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: */
79 enum 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 */
87 struct 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 round_start:1, /* start of packet-timed tx->ack round? */
99 idle_restart:1, /* restarting after idle? */
100 probe_rtt_round_done:1, /* a BBR_PROBE_RTT round at 4 pkts? */
101 unused:13,
102 lt_is_sampling:1, /* taking long-term ("LT") samples now? */
103 lt_rtt_cnt:7, /* round trips in long-term interval */
104 lt_use_bw:1; /* use lt_bw as our bw estimate? */
105 u32 lt_bw; /* LT est delivery rate in pkts/uS << 24 */
106 u32 lt_last_delivered; /* LT intvl start: tp->delivered */
107 u32 lt_last_stamp; /* LT intvl start: tp->delivered_mstamp */
108 u32 lt_last_lost; /* LT intvl start: tp->lost */
109 u32 pacing_gain:10, /* current gain for setting pacing rate */
110 cwnd_gain:10, /* current gain for setting cwnd */
111 full_bw_reached:1, /* reached full bw in Startup? */
112 full_bw_cnt:2, /* number of rounds without large bw gains */
113 cycle_idx:3, /* current index in pacing_gain cycle array */
114 has_seen_rtt:1, /* have we seen an RTT sample yet? */
115 unused_b:5;
116 u32 prior_cwnd; /* prior cwnd upon entering loss recovery */
117 u32 full_bw; /* recent bw, to estimate if pipe is full */
118
119 /* For tracking ACK aggregation: */
120 u64 ack_epoch_mstamp; /* start of ACK sampling epoch */
121 u16 extra_acked[2]; /* max excess data ACKed in epoch */
122 u32 ack_epoch_acked:20, /* packets (S)ACKed in sampling epoch */
123 extra_acked_win_rtts:5, /* age of extra_acked, in round trips */
124 extra_acked_win_idx:1, /* current index in extra_acked array */
125 unused_c:6;
126 };
127
128 #define CYCLE_LEN 8 /* number of phases in a pacing gain cycle */
129
130 /* Window length of bw filter (in rounds): */
131 static const int bbr_bw_rtts = CYCLE_LEN + 2;
132 /* Window length of min_rtt filter (in sec): */
133 static const u32 bbr_min_rtt_win_sec = 10;
134 /* Minimum time (in ms) spent at bbr_cwnd_min_target in BBR_PROBE_RTT mode: */
135 static const u32 bbr_probe_rtt_mode_ms = 200;
136 /* Skip TSO below the following bandwidth (bits/sec): */
137 static const int bbr_min_tso_rate = 1200000;
138
139 /* Pace at ~1% below estimated bw, on average, to reduce queue at bottleneck.
140 * In order to help drive the network toward lower queues and low latency while
141 * maintaining high utilization, the average pacing rate aims to be slightly
142 * lower than the estimated bandwidth. This is an important aspect of the
143 * design.
144 */
145 static const int bbr_pacing_margin_percent = 1;
146
147 /* We use a high_gain value of 2/ln(2) because it's the smallest pacing gain
148 * that will allow a smoothly increasing pacing rate that will double each RTT
149 * and send the same number of packets per RTT that an un-paced, slow-starting
150 * Reno or CUBIC flow would:
151 */
152 static const int bbr_high_gain = BBR_UNIT * 2885 / 1000 + 1;
153 /* The pacing gain of 1/high_gain in BBR_DRAIN is calculated to typically drain
154 * the queue created in BBR_STARTUP in a single round:
155 */
156 static const int bbr_drain_gain = BBR_UNIT * 1000 / 2885;
157 /* The gain for deriving steady-state cwnd tolerates delayed/stretched ACKs: */
158 static const int bbr_cwnd_gain = BBR_UNIT * 2;
159 /* The pacing_gain values for the PROBE_BW gain cycle, to discover/share bw: */
160 static const int bbr_pacing_gain[] = {
161 BBR_UNIT * 5 / 4, /* probe for more available bw */
162 BBR_UNIT * 3 / 4, /* drain queue and/or yield bw to other flows */
163 BBR_UNIT, BBR_UNIT, BBR_UNIT, /* cruise at 1.0*bw to utilize pipe, */
164 BBR_UNIT, BBR_UNIT, BBR_UNIT /* without creating excess queue... */
165 };
166 /* Randomize the starting gain cycling phase over N phases: */
167 static const u32 bbr_cycle_rand = 7;
168
169 /* Try to keep at least this many packets in flight, if things go smoothly. For
170 * smooth functioning, a sliding window protocol ACKing every other packet
171 * needs at least 4 packets in flight:
172 */
173 static const u32 bbr_cwnd_min_target = 4;
174
175 /* To estimate if BBR_STARTUP mode (i.e. high_gain) has filled pipe... */
176 /* If bw has increased significantly (1.25x), there may be more bw available: */
177 static const u32 bbr_full_bw_thresh = BBR_UNIT * 5 / 4;
178 /* But after 3 rounds w/o significant bw growth, estimate pipe is full: */
179 static const u32 bbr_full_bw_cnt = 3;
180
181 /* "long-term" ("LT") bandwidth estimator parameters... */
182 /* The minimum number of rounds in an LT bw sampling interval: */
183 static const u32 bbr_lt_intvl_min_rtts = 4;
184 /* If lost/delivered ratio > 20%, interval is "lossy" and we may be policed: */
185 static const u32 bbr_lt_loss_thresh = 50;
186 /* If 2 intervals have a bw ratio <= 1/8, their bw is "consistent": */
187 static const u32 bbr_lt_bw_ratio = BBR_UNIT / 8;
188 /* If 2 intervals have a bw diff <= 4 Kbit/sec their bw is "consistent": */
189 static const u32 bbr_lt_bw_diff = 4000 / 8;
190 /* If we estimate we're policed, use lt_bw for this many round trips: */
191 static const u32 bbr_lt_bw_max_rtts = 48;
192
193 /* Gain factor for adding extra_acked to target cwnd: */
194 static const int bbr_extra_acked_gain = BBR_UNIT;
195 /* Window length of extra_acked window. */
196 static const u32 bbr_extra_acked_win_rtts = 5;
197 /* Max allowed val for ack_epoch_acked, after which sampling epoch is reset */
198 static const u32 bbr_ack_epoch_acked_reset_thresh = 1U << 20;
199 /* Time period for clamping cwnd increment due to ack aggregation */
200 static const u32 bbr_extra_acked_max_us = 100 * 1000;
201
202 static void bbr_check_probe_rtt_done(struct sock *sk);
203
204 /* Do we estimate that STARTUP filled the pipe? */
205 static bool bbr_full_bw_reached(const struct sock *sk)
206 {
207 const struct bbr *bbr = inet_csk_ca(sk);
208
209 return bbr->full_bw_reached;
210 }
211
212 /* Return the windowed max recent bandwidth sample, in pkts/uS << BW_SCALE. */
213 static u32 bbr_max_bw(const struct sock *sk)
214 {
215 struct bbr *bbr = inet_csk_ca(sk);
216
217 return minmax_get(&bbr->bw);
218 }
219
220 /* Return the estimated bandwidth of the path, in pkts/uS << BW_SCALE. */
221 static u32 bbr_bw(const struct sock *sk)
222 {
223 struct bbr *bbr = inet_csk_ca(sk);
224
225 return bbr->lt_use_bw ? bbr->lt_bw : bbr_max_bw(sk);
226 }
227
228 /* Return maximum extra acked in past k-2k round trips,
229 * where k = bbr_extra_acked_win_rtts.
230 */
231 static u16 bbr_extra_acked(const struct sock *sk)
232 {
233 struct bbr *bbr = inet_csk_ca(sk);
234
235 return max(bbr->extra_acked[0], bbr->extra_acked[1]);
236 }
237
238 /* Return rate in bytes per second, optionally with a gain.
239 * The order here is chosen carefully to avoid overflow of u64. This should
240 * work for input rates of up to 2.9Tbit/sec and gain of 2.89x.
241 */
242 static u64 bbr_rate_bytes_per_sec(struct sock *sk, u64 rate, int gain)
243 {
244 unsigned int mss = tcp_sk(sk)->mss_cache;
245
246 rate *= mss;
247 rate *= gain;
248 rate >>= BBR_SCALE;
249 rate *= USEC_PER_SEC / 100 * (100 - bbr_pacing_margin_percent);
250 return rate >> BW_SCALE;
251 }
252
253 /* Convert a BBR bw and gain factor to a pacing rate in bytes per second. */
254 static unsigned long bbr_bw_to_pacing_rate(struct sock *sk, u32 bw, int gain)
255 {
256 u64 rate = bw;
257
258 rate = bbr_rate_bytes_per_sec(sk, rate, gain);
259 rate = min_t(u64, rate, sk->sk_max_pacing_rate);
260 return rate;
261 }
262
263 /* Initialize pacing rate to: high_gain * init_cwnd / RTT. */
264 static void bbr_init_pacing_rate_from_rtt(struct sock *sk)
265 {
266 struct tcp_sock *tp = tcp_sk(sk);
267 struct bbr *bbr = inet_csk_ca(sk);
268 u64 bw;
269 u32 rtt_us;
270
271 if (tp->srtt_us) { /* any RTT sample yet? */
272 rtt_us = max(tp->srtt_us >> 3, 1U);
273 bbr->has_seen_rtt = 1;
274 } else { /* no RTT sample yet */
275 rtt_us = USEC_PER_MSEC; /* use nominal default RTT */
276 }
277 bw = (u64)tp->snd_cwnd * BW_UNIT;
278 do_div(bw, rtt_us);
279 sk->sk_pacing_rate = bbr_bw_to_pacing_rate(sk, bw, bbr_high_gain);
280 }
281
282 /* Pace using current bw estimate and a gain factor. */
283 static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain)
284 {
285 struct tcp_sock *tp = tcp_sk(sk);
286 struct bbr *bbr = inet_csk_ca(sk);
287 unsigned long rate = bbr_bw_to_pacing_rate(sk, bw, gain);
288
289 if (unlikely(!bbr->has_seen_rtt && tp->srtt_us))
290 bbr_init_pacing_rate_from_rtt(sk);
291 if (bbr_full_bw_reached(sk) || rate > sk->sk_pacing_rate)
292 sk->sk_pacing_rate = rate;
293 }
294
295 /* override sysctl_tcp_min_tso_segs */
296 static u32 bbr_min_tso_segs(struct sock *sk)
297 {
298 return sk->sk_pacing_rate < (bbr_min_tso_rate >> 3) ? 1 : 2;
299 }
300
301 static u32 bbr_tso_segs_goal(struct sock *sk)
302 {
303 struct tcp_sock *tp = tcp_sk(sk);
304 u32 segs, bytes;
305
306 /* Sort of tcp_tso_autosize() but ignoring
307 * driver provided sk_gso_max_size.
308 */
309 bytes = min_t(unsigned long,
310 sk->sk_pacing_rate >> READ_ONCE(sk->sk_pacing_shift),
311 GSO_MAX_SIZE - 1 - MAX_TCP_HEADER);
312 segs = max_t(u32, bytes / tp->mss_cache, bbr_min_tso_segs(sk));
313
314 return min(segs, 0x7FU);
315 }
316
317 /* Save "last known good" cwnd so we can restore it after losses or PROBE_RTT */
318 static void bbr_save_cwnd(struct sock *sk)
319 {
320 struct tcp_sock *tp = tcp_sk(sk);
321 struct bbr *bbr = inet_csk_ca(sk);
322
323 if (bbr->prev_ca_state < TCP_CA_Recovery && bbr->mode != BBR_PROBE_RTT)
324 bbr->prior_cwnd = tp->snd_cwnd; /* this cwnd is good enough */
325 else /* loss recovery or BBR_PROBE_RTT have temporarily cut cwnd */
326 bbr->prior_cwnd = max(bbr->prior_cwnd, tp->snd_cwnd);
327 }
328
329 static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event)
330 {
331 struct tcp_sock *tp = tcp_sk(sk);
332 struct bbr *bbr = inet_csk_ca(sk);
333
334 if (event == CA_EVENT_TX_START && tp->app_limited) {
335 bbr->idle_restart = 1;
336 bbr->ack_epoch_mstamp = tp->tcp_mstamp;
337 bbr->ack_epoch_acked = 0;
338 /* Avoid pointless buffer overflows: pace at est. bw if we don't
339 * need more speed (we're restarting from idle and app-limited).
340 */
341 if (bbr->mode == BBR_PROBE_BW)
342 bbr_set_pacing_rate(sk, bbr_bw(sk), BBR_UNIT);
343 else if (bbr->mode == BBR_PROBE_RTT)
344 bbr_check_probe_rtt_done(sk);
345 }
346 }
347
348 /* Calculate bdp based on min RTT and the estimated bottleneck bandwidth:
349 *
350 * bdp = ceil(bw * min_rtt * gain)
351 *
352 * The key factor, gain, controls the amount of queue. While a small gain
353 * builds a smaller queue, it becomes more vulnerable to noise in RTT
354 * measurements (e.g., delayed ACKs or other ACK compression effects). This
355 * noise may cause BBR to under-estimate the rate.
356 */
357 static u32 bbr_bdp(struct sock *sk, u32 bw, int gain)
358 {
359 struct bbr *bbr = inet_csk_ca(sk);
360 u32 bdp;
361 u64 w;
362
363 /* If we've never had a valid RTT sample, cap cwnd at the initial
364 * default. This should only happen when the connection is not using TCP
365 * timestamps and has retransmitted all of the SYN/SYNACK/data packets
366 * ACKed so far. In this case, an RTO can cut cwnd to 1, in which
367 * case we need to slow-start up toward something safe: TCP_INIT_CWND.
368 */
369 if (unlikely(bbr->min_rtt_us == ~0U)) /* no valid RTT samples yet? */
370 return TCP_INIT_CWND; /* be safe: cap at default initial cwnd*/
371
372 w = (u64)bw * bbr->min_rtt_us;
373
374 /* Apply a gain to the given value, remove the BW_SCALE shift, and
375 * round the value up to avoid a negative feedback loop.
376 */
377 bdp = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT;
378
379 return bdp;
380 }
381
382 /* To achieve full performance in high-speed paths, we budget enough cwnd to
383 * fit full-sized skbs in-flight on both end hosts to fully utilize the path:
384 * - one skb in sending host Qdisc,
385 * - one skb in sending host TSO/GSO engine
386 * - one skb being received by receiver host LRO/GRO/delayed-ACK engine
387 * Don't worry, at low rates (bbr_min_tso_rate) this won't bloat cwnd because
388 * in such cases tso_segs_goal is 1. The minimum cwnd is 4 packets,
389 * which allows 2 outstanding 2-packet sequences, to try to keep pipe
390 * full even with ACK-every-other-packet delayed ACKs.
391 */
392 static u32 bbr_quantization_budget(struct sock *sk, u32 cwnd)
393 {
394 struct bbr *bbr = inet_csk_ca(sk);
395
396 /* Allow enough full-sized skbs in flight to utilize end systems. */
397 cwnd += 3 * bbr_tso_segs_goal(sk);
398
399 /* Reduce delayed ACKs by rounding up cwnd to the next even number. */
400 cwnd = (cwnd + 1) & ~1U;
401
402 /* Ensure gain cycling gets inflight above BDP even for small BDPs. */
403 if (bbr->mode == BBR_PROBE_BW && bbr->cycle_idx == 0)
404 cwnd += 2;
405
406 return cwnd;
407 }
408
409 /* Find inflight based on min RTT and the estimated bottleneck bandwidth. */
410 static u32 bbr_inflight(struct sock *sk, u32 bw, int gain)
411 {
412 u32 inflight;
413
414 inflight = bbr_bdp(sk, bw, gain);
415 inflight = bbr_quantization_budget(sk, inflight);
416
417 return inflight;
418 }
419
420 /* With pacing at lower layers, there's often less data "in the network" than
421 * "in flight". With TSQ and departure time pacing at lower layers (e.g. fq),
422 * we often have several skbs queued in the pacing layer with a pre-scheduled
423 * earliest departure time (EDT). BBR adapts its pacing rate based on the
424 * inflight level that it estimates has already been "baked in" by previous
425 * departure time decisions. We calculate a rough estimate of the number of our
426 * packets that might be in the network at the earliest departure time for the
427 * next skb scheduled:
428 * in_network_at_edt = inflight_at_edt - (EDT - now) * bw
429 * If we're increasing inflight, then we want to know if the transmit of the
430 * EDT skb will push inflight above the target, so inflight_at_edt includes
431 * bbr_tso_segs_goal() from the skb departing at EDT. If decreasing inflight,
432 * then estimate if inflight will sink too low just before the EDT transmit.
433 */
434 static u32 bbr_packets_in_net_at_edt(struct sock *sk, u32 inflight_now)
435 {
436 struct tcp_sock *tp = tcp_sk(sk);
437 struct bbr *bbr = inet_csk_ca(sk);
438 u64 now_ns, edt_ns, interval_us;
439 u32 interval_delivered, inflight_at_edt;
440
441 now_ns = tp->tcp_clock_cache;
442 edt_ns = max(tp->tcp_wstamp_ns, now_ns);
443 interval_us = div_u64(edt_ns - now_ns, NSEC_PER_USEC);
444 interval_delivered = (u64)bbr_bw(sk) * interval_us >> BW_SCALE;
445 inflight_at_edt = inflight_now;
446 if (bbr->pacing_gain > BBR_UNIT) /* increasing inflight */
447 inflight_at_edt += bbr_tso_segs_goal(sk); /* include EDT skb */
448 if (interval_delivered >= inflight_at_edt)
449 return 0;
450 return inflight_at_edt - interval_delivered;
451 }
452
453 /* Find the cwnd increment based on estimate of ack aggregation */
454 static u32 bbr_ack_aggregation_cwnd(struct sock *sk)
455 {
456 u32 max_aggr_cwnd, aggr_cwnd = 0;
457
458 if (bbr_extra_acked_gain && bbr_full_bw_reached(sk)) {
459 max_aggr_cwnd = ((u64)bbr_bw(sk) * bbr_extra_acked_max_us)
460 / BW_UNIT;
461 aggr_cwnd = (bbr_extra_acked_gain * bbr_extra_acked(sk))
462 >> BBR_SCALE;
463 aggr_cwnd = min(aggr_cwnd, max_aggr_cwnd);
464 }
465
466 return aggr_cwnd;
467 }
468
469 /* An optimization in BBR to reduce losses: On the first round of recovery, we
470 * follow the packet conservation principle: send P packets per P packets acked.
471 * After that, we slow-start and send at most 2*P packets per P packets acked.
472 * After recovery finishes, or upon undo, we restore the cwnd we had when
473 * recovery started (capped by the target cwnd based on estimated BDP).
474 *
475 * TODO(ycheng/ncardwell): implement a rate-based approach.
476 */
477 static bool bbr_set_cwnd_to_recover_or_restore(
478 struct sock *sk, const struct rate_sample *rs, u32 acked, u32 *new_cwnd)
479 {
480 struct tcp_sock *tp = tcp_sk(sk);
481 struct bbr *bbr = inet_csk_ca(sk);
482 u8 prev_state = bbr->prev_ca_state, state = inet_csk(sk)->icsk_ca_state;
483 u32 cwnd = tp->snd_cwnd;
484
485 /* An ACK for P pkts should release at most 2*P packets. We do this
486 * in two steps. First, here we deduct the number of lost packets.
487 * Then, in bbr_set_cwnd() we slow start up toward the target cwnd.
488 */
489 if (rs->losses > 0)
490 cwnd = max_t(s32, cwnd - rs->losses, 1);
491
492 if (state == TCP_CA_Recovery && prev_state != TCP_CA_Recovery) {
493 /* Starting 1st round of Recovery, so do packet conservation. */
494 bbr->packet_conservation = 1;
495 bbr->next_rtt_delivered = tp->delivered; /* start round now */
496 /* Cut unused cwnd from app behavior, TSQ, or TSO deferral: */
497 cwnd = tcp_packets_in_flight(tp) + acked;
498 } else if (prev_state >= TCP_CA_Recovery && state < TCP_CA_Recovery) {
499 /* Exiting loss recovery; restore cwnd saved before recovery. */
500 cwnd = max(cwnd, bbr->prior_cwnd);
501 bbr->packet_conservation = 0;
502 }
503 bbr->prev_ca_state = state;
504
505 if (bbr->packet_conservation) {
506 *new_cwnd = max(cwnd, tcp_packets_in_flight(tp) + acked);
507 return true; /* yes, using packet conservation */
508 }
509 *new_cwnd = cwnd;
510 return false;
511 }
512
513 /* Slow-start up toward target cwnd (if bw estimate is growing, or packet loss
514 * has drawn us down below target), or snap down to target if we're above it.
515 */
516 static void bbr_set_cwnd(struct sock *sk, const struct rate_sample *rs,
517 u32 acked, u32 bw, int gain)
518 {
519 struct tcp_sock *tp = tcp_sk(sk);
520 struct bbr *bbr = inet_csk_ca(sk);
521 u32 cwnd = tp->snd_cwnd, target_cwnd = 0;
522
523 if (!acked)
524 goto done; /* no packet fully ACKed; just apply caps */
525
526 if (bbr_set_cwnd_to_recover_or_restore(sk, rs, acked, &cwnd))
527 goto done;
528
529 target_cwnd = bbr_bdp(sk, bw, gain);
530
531 /* Increment the cwnd to account for excess ACKed data that seems
532 * due to aggregation (of data and/or ACKs) visible in the ACK stream.
533 */
534 target_cwnd += bbr_ack_aggregation_cwnd(sk);
535 target_cwnd = bbr_quantization_budget(sk, target_cwnd);
536
537 /* If we're below target cwnd, slow start cwnd toward target cwnd. */
538 if (bbr_full_bw_reached(sk)) /* only cut cwnd if we filled the pipe */
539 cwnd = min(cwnd + acked, target_cwnd);
540 else if (cwnd < target_cwnd || tp->delivered < TCP_INIT_CWND)
541 cwnd = cwnd + acked;
542 cwnd = max(cwnd, bbr_cwnd_min_target);
543
544 done:
545 tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp); /* apply global cap */
546 if (bbr->mode == BBR_PROBE_RTT) /* drain queue, refresh min_rtt */
547 tp->snd_cwnd = min(tp->snd_cwnd, bbr_cwnd_min_target);
548 }
549
550 /* End cycle phase if it's time and/or we hit the phase's in-flight target. */
551 static bool bbr_is_next_cycle_phase(struct sock *sk,
552 const struct rate_sample *rs)
553 {
554 struct tcp_sock *tp = tcp_sk(sk);
555 struct bbr *bbr = inet_csk_ca(sk);
556 bool is_full_length =
557 tcp_stamp_us_delta(tp->delivered_mstamp, bbr->cycle_mstamp) >
558 bbr->min_rtt_us;
559 u32 inflight, bw;
560
561 /* The pacing_gain of 1.0 paces at the estimated bw to try to fully
562 * use the pipe without increasing the queue.
563 */
564 if (bbr->pacing_gain == BBR_UNIT)
565 return is_full_length; /* just use wall clock time */
566
567 inflight = bbr_packets_in_net_at_edt(sk, rs->prior_in_flight);
568 bw = bbr_max_bw(sk);
569
570 /* A pacing_gain > 1.0 probes for bw by trying to raise inflight to at
571 * least pacing_gain*BDP; this may take more than min_rtt if min_rtt is
572 * small (e.g. on a LAN). We do not persist if packets are lost, since
573 * a path with small buffers may not hold that much.
574 */
575 if (bbr->pacing_gain > BBR_UNIT)
576 return is_full_length &&
577 (rs->losses || /* perhaps pacing_gain*BDP won't fit */
578 inflight >= bbr_inflight(sk, bw, bbr->pacing_gain));
579
580 /* A pacing_gain < 1.0 tries to drain extra queue we added if bw
581 * probing didn't find more bw. If inflight falls to match BDP then we
582 * estimate queue is drained; persisting would underutilize the pipe.
583 */
584 return is_full_length ||
585 inflight <= bbr_inflight(sk, bw, BBR_UNIT);
586 }
587
588 static void bbr_advance_cycle_phase(struct sock *sk)
589 {
590 struct tcp_sock *tp = tcp_sk(sk);
591 struct bbr *bbr = inet_csk_ca(sk);
592
593 bbr->cycle_idx = (bbr->cycle_idx + 1) & (CYCLE_LEN - 1);
594 bbr->cycle_mstamp = tp->delivered_mstamp;
595 }
596
597 /* Gain cycling: cycle pacing gain to converge to fair share of available bw. */
598 static void bbr_update_cycle_phase(struct sock *sk,
599 const struct rate_sample *rs)
600 {
601 struct bbr *bbr = inet_csk_ca(sk);
602
603 if (bbr->mode == BBR_PROBE_BW && bbr_is_next_cycle_phase(sk, rs))
604 bbr_advance_cycle_phase(sk);
605 }
606
607 static void bbr_reset_startup_mode(struct sock *sk)
608 {
609 struct bbr *bbr = inet_csk_ca(sk);
610
611 bbr->mode = BBR_STARTUP;
612 }
613
614 static void bbr_reset_probe_bw_mode(struct sock *sk)
615 {
616 struct bbr *bbr = inet_csk_ca(sk);
617
618 bbr->mode = BBR_PROBE_BW;
619 bbr->cycle_idx = CYCLE_LEN - 1 - prandom_u32_max(bbr_cycle_rand);
620 bbr_advance_cycle_phase(sk); /* flip to next phase of gain cycle */
621 }
622
623 static void bbr_reset_mode(struct sock *sk)
624 {
625 if (!bbr_full_bw_reached(sk))
626 bbr_reset_startup_mode(sk);
627 else
628 bbr_reset_probe_bw_mode(sk);
629 }
630
631 /* Start a new long-term sampling interval. */
632 static void bbr_reset_lt_bw_sampling_interval(struct sock *sk)
633 {
634 struct tcp_sock *tp = tcp_sk(sk);
635 struct bbr *bbr = inet_csk_ca(sk);
636
637 bbr->lt_last_stamp = div_u64(tp->delivered_mstamp, USEC_PER_MSEC);
638 bbr->lt_last_delivered = tp->delivered;
639 bbr->lt_last_lost = tp->lost;
640 bbr->lt_rtt_cnt = 0;
641 }
642
643 /* Completely reset long-term bandwidth sampling. */
644 static void bbr_reset_lt_bw_sampling(struct sock *sk)
645 {
646 struct bbr *bbr = inet_csk_ca(sk);
647
648 bbr->lt_bw = 0;
649 bbr->lt_use_bw = 0;
650 bbr->lt_is_sampling = false;
651 bbr_reset_lt_bw_sampling_interval(sk);
652 }
653
654 /* Long-term bw sampling interval is done. Estimate whether we're policed. */
655 static void bbr_lt_bw_interval_done(struct sock *sk, u32 bw)
656 {
657 struct bbr *bbr = inet_csk_ca(sk);
658 u32 diff;
659
660 if (bbr->lt_bw) { /* do we have bw from a previous interval? */
661 /* Is new bw close to the lt_bw from the previous interval? */
662 diff = abs(bw - bbr->lt_bw);
663 if ((diff * BBR_UNIT <= bbr_lt_bw_ratio * bbr->lt_bw) ||
664 (bbr_rate_bytes_per_sec(sk, diff, BBR_UNIT) <=
665 bbr_lt_bw_diff)) {
666 /* All criteria are met; estimate we're policed. */
667 bbr->lt_bw = (bw + bbr->lt_bw) >> 1; /* avg 2 intvls */
668 bbr->lt_use_bw = 1;
669 bbr->pacing_gain = BBR_UNIT; /* try to avoid drops */
670 bbr->lt_rtt_cnt = 0;
671 return;
672 }
673 }
674 bbr->lt_bw = bw;
675 bbr_reset_lt_bw_sampling_interval(sk);
676 }
677
678 /* Token-bucket traffic policers are common (see "An Internet-Wide Analysis of
679 * Traffic Policing", SIGCOMM 2016). BBR detects token-bucket policers and
680 * explicitly models their policed rate, to reduce unnecessary losses. We
681 * estimate that we're policed if we see 2 consecutive sampling intervals with
682 * consistent throughput and high packet loss. If we think we're being policed,
683 * set lt_bw to the "long-term" average delivery rate from those 2 intervals.
684 */
685 static void bbr_lt_bw_sampling(struct sock *sk, const struct rate_sample *rs)
686 {
687 struct tcp_sock *tp = tcp_sk(sk);
688 struct bbr *bbr = inet_csk_ca(sk);
689 u32 lost, delivered;
690 u64 bw;
691 u32 t;
692
693 if (bbr->lt_use_bw) { /* already using long-term rate, lt_bw? */
694 if (bbr->mode == BBR_PROBE_BW && bbr->round_start &&
695 ++bbr->lt_rtt_cnt >= bbr_lt_bw_max_rtts) {
696 bbr_reset_lt_bw_sampling(sk); /* stop using lt_bw */
697 bbr_reset_probe_bw_mode(sk); /* restart gain cycling */
698 }
699 return;
700 }
701
702 /* Wait for the first loss before sampling, to let the policer exhaust
703 * its tokens and estimate the steady-state rate allowed by the policer.
704 * Starting samples earlier includes bursts that over-estimate the bw.
705 */
706 if (!bbr->lt_is_sampling) {
707 if (!rs->losses)
708 return;
709 bbr_reset_lt_bw_sampling_interval(sk);
710 bbr->lt_is_sampling = true;
711 }
712
713 /* To avoid underestimates, reset sampling if we run out of data. */
714 if (rs->is_app_limited) {
715 bbr_reset_lt_bw_sampling(sk);
716 return;
717 }
718
719 if (bbr->round_start)
720 bbr->lt_rtt_cnt++; /* count round trips in this interval */
721 if (bbr->lt_rtt_cnt < bbr_lt_intvl_min_rtts)
722 return; /* sampling interval needs to be longer */
723 if (bbr->lt_rtt_cnt > 4 * bbr_lt_intvl_min_rtts) {
724 bbr_reset_lt_bw_sampling(sk); /* interval is too long */
725 return;
726 }
727
728 /* End sampling interval when a packet is lost, so we estimate the
729 * policer tokens were exhausted. Stopping the sampling before the
730 * tokens are exhausted under-estimates the policed rate.
731 */
732 if (!rs->losses)
733 return;
734
735 /* Calculate packets lost and delivered in sampling interval. */
736 lost = tp->lost - bbr->lt_last_lost;
737 delivered = tp->delivered - bbr->lt_last_delivered;
738 /* Is loss rate (lost/delivered) >= lt_loss_thresh? If not, wait. */
739 if (!delivered || (lost << BBR_SCALE) < bbr_lt_loss_thresh * delivered)
740 return;
741
742 /* Find average delivery rate in this sampling interval. */
743 t = div_u64(tp->delivered_mstamp, USEC_PER_MSEC) - bbr->lt_last_stamp;
744 if ((s32)t < 1)
745 return; /* interval is less than one ms, so wait */
746 /* Check if can multiply without overflow */
747 if (t >= ~0U / USEC_PER_MSEC) {
748 bbr_reset_lt_bw_sampling(sk); /* interval too long; reset */
749 return;
750 }
751 t *= USEC_PER_MSEC;
752 bw = (u64)delivered * BW_UNIT;
753 do_div(bw, t);
754 bbr_lt_bw_interval_done(sk, bw);
755 }
756
757 /* Estimate the bandwidth based on how fast packets are delivered */
758 static void bbr_update_bw(struct sock *sk, const struct rate_sample *rs)
759 {
760 struct tcp_sock *tp = tcp_sk(sk);
761 struct bbr *bbr = inet_csk_ca(sk);
762 u64 bw;
763
764 bbr->round_start = 0;
765 if (rs->delivered < 0 || rs->interval_us <= 0)
766 return; /* Not a valid observation */
767
768 /* See if we've reached the next RTT */
769 if (!before(rs->prior_delivered, bbr->next_rtt_delivered)) {
770 bbr->next_rtt_delivered = tp->delivered;
771 bbr->rtt_cnt++;
772 bbr->round_start = 1;
773 bbr->packet_conservation = 0;
774 }
775
776 bbr_lt_bw_sampling(sk, rs);
777
778 /* Divide delivered by the interval to find a (lower bound) bottleneck
779 * bandwidth sample. Delivered is in packets and interval_us in uS and
780 * ratio will be <<1 for most connections. So delivered is first scaled.
781 */
782 bw = div64_long((u64)rs->delivered * BW_UNIT, rs->interval_us);
783
784 /* If this sample is application-limited, it is likely to have a very
785 * low delivered count that represents application behavior rather than
786 * the available network rate. Such a sample could drag down estimated
787 * bw, causing needless slow-down. Thus, to continue to send at the
788 * last measured network rate, we filter out app-limited samples unless
789 * they describe the path bw at least as well as our bw model.
790 *
791 * So the goal during app-limited phase is to proceed with the best
792 * network rate no matter how long. We automatically leave this
793 * phase when app writes faster than the network can deliver :)
794 */
795 if (!rs->is_app_limited || bw >= bbr_max_bw(sk)) {
796 /* Incorporate new sample into our max bw filter. */
797 minmax_running_max(&bbr->bw, bbr_bw_rtts, bbr->rtt_cnt, bw);
798 }
799 }
800
801 /* Estimates the windowed max degree of ack aggregation.
802 * This is used to provision extra in-flight data to keep sending during
803 * inter-ACK silences.
804 *
805 * Degree of ack aggregation is estimated as extra data acked beyond expected.
806 *
807 * max_extra_acked = "maximum recent excess data ACKed beyond max_bw * interval"
808 * cwnd += max_extra_acked
809 *
810 * Max extra_acked is clamped by cwnd and bw * bbr_extra_acked_max_us (100 ms).
811 * Max filter is an approximate sliding window of 5-10 (packet timed) round
812 * trips.
813 */
814 static void bbr_update_ack_aggregation(struct sock *sk,
815 const struct rate_sample *rs)
816 {
817 u32 epoch_us, expected_acked, extra_acked;
818 struct bbr *bbr = inet_csk_ca(sk);
819 struct tcp_sock *tp = tcp_sk(sk);
820
821 if (!bbr_extra_acked_gain || rs->acked_sacked <= 0 ||
822 rs->delivered < 0 || rs->interval_us <= 0)
823 return;
824
825 if (bbr->round_start) {
826 bbr->extra_acked_win_rtts = min(0x1F,
827 bbr->extra_acked_win_rtts + 1);
828 if (bbr->extra_acked_win_rtts >= bbr_extra_acked_win_rtts) {
829 bbr->extra_acked_win_rtts = 0;
830 bbr->extra_acked_win_idx = bbr->extra_acked_win_idx ?
831 0 : 1;
832 bbr->extra_acked[bbr->extra_acked_win_idx] = 0;
833 }
834 }
835
836 /* Compute how many packets we expected to be delivered over epoch. */
837 epoch_us = tcp_stamp_us_delta(tp->delivered_mstamp,
838 bbr->ack_epoch_mstamp);
839 expected_acked = ((u64)bbr_bw(sk) * epoch_us) / BW_UNIT;
840
841 /* Reset the aggregation epoch if ACK rate is below expected rate or
842 * significantly large no. of ack received since epoch (potentially
843 * quite old epoch).
844 */
845 if (bbr->ack_epoch_acked <= expected_acked ||
846 (bbr->ack_epoch_acked + rs->acked_sacked >=
847 bbr_ack_epoch_acked_reset_thresh)) {
848 bbr->ack_epoch_acked = 0;
849 bbr->ack_epoch_mstamp = tp->delivered_mstamp;
850 expected_acked = 0;
851 }
852
853 /* Compute excess data delivered, beyond what was expected. */
854 bbr->ack_epoch_acked = min_t(u32, 0xFFFFF,
855 bbr->ack_epoch_acked + rs->acked_sacked);
856 extra_acked = bbr->ack_epoch_acked - expected_acked;
857 extra_acked = min(extra_acked, tp->snd_cwnd);
858 if (extra_acked > bbr->extra_acked[bbr->extra_acked_win_idx])
859 bbr->extra_acked[bbr->extra_acked_win_idx] = extra_acked;
860 }
861
862 /* Estimate when the pipe is full, using the change in delivery rate: BBR
863 * estimates that STARTUP filled the pipe if the estimated bw hasn't changed by
864 * at least bbr_full_bw_thresh (25%) after bbr_full_bw_cnt (3) non-app-limited
865 * rounds. Why 3 rounds: 1: rwin autotuning grows the rwin, 2: we fill the
866 * higher rwin, 3: we get higher delivery rate samples. Or transient
867 * cross-traffic or radio noise can go away. CUBIC Hystart shares a similar
868 * design goal, but uses delay and inter-ACK spacing instead of bandwidth.
869 */
870 static void bbr_check_full_bw_reached(struct sock *sk,
871 const struct rate_sample *rs)
872 {
873 struct bbr *bbr = inet_csk_ca(sk);
874 u32 bw_thresh;
875
876 if (bbr_full_bw_reached(sk) || !bbr->round_start || rs->is_app_limited)
877 return;
878
879 bw_thresh = (u64)bbr->full_bw * bbr_full_bw_thresh >> BBR_SCALE;
880 if (bbr_max_bw(sk) >= bw_thresh) {
881 bbr->full_bw = bbr_max_bw(sk);
882 bbr->full_bw_cnt = 0;
883 return;
884 }
885 ++bbr->full_bw_cnt;
886 bbr->full_bw_reached = bbr->full_bw_cnt >= bbr_full_bw_cnt;
887 }
888
889 /* If pipe is probably full, drain the queue and then enter steady-state. */
890 static void bbr_check_drain(struct sock *sk, const struct rate_sample *rs)
891 {
892 struct bbr *bbr = inet_csk_ca(sk);
893
894 if (bbr->mode == BBR_STARTUP && bbr_full_bw_reached(sk)) {
895 bbr->mode = BBR_DRAIN; /* drain queue we created */
896 tcp_sk(sk)->snd_ssthresh =
897 bbr_inflight(sk, bbr_max_bw(sk), BBR_UNIT);
898 } /* fall through to check if in-flight is already small: */
899 if (bbr->mode == BBR_DRAIN &&
900 bbr_packets_in_net_at_edt(sk, tcp_packets_in_flight(tcp_sk(sk))) <=
901 bbr_inflight(sk, bbr_max_bw(sk), BBR_UNIT))
902 bbr_reset_probe_bw_mode(sk); /* we estimate queue is drained */
903 }
904
905 static void bbr_check_probe_rtt_done(struct sock *sk)
906 {
907 struct tcp_sock *tp = tcp_sk(sk);
908 struct bbr *bbr = inet_csk_ca(sk);
909
910 if (!(bbr->probe_rtt_done_stamp &&
911 after(tcp_jiffies32, bbr->probe_rtt_done_stamp)))
912 return;
913
914 bbr->min_rtt_stamp = tcp_jiffies32; /* wait a while until PROBE_RTT */
915 tp->snd_cwnd = max(tp->snd_cwnd, bbr->prior_cwnd);
916 bbr_reset_mode(sk);
917 }
918
919 /* The goal of PROBE_RTT mode is to have BBR flows cooperatively and
920 * periodically drain the bottleneck queue, to converge to measure the true
921 * min_rtt (unloaded propagation delay). This allows the flows to keep queues
922 * small (reducing queuing delay and packet loss) and achieve fairness among
923 * BBR flows.
924 *
925 * The min_rtt filter window is 10 seconds. When the min_rtt estimate expires,
926 * we enter PROBE_RTT mode and cap the cwnd at bbr_cwnd_min_target=4 packets.
927 * After at least bbr_probe_rtt_mode_ms=200ms and at least one packet-timed
928 * round trip elapsed with that flight size <= 4, we leave PROBE_RTT mode and
929 * re-enter the previous mode. BBR uses 200ms to approximately bound the
930 * performance penalty of PROBE_RTT's cwnd capping to roughly 2% (200ms/10s).
931 *
932 * Note that flows need only pay 2% if they are busy sending over the last 10
933 * seconds. Interactive applications (e.g., Web, RPCs, video chunks) often have
934 * natural silences or low-rate periods within 10 seconds where the rate is low
935 * enough for long enough to drain its queue in the bottleneck. We pick up
936 * these min RTT measurements opportunistically with our min_rtt filter. :-)
937 */
938 static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs)
939 {
940 struct tcp_sock *tp = tcp_sk(sk);
941 struct bbr *bbr = inet_csk_ca(sk);
942 bool filter_expired;
943
944 /* Track min RTT seen in the min_rtt_win_sec filter window: */
945 filter_expired = after(tcp_jiffies32,
946 bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ);
947 if (rs->rtt_us >= 0 &&
948 (rs->rtt_us <= bbr->min_rtt_us ||
949 (filter_expired && !rs->is_ack_delayed))) {
950 bbr->min_rtt_us = rs->rtt_us;
951 bbr->min_rtt_stamp = tcp_jiffies32;
952 }
953
954 if (bbr_probe_rtt_mode_ms > 0 && filter_expired &&
955 !bbr->idle_restart && bbr->mode != BBR_PROBE_RTT) {
956 bbr->mode = BBR_PROBE_RTT; /* dip, drain queue */
957 bbr_save_cwnd(sk); /* note cwnd so we can restore it */
958 bbr->probe_rtt_done_stamp = 0;
959 }
960
961 if (bbr->mode == BBR_PROBE_RTT) {
962 /* Ignore low rate samples during this mode. */
963 tp->app_limited =
964 (tp->delivered + tcp_packets_in_flight(tp)) ? : 1;
965 /* Maintain min packets in flight for max(200 ms, 1 round). */
966 if (!bbr->probe_rtt_done_stamp &&
967 tcp_packets_in_flight(tp) <= bbr_cwnd_min_target) {
968 bbr->probe_rtt_done_stamp = tcp_jiffies32 +
969 msecs_to_jiffies(bbr_probe_rtt_mode_ms);
970 bbr->probe_rtt_round_done = 0;
971 bbr->next_rtt_delivered = tp->delivered;
972 } else if (bbr->probe_rtt_done_stamp) {
973 if (bbr->round_start)
974 bbr->probe_rtt_round_done = 1;
975 if (bbr->probe_rtt_round_done)
976 bbr_check_probe_rtt_done(sk);
977 }
978 }
979 /* Restart after idle ends only once we process a new S/ACK for data */
980 if (rs->delivered > 0)
981 bbr->idle_restart = 0;
982 }
983
984 static void bbr_update_gains(struct sock *sk)
985 {
986 struct bbr *bbr = inet_csk_ca(sk);
987
988 switch (bbr->mode) {
989 case BBR_STARTUP:
990 bbr->pacing_gain = bbr_high_gain;
991 bbr->cwnd_gain = bbr_high_gain;
992 break;
993 case BBR_DRAIN:
994 bbr->pacing_gain = bbr_drain_gain; /* slow, to drain */
995 bbr->cwnd_gain = bbr_high_gain; /* keep cwnd */
996 break;
997 case BBR_PROBE_BW:
998 bbr->pacing_gain = (bbr->lt_use_bw ?
999 BBR_UNIT :
1000 bbr_pacing_gain[bbr->cycle_idx]);
1001 bbr->cwnd_gain = bbr_cwnd_gain;
1002 break;
1003 case BBR_PROBE_RTT:
1004 bbr->pacing_gain = BBR_UNIT;
1005 bbr->cwnd_gain = BBR_UNIT;
1006 break;
1007 default:
1008 WARN_ONCE(1, "BBR bad mode: %u\n", bbr->mode);
1009 break;
1010 }
1011 }
1012
1013 static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)
1014 {
1015 bbr_update_bw(sk, rs);
1016 bbr_update_ack_aggregation(sk, rs);
1017 bbr_update_cycle_phase(sk, rs);
1018 bbr_check_full_bw_reached(sk, rs);
1019 bbr_check_drain(sk, rs);
1020 bbr_update_min_rtt(sk, rs);
1021 bbr_update_gains(sk);
1022 }
1023
1024 static void bbr_main(struct sock *sk, const struct rate_sample *rs)
1025 {
1026 struct bbr *bbr = inet_csk_ca(sk);
1027 u32 bw;
1028
1029 bbr_update_model(sk, rs);
1030
1031 bw = bbr_bw(sk);
1032 bbr_set_pacing_rate(sk, bw, bbr->pacing_gain);
1033 bbr_set_cwnd(sk, rs, rs->acked_sacked, bw, bbr->cwnd_gain);
1034 }
1035
1036 static void bbr_init(struct sock *sk)
1037 {
1038 struct tcp_sock *tp = tcp_sk(sk);
1039 struct bbr *bbr = inet_csk_ca(sk);
1040
1041 bbr->prior_cwnd = 0;
1042 tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
1043 bbr->rtt_cnt = 0;
1044 bbr->next_rtt_delivered = 0;
1045 bbr->prev_ca_state = TCP_CA_Open;
1046 bbr->packet_conservation = 0;
1047
1048 bbr->probe_rtt_done_stamp = 0;
1049 bbr->probe_rtt_round_done = 0;
1050 bbr->min_rtt_us = tcp_min_rtt(tp);
1051 bbr->min_rtt_stamp = tcp_jiffies32;
1052
1053 minmax_reset(&bbr->bw, bbr->rtt_cnt, 0); /* init max bw to 0 */
1054
1055 bbr->has_seen_rtt = 0;
1056 bbr_init_pacing_rate_from_rtt(sk);
1057
1058 bbr->round_start = 0;
1059 bbr->idle_restart = 0;
1060 bbr->full_bw_reached = 0;
1061 bbr->full_bw = 0;
1062 bbr->full_bw_cnt = 0;
1063 bbr->cycle_mstamp = 0;
1064 bbr->cycle_idx = 0;
1065 bbr_reset_lt_bw_sampling(sk);
1066 bbr_reset_startup_mode(sk);
1067
1068 bbr->ack_epoch_mstamp = tp->tcp_mstamp;
1069 bbr->ack_epoch_acked = 0;
1070 bbr->extra_acked_win_rtts = 0;
1071 bbr->extra_acked_win_idx = 0;
1072 bbr->extra_acked[0] = 0;
1073 bbr->extra_acked[1] = 0;
1074
1075 cmpxchg(&sk->sk_pacing_status, SK_PACING_NONE, SK_PACING_NEEDED);
1076 }
1077
1078 static u32 bbr_sndbuf_expand(struct sock *sk)
1079 {
1080 /* Provision 3 * cwnd since BBR may slow-start even during recovery. */
1081 return 3;
1082 }
1083
1084 /* In theory BBR does not need to undo the cwnd since it does not
1085 * always reduce cwnd on losses (see bbr_main()). Keep it for now.
1086 */
1087 static u32 bbr_undo_cwnd(struct sock *sk)
1088 {
1089 struct bbr *bbr = inet_csk_ca(sk);
1090
1091 bbr->full_bw = 0; /* spurious slow-down; reset full pipe detection */
1092 bbr->full_bw_cnt = 0;
1093 bbr_reset_lt_bw_sampling(sk);
1094 return tcp_sk(sk)->snd_cwnd;
1095 }
1096
1097 /* Entering loss recovery, so save cwnd for when we exit or undo recovery. */
1098 static u32 bbr_ssthresh(struct sock *sk)
1099 {
1100 bbr_save_cwnd(sk);
1101 return tcp_sk(sk)->snd_ssthresh;
1102 }
1103
1104 static size_t bbr_get_info(struct sock *sk, u32 ext, int *attr,
1105 union tcp_cc_info *info)
1106 {
1107 if (ext & (1 << (INET_DIAG_BBRINFO - 1)) ||
1108 ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
1109 struct tcp_sock *tp = tcp_sk(sk);
1110 struct bbr *bbr = inet_csk_ca(sk);
1111 u64 bw = bbr_bw(sk);
1112
1113 bw = bw * tp->mss_cache * USEC_PER_SEC >> BW_SCALE;
1114 memset(&info->bbr, 0, sizeof(info->bbr));
1115 info->bbr.bbr_bw_lo = (u32)bw;
1116 info->bbr.bbr_bw_hi = (u32)(bw >> 32);
1117 info->bbr.bbr_min_rtt = bbr->min_rtt_us;
1118 info->bbr.bbr_pacing_gain = bbr->pacing_gain;
1119 info->bbr.bbr_cwnd_gain = bbr->cwnd_gain;
1120 *attr = INET_DIAG_BBRINFO;
1121 return sizeof(info->bbr);
1122 }
1123 return 0;
1124 }
1125
1126 static void bbr_set_state(struct sock *sk, u8 new_state)
1127 {
1128 struct bbr *bbr = inet_csk_ca(sk);
1129
1130 if (new_state == TCP_CA_Loss) {
1131 struct rate_sample rs = { .losses = 1 };
1132
1133 bbr->prev_ca_state = TCP_CA_Loss;
1134 bbr->full_bw = 0;
1135 bbr->round_start = 1; /* treat RTO like end of a round */
1136 bbr_lt_bw_sampling(sk, &rs);
1137 }
1138 }
1139
1140 static struct tcp_congestion_ops tcp_bbr_cong_ops __read_mostly = {
1141 .flags = TCP_CONG_NON_RESTRICTED,
1142 .name = "bbr",
1143 .owner = THIS_MODULE,
1144 .init = bbr_init,
1145 .cong_control = bbr_main,
1146 .sndbuf_expand = bbr_sndbuf_expand,
1147 .undo_cwnd = bbr_undo_cwnd,
1148 .cwnd_event = bbr_cwnd_event,
1149 .ssthresh = bbr_ssthresh,
1150 .min_tso_segs = bbr_min_tso_segs,
1151 .get_info = bbr_get_info,
1152 .set_state = bbr_set_state,
1153 };
1154
1155 static int __init bbr_register(void)
1156 {
1157 BUILD_BUG_ON(sizeof(struct bbr) > ICSK_CA_PRIV_SIZE);
1158 return tcp_register_congestion_control(&tcp_bbr_cong_ops);
1159 }
1160
1161 static void __exit bbr_unregister(void)
1162 {
1163 tcp_unregister_congestion_control(&tcp_bbr_cong_ops);
1164 }
1165
1166 module_init(bbr_register);
1167 module_exit(bbr_unregister);
1168
1169 MODULE_AUTHOR("Van Jacobson <vanj@google.com>");
1170 MODULE_AUTHOR("Neal Cardwell <ncardwell@google.com>");
1171 MODULE_AUTHOR("Yuchung Cheng <ycheng@google.com>");
1172 MODULE_AUTHOR("Soheil Hassas Yeganeh <soheil@google.com>");
1173 MODULE_LICENSE("Dual BSD/GPL");
1174 MODULE_DESCRIPTION("TCP BBR (Bottleneck Bandwidth and RTT)");