiply without overflow */
	if (t >= ~0U / USEC_PER_MSEC) {
		bbr_reset_lt_bw_sampling(sk);  /* interval too long; reset */
		return;
	}
	t *= USEC_PER_MSEC;
	bw = (u64)delivered * BW_UNIT;
	do_div(bw, t);
	bbr_lt_bw_interval_done(sk, bw);
}

/* Estimate the bandwidth based on how fast packets are delivered */
static void bbr_update_bw(struct sock *sk, const struct rate_sample *rs)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);
	u64 bw;

	bbr->round_start = 0;
	if (rs->delivered < 0 || rs->interval_us <= 0)
		return; /* Not a valid observation */

	/* See if we've reached the next RTT */
	if (!before(rs->prior_delivered, bbr->next_rtt_delivered)) {
		bbr->next_rtt_delivered = tp->delivered;
		bbr->rtt_cnt++;
		bbr->round_start = 1;
		bbr->packet_conservation = 0;
	}

	bbr_lt_bw_sampling(sk, rs);

	/* Divide delivered by the interval to find a (lower bound) bottleneck
	 * bandwidth sample. Delivered is in packets and interval_us in uS and
	 * ratio will be <<1 for most connections. So delivered is first scaled.
	 */
	bw = (u64)rs->delivered * BW_UNIT;
	do_div(bw, rs->interval_us);

	/* If this sample is application-limited, it is likely to have a very
	 * low delivered count that represents application behavior rather than
	 * the available network rate. Such a sample could drag down estimated
	 * bw, causing needless slow-down. Thus, to continue to send at the
	 * last measured network rate, we filter out app-limited samples unless
	 * they describe the path bw at least as well as our bw model.
	 *
	 * So the goal during app-limited phase is to proceed with the best
	 * network rate no matter how long. We automatically leave this
	 * phase when app writes faster than the network can deliver :)
	 */
	if (!rs->is_app_limited || bw >= bbr_max_bw(sk)) {
		/* Incorporate new sample into our max bw filter. */
		minmax_running_max(&bbr->bw, bbr_bw_rtts, bbr->rtt_cnt, bw);
	}
}

/* Estimate when the pipe is full, using the change in delivery rate: BBR
 * estimates that STARTUP filled the pipe if the estimated bw hasn't changed by
 * at least bbr_full_bw_thresh (25%) after bbr_full_bw_cnt (3) non-app-limited
 * rounds. Why 3 rounds: 1: rwin autotuning grows the rwin, 2: we fill the
 * higher rwin, 3: we get higher delivery rate samples. Or transient
 * cross-traffic or radio noise can go away. CUBIC Hystart shares a similar
 * design goal, but uses delay and inter-ACK spacing instead of bandwidth.
 */
static void bbr_check_full_bw_reached(struct sock *sk,
				      const struct rate_sample *rs)
{
	struct bbr *bbr = inet_csk_ca(sk);
	u32 bw_thresh;

	if (bbr_full_bw_reached(sk) || !bbr->round_start || rs->is_app_limited)
		return;

	bw_thresh = (u64)bbr->full_bw * bbr_full_bw_thresh >> BBR_SCALE;
	if (bbr_max_bw(sk) >= bw_thresh) {
		bbr->full_bw = bbr_max_bw(sk);
		bbr->full_bw_cnt = 0;
		return;
	}
	++bbr->full_bw_cnt;
}

/* If pipe is probably full, drain the queue and then enter steady-state. */
static void bbr_check_drain(struct sock *sk, const struct rate_sample *rs)
{
	struct bbr *bbr = inet_csk_ca(sk);

	if (bbr->mode == BBR_STARTUP && bbr_full_bw_reached(sk)) {
		bbr->mode = BBR_DRAIN;	/* drain queue we created */
		bbr->pacing_gain = bbr_drain_gain;	/* pace slow to drain */
		bbr->cwnd_gain = bbr_high_gain;	/* maintain cwnd */
	}	/* fall through to check if in-flight is already small: */
	if (bbr->mode == BBR_DRAIN &&
	    tcp_packets_in_flight(tcp_sk(sk)) <=
	    bbr_target_cwnd(sk, bbr_max_bw(sk), BBR_UNIT))
		bbr_reset_probe_bw_mode(sk);  /* we estimate queue is drained */
}

/* The goal of PROBE_RTT mode is to have BBR flows cooperatively and
 * periodically drain the bottleneck queue, to converge to measure the true
 * min_rtt (unloaded propagation delay). This allows the flows to keep queues
 * small (reducing queuing delay and packet loss) and achieve fairness among
 * BBR flows.
 *
 * The min_rtt filter window is 10 seconds. When the min_rtt estimate expires,
 * we enter PROBE_RTT mode and cap the cwnd at bbr_cwnd_min_target=4 packets.
 * After at least bbr_probe_rtt_mode_ms=200ms and at least one packet-timed
 * round trip elapsed with that flight size <= 4, we leave PROBE_RTT mode and
 * re-enter the previous mode. BBR uses 200ms to approximately bound the
 * performance penalty of PROBE_RTT's cwnd capping to roughly 2% (200ms/10s).
 *
 * Note that flows need only pay 2% if they are busy sending over the last 10
 * seconds. Interactive applications (e.g., Web, RPCs, video chunks) often have
 * natural silences or low-rate periods within 10 seconds where the rate is low
 * enough for long enough to drain its queue in the bottleneck. We pick up
 * these min RTT measurements opportunistically with our min_rtt filter. :-)
 */
static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);
	bool filter_expired;

	/* Track min RTT seen in the min_rtt_win_sec filter window: */
	filter_expired = after(tcp_jiffies32,
			       bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ);
	if (rs->rtt_us >= 0 &&
	    (rs->rtt_us <= bbr->min_rtt_us || filter_expired)) {
		bbr->min_rtt_us = rs->rtt_us;
		bbr->min_rtt_stamp = tcp_jiffies32;
	}

	if (bbr_probe_rtt_mode_ms > 0 && filter_expired &&
	    !bbr->idle_restart && bbr->mode != BBR_PROBE_RTT) {
		bbr->mode = BBR_PROBE_RTT;  /* dip, drain queue */
		bbr->pacing_gain = BBR_UNIT;
		bbr->cwnd_gain = BBR_UNIT;
		bbr_save_cwnd(sk);  /* note cwnd so we can restore it */
		bbr->probe_rtt_done_stamp = 0;
	}

	if (bbr->mode == BBR_PROBE_RTT) {
		/* Ignore low rate samples during this mode. */
		tp->app_limited =
			(tp->delivered + tcp_packets_in_flight(tp)) ? : 1;
		/* Maintain min packets in flight for max(200 ms, 1 round). */
		if (!bbr->probe_rtt_done_stamp &&
		    tcp_packets_in_flight(tp) <= bbr_cwnd_min_target) {
			bbr->probe_rtt_done_stamp = tcp_jiffies32 +
				msecs_to_jiffies(bbr_probe_rtt_mode_ms);
			bbr->probe_rtt_round_done = 0;
			bbr->next_rtt_delivered = tp->delivered;
		} else if (bbr->probe_rtt_done_stamp) {
			if (bbr->round_start)
				bbr->probe_rtt_round_done = 1;
			if (bbr->probe_rtt_round_done &&
			    after(tcp_jiffies32, bbr->probe_rtt_done_stamp)) {
				bbr->min_rtt_stamp = tcp_jiffies32;
				bbr->restore_cwnd = 1;  /* snap to prior_cwnd */
				bbr_reset_mode(sk);
			}
		}
	}
	bbr->idle_restart = 0;
}

static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)
{
	bbr_update_bw(sk, rs);
	bbr_update_cycle_phase(sk, rs);
	bbr_check_full_bw_reached(sk, rs);
	bbr_check_drain(sk, rs);
	bbr_update_min_rtt(sk, rs);
}

static void bbr_main(struct sock *sk, const struct rate_sample *rs)
{
	struct bbr *bbr = inet_csk_ca(sk);
	u32 bw;

	bbr_update_model(sk, rs);

	bw = bbr_bw(sk);
	bbr_set_pacing_rate(sk, bw, bbr->pacing_gain);
	bbr_set_tso_segs_goal(sk);
	bbr_set_cwnd(sk, rs, rs->acked_sacked, bw, bbr->cwnd_gain);
}

static void bbr_init(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bbr *bbr = inet_csk_ca(sk);
	u64 bw;

	bbr->prior_cwnd = 0;
	bbr->tso_segs_goal = 0;	 /* default segs per skb until first ACK */
	bbr->rtt_cnt = 0;
	bbr->next_rtt_delivered = 0;
	bbr->prev_ca_state = TCP_CA_Open;
	bbr->packet_conservation = 0;

	bbr->probe_rtt_done_stamp = 0;
	bbr->probe_rtt_round_done = 0;
	bbr->min_rtt_us = tcp_min_rtt(tp);
	bbr->min_rtt_stamp = tcp_jiffies32;

	minmax_reset(&bbr->bw, bbr->rtt_cnt, 0);  /* init max bw to 0 */

	/* Initialize pacing rate to: high_gain * init_cwnd / RTT. */
	bw = (u64)tp->snd_cwnd * BW_UNIT;
	do_div(bw, (tp->srtt_us >> 3) ? : USEC_PER_MSEC);
	sk->sk_pacing_rate = 0;		/* force an update of sk_pacing_rate */
	bbr_set_pacing_rate(sk, bw, bbr_high_gain);

	bbr->restore_cwnd = 0;
	bbr->round_start = 0;
	bbr->idle_restart = 0;
	bbr->full_bw = 0;
	bbr->full_bw_cnt = 0;
	bbr->cycle_mstamp = 0;
	bbr->cycle_idx = 0;
	bbr_reset_lt_bw_sampling(sk);
	bbr_reset_startup_mode(sk);

	cmpxchg(&sk->sk_pacing_status, SK_PACING_NONE, SK_PACING_NEEDED);
}

static u32 bbr_sndbuf_expand(struct sock *sk)
{
	/* Provision 3 * cwnd since BBR may slow-start even during recovery. */
	return 3;
}

/* In theory BBR does not need to undo the cwnd since it does not
 * always reduce cwnd on losses (see bbr_main()). Keep it for now.
 */
static u32 bbr_undo_cwnd(struct sock *sk)
{
	return tcp_sk(sk)->snd_cwnd;
}

/* Entering loss recovery, so save cwnd for when we exit or undo recovery. */
static u32 bbr_ssthresh(struct sock *sk)
{
	bbr_save_cwnd(sk);
	return TCP_INFINITE_SSTHRESH;	 /* BBR does not use ssthresh */
}

static size_t bbr_get_info(struct sock *sk, u32 ext, int *attr,
			   union tcp_cc_info *info)
{
	if (ext & (1 << (INET_DIAG_BBRINFO - 1)) ||
	    ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
		struct tcp_sock *tp = tcp_sk(sk);
		struct bbr *bbr = inet_csk_ca(sk);
		u64 bw = bbr_bw(sk);

		bw = bw * tp->mss_cache * USEC_PER_SEC >> BW_SCALE;
		memset(&info->bbr, 0, sizeof(info->bbr));
		info->bbr.bbr_bw_lo		= (u32)bw;
		info->bbr.bbr_bw_hi		= (u32)(bw >> 32);
		info->bbr.bbr_min_rtt		= bbr->min_rtt_us;
		info->bbr.bbr_pacing_gain	= bbr->pacing_gain;
		info->bbr.bbr_cwnd_gain		= bbr->cwnd_gain;
		*attr = INET_DIAG_BBRINFO;
		return sizeof(info->bbr);
	}
	return 0;
}

static void bbr_set_state(struct sock *sk, u8 new_state)
{
	struct bbr *bbr = inet_csk_ca(sk);

	if (new_state == TCP_CA_Loss) {
		struct rate_sample rs = { .losses = 1 };

		bbr->prev_ca_state = TCP_CA_Loss;
		bbr->full_bw = 0;
		bbr->round_start = 1;	/* treat RTO like end of a round */
		bbr_lt_bw_sampling(sk, &rs);
	}
}

static struct tcp_congestion_ops tcp_bbr_cong_ops __read_mostly = {
	.flags		= TCP_CONG_NON_RESTRICTED,
	.name		= "bbr",
	.owner		= THIS_MODULE,
	.init		= bbr_init,
	.cong_control	= bbr_main,
	.sndbuf_expand	= bbr_sndbuf_expand,
	.undo_cwnd	= bbr_undo_cwnd,
	.cwnd_event	= bbr_cwnd_event,
	.ssthresh	= bbr_ssthresh,
	.tso_segs_goal	= bbr_tso_segs_goal,
	.get_info	= bbr_get_info,
	.set_state	= bbr_set_state,
};

static int __init bbr_register(void)
{
	BUILD_BUG_ON(sizeof(struct bbr) > ICSK_CA_PRIV_SIZE);
	return tcp_register_congestion_control(&tcp_bbr_cong_ops);
}

static void __exit bbr_unregister(void)
{
	tcp_unregister_congestion_control(&tcp_bbr_cong_ops);
}

module_init(bbr_register);
module_exit(bbr_unregister);

MODULE_AUTHOR("Van Jacobson <vanj@google.com>");
MODULE_AUTHOR("Neal Cardwell <ncardwell@google.com>");
MODULE_AUTHOR("Yuchung Cheng <ycheng@google.com>");
MODULE_AUTHOR("Soheil Hassas Yeganeh <soheil@google.com>");
MODULE_LICENSE("Dual BSD/GPL");
MODULE_DESCRIPTION("TCP BBR (Bottleneck Bandwidth and RTT)");