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1/* Bottleneck Bandwidth and RTT (BBR) congestion control
2 *
3 * BBR congestion control computes the sending rate based on the delivery
4 * rate (throughput) estimated from ACKs. In a nutshell:
5 *
6 * On each ACK, update our model of the network path:
7 * bottleneck_bandwidth = windowed_max(delivered / elapsed, 10 round trips)
8 * min_rtt = windowed_min(rtt, 10 seconds)
9 * pacing_rate = pacing_gain * bottleneck_bandwidth
10 * cwnd = max(cwnd_gain * bottleneck_bandwidth * min_rtt, 4)
11 *
12 * The core algorithm does not react directly to packet losses or delays,
13 * although BBR may adjust the size of next send per ACK when loss is
14 * observed, or adjust the sending rate if it estimates there is a
15 * traffic policer, in order to keep the drop rate reasonable.
16 *
17 * Here is a state transition diagram for BBR:
18 *
19 * |
20 * V
21 * +---> STARTUP ----+
22 * | | |
23 * | V |
24 * | DRAIN ----+
25 * | | |
26 * | V |
27 * +---> PROBE_BW ----+
28 * | ^ | |
29 * | | | |
30 * | +----+ |
31 * | |
32 * +---- PROBE_RTT <--+
33 *
34 * A BBR flow starts in STARTUP, and ramps up its sending rate quickly.
35 * When it estimates the pipe is full, it enters DRAIN to drain the queue.
36 * In steady state a BBR flow only uses PROBE_BW and PROBE_RTT.
37 * A long-lived BBR flow spends the vast majority of its time remaining
38 * (repeatedly) in PROBE_BW, fully probing and utilizing the pipe's bandwidth
39 * in a fair manner, with a small, bounded queue. *If* a flow has been
40 * continuously sending for the entire min_rtt window, and hasn't seen an RTT
41 * sample that matches or decreases its min_rtt estimate for 10 seconds, then
42 * it briefly enters PROBE_RTT to cut inflight to a minimum value to re-probe
43 * the path's two-way propagation delay (min_rtt). When exiting PROBE_RTT, if
44 * we estimated that we reached the full bw of the pipe then we enter PROBE_BW;
45 * otherwise we enter STARTUP to try to fill the pipe.
46 *
47 * BBR is described in detail in:
48 * "BBR: Congestion-Based Congestion Control",
49 * Neal Cardwell, Yuchung Cheng, C. Stephen Gunn, Soheil Hassas Yeganeh,
50 * Van Jacobson. ACM Queue, Vol. 14 No. 5, September-October 2016.
51 *
52 * There is a public e-mail list for discussing BBR development and testing:
53 * https://groups.google.com/forum/#!forum/bbr-dev
54 *
55 * NOTE: BBR might be used with the fq qdisc ("man tc-fq") with pacing enabled,
56 * otherwise TCP stack falls back to an internal pacing using one high
57 * resolution timer per TCP socket and may use more resources.
58 */
59#include <linux/module.h>
60#include <net/tcp.h>
61#include <linux/inet_diag.h>
62#include <linux/inet.h>
63#include <linux/random.h>
64#include <linux/win_minmax.h>
65
66/* Scale factor for rate in pkt/uSec unit to avoid truncation in bandwidth
67 * estimation. The rate unit ~= (1500 bytes / 1 usec / 2^24) ~= 715 bps.
68 * This handles bandwidths from 0.06pps (715bps) to 256Mpps (3Tbps) in a u32.
69 * Since the minimum window is >=4 packets, the lower bound isn't
70 * an issue. The upper bound isn't an issue with existing technologies.
71 */
72#define BW_SCALE 24
73#define BW_UNIT (1 << BW_SCALE)
74
75#define BBR_SCALE 8 /* scaling factor for fractions in BBR (e.g. gains) */
76#define BBR_UNIT (1 << BBR_SCALE)
77
78/* BBR has the following modes for deciding how fast to send: */
79enum bbr_mode {
80 BBR_STARTUP, /* ramp up sending rate rapidly to fill pipe */
81 BBR_DRAIN, /* drain any queue created during startup */
82 BBR_PROBE_BW, /* discover, share bw: pace around estimated bw */
83 BBR_PROBE_RTT, /* cut inflight to min to probe min_rtt */
84};
85
86/* BBR congestion control block */
87struct bbr {
88 u32 min_rtt_us; /* min RTT in min_rtt_win_sec window */
89 u32 min_rtt_stamp; /* timestamp of min_rtt_us */
90 u32 probe_rtt_done_stamp; /* end time for BBR_PROBE_RTT mode */
91 struct minmax bw; /* Max recent delivery rate in pkts/uS << 24 */
92 u32 rtt_cnt; /* count of packet-timed rounds elapsed */
93 u32 next_rtt_delivered; /* scb->tx.delivered at end of round */
94 u64 cycle_mstamp; /* time of this cycle phase start */
95 u32 mode:3, /* current bbr_mode in state machine */
96 prev_ca_state:3, /* CA state on previous ACK */
97 packet_conservation:1, /* use packet conservation? */
98 restore_cwnd:1, /* decided to revert cwnd to old value */
99 round_start:1, /* start of packet-timed tx->ack round? */
100 idle_restart:1, /* restarting after idle? */
101 probe_rtt_round_done:1, /* a BBR_PROBE_RTT round at 4 pkts? */
102 unused:12,
103 lt_is_sampling:1, /* taking long-term ("LT") samples now? */
104 lt_rtt_cnt:7, /* round trips in long-term interval */
105 lt_use_bw:1; /* use lt_bw as our bw estimate? */
106 u32 lt_bw; /* LT est delivery rate in pkts/uS << 24 */
107 u32 lt_last_delivered; /* LT intvl start: tp->delivered */
108 u32 lt_last_stamp; /* LT intvl start: tp->delivered_mstamp */
109 u32 lt_last_lost; /* LT intvl start: tp->lost */
110 u32 pacing_gain:10, /* current gain for setting pacing rate */
111 cwnd_gain:10, /* current gain for setting cwnd */
112 full_bw_reached:1, /* reached full bw in Startup? */
113 full_bw_cnt:2, /* number of rounds without large bw gains */
114 cycle_idx:3, /* current index in pacing_gain cycle array */
115 has_seen_rtt:1, /* have we seen an RTT sample yet? */
116 unused_b:5;
117 u32 prior_cwnd; /* prior cwnd upon entering loss recovery */
118 u32 full_bw; /* recent bw, to estimate if pipe is full */
119};
120
121#define CYCLE_LEN 8 /* number of phases in a pacing gain cycle */
122
123/* Window length of bw filter (in rounds): */
124static const int bbr_bw_rtts = CYCLE_LEN + 2;
125/* Window length of min_rtt filter (in sec): */
126static const u32 bbr_min_rtt_win_sec = 10;
127/* Minimum time (in ms) spent at bbr_cwnd_min_target in BBR_PROBE_RTT mode: */
128static const u32 bbr_probe_rtt_mode_ms = 200;
129/* Skip TSO below the following bandwidth (bits/sec): */
130static const int bbr_min_tso_rate = 1200000;
131
132/* We use a high_gain value of 2/ln(2) because it's the smallest pacing gain
133 * that will allow a smoothly increasing pacing rate that will double each RTT
134 * and send the same number of packets per RTT that an un-paced, slow-starting
135 * Reno or CUBIC flow would:
136 */
137static const int bbr_high_gain = BBR_UNIT * 2885 / 1000 + 1;
138/* The pacing gain of 1/high_gain in BBR_DRAIN is calculated to typically drain
139 * the queue created in BBR_STARTUP in a single round:
140 */
141static const int bbr_drain_gain = BBR_UNIT * 1000 / 2885;
142/* The gain for deriving steady-state cwnd tolerates delayed/stretched ACKs: */
143static const int bbr_cwnd_gain = BBR_UNIT * 2;
144/* The pacing_gain values for the PROBE_BW gain cycle, to discover/share bw: */
145static const int bbr_pacing_gain[] = {
146 BBR_UNIT * 5 / 4, /* probe for more available bw */
147 BBR_UNIT * 3 / 4, /* drain queue and/or yield bw to other flows */
148 BBR_UNIT, BBR_UNIT, BBR_UNIT, /* cruise at 1.0*bw to utilize pipe, */
149 BBR_UNIT, BBR_UNIT, BBR_UNIT /* without creating excess queue... */
150};
151/* Randomize the starting gain cycling phase over N phases: */
152static const u32 bbr_cycle_rand = 7;
153
154/* Try to keep at least this many packets in flight, if things go smoothly. For
155 * smooth functioning, a sliding window protocol ACKing every other packet
156 * needs at least 4 packets in flight:
157 */
158static const u32 bbr_cwnd_min_target = 4;
159
160/* To estimate if BBR_STARTUP mode (i.e. high_gain) has filled pipe... */
161/* If bw has increased significantly (1.25x), there may be more bw available: */
162static const u32 bbr_full_bw_thresh = BBR_UNIT * 5 / 4;
163/* But after 3 rounds w/o significant bw growth, estimate pipe is full: */
164static const u32 bbr_full_bw_cnt = 3;
165
166/* "long-term" ("LT") bandwidth estimator parameters... */
167/* The minimum number of rounds in an LT bw sampling interval: */
168static const u32 bbr_lt_intvl_min_rtts = 4;
169/* If lost/delivered ratio > 20%, interval is "lossy" and we may be policed: */
170static const u32 bbr_lt_loss_thresh = 50;
171/* If 2 intervals have a bw ratio <= 1/8, their bw is "consistent": */
172static const u32 bbr_lt_bw_ratio = BBR_UNIT / 8;
173/* If 2 intervals have a bw diff <= 4 Kbit/sec their bw is "consistent": */
174static const u32 bbr_lt_bw_diff = 4000 / 8;
175/* If we estimate we're policed, use lt_bw for this many round trips: */
176static const u32 bbr_lt_bw_max_rtts = 48;
177
178/* Do we estimate that STARTUP filled the pipe? */
179static bool bbr_full_bw_reached(const struct sock *sk)
180{
181 const struct bbr *bbr = inet_csk_ca(sk);
182
183 return bbr->full_bw_reached;
184}
185
186/* Return the windowed max recent bandwidth sample, in pkts/uS << BW_SCALE. */
187static u32 bbr_max_bw(const struct sock *sk)
188{
189 struct bbr *bbr = inet_csk_ca(sk);
190
191 return minmax_get(&bbr->bw);
192}
193
194/* Return the estimated bandwidth of the path, in pkts/uS << BW_SCALE. */
195static u32 bbr_bw(const struct sock *sk)
196{
197 struct bbr *bbr = inet_csk_ca(sk);
198
199 return bbr->lt_use_bw ? bbr->lt_bw : bbr_max_bw(sk);
200}
201
202/* Return rate in bytes per second, optionally with a gain.
203 * The order here is chosen carefully to avoid overflow of u64. This should
204 * work for input rates of up to 2.9Tbit/sec and gain of 2.89x.
205 */
206static u64 bbr_rate_bytes_per_sec(struct sock *sk, u64 rate, int gain)
207{
208 rate *= tcp_mss_to_mtu(sk, tcp_sk(sk)->mss_cache);
209 rate *= gain;
210 rate >>= BBR_SCALE;
211 rate *= USEC_PER_SEC;
212 return rate >> BW_SCALE;
213}
214
215/* Convert a BBR bw and gain factor to a pacing rate in bytes per second. */
216static u32 bbr_bw_to_pacing_rate(struct sock *sk, u32 bw, int gain)
217{
218 u64 rate = bw;
219
220 rate = bbr_rate_bytes_per_sec(sk, rate, gain);
221 rate = min_t(u64, rate, sk->sk_max_pacing_rate);
222 return rate;
223}
224
225/* Initialize pacing rate to: high_gain * init_cwnd / RTT. */
226static void bbr_init_pacing_rate_from_rtt(struct sock *sk)
227{
228 struct tcp_sock *tp = tcp_sk(sk);
229 struct bbr *bbr = inet_csk_ca(sk);
230 u64 bw;
231 u32 rtt_us;
232
233 if (tp->srtt_us) { /* any RTT sample yet? */
234 rtt_us = max(tp->srtt_us >> 3, 1U);
235 bbr->has_seen_rtt = 1;
236 } else { /* no RTT sample yet */
237 rtt_us = USEC_PER_MSEC; /* use nominal default RTT */
238 }
239 bw = (u64)tp->snd_cwnd * BW_UNIT;
240 do_div(bw, rtt_us);
241 sk->sk_pacing_rate = bbr_bw_to_pacing_rate(sk, bw, bbr_high_gain);
242}
243
244/* Pace using current bw estimate and a gain factor. In order to help drive the
245 * network toward lower queues while maintaining high utilization and low
246 * latency, the average pacing rate aims to be slightly (~1%) lower than the
247 * estimated bandwidth. This is an important aspect of the design. In this
248 * implementation this slightly lower pacing rate is achieved implicitly by not
249 * including link-layer headers in the packet size used for the pacing rate.
250 */
251static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain)
252{
253 struct tcp_sock *tp = tcp_sk(sk);
254 struct bbr *bbr = inet_csk_ca(sk);
255 u32 rate = bbr_bw_to_pacing_rate(sk, bw, gain);
256
257 if (unlikely(!bbr->has_seen_rtt && tp->srtt_us))
258 bbr_init_pacing_rate_from_rtt(sk);
259 if (bbr_full_bw_reached(sk) || rate > sk->sk_pacing_rate)
260 sk->sk_pacing_rate = rate;
261}
262
263/* override sysctl_tcp_min_tso_segs */
264static u32 bbr_min_tso_segs(struct sock *sk)
265{
266 return sk->sk_pacing_rate < (bbr_min_tso_rate >> 3) ? 1 : 2;
267}
268
269static u32 bbr_tso_segs_goal(struct sock *sk)
270{
271 struct tcp_sock *tp = tcp_sk(sk);
272 u32 segs, bytes;
273
274 /* Sort of tcp_tso_autosize() but ignoring
275 * driver provided sk_gso_max_size.
276 */
277 bytes = min_t(u32, sk->sk_pacing_rate >> sk->sk_pacing_shift,
278 GSO_MAX_SIZE - 1 - MAX_TCP_HEADER);
279 segs = max_t(u32, bytes / tp->mss_cache, bbr_min_tso_segs(sk));
280
281 return min(segs, 0x7FU);
282}
283
284/* Save "last known good" cwnd so we can restore it after losses or PROBE_RTT */
285static void bbr_save_cwnd(struct sock *sk)
286{
287 struct tcp_sock *tp = tcp_sk(sk);
288 struct bbr *bbr = inet_csk_ca(sk);
289
290 if (bbr->prev_ca_state < TCP_CA_Recovery && bbr->mode != BBR_PROBE_RTT)
291 bbr->prior_cwnd = tp->snd_cwnd; /* this cwnd is good enough */
292 else /* loss recovery or BBR_PROBE_RTT have temporarily cut cwnd */
293 bbr->prior_cwnd = max(bbr->prior_cwnd, tp->snd_cwnd);
294}
295
296static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event)
297{
298 struct tcp_sock *tp = tcp_sk(sk);
299 struct bbr *bbr = inet_csk_ca(sk);
300
301 if (event == CA_EVENT_TX_START && tp->app_limited) {
302 bbr->idle_restart = 1;
303 /* Avoid pointless buffer overflows: pace at est. bw if we don't
304 * need more speed (we're restarting from idle and app-limited).
305 */
306 if (bbr->mode == BBR_PROBE_BW)
307 bbr_set_pacing_rate(sk, bbr_bw(sk), BBR_UNIT);
308 }
309}
310
311/* Find target cwnd. Right-size the cwnd based on min RTT and the
312 * estimated bottleneck bandwidth:
313 *
314 * cwnd = bw * min_rtt * gain = BDP * gain
315 *
316 * The key factor, gain, controls the amount of queue. While a small gain
317 * builds a smaller queue, it becomes more vulnerable to noise in RTT
318 * measurements (e.g., delayed ACKs or other ACK compression effects). This
319 * noise may cause BBR to under-estimate the rate.
320 *
321 * To achieve full performance in high-speed paths, we budget enough cwnd to
322 * fit full-sized skbs in-flight on both end hosts to fully utilize the path:
323 * - one skb in sending host Qdisc,
324 * - one skb in sending host TSO/GSO engine
325 * - one skb being received by receiver host LRO/GRO/delayed-ACK engine
326 * Don't worry, at low rates (bbr_min_tso_rate) this won't bloat cwnd because
327 * in such cases tso_segs_goal is 1. The minimum cwnd is 4 packets,
328 * which allows 2 outstanding 2-packet sequences, to try to keep pipe
329 * full even with ACK-every-other-packet delayed ACKs.
330 */
331static u32 bbr_target_cwnd(struct sock *sk, u32 bw, int gain)
332{
333 struct bbr *bbr = inet_csk_ca(sk);
334 u32 cwnd;
335 u64 w;
336
337 /* If we've never had a valid RTT sample, cap cwnd at the initial
338 * default. This should only happen when the connection is not using TCP
339 * timestamps and has retransmitted all of the SYN/SYNACK/data packets
340 * ACKed so far. In this case, an RTO can cut cwnd to 1, in which
341 * case we need to slow-start up toward something safe: TCP_INIT_CWND.
342 */
343 if (unlikely(bbr->min_rtt_us == ~0U)) /* no valid RTT samples yet? */
344 return TCP_INIT_CWND; /* be safe: cap at default initial cwnd*/
345
346 w = (u64)bw * bbr->min_rtt_us;
347
348 /* Apply a gain to the given value, then remove the BW_SCALE shift. */
349 cwnd = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT;
350
351 /* Allow enough full-sized skbs in flight to utilize end systems. */
352 cwnd += 3 * bbr_tso_segs_goal(sk);
353
354 /* Reduce delayed ACKs by rounding up cwnd to the next even number. */
355 cwnd = (cwnd + 1) & ~1U;
356
357 return cwnd;
358}
359
360/* An optimization in BBR to reduce losses: On the first round of recovery, we
361 * follow the packet conservation principle: send P packets per P packets acked.
362 * After that, we slow-start and send at most 2*P packets per P packets acked.
363 * After recovery finishes, or upon undo, we restore the cwnd we had when
364 * recovery started (capped by the target cwnd based on estimated BDP).
365 *
366 * TODO(ycheng/ncardwell): implement a rate-based approach.
367 */
368static bool bbr_set_cwnd_to_recover_or_restore(
369 struct sock *sk, const struct rate_sample *rs, u32 acked, u32 *new_cwnd)
370{
371 struct tcp_sock *tp = tcp_sk(sk);
372 struct bbr *bbr = inet_csk_ca(sk);
373 u8 prev_state = bbr->prev_ca_state, state = inet_csk(sk)->icsk_ca_state;
374 u32 cwnd = tp->snd_cwnd;
375
376 /* An ACK for P pkts should release at most 2*P packets. We do this
377 * in two steps. First, here we deduct the number of lost packets.
378 * Then, in bbr_set_cwnd() we slow start up toward the target cwnd.
379 */
380 if (rs->losses > 0)
381 cwnd = max_t(s32, cwnd - rs->losses, 1);
382
383 if (state == TCP_CA_Recovery && prev_state != TCP_CA_Recovery) {
384 /* Starting 1st round of Recovery, so do packet conservation. */
385 bbr->packet_conservation = 1;
386 bbr->next_rtt_delivered = tp->delivered; /* start round now */
387 /* Cut unused cwnd from app behavior, TSQ, or TSO deferral: */
388 cwnd = tcp_packets_in_flight(tp) + acked;
389 } else if (prev_state >= TCP_CA_Recovery && state < TCP_CA_Recovery) {
390 /* Exiting loss recovery; restore cwnd saved before recovery. */
391 bbr->restore_cwnd = 1;
392 bbr->packet_conservation = 0;
393 }
394 bbr->prev_ca_state = state;
395
396 if (bbr->restore_cwnd) {
397 /* Restore cwnd after exiting loss recovery or PROBE_RTT. */
398 cwnd = max(cwnd, bbr->prior_cwnd);
399 bbr->restore_cwnd = 0;
400 }
401
402 if (bbr->packet_conservation) {
403 *new_cwnd = max(cwnd, tcp_packets_in_flight(tp) + acked);
404 return true; /* yes, using packet conservation */
405 }
406 *new_cwnd = cwnd;
407 return false;
408}
409
410/* Slow-start up toward target cwnd (if bw estimate is growing, or packet loss
411 * has drawn us down below target), or snap down to target if we're above it.
412 */
413static void bbr_set_cwnd(struct sock *sk, const struct rate_sample *rs,
414 u32 acked, u32 bw, int gain)
415{
416 struct tcp_sock *tp = tcp_sk(sk);
417 struct bbr *bbr = inet_csk_ca(sk);
418 u32 cwnd = 0, target_cwnd = 0;
419
420 if (!acked)
421 return;
422
423 if (bbr_set_cwnd_to_recover_or_restore(sk, rs, acked, &cwnd))
424 goto done;
425
426 /* If we're below target cwnd, slow start cwnd toward target cwnd. */
427 target_cwnd = bbr_target_cwnd(sk, bw, gain);
428 if (bbr_full_bw_reached(sk)) /* only cut cwnd if we filled the pipe */
429 cwnd = min(cwnd + acked, target_cwnd);
430 else if (cwnd < target_cwnd || tp->delivered < TCP_INIT_CWND)
431 cwnd = cwnd + acked;
432 cwnd = max(cwnd, bbr_cwnd_min_target);
433
434done:
435 tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp); /* apply global cap */
436 if (bbr->mode == BBR_PROBE_RTT) /* drain queue, refresh min_rtt */
437 tp->snd_cwnd = min(tp->snd_cwnd, bbr_cwnd_min_target);
438}
439
440/* End cycle phase if it's time and/or we hit the phase's in-flight target. */
441static bool bbr_is_next_cycle_phase(struct sock *sk,
442 const struct rate_sample *rs)
443{
444 struct tcp_sock *tp = tcp_sk(sk);
445 struct bbr *bbr = inet_csk_ca(sk);
446 bool is_full_length =
447 tcp_stamp_us_delta(tp->delivered_mstamp, bbr->cycle_mstamp) >
448 bbr->min_rtt_us;
449 u32 inflight, bw;
450
451 /* The pacing_gain of 1.0 paces at the estimated bw to try to fully
452 * use the pipe without increasing the queue.
453 */
454 if (bbr->pacing_gain == BBR_UNIT)
455 return is_full_length; /* just use wall clock time */
456
457 inflight = rs->prior_in_flight; /* what was in-flight before ACK? */
458 bw = bbr_max_bw(sk);
459
460 /* A pacing_gain > 1.0 probes for bw by trying to raise inflight to at
461 * least pacing_gain*BDP; this may take more than min_rtt if min_rtt is
462 * small (e.g. on a LAN). We do not persist if packets are lost, since
463 * a path with small buffers may not hold that much.
464 */
465 if (bbr->pacing_gain > BBR_UNIT)
466 return is_full_length &&
467 (rs->losses || /* perhaps pacing_gain*BDP won't fit */
468 inflight >= bbr_target_cwnd(sk, bw, bbr->pacing_gain));
469
470 /* A pacing_gain < 1.0 tries to drain extra queue we added if bw
471 * probing didn't find more bw. If inflight falls to match BDP then we
472 * estimate queue is drained; persisting would underutilize the pipe.
473 */
474 return is_full_length ||
475 inflight <= bbr_target_cwnd(sk, bw, BBR_UNIT);
476}
477
478static void bbr_advance_cycle_phase(struct sock *sk)
479{
480 struct tcp_sock *tp = tcp_sk(sk);
481 struct bbr *bbr = inet_csk_ca(sk);
482
483 bbr->cycle_idx = (bbr->cycle_idx + 1) & (CYCLE_LEN - 1);
484 bbr->cycle_mstamp = tp->delivered_mstamp;
485 bbr->pacing_gain = bbr->lt_use_bw ? BBR_UNIT :
486 bbr_pacing_gain[bbr->cycle_idx];
487}
488
489/* Gain cycling: cycle pacing gain to converge to fair share of available bw. */
490static void bbr_update_cycle_phase(struct sock *sk,
491 const struct rate_sample *rs)
492{
493 struct bbr *bbr = inet_csk_ca(sk);
494
495 if (bbr->mode == BBR_PROBE_BW && bbr_is_next_cycle_phase(sk, rs))
496 bbr_advance_cycle_phase(sk);
497}
498
499static void bbr_reset_startup_mode(struct sock *sk)
500{
501 struct bbr *bbr = inet_csk_ca(sk);
502
503 bbr->mode = BBR_STARTUP;
504 bbr->pacing_gain = bbr_high_gain;
505 bbr->cwnd_gain = bbr_high_gain;
506}
507
508static void bbr_reset_probe_bw_mode(struct sock *sk)
509{
510 struct bbr *bbr = inet_csk_ca(sk);
511
512 bbr->mode = BBR_PROBE_BW;
513 bbr->pacing_gain = BBR_UNIT;
514 bbr->cwnd_gain = bbr_cwnd_gain;
515 bbr->cycle_idx = CYCLE_LEN - 1 - prandom_u32_max(bbr_cycle_rand);
516 bbr_advance_cycle_phase(sk); /* flip to next phase of gain cycle */
517}
518
519static void bbr_reset_mode(struct sock *sk)
520{
521 if (!bbr_full_bw_reached(sk))
522 bbr_reset_startup_mode(sk);
523 else
524 bbr_reset_probe_bw_mode(sk);
525}
526
527/* Start a new long-term sampling interval. */
528static void bbr_reset_lt_bw_sampling_interval(struct sock *sk)
529{
530 struct tcp_sock *tp = tcp_sk(sk);
531 struct bbr *bbr = inet_csk_ca(sk);
532
533 bbr->lt_last_stamp = div_u64(tp->delivered_mstamp, USEC_PER_MSEC);
534 bbr->lt_last_delivered = tp->delivered;
535 bbr->lt_last_lost = tp->lost;
536 bbr->lt_rtt_cnt = 0;
537}
538
539/* Completely reset long-term bandwidth sampling. */
540static void bbr_reset_lt_bw_sampling(struct sock *sk)
541{
542 struct bbr *bbr = inet_csk_ca(sk);
543
544 bbr->lt_bw = 0;
545 bbr->lt_use_bw = 0;
546 bbr->lt_is_sampling = false;
547 bbr_reset_lt_bw_sampling_interval(sk);
548}
549
550/* Long-term bw sampling interval is done. Estimate whether we're policed. */
551static void bbr_lt_bw_interval_done(struct sock *sk, u32 bw)
552{
553 struct bbr *bbr = inet_csk_ca(sk);
554 u32 diff;
555
556 if (bbr->lt_bw) { /* do we have bw from a previous interval? */
557 /* Is new bw close to the lt_bw from the previous interval? */
558 diff = abs(bw - bbr->lt_bw);
559 if ((diff * BBR_UNIT <= bbr_lt_bw_ratio * bbr->lt_bw) ||
560 (bbr_rate_bytes_per_sec(sk, diff, BBR_UNIT) <=
561 bbr_lt_bw_diff)) {
562 /* All criteria are met; estimate we're policed. */
563 bbr->lt_bw = (bw + bbr->lt_bw) >> 1; /* avg 2 intvls */
564 bbr->lt_use_bw = 1;
565 bbr->pacing_gain = BBR_UNIT; /* try to avoid drops */
566 bbr->lt_rtt_cnt = 0;
567 return;
568 }
569 }
570 bbr->lt_bw = bw;
571 bbr_reset_lt_bw_sampling_interval(sk);
572}
573
574/* Token-bucket traffic policers are common (see "An Internet-Wide Analysis of
575 * Traffic Policing", SIGCOMM 2016). BBR detects token-bucket policers and
576 * explicitly models their policed rate, to reduce unnecessary losses. We
577 * estimate that we're policed if we see 2 consecutive sampling intervals with
578 * consistent throughput and high packet loss. If we think we're being policed,
579 * set lt_bw to the "long-term" average delivery rate from those 2 intervals.
580 */
581static void bbr_lt_bw_sampling(struct sock *sk, const struct rate_sample *rs)
582{
583 struct tcp_sock *tp = tcp_sk(sk);
584 struct bbr *bbr = inet_csk_ca(sk);
585 u32 lost, delivered;
586 u64 bw;
587 u32 t;
588
589 if (bbr->lt_use_bw) { /* already using long-term rate, lt_bw? */
590 if (bbr->mode == BBR_PROBE_BW && bbr->round_start &&
591 ++bbr->lt_rtt_cnt >= bbr_lt_bw_max_rtts) {
592 bbr_reset_lt_bw_sampling(sk); /* stop using lt_bw */
593 bbr_reset_probe_bw_mode(sk); /* restart gain cycling */
594 }
595 return;
596 }
597
598 /* Wait for the first loss before sampling, to let the policer exhaust
599 * its tokens and estimate the steady-state rate allowed by the policer.
600 * Starting samples earlier includes bursts that over-estimate the bw.
601 */
602 if (!bbr->lt_is_sampling) {
603 if (!rs->losses)
604 return;
605 bbr_reset_lt_bw_sampling_interval(sk);
606 bbr->lt_is_sampling = true;
607 }
608
609 /* To avoid underestimates, reset sampling if we run out of data. */
610 if (rs->is_app_limited) {
611 bbr_reset_lt_bw_sampling(sk);
612 return;
613 }
614
615 if (bbr->round_start)
616 bbr->lt_rtt_cnt++; /* count round trips in this interval */
617 if (bbr->lt_rtt_cnt < bbr_lt_intvl_min_rtts)
618 return; /* sampling interval needs to be longer */
619 if (bbr->lt_rtt_cnt > 4 * bbr_lt_intvl_min_rtts) {
620 bbr_reset_lt_bw_sampling(sk); /* interval is too long */
621 return;
622 }
623
624 /* End sampling interval when a packet is lost, so we estimate the
625 * policer tokens were exhausted. Stopping the sampling before the
626 * tokens are exhausted under-estimates the policed rate.
627 */
628 if (!rs->losses)
629 return;
630
631 /* Calculate packets lost and delivered in sampling interval. */
632 lost = tp->lost - bbr->lt_last_lost;
633 delivered = tp->delivered - bbr->lt_last_delivered;
634 /* Is loss rate (lost/delivered) >= lt_loss_thresh? If not, wait. */
635 if (!delivered || (lost << BBR_SCALE) < bbr_lt_loss_thresh * delivered)
636 return;
637
638 /* Find average delivery rate in this sampling interval. */
639 t = div_u64(tp->delivered_mstamp, USEC_PER_MSEC) - bbr->lt_last_stamp;
640 if ((s32)t < 1)
641 return; /* interval is less than one ms, so wait */
642 /* Check if can multiply without overflow */
643 if (t >= ~0U / USEC_PER_MSEC) {
644 bbr_reset_lt_bw_sampling(sk); /* interval too long; reset */
645 return;
646 }
647 t *= USEC_PER_MSEC;
648 bw = (u64)delivered * BW_UNIT;
649 do_div(bw, t);
650 bbr_lt_bw_interval_done(sk, bw);
651}
652
653/* Estimate the bandwidth based on how fast packets are delivered */
654static void bbr_update_bw(struct sock *sk, const struct rate_sample *rs)
655{
656 struct tcp_sock *tp = tcp_sk(sk);
657 struct bbr *bbr = inet_csk_ca(sk);
658 u64 bw;
659
660 bbr->round_start = 0;
661 if (rs->delivered < 0 || rs->interval_us <= 0)
662 return; /* Not a valid observation */
663
664 /* See if we've reached the next RTT */
665 if (!before(rs->prior_delivered, bbr->next_rtt_delivered)) {
666 bbr->next_rtt_delivered = tp->delivered;
667 bbr->rtt_cnt++;
668 bbr->round_start = 1;
669 bbr->packet_conservation = 0;
670 }
671
672 bbr_lt_bw_sampling(sk, rs);
673
674 /* Divide delivered by the interval to find a (lower bound) bottleneck
675 * bandwidth sample. Delivered is in packets and interval_us in uS and
676 * ratio will be <<1 for most connections. So delivered is first scaled.
677 */
678 bw = (u64)rs->delivered * BW_UNIT;
679 do_div(bw, rs->interval_us);
680
681 /* If this sample is application-limited, it is likely to have a very
682 * low delivered count that represents application behavior rather than
683 * the available network rate. Such a sample could drag down estimated
684 * bw, causing needless slow-down. Thus, to continue to send at the
685 * last measured network rate, we filter out app-limited samples unless
686 * they describe the path bw at least as well as our bw model.
687 *
688 * So the goal during app-limited phase is to proceed with the best
689 * network rate no matter how long. We automatically leave this
690 * phase when app writes faster than the network can deliver :)
691 */
692 if (!rs->is_app_limited || bw >= bbr_max_bw(sk)) {
693 /* Incorporate new sample into our max bw filter. */
694 minmax_running_max(&bbr->bw, bbr_bw_rtts, bbr->rtt_cnt, bw);
695 }
696}
697
698/* Estimate when the pipe is full, using the change in delivery rate: BBR
699 * estimates that STARTUP filled the pipe if the estimated bw hasn't changed by
700 * at least bbr_full_bw_thresh (25%) after bbr_full_bw_cnt (3) non-app-limited
701 * rounds. Why 3 rounds: 1: rwin autotuning grows the rwin, 2: we fill the
702 * higher rwin, 3: we get higher delivery rate samples. Or transient
703 * cross-traffic or radio noise can go away. CUBIC Hystart shares a similar
704 * design goal, but uses delay and inter-ACK spacing instead of bandwidth.
705 */
706static void bbr_check_full_bw_reached(struct sock *sk,
707 const struct rate_sample *rs)
708{
709 struct bbr *bbr = inet_csk_ca(sk);
710 u32 bw_thresh;
711
712 if (bbr_full_bw_reached(sk) || !bbr->round_start || rs->is_app_limited)
713 return;
714
715 bw_thresh = (u64)bbr->full_bw * bbr_full_bw_thresh >> BBR_SCALE;
716 if (bbr_max_bw(sk) >= bw_thresh) {
717 bbr->full_bw = bbr_max_bw(sk);
718 bbr->full_bw_cnt = 0;
719 return;
720 }
721 ++bbr->full_bw_cnt;
722 bbr->full_bw_reached = bbr->full_bw_cnt >= bbr_full_bw_cnt;
723}
724
725/* If pipe is probably full, drain the queue and then enter steady-state. */
726static void bbr_check_drain(struct sock *sk, const struct rate_sample *rs)
727{
728 struct bbr *bbr = inet_csk_ca(sk);
729
730 if (bbr->mode == BBR_STARTUP && bbr_full_bw_reached(sk)) {
731 bbr->mode = BBR_DRAIN; /* drain queue we created */
732 bbr->pacing_gain = bbr_drain_gain; /* pace slow to drain */
733 bbr->cwnd_gain = bbr_high_gain; /* maintain cwnd */
734 tcp_sk(sk)->snd_ssthresh =
735 bbr_target_cwnd(sk, bbr_max_bw(sk), BBR_UNIT);
736 } /* fall through to check if in-flight is already small: */
737 if (bbr->mode == BBR_DRAIN &&
738 tcp_packets_in_flight(tcp_sk(sk)) <=
739 bbr_target_cwnd(sk, bbr_max_bw(sk), BBR_UNIT))
740 bbr_reset_probe_bw_mode(sk); /* we estimate queue is drained */
741}
742
743/* The goal of PROBE_RTT mode is to have BBR flows cooperatively and
744 * periodically drain the bottleneck queue, to converge to measure the true
745 * min_rtt (unloaded propagation delay). This allows the flows to keep queues
746 * small (reducing queuing delay and packet loss) and achieve fairness among
747 * BBR flows.
748 *
749 * The min_rtt filter window is 10 seconds. When the min_rtt estimate expires,
750 * we enter PROBE_RTT mode and cap the cwnd at bbr_cwnd_min_target=4 packets.
751 * After at least bbr_probe_rtt_mode_ms=200ms and at least one packet-timed
752 * round trip elapsed with that flight size <= 4, we leave PROBE_RTT mode and
753 * re-enter the previous mode. BBR uses 200ms to approximately bound the
754 * performance penalty of PROBE_RTT's cwnd capping to roughly 2% (200ms/10s).
755 *
756 * Note that flows need only pay 2% if they are busy sending over the last 10
757 * seconds. Interactive applications (e.g., Web, RPCs, video chunks) often have
758 * natural silences or low-rate periods within 10 seconds where the rate is low
759 * enough for long enough to drain its queue in the bottleneck. We pick up
760 * these min RTT measurements opportunistically with our min_rtt filter. :-)
761 */
762static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs)
763{
764 struct tcp_sock *tp = tcp_sk(sk);
765 struct bbr *bbr = inet_csk_ca(sk);
766 bool filter_expired;
767
768 /* Track min RTT seen in the min_rtt_win_sec filter window: */
769 filter_expired = after(tcp_jiffies32,
770 bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ);
771 if (rs->rtt_us >= 0 &&
772 (rs->rtt_us <= bbr->min_rtt_us ||
773 (filter_expired && !rs->is_ack_delayed))) {
774 bbr->min_rtt_us = rs->rtt_us;
775 bbr->min_rtt_stamp = tcp_jiffies32;
776 }
777
778 if (bbr_probe_rtt_mode_ms > 0 && filter_expired &&
779 !bbr->idle_restart && bbr->mode != BBR_PROBE_RTT) {
780 bbr->mode = BBR_PROBE_RTT; /* dip, drain queue */
781 bbr->pacing_gain = BBR_UNIT;
782 bbr->cwnd_gain = BBR_UNIT;
783 bbr_save_cwnd(sk); /* note cwnd so we can restore it */
784 bbr->probe_rtt_done_stamp = 0;
785 }
786
787 if (bbr->mode == BBR_PROBE_RTT) {
788 /* Ignore low rate samples during this mode. */
789 tp->app_limited =
790 (tp->delivered + tcp_packets_in_flight(tp)) ? : 1;
791 /* Maintain min packets in flight for max(200 ms, 1 round). */
792 if (!bbr->probe_rtt_done_stamp &&
793 tcp_packets_in_flight(tp) <= bbr_cwnd_min_target) {
794 bbr->probe_rtt_done_stamp = tcp_jiffies32 +
795 msecs_to_jiffies(bbr_probe_rtt_mode_ms);
796 bbr->probe_rtt_round_done = 0;
797 bbr->next_rtt_delivered = tp->delivered;
798 } else if (bbr->probe_rtt_done_stamp) {
799 if (bbr->round_start)
800 bbr->probe_rtt_round_done = 1;
801 if (bbr->probe_rtt_round_done &&
802 after(tcp_jiffies32, bbr->probe_rtt_done_stamp)) {
803 bbr->min_rtt_stamp = tcp_jiffies32;
804 bbr->restore_cwnd = 1; /* snap to prior_cwnd */
805 bbr_reset_mode(sk);
806 }
807 }
808 }
809 /* Restart after idle ends only once we process a new S/ACK for data */
810 if (rs->delivered > 0)
811 bbr->idle_restart = 0;
812}
813
814static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)
815{
816 bbr_update_bw(sk, rs);
817 bbr_update_cycle_phase(sk, rs);
818 bbr_check_full_bw_reached(sk, rs);
819 bbr_check_drain(sk, rs);
820 bbr_update_min_rtt(sk, rs);
821}
822
823static void bbr_main(struct sock *sk, const struct rate_sample *rs)
824{
825 struct bbr *bbr = inet_csk_ca(sk);
826 u32 bw;
827
828 bbr_update_model(sk, rs);
829
830 bw = bbr_bw(sk);
831 bbr_set_pacing_rate(sk, bw, bbr->pacing_gain);
832 bbr_set_cwnd(sk, rs, rs->acked_sacked, bw, bbr->cwnd_gain);
833}
834
835static void bbr_init(struct sock *sk)
836{
837 struct tcp_sock *tp = tcp_sk(sk);
838 struct bbr *bbr = inet_csk_ca(sk);
839
840 bbr->prior_cwnd = 0;
841 tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
842 bbr->rtt_cnt = 0;
843 bbr->next_rtt_delivered = 0;
844 bbr->prev_ca_state = TCP_CA_Open;
845 bbr->packet_conservation = 0;
846
847 bbr->probe_rtt_done_stamp = 0;
848 bbr->probe_rtt_round_done = 0;
849 bbr->min_rtt_us = tcp_min_rtt(tp);
850 bbr->min_rtt_stamp = tcp_jiffies32;
851
852 minmax_reset(&bbr->bw, bbr->rtt_cnt, 0); /* init max bw to 0 */
853
854 bbr->has_seen_rtt = 0;
855 bbr_init_pacing_rate_from_rtt(sk);
856
857 bbr->restore_cwnd = 0;
858 bbr->round_start = 0;
859 bbr->idle_restart = 0;
860 bbr->full_bw_reached = 0;
861 bbr->full_bw = 0;
862 bbr->full_bw_cnt = 0;
863 bbr->cycle_mstamp = 0;
864 bbr->cycle_idx = 0;
865 bbr_reset_lt_bw_sampling(sk);
866 bbr_reset_startup_mode(sk);
867
868 cmpxchg(&sk->sk_pacing_status, SK_PACING_NONE, SK_PACING_NEEDED);
869}
870
871static u32 bbr_sndbuf_expand(struct sock *sk)
872{
873 /* Provision 3 * cwnd since BBR may slow-start even during recovery. */
874 return 3;
875}
876
877/* In theory BBR does not need to undo the cwnd since it does not
878 * always reduce cwnd on losses (see bbr_main()). Keep it for now.
879 */
880static u32 bbr_undo_cwnd(struct sock *sk)
881{
882 struct bbr *bbr = inet_csk_ca(sk);
883
884 bbr->full_bw = 0; /* spurious slow-down; reset full pipe detection */
885 bbr->full_bw_cnt = 0;
886 bbr_reset_lt_bw_sampling(sk);
887 return tcp_sk(sk)->snd_cwnd;
888}
889
890/* Entering loss recovery, so save cwnd for when we exit or undo recovery. */
891static u32 bbr_ssthresh(struct sock *sk)
892{
893 bbr_save_cwnd(sk);
894 return tcp_sk(sk)->snd_ssthresh;
895}
896
897static size_t bbr_get_info(struct sock *sk, u32 ext, int *attr,
898 union tcp_cc_info *info)
899{
900 if (ext & (1 << (INET_DIAG_BBRINFO - 1)) ||
901 ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
902 struct tcp_sock *tp = tcp_sk(sk);
903 struct bbr *bbr = inet_csk_ca(sk);
904 u64 bw = bbr_bw(sk);
905
906 bw = bw * tp->mss_cache * USEC_PER_SEC >> BW_SCALE;
907 memset(&info->bbr, 0, sizeof(info->bbr));
908 info->bbr.bbr_bw_lo = (u32)bw;
909 info->bbr.bbr_bw_hi = (u32)(bw >> 32);
910 info->bbr.bbr_min_rtt = bbr->min_rtt_us;
911 info->bbr.bbr_pacing_gain = bbr->pacing_gain;
912 info->bbr.bbr_cwnd_gain = bbr->cwnd_gain;
913 *attr = INET_DIAG_BBRINFO;
914 return sizeof(info->bbr);
915 }
916 return 0;
917}
918
919static void bbr_set_state(struct sock *sk, u8 new_state)
920{
921 struct bbr *bbr = inet_csk_ca(sk);
922
923 if (new_state == TCP_CA_Loss) {
924 struct rate_sample rs = { .losses = 1 };
925
926 bbr->prev_ca_state = TCP_CA_Loss;
927 bbr->full_bw = 0;
928 bbr->round_start = 1; /* treat RTO like end of a round */
929 bbr_lt_bw_sampling(sk, &rs);
930 }
931}
932
933static struct tcp_congestion_ops tcp_bbr_cong_ops __read_mostly = {
934 .flags = TCP_CONG_NON_RESTRICTED,
935 .name = "bbr",
936 .owner = THIS_MODULE,
937 .init = bbr_init,
938 .cong_control = bbr_main,
939 .sndbuf_expand = bbr_sndbuf_expand,
940 .undo_cwnd = bbr_undo_cwnd,
941 .cwnd_event = bbr_cwnd_event,
942 .ssthresh = bbr_ssthresh,
943 .min_tso_segs = bbr_min_tso_segs,
944 .get_info = bbr_get_info,
945 .set_state = bbr_set_state,
946};
947
948static int __init bbr_register(void)
949{
950 BUILD_BUG_ON(sizeof(struct bbr) > ICSK_CA_PRIV_SIZE);
951 return tcp_register_congestion_control(&tcp_bbr_cong_ops);
952}
953
954static void __exit bbr_unregister(void)
955{
956 tcp_unregister_congestion_control(&tcp_bbr_cong_ops);
957}
958
959module_init(bbr_register);
960module_exit(bbr_unregister);
961
962MODULE_AUTHOR("Van Jacobson <vanj@google.com>");
963MODULE_AUTHOR("Neal Cardwell <ncardwell@google.com>");
964MODULE_AUTHOR("Yuchung Cheng <ycheng@google.com>");
965MODULE_AUTHOR("Soheil Hassas Yeganeh <soheil@google.com>");
966MODULE_LICENSE("Dual BSD/GPL");
967MODULE_DESCRIPTION("TCP BBR (Bottleneck Bandwidth and RTT)");