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