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 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): */ 124 static const int bbr_bw_rtts = CYCLE_LEN + 2; 125 /* Window length of min_rtt filter (in sec): */ 126 static const u32 bbr_min_rtt_win_sec = 10; 127 /* Minimum time (in ms) spent at bbr_cwnd_min_target in BBR_PROBE_RTT mode: */ 128 static const u32 bbr_probe_rtt_mode_ms = 200; 129 /* Skip TSO below the following bandwidth (bits/sec): */ 130 static 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 */ 137 static 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 */ 141 static const int bbr_drain_gain = BBR_UNIT * 1000 / 2885; 142 /* The gain for deriving steady-state cwnd tolerates delayed/stretched ACKs: */ 143 static const int bbr_cwnd_gain = BBR_UNIT * 2; 144 /* The pacing_gain values for the PROBE_BW gain cycle, to discover/share bw: */ 145 static 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: */ 152 static 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 */ 158 static 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: */ 162 static const u32 bbr_full_bw_thresh = BBR_UNIT * 5 / 4; 163 /* But after 3 rounds w/o significant bw growth, estimate pipe is full: */ 164 static 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: */ 168 static const u32 bbr_lt_intvl_min_rtts = 4; 169 /* If lost/delivered ratio > 20%, interval is "lossy" and we may be policed: */ 170 static const u32 bbr_lt_loss_thresh = 50; 171 /* If 2 intervals have a bw ratio <= 1/8, their bw is "consistent": */ 172 static const u32 bbr_lt_bw_ratio = BBR_UNIT / 8; 173 /* If 2 intervals have a bw diff <= 4 Kbit/sec their bw is "consistent": */ 174 static const u32 bbr_lt_bw_diff = 4000 / 8; 175 /* If we estimate we're policed, use lt_bw for this many round trips: */ 176 static const u32 bbr_lt_bw_max_rtts = 48; 177 178 /* Do we estimate that STARTUP filled the pipe? */ 179 static 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. */ 187 static 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. */ 195 static 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 */ 206 static u64 bbr_rate_bytes_per_sec(struct sock *sk, u64 rate, int gain) 207 { 208 unsigned int mss = tcp_sk(sk)->mss_cache; 209 210 if (!tcp_needs_internal_pacing(sk)) 211 mss = tcp_mss_to_mtu(sk, mss); 212 rate *= mss; 213 rate *= gain; 214 rate >>= BBR_SCALE; 215 rate *= USEC_PER_SEC; 216 return rate >> BW_SCALE; 217 } 218 219 /* Convert a BBR bw and gain factor to a pacing rate in bytes per second. */ 220 static u32 bbr_bw_to_pacing_rate(struct sock *sk, u32 bw, int gain) 221 { 222 u64 rate = bw; 223 224 rate = bbr_rate_bytes_per_sec(sk, rate, gain); 225 rate = min_t(u64, rate, sk->sk_max_pacing_rate); 226 return rate; 227 } 228 229 /* Initialize pacing rate to: high_gain * init_cwnd / RTT. */ 230 static void bbr_init_pacing_rate_from_rtt(struct sock *sk) 231 { 232 struct tcp_sock *tp = tcp_sk(sk); 233 struct bbr *bbr = inet_csk_ca(sk); 234 u64 bw; 235 u32 rtt_us; 236 237 if (tp->srtt_us) { /* any RTT sample yet? */ 238 rtt_us = max(tp->srtt_us >> 3, 1U); 239 bbr->has_seen_rtt = 1; 240 } else { /* no RTT sample yet */ 241 rtt_us = USEC_PER_MSEC; /* use nominal default RTT */ 242 } 243 bw = (u64)tp->snd_cwnd * BW_UNIT; 244 do_div(bw, rtt_us); 245 sk->sk_pacing_rate = bbr_bw_to_pacing_rate(sk, bw, bbr_high_gain); 246 } 247 248 /* Pace using current bw estimate and a gain factor. In order to help drive the 249 * network toward lower queues while maintaining high utilization and low 250 * latency, the average pacing rate aims to be slightly (~1%) lower than the 251 * estimated bandwidth. This is an important aspect of the design. In this 252 * implementation this slightly lower pacing rate is achieved implicitly by not 253 * including link-layer headers in the packet size used for the pacing rate. 254 */ 255 static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain) 256 { 257 struct tcp_sock *tp = tcp_sk(sk); 258 struct bbr *bbr = inet_csk_ca(sk); 259 u32 rate = bbr_bw_to_pacing_rate(sk, bw, gain); 260 261 if (unlikely(!bbr->has_seen_rtt && tp->srtt_us)) 262 bbr_init_pacing_rate_from_rtt(sk); 263 if (bbr_full_bw_reached(sk) || rate > sk->sk_pacing_rate) 264 sk->sk_pacing_rate = rate; 265 } 266 267 /* override sysctl_tcp_min_tso_segs */ 268 static u32 bbr_min_tso_segs(struct sock *sk) 269 { 270 return sk->sk_pacing_rate < (bbr_min_tso_rate >> 3) ? 1 : 2; 271 } 272 273 static u32 bbr_tso_segs_goal(struct sock *sk) 274 { 275 struct tcp_sock *tp = tcp_sk(sk); 276 u32 segs, bytes; 277 278 /* Sort of tcp_tso_autosize() but ignoring 279 * driver provided sk_gso_max_size. 280 */ 281 bytes = min_t(u32, sk->sk_pacing_rate >> sk->sk_pacing_shift, 282 GSO_MAX_SIZE - 1 - MAX_TCP_HEADER); 283 segs = max_t(u32, bytes / tp->mss_cache, bbr_min_tso_segs(sk)); 284 285 return min(segs, 0x7FU); 286 } 287 288 /* Save "last known good" cwnd so we can restore it after losses or PROBE_RTT */ 289 static void bbr_save_cwnd(struct sock *sk) 290 { 291 struct tcp_sock *tp = tcp_sk(sk); 292 struct bbr *bbr = inet_csk_ca(sk); 293 294 if (bbr->prev_ca_state < TCP_CA_Recovery && bbr->mode != BBR_PROBE_RTT) 295 bbr->prior_cwnd = tp->snd_cwnd; /* this cwnd is good enough */ 296 else /* loss recovery or BBR_PROBE_RTT have temporarily cut cwnd */ 297 bbr->prior_cwnd = max(bbr->prior_cwnd, tp->snd_cwnd); 298 } 299 300 static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event) 301 { 302 struct tcp_sock *tp = tcp_sk(sk); 303 struct bbr *bbr = inet_csk_ca(sk); 304 305 if (event == CA_EVENT_TX_START && tp->app_limited) { 306 bbr->idle_restart = 1; 307 /* Avoid pointless buffer overflows: pace at est. bw if we don't 308 * need more speed (we're restarting from idle and app-limited). 309 */ 310 if (bbr->mode == BBR_PROBE_BW) 311 bbr_set_pacing_rate(sk, bbr_bw(sk), BBR_UNIT); 312 } 313 } 314 315 /* Find target cwnd. Right-size the cwnd based on min RTT and the 316 * estimated bottleneck bandwidth: 317 * 318 * cwnd = bw * min_rtt * gain = BDP * gain 319 * 320 * The key factor, gain, controls the amount of queue. While a small gain 321 * builds a smaller queue, it becomes more vulnerable to noise in RTT 322 * measurements (e.g., delayed ACKs or other ACK compression effects). This 323 * noise may cause BBR to under-estimate the rate. 324 * 325 * To achieve full performance in high-speed paths, we budget enough cwnd to 326 * fit full-sized skbs in-flight on both end hosts to fully utilize the path: 327 * - one skb in sending host Qdisc, 328 * - one skb in sending host TSO/GSO engine 329 * - one skb being received by receiver host LRO/GRO/delayed-ACK engine 330 * Don't worry, at low rates (bbr_min_tso_rate) this won't bloat cwnd because 331 * in such cases tso_segs_goal is 1. The minimum cwnd is 4 packets, 332 * which allows 2 outstanding 2-packet sequences, to try to keep pipe 333 * full even with ACK-every-other-packet delayed ACKs. 334 */ 335 static u32 bbr_target_cwnd(struct sock *sk, u32 bw, int gain) 336 { 337 struct bbr *bbr = inet_csk_ca(sk); 338 u32 cwnd; 339 u64 w; 340 341 /* If we've never had a valid RTT sample, cap cwnd at the initial 342 * default. This should only happen when the connection is not using TCP 343 * timestamps and has retransmitted all of the SYN/SYNACK/data packets 344 * ACKed so far. In this case, an RTO can cut cwnd to 1, in which 345 * case we need to slow-start up toward something safe: TCP_INIT_CWND. 346 */ 347 if (unlikely(bbr->min_rtt_us == ~0U)) /* no valid RTT samples yet? */ 348 return TCP_INIT_CWND; /* be safe: cap at default initial cwnd*/ 349 350 w = (u64)bw * bbr->min_rtt_us; 351 352 /* Apply a gain to the given value, then remove the BW_SCALE shift. */ 353 cwnd = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT; 354 355 /* Allow enough full-sized skbs in flight to utilize end systems. */ 356 cwnd += 3 * bbr_tso_segs_goal(sk); 357 358 /* Reduce delayed ACKs by rounding up cwnd to the next even number. */ 359 cwnd = (cwnd + 1) & ~1U; 360 361 return cwnd; 362 } 363 364 /* An optimization in BBR to reduce losses: On the first round of recovery, we 365 * follow the packet conservation principle: send P packets per P packets acked. 366 * After that, we slow-start and send at most 2*P packets per P packets acked. 367 * After recovery finishes, or upon undo, we restore the cwnd we had when 368 * recovery started (capped by the target cwnd based on estimated BDP). 369 * 370 * TODO(ycheng/ncardwell): implement a rate-based approach. 371 */ 372 static bool bbr_set_cwnd_to_recover_or_restore( 373 struct sock *sk, const struct rate_sample *rs, u32 acked, u32 *new_cwnd) 374 { 375 struct tcp_sock *tp = tcp_sk(sk); 376 struct bbr *bbr = inet_csk_ca(sk); 377 u8 prev_state = bbr->prev_ca_state, state = inet_csk(sk)->icsk_ca_state; 378 u32 cwnd = tp->snd_cwnd; 379 380 /* An ACK for P pkts should release at most 2*P packets. We do this 381 * in two steps. First, here we deduct the number of lost packets. 382 * Then, in bbr_set_cwnd() we slow start up toward the target cwnd. 383 */ 384 if (rs->losses > 0) 385 cwnd = max_t(s32, cwnd - rs->losses, 1); 386 387 if (state == TCP_CA_Recovery && prev_state != TCP_CA_Recovery) { 388 /* Starting 1st round of Recovery, so do packet conservation. */ 389 bbr->packet_conservation = 1; 390 bbr->next_rtt_delivered = tp->delivered; /* start round now */ 391 /* Cut unused cwnd from app behavior, TSQ, or TSO deferral: */ 392 cwnd = tcp_packets_in_flight(tp) + acked; 393 } else if (prev_state >= TCP_CA_Recovery && state < TCP_CA_Recovery) { 394 /* Exiting loss recovery; restore cwnd saved before recovery. */ 395 bbr->restore_cwnd = 1; 396 bbr->packet_conservation = 0; 397 } 398 bbr->prev_ca_state = state; 399 400 if (bbr->restore_cwnd) { 401 /* Restore cwnd after exiting loss recovery or PROBE_RTT. */ 402 cwnd = max(cwnd, bbr->prior_cwnd); 403 bbr->restore_cwnd = 0; 404 } 405 406 if (bbr->packet_conservation) { 407 *new_cwnd = max(cwnd, tcp_packets_in_flight(tp) + acked); 408 return true; /* yes, using packet conservation */ 409 } 410 *new_cwnd = cwnd; 411 return false; 412 } 413 414 /* Slow-start up toward target cwnd (if bw estimate is growing, or packet loss 415 * has drawn us down below target), or snap down to target if we're above it. 416 */ 417 static void bbr_set_cwnd(struct sock *sk, const struct rate_sample *rs, 418 u32 acked, u32 bw, int gain) 419 { 420 struct tcp_sock *tp = tcp_sk(sk); 421 struct bbr *bbr = inet_csk_ca(sk); 422 u32 cwnd = 0, target_cwnd = 0; 423 424 if (!acked) 425 return; 426 427 if (bbr_set_cwnd_to_recover_or_restore(sk, rs, acked, &cwnd)) 428 goto done; 429 430 /* If we're below target cwnd, slow start cwnd toward target cwnd. */ 431 target_cwnd = bbr_target_cwnd(sk, bw, gain); 432 if (bbr_full_bw_reached(sk)) /* only cut cwnd if we filled the pipe */ 433 cwnd = min(cwnd + acked, target_cwnd); 434 else if (cwnd < target_cwnd || tp->delivered < TCP_INIT_CWND) 435 cwnd = cwnd + acked; 436 cwnd = max(cwnd, bbr_cwnd_min_target); 437 438 done: 439 tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp); /* apply global cap */ 440 if (bbr->mode == BBR_PROBE_RTT) /* drain queue, refresh min_rtt */ 441 tp->snd_cwnd = min(tp->snd_cwnd, bbr_cwnd_min_target); 442 } 443 444 /* End cycle phase if it's time and/or we hit the phase's in-flight target. */ 445 static bool bbr_is_next_cycle_phase(struct sock *sk, 446 const struct rate_sample *rs) 447 { 448 struct tcp_sock *tp = tcp_sk(sk); 449 struct bbr *bbr = inet_csk_ca(sk); 450 bool is_full_length = 451 tcp_stamp_us_delta(tp->delivered_mstamp, bbr->cycle_mstamp) > 452 bbr->min_rtt_us; 453 u32 inflight, bw; 454 455 /* The pacing_gain of 1.0 paces at the estimated bw to try to fully 456 * use the pipe without increasing the queue. 457 */ 458 if (bbr->pacing_gain == BBR_UNIT) 459 return is_full_length; /* just use wall clock time */ 460 461 inflight = rs->prior_in_flight; /* what was in-flight before ACK? */ 462 bw = bbr_max_bw(sk); 463 464 /* A pacing_gain > 1.0 probes for bw by trying to raise inflight to at 465 * least pacing_gain*BDP; this may take more than min_rtt if min_rtt is 466 * small (e.g. on a LAN). We do not persist if packets are lost, since 467 * a path with small buffers may not hold that much. 468 */ 469 if (bbr->pacing_gain > BBR_UNIT) 470 return is_full_length && 471 (rs->losses || /* perhaps pacing_gain*BDP won't fit */ 472 inflight >= bbr_target_cwnd(sk, bw, bbr->pacing_gain)); 473 474 /* A pacing_gain < 1.0 tries to drain extra queue we added if bw 475 * probing didn't find more bw. If inflight falls to match BDP then we 476 * estimate queue is drained; persisting would underutilize the pipe. 477 */ 478 return is_full_length || 479 inflight <= bbr_target_cwnd(sk, bw, BBR_UNIT); 480 } 481 482 static void bbr_advance_cycle_phase(struct sock *sk) 483 { 484 struct tcp_sock *tp = tcp_sk(sk); 485 struct bbr *bbr = inet_csk_ca(sk); 486 487 bbr->cycle_idx = (bbr->cycle_idx + 1) & (CYCLE_LEN - 1); 488 bbr->cycle_mstamp = tp->delivered_mstamp; 489 bbr->pacing_gain = bbr->lt_use_bw ? BBR_UNIT : 490 bbr_pacing_gain[bbr->cycle_idx]; 491 } 492 493 /* Gain cycling: cycle pacing gain to converge to fair share of available bw. */ 494 static void bbr_update_cycle_phase(struct sock *sk, 495 const struct rate_sample *rs) 496 { 497 struct bbr *bbr = inet_csk_ca(sk); 498 499 if (bbr->mode == BBR_PROBE_BW && bbr_is_next_cycle_phase(sk, rs)) 500 bbr_advance_cycle_phase(sk); 501 } 502 503 static void bbr_reset_startup_mode(struct sock *sk) 504 { 505 struct bbr *bbr = inet_csk_ca(sk); 506 507 bbr->mode = BBR_STARTUP; 508 bbr->pacing_gain = bbr_high_gain; 509 bbr->cwnd_gain = bbr_high_gain; 510 } 511 512 static void bbr_reset_probe_bw_mode(struct sock *sk) 513 { 514 struct bbr *bbr = inet_csk_ca(sk); 515 516 bbr->mode = BBR_PROBE_BW; 517 bbr->pacing_gain = BBR_UNIT; 518 bbr->cwnd_gain = bbr_cwnd_gain; 519 bbr->cycle_idx = CYCLE_LEN - 1 - prandom_u32_max(bbr_cycle_rand); 520 bbr_advance_cycle_phase(sk); /* flip to next phase of gain cycle */ 521 } 522 523 static void bbr_reset_mode(struct sock *sk) 524 { 525 if (!bbr_full_bw_reached(sk)) 526 bbr_reset_startup_mode(sk); 527 else 528 bbr_reset_probe_bw_mode(sk); 529 } 530 531 /* Start a new long-term sampling interval. */ 532 static void bbr_reset_lt_bw_sampling_interval(struct sock *sk) 533 { 534 struct tcp_sock *tp = tcp_sk(sk); 535 struct bbr *bbr = inet_csk_ca(sk); 536 537 bbr->lt_last_stamp = div_u64(tp->delivered_mstamp, USEC_PER_MSEC); 538 bbr->lt_last_delivered = tp->delivered; 539 bbr->lt_last_lost = tp->lost; 540 bbr->lt_rtt_cnt = 0; 541 } 542 543 /* Completely reset long-term bandwidth sampling. */ 544 static void bbr_reset_lt_bw_sampling(struct sock *sk) 545 { 546 struct bbr *bbr = inet_csk_ca(sk); 547 548 bbr->lt_bw = 0; 549 bbr->lt_use_bw = 0; 550 bbr->lt_is_sampling = false; 551 bbr_reset_lt_bw_sampling_interval(sk); 552 } 553 554 /* Long-term bw sampling interval is done. Estimate whether we're policed. */ 555 static void bbr_lt_bw_interval_done(struct sock *sk, u32 bw) 556 { 557 struct bbr *bbr = inet_csk_ca(sk); 558 u32 diff; 559 560 if (bbr->lt_bw) { /* do we have bw from a previous interval? */ 561 /* Is new bw close to the lt_bw from the previous interval? */ 562 diff = abs(bw - bbr->lt_bw); 563 if ((diff * BBR_UNIT <= bbr_lt_bw_ratio * bbr->lt_bw) || 564 (bbr_rate_bytes_per_sec(sk, diff, BBR_UNIT) <= 565 bbr_lt_bw_diff)) { 566 /* All criteria are met; estimate we're policed. */ 567 bbr->lt_bw = (bw + bbr->lt_bw) >> 1; /* avg 2 intvls */ 568 bbr->lt_use_bw = 1; 569 bbr->pacing_gain = BBR_UNIT; /* try to avoid drops */ 570 bbr->lt_rtt_cnt = 0; 571 return; 572 } 573 } 574 bbr->lt_bw = bw; 575 bbr_reset_lt_bw_sampling_interval(sk); 576 } 577 578 /* Token-bucket traffic policers are common (see "An Internet-Wide Analysis of 579 * Traffic Policing", SIGCOMM 2016). BBR detects token-bucket policers and 580 * explicitly models their policed rate, to reduce unnecessary losses. We 581 * estimate that we're policed if we see 2 consecutive sampling intervals with 582 * consistent throughput and high packet loss. If we think we're being policed, 583 * set lt_bw to the "long-term" average delivery rate from those 2 intervals. 584 */ 585 static void bbr_lt_bw_sampling(struct sock *sk, const struct rate_sample *rs) 586 { 587 struct tcp_sock *tp = tcp_sk(sk); 588 struct bbr *bbr = inet_csk_ca(sk); 589 u32 lost, delivered; 590 u64 bw; 591 u32 t; 592 593 if (bbr->lt_use_bw) { /* already using long-term rate, lt_bw? */ 594 if (bbr->mode == BBR_PROBE_BW && bbr->round_start && 595 ++bbr->lt_rtt_cnt >= bbr_lt_bw_max_rtts) { 596 bbr_reset_lt_bw_sampling(sk); /* stop using lt_bw */ 597 bbr_reset_probe_bw_mode(sk); /* restart gain cycling */ 598 } 599 return; 600 } 601 602 /* Wait for the first loss before sampling, to let the policer exhaust 603 * its tokens and estimate the steady-state rate allowed by the policer. 604 * Starting samples earlier includes bursts that over-estimate the bw. 605 */ 606 if (!bbr->lt_is_sampling) { 607 if (!rs->losses) 608 return; 609 bbr_reset_lt_bw_sampling_interval(sk); 610 bbr->lt_is_sampling = true; 611 } 612 613 /* To avoid underestimates, reset sampling if we run out of data. */ 614 if (rs->is_app_limited) { 615 bbr_reset_lt_bw_sampling(sk); 616 return; 617 } 618 619 if (bbr->round_start) 620 bbr->lt_rtt_cnt++; /* count round trips in this interval */ 621 if (bbr->lt_rtt_cnt < bbr_lt_intvl_min_rtts) 622 return; /* sampling interval needs to be longer */ 623 if (bbr->lt_rtt_cnt > 4 * bbr_lt_intvl_min_rtts) { 624 bbr_reset_lt_bw_sampling(sk); /* interval is too long */ 625 return; 626 } 627 628 /* End sampling interval when a packet is lost, so we estimate the 629 * policer tokens were exhausted. Stopping the sampling before the 630 * tokens are exhausted under-estimates the policed rate. 631 */ 632 if (!rs->losses) 633 return; 634 635 /* Calculate packets lost and delivered in sampling interval. */ 636 lost = tp->lost - bbr->lt_last_lost; 637 delivered = tp->delivered - bbr->lt_last_delivered; 638 /* Is loss rate (lost/delivered) >= lt_loss_thresh? If not, wait. */ 639 if (!delivered || (lost << BBR_SCALE) < bbr_lt_loss_thresh * delivered) 640 return; 641 642 /* Find average delivery rate in this sampling interval. */ 643 t = div_u64(tp->delivered_mstamp, USEC_PER_MSEC) - bbr->lt_last_stamp; 644 if ((s32)t < 1) 645 return; /* interval is less than one ms, so wait */ 646 /* Check if can multiply without overflow */ 647 if (t >= ~0U / USEC_PER_MSEC) { 648 bbr_reset_lt_bw_sampling(sk); /* interval too long; reset */ 649 return; 650 } 651 t *= USEC_PER_MSEC; 652 bw = (u64)delivered * BW_UNIT; 653 do_div(bw, t); 654 bbr_lt_bw_interval_done(sk, bw); 655 } 656 657 /* Estimate the bandwidth based on how fast packets are delivered */ 658 static void bbr_update_bw(struct sock *sk, const struct rate_sample *rs) 659 { 660 struct tcp_sock *tp = tcp_sk(sk); 661 struct bbr *bbr = inet_csk_ca(sk); 662 u64 bw; 663 664 bbr->round_start = 0; 665 if (rs->delivered < 0 || rs->interval_us <= 0) 666 return; /* Not a valid observation */ 667 668 /* See if we've reached the next RTT */ 669 if (!before(rs->prior_delivered, bbr->next_rtt_delivered)) { 670 bbr->next_rtt_delivered = tp->delivered; 671 bbr->rtt_cnt++; 672 bbr->round_start = 1; 673 bbr->packet_conservation = 0; 674 } 675 676 bbr_lt_bw_sampling(sk, rs); 677 678 /* Divide delivered by the interval to find a (lower bound) bottleneck 679 * bandwidth sample. Delivered is in packets and interval_us in uS and 680 * ratio will be <<1 for most connections. So delivered is first scaled. 681 */ 682 bw = (u64)rs->delivered * BW_UNIT; 683 do_div(bw, rs->interval_us); 684 685 /* If this sample is application-limited, it is likely to have a very 686 * low delivered count that represents application behavior rather than 687 * the available network rate. Such a sample could drag down estimated 688 * bw, causing needless slow-down. Thus, to continue to send at the 689 * last measured network rate, we filter out app-limited samples unless 690 * they describe the path bw at least as well as our bw model. 691 * 692 * So the goal during app-limited phase is to proceed with the best 693 * network rate no matter how long. We automatically leave this 694 * phase when app writes faster than the network can deliver :) 695 */ 696 if (!rs->is_app_limited || bw >= bbr_max_bw(sk)) { 697 /* Incorporate new sample into our max bw filter. */ 698 minmax_running_max(&bbr->bw, bbr_bw_rtts, bbr->rtt_cnt, bw); 699 } 700 } 701 702 /* Estimate when the pipe is full, using the change in delivery rate: BBR 703 * estimates that STARTUP filled the pipe if the estimated bw hasn't changed by 704 * at least bbr_full_bw_thresh (25%) after bbr_full_bw_cnt (3) non-app-limited 705 * rounds. Why 3 rounds: 1: rwin autotuning grows the rwin, 2: we fill the 706 * higher rwin, 3: we get higher delivery rate samples. Or transient 707 * cross-traffic or radio noise can go away. CUBIC Hystart shares a similar 708 * design goal, but uses delay and inter-ACK spacing instead of bandwidth. 709 */ 710 static void bbr_check_full_bw_reached(struct sock *sk, 711 const struct rate_sample *rs) 712 { 713 struct bbr *bbr = inet_csk_ca(sk); 714 u32 bw_thresh; 715 716 if (bbr_full_bw_reached(sk) || !bbr->round_start || rs->is_app_limited) 717 return; 718 719 bw_thresh = (u64)bbr->full_bw * bbr_full_bw_thresh >> BBR_SCALE; 720 if (bbr_max_bw(sk) >= bw_thresh) { 721 bbr->full_bw = bbr_max_bw(sk); 722 bbr->full_bw_cnt = 0; 723 return; 724 } 725 ++bbr->full_bw_cnt; 726 bbr->full_bw_reached = bbr->full_bw_cnt >= bbr_full_bw_cnt; 727 } 728 729 /* If pipe is probably full, drain the queue and then enter steady-state. */ 730 static void bbr_check_drain(struct sock *sk, const struct rate_sample *rs) 731 { 732 struct bbr *bbr = inet_csk_ca(sk); 733 734 if (bbr->mode == BBR_STARTUP && bbr_full_bw_reached(sk)) { 735 bbr->mode = BBR_DRAIN; /* drain queue we created */ 736 bbr->pacing_gain = bbr_drain_gain; /* pace slow to drain */ 737 bbr->cwnd_gain = bbr_high_gain; /* maintain cwnd */ 738 tcp_sk(sk)->snd_ssthresh = 739 bbr_target_cwnd(sk, bbr_max_bw(sk), BBR_UNIT); 740 } /* fall through to check if in-flight is already small: */ 741 if (bbr->mode == BBR_DRAIN && 742 tcp_packets_in_flight(tcp_sk(sk)) <= 743 bbr_target_cwnd(sk, bbr_max_bw(sk), BBR_UNIT)) 744 bbr_reset_probe_bw_mode(sk); /* we estimate queue is drained */ 745 } 746 747 /* The goal of PROBE_RTT mode is to have BBR flows cooperatively and 748 * periodically drain the bottleneck queue, to converge to measure the true 749 * min_rtt (unloaded propagation delay). This allows the flows to keep queues 750 * small (reducing queuing delay and packet loss) and achieve fairness among 751 * BBR flows. 752 * 753 * The min_rtt filter window is 10 seconds. When the min_rtt estimate expires, 754 * we enter PROBE_RTT mode and cap the cwnd at bbr_cwnd_min_target=4 packets. 755 * After at least bbr_probe_rtt_mode_ms=200ms and at least one packet-timed 756 * round trip elapsed with that flight size <= 4, we leave PROBE_RTT mode and 757 * re-enter the previous mode. BBR uses 200ms to approximately bound the 758 * performance penalty of PROBE_RTT's cwnd capping to roughly 2% (200ms/10s). 759 * 760 * Note that flows need only pay 2% if they are busy sending over the last 10 761 * seconds. Interactive applications (e.g., Web, RPCs, video chunks) often have 762 * natural silences or low-rate periods within 10 seconds where the rate is low 763 * enough for long enough to drain its queue in the bottleneck. We pick up 764 * these min RTT measurements opportunistically with our min_rtt filter. :-) 765 */ 766 static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs) 767 { 768 struct tcp_sock *tp = tcp_sk(sk); 769 struct bbr *bbr = inet_csk_ca(sk); 770 bool filter_expired; 771 772 /* Track min RTT seen in the min_rtt_win_sec filter window: */ 773 filter_expired = after(tcp_jiffies32, 774 bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ); 775 if (rs->rtt_us >= 0 && 776 (rs->rtt_us <= bbr->min_rtt_us || 777 (filter_expired && !rs->is_ack_delayed))) { 778 bbr->min_rtt_us = rs->rtt_us; 779 bbr->min_rtt_stamp = tcp_jiffies32; 780 } 781 782 if (bbr_probe_rtt_mode_ms > 0 && filter_expired && 783 !bbr->idle_restart && bbr->mode != BBR_PROBE_RTT) { 784 bbr->mode = BBR_PROBE_RTT; /* dip, drain queue */ 785 bbr->pacing_gain = BBR_UNIT; 786 bbr->cwnd_gain = BBR_UNIT; 787 bbr_save_cwnd(sk); /* note cwnd so we can restore it */ 788 bbr->probe_rtt_done_stamp = 0; 789 } 790 791 if (bbr->mode == BBR_PROBE_RTT) { 792 /* Ignore low rate samples during this mode. */ 793 tp->app_limited = 794 (tp->delivered + tcp_packets_in_flight(tp)) ? : 1; 795 /* Maintain min packets in flight for max(200 ms, 1 round). */ 796 if (!bbr->probe_rtt_done_stamp && 797 tcp_packets_in_flight(tp) <= bbr_cwnd_min_target) { 798 bbr->probe_rtt_done_stamp = tcp_jiffies32 + 799 msecs_to_jiffies(bbr_probe_rtt_mode_ms); 800 bbr->probe_rtt_round_done = 0; 801 bbr->next_rtt_delivered = tp->delivered; 802 } else if (bbr->probe_rtt_done_stamp) { 803 if (bbr->round_start) 804 bbr->probe_rtt_round_done = 1; 805 if (bbr->probe_rtt_round_done && 806 after(tcp_jiffies32, bbr->probe_rtt_done_stamp)) { 807 bbr->min_rtt_stamp = tcp_jiffies32; 808 bbr->restore_cwnd = 1; /* snap to prior_cwnd */ 809 bbr_reset_mode(sk); 810 } 811 } 812 } 813 /* Restart after idle ends only once we process a new S/ACK for data */ 814 if (rs->delivered > 0) 815 bbr->idle_restart = 0; 816 } 817 818 static void bbr_update_model(struct sock *sk, const struct rate_sample *rs) 819 { 820 bbr_update_bw(sk, rs); 821 bbr_update_cycle_phase(sk, rs); 822 bbr_check_full_bw_reached(sk, rs); 823 bbr_check_drain(sk, rs); 824 bbr_update_min_rtt(sk, rs); 825 } 826 827 static void bbr_main(struct sock *sk, const struct rate_sample *rs) 828 { 829 struct bbr *bbr = inet_csk_ca(sk); 830 u32 bw; 831 832 bbr_update_model(sk, rs); 833 834 bw = bbr_bw(sk); 835 bbr_set_pacing_rate(sk, bw, bbr->pacing_gain); 836 bbr_set_cwnd(sk, rs, rs->acked_sacked, bw, bbr->cwnd_gain); 837 } 838 839 static void bbr_init(struct sock *sk) 840 { 841 struct tcp_sock *tp = tcp_sk(sk); 842 struct bbr *bbr = inet_csk_ca(sk); 843 844 bbr->prior_cwnd = 0; 845 tp->snd_ssthresh = TCP_INFINITE_SSTHRESH; 846 bbr->rtt_cnt = 0; 847 bbr->next_rtt_delivered = 0; 848 bbr->prev_ca_state = TCP_CA_Open; 849 bbr->packet_conservation = 0; 850 851 bbr->probe_rtt_done_stamp = 0; 852 bbr->probe_rtt_round_done = 0; 853 bbr->min_rtt_us = tcp_min_rtt(tp); 854 bbr->min_rtt_stamp = tcp_jiffies32; 855 856 minmax_reset(&bbr->bw, bbr->rtt_cnt, 0); /* init max bw to 0 */ 857 858 bbr->has_seen_rtt = 0; 859 bbr_init_pacing_rate_from_rtt(sk); 860 861 bbr->restore_cwnd = 0; 862 bbr->round_start = 0; 863 bbr->idle_restart = 0; 864 bbr->full_bw_reached = 0; 865 bbr->full_bw = 0; 866 bbr->full_bw_cnt = 0; 867 bbr->cycle_mstamp = 0; 868 bbr->cycle_idx = 0; 869 bbr_reset_lt_bw_sampling(sk); 870 bbr_reset_startup_mode(sk); 871 872 cmpxchg(&sk->sk_pacing_status, SK_PACING_NONE, SK_PACING_NEEDED); 873 } 874 875 static u32 bbr_sndbuf_expand(struct sock *sk) 876 { 877 /* Provision 3 * cwnd since BBR may slow-start even during recovery. */ 878 return 3; 879 } 880 881 /* In theory BBR does not need to undo the cwnd since it does not 882 * always reduce cwnd on losses (see bbr_main()). Keep it for now. 883 */ 884 static u32 bbr_undo_cwnd(struct sock *sk) 885 { 886 struct bbr *bbr = inet_csk_ca(sk); 887 888 bbr->full_bw = 0; /* spurious slow-down; reset full pipe detection */ 889 bbr->full_bw_cnt = 0; 890 bbr_reset_lt_bw_sampling(sk); 891 return tcp_sk(sk)->snd_cwnd; 892 } 893 894 /* Entering loss recovery, so save cwnd for when we exit or undo recovery. */ 895 static u32 bbr_ssthresh(struct sock *sk) 896 { 897 bbr_save_cwnd(sk); 898 return tcp_sk(sk)->snd_ssthresh; 899 } 900 901 static size_t bbr_get_info(struct sock *sk, u32 ext, int *attr, 902 union tcp_cc_info *info) 903 { 904 if (ext & (1 << (INET_DIAG_BBRINFO - 1)) || 905 ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 906 struct tcp_sock *tp = tcp_sk(sk); 907 struct bbr *bbr = inet_csk_ca(sk); 908 u64 bw = bbr_bw(sk); 909 910 bw = bw * tp->mss_cache * USEC_PER_SEC >> BW_SCALE; 911 memset(&info->bbr, 0, sizeof(info->bbr)); 912 info->bbr.bbr_bw_lo = (u32)bw; 913 info->bbr.bbr_bw_hi = (u32)(bw >> 32); 914 info->bbr.bbr_min_rtt = bbr->min_rtt_us; 915 info->bbr.bbr_pacing_gain = bbr->pacing_gain; 916 info->bbr.bbr_cwnd_gain = bbr->cwnd_gain; 917 *attr = INET_DIAG_BBRINFO; 918 return sizeof(info->bbr); 919 } 920 return 0; 921 } 922 923 static void bbr_set_state(struct sock *sk, u8 new_state) 924 { 925 struct bbr *bbr = inet_csk_ca(sk); 926 927 if (new_state == TCP_CA_Loss) { 928 struct rate_sample rs = { .losses = 1 }; 929 930 bbr->prev_ca_state = TCP_CA_Loss; 931 bbr->full_bw = 0; 932 bbr->round_start = 1; /* treat RTO like end of a round */ 933 bbr_lt_bw_sampling(sk, &rs); 934 } 935 } 936 937 static struct tcp_congestion_ops tcp_bbr_cong_ops __read_mostly = { 938 .flags = TCP_CONG_NON_RESTRICTED, 939 .name = "bbr", 940 .owner = THIS_MODULE, 941 .init = bbr_init, 942 .cong_control = bbr_main, 943 .sndbuf_expand = bbr_sndbuf_expand, 944 .undo_cwnd = bbr_undo_cwnd, 945 .cwnd_event = bbr_cwnd_event, 946 .ssthresh = bbr_ssthresh, 947 .min_tso_segs = bbr_min_tso_segs, 948 .get_info = bbr_get_info, 949 .set_state = bbr_set_state, 950 }; 951 952 static int __init bbr_register(void) 953 { 954 BUILD_BUG_ON(sizeof(struct bbr) > ICSK_CA_PRIV_SIZE); 955 return tcp_register_congestion_control(&tcp_bbr_cong_ops); 956 } 957 958 static void __exit bbr_unregister(void) 959 { 960 tcp_unregister_congestion_control(&tcp_bbr_cong_ops); 961 } 962 963 module_init(bbr_register); 964 module_exit(bbr_unregister); 965 966 MODULE_AUTHOR("Van Jacobson <vanj@google.com>"); 967 MODULE_AUTHOR("Neal Cardwell <ncardwell@google.com>"); 968 MODULE_AUTHOR("Yuchung Cheng <ycheng@google.com>"); 969 MODULE_AUTHOR("Soheil Hassas Yeganeh <soheil@google.com>"); 970 MODULE_LICENSE("Dual BSD/GPL"); 971 MODULE_DESCRIPTION("TCP BBR (Bottleneck Bandwidth and RTT)"); 972