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